we llllllllllllIll!IllHJNIHUIIUllllllllllllllllllllllllllll g“ _ [Jinnah erate ‘ University 310514 5837 This is to certify that the thesis entitled STUDIES AND APPLICATIONS IN TRIPLE QUADRUPOLE MASS SPECTROMETRY presented by ROBERT KAZMER LATVEN has been accepted towards fulfillment of the requirements for Ph . D. Jame in Chemistry a./9.&éc/ Major professor 0-7639 if a g,‘ f 21' 5" ' '- ~ :3 h I a I“ F 5 r: ‘. 3.3 "it; a , . )VIESI_} RETURNING MATERIALS: lace in book drop to LJBRAfiJEs remove this checkout from “or. your record. fig W1” be charged if book is returned after the date stamped below. STUDIES AND APPLICATIONS IN TRIPLE QUADRUPOLE MASS SPECTROMETRY BY ROBERT KAZMER LATVEN A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1981 67/ 7/77 ABSTRACT Studies and Applications in Triple Quadrupole Mass Spectrometry by Robert Kazmer Latven The energetics of collisionally activated decomposition is studied and applications in structure elucidation and mixture analysis are explored by triple quadrupole mass spectrometry. In this technique, ions formed in the mass Spectrometer ion source are selected by their mass to charge ratio in the first quadrupole mass filter to pass into an "RF-only" quadrupole collision cell. The selected ion undergoes collisionally activated decomposition by low energy ((100 eV) impact with argon or other target gas. The resulting fragments are mass analyzed by a third quadrupole and detected. Collision cross sections are studied as a function of internal energy. Carbon monoxide ion is studied as it is formed from carbon monoxide, carbon dioxide, acetaldehyde, formic acid, and methanol. Cross sections are determined as a function of electron energy for each of the stated compounds. Metastable peaks are observed in the triple quadrupole mass Spectrometer which heretofore have been reported only for sector instruments. Ions which result from unimolecular decomposition are characterized by their independence from target pressure at low pressures. The translational, or axial kinetic energy of ions undergoing collisionally activated decomposition significantly influences the cross sections of the processes available to the selected ion. Discussed are axial energy characteristics of parent and unimolecular decomposition product ions, daughter ions, and charge transfer and proton transfer products. Carbon monoxide is determined at parts-per-million concentration in the presence of C02, N2 and hydrocarbons. Quantification of the CO concentration is achieved by the addition of a known amount of labeled internal standard, 13C0. The accuracy of the determination is better than 20% in concentrations above 10 ppm. The molecular ion and some important ions of ethanol, ethanol-OD, and 2,2,2-d3 ethanol are studied by TQMS to obtain information regarding the ions' structures and fragmentation pathways. Unambiguous assignment of parent ion structure and fragmentation mechanisms is possible in many cases. C: Copyri ght by ROBERT KAZMER LATVEN 1981 ii To Alix and Robby iii ACKNOWLEDGEMENTS This culmination of academic effort is the net result of the unselfish assistance of many. I would like to acknowledge first those whose guidance has brought me to these pages. First and foremost is my father Albert R. Latven, who early instilled the foundation of appreciation for the beauty in scientific truth. I then must thank Dr. and Mrs. M.L. "Clem" Clevenger, for their strong but gentle guidance which saved me from the path of self-destruction. At the University of New Mexico, it was Professor R. D. Caton's enthusiastic lectures which first directed my interest to the field of chemistry. Professor Tom Niemczyk was instrumental in my focusing on the analytical applications of electronic control and measurement. At Michigan State, the research project I inherited from now Professor Rick Yost turns out to have charted my postgraduate field. Last in time, but not in contribution, is my research director, Professor Chris Enke, without whose guidance, inspiration and insights my graduate career could not have been so completely rewarding. In the art of computer graphics, the assistance of Dr. Tom Atkinson ensured nunerous deadlines to be successfully passed; the critical and helpful comments of my second reader, Professor J. Allison, were crucial for the development of the manuscripts iv contained herein. The careful proofreading of Ms. Meg McFarland and faultless typing of Ms. Theresa Fillwock were also very appreciated as were the help and encouragement received from my associates and fellow group members, especially Mr. Milton Nebber. I would also like to acknowledge the generous support of the National Institutes of Health under whose grant most of my research was performed, and to the General Electric Company for a summer research fellowship they provided. Finally, I want to express my appreciation and gratitude for the support I received from the very start from my wife, Katie. Her unflagging effort in pulling the family through these sometimes arduous, sometimes lovely days in Michigan will always be remembered. TABLE OF CONTENTS Page List of Tables ..................... . . . . .viii List of Figures . . . . ................... . . ix Chapter 1. Introduction .............. . ...... 1 Organization . . ..... . . . . . . . . . . ...... . 2 Overview . . . . . . . . . ......... . . . . . . . . 3 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Introduction to the Research ........ . . . . . . . . 10 References . . . . . . . . . . . . . . . . . . . . . . . . . 13 Chapter 2. Precursor Effects on the Fragmentation of CO+ by Triple Quadrupole Mass Spectrometry . . . . . . . . . 15 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Introduction . . . . . . . . . . . . . . .......... 17 Experimental . ....... . . . . . . . . . . . . . . . . 19 Results. . ........... . ............. 20 Discussion ...... . .................. 26 References ........ . . ............... 34 Chapter 3. Observations of Metastable Decompositions in a Triple Quadrupole Mass Spectrometer ........ . 36 References . . . . . . . . . . . . . . . . . . . . . . . . . 41 Chapter 4. Characterization of Collisionally Activated Decomposition Product Ions by Axial Energy Profiling in Triple Quadrupole Mass Spectrometry. . . 42 Smmmy... ... ... ... ... ... ... ... ... 43 Introduction . . . . . ................... 44 Experimental . . . . . . . . ....... . ....... . 45 Results and Discussion ......... . . . . . . . . . . 46 References . . . . . . . . . . . . . . . . . . . . . . . . . 68 Chapter 5. Determination of Carbon Monoxide by Air by Triple Quadrupole Mass Spectrometry . . ....... 69 Abstract .......................... 70 Introduction . . . . ............ . ....... 71 Experimental . . . . . ................... 73 Results and Discussion ................... 74 References ......................... 84 vi Page Chapter 6. Structures and Fragmentation Mechanisms of the Ions of Ethanol by Triple Quadrupole Mass Spectrometry . . 86 Abstract 0 O O I O I O O O O O O 0 O O O O O O O O O O I O 0 87 Introduction. . . . . . . . . . . . . . . . . . . . . . . . 88 Experimental. . . . . . . . . . . . . . . . . . . . . . . . 89 Results and Discussion. . . . . . . . . . . ..... . . . 91 Conclusions . . . . . . . . . . . . . . . . . . . . . . . 105 References. . . . . . . . . . . . . . . . . . . . . 111 Chapter 7. Comments and Conclusions . . ....... . . . . . 112 Appendix 1. A New Electron Impact Ion Source. . . . . . . . . . 115 vii Table Table Table Table Table Table Table Table Table 4.1 4.2 5.1 6.1 6.2 6.3 6.4 6.5 6.6 LIST OF TABLES Page EAX values for two transitions of the [C0]+° parent ion from various samples. Q3 Offset f1. XEd at -25 v. C O O O O O O O O O O O 0 O O 57 Charge and proton exchange reactions seen in TQMS ....................... . 66 Relative precision of C0 determinations ....... 81 Low energy (EAX = 25 eV) CAD spectra of ethanol ions. Intensities are given as 1 of most abundant daughter ions . . ......... . . . . 92 Low energy (EAX = 25 eV) CAD spectra of ethanol-OD ions ....... . .......... . 93 Low energy (EAX = 25 eV) CAD spectra of 2,2,2'd3 EthanOI TONS. o o o o o o o o o o o o o o o 94 Low energy (EAX = 25 eV) CAD spectra of methanola and methanol—ODb ions. . . . . . . . . . . 95 Structure of the major ions of ethanola and methan01b0 O O O O O O O O O O O O O O O O O O O O 0101 Comparison of high energy CID in MIKES with low-energy CAD in TQMS for the fragmentation of ions of ethanol. . . . . . . . . . . . . . . . . . .105 viii Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 1.1 2.1 2.2 2.3 2.4 2.5 3.1 4.1 4.2 4.3 LIST OF FIGURES Schematic of triple quadrupole mass spectrometer. O O O O O O O O O O O O O O O O O O Ionization efficiency curve for C0 C0+ by El; fragmentation efficiency curve for CO+ C+ + 0 by Ar CAD, from C0 sample . . . . . . . . . . . . Ionization efficiency curve for C02 C0+ + 0 by El, fragmentation efficiency curve for C0+ C+ + D by Ar CAD, from C02 sample . . . . . CAD cross sections for the C0+ C+ + 0 transition for C0, C02, methanol, acetaldehyde O and formic aCi d O O O O O O O O O O O O O O O O O I O Morse potential curve of the X2 + state of C0+ showing vibrational levels and equilibrium distances for CO+ precursors. For CO+, r0 = 1.12 A . . . . . . . . . . . . . . . . . . . Axial (translational) energy profiles of CAD product ions for the C0+ C+ + 0 transition from C0 and C02 precursors. Cross sections are normalized to constant parent ion flux. . . . . . . . Collisionally activated decomposition of propane molecular ion (m/z = 44). Ion intensities are shown as a function of target pressure. Ion at m/z 43 is unimolecular decomposition product. . . . Ion current as a function of axial energy for m/z 46 parent ion from ethanol, its first derivative and the [M-H]+ unimolecular decomposition product . . . . . . . . . . . . . . . . . . . . . Axial energy profiles for parent and daughter for N2 N+ + N. VQZ = VQ3. . . . . . . . . . . . . Axial energy profiles for carbon monoxide system; [CO]+' parent, and daughters C+ and 0+. . . . . . . ix Page . 21 . 22 . 25 O 29 32 39 . 48 . 54 . 56 Figure Figure Figure Figure Figure Figure Figure Figure Figure 4.4 4.5 4.6 5.1 5.2 5.3 5.4 6.1 A.1 Axial energy profiles for [C0]+- C+ in carbon monoxide at various ionizing electron energies. O O O O O O O O O O O O O O O O O O O O O 0 Axial energy profiles for [C0]+ C+ in carbon dioxide at various Ar target gas pressures O O O O C O O O O O O O O O O O O O O O O 0 Axial energy profiles for [CH0]+ parent ion, daughter ions [CH]+ and CT, and proton transfer product [H30]+-. . . . . . . . . . . . . . . CAD spectrum of 10 ppm CO in N2 . . . . . . . . . . . Electron impact ionization efficiency curves for C0+; fragmentation efficiency curves for C0+ C+ + 0 for CO and C02 samples . . . . . . . . . C02 and 13C0 neutral loss of 16 scan, Ar CAD, at 70, 30, and 20 eV electron energy . . . . . . 20 ppm CO, 20 ppm 13CO in air, neutral loss Of 16 scan, Ar CAD. O I O O O O O O O O O I O O O O 0 CAD product ion intensity vs. pressure for the fragmentation of 46+ from ethanol . . ........ Schematic of ion source .......... . . . . . Page . 59 . 97 .116 Chapter 1. INTRODUCTION ORGANIZATION This first chapter encompasses an overview of selected ion fragmentation techniques in mass spectrometry, which includes the detection of metastable decompositions as well as those of high energy (keV) collision-induced dissociations (CID) in normal and reversed geometry sector instruments. The more recent introduction of triple quadrupole mass spectrometry (TQMS) for studies of the low energy (less than 100 eV) collisionally activated decomposition (CAD) process is the subject of the research presented here; its development and theory of Operation are therefore presented in greater detail. Chapters 2 through 6 explore both the fundamental behavior of low energy ion-molecule reactions upon which this technique is built, and applications of TQMS in the areas of structure elucidation and mixture analysis. These chapters are presented in manuscript format. Chapter 7 includes conjectures, comments and suggestions for future work; the appendix details the design, construction and perfor- mance evaluation of a new electron impact ion source. w Selected ion fragmentation involves the determination of both the precursor ion and product ion(s) of the ionic reaction A++B++C The kinetics of this reaction may be either “zeroth” order (unimolecular), or first order (collision-induced) with respect to any target gas present. The earliest and still most extensive observations of selected ion fragmentation mass spectra are from metastable ions (1-4) observed in a sector mass spectrometer. A metastable ion is one that is sufficiently stable to leave the ionization chamber, but which unimolecularly decomposes before reaching the detector. Ions which decompose in the field free region in front of the magnetic sector will be focused and detected under normal operating conditions (5). These "metastable peaks" are broad and appear at non-integer mass units superimposed on the normal mass Spectrum. Nevertheless, information regarding the nature of the transition is contained in the value of the fractional mass at which the metastable peak appears. If m* is the apparent mass of the metastable peak, the relation between this value, and the true mass of the parent (m1) and daughter (m2) is given by m* = (m2)2/m1 Since m1 and m2 are integers, their values can often be determined uniquely. Often, however, few ions from a given molecule provide observable metastable transitions (6). In addition, the low collection efficiency for metastable products results in many metastable transitions remaining undetected. Ions which decompose in front of the electric sector of a normal geometry double-focusing mass spectrometer will be unable to pass through this sector because they will have lost kinetic energy (5). The solution to this problem was presented by Jennings in 1965 (7) with the introduction of the high voltage scanning technique (HV). The ion accelerating voltage is scanned from its normal value V with constant electric sector voltage and magnetic field. Normal ions are no longer transmitted, but the daughter ions of mass m2 formed from parents of mass m1 will be collected when the accelerating voltage is V (ml/m2). This leads to the observation and identification of more metastable transitions than is possible using the second field free region between the analyzers (8). However, conditions in the ion source are affected by changes in the accelerating voltage and this limits the mass range over which such scans can be carried out to less than 8:1 (7). This makes it impossible to examine very low mass daughter ions. If the voltage applied between the plates of the electric sector is lowered by E1 = E(m2/m1), metastable decomposition product ions can be detected behind the B-slit on the energy focusing plane after the electric sector (9). Since the electric sector is an energy focusing device, a spectrum produced from its scan (V constant) is known as an ion kinetic energy spectrun (IKES). A more recent technique in the evolution of sector instruments involves reversing the geometry of the magnetic and electric sectors, i.e., positioning the magnetic sector first, in order to mass-select the parent ion prior to an IKES analysis (10). This mass-analyzed ion kinetic energy spectrum (MIKES) yields unequivocal information concerning ion fragmentations and avoids the overlapping of IKES peaks. The MIKES instrument is well-suited for the study of high energy collision-induced dissociation of selected ions (11-15), but peak broadening from the kinetic energy release upon fragmentation reduces resolution to below the unit mass level, thus restricting many applications. The use of quadrupoles for selected ion fragmentation was first proposed by Yost and Enke in 1978 (17). The configuration suggested by them involves the use of three quadrupoles in series. The first quadrupole selects the mass of the parent ion; the second quadrupole is used in "RF-only“ mode, which means ions of all m/z are stable within the RF field. A target gas is bled into this quadrupole and collisions between target and selected ions result in fragmentations which are mass analyzed by the third quadrupole and detected. The quadrupole implementation of selected ion fragmentation affords high collision efficiency and throughput, with better than unit mass resolution for both analyzer sections (18). Its capabilities have been demonstrated for several applications (19-20). m The triple quadrupole mass spectrometer (TQMS) consists of, in series, an ionization source, a quadrupole mass filter, an “RF-only" collision cell quadrupole, a second mass filter and an ion detector. Figure 1 shows a schematic of the TQMS. ION QUAD QUAD COLLISION OUAD PAPTICLE SOURCE MASS FILTER CHAMBER MASS FILTER MULTIPLIER ________-_ _ __ -_,’Tx I) n '___________J_“ Q} j 'J___________- __,:/~‘ (I I (E - ;:::::::::::1_,u q 41 E3 J --------------- J jQ j w ‘l -\ SAMPLE ION ION FRAGMENTATION PRODUCT ION ION IONIZATION SELECTION f/on REACTION SELECTION DETECTION Figure 1.1 Schematic of Triple Quadrupole Mass Spectrometer All components described above are housed in a vacuum chamber which is pumped by turbomolecular pumps to a base pressure of ca. 10‘7 torr. The sample is introduced into the ion source by either a direct probe for solid or nonvolatile liquid samples, or via a line from a batch inlet system for volatile liquids or gases. An electron impact ion source consists of an enclosed volume through which pass sample molecules and a beam of electrons. The kinetic energy of the electrons in the beam is variable, but usually held to 70 eV or lower. The electron current is typically 10 to 500 uA. A bombarding electron passing closely by a molecule of several A diameter can only interact for about 10'16 seconds. In this short time, the heavy nuclei cannot undergo significant vibrational motion, and thus the electronic excitations nay be considered vertical processes which satisfy the Franck-Condon principle (21). Vibrational excitation of an ion can occur when it undergoes internal conversion from a low vibrational level of a highly excited electronic state to a high vibrational level of a lower lying electronic state. Eventually, the energy deposited by the electron will be randomly distributed throughout the vibrational degrees of freedom of the molecule. The molecular ion may then decompose into fragment ions to carry away the excess internal energy. Fragmentation can also arise from vertical excitation to a dissociative vibrational level of a bound state as well as excitation to a dissociative electronic state directly. Ions thus formed are pushed from their point of ionization by action of an electrostatic repeller plate, are withdrawn from the ion source by an extracting lens, and are then focused, with a certain amount of translational energy, into the first quadrupole mass filter. In the quadrupole, the ions experience both RF and DC fields. Across one set of diagonally paired cyclindrical rods are impressed an RF voltage (v = 1-3 Mhz 0.3 3000 V) which is offset from ground by a given DC voltage (5 500 V). The opposite pair of rods are given the same RF amplitude and frequency, but phase shifted 180°. In addition, the DC voltage is of the opposite polarity. A light ion feels the effect of the RF voltage more strongly than it feels the DC voltage. There is therefore a low mass cutoff based on the amplitude of the radio frequency. Similarly, heavy mass ions are more influenced by DC voltage, and as this voltage increases, the high mass cutoff decreases. There can be found, then, a unique combination of RF and DC voltages, (at a given RF frequency) at which an ion of only a particular mass is stable. The first quadrupole is then set to pass this ion of interest, and reject transmission of all others. This selected ion exits the first quadrupole through a lens and then enters the second, RF-only quadrupole with a kinetic energy of ca. 0 to 50 eV. As implied by its name, the RF-only quadrupole carries no DC offset. There is therefore no high mass cutoff. Also, the RF amplitude is kept low enough (ca. 25 V) that the low mass cutoff is below any mass of interest (usually 12 amu). The RF-only quadrupole is therefore, stable for the transmission of ions of all m/z. In addition, the RF field is positive focusing, which results in the transmission of ions which have up to several electron volts off-axis energy. Into the RF-only quadrupole is bled a target gas, which undergoes collision with the selected ion. The time of interaction of the ion and target is > 10"14 seconds. Since this is on the time scale for vibrations, it is likely that these collisions directly excite the higher vibrational energy levels of the ion. There is evidence of long-lived collision complexes (22,23). Internal energy is randomized and collisional spectra are produced which are qualitatively similar to those produced by electron impact. A number of other important factors can influence the collisionally activated decomposition process. These include the axial (translational) energy, nature and pressure of the target gas, and the internal energy of the selected ion. The effects of these variables are discussed further in Chapters 2 and 4. After collision, the resulting products exit the collision cell and are accelerated into the third quadrupole, a mass filter. A mass range is scanned and the m/z values of the CAD fragment ions are detected and recorded. 10 INTRODUCTION TO THE RESEARCH The overall objective of the author's research involves developing a clearer understanding of the processes which govern low energy collisionally activated decompositions in the triple quadrupole mass spectrometer; in addition, the usefulness of the technique is demonstrated by its application in the areas of structure elucidation and mixture analysis. Chapters 2-6 are presented in manuscript format. The nature of the collision process is probed by studying the effects of the selected ion's internal and kinetic energy on fragmentation, as well as changes which result from varying the atomic weight of the target and the target pressure. The second chapter, entitled, "Precursor Effects on the Fragmentation of CO+ by Triple Quadrupole M.S." focuses on the internal energy effects of the ion undergoing collision. Carbon monoxide ion is studied and the cross section for fragmentation is determined for electron energies which vary from appearance to 70 eV. Ions of CO+ are generated from five precursors: carbon monoxide, carbon dioxide, methanol, acetaldehyde, and formic acid. Each sample produces a C0+ ion with an apparent difference in internal energy. Ions of C0+ are seen to fragment with different efficiency (cross section) based on both electron energy and nature of the precursor. ll The third chapter is a note which announces the detection and characterization of unimolecular decomposition or "metastable ion" products in the triple quadrupole. Pressure studies of several molecules Show that [M-H]+ and [M-H2]+° ion intensity is always independent of target pressure at low pressures, although there is a slight collisional contribution to the ion current at pressures above 5 x 10'5 torr. Target pressure independence (zero order kinetics) demonstrates unimolecular decompositions and hence, the metastable nature of the parent ion. “Characterization of Collisionally Activated Decomposition Product Ions by Axial Energy Profiling in Triple Quadrupole Mass Spectrometry," (Chapter 4) demonstrates the effect on fragmentation efficiency by variations in axial energy (translational kinetic energy), target gas pressure and nature of target. Several classes of CAD product ions provide unique axial energy profiles; that is, the intensity response of a monitored CAD transition with respect to axial energy yields peak shapes or profiles which can indicate the nature of the reaction by ’ which the product is formed. Parent ions, unimolecular decomposition products, daughter ions, and charge or proton transfer products each provide a unique class of axial energy profiles. Chapters 5 and 6 are applications papers, the first of which, "The Determination of Carbon Monoxide in Air“ is an analytical study of a traditionally difficult problem in mass spectrometry. Three inter- ferences are important in a mass spectrometric determination of carbon monoxide: N2, C02, and hydrocarbon. All produce either molecular or fragment ions at m/z 28 as does C0. The interference from nitrogen 12 is removed by the selectivity provided by the collision process and subsequent CAD daughter detection. Hydrocarbon at m/z 28 has a very low cross section for the production of a CAD daughter ion at m/z 12, and does not interfere, except at very large (percent) concentrations. Carbon dioxide also produces a C0+ ion at m/z 28, which, at 70 eV electron energy has a cross section nearly identical with carbon monoxide for both the production of CO+ in the ion source and C+ from CO+ in the collision cell by CAD. However, advantage can be taken of the differences in internal energy, and hence collision cross section at low (20 eV) ionizing electron energy which reduces C02 interference several thousand-fold. Detection limits for C0 in air are about 10 ppm. The chapter entitled, "Structures and Fragmentation Mechanisms of the Ions of Ethanol by Triple Quadrupole MS" demonstrates the molecular structure determination capability of the triple quadrupole. The structure of six ions of ethanol are shown, and the mechanisms of their formation and fragmentation are determined with the aid of deuterium labelled isotopes. The last chapter is a forum which the author uses to elaborate on more conjectual conclusions than are permissible in manuscripts destined for publication. In addition, instrumental modifications and suggestions for future work are presented. Finally, an appendix is given which documents the design criteria, construction techniques, and performance evaluation of a new ion source. The primary characteristic for which the source is designed is electron energy homogeneity. Appearance potential studies indicate that the inhomogeneity in electron energy is less than 0.25 eV. 11. 12. 13. 14. 15. 16. 17. 13 REFERENCES Coggeshall, N.D., J. Chem. Phys. 31, 2167 (1962). Schug, J.C., J. Chem. Phys. 49, 1283 (1964). Barber, M., N.A. Halstenholme, and K.R., Jennings, Nature 214, 664 (1967). Smyth, K.C., and T.N. Shannon, J. Chem. Phys. 51, 4633 (1969). Cooks, R.G., J.H. Beynon, R.M. Caprioli, and G.R. Lester, "Metastable Ions," Elsevier, Amsterdam, 1973, p. 29. Milne, G.N., ed., "Mass Spectrometry, Techniques and Applications," Wiley Interscience, New York, 1971, p. 425. Jennings, K.R., J. Chem. Phys. 41, 4176 (1965). Cooks, et al., op. cit, p. 41. ibid., p. 40. Mach, S.T., B.F. Bente, III, and F.w. McLafferty, Int. J. Mass Spectrom. Ion Phys. 2, 333 (1972). McLafferty, F.N., R. Kornfeld, H.P. Hadden, K. Levson, I. Sakai, B.F. Bente, III, So'Co Tsai, 311d HoDoRo SChUddermage, Jo AI“. Chem. Soc. g5_, 3886 (1973). McLafferty, F.w., B.F. Bente, III, R. Kornfeld, S.-C. Tsai, and I. Howe, J. Am. Chem. Soc. 95, 2170 (1973). Levson, K., Org. Mass Spectrom. 1Q, 55 (1975). Cooks, R.G., J.H. Beynon, and J.F. Litton, Org. Mass. Spectrom. .19, 503 (1975). Bozorgzadeh, M.H., R.P. Morgan, and J.H. Beynon, Analyst 191, 613 (1978). . Paul, M., H.P. Reinhard, and V. von Zahn, Z. Phys. 152, 143 (1958). Yost, R.A., and C.G. Enke, J. Am. Chem. Soc. 199, 2274 (1978). 18. 19. 20. 21. 22. 23. 14 Yost, R.A., C.G. Enke, D.G. McGilvery, D. Smith, and J.D. Morrison, Int. J. Mass Spectrom. Ion Phys. 99, 127 (1979). Yost, R.A., and C.G. Enke, Anal. Chem. 99, 1251A (1979). Yost, R.A., and C.G. Enke, Org. Mass Spectrom. 19, 171 (1981). Cooks, et al., 0p. cit., p. 218. Friedman, L., and 8.6. Reuben, Adv. Chem. Phys. 19, 33 (1970). Rafey, K.M., and H.A. Chupka, J. Chem. Phys. 99, 2544 (1965). Chapter 2. PRECURSOR EFFECTS ON THE FRAGMENTATION OF c0+ BY TRIPLE QUADRUPOLE MASS SPECTROMETRY l5 Precursor Effects on the Fragmentation of CDT by Triple Quadrupole Mass Spectrometry R.K. Latven and C.G. Enke Department of Chemistry Michigan State University East Lansing, Michigan 48824 ABSTRACT Collision cross sections were studied as a function of carbon monoxide ion internal energy by low energy collisionally activated decomposition in the triple quadrupole mass spectrometer. This ion was studied as it arose from a number of precursor molecules: carbon monoxide, carbon dioxide, methanol, acetaldehyde, and formic acid. These samples produced only C0+ at m/z 28. Cross sections were determined as a function of internal energy and axial energy for the stated compounds. The axial energy which gave the greatest intensity was independent of electron energy, while the collision cross section was strongly dependent on both electron energy and the nature of the C0+ precursor. These results indicate that the internal energy of the selected ion is important in the total energy available for fragmentation. Carbon monoxide had the greatest cross section for fragmentation followed by carbon dioxide, acetaldehyde, formic acid, l6 l7 and methanol. This ordering follows a trend in increasing CO bond lengths in the undissociated moleule. INTRODUCTION For any collisional decomposition, there corresponds a conversion of kinetic energy to internal energy and the subsequent loss of a fraction of that energy through fragmentation. In the present work, data are presented which Show the effect of an ion's internal energy on its cross section for low energy (less than 100 eV) collisionally activated decomposition. Collision induced dissociation at kilovolt energies in normal (1,2) and reversed (3-5) sector instruments have provided energy-change spectra from which kinetic energy loss from collision, kinetic energy release from dissociation, and collision cross section have been obtained. High energy systems involve minimum momentum transfer, small scattering angles, and usually electronic interactions (2). In addition, the time frame for the interaction of the colliding species is short and nuclear motion can be ignored when discussing the associated electronic excitation (Franck-Condon Principle). Although the inelasticity of high energy collisions provides the mechanism for the transformation of kinetic into internal energy, McLafferty (6) has pointed out that the internal energy of an ion before collision has a negligible effect on the ion's collisional spectrum, except in special cases. In his experiments the daughter to parent intensity ratios were 18 calculated for electron energies from threshold to 50 eV. In most cases this ratio was independent of electron energy. The essential features of the low energy collision process of interest here are large momentum exchanges which occur on a much longer time scale (7,8). The Massey adiabatic criterion (9) further suggests that the interaction does not involve electronic excitation. At low energies this criterion assumes that the electrons can adjust adiabatically to the perturbation resulting from the interaction between ion and target, thus making an electronic transition unlikely. However, electronic excitations have been shown to compete with vibrational excitation at low energies in some cases (10). That the collisionally activated decomposition (CAD) spectra are qualitatively similar to both electron impact and high energy CID spectra is consistent with the quasi-equilibrium theory: the long lifetime (7,8) of the collision complex postulated for low energy collisions enables the internal energy to be distributed among the many internal degrees of freedom, as it is in high energy systems. However, the present work indicates that the internal energy of a slow ion plays an important part in the facility with which the ion undergoes CAD. This is consistent with trajectory calculations in one and three dimensions, as performed by Duff and coworkers (11). Recently, the introduction of the tandem quadrupole application of MS/MS (12,13) has provided a simple yet powerful tool for the observa- tion of low energy CAD processes. The positive focusing nature of the quadrupole collision cell enables even collisions with large scattering angles to be transmitted to the detector. In the following sections data are presented which explore the dependence of the collision cross 19 section on the electron energy for a number of sample molecules including carbon monoxide, carbon dioxide, methanol, acetaldehyde, and formic acid. These compounds were chosen because their contribution to the ion current at m/z 28 is only the [C0]+° species. Acetaldehyde alone holds the possibility for C2H4 formation, but an appearance potential study by Dorman (14) demonstrated no observable hydrocarbon at m/z 28. The C0+-+ C+ + 0 transition via argon CAD was studied in detail in these experiments. By comparing parent (28+) and daughter (12*) intensities as a function of ionizing energy, the CAD collision cross-section as a function of internal energy was determined. EXPERIMENTAL The triple quadrupole mass spectrometer has been described in detail in earlier work (12). The essential elements are two quadrupole mass filters bounding an "RF only" quadrupole collision cell. An electron impact ionization source was employed, and ions were detected by a channel electron multiplier. Data were recorded on an X-Y recorder, digitized, and replotted logarithmically. Liquid samples were treated by standard freeze-pump-thaw techniques; gaseous samples were transferred using standard transfer techniques. All samples were reagent grade and used without further purification. Sample pressure in the ion source was below 10"3 torr. Background chamber pressure was 5 x 10'8 torr. Target gas 20 was argon, unless otherwise noted. Target pressure was 5 x 10'4 torr. For cross section experiments, m/z 28 was selected from electrbn impact ions by the first quadrupole mass filter. This ion was assumed to be composed of only the CO species. The ion was focused into the RF-only quadrupole where collisions with the target produced product C and 0 ions at m/z 12 and 16. The second mass filter was set to pass m/z 28, 16, and 12, respectively; the intensities were recorded for each ion as a function of axial energy. Ionization efficiency curves (1 vs. eV) were taken for parent and carbon daughter, and collision cross sections were calculated. RESULTS Figure 2.1 compares the ionization efficiency of carbon monoxide sample by electron impact (left, labeled COT) with the fragmentation efficiency for CO+ +C+ + O by Ar CAD (right, CT). The intensity of the CAD fragmentation efficiency curve is normalized to that of the ionization efficiency at 70 eV. Little difference is seen between both responses. Figure 2.2 shows a similar set of curves, except that the sample molecule in this case is C02. The plot labeled C0+ is the appearance potential curve for the production of C0+ from C02 in the ion source while the C+ curve indicates the fragmentation efficiency for the same C0+ + C+ + 0 Ar CAD reaction as seen in 2l IOO .. a} 5 I C01 01 r I, Relative '01 I .O' 11]] llJ 0 I0 20 3040506070 Lab electron energ)’. eV Figure 2.1 Ionization efficiency curve for CO-+ C0+ byEI fragmentation efficiency curve for C0++ + O by Ar CAD, from C0 sample. 100 22 50- I0- .05 - .0I I, Relative , co, re . ..5 l IJULJILIIJLJ 0 Figure 2.2 IO 20 3O 4O 5O 60 70 Lab electron energy,ev Ionization efficiency curve for C02 + CO+ + O by EI; fragmentation efficiency curve for C0”:+ C+ + 0 by Ar CAD, from C02 sample. 23 Figure 1. In this case there is a notable decrease in fragmentation cross section at the lower ionization energies. One percent of the maximum C+ produced from C0+ by Ar CAD is achieved at only 0.2 eV more electron energy than is required to produce 1% of the maximum CO+ produced from carbon monoxide in the source. However, with C0+ from C02, a 2.9 eV higher electron energy is required to produce the same 1% fraction of decomposition. Since all conditions are identical in in both experiments, except for the nature of the CO+ precursor, it is reasonable to conclude that the differences in cross section are due to a difference in internal energy carried by the ion into the collision cell. In Figure 2.3, relative cross sections are plotted for carbon monoxide, carbon dioxide, methanol, acetaldehyde and formic acid. Absolute cross sections range from 0.67 A2 to 0.62 A2 at 70 eV and 20 eV electron energy for C0+ from carbon monoxide, while from methanol, the same ion's effective collision area ranges from 0.24 to 0.038 32, a sixteen-fold difference at the 20 eV value. In addition, the cross section of the C0+ + C+ transition from 002 is 0.64 A2 at 70 eV, very close to that of carbon monoxide, while the cross section drops to less than half that value at 20 eV. This implies that given enough electron energy, C0+ from C02 can have as much internal energy as C0+ from C0, and hence display similar cross sections. 24 Figure 2.3 CAD cross sections for the CO+-+ CT + 0 transition for CO, C02, methanol, acetaldehyde and formic acid. 25 >0 .xmeocw .3580 9... on: Obi 00: out _ _ _ _ m.~ mesa on: out Wu opt .anmég IOOUI I I [61+ dn/h - llUI- - 0 Houses 8801:) ova 26 DISCUSSION An ion undergoing collisionally activated decomposition (CAD) is given translational or axial kinetic energy (EAX) by the electro- static potentials through which the ion is accelerated. The influence of axial energy on the CAD process has been studied (15), and the results demonstrate that for a given offset potential on the third quadrupole, there exist no large shifts in the axial energy which corresponds to a maximum cross section for the transition. This axial energy value is independent of both source electron energy, target gas pressure, nature of target gas (using center of mass coordinates) and sample molecule (15). The selected ion also possesses a critical amount of internal energy. The difference between this amount and the energy needed for dissociation is supplied by energy transfer in the collision (2). For a given axial energy, variations in internal energy may result in differences in collisional cross section. Since the responses of the collision cross sections to variations in electron energy are dissimilar for the low and high energy collision processes (5) there may be different mechanisms involved in the formation of the excited transition state prior to fragmentation. Collisional excitation in high energy C10 is related to electron impact in that both the average energy at activation and the time allowed for fragmentation are similar for both processes (7). High energy collisional activation is believed to involve simple electronic excitation (16), and thus the fragmentation pattern depends on the range of internal energies acquired on activation. The internal energy in a fragment depends upon the ionizing electron energy and the nature 27 of the precursor for electron impact, and the relative translational energy of the ion in the molecular impact case (4). Cooks here emphasizes that identical energy deposition functions for the two processes is not assumed, but that fragnentation does arise from the same electronic states in both cases. For the low energy case, although the average energy deposited Rey be similar to E1 and C10, the time allowed for fragmentation is much longer. Since the Massey adiabatic criterion (9) states that elec- tronic excitation is not important at these low kinetic energies, it is likely that there the collision energy is adsorbed by the vibrational modes directly. This may explain the disparity in cross section response to ionizing electron energy in the high and low energy colli- sional systems. In the former, the particular vibronic state as dictated by the electron energy is not important since the mode of excitation is electronic. The vibrational states which specify the internal energy distribution of the reactant ion, while strongly depen- dent on the ionizing energy, are quantitatively important in prescrib- ing the cross section associated with low energy fragmentation. The ordering of the collision cross sections seen in Figure 2.3 start at carbon monoxide with the greatest value, followed by carbon dioxide acetaldehyde, formic acid and methanol. Since all external factors influencing the CO+ selected ion are identical for each precursor, it follows that the differences in cross section must reflect differences in the internal properties of the ion. Since vibrational motion occurs on a much longer time scale than electronic motion (including the electronic motion of bond breaking) it is reasonable to assume that at least initially, the equilibrum bond 28 distance of the nascent C0+ ion reflects the CO bond distance of the prneetJnd (XZZT) state of the carbon monoxide ion. The lowest vi t>lcational level of the ground state ion has an equilibrium distance o1r 11.12 A. Since the bond distance of the CO molecule is 1.11 A, it does not have cause to gain vibrational energy on formation. However, C02 has an equilibrium CO bond distance of 1.18 A. When C0+ is formed from C02 at its appearance potential, this distance is nearly equal to that ascribed to the tenth vibrational level of the CO+ ground state. Similarly, the length of the C0 single bond in methanol, (‘TC> = 1.43 A,) corresponds approximately to the v = 42 vibrational 'I LU l0 It >: _ ————————————— {- L9 0'; LL] _ Z LLJ 8 . v = 42 CH3OH .... r. = 1.43 A v = 29 6 q HCOOH ..... r0 = 1.31 A - > 4 HCOOH ..... r. = 1.24 A V ‘ 2‘ e on v = l7 J/l °‘ 4 . CH3CHO .... r. = 1.22 A 1 a" 'C02 ooooooo rag-118A \v=-IO 2-1 c0 r. =1.111l ”0 ‘1 0 . . . 1 0 0.5 1.0 1.5 2.0 Figure 2.4 Morse potential curve of the X BOND LENGTH. 3 22+ state of C0+ showing vibrational levels and equilibrium distances for C0 precursors. For COT, re = l.12 A. 30 significant fraction of the ions at 35 eV axial energy to reach the collision cell. Studies by Ajello (18,19) of the emissions from CD and C02 excited by electron impact were used to determine the populations of the vibrational levels. This work indicates that while the ground 22+ state in COT is more highly vibrational level of the B populated than that of COZT, the latter has significantly greater populations in the higher vibrational levels. This follows the nascent equilibrium bond distance calculations in that C0+ from C02 has greater internal energy than that from C0. In a similar study (20), the production of C01 from methanol was seen to proceed from a very highly excited parent. The C0+ thus produced would likely also be highly vibrationally excited, as is suggested by bond distance calculations. If C0+ from MeOH is much more excited than CO+ from C0, then (from Fig. 2.3 and 2.4) the trend of decreasing collision cross section follows one of increasing vibrational temperature. This inverse relationship between vibrational temperature and collision cross section is known as vibrational inhibition (21) and has been described theoretically for colinear systems (22). In this theory, vibrational inhibition of the dissociation probability dominates at higher collisional (axial) energies, while a concomitant vibrational enhancement operates at lower collisional energies. Examination of the CO and 002 systems at lower axial energies in our system shows such a reversing of cross sections. Figure 2.5 shows the collision cross section of COT + C+ + 0 product ions as a function of axial energy, from C0 and C02 precursors. Above 24 eV, C0+ from carbon monoxide has the greater cross section while for axial energies below that value, the more highly excited C0+ from C02 undergoes 31 Figure 2.5 Axial (translational) energy profiles of CAD product ions for the C0+ + C+ + 0 transition from C0 and C02 precursors. Cross sections are normalized to constant parent ion flux. IO 32 (D I a', Relative .b I CO co2 l J 15 20 2'5 30 Axial Energy, eV 35 33 decomposition more efficiently. This reversal is not observed for the other compounds studied. However, the decreased signal to noise ratio encountered at very low axial energies prevents accurate determination of the cross section in this region. Although relative parent-daughter intensity ratios may change with internal energy and nature of precursor, a fixed ionizing electron energy will provide similar CAD daughter ion spectra (except for parent ion intensity) regardless of precursor. The relative constancy of daughter ion spectra with respect to parent structure is essential for parent ion identification; in addition, relative collision cross sections can provide clues regarding the structure of the precursor from which the parent ion is formed, as well as information on the internal energy of the parent ion. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 34 REFERENCES Rosenstock, H.M., and C.E. Melton, J. Chem. Phys. gg, 314 (1957). Jennings, K.R., Int. J. Mass Spectrom. Ion Phys. 1, 227 (1968). Nachs, T., C.C. Van de Sande, and F.H. McLafferty, Org. Mass Spectrom. 11, 1308 (1976). Cooks, R.G., ed., "Collision Spectroscopy," Plenum Books, New York (1978). McLafferty, F.H., R. Kornfeld, H.F. Haddon, K. Levson, I. Sakai, P.F. Bente, S.C. Tsai, and H.D.R. Schuddemage, J. Am. Chem. Soc. 99, 3386 (1973). McLafferty, F.H., P.F. Bente, R. Kornfeld, S.C. Tsai, and I. Howe, J. Am. Chem. Soc. 99, 2120 (1973). Frideman, L., and B.G. Reuben, Adv. Chem. Phys. 19, 33 (1970). Rafey, K.M., and N.A. Chupka, J. Chem. Phys. 99, 2544 (1965). Massey, H.S.w., and E.H.S. Burhop, ”Electronic and Ionic Impact Phenomena," Oxford University Press, New York (1952) Pauly, H., Faraday Disc. Chem. Soc. 99, 193 (1973). Duff, J.H., N.C. Blais, and 0.0. Truhlar, J. Chem. Phys. 11, 4304 (1980). Yost, R.A., and C.G. Enke, Anal. Chem. 91, 1251A (1979). Zakett, D., and R.G. Cooks, Analytica Chemica Acta 119, 129 (1980). Dorman, F.H., J. Chem. Phys. 99, 65 (1965). Latven, R.K., and C.G. Enke, in preparation. Durup, J., in “Recent DevelOpments in Mass Spectrometry," K. Ogato and T. Hayakawa, eds., University Park Press, Baltimore, p. 921 (1970). Bondybey, V.E., and T.A. Miller, J. Chem. Phys. 99, 3597 (1978). Ajello, J.M., J. Chem. Phys. 99, 3158 (1971). .pp. cit., p. 3169. Hirota, K., M. Hotada, and T. Ogawa, Int. J. Radiat. Phys. Chem. 9, 205 (1976). 35 21. Rusinek, 1., and R.E. Roberts, J. Chem. Phys. 99, 1147 (1978). 22. Hunt, P.M., and M.S. Child, in press. 23. Allison, J., and Zare, R.N., Chem. Phys. 99, 263 (1978). Chapter 3. OBSERVATIONS OF METASTABLE IONS IN A TRIPLE QUADRUPOLE MASS SPECTROMETER 36 Observations of Metastable Ions in a Triple Quadrupole Mass Spectrometer R. K. Latven and C. G. Enke Department of Chemistry Michigan State University East Lansing, Michigan 48824 Sir: In studying the collisionally activated decomposition in the triple quadrupole mass spectrometer (1) transitions were Observed which resemble metastable decompositions heretofore reported only for sector instruments (2-6). Metastable peaks arise in a mass spectrum from decomposition processes which are slow (k = 105 - 106 5‘1) with respect to normal fragmentation. The geometry of the triple quadrupole instrunent is such that the center “RF-only" collision quadrupole, which is bounded by two quadrupole mass filters, is analogous to the field free region in sector instrunents. For this work, a pure sample was ionized by electron impact; a single ionic species was selected by mass from the ion source by the first mass filter and focused into the collision cell. There, impact 37 38 with argon (or other inert or reactive gas) effects fragmentation. The resulting charged products are mass selected by the second mass filter to be detected. Since the quadrupole is a true mass filtering device and not an energy analyzer, products from unimolecular decomposition are seen at their nominal mass without the energy spreading which characterizes their broad peak shape in sector instruments. Ions resulting from metastable decay were first characterized in our instrument by a study of the pressure dependence of collisionally activated decompositions. Figure 3.1 shows such a pressure response for the collisionally activated decomposition of propane molecular ion at m/z 44. The intensity of the parent ion at m/z 44 decreases at increasing pressure due to fragmentation; at higher pressures, some scattering loss is also observed. Ions resulting from collisions between the parent and target appear at m/z 14, 15, 27, 28, and 29, and show a strong dependence on target pressure. However, the species observed at m/z 43 does not show a pressure dependence below ca. 10'5 torr. This ion must therefore result from a unimolecular decomposition of the parent ion. At higher pressures, however, a collisional contribution adds to the total ion current. If the metastable contribution (m2 x 10'8A) is subtracted from the pressure dependent part of the ion current for m/z 43, a first order response is observed, indicating that at higher pressures a collisional as well as a unimolecular decomposition contributes to the total ion current. Great care was exercised to ensure that the appearance of the [P-H]+ metastable product ion did not result from "leakage" due to poor resolution of the parent ion. At values of quad 1 resolution 100 50 l0 1, relative .05 39 44' .0I 1. Figure 3.1 ,0... 10--5 10-4 IO" Pressure, torr, QZ Collisionally activated decomposition of propane molecular ion (m/z = 44). Ion intensities are shown as a function of target pressure. Ion at m/z 43 is unimolecular decomposi- tion product. 40 greater than unity, the [P]+-/[P-1]+ ratio is independent of the applied resolution. In addition, even when large peaks appear at [P+1]+ in the E1 spectrum, they are not observed in the CAD spectrum when P+ is selected at proper resolution. Residence times in the collision cell were determined experimentally at various axial energies by impressing a sine wave on the entrance aperture of the first quadrupole and observing the phase shift of the ions as they were detected. After correcting for amplifier response, the velocity of an ion traversing the three quadrupoles agreed with the theoretical value to within experimental error. Residence times can then be given by: t = 1 (m/zevAX)1/2 where t is residence time, 1 is the length of the field free region, m is mass of selected ion, e is its charge and VAX is the axial voltage, i.e. the offset voltage betwen the ion volume and the quadrupole rods of the collision cell. For an ion of m/z 44 accelerated to 15 V (Figure 3.1 experimental conditions) to undergo metastable decomposition and be detected in our instrument, the transition must occur between 28 and 56 us after exiting the source. The metastable transitions so far observed in the triple quadrupole mass spectrometer have only involved loss of H or H2; these unimolecular decompositions are formed typically from odd electron and even electron ions respectively. In addition, all loss of H or H2 have been from unimolecular decomposition of the parent ion. 5. 6. 41 REFERENCES Yost, R.A., and C.G. Enke, J. Am. Chem. Soc. 199, 2274 (1978). Coggeshall, N.D., J. Chem. Phys. 91, 2167 (1962). Schug, J.C., J. Chem. Phys. 99, 1283 (1964). Barber, M., H.A. Halstenholem, and K.R. Jennings, Nature 911, 664 1967 . Smyth, K.C., and T.H. Shannon, J. Chem. Phys. 91, 4633 (1969). Cooks, R.C., J.H. Beynon, R.M. Caprioli, and G.R. Lester, "Metastable Ions," Elsevier, Amsterdam (1973). 41 Chapter 4. CHARACTERIZATION OF COLLISIONALLY ACTIVATED DECOMPOSITION PRODUCT IONS BY AXIAL ENERGY PROFILING IN TRIPLE QUADRUPOLE MASS SPECTROMETRY 42 Characterization of Collisionally Activated Decomposition Product Ions by Axial Energy Profiling in Triple Quadrupole Mass Spectrometry R.K. Latven and C.G. Enke Department of Chemistry Michigan State University East Lansing, Michigan 48824 SUMMARY In the triple quadrupole implementation of MS/MS, the transla- tional, or axial kinetic energy of ions undergoing collisionally activated decomposition significantly influences the cross sections of the processes available to the selected ion. In addition, a product ion's axial energy profile can characterize its genesis; parent and unimolecular decomposition product ions, CAD daughter ions, and charge transfer and proton transfer products all have distinctive axial energy profiles. Parent and metastable ion intensities rise sigmoidally at the turn-on potential and plateau as axial energy is further increased; for a given set of instrumental conditions, CAD daughter ions can display a peak, the distribution of which is generally centered from 43 44 between 5 and 40 eV and is characteristic of the particular parent-daughter transition. For a given transition, the axial energy profile is independent of the sample molecule from which the parent is derived. The profile is also independent of parent ion internal energy, the nature of the target gas, and the target gas pressure. The ion intensities of charge transfer and proton transfer products rise sigmoidally at the turn on voltage, but are attenuated sharply as the axial energy of the parent ion exceeds several volts. INTRODUCTION In the quadrupole collision cell (1) of the triple quadrupole mass spectrometer (TQMS) (2), ions which are selected by the first quadrupole mass filter collide with an inert or other target gas; the resulting collision products are analyzed by a second quadrupole mass filter. Ions which appear at the detector can include the parent ion, CAD daughter ions, charge or proton exchange reaction products as well as ions formed by unimolecular decomposition of a metastable parent. A method to characterize these products by the nature of their formation is the focus of this study. High energy (keV) collision-induced dissociation (C10) in normal and reversed sector instruments has provided much information on the collision process (3-7). At these high energies, small variations in kinetic energy do not provide significant structural information. Recently, low energy (less than 100 eV) collisionally activated decomposition (CAD) has been demonstrated useful in structure elucidation (8-10), mixture analysis (8,11,12), and charge exchange 45 studies (13). The kinetic energy of the selected ion can influence greatly the cross sections for fragmentation which result from the low energy collision process (14). For these applications, attention to the axial energy response is critical. EXPERIMENTAL The triple quadrupole mass spectrometer used in this study has been described in detail in earlier work (8). Briefly, the instrument consists of, in series, an electron impact ion source of special design, a quadrupole mass filter (Q1), an RF-only quadrupole collision cell (Q2), and a second quadrupole mass filter (Q3). The ion detection circuit consists of a channel electron multiplier and picoammeter; data were recorded with an X-Y recorder. All samples were commercially available and used without further purification. Samples were treated by standard freeze-pump-thaw techniques and then bled into the source through a Granville-Phillips variable leak, to a pressure of 5 millitorr (1 Torr = 133.3 Pa). The electron energy was 70 eV with an emission current of 0.25 mA. The argon target gas was from Airco Industrial Gases (Southfield, MI 48075), 99.998% pure. Target gas pressure was measured by a calibrated Bayard-Albert type ionization gauge and regulated by a Granville-Phillips model 216 flow controller. An ion of particular m/z produced in the source and chosen for study was selected by quad 1; argon target gas was then bled into the collision cell and automatically regulated to 4 x 10"4 torr. The product ion of interest was selected by the second quadrupole mass 46 filter (Q3). Quad 1 and 3 offset voltages were held at -25 V while quad 2 offset voltage was scanned. Energy calibration was done through a zero point calculation of the first derivative of the parent ion intensity profile (see below). To determine the ion transit times in the triple quadrupole flight path, a sine wave was impressed on lens 3 of the ion source. The ion current thus modulated was phase-corrected to the input. When the output was out of phase by one wavelength, the transit time was calculated to be one period. Since a single quadrupole occupies one-third the total length of the ion path, transit times were divided by three to give the residence times in each quadrupole. RESULTS AND DISCUSSION Axial voltage (VAX) is the measured offset voltage between the volume in which the ions are formed and the collision quadrupole rods (V02). This measurement, although precise and reproducible, may only approximate the true axial energy, due to the field penetration effects of the repeller and extractor lens in the ion source. The axial voltage is easily corrected to axial energy (EAX), by denoting as zero electron volts the maximum of the function where ID is the ion current of the parent selected by both mass analyzers (no target gas); the axial voltage corresponding to the point EAX = 0 is also termed turn-on voltage. Figure 4.1 shows the axial 47 Figure 4.1 Ion current as a function of axial energy for m/z 46 parent ion from ethanol, its first derivative and the [M-H]+ unimolecular decomposition product. 48 00 _.e deemed >o «65cm. .23 0? On ON 0. O O... u q . _ _ O n e i ON III) A .8 . 9» H. mm. . m... .. 8 ... be. i 1 0m B - Ho 00. 49 energy profile and its first derivative for parent ion m/z 46 from ethanol. For all experiments, except where otherwise noted, the first quadrupole mass filter selects the parent ion of interest, and the second mass filter (Q3) selects the CAD daughter or product ion. VQ2 is scanned and the response is plotted with respect to corrected EAX to produce the given axial energy profile. Several hundred axial energy profiles have been determined for parent ions, metastable decay products and CAD daughter ions, from more than twenty compounds which include hydrocarbons, alcohols, aldehydes, ketones, and acids. From this data base, the following results are taken; axial energy profile types are detailed below. 1. Parent Ions Any ion selected for fragmentation by the first analyzer (01) is termed a parent ion [PJTZ All parent ions display qualitatively similar axial energy spectra. Figure 4.1 shows the axial energy profile of the molecular parent ion m/z 46 from ethanol E1. The response increases abruptly at the turn on potential, and rises to 90% of maximum value within about five electron volts ion kinetic energy. At higher axial energies, there is usually no significant increase in ion current. The Gaussian distribution of the derivative function arises from the kinetic energy spread of the ions as they are formed in the source. Full width at half-maximum is about 1.5 volts. Undulations in the plateau are believed due to the effect of asynchronous RF frequencies from the three quadrupoles on the transmitted ion. This reproducible fine structure in the plateau region is a strong function of quad 2 RF voltage and frequency. The amplitude of the oscillations increases as 50 RF voltage decreases; the RF frequency influences the waveform itself, but limitations imposed by the quadrupole control electronics preclude detailed study of this parameter. However, variations in intensity in the plateau region do not appear to influence daughter ion intensities in the same region. 2. Unimolecular Decomposition Products In the triple quadrupole instrument, ions which are formed from unimolecular decomposition of the selected parent ion while the parent ion is in the RF only region of the quadrupole collision cell will be detected as 'metastable' decay. The intensity of the unimolecular decomposition product is therefore independent of CAD target gas pressure. A separate study (15) details the effect of target pressure on CAD fragmentation. For ethanol molecular ion, for example, loss of hydrogen is shown to be independent of pressure below about 2 x 10'5 torr. Above this pressure, the probability of the 46+ +45+ + 1 transition is only slightly augmented by the collision process. Unimolecular decomposition products can be easily characterized by their axial energy response. Figure 4.1 shows the [P-H]+ profile which arises from the m/z 46 parent ion from ethanol. The intensity of ions formed by unimolecular decomposition is fairly independent of axial energy after the turn on potential. In addition, the profile is qualitatively similar to that of the parent ion: a steep rise at the turn on potential followed by a plateau. However, unlike the parent ion, RF 'beats' are typically not detected in the plateau region of ions formed from metastable decay. 51 The residence time of an ion in a single quadrupole has been experimentally determined as a function of axial voltage. After correcting for amplifier response, the results agree with theoretical values (EAX = 1/2 mV2) to within experimental error (20%). Residence time in microseconds can be calculated for singly charged ions from: t = .72 1 (m/EAX)1/2 where l is the length of the collision cell in cm, m is mass of parent ion in amu and EAX is the axial energy. At mass 46 and EAX = 25 eV, the residence time in each quadrupole is about 22 us. Therefore, for the [M]+--+ [M-H]+ metastable transition of ethanol to be detected, the decay must occur between about 22 and 44 microseconds after the parent ion is extracted from the source ionization region. In the compounds studied so far, unimolecular decomposition products have only been observed for loss of H or H2; moreover, all transitions of these types have been metastable (15). 3. CAD Daughter Ions Products which result from collisionally activated decomposition of the selected parent are termed CAD daughter ions. The cross section of a particular parent-daughter transition is highly dependent on the kinetic energy of the parent ion at the time of collision. If the DC offset of Q3 is tied to that of Q2, a sigmoidal rise plus plateau is observed for the axial energy response of a daughter ion. Similar in shape to that of a parent ion, the daughter ion profile with tied Q2 and Q3 offsets shows a turn-on voltage of greater than zero eV 52 axial energy. Figure 4.2 shows this response for the N2+ e'N+ + N transition. Although the increased turn on voltage of the daughter ion with respect to its parent probably reflects some change 1'0 kTHETIC energy of the ion undergoing transition, the exact mechanism describing this behavior is unclear at this time. Many other daughter ion profiles of this type show more shallow increases in intensity at turn-on, which makes characterization of daughter ion axial energy spectra difficult with tied Q2 and 03 offset voltages. In addition, Q3 resolution is seriously degraded at Q3 offsets much greater than 30 V. Setting Q3 offset at a fixed value, however, provides a reproduci- ble peak-shaped response in all cases studied. Figure 4.3 shows the fixed Q3 offset axial energy profile for the collisionally activated decomposition of carbon monoxide molecular ion. The carbon daughter ion appearing at m/z 12 displays a maximum intensity (EAX) at 33 eV, while the oxygen daughter ion displays its maximum at 18 eV. Moreover, for C0+ selected from six unlike compounds, EAX values for a given transition occur within a range of several electron volts (Table 4.1). This constancy of EAX for a particular parent-daughter transition, when parent ions are of like structure, appears to be quite general and has been observed for all cases studied. It is analytically useful therefore to study daughter ion axial energy response with fixed Q3 offset. In this laboratory, for axial energy studies, 01 and Q3 offset voltages are set at -25 V, while Q2 offset voltage is scanned to provide axial energy profiles. The location of the maximum, EAX’ is not dependent on the source ionization energy. Figure 4.4 shows the C0+ + C+ transition for carbon monoxide molecular ion, for a number of ionizing electron 53 Figure 4.2 Axial energy profiles for parent and daughter for N2+ + NT + N. VQZ = Vq3. 54 N.e deemed >m «3.55 .33. Om Om O? on ON 0_ O . 1 q . _ T q A q 1 . A00. 5 4V. .mN 1 0 q- 1 O (O 00_ 9411098 ‘I 55 Figure 4.3 Axial energy profiles for carbon monoxide system; [COJT' parent, and daughters C+ and 0+. 56 om >o .3..ch .23. O m.e deemed OT :03 .m. «.03 .o. x .m. _ 0 -ON .0? ..Om 10m 00. 84110198 ‘I 57 Table 4.1. EAX values for two transitions of the [C0]+° parent ion from various samples. Q3 offset fixed at -25 V. (All values are accurate to within 10%.) Sample 28+-+ 16+ 28+ + 12+ C0 21 33 C02 22 33 Methanol 20 32 Formaldehyde 20 28 Acetaldehyde 20 28 Formic Acid 23 33 58 Figure 4.4 Axial energy profiles for [C0]+° + C+ in carbon monoxide at various ionizing electron energies. 59 >m .335 6:3. e.e menace Om 00. 84110188 ‘I 6O energies. Target gas pressure, while strongly affecting the collision cross-section, likewise does not affect the position of sz. The axial energy distribution independence from target gas pressure for [C0]+° + CT in a carbon dioxide sample is shown in Figure 4.5. No significant shift in EAX is observed over a fifty-fold range in target gas pressure. In addition, the nature of the target gas does not influence sz if center of mass coordinates are used. For example, in the 28+ +-12+ transition from carbon monoxide sample, EAX appears at 33 and 24 eV for argon and xenon targets respectively. If ECM = EAX [Mp/(Mt + Mp)], where Mt and Mp are the masses of the target and parent ion, then for the 28+ '*12+ system, xenon and argon targets yeild ECM values which agree to within .5 eV (ECM = 20 eV). The offset voltage on Q3 does affect the position of ERX, however, especially as the offset approaches zero. When the voltage is greater than -15 or -20 volts, little change in EAX is seen. The value of -25 volts was chosen as the standard Q3 offset for axial energy profiling studies because it appears to be a good compromise between Q3 resolution and constancy of 51x- 4. Charge and Proton Transfer Cooks and co-workers have discussed charge exchange using a double quadrupole mass spectrometer (13). In these and other studies (15,16), a selected ion of well defined internal energy, usually an inert gas, is used to bombard an analyte target. For a resonant process, the charge exchange deposits a known quantity of internal energy into the molecular ion of the target. 61 Figure 4.5 Axial energy profiles for [CO]+ +~C+ in carbon dioxide at various Ar target gas pressures. 62 m.e deemed >m $955 .23. cm 00 co on ON 0. o J . J R _ . . 4 . . a d l .70.; ., om 21:} . .10 x0. , . {(1171 _ . / . 2... cc .3 , Clo—Km \ l I . 9.6 x 1 cm , ..- .\ T0:6 , . 1 t2 {... c. 3:88.. i 00 00. all 11018.1 ‘1 63 In this work, however, we will focus on only those exchange products which result from CAD of a selected analyte ion. These exchange products usually appear as interferences. For example, m/z 19 was observed in the CAD spectrum of [CH0]+ from methanol. Since H30+ is not a possible fragment of [CHO]+, nor is any other ion at m/z 29, it must arise from a proton transfer reaction with water by CH0+ + H20 + H3O+ + C0. The H3O+ intensity was seen to vary with the pressure of the target gas, which contained residual water vapor. Characterization of this proton transfer product was readily achieved through axial energy profiling. The ion current increases sigmoidally at the turn on potential, but is attenuated sharply within several volts of increasing axial energy (Figure 4.6). The appearance in CAD spectra of m/z 29 has at times been difficult to explain, for example when the selected ion is m/z 39 from cyclohexane, or CH3+ from ethane. In the latter case, the 29+ could have arisen from either an ion-molecule reaction with the sample or a proton transfer reaction with a contaminant at m/z 28. The use of deuterium-labeled ethane enabled the determination of the process by which the ion at m/z 29 arises. If this ion were C2H5+, its deuterium analog would appear as C205+ at m/z 34. A deuteron transfer product, however, would appear at m/z 30. Argon CAD on CD3+ provided a signal at m/z 30 demonstrating that the spurious peak at m/z 29 resulted from a proton transfer to contaminant at mass 28. A number of processes have been so characterized, and are enumerated in Table 4.2. 64 Figure 4.6 Axial energy profiles for [CH0]+ parent ion, daughter ions [CHJT and CT, and proton transfer product [H30]+. 65 >m $9.23 823. e.e deemed Om . 0V . Om ON 0. O 0.- . . 1 4 . 4 . O .m. i 63.9 .ON 85.9 . nvéuuw. H mw - m. -oom .8 .0m 00. la 66 Table 4.2. Charge and proton exchange reactions seen in TQMS. Charge Exchange Proton Exchange P+ + H20 + H20+ + P P+ + H20 + H3O+ + [P-H] 2* + N2 + N2+ + P P+ + N2 + N2H+ + [P-H] PT + 02 't 02+ + P P+ + Ar + ArHI’ + [P-H] P+ + Ar + Ar+ + P P+ + CH4 + CH4+ + P 67 In addition to providing qualitative information regarding genesis of CAD product ions in triple quadrupole mass spectrometry, these initial studies indicate a degree of detail in the axial energy scans which may yield quantitative descriptions of the thermochemistry which governs these processes. Recent advances in instrunentation are making the acquisition and correlation of axial energy data more efficient, and should aid in the exploitation of this important mass spectral dimension. 11. 12. 13. 14. 15. 16. 68 REFERENCES Yost, R.A., C.G. Enke, D.C. McGilvery, D. Smith, and J.D. Morrison, Int. J. Mass Spectrom. Ion Phys. 99, 127 (1979). Yost, R.A., and C.G. Enke, J. Am. Chem. Soc. 199, 2274 (1978). Cooks, R.G., in "Collisional Spectroscopy," R.G. Cooks, ed., Plenum Press, New York (1978). Rosenstock, H.M., and C.E. Melton, J. Chem. Phys. 99, 314 (1957). Jennings, K.R., Int. J. Mass Spectrom. Ion Phys. 1, 227 (1968). Hachs, T., C.C. Van de Sande, and F.H. McLafferty, Org. Mass Spectrom. 11, 1308 (1976). Laramee, J.A., 0. Cameron, and R.G. Cooks, J. Am. Soc. 199, 103 (1981). Yost, R.A., and C.G. Enke, Anal. Chem. 91_1251A (1979). Yost, R.A., and C.G. Enke, Org. Mass Spectrom. 19, 171 (1981). Glish, G.L., P.H. Hemberger, and R.G. Cooks, Anal. Chim. Acta 119, 137 (1980). Glish, G.L., and R.G. Cooks, ibid, 119, 145 (1980). Zackett, 0., P.H. Hemberger, and R.G. Cooks, ibid, 119, 149 (1980). Busch, K.L., T.L. Kruger, and R.G. Cooks, ibid, 119, 153 (1980). Latven, R.K., B. Newcomb, and C.G. Enke, 28th Annual Conference on Mass Spectrometry and Allied Topics (1979). Lifshitz, C., and T.O. Tierman, J. Chem. Phys. 91, 1515 (1972). Hsu, 0.5., and R.G. Cooks, Org. Mass Spectrom. 9, 1373 (1971). Chapter 5. DETERMINATION OF CARBON MONOXIDE IN AIR BY TRIPLE QUADRUPOLE MASS SPECTROMETRY 69 Determination of Carbon Monoxide in Air by Triple Quadrupole Mass Spectrometry R. Kazmer Latven and Christie G. Enke Department of Chemistry Michigan State University East Lansing, Michigan 48824 ABSTRACT Carbon monoxide has been determined at parts-per-million concentra- tion in the presence of C02, N2 and hydrocarbons by triple quadru- pole mass spectrometry (TQMS). In this work, advantage is taken of the ability of TQMS to distinguish differing ions of the same mass. The first quadrupole mass filter selects m/z 28; these selected ions undergo collisionally activated decomposition in the RF-only quadrupole collision cell and the C0+ product ion at m/z 12 is mass filtered by the third quadrupole and detected. Quantification of the CO concentra- tion is achieved by the addition of a known amount of labeled internal standard, 1300, which displays a fragmentation peak at m/z 13 when the first mass filter selects the parent at m/z 29. Side-by-side comparison with the m/z 12 peak from C0 allows the determination of CO concentration to within 20% down to 10 ppm. 70 71 INTRODUCTION The application of mass spectrometry/mass spectrometry (MS/MS) to the analysis of mixtures is currently the focus of considerable study (1-4); several reviews have been published (5,6) which present a rather complete guide to the recent literature. The isolation of components in a complex mixture by ionization and mass separation offers specific advantages over other techniques. For example, the component of interest is available continuously during the analysis, albeit at a lower flux, which contrasts the sequential delivery of the separated components by chromatography. For air analysis this is a distinct advantage since essentially continuous monitoring of a number of components is possible. For other samples the analysis is also extremely rapid (2), especially when compared to chromatographic retention times. In many cases little or no sample purification or derivitization is required (7,8). The primary constraint in the use of MS/MS for mixture work is the necessity that the components of interest produce a strong molecular or other characteristic ion for selection and analysis. An ion which does not produce abundant molecular ion is difficult to analyze in a complex mixture. Also, abundant fragment ions may interfere with the analysis. Soft ionization techniques, for example, chemical ionization (9), field ionization (10) or field desorption (10,11) may be necessary to produce an abundant molecular or protonated molecular ion with little fragmen- tation. The triple (12) and double (13) quadrupole implementations of MS/MS have been described for several applications in mixture analysis (1,14-16); the unit resolution of both analyzer sections together with inherently high efficiency and sensitivity make the quadrupole MS/MS 72 ideal for analytical applications. In addition, since spectra generated by quadrupole MS/MS are in many ways similar to their MIKES counterpart, at least for electron impact generated ions (16), the considerable library of high energy C10 and CA spectra may be useful for quadrupole MS/MS reference. The application of mass spectrometry for the direct determination of trace amounts of carbon monoxide in air is a particularly difficult problem. Interferences from nitrogen at the same m/z preclude the use of low (unit) resolution instrumentation, although indirect methods (e.g., CO over CuO at 800° + C02, then cryogenic trapping (17)) are available. Kambara et al. (18) have employed atmospheric pressure ionization to achieve impressive sensitivity for the determination of CO in N2 but did not address the question of interferences from carbon dioxide and hydrocarbon. High resolution MS can easily differ- entiate the .003 amu difference between C0 and N2 at equimolar concentrations; however, the resolution required when the concentration ratio is as large as 105:1 has not been attained. The use of chemical ionization has not been reported for this determination. This application of MS/MS, however, is ideally suited for the determination of C0 in air. In the triple quadrupole implementation of MS/MS, the first quadrupole selects only those ions of m/z 28. Fragmentation of the N2 component produces daughter ions at m/z 14, while for the trace CO component, the ion current at m/z 12 is detected. 73 EXPERIMENTAL The triple quadrupole mass spectrometer used in this study has been described in detail (14). The instrument consists of, in series, an electron impact ionization source, an Extranuclear Labs quadrupole mass filter (Q1) an in-house designed and fabricated "RF-only“ quadrupole collision cell (02), and a second Extranuclear Labs mass filter (Q3). Ion current is detected by a Galileo channel electron multiplier, amplified by a Kiethley #18000-20 picoammeter, and recorded on an H-P 7044-A X-Y recorder. The cross axial electron impact ionization source, designed and constructed in our laboratory for this work is more efficient than the commercial model it replaced and shows an apparent electron energy inhomogeneity of less than 0.25 eV based on appearance potential experiments (19). The ionizing electron energy was set at 20 eV, and the emission current used was 50 microamperes. Repeller voltage was held at 10 V above the source block; sample pressure was 10 millitorr. Quadrupoles 1 and 3 offset voltages were ~25 V while the axial energy (02 offset) was held at 33 eV, the Optimum energy for the COT-+ C+ transition studied. Target gas was 99.998% pure argon (Airco Industrial Gases, Southfield, MI, 48075), and controlled to 5 x 10'4 torr by a Granville Phillips model 216 flow controller. Carbon monoxide and carbon dioxide gases were high purity, also from Airco; the isotopic standard (13C0) was 99 atom % 13C and purchased from Merck & Co. (St. Louis, MO 63116). For these experiments, the ion of interest produced in the source and chosen for study (m/z 28 or 29) was selected by Q1; the selected ion was transmitted into the collision cell (Q2) where impact with 74 argon caused fragmentation. The positive focusing nature of the quadrupole collision cell allows little scattering loss (20) and thereby transmits nearly all CAD product ions to Q3 where they are mass analyzed and subsequently detected. RESULTS AND DISCUSSION A number of interferences can influence the ability to determine carbon monoxide in air by MS/MS. The first mass filter will select only ions of m/z 28 from the air matrix for study. Unfortunately, the major constituent of air, molecular nitrogen, is also seen at m/z 28. In addition, any hydrocarbon present larger than ethylene can have a C2H4+ fragment at m/z 28. The most insidious interferent however, is carbon dioxide, which, at ambient concentrations produces an abundant C0+ ion at m/z 28. Interference from N2 Certainly the easiest and most straightforward interference to eliminate is that from molecular nitrogen, even though it is present at some hundred thousand fold excess over carbon monoxide. The reactons: "2+ CAD/Ar! N+ + N and c0+ AFZCAD C+ + O are easily distinguished by quadrupole MS/MS. Figure 5.1 shows sensitivity and resolution easily attainable in the detection of 10 ppm CO in N2. Here, Q1 is set to pass mass 28; Q3 is scanned. ABL RH. 75 N2/10 PPM co 28+ AR CAD 188'“ Figure 5.1 CAD spectrun of 10 PP“ C0 1" "2° 38 76 Interference from Hydrocarbon In rare cases does the concentration of hydrocarbon in ambient air rise to appreciable levels. However, in an industrial or research environment it is possible that hydrocarbon concentration may rise to several thousand parts per million. Often, hydrocarbon produces fragments at m/z 28 with the composition C2H4+. To ascertain the effect of this ion on the determination of CO, we chose as our contam- inant ethane, which forms an abundant EI fragment ion at m/z 28. Per- forming CAD on this particular ion shows an ion intensity at m/z 12 of less than 0.5% of the base daughter peak (m/z 14). Together with fragmentation efficiency data for C2H4+, this results in an absolute cross section of 0.0013 A2 at 5 x 10'4 torr target pressure, as compared to 0.64 A2 for carbon monoxide. In addition, the translational energy (EAx) of the selected ion plays an impor- tant part in the efficiency for a particular transition (21). If the axial energy is chosen judiciously, (EAX = 33 eV, V03 = -25V) the hydrocarbon is attenuated further by at least a factor of two. A one thousand-fold excess of hydrocarbon ion at m/z 28 over the CO con- centration is therefore required to produce equal signals at m/z 12. Interference from Carbon Dioxide. The most troublesome interference for CO determination by quadru- pole MS/MS arises from the presence of carbon dioxide in air and other samples. At an ionizing electron energy of 70 eV, the cross sections for the production of CO+ are virtually the same for both C0 and C02. Since the concentration of C02 in the atmosphere is 330 ppm, its potential for interference is significant. Advantage is taken of the difference in collision cross section for C0+ ions formed at low ionizing electron energies. The ionization 77 potential of carbon monoxide is 14.01 eV (22), while the appearance potential of COT from carbon dioxide is 19.5 eV (23). Hhile this circumstance is fortuitous, the more important criteria are the relative CAD product intensities at these low electron energies. Figure 5.2 shows the ionization efficiency curve for C01 from C0 by electron impact (labeled CO+ under CO in figure), and the appearance potential curve for CO+ from C02, also by EI (labeled C0+ under C02). Adjacent to these parent ionization efficiency curves are their respective daughter fragmentation efficiency curves for the reaction C0+ + C+ + 0 by Ar CAD. The cross sections for CO+ formation from C0 and C02 are within 10% at 70 eV electron energy; also, the cross section for the C0+ + C+ + 0 reaction is within 5% for the fragmentation of CO+ from both precursors at this same energy. Therefore the processes shown in Figure 5.2 can all be normalized to 70 eV to facilitate comparison at lower energies. At 20 eV, the cross section for the m/z 28 *'12 transition from C02 sample decreases by more than a factor of 10 compared to that at 70 eV while for carbon monoxide samples there is essentially no attenuation at 20 eV. The net result is that the CAD product intensity is reduced by a factor of four at 20 eV for carbon monoxide, while for C02 this intensity is reduced by 105. Carbon dioxide interference can be reduced even more by further lowering the electron energy; however, this is done at great expense of carbon monoxide signal strength, which falls sharply below 20 eV. This dramatic effect of the ionizing energy on the C02 interference is demonstrated by the simple, but graphic isotope experiment shown in Figure 5.3. From an equimolar sample of C02 and I, Relative percent 78 100 50 =/ ' IO 15 20 25 30 Lab electron energy, eV Figure 5.2 Electron impact ionization efficiency curves for COT; fragmentation efficiency curves for CO+ + C+ + 0 for CO and C02 samples. ‘Zcoz‘aco AR cno -7o EV In. AIUI. m r m m m m w to m 12cozlaco AR CA0 -30 Ev ii) 1"’cozl-"co AR CA0 -20 EV III. HUI. x a nu . AIUI. M s on m a a m a m Figure 5.3 C02 and 13CO neutral loss of 16 scan, Ar CAD, at 70, 30, and 20 eV electron energy. 80 and 13CD, a neutral loss (MQ1 — M03 = 16 amu) scan was taken. The first analyzer (Q1) was scanned from m/z 27 to 30 while the second analyzer (Q3) was scanned from 11 to 14. The peak at m/z 12 arises from the C02 sample while that at mjz 13, from 13C0* + 13C+ + 0. The ion currents are nearly equal at 70 eV electron energy; at 30 eV, the peak areas are in a ratio of about 3:1. At 20 eV, essentially no contribution from C02 remains. The reasons underlying the reduction in the collision cross section for C0+ from carbon dioxide at low electron energies appear to arise from an increased amount of vibrational internal energy as compared to that of singly ionized C0. These effects are the subject of a separate study (21). Quantification of CO The standard addition of isotopic CO enables the determination of the carbon monoxide concentration in the sample. Care must be taken to ensure thorough mixing of the standard prior to analysis. With the electron energy at 20 eV and the axial energy at 33 eV, the mass spectrometer is scanned to detect a neutral loss of 16. Samples from room air were spiked with approximately 1000, 500, 100, 50, 20, 10, and 1 ppm each of C0 and 13C0; all were spiked with 1000 ppm ethane. At 20 ppm the differences in ion intensity of the sample and its isotope were 20% at a maximum. For samples over 100 ppm, these differences were typically less than 10% (Table 5.1). Figure 5.4 shows the neutral loss of 16 scan from sz 10 to 15 (daughter mass), of 20 ppm each of C0 and 13C0 in room air, with 1000 ppm ethane added. The peak at m/z 12 represents the analyte, m/z 81 Table 5.1. Relative precision of C0 determinations. [C0] = [13CO] Mean Deviation PPm % 1000 3.8 500 5.8 100 8.1 50 15 20 18 10 15 82 AIR/20 PPM co, 1300 AR CA0 57 ‘Q —4 .—4 86— .1 ‘ Pr "1 5rd“ _7_' __ .2: ,~ < LL?— , ‘I -.J ._4 91 CI _ a 28— ——4 ..4 '7... L T l '3 23 58 vp‘ Av Figure 5.4 20 ppm CO, 20 ppm 13CO in air, neutral loss of 16 scan, Ar CAD. 83 13, the isotopic standard, and m/z 14, the C2H5+-+ CH2+ + CH4 transition from ethane. The practical detection limit for our instrument in its current configuration is about 10 ppm. This limitation is imposed by the analog detection circuitry. Dual mode analog/pulse counting detection will be implemented soon and should improve the detection to below the interference limit. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 84 REFERENCES Hunt, D.F., J. Shabanowitz, and A.B. Giordani, Anal. Chem. 99, 386-390 (1980). Glish, G.L., V.M. Shaddock, K. Harmon, and R.G. Cooks, Anal. Chem. s2, 165-167 (1980). %aket§, D., J.D. Ciupac, and R.G. Cooks, Anal. Chem. 99, 723-726 1981 . McLafferty, F.H., and E.R. Lory, J. Chromatogr. 999, 109-116 1981 . McLafferty, F.H., P.J. Todd, D.H. McGilvery, M.A. Baldwin, F.M. Bockhoff, G.J. Mendel, M. Hixom, and T.E. Neimi, Adv. Mass Spectrom. 99, 1589-1596 (1980). Cooks, R.G., R.M. Kondrat, M. Youssefi, and J.L. McLaughlin, J. Enthnopharmacol. 9, 299-312 (1981). Kondrat, R.N., G.A. McClusky, and R.G. Cooks, Anal. Chem. 99, 1222-1223 (1978). Kondrat, K.R., R.G. Cooks, and J.L. McLaughlin, Science 199, 978-980 (1978). Munson, M.S.B., and F.H. Field, J. Am. Chem. Soc. 99, 1047 (1966). Beckey, H.D., J. Phys. E. 19, 72 (1979). Schulten, H.-R., Int. J. Mass Spectrom. Ion Phys. 99, 97 (1979). Yost, R.A., and C.G. Enke, J. Am. Chem. Soc. 199, 2274 (1978). Zakett, D., and R.G. Cooks, Anal. Chim. Acta 119, 129-135 (1980). Yost, R.A., and C.G. Enke, Anal. Chem. 91, 1251A (1979). Glish, G.L., and R.G. Cooks, Anal. Chim. Acta 112, 145-148 (1980). Zakett, D., P.H. Hemberger, and R.G. Cooks, Anal. Chim. Acta 119, 149-152 (1980). Gardner, L.R., and C.T. Dillinger, Anal. Chem. 91, 1230-1236 (1979). Kambara, H., Y. Ogawa, Y. Mitsui, and I. Kanomata, Anal. Chem. 9;, 1500-1503 (1980). Latven, R.K., Dissertation, Michigan State University, 1981. 20. 21. 22. 23. 85 Yost, R.A., C.G. Enke, D. McGilvery, D. Smith, and J.D. Morrison, Int. J. Mass Spectrom. Ion Phys. 99, 127 (1979). Latven, R.K., and C.G. Enke, in preparation. Krupenee, P.H., “The Band Spectrum of Carbon Monoxide," Natl. Stand. Ref. Data Ser., Nat. Bur. Stand NSRDS—NBS 5 (1966). Heissler, G.L., J.A.R. Samson, M. Ogawa, and R.G. Cooks, J. Opt. Soc. Am. 99, 388 (1959). Chapter 6. STRUCTURES AND FRAGMENTATION MECHANISMS OF THE IONS OF ETHANOL BY TRIPLE QUADRUPOLE MASS SPECTROMETRY 86 Structures and Fragmentation Mechanisms of the Ions of Ethanol by Triple Quadrupole MS R. Kazmer Latven, Margaret B. McFarland, and Christie G. Enke Department of Chemistry Michigan State University East Lansing, Michigan 48824 ABSTRACT The low energy (IO-25 eV) collisionally activated decomposition of the molecular ion and of some important fragment ions of ethanol, ethanol-OD, and 2,2,2-d3 ethanol has been studied by triple quadrupole mass spectrometry to obtain information regarding the ions' structures and fragmentation pathways. In this technique, ions generated by electron impact in the mass spectrometer ion source are mass selected in the first quadrupole, undergo collisionally activated decomposition (CAD) in the second, RF-only, quadrupole collision cell, and the resulting daughter ions are mass analyzed by the third quadrupole. The unit resolving power of the final quadrupole analyzer is a distinct advantage over other implementations of MS/MS in that structural information contained in deuterated species is readily obtainable. Ethanol ions thus studied include [CH3CH20H]+‘, [CH3CHOH]+, [CHZCOH]+, [CHZCO]+ , [CH3OH]+' and [CHZOH]+. 87 88 Fragmentation mechanisms based on CAD spectra are consistent with the fragmentation of a particular ion's deuterated analogs. The elucidation of the decomposition mechanism leads to an unambiguous assignment of parent ion structure in many cases. The observed dissociations parallel closely those observed in both electron impact and high energy C10 in sector instruments. The data suggest that fragmentation reactions which follow electron impact and collisional activation at both high and low energies are at least qualitatively independent of the method of excitation. Thus isotopic labeling in conjunction with low-energy collisionally activated decomposition in the triple quadrupole mass spectrometer is a convenient and useful tool for the determination of ion structure and fragmentation mechanisms, and is complementary to high energy collisional techniques. INTRODUCTION Low energy collisionally activated decomposition of gaseous ions provides a direct yet simple method for the elucidation of ionic structure. Triple quadrupole mass spectrometry (1) is a technique which embodies the low energy collision process; when coupled with isotopic labeling, it can yield unambiguous structural and mechanistic information (2). A major advantage of triple quadrupole mass spectrometry (TQMS) for the study of ionic structure and fragmentation is the unit mass resolving power of both analyzer sections which allows fragments which differ by only one amu to be independently observed. In the present paper, the structure of some important ions of ethanol are determined by low energy collisionally activated decomposition 89 (CAD) and the mechanisms involved in their dissociation are explored with the aid of deuterium labeled isotopes. Ethanol has been a popular molecule for the test of new techniques or theories in ionic structures (3-8). Friedman 99.91. (3) in 1957, predicted a fragmentation scheme for ethanol based on appearance potentials and thermochemical calculations. However, more recent proposals for ethanol fragmentation have been made by Danchevskaya and Torbin (4). Their work, based on electron impact studies of ethanol and several deuterated analogs, predicts both ion structure and fragmentation mechanisms. Selected ion fragmentation in reversed geometry sector instruments has been well demonstrated for organic structure determination (9,10). Cooks (7) and coworkers in 1975 have performed high energy (m8 keV) collision-induced dissociation of the ions of ethanol in a thermochemical study, which allowed the description of the reactions of highly excited ions. In the present work, the ethanol fragments at m/z 46, 45, 43, 42, 32, and 31 are studied by low energy (10-25 eV) CAD. In addition, a comparison of the methods and results from electron impact, C10 and CAD is offered. EXPERIMENTAL The triple quadrupole mass spectrometer used in this study has been described in detail in earlier work (11). Briefly, the instrument consists of, in series, an electron impact ion source of special design, a quadrupole mass filter (Q1), an RF-only quadrupole collision 90 cell (Q2), and a second quadrupole mass filter (Q3). The ion source, not previously described, is a cross axial type with a magnetically confined electron beam. An elongated, wire mesh enclosed ionization region provides electron energy homogeneity and minimizes interactions with unionized molecules. The ion volume exit aperture leads to three focusing lenses and an Extranuclear Labs 'ELFS' leaky dielectric quadrupole entrance aperture to avoid effects of fringing fields. Detection is via a Galileo 4800 channel electron multiplier. The output current is amplified by a Kiethly 18000-20 picoammeter, and data recorded by an H-P 7044A X-Y recorder. The ethanol used for analysis was reagent grade and used without further purification. Ethanol-OD (min 99 atom %D) and 2,2,2-d3 ethanol (98 atom %D) were purchased from Merck and Co. (St. Louis, MO 63116). The sample was degassed by standard freeze-punp-thaw cycle and was then bled into the source through a Granville-Phillips variable leak, to a pressure of 5 millitorr (1 torr = 133 Pa). The electron energy was 70 eV with an emission current of 0.25 mA. The argon target gas was from Airco Industrial Gases (Southfield, MI 48075), 99.998% pure. Target gas pressure was measured by a calibrated Bayard-Alpert type ionization gauge and regulated by a Granville-Phillips model 216 flow controller. An ion of particular m/z produced in the source and chosen for study was selected by quad 1; argon target gas was then bled into the collision cell and automatically regulated to 4 x 10'4 torr. The resulting fragmentations were determined by scanning the second 91 analyzer (Q3). Spectra were recorded with 10 V and 25 V axial energy. The axial energy, EAX’ is approximately equal to the offset voltage between source ion volume and the quadrupole rods of the collision region. Axial energy profiles (I vs. EAX) give characteristic information regarding CAD product ions' nature and origin (12). Profiles were obtained for all ions suspected of arising from charge exchange or proton transfer to determine the nature of the transition. CAD spectrum were obtained for every ion in the E1 Spectra of ethanol ethanol-OD, and 2,2,2-d3 ethanol, having an intensity greater than 5% of the base peak. Data given in Tables 1-4 are accurate to within 10%. RESULTS AND DISCUSSION Tables 6.1-6.4 show low energy (EAX = 25V) CAD spectra of the major ions formed by electron impact from ethanol, ethanol-OD, 2,2,2-d3 ethanol, and methanol/methanol-OD. The intensities given for the parent ion relative to the base daughter ion peak are indicative of the collision cross-sections for the parent ion. A higher parent peak implies a lower cross-section. The structures and fragmentation mechanisms reported for the ions of ethanol are consistent with those of their isotopically labeled analogs. m/z 46: [CH3CH20H]+' The structure of the molecular ion is unambiguous in this case since it arises from a saturated molecule, and has only one site of nonbonding electrons available for ionization. Upon collision (EAX Table 6.1. Low Energy (EAX = 25 eV) CAD Spectra of Ethanol Ions. 92 Intensities are given as % of most abundant daughter ion. CAD Ions Parent Ions, m/z m/z 46 45 43 42 31 30 46 300 45 45 3600 43 12 450 42 15 420 31 100 5900 30 3.0 64 920 29 10.6 100 2.4 99 100 28 7.1 27 4.8 98 26 7.1 1.2 25 19 1.2 66 15 1.9 21 100 14 4.3 100 100 2.9 13 2.3 7.7 4.6 93 Table 6.2. Low Energy (EAX = 25 eV) CAD Spectra of Ethanol-OD Ions. CAD Ions Parent Ions, m/z m/z 47 46 44 42 32 31 47 265 46 54 3000 44 9.4 1100 42 12 750 32 100 7000 31 2.9 100 3000 30 6.2 8.3 30 29 7.0 53 13 100 100 28 6.2 27 2.9 75 26 5.6 20 .58 53 19 .35 7.8 16 .19 7.8 100 15 .69 3.7 10 14 10 100 48 10 13 2.7 5.0 3.1 94 Table 6.3. Low Energy (EAX = 25 eV) CAD Spectra of 2,2,2-d3 Ethanol Ions. CAD Ions Parent Ions, m/z m/z 49 48 45 44 31 49 450 48 55 2600 45 4.0 830 44 3.1 2.0 560 32 6.2 2.1 31 100 21 6250 3D 7.1 100 1.5 .6 29 6.1 66 3.7 1.3 100 28 .76 4.0 .67 .4 27 1.6 2.1 .8 22 1.0 1.5 21 1.9 17 20 .33 21 19 18 .71 2.6 17 .52 8.4 100 16 5.3 2.2 100 15 1.2 1.2 14 1.0 62 13 6.2 95 Table 6.4. Low Energy (EAX = 25 eV) CAD Spectra of Methanola and Methanol-ODb Ions. CAD Ions Parent Ions, m/z m/z a32 a31 P33 b32 b31 33 1700 32 1400 100 1800 31 100 3400 7000 29 100 100 100 28 16 15 18 32 12 14 4.8 25 5.8 11 41 13 3.6 6 96 = 25V into argon) the molecular ion produces [45]+, and [31]+ directly, both by a-cleavage, as seen in Schemes 1 and 2. This is consistent with the labeled analogs' behavior: ethanol-OD shows [M]+° +‘[CH3CHOD]+ + H, and [M]+° + [OH200]+ + 'CH3, while d3 ethanol yields the analogous [M]+- + [CD3CHOH]+ + H and [M]+° +_[CH20H] + °CD3. Loss of methyl provides the base peak, and loss of H forms an ion of approximately 50% base peak in all cases. Other peaks seen in CAD spectrum of ethanol molecular ion, viz. m/z 30, 27, and 15, may arise from [M-H]+ at m/z 45, since these transitions are strong in the [451+ CAD spectrum itself, and appear there in the same ordering as observed in the [46]+- CAD spectrum. The target pressure response for this fragmentation is shown in Figure 6.1. The parent ion at m/z 46 is attenuated at increasing pressure due to fragmentation; at higher pressures some scattering loss is observed. The [M-H]+ fragment at m/z 45I is composed of both a metastable and a collisional contribution. 1f the metastable contributor (~8%) is removed, the contribution from collisionally activated decomposition can be determined, and is shown by the dashed line at m/z 45 in the figure. Other daughter ions at m/z 31, 30, 27, and 15 show their pressure dependence as indicated. If the fragment ion response is normalized to the scattering-corrected parent ion intensity, the log-log slope of the fragments indicates the order of the reaction: m/z 45I 45 31 30 27 15 Slope 0 1.0 1.1 1.0 1.9 1.9 1, relative 97 l00’ 1 J l0" l0" IO" Pressure, torr, 02 Figure 6.1 CAD product ion intensity vs. pressure for the fragmentation of 46* from ethanol. 98 These reaction orders demonstrate that the parent ion produces m/z 45, 31 and possibly 30 by a first order (single collision) process. Analogous to the [461+' + [31]+ transition may be the loss of methyl from [45]+ +'[30]+- The cross-sections for these reactions at 10"5 torr target pressure are well within a factor of 3: 13 and 5.9 A2, respectively. The second order processes (two collisions) may follow [46]+'-* [451+‘+ [27]+ and [46]+'-+ [45]+ + [15]+ pathways; it is possible, however, for the second order processes to follow a first order collisional excitation of the parent ion [46]+- + [46*]T- + [27]+ and [46]+' +-[46*]+' + [15]+. The ion at m/z 29 is a mixture of [C2H5]+ and [CHO]+. The library reference spectra of [CZH5]+ Show a 15+:14+:13+:12+ ratio of 100:55:2:O while [CH0]+ reference spectra give a 13+:12+ ratio of about 10:4. Ethanol [29]+ provides a 10:7:8:3 ratio of the 15+-to-12+ series which, assuming the molecular ion fragments similarly to [291+ by El and CAD, indicates the presence of a mixture which is approximately 80 t 10% [CHO]+. Friedman and coworkers (3) have determined this value by high resolution MS to be 77%. The [C2H5]+ arises from a charge site-initiated loss of 0H from the molecular ion, while the [CH0]+ is probably formed through the [451+ intermediate. Protonated or deuterated water ions are formed at m/z 19 from ethanol molecular ion, at m/z 19 and 20 from ethanol-OD and at m/z 22*, 21+, and 20+ from d3 ethanol from both [46]+ and [45]+ parent ions by rearrangement, the exact mechanisms of which are highly complex (7). 99 m/z 45: [CH3CHOH]+ The [45]+ ion was seen to have arisen from a cleavage of the molecular ion at [46]+°. As such, its CAD spectrum should be analogous to the [46]+ from ethanol-OD and the [481+ from d3 ethanol. Scheme 3 shows a four centered loss of H2 from the carbonyl and a carbon. Ethanol-OD and d3 ethanol show an analogous reaction, loss of H2 and HD, respectively. Scheme 4 shows odd electron ion formation from an even electron species. Although decompositions of even electron species have been poorly understood (13), a recent review article by McLafferty (14) describes their utility in characterizing structure. This particular even electron mechanism is seen often in CAD of even electron ions (15,16). The driving force of the fragmentation is necessarily induc- tive in nature, but instead of attracting an electron pair, only one of the electrons of the bond is attracted to the charge site. The other electron leaves with the neutral radical, molecule or atom. Since this mechanism does not involve a true electron pair induction, for purposes of this discussion we will refer to it as electron unpairing, after Sigsby, Day and Cooks (15,16) and give it the notation i*. A loss of methyl radical from [45]+ by electron unpairing forms [HCOH]+- at m/z 30. Ethanol-OD [461* and 2,2,2-d3 ethanol [48]+ similarly lose °CH3 and °CD3 to form [31]+' and [30]+', respectively. The formation of [27]+ and H20 from [45]+ involves the loss of hydroxyl and the competing loss of hydrogen from both the number one and two positions. While the ethanol-OD analog at m/z 46 shows pre- dominantly a loss of 19 (00 plus unspecified H), the 2,2,2-d3 ethanol demonstrates loss of 18 (H from no. 1 carbon) and the loss of 19 (D from no. 2 carbon) in a 3:2 ratio respectively (Scheme 5). 100 The [CH0]+ daughter ion at m/z 29 is formed by the loss of the terminal methyl and the hydroxyl proton. This is indicated by the observation that [29]+ is similarly intense in the spectra of all three analogs, although the exact mechanism of formation remains unclear. The m/z 45 parent can also form [CH3]+ by induction from the carbonium ion as shown in Scheme 6. The d3 analog similarly forms [6033+ at m/z 18, as well as [60211]+ and [cowth from rearrangement prior to fragmentation. m/z 43: [CHZCOH]+ This ion at m/z 43 arises from loss of vicinal H2 from [4SJ+. The important resonance contributors of this ion can be seen in Table 6.5. The charge site can exist on the oxygen or carbonyl carbon (Schemes 7 and 8). From the former, electron unpairing results in the loss of H, (or D from the -OD analog) while the similar mechanism induced by the charge on carbon results in the formation of the acetylene ion in both cases, and loss of neutral OH (or 00). The migration of the hydroxyl proton to the terminal carbon is very important in this ion due to the high stability of the carbon monoxide leaving group. This results in the inductive formation of [151* as the base peak for the ethanol sample (Scheme 9), and [16]+ for the ethanol-OD analog whose parent appears at m/z 44. The corresponding d3 ethanol ion at m/z 45 contains rearrangement ions, e.g. CDH2C0H+. which can cause ambiguity in interpretation. 101 Table 6.5. Structure of the major ions of ethanola and methanolb. -+ a[46]+- H3C - CH2 - 0H + a[451+ H3C - CH - OH + + a[43]+ HzC - C - OH <—» H2C = C = OH .+ -+ a[42]+- HZC - C = 0 .2, H26 = C = 0 b[32]+ H3C - OH 0+ I a»b[31]+ H20 - OH -++ H26 = 102 m/z 42: [CHZCO]+- The structure of the ion at m/z 42 is seen to arise from a loss of four protons, one of which is the hydroxyl proton (cf. Scheme 7). The fragmentation of m/z 42 is shown in Scheme 10 and is equivalent to the ethanol-OD spectrum at the same m/z; both show loss of CO to form the [CH2]+- species as the base peak. The d3 ethanol spectrum which corresponds to this structure occurs at m/z 44. Although this ion fragments to the analogous [CDz]+- at m/z 16, this [44]+° may contain rearranged [CDHCOH]+ as well, which would also produce a peak at m/z 16, i.e. [CDHz]+ by Scheme 9. m/z 32: [CH3OH]+- For completeness, the ions of methanol and methanol-OD are included. For the molecular ions, two important reactions are observed: the radical site initiated a Cleavage to form [M-H]+ as the base peak, and the charged induced formation of methyl ion (Schemes 11 and 12). m/z 31: [CHZOH]+ The species at m/z 31 has two important resonance contributors shown in Table 5. Four centered loss of hydrogen is shown in Scheme 13; electron unpairing forms [CH2]+- at m/z 14, leaving behind the neutral hydroxyl molecule (Scheme 14). Analogous behavior is again observed in both ethanol-OD and methanol-OD ions at [3211. In addition, the ethanol spectrun is identical to the d3 ethanol spectrum at m/z 31, which is consistent with the mechanism for formation of this ion. 103 Agreement with Other Work The fragmentation of ethanol has been the subject of a nunber of studies. Friedman gt 91. (3) employed a semiempirical application of the statistical theory of mass spectra to assign structures and fragmentation pathways to the mass spectrum of ethanol. Although important differences exist in the nature of electron impact and low energy CAD which preclude direct correlations of fragmentation pathways, a general comparison an be made. For example, Friedman predicts two products from direct decomposition of the molecular ion which are unobserved in the CAD study, viz. [4611- + [44]+' and [46]+--+ [30]+-. In addition, CAD shows six processes in the fragmentation of [45]+ which are not predicted in the theoretical model. A direct comparison of the assignment of ion structures can be made, however, since the parent ion in the CAD study arises from electron impact. The earlier work and the present work concur on the assignment of ionic structures at m/z 46, 45, 44, 31, and 30. However, the structure at m/z 43 is postulated by Friedman to be [CH3CO]+; the present work proposes the structure to be [CHZCOH]+. The evidence which suggests that [CH3CO]+ may not be the structure of [43]+ is that its precursor loses H2 from vicinal hydrogens to £959 [CHZCOH]+ (Scheme 3) and also that the terminal methyl structure is unlikely to lose OH, required for the formation of [Csz]+-. As mentioned earlier in the discussion regarding [CHZCOH]+, migration of the hydroxyl proton is important for the elimination of CO, and therefore, [CH3CO]+ must exist for at least some time prior to CO loss. 104 Danchevskaya and Torbin (4) propose the structure of this ion to be [CHZCH0]+. Their work does imply a migration of the acidic proton to the carbonyl site, however. In all other cases their hypotheses of ion structures agree with ours. Cooks and coworkers (7) have studied the fragmentation of ethanol in a work on the thermochemistry of reactions of highly excited ions. 0f the 16 high energy (8 kV) collision-induced dissociations reported by Cooks, all but one are observed in the present low energy (25 eV) study, although in some cases, relative intensities do not correspond (Table 6.6). Jennings (17) has pointed out that distribution within groups of C10 peaks was very similar to that observed within the same groups in a 70 eV mass spectrum. This suggests that despite the different methods of excitation, the distribution of excitation energies is not very different in the two cases. He therefore concludes that relative abundance of fragments depends only on the initial energy transferred to the molecules and not upon the method by which the energy is transferred. By comparing the present study with the results of Cooks (7) we have shown that low energy CAD fragmentation spectra are quite similar to those resulting from both high energy C10 and electron impact. Although collisional activation in high and low energy systems involves electronic (18) and vibrational (19) excitation respectively, the amount of energy thus deposited must be approximately the same in both C6585 . 105 Table 6.6. Comparison of High Energy CID in MIKES with Low-energy CAD in TQMS for the Fragmentation of Ions of Ethanol. Key: 5: Strong, > 10%; m: medium, 1-10%; w: weak, (1%. DECOMPOSITION INTENSITIES MIKES TQMS [46]+' +[45]+ S S [46]+° + [43]+ m not observed [46]+' + [31]+ m S [46]+- + [30]+° not reported m [46]+° + [29]+ w m [46]+° + [28]+° m w [46]+' + [27]+ w m [46]“ + [193+ w w [46]+- + [15]+ not reported m [45]+ +[43]+ m w [45]+ + [31]+ w w [45]+ + [30]+° not reported m [45]+ + [29]+ m m [45]+ + [283+° w w [45]+ + [27]+ m m [45]+ + [19]... s m [45]+ +[15]+ w w [43]+ + [29]+ w w [31]+ * [29]+ s m 106 CONCLUSIONS Ions of ethanol, ethanol-OD, and 2,2,2-d3 ethanol were directly characterized by low energy CAD in the triple quadrupole mass spectrometer. The structure of ethanol ions at m/z 46, 45, 43, 42, 32, and 31 are postulated to have the following structures: [CH3CH20HJ+', [CH3CH0H]+, [CHZCOH]+, [cuzc01*', [CH30H]+', and [CHZOH]+. In addition, the mechanisms for the formation and fragmentation of these ions are demonstrated. Fragmentations can be the result of multiple collisions in the “RF only" quadrupole. A mechanism is described which rationalizes the appearance of odd-electron collision products from even-electron parent ions. This mechanism, termed electron unpairing (14,15), is initiated by the charge site and involves the breaking of an adjacent bond, only one of whose electrons is captured by the charge site. The other electron leaves with the neutral species as a radical or H. In addition, the similarities in the nature of low-energy CAD, high-energy CID and electron impact spectra suggest that the resulting spectra depend only on the initial energy transferred to the molecule or ion, and not upon the method by which this energy is transferred. These similarities appear to provide complementary information which may prove useful in areas of structure elucidation and ion physics. Triple quadrupole mass spectrometry, when applied to studies of isotopically labeled analogs can yield unambiguous information regarding the structure of the selected ion, and the mechanisms of its collisionally activated decomposition. 107 H H '(‘4- a, / HC-C-OH ——> HC-C +H 3 'J 3 C \ H OH + SCHEME 1: [46]*-~+ [451* H H 'roi’ a \ + H - CdJOH -—+. /C = OH + CH3 (CH3 H SCHEME 2: [46]+°-+ [31]+ H OH H ' +/ \ + H - E/“x? .__. /C = C - 0H + H2 \s/ H H H SCHEME 3: [45]+-+ [43]+ H OH (\+ / 7:1: 4" H - C —-+ HC - 0H 4' .CH3 IJ \ H SCHEME 4: [45]+-+ [303+- 108 H OH 'jr/ + H2C " C —+ H2O = CH + H20 '+ \ H OH P/ + + H3C-Cr,—> H3C-C :3 H2C=CH + H20 + H SCHEME 5: [453+.. [27]+ OH +/ 71 + H SCHEME 6: [45]+-> [15]+ + 0* '+ HZC = C = dbl H .3—. HZC = C = o + H \A SCHEME 7: [43]+ + [421*- .* f HZC = Ckl 0H -3—+ HC = CH + OH V SCHEME 8: [43]+ + [26]+° 109 + + HZC = C = OH -——+ H3C - C = 0 .__. +CH3 + CO U SCHEME 9: [433* + [15]+ H H + 7; \+ C - C = 0 -——+ C + CO U / H H SCHEME 10: [421+' + [14J+° H H l ("t a \ + H - C - 0H ——. C - OH + H C") / H H SCHEME 11: [32]+'-+ [31]+ + . H3C - 0H .;3. +CH3 + 0H \J SCHEME 12: [32]+° + [15]+ 110 H \ + + C=O —+ HCO + H /J\.\ 2 HN/H SCHEME 13: [31]++ [29]+ \ 71* +° CCJAOH —. HZC + 0H SCHEME 14: [311* + [14]“ 2. 3. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. lll REFERENCES Yost, R.A., and C.G. Enke, J. Am. Chem. Soc. 199, 2274 (1978). Yost, R.A., and C.G. Enke, Org. Mass Spectrom. 16, 171 (1981). Eriedman, L., F.A. Long, and M. Holfsburg, J. Chem. Phys. g1, 613 1957 . Danchevskaya, M.N., and S.N. Torbin, Zhurnal Fizicheskoi Chimii 51, 2843 (1977). Sieck, L.H., F.P. Abramson, and J.H. Futrell, J. Chem. Phys. 55, 2859 (1966). Ryan, K.R., L.H. Sieck, and J.H. Futrell, J. Chem. Phys. 51, 111 (1964). Cooks, R.G., L. Hendricks, and J.H. Benyon, Org. Mass Spectrom. 19, 625 (1975). Munson, M.S.B., J. Am. Chem. Soc. §Z, 5313 (1965). McLafferty, F.H., P.F. Bente III, R. Kornfeld, S.C. Tsai, and I. Howe, J. Am. Chem. Soc. 96, 2120 (1973). McLafferty, F.H., R. Kornfeld, H.F. Hadon, K. Levsen, I. Sakai, P.F. Bente III, S.C. Tsai, and H.D.R. Schuddemagl, J. Am. Chem. 95, 3886 (1973). Yost, R.A., and C.G. Enke, Anal. Chem. 61, 1251 A (1979). Latven, R.K., and C.G. Enke, in preparation. McLafferty, F.H., Interpretation of Mass Spectra, Benjamin, Reading, MA, p. 67 (1978). McLafferty, F.H., Org. Mass Spectrom. 15, 115 (1980). Sigsby, M.L., R.J. Day, and R.G. Cooks, Org. Mass Spectrom. 15, 273 1979 . Sigsby, M.L., R.J. Day, and R.G. Cooks, Org. Mass Spectrom. 11, 556 1979 . Jennings, K.R., Int. J. Mass Spectrom. Ion Phys. 1, 227 (1968). Cooks, R.G., in R.G. Cooks (ed.) Collision Spectroscopy, Plenum Press, New York, p. 369 (1970). Friedman, L., and B.G. Reuben, Adv. Chem. Phys. 12, 33 (1970). Chapter 7: COMMENTS AND CONCLUSIONS 112 113 1. Energy Studies A thorough understanding of the physical parameters which affect collision cross section is necessary for elucidation of the processes which goverH low energy collisionally activated decomposition. It is clear that collision cross section depends primarily on the selected ion's axial energy and internal energy. While the ion's axial energy is simply a function of the offset potential between the ionization region and the quadrupole rods of the collision cell, the internal energy depends on ionizing electron energy, the nature of the precursor and the excitation lifetimes of the electronic and vibra- tional states with which the selected ion is formed. Studies of the effects of these parameters will further enhance the conceptual framework around which significant new knowledge may develop. Useful information regarding the thermochemistry of collisionally activated decompositions may be provided by energy-loss spectra derived from studies of the interaction of 02 and Q3 offsets on the collision cross section. Energy-loss spectra are currently utilized in MIKES instrumentation to measure the amount of energy converted from the translational to internal domains. Energy which is so converted manifests as a reduction in translational energy. This loss of translational energy can be measured under appropriate conditions in the triple quadrupole MS by observing the change in stopping potential required of the 03 offset when 02 offset is set to pass a particular daughter product. In addition, since Q3 offset affects the apparent axial energy profile, the understanding of its role in ion transmission is critical for not only axial energy studies, but also for the best settings of 02 and DB offsets for collecting library CAD spectra. 114 2. Applications For mixture analysis, the limiting factor at present is the current state of the instrumentation. Advances in detection circuitry with the implementation of pulse counting will lower detection limits consider- ably. Advances in the software for both instrument control and data management are acutely required although significant improvements have been made recently which are as yet untested. The characterization of organic structures is an application which has been well described but little implemented since the introduction of TQMS. A systematic study of the lower hydrocarbons followed by their fragmentation characteristics as influenced by neighboring functional groups is the logical first step. Once this framework has been established, the characterization of classes of compounds can begin. This author sees the class of esters to be particularly significant since they are economically important and ideally suited for study by TQMS. APPENDIX Appendix 1. A New Electron Impact Ion Source 115 A NEW ELECTRON IMPACT ION SOURCE A new electron impact ionization source was designed and constructed for the purpose of providing ionizing electrons with a small energy spread, while still maintaining a strong ion current. Key elements in the design are: (1) short filament length; (2) slit shaped electron entrance aperture, (3) electron collimation magnet, (4) elongated, wire mesh-enclosed ionization region; (5) cross axial beam paths, (6) leaky dielectric quadrupole entrance aperture; (7) temperature programmability to 250°C. See Figure A.1. The short filament length minimizes IR drop across the filament. For Rhenium ribbon of 5 mm length (Finnigan 51737, .0045 x .009 in.) the resistance is measured at .042 ohms. For 3.5 A filament current, the electrons on each side of the filament will only see a potential difference of 150 mv. The slit directly below the filament ensures that the electrons which enter the source are only those which are emitted on the cylindrical axis of the ion volume. The 500 gauss magnet minimizes beam spread ensuring a maximum electron density on the ionizing axis. Electrons which have an initial non-normal component of their energy will be forced to direct that component in a helical path between the poles of the magnet. For a 70 eV electron with 10 eV off axis energy, a magnetic field of 500 gauss will produce a helical path of .03 nm in radius. The ionization region is cylindrically shaped with a length to width ratio of 1.5. The electron and molecular beams cross at the cylindrical axis, centered on a point halfway down its length. The volume is an open stainless frame, with a silver screen encompassing 116 117 .mogsom cow we urumemgum H.< mczmwm @mnjw macs“. 05522 $23 3:82 362.00 5.5 29:8 :55? mE:_o> coH 5.8mm '0 I EVN\\\\\\\\\\\\\ o0 I \\\\\\V I \\\\\\\‘ \ H V E\\\\\\\\\\\| I (QUOLIJLLOI... 118 the open volume. The openness allows unionized sample to escape into the vacuum before it can interact with newly formed ions. The screen insures that the volume sees an equal potential at all points. However, since the repeller used to push the ions toward the exit aperture is inside the volume, its field can influence the potentials seen by the ionizing electrons. Moving the repeller away from the ionizing region (elongation of volume) allows the field to be more uniform at the points in it through which the electrons pass. The Extranuclear Labs ELFS leaky dielectric at the exit lens and protruding in to the first quadrupole field reduces the effect of the fringing fields of the quadrupoles, allowing higher transmission, especially of high mass ions. The source can also be heated to 250°C. The performance of the source was evaluated by comparison with the commercially available "CI-El“ ion source which it replaced. To test electron energy homogeneity, appearance potential studies revealed fine structure in an ionization efficiency curves to within 0.25 eV. The electron spread of the original source was so broad (10 ev) that no reasonable appearance potential curves could be obtained. The new source was shown to be more efficient than the original, in studies in the signal strength of perfluorotributyl amine. For this compound, the peak at m/z 502 was undetectable with the old source, but present at 2% of the base peak with the new design. "11111111111111111T