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This is to certify that the thesis entitled THE ISOTOPE RATIO CAPABILITIES OF A CONVENTIONAL DOUBLE FOCUSING MASS SPECTRWEI‘ER AS APPLIED TO NINETEENTH-CENTURY LEAD-GLAZED CERAMICS USING FAST ATOM BOVlBARIMENT IONIZATION presented by Gregory C. Dolnikowski has been accepted towards fulfillment of the requirements for ”.8. degree in Chfl'fliStSY />(Wd\\ , Major professor Date W53 / 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution I MSU 1 RETURNING MATERIALS: P1= ~ in r“ ‘ '“op * THE ISOTOPE RATIO CAPABILITIES OF A CONVENTIONAL DOUBLE FOCUSING MASS SPECTROMETER AS APPLIED TO NINETEENTH-CENTURY LEADASLAZED CERAMICS USING FAST ATOM BOMBARDMENT IONIZATION By Gregory C. Dolnikowski A THESIS Submitted to Michigan State University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Department of Chemistry 1983 K) ‘45: ABSTRACT THE ISOTOPE RATIO CAPABILITIES OF A CONVENTIONAL DOUBLE FOCUSING MASS SPECTROMETER AS APPLIED TO NINETEENTH-CENTURY LEAELGLAZED CERAMICS USING FAST ATOM BOMBARDMENT IONIZATION By Gregory C. Dolnikowski The origin of archaeological artifacts containing lead can be determined using precise measurements of lead isotope ratios by mass spectrometry. Normal isotOpe ratio techniques are precise but laborious and costly. Direct analysis of lead-based glasses and glazes by fast atom bombardment mass spectrometry was studied as an alternative technique. Isotope ratio data were collected with a computerized doUble fecusing mass spectrometer, by means of limited-range magnetic scanning or computer-controlled accelerating voltage switching. Many elements including lead, copper, and iron were detected in nineteenth—century American ceramic glazes. It was possible to measure lead isotope ratios to a precision of three—tenths to one percent in the ceramic glazes and in a glass containing an NBS isotOpic lead standard. A.molecular interference and several isotope discriminations were noted and their effects on the lead isotope ratios have been docunented. ACKNOWLEDGMENTS I would like to thank all of the persons who made this project possible: Dr. Eraselton and Beth Martin for perflonning the ICP analyses; Jo KOtarski for drawing many of the figures; all of the people in the Mass Spectrometry Facility fer help and suggestions; Dr. Crouch, D. Allison, D. Holland and D“. Watson for advice and ideas; Kevin Smith for lending the archaeological artifacts; and my wife Edie for typing and support. 11 II. III. IV. TABLE OF CONTENTS List of Tables List of Figures List of Abbreviations Introduction A. Stable Isotopes of Lead B. Geological and Archaeological Importance of Lead C. Isotope Ratio Precision and Accuracy D. Isotope Ratio Mass Spectrometer E. Alternative Approaches to Measuring Isotopes by MS F. Archaeological Samples Experimental A. Separation Studies of Metals 1. Metal-DEDTC Chromatography 2. Dowex 1-X8 Chromatography 3. Electrodeposition B. Mass Spectrometry Instrunentation 1. Varian MAT CH-SDF Mass Spectrometer 2 DEC PDP-8e Data System 3. Control Amplifier for Accelerating Voltage A Accelerating Voltage Alternater Programs 5. Nbgnetic Scanning Using AVA C. EI/D Mass Spectrometry of Lead 111 vi. viii. 10. 12. 12. 12. 13. 1M. 15. 15. 18. 20. 26. 28. 29. iv D. Fast Atom Bombardment Mass Spectrometry 30. 1. Physical Arrangement of FAB Ion Source 30. 2. FAB Conditions 30. 3. Lead Compounds by FAB 32. 11. Production of Lead Glass 32. 5. FAB of Lead Glass and Lead Glazes 33. 6. Isotope Ratios by FAB and El 311. 7. ICP Analysis of Lead Glazes 35. VI. Results and Discussion 36. A. Mass Spectra of Lead Compounds 36. 1. Lead by HUD 36. 2. Pb(DEDI‘C)2 by EI 39. 3. Isotope Ratios Using Pb(DElTl‘C)2 112. ll. KCl/Glycerol by FAB All. 5. Lead Acetate/Glycerol by FAB All. 6. Po(N03)2, Pb(DI~3DI'C)2 and Pb02 by FAB 50. 7. Lead Acetate/Glycerol/KCI by FAB 50. 8. Isotope Ratios Using Lead Acetate/Glycerol/KCl 55. B. FAB Pbss Spectra of Lead Glasses and Glazes 56. 1. Lead Class by FAB 56. 2. 'Ilvo-Sided Tape by FAB 60. 3. Lead-Glazed Redware by FAB 63. ll. Lead-Glazed Slipware by FAB 67. 5. Pearlware, Creanware, and Mn-Glazed Redware by FAB 67. 6. ICP Results for Ceramic Pottery Glazes 75. C. Isotope Ratio Mass Spectrometry of Lead Class by FAB 77. V 1. NBS Isotopic Standard 981 "Cemmon" Lead 2. Lead Isotope Ratio Precision and Accuracy by FAB 3. Xenon Isotope Ratio Precision and Accuracy by EI A. Sample Probe Positioning Error 5. Isotope Ratios by SIMS 6. PbH+Interference in Pb+Isotope Ratios q Isotope Ratio Dependence on Accelerating Voltage Isotope Ratio Dependence on FAB Voltage D. Lead Isotope Ratios from Archaeological Artifacts 1. Possible Origins of the Highland's ceramics 2. Lead IsotOpe Ratios fbr Lead-Glazed Redware Samples VII. Summary VIII. References 77. 77. 85. 87. 90. 91. 9M. 96. 101. 101. 101. 107. 112. Table 1 Table 2 Table 3 Table u Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 1“ LIST OF TABLES Stable Lead Isotopes Standard FAB-MS Operating Conditions Major Ions in the ET Mass Spectrun of Pb(DED1‘C)2 Major Ions in the FAB Mass Spectrun of Lead Acetate in Glycerol Ions in the FAB Mass Spectrun of Lead Glass Major Ions in the FAB Vass Spectrun of Ceramic Glaze sample HL790(GREEN) Major Ions in the FAB Mass Spectrun of Ceramic Glaze Sample HL122UA ICP Multielement Analysis of ceramic Glazes NBS Isotopic Standard 981 "Common" Lead Lead Isotope Ratio Internal Precision for Accelerating Voltage Switching Lead Isotope Ratio Internal Precision for Magnetic Scanning Lead Isotope Ratio External Precision Between Runs Lead Isotope Ratios Corrected fbr Interferences and Isotope Discrimination Xenon Isotope Ratios External Precision for Magnetic Scanning 'v1 31. H1. H9. 61. 65. 70. 76. 78. 79. 80. 82. 8D. 86. Table Table Table Table Table Table Table Table Table 15 16 17 18 19 20 21 22 23 ‘vii Probe Position versus Signal Intensity Probe Position Versus Isotope Ratio Accelerating Voltage versus Isotope Ratio FAB Voltage versus Isotope Ratio Isotope Ratio Changes Due to Xe Pressure Drop Selected Isotope Ratios frcm Great B'itain Selected Isotope Ratios from the United States Lead Isotope Ratios of Lead—Glazed Redware Corrected Lead Isotope Ratios for Lead-Glazed Redware 88. 89. 95. 98. 100. 102. 103. 10“. 106. Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 7 10 11 12 13 111 15 16 17 18 19 21 22 LIST OF FIGURES Isotope Ratio Mass Spectrometer Accelerating Voltage Control Amplifier Circuit Design Control Ampl ifier Linearity Overview of Vass Spectrometer and Data System Flowchart for AVA Data Collection EI/D Mass Spectrum of PbO 2 EI/D Ion OJrrent Profile of PbO2 EI Mass Spectrum of PMDEDTCE Isotope Fractionation in Pb(DEDTC)2 FAB FAB FAB FAB FAB FAB FAB FAB FAB FAB FAB FAB FAB Mass Spectrum of KCl/Glycerol Mass Mass Mass Mass Mass Mass Mass Mass Mass Mass Mass Mass Spectrum Spectrum Spectrum Spec trun Spectrum Spec trun Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum of Lead Acetate/Glycerol (UpJohn) of Lead Acetate/Glycerol (MSU) of Lead Nitrate/Glycerol of Pb(DEDTC)2 /Glycerol of Pb02/G1ycerol/8202 of Lead Acetate/Glycerol/KCl of Lead Glass (Major Ions) of Lead Glass (Minor Tons) of Bio-Sided Tape of Lead-Glazed Redware (HL122HB) of Lead-Glazed Redware (HL12211A) of Lead-Glazed Slipware (HL790(RED)) viii Figure Figure Figure Figure Figure 23 2’4 26 27 ix FAB Mass Spectrum of Lead-Glazed Slipware (HL790(GREEN))69. FAB Mass Spectrum of Pearlware Body Sherd (HL537A) 72. FAB Mass Spectrum of Creamware (HL537B) 73. FAB Mass Spectrum of Nanganese-Glazed Redware (HL539) 711. Pb“ and PbH+ Isotopes 92. ADC AVA DAC DEC DE DTC ECP EI EI/D FAB-MS FD FI GC-MS MS NBS SIMS LIST OF ABBREVIATIONS Analog to Digital Converter Accelerating Voltage Alternator [figital to Analog Converter Ifigital Equipment Corporation Diethyldithiocarbamate Emitter Current Programmer Electron Impact Electron Impact - Thermal Desorption Fast Atom Bombardment - Mass Spectrometry Field Desorption Field Ionization Gas Chromatography Gas Chromatography - Mass Spectrometry Mass Spectrometry National Bureau of Standards Secondary Ion Mass Spectrometry INTRODUCTION A. Stable Isotopes of Lead The element lead has fbur stable isotopes (20A, 206, 207, and 208), with the heaviest three being end products of radioactive decomposition. Table 1 shows the radioactive precursors of lead and their half lives. The concentration of these uranium and thorium isotopes varies depending on the geographic location of the lead ore deposit. The combination of differing half>lives and differing initial concentrations of the radioactive elements leads to two important consequences. First, the total abundance of the four lead isotopes and the abundance ratios among lead isotopes vary continuously over time in a given geological deposit; second, the lead isotope ratios vary according to the geographic location of the deposit (1,2,3,A). During the geological fermation of lead ores the lead isotopes are nearly always separated from the uranium and thorium isotopes because lead has a lower melting point (1). when the lead is geologically separated from the radioactive precursors, its isotope ratios are "frozen" in time and in space. It is therefore theoretically possible for each lead ore deposit that contains no uranium or thorium to have a unique isotopic composition. In fact, lead isotope ratios vary widely (by as much as 10% for a given isotope pair) over the face of the earth (A). 2 Table 1 Stable Lead IsotOpes Pb Isotope Atom Percent Radioactive Radioactive Mass Composition Precursor Half Life 20A u 1.3% Non-radiogenic ------ 238 9 206 u 25.2% U u.u683 X 10 yr 235 8 207 u 21.0% U 7.0381 X 10 yr 232 10 208 u 52.5% Th 1.U01 X 10 yr 3 B. Geological and Archaeological Importance of Lead This enormous variability of lead isotope ratios has been extensively exploited in the areas of geochronology (1,2,3) and archaeology (N,S,6,7,8) and pollution studies (9,10). Where the lead isotopes and their radioactive precursors have not been separated over time, such as in mineral deposits of zircon crystals, it is possible to date those deposits by measuring the U to Pb ratio. This method is entirely analogous to radiocarbon dating which has been widely used in archaeology, but it is applicable to entirely different geological deposits. In fact, the best estimate of the age of the earth has been determined by measuring U/Pb ratios (1). As mentioned previously, however, most lead ore deposits are found to have been separated from their radioactive precursors. Such deposits are essentially useless to geochronology but are potentially very important to archaeology. It has been shown that lead isotope ratios are not changed by the smelting of lead ores into lead, by the manufacture of lead into objects, or by the subsequent weathering of these artifacts (11,12). Thus, it is possible to relate an artifact containing lead to the geographical location of the mine from which the lead originated, by measuring the isotope ratios of lead in the artifact and comparing than to isotope ratios of lead ores from around the world. If lead were an uncommon element or not widely used, this method would find little application, but in fact, lead is plentiful and was one of the first metals mined (approxnmately A000 BC) (A). Lead is also easily separated from its most common ore, galena, whose chemical formula is PbS. Lead was extensively used in the 4 ancient world in a wide variety of products including bronze alloys, pigments, cosmetics, medicines, glasses, and pottery glazes (A). Therefore, lead isotope analysis is a technique which is widely applicable to archaeological artifacts for the purpose of determining the geographical origin of the lead in those artifacts. C. IsotOpe Ratio Precision and Accuracy In order to determine the exact geographical origin of the lead in an artifact it is necessary to measure isotope ratios very precisely and accurately. In order for a lead isotope measurement to pinpoint the origin of an archaeological sample the precision of the isotope ratio analysis must generally be equal to or better than 0.1% relative standard deviation from the mean fer replicate analyses of the same sample (13). The accuracy must also be equal to or better than 0.1% relative standard deviation, which can be evaluated by determining the isotope ratios of certified isotope standards such as those prepared by the National Bureau of Standards (NBS) (1M). D. IsotOpe Ratio Mass Spectrometer Mass spectrometry is the ideal method for conducting isotopic analyses. However, when operated in a conventional scanning mode, most commercial mass spectrometers attain approximately 10% precision for isotope ratios (15). The reason for this lack of precision is that there is often a great difference in the relative intensities among 5 isotopes of an element, fer example the approthate natural abundance ratio of lead isotopes 20A and 206 is only 0.05. Because it is necessary to divide small numbers by large numbers, a small uncertainty in either number results in a large uncertainty in the ratio. Due to the difficulty in obtaining highly precise and accurate isotope ratios, a fewimass spectrometers have been designed and built for the sole purpose of measuring isotope ratios (16,17). Figure 1 shows a diagram of such an isotope ratio mass spectrometer. The salient features of this kind of instrument are that it is equipped with a thermal ionization source fer ionization of non-volatile elements, a magnet for mass analysis, and dual ion collectors (usually Faraday cages) fer simultaneous measurement of two isotopic ion beam intensities. A thermal ionization source (17) consists of a metal filament (usually rhenium) , onto which a sample may be carefully electrodeposited or precipitated, surrounded by a chamber at very low pressure (less than one microtorr) . The filament is resistively heated mtil it is 1200 degrees Celcius in the case of Pb (17). Only elements with low ionization potentials (less than 7 eV) such as lead can be volatilized and stripped of an electron under these mild ionization conditions. A suspension of silica gel can be added to the filament. This acts as an ionization enhancer and allows elements with ionization potentials up to 9eV to be thermally ionized (17). The isotopic ions thus formed are accelerated out of the ion source using a high voltage, usually 3-A kv. The ions are deflected by the 6 Figure 1 Isotope Ratio Mass Spectrometer DUAL COLLECTORS MAGNETIC AMPLIFIERS SECTOR DATA SYSTEM -——-|ON BEAM I I : ACCELERATING VOLTAGE I z/AND ION OPTICS I l l._'__ ___,.. I oum.GAs / ‘ RESERVOIRS DIFFUSION PUM THERMAL IoumAHON SOURCE MECHANICAL PUMP 7 magnetic field and separated into beams according to their mass to charge ratios (15). The detectors are arranged in space so that the two isotopic ion beams of interest strike both detectors simultaneously. An electronic circuit obtains the ratio of the signals from these two detectors. The ratio is either displayed on chart paper or transferred to a data system. The advantages of this kind of isotope ratio mass spectrometer are its inherent high precision and accuracy. There are several reasons for the high precision. The first reason is that the thermal ionization source exhibits very little isotopic fractionation (18). This means that the ionization system itself does not cause the sample's isotopic composition to change much over the course of the measurement. Nbst ionization sources cause some isotOpic fractionation, but for the triple filament thermal ionization source (which include two heated sample filaments and an extra filament for bombarding the sample with electrons) the change in the lead isotope ratio 20u/206 was measured to be only 0.03% over a 2” minute analysis (13). The second reason for high precision is that all experimental variables are held constant throughout the measurement, including the accelerating voltage and the magnet current. The third reason is that Faraday ion collectors are extremely stable and highly accurate ion current measuring devices. The kind of instrumentation just described is capable of'measuring lead isotope ratios to a precision of 0.01% relative standard deviation from the mean, and was used to determine the values for so called "absolute" lead isotope ratio standards at the NBS (1U). 8 UnfOrtunately, the instrumental method fer detenmining lead isotope ratios just described has some major practical disadvantages: only two isotopes can be measured at one time, the analysis and sample preparation procedures are laborious and tflme consuming, and the instrumentation is not widely available because it must be designed for a particular analysis and consequently is usually custom—built (17). The first disadvantage is no longer strictly a disadvantage because very recently, an isotope ratio mass spectrometer was built that includes six moveable ion collectors (19), but the other disadvantages are of such magnitude that most laboratories cannot perform lead isotope analysis using this instrumentation. The goal of this project, therefore, is to design a simple, reproducible sample preparation and analysis procedure fer accurate and precise lead isotope ratio mass spectrometry using a commercially available conventional type instrument. E. Alternative Approaches to Measuring Isotopes by MS Mass spectrometric analysis of lead and other metals has been reported using the following ionization techniques: field desorption (FD) (20,21); laser assisted FD (22,23,2A,25); spark source mass spectrometry (26); electron impact of volatile metal compounds (27), including tetraethyl compounds (1,2,3) and dithiocarbamate compounds (28,29,30,31,32,33,3A); secondary ion mass spectrometry (SIMS)(35,36,37,38,39); inductively coupled plasma mass spectrometry 9 (A0); and recently fast atom bombardment mass spectrometry (FAB—MS) (M1). The commercial mass spectrometer that was available for this project was the Varian-MAT CH-SDF. Because of the versatility of the instrument's design, it was decided to investigate as many as possible of the ionization techniques for lead in order to determine which one was best for performing lead isotope analysis. Dual detectors were not available nor are they even usable on a reversed geometry doUble—focusing mass spectrometer of this kind unless the electrostatic sector is removed. In order to approximate the performance of a multiple detector arrangement, one can rapidly focus on each ion of interest by accelerating voltage switching (A2,“3,MU), or by scanning the magnet over a short mass range (23,25). Both of these techniques were investigated during the course of this project. Due to the double focusing characteristics of the CH-5DF the total number of ions reaching the detector is smaller than in an isotope ratio mass spectrometer, so the signal to noise ratio is also diminished. An electron multiplier was available instead of the Faraday cup normally employed for isotOpe ratio analysis. The electron multiplier can detect smaller ion currents than a Faraday cup but its output is not as accurate(fl5). The decreases in precision due to the smaller ion currents of doublebfocusing and the electron multiplier detector can be offset to some degree by employing a method of averaging random noise. This can be accomplished by a multi-channel integrator (23,25), or by a digital computer (A2). Since a sophisticated digital data system was already available, it was the natural choice for data collection and 1O analysis (A2). F. Lead in Archaeological Samples Lead is rarely found in a pure form in archaeological artifacts. It is often found as an alloy with other metals, especially copper, in bronze and brass materials (5,6). Lead and other metal oxides were commonly added to pottery glazes for color and as fluxing agents (A6,A7,A8,A9). Lead has been separated from other metals in a variety of ways including: electro—deposition (13), cation exchange chromatography (50,51), anion exchange chromatography (52,53,5A); high performance liquid chromatography (HPLC) (55), and gas chromatography (56,57,58). we decided to investigate several of these separation procedures should our archaeological samples require them. The archaeological samples that were used for mass spectrometric analysis were lead-glazed ceramic sherds from a nineteenth—century Pennsylvania country estate site (59). Glazed pottery and similar materials such as colored glasses have been quantitatively analyzed for major and trace components by a wide variety of spectroscopic methods including: atomic emission (A7), X-ray fluorescence (60), neutron activation (A8), atomic absorption (61), and electron microprobe analysis (A9). Glazes and glasses have also been analyzed by SIMS (35) and very recently FABAMS (A1). Since the Varian CH-SDF had a FAB ionization source associated with it, this method presented an Opportunity to study the elemental composition of the glazes in 11 conjunction with their lead isotope ratios. Atomic information of this kind can also be used to determine the origin of artifacts (A7). It is fortunate that SIMS and FAB-MS are closely related phenomena (A1), for it was possible to benefit from the articles about the theoretical and practical considerations involved in using SIMS for isotope ratio determinations and quantitative analysis (35,38). EXPERIMENTAL A. Separation Studies of Metals 1. Metal-DEDTC O'Iromatography The first studies that were done in the project involved separating lead from other metals, especially copper, by a number of common techniques in order to compare them. The first technique involved derivatization of the metals with diethyldithiocarbamate (DEDTC) ligands and separation via HPLC or GC. The synthesis of the DEDTC ligand and the Pb(DEDI‘C)a complex was published by Tavlaridis and Neeb (58,62). The Na(DEDTC) salt is commercially available, but is easily produced in a single step in nearly 100% yield by carefully mixing equimolar amounts of carbon disulfide and diethylamine dissolved in dilute NaOH solution in an ice bath. The reaction is extremely exothermic and proceeds to completion immediately according to reaction 1. 5% NaOH S ll. - 4- CS 4» (CH - CH) -NH : (CH - CH) -N-C-S Na 1) 2 3 22 o 3 22 0C Pb(DEDTC) is prepared by adding 0.1 M lead acetate or lead nitrate solution dropwise to the aqueous Na(DEDTC) solution as shown in reaction 2. aq. S S - 2+ / \\ / \ 2(DEDTC) + Pb ———-- (CH - CH ) -N-C Pb C—N—(CH — CH ) 2) o 3 22 \S/ \S/ 2 32 250 12 13 Pb(DEDlC%ais a thick white precipitate that is easily isolated by vacuum filtration. The precipitate is washed with water to remove salts and recrystalized from chloroform. The same procedure was followed for a bronze sample containing Cu, Sn, Zn, and Pb after the sample was first dissolved in dilute nitric acid and neutralized with 5% NaOH. Both the Pb(DEUl‘C)2 and the mixture of metal DEDTC's were run on an HP—AO2 GC with a 2% 0V-1 column and on a home-built HPLC using a Licrosorb SI-60 silica gel column with 1Qum packing. The conditions for the HPLC (55) and GC (62) were the same as the literature values. In both the GC and HPLC separations, the components of the mixture were less well resolved than in the literature chromatograms, but this technique is clearly feasible for our experiment. 2. Dowex 1-X8 Chromatography Anion exchange on Dowex 1—X8 resin is another technique for separating lead from other metals (52). A 50 ml buret was cleaned and a plug of glass wool placed next to the stopcock. 10 ml of Dowex 1-X8 (200-A00 mesh) was slurry-packed into the buret. The resin was washed with 50 ml dilute NaOH, 50 ml dilute HCl, 50 ml dilute nitric acid and 50 ml of a 90% glacial acetic acid/10% concentrated nitric acid mixture. Fiftyiml of a bronze solution in dilute nitric acid was sorbed onto the column. The copper and other non-absorbed components were washed from the column with 50 ml of 90% glacial acetic acid/10% concentrated nitric acid. The lead acetate, being strongly absorbed on the column, did not elute. The lead was eluted with 100 ml of 1'N nitric acid. This final 14 eluate was analysed by flame atomic absorption spectrophotometry and was found to contain at least 5000 ppm lead and no detectable c0pper. 3. Electrodeposition Both the anion exchange separation and the DEDTC chromatography were time consuming separation procedures requiring a fair amount of operator skill and attention. Since an extremely shmple sample separation procedure that was also very quick and required little attention was desired, anodic deposition of lead as lead dioxide was investigated (13). Most metals deposit on the cathode in an electrolytic cell mder the usual analytical conditions except for lead, which can be quantitatively separated from nearly all other metals by electrodeposition as lead dioxide onto a platinum gauze anode in a short period of time (13). Because no platinum was available, two gold electrodes were used and excellent separations of lead dioxide from bronze solutions were achieved in less than fifteen minutes. A procedure for electrodepositing lead dioxide directly onto field desorption emitter wires for the purpose of doing electron impact/thermal desorption (ET/D) mass spectrometry was also devised (63). The procedure for performing EI/D MS is given in a later section of this paper. The procedure for direct electrodeposition of lead dioxide onto FD emitter wires is as follows. The sample containing lead and other metals was dissolved in dilute nitric acid solution and made just basic with NaOH solid, forming a gelatinous precipitate of‘metal hydroxides. This mixture is then neutralized with dilute nitric acid 15 solution until clear. A drop of this solution was then suspended from two copper wires. An FD emitter wire (tungsten with no carbon dendrites) spot welded to emitter posts, is placed in the drop of solution but not touching the c0pper wires. The copper wires are connected to the negative terminal of a 0-15 V variable voltage power supply, and the emitter is connected to the positive terminal. The voltage is increased slowly until a black coating appears on the emitter. The emitter is removed from the solution without turning off the voltage and allowed to dry in an oven for one hour at 150 degrees Celsius. This procedure causes an even coating of lead dioxide to be applied to the emitter wire with no lead dioxide sticking to the emitter posts. The electrodeposition method is clearly the method of choice for separating lead from other metals due to its extreme simplicity, speed, and separating power. It will be seen later in this thesis, however, that for some lead samples no separation at all was needed for lead isotope analysis due to the mass spectrometric analytical procedure. B. Mass Spectrometry Instrumentation 1. Varian MAT CH-5DF Mass Spectrometer The instrument used for this project was a Varian MAT CH—SDF, a reversed Nier-Johnson geometry double focusing mass spectrometer. The CH-5DF is equipped with a combination ion source for several different I6 modes of ionization, including electron hmpact (EI), field desorption (FD) and field ionization (F1). The CH-SDF is equipped with numerous sample inlets, including a GC inlet, a batch inlet for volatile liquids, a direct probe for solids and a field desorption probe that had been modified to accept EI/D samples (63). During the course of this project, the ion source was also modified to perform fast atom bombardment mass spectrometry (FAB-MS) by replacing the gas chromatograph inlet with Ion-Tech Model B11NF Fast Atom gun from Tedington Ltd. with a B-50 current regulated power supply. The GC was a Varian Aerograph Abdel Series 2100 packed column GC with a two-stage watson-Biemann effusion separator. Two holes have been drilled in the outer casing of the probe and threaded so that screws can be inserted, and used as probe stops. The first stop is used for FAB samples and isotope ratio analysis and the second stop is used for EI/D analysis. In the EI/D position a tab on the ion source makes contact with the probe and allows current from an emitter current programmer (63) to pass through the emitter wire causing it to become hot due to resistive heating. The CH-5DF can be used in a magnetic scanning mode for obtaining low' mass resolution mass spectra. Because it is a doUble focusing mass Spectrometer with both magnetic and electrostatic sector fields, it can also provide high resolution mass measurements via a peak matching procedure, in which the ion beam to be measured is viewed on an oscilloscope and alternately compared to a reference ion of known mass to charge ratio by means of accelerating voltage switching. The 17 reference ion is typically from perf'luorokerosene in the El mode, or from a mixture of KCl and glycerol in FAB mode. The ratio of accelerating voltage for the reference ion beam divided by the accelerating voltage for the unknown ion beam is inversely proportional to the ratio of the two masses. Therefore, knowing the exact mass of the reference compound ion, one can calculate the mass of the unknown compound to six significant figures. The maximum resolution of which the CH-SDF is capable according to the manufacturer is 10,000 by the 5% valley definition (6A). Newer models such as the Varian MAT 731 have significantly greater resolving capabilities due to an increase in accelerating voltage from 3 kv to 8-10 kV and improved ion-optics. The vacuum system of the CH-5DF consisted of three oil diffusion pumps and three direct drive mechanical pumps. The analyser and ion source are differentially pumped with the ion source diffusion pump having its own mechanical rough pump, and the two analyser diffusion pumps sharing another rolgh pump. All of the sample inlets previously mentioned and also the specially designed glass inert gas inlet for the FAB gun are connected to the third mechanical pump. A cryogenic cooling unit cools a baffle above each of the three diffusion pumps which are filled with 95% ethanol. When the vacuun system is functioning properly the pressure in the analyser is below 1 microtorr as measured by a Penning gauge in the electrostatic analyser. The operating pressure in the ion source is 1 microtorr under E1 or EI/D conditions and 10 microtorr mder FAB conditions as measured by a 18 Penning gauge next to the ion source. During Operation of the CH-SDF under low mass resolution conditions, the magnetic field is monitored by a Hall probe whose current signal is converted to a voltage by a Hall amplifier. During the course of this project the Hall amplifier was modified by addition of a AD533KH squaring unit which allows a digital panel meter on the front of the instrument to display the nominal ion mass which the magnet is currently set to transmit to the detector. The Hall amplifier also sends an unsquared voltage to a digital computer interface where it is subsequently squared and digitized so that the computer can determine the mass axis of each mass spectrum during magnetic scanning. The computer system will be described in greater detail later in this paper. The detector system currently in use is a discrete 1A-dynode electron multiplier model C31019B made by RCA. It can at present be used only fOr positive ions, but a conversion dynode is being designed which will allow it to detect negative ions as well. The output of the electron multiplier is amplified by standard CH-SDF components and sent both to an oscillOSCOpe display and to the same digital computer interface as the Hall voltage. 2. DEC PDP-Re Data System The~data system for the CH-5DF consists of two Digital Equipment Corporation (DEC) PDP-8e computers, a Plessey'model PM-DD/RC disk drive, 19 a paper tape reader, a DEC-tape reader, and a Tektronics A010 graphics terminal. The interface to the mass spectrometer includes a 12—bit Analogic analog to digital converter and a 1A-bit Analogic digital to analog converter with associated electronics. This system is also connected with a time-shared DEC PDP-11/AA computer which is used fOr off-line data processing and data archiving on 9-track magnetic tapes. The dual PDP—8e computers act separately. One computer, called the front end, takes data and gives instructions to the mass spectrometer, while the other, called the host, stores data on the magnetic disk, communicates with the operator, and displays data in real time. The system contains a number of programs written in assembly language that are flexible, interactive, and extremely fast. The major programs are MSSIN, MSSOUT, MSSTST and MSSAVA. MSSIN collects data when the CH-SDF is in low resolution scanning mode. MSSAVA collects data in selected ion monitoring mode, but is capable of other modes of data collection and will be discussed at greater length later. MSSOUT displays the mass Spectral data as it is being collected, or after the run has been terminated; it can generate mass spectral bargraphs, mass chromatograms, and ion current profiles of various types. A. more sophisticated version of MSSOUT written in FORTRAN with more flexible and powerfUl data handling capabilities is available on the PDP-11/AA computer. MSSTST is a program designed to test all of the various parameters of the computer-mass spectrometer system that are under direct computer control or observation. It is a very userl program fior determining malfUnctions when a part of this complicated system is not 2O functioning prOperly. 3. Control Amplifier for Accelerating Voltage The main function of MSSAVA is to control the accelerating voltage and the electrostatic sector voltage of the CH-5DF in tandem so that several ions of interest can be fOCused on the detector sequentially in rapid succession so as to achieve nearly simultaneous multiple ion monitoring. This capability is very useful when performing trace component analyses, isotOpe dilution studies, or isotope ratio measurements. MSSAVA accomplishes control of the CH-SDF's accelerating and electrostatic sector voltages by means of a digital to analog converter and an analog control amplifier designed and built during the course of this project. The circuit diagram of the control amplifier is given in Figure 2. The major component in the control amplifier circuit is an Analog Devices 277K isolation amplifier which is a two-stage operational amplifier with the stages separated by an isolation transformer. The purpose of the isolation amplifier is to provide electrical isolation between the +/-15 volt computer driven analog circuit and the +/-250V circuit that controls the electrostatic sector voltages. Early in the project, a photodiode - photoconductor couple was used to provide electrical isolation but this circuit was not able to respond quickly enOIgh to control the accelerating voltage without causing signal oscillation. The isolation amplifier on the other hand provides rapid, 21 Hmre2 Accelerating Voltage Control Amplifier Circuit Design ELECTRosrmC secron a: Tmc REFERENCE TA C I "05 .. VOL 65 SECTOR VOLTAGE NT 3 "a V CO ROLLER \ARIAN CH'SU 3.33.0. >——-Jv\~——-——— 00 RR ‘ . mg)“ POTENTIOMETER m - 45v Kati! V-‘5:XCV--‘ uuvcn 50F new I \ ECC I28 TRAIvsrerR TL 082 DUAL OP. AMP. DATA SYSTEM .1 330 kam) ANALm DEVICES 277K ISOLAT 1m AMPLIFIER I -Isvour 2 FEEDBACK 5 nsvour 9 SHIELD ID v.0, II COMMON I2 FEEDBACK I4 45v n l5 nsvm I6 SHIELD 239V FLUK HIGH VOLTAGE POWER SJPPLY 22 stable, and linear control over the accelerating voltage. The first stage of the isolation amplifier is a high input flmpedance, high gain operational amplifier that is configured as an inverting amplifier in Figure 2 and Operates between +/- 15 volts. The second stage is floated at +239 volts by a stable high voltage Fluke model AO7DR power supply. A Polytron model P31 +/-15 volt power supply is also floated at +239 volts and provides the +/-15 volt power for all the operational amplifiers in the circuit via special circuitry in the isolation amplifier. There is a potentiometer on the front of the peak matching box of the Varian CH-SDF called the "Find" potentiometer which is used to vary the electrostatic sector voltage manually. The accelerating voltage has a tracking power supply which responds automatically to any changes in the electrostatic sector voltage, so it is not necessary to control the accelerating voltage directly. The isolation amplifier output goes to the base of an ECG 128 transistor which is the actual control element in Figure 2. The emitter and collector terminals of the transistor are connected in parallel with the "Find" potentiometer. As the base voltage is varied the emitter to collector resistance changes, causing the voltage across the "Find" potentiometer to change as if one had manually moved the knob. This changes the electrostatic sector voltage and the accelerating voltage, causing the mass spectrometer to foCus on ions of different mass to charge ratios. A convenient reference point in the electrostatic sector circuit was chosen to provide feedback for the isolation amplifier to insure maximum linearity of response. A plot of accelerating voltage versus computer control voltage is given in Figure 3. One can see that the relationship is 23 Figure 3 Control Ampl ifier Linearity 2790 “II- 2750 +~ ¢ I 2710‘“ q- 2670 " 4. Accelerating .0. 4. Voltage + 2630 - I .g. DAC Voltage 2A- highly linear and that the accelerating voltage is variable over approthately 213V, which corresponds to about 10% of the mass range covered by the magnet. Circuit connections in Figure 2 were made with a wire wrapping tool and high voltage connections to the circuit were made by means of an amphenol connector and 3 ENG connector. The circuit board was covered in plastic and placed in grounded aluminum box. The circuit is engaged by connecting the high voltage cables and flipping a toggle switch mounted on the front of the box. It was necessary to connect the grounds fer the computer system, the control amplifier, and the Varian CH-SDF mass spectrometer together with a half inch diameter cable to reduce 60 Hz noise. It was also determined that a heater in the LKB—9000 mass spectrometer was causing common mode noise that was easily measured during isotope ratio work on the Varian CH-SDF. A switch was installed on the front of the LKB-9000 to disconnect the LKB-9000 from the Varian CH-SDF's grounding system during isotope ratio determinations. Also, during isotope ratio determinations, the signal from the electron multiplier was filtered with a 10 Hz low—pass filter in order to decrease the high frequency noise from the multiplier. Figure A shows an overview of the complete computer-mass spectrometer system. At present two mass spectrometers, (the Varian MAT CH-SDF and the LKB 9000) share the same data system. A single switch allows either mass spectrometer to be used but not both simultaneously. In the near future both mass spectrometers will have their own data systems, in order to alleviate the grounding problems and to enable both mass spectrometers to be used simultaneously by different 25 Figure A Overview of Mass Spectrometer and Data System CONTROLLER F OR ACCELERAHNG AND E LECTRO STATIC MAGNETIC VOLTAGES SE CTOR ELECTROSTATIC SECTOR ELECTRON ACCELERATING MULTIPLIER VOLTAGE . AND ION OPTICS\ — COMBINED ARGO” I- /F%Bl'gl{l' | AMPLIFIER FAB GUN SAMPLE PROBE CONTROL DATA ADC AMPLIFIER DAC SYSTEM 26 operators. A. Accelerating Voltage Alternater Programs Another data system has been used to collect some of the results in this paper. It is called the Riber System 150. The major differences are that the System 150 uses only one PEP-8e and uses the Diabolo not the Plessey' magnetic disk. Also the program used for selected ion monitoring is called AVA not MSSAVA. AVA is a different version of the program MSSAVA and is in some respects superior to MSSAVA from the standpoint of'measuring isotope ratios, due to greater capabilities fer speed, signal averaging, and data reduction. AVA'S effects on the CH-SDF's accelerating voltage are, however, identical to those of MSSAVA. Both programs have a feature called "tracking" which allows the computer to determine the centroid of the isotope peak to be measured and collect data at that point. This feature is used to compensate for magnet drift. During the relatively short isotope ratio runs the tracking feature was often not employed because magnet drift was inconsequential over the course of a fewIminutes, and because without the tracking procedure more data points could be taken per unit time interval. The algorithm for AVA or MSSAVA data collection is shown in flowchart form in Figure 5. At any time this algorithm may be interrupted by a CONTROL C in the AVA program, or a KR or CR command in the MSSAVA program. The overall result is that the operator manually fecuses the mass 27 Figure 5 Flowchart for AVA Data Collection Operator enters: 1) Ions to collect 2) Adjustable Parameters. Cperator focuses the magnet manually. Operator starts automated data collection. Computer switches accelerating voltage to find first ion. CDmputer switches Adjustable delay 1 accelerating voltage¢——o-to let voltages to find next ion. settle. I Computer completes Yes autofbcus routine Yes including delay 2 Computer takes data points, averages them , and stores the average. another ion in the data 28 spectrometer, sets various parameters such as the delays, the amount of averaging during data collection and tracking and tells the computer which ions to collect. The computer then automatically collects data fer each ion in the data set sequentially, then repeats the cycle until the Operator stops the data collection with a keyboard command. After data collection the Operator can find ratios between the ion intensities using MSSOUT or other overlays in AVA. For a more complete description of AVA see a previously published paper by Holland_gt.§l (A2). 5. Magnetic Scanning Using AVA There is another method of collecting isotOpe data using AVA. One can disable tracking and averaging, set the delays to their smallest values, disconnect the accelerating voltage control amplifier from the mass spectrometer, and scan the magnet repetitively at a moderate rate (scan rate A) over a short range, while collecting only a single ion current with the AVA program. When the delays are set to their smallest values, AVA collects data very rapidly, and it is possible to acquire a thousand data points across a peak in this mode. The result looks like an analog tracing of the mass spectrum that an oscillograph might produce. After taking data the Operator can integrate the resulting peak areas using other AVA or MSSOUT routines and ratio them to determine isotope ratios. The second method is admittedly less elegant than the first but has the advantage of greater sflmplicity. Both methods were used for determining isotOpe ratios of lead in order to compare the precisions of'magnetic scanning versus accelerating voltage 29 switching . C. Electron Impact/Thermal Desorption Mass Spectrometry of Lead The procedure for preparing lead oxide coated tungsten FD emitter filaments is given in part A of the experimental section. Lead oxide coated emitters of this sort were placed on the end of a field desorption probe and moved thrOIgh the vaccuum lock to a position marked on the outside of the probe which corresponds to a position A mm from the electron beam (63). The electron beam was then turned on, and the emitter current programmer (ECP) was also turned on. If the test circuit in the ECP indicated that there was electrical connection between its input and output; this meant that the emitter was touching the probe tabs in the ion source and that the tungsten wire was unbroken. The ECP was then switched to programming mode, and the current was increased thrOLgh the circuit at a slow rate. The ECP was turned off either when the data indicated that the lead oxide was entirely desorbed or when the tungsten wire broke due to thermal expansion, which caused the current programming to stop automatically. Data for the EI/D mass spectrometry of lead are given in the Results Section of this paper. 30 D. Fast Atom Bombardment Mass Spectrometry 1. Physical Arrangement of the FAB Ion Source The FAB gun is located at an angle of 90 degrees to the path of the secondary ions that are collected. Several tips of various metals including stainless steel, aluminum, and copper were made to hold the samples which were to be bombarded by fast atoms. These tips were fashioned so that they would fit into the emitter posts of the FD probe and would present a grazing target to the incident fast atoms. The angle of incidence of the FAB beam onto the probe tip was approthately 30 degrees. 2. FAB Conditions Argon gas was used initially as the FAB gas, but Efiemann's article (65) made it apparent that xenon was a better choice. xenon was in fact observed to increase the secondary ion yield in the varian CH;5DF but no attempt has been made in this laboratory to quantify the increase in sensitivity due to switching to xenon. Results fOr the fast atom bombardment of lead samples are given in the next section. If the conditions are not given fbr the mass spectrum or the isotope ratio determination, then the conditions used were the standard ones which were used in this laboratory fOr nearly all FAB samples both organic and inorganic. Standard FAB conditions are given in Table 2. 31 Table 2 Standard FAB-MS Operating Conditions FAB Gun VOltage FAB Gun Current Electron Multiplier VOltage Mass Resolution Accelerating Voltage Xenon Pressure 8.0 kV 1.0 mA 2.0 - 2.3 kV 500 (5% Valley) 2817 V 10 microtorr 32 3. Lead Compounds by FAB The following lead compounds were dissolved in glycerol or in glycerol saturated with KCl: lead(II)nitrate, lead(II)acetate, lead dioxide, and lead(II)diethydithiocarbamate. The first three were easily brought into solution by vigorous stirring with a glass rod over mild heat on a hot plate. In order to dissolve the lead oxide, 30% hydrogen peroxide was added drOpwise to the mixture of lead oxide and glycerol with stirring until the lead oxide dissolved. One microliter of these solutions were applied to the surface of a FAB sample probe tip with a calibrated disposable micropipette, and inserted into the FAB 'beam flor analysis. The FAB Imass spectra of these samples are given in the results section of this paper. A. Production of Lead Class Other experimenters in this laboratory determined that certain glasses containing potassium when heated or introduced into a FAB'beam emitted large quantities of potassium ions. Lead glasses of shmilar composition were fabricated by this experimenter and examined under FAB conditions. A lead glass was produced by weighing on an analytical balance, 0.1 millimoles of lead nitrate, 0.1 millimoles of alumina and 0.2 millimoles of silica. The ratio of 1:1:2 was taken from an early paper on thermionic emitters (66). This mixture was ground to a fine white powder in a mortar and pestal which had been cleaned with concentrated nitric acid and rinsed with distilled water and dried with 33 acetone. A small portion of the powder was placed on a piece of platinum and carefully heated from the platinum side with an oxygen-acetylene torch until the glass was red hot. This fOrms a rough uneven glass. Another portion of the powder was heated until white hot fOrming a smooth even glass. The smooth glasses produced no FAB Imass spectra. The FAB Imass spectra of the rough glasses are given in the results section of this paper. Another lead glass of the same composition was created but NBS standard 981 "common" lead (1A) was used instead of reagent grade lead nitrate. The NBS lead was dissolved in concentrated analytical grade nitric acid (Rallinkrodt), and then heated until dryness on a watch glass on a hot plate to fOrm the lead nitrate. This glass was used both fbr FAB low resolution mass spectra and flDr determining FAB-induced lead isotope ratio measurements. 5. FAB of Lead Glass and Lead Glazes A number of glazed pottery sherds were donated by Kevin Smith from the AnthrOpology department at the University of Michigan fOr FAB analysis and possible determination of lead isotope ratios. Some of these sherds were considered to contain lead as a major ingredient in the glaze, others were not supposed to contain lead, but no elemental analysis of these glazes had been done previously. These pottery sherds are from Whitemarsh TOwnship, Pennsylvania, and date from the late 1800's. The samples were taken from the remains of a country estate called the Highlands that was built by Anthony Morris. 34 The pottery glazes could be chipped off easily in most cases by using a razor blade. A small chip of glaze (on average 1 mg) was removed from each pottery sample and mounted on a FAB probe tip in order to record FAB mass spectra, and to determine lead isotope ratios if sufficient lead was found in the samples. Initially glycerol was used to adhere the samples to the probe surface, but this proved to be hazardous as some samples fell off into the ion source requiring the source to be removed and cleaned. TWO-Sided cellophane tape proved to be a reliable and simple method for adhering both the lead glasses and the lead glaze chips to the FAB probe surface. The surface of the lead glazes were cleaned with acetone prior to mass spectrometry to remove any residual dirt and fingerprints. 6. Isotope Ratios by FAB and EI FAB mass spectra of the glazes, isotope ratios of the lead ions in those samples and exact mass data mder FAB conditions concerning several ions in the samples are given in the results section. Isotope ratio data were also obtained under EI conditions for xenon and Pb(DED’1‘C)2 . Xenon was simply allowed to enter through the FAB gun with the voltage turned off, and the EI filament was turned on when the source pressure reached 5 microtorr. Pb(DEDTC)2 , which had been synthesized earlier in this work, was investigated by both high resolution exact mass determination of its molecular ion, Pb(DEDI'C);, and the ion corresponding to Pb(DEDI‘C)+, and also by accelerating voltage switching isotope ratio determination of the isotope peaks of the 35 molecular ion. In both cases a few crystals of Pb(DEDTC%3were placed in the direct probe inlet of the CH-5DF and sublimed into the ionization chamber with the electron beam on. This was a very dirty procedure and the ion source had to be cleaned after several of these runs. Other researchers have noted that DEDTC complexes often engage in reactions with the ion source (28). 7. ICP Analysis of Lead Glass The same chips of ceramic glazes that had been used fer mass spectrometric analysis were weighed on an analytical balance and sent to Dr. Braselton's laboratory in the Life Sciences Building at Michigan State University fOr quantitative elemental analysis by inductively coupled plasma (ICP) emission spectroscopy. The samples were wet ashed in 1 ml of concentrated nitric acid and diluted with 5 ml of distilled water. Wet ashing consists of enclosing the sample with the nitric acid in a Teflon container and heating it to 80 degrees Celsius for 2A hours. This procedure dissolves nearly all materials except the silicates. Since there are considerable quantities of silicates present in the glazes, a residue remains in the vials, and the analysis is therefore termed a "total extractable metalS" analysis. The ICP used was a Jarrel-Ash Model 955 Plasma Atomcomp. Results of this study appear in the results section of the thesis. RESULTS AND DISCUSSION A. Mass Spectra of Lead COmpounds 1. Lead by EI/D This chapter will be divided into three main sections. The first section is devoted to the mass spectra of lead and its compounds that have been obtained during this project by a variety ionization techniques. The second section is devoted to the data obtained from the pottery glazes and contains both mass spectra and some quantitative elemental analyses. The third section is devoted to lead isotope ratio results as obtained from the Varian MAT CH-5DF mass spectrometer. Figure 6 is the mass spectrum of lead dioxide coated on a FD emitter and ionized by electron hmpact/thermal desorption. The major ions in the spectrum belong to lead; the other ions are presumably decomposition products of trace organic compounds on the FD emitters. No peaks were observed which would correspond to the intact lead dioxide molecular ion. Figure 7 is a total ion trace of the lead EI/D run which produced the mass spectrum in Figure 6. NOtice that the lead desorbs over the course of only four Scans. This is an undesirable characteristic of this method fbr obtaining isotope ratios since a stable long-lived signal is needed for precise determination of signal intensities. 36 37 Figure 6 EI/D Mass Spectrum of PbO2 82 99 LL. .. IJ rvv‘lva—v vv—Y wv—v vvv VYV vvv vvvv vrvw wwvv fl I ""l 188105110115121313513331351431451513 \I— 01 0"..— CI (0.: ('1 0.. O 0 L" < 2 I3 :3 .rr-.--1m.l .'"l. . J‘.‘ _-I.' jam ’"ITYIYII. r:l.77l.-fi';l'1'li"':L- '2’ 18812519» 145 221»: EDS :18 215 2w .525 am 2.55 am: :45 .:.'-':»U 38 Figure 7 EI/D Ion Current Profile of P602 180".= 28366. \ 293 --._ _ 39 2. Pb(DEDTC) By ET 2 In order to obtain a longer lasting signal the volatile lead complex, Pb(DEDTC£ was synthesized. Figure 8 shows the EI mass spectrum of Pb(DED1‘C)2 as introduced by direct probe. This Spectrum is extremely similar to the mass spectra obtained by GC-MS on the Varian CH-5DF, and on the HP-5985 quadrupole GC-MS. Table 3 lists the major EI mass spectral ions of Pb(DEDTC)2 in Figure 8 and the chemical formulas of those ions. Several of the formulas were confirmed by high resolution mass spectrometry on the Varian CH-SDF, including lead itself, lead plus one DEDTC ligand and lead plus two DEDTC ligands. From the data in the chemical literature it is possible to make several generalizations about the DEDTC complexes. First, the molecular ions are observed in all cases. Second, the high mass regions show rather simple fragmentation patterns, which are characterized by loss of $2CNR2, S, S or 2’ SCNRagroups (33). Third, several elements including Fe (33) and Ni (28) are evidently stripped from mass spectrometers sources by DEDTC ligands. The structure of metal dithiocarbamates are said to involve four membered quazi-antiaromatic rings chelated through sulfur bonds to the metal atom (3A). This is possible because sulfur is able to engage in sigma-pi bonding to metals. Dias _e_t _a_l (3A) prOpose, furthermore, that lead complexed to single dithiocarbamate ligand should be a very stable structure, due to an inert gas configuration around the lead, and back-bonding of the filled 5d orbital on lead to the empty 3d orbital on sulfur. Data in Figure 8 tend to support this claim, in that the 40 Figure 8 El Mass Spectrum of Pb(DEDTC)2 116 _ x 2 ‘ 83 J 149 2‘33 iJAILL V ‘1 V v j'A; fj iv 1]! A 'Yul'i j fif' "'l_ I_ [ I_ I” I I. ,1 F” l'_' _ 60 BO 18H 1'20 :40 16v to: 200 2:53 240 2c»: 28w 3.)»: d BSD «I - 504 q . "I I " _ T"'T*T"*7:T*TI_'_*" '_"'IT_'T'I.' "1' I_ 'I_ 309 320 34v 360 '80 4I .1 4EU 44v 46v 4bL‘ SUB SZI S-tI 41 Table 3 Major Ions in the ET Mass Spectrum of Pb(DEDTC)22 M/Z Formula * + 60 SCNH 2 * + 72 N-(C H ) 2 5 * + 88 HSCN-(C H ) 2 5 * + 116 SCN—(C H ) 2 5 2 * + 1A9 HS CN—(C H ) 2 2 5 2 9 + 208 PD 9 + 356 PbS CN-(C H ) 2 2 5 2 Q + 50A Pb(S CN—(C H ) ) 2 2 5 2 2 & See Reference (33) 6 Confirmed by exact mass determination A2 Pb(DEDTC)+ ion has a much greater abundance than the Pb(DEDI‘C)2+molecular ion, whereas most dithiocarbamates have more intense molecular ions (33). 3. Isotope Ratios Using Pb(DEDI1'C)2 It has also been reported (29) that Pb(DEDl‘C)2 exhibits double or triple maxima when sublimed from a direct insertion probe at temperatures less than 300 degrees Celsius. These sublimation characteristics have also been observed in the Varian CH-5DF and may be due to geometric isomers in the DEDTC ligands or to excess chelating agent still present as an impurity in the PMDED'I‘C)2 crystals (29). In any case, it has been determined in this research that the uneven sublimation profile of Pb(DEDI'C)2 makes performing precise isotope ratio determination extremely difficult even though the ion signal is longer lasting than that Obtained by either EI/D or GC-MS of Pb. FUrthermore, even thoLgh the ion signal is intense, severe isotope fractionation effects were observed using MSSAVA (see Figure 9), and due to the DEDTC interaction with the ion source, the source needed to be cleaned after only a few introductions of Pb(DEDTC)2. These practical considerations do not limit the general use of dithiocarbamate complexes for quantitative analysis of metals using CC or GC-MS but make precise isotope ratio determinations very difficult. 43 Figure 9 Isotope Fractionation in Pb(DEDTC)2 1.000 0.960 0.920 0.880 * Isotope Ratio 0.840 0.800 0.780 I l _h .L l l l I I 0 10 20 30 40 50 Time (seconds) * + This isotope ratio is the 1503/1502 for Pb(DEDi‘0)2 q. and corresponds to the I /1206 for Pb. 207 44 A. KCl/Glycerol by FAB While the work on Pb(DEDTC%3waS in progress, the Mass Spectrometry Facility bought a FAB ionization gun from Ion Tech. The FAB gun was installed on the Varian CH-5DF, and a calibration mixture was prepared from glycerol saturated with KCl. Figure 10 is a mass spectrum of the KCl/glycerol solution. This solution gives clusters of peaks at regular intervals from m/z 19 to at least m/z 600, and as such was ideally suited as a calibration standard fOr the mass axis and has also been used as a reference standard for determining exact masses of compounds of unknown molecular formula. The clusters are fOrmed from potassium attachments to glycerol according to the following formulas: K (glycerolF' and K (glycerol - water)+ n m n m where n = 1 to 3 and m = 1 to 7. One also observes the potassium atomic ion. 5. Lead Acetate/Glycerol by FAB It seemed reasonable to assume that lead might ferm similar attachments to glycerol under FAB conditions if it was added to the glycerol as a solUble salt, such as lead acetate or lead nitrate. When a saturated solution of lead acetate in glycerol was introduced into the FAB beam the mass spectrum in Figure 11 was the result. The mass spectrum in Figure 11 was actually obtained at UpJohn Corporation in Kalamazoo on a Varian CH-SDF equipped with the same FAB gun used in the 45 Figure 10 FAB Mass Spectrum of KCl/Glycerol _: 39 113 131 '2 1b? can: " s]? 75 93 L I 1R? 1 -L Ll . m I .l i A II I T T I I 'T—I I '_I I I l’ ‘30 40 '50 E: 70 30 913 1H3 110 1.10 1’0 140 193 160 17121 1:0 1:0 mm W 223 243 .263 279 29? 31? X 25 1,7,. 355 373 '1 ll [I‘ll'ld II I! 11:]! [VIII‘ l LAJ;J Lil.h!l[lil:lT_1 ll I‘LL. LI'A IIIIJJ 'Ié‘ .lJrulJ 1.11 ”1.11.39 .LI'IIIIII A'lALAIU; 210 320 23:0 ‘40 250 268 am 2333 2‘90 380 31le 320 32-4. 34:3 BEN-II 2:5»: ETD 350 32?.3 J .2393 d 409 - o ‘- .- ‘ I ll 1"" EE‘ 1"” “’5 5:3 52‘ 51‘ 55: "‘ 11. J 11.. IIII I '4 Al I]. ”11.1 1111 1'11 ill 11 ELIIL I I I I I I I I I I I I I I T l I 390 400 410 420 430 4-461 450 Ari-III 47‘») 430 4'3”) 500 510 5.33 533 54:1 SE} 560 57.1 S d 'S ‘ a”? 97 Is «a c. d 1111‘ LP![. Lilli tell-1:. EVIL 411 A .11. LA +4 _. I. I_ - _T. ’T ‘I- I ’I - 'I _ _I.- __ _T_ _T__ :12- I - -1. -1 -I . T—‘l' _ 5:0 586 8:4»: 05.10 e10 e20 6.36 am BEL-I emu are use as»: IL: 711 (:0 .30 .w 750 46 Figure 11 FAB Pass Spectrum of Lead Acetate/Glycerol (UpJohn) W 0. r_.x I q 8.. _. ‘0. 7. W .' J 1141 lJ. 14 J. u x I A - v v V v v w v—w V v V f v W f 1 'fi v ffi V v .9 99 0.0 '00 .80 30° .00 ‘0. GOO .96 U I 1 A no. ... I -4 7" ,.. IO. '1 v W vvvvvv v w—v 1 1 Vi v .90 .00 .00 .00 700 VI: see 0.0 no. .00 coo U I '3'. 30. L Hm 4'- vvvvvv we- ,--, ‘099 10.0 '100 lilo loo I800 '300 18.0 47 MSU Mass Spectrometry Facility. Figure 11 shows intense lead + glycerol clusters to m/z 1306 which is near the mass limit of the instrument. The most important feature of the spectrum is, however, that the Pb+ atomic ions are present. Because FAB produces very stable and extremely long-lived (up to an hour) ion signals, FAB of lead in glycerol was at the time considered to be a good candidate for conducting measurements of isotope ratios. Figure 12 is a mass spectrum of the same sample of glycerol saturated with lead acetate, but this spectrum was taken on the Varian CH-5DF in the Mass Spectrometry Facility and the emphasis is on the less intense ions below mass 300. Table A is a list of the major ions in the mass spectrum of saturated lead acetate in glycerol from Figures 11 and 12. In Figure 12 one can see two things that are not observable in Figure 11. First, there is a peak from the glycerol matrix at nearly every mass, some of which clearly are isobaric with lead under low resolution, therefOre interfering with the isotope ratios. This of course meant that a method had to be found for eliminating or at least drastically reducing this chemical background. Second, in addition to the glycerol and lead plus glycerol adducts, there is a cluster at m/z 225 which corresponds to PbOHI ‘48 figwe12 FAB Mass Spectrum of Lead Acetate/Glycerol (MSU) - 93 ‘ 61 7s ‘31 1'65 d - - ' 207: d —I u.l|l.| l. ....l.ll l. .4 .4 .I II . ... L . . _x x A .I I] L x . . ..- .1. A . .nllug I I I I fi_I_ I - I - L I . I 1.- I—_ L- T._ .-' ".I ‘7’- SO 68 P6 88 90 U0 11v 126 1:0 14v ISO tee 1.U 1au 19a :00 :10 Bio + 299 d a 223 -I -I 251 2C? l A III 11L .2 " I 1 All . .I .1, I I TI .l_ _ . _ _l_’ '_I_‘ ‘I. I__ PI’_ ‘I_ -T‘ AI__ Pu- _C'. T-_ 226 23a 248 253 sou _Tu _-e ass sue -1» 320 ssu 34v 550 see S.v saw Bau _ 5?? q .4 a, 4’4 .592 ‘ l H LL . u. “I. III 11111. at“. I] I A x n I I “u. vvvrlfifv—rvvrfitv.fV*LY-1+wlvv vlvvvrl.rvvaYVVWVTrvvr.v.VTYIv.vavvvav-vaAYTrvIv.vY~r v.vrv‘.- 39B 408 418 4;U 4su 440 450 Aeo 4r6 Aau 49v 58v 510 See SJJ S4u SSU Sou ‘49 Table A Major Ions in the FAB Mass Spectrum of Lead Acetate in Glycerol M/Z Fermula OH I + 61 OH-CH -CH 2 OH | + 75 OH-CH -CH-CH 2 2 OH + l + 93 (gly)+H : OH-CH -CH-CH-OH 2 2 2 + 185 (gly) +H 2 + 20A-208 Pb 4. 221-225 PbOH ‘ + 295-300 Pb(gly) 4. 501—507 Pb (gly) 2 4. 593-597 Pb (gly) 2 2 4. 797-80A Pb (gly) 3 2 + 1095-1100 Pb (gly) A 3 + 1302-1306 Pb (gly) 5 3 50 6. Pb(NO ) , Pb(DEDTC) and PbO by FAB 3 2 2 2 Figure 13 is the FAB mass spectrum of saturated lead nitrate in glycerol, and Figure 1A is the FAB mass spectrum of saturated Pb(DEDTC)2 in glycerol. The major item of interest in these two spectra is that they are nearly identical except the lead nitrate gives less intense Pb+and (Pb + glycerol)+ ions. Lead dioxide is not soluble in glycerol and gives a FAB spectrum that does not indicate the presence of lead. However, when 30% hydrogen peroxide is carefully added to the mixture of lead dioxide and glycerol, the lead dioxide dissolves. The hydrogen peroxide also reacts with the glycerol so that the matrix peaks are different, but if they are subtracted from the spectrum, the Pb+ ions are observed as is shown in Figure 15. None of these spectra of lead compounds in glycerol however, is suitable for isotope ratio analysis due to the background of the glycerol matrix. 7. Lead Acetate/Glycerol/KCl by FAB It was presumed that if glycerol was saturated with both KCl and lead acetate then both the potassium-glycerol adducts and the lead-glycerol adducts would appear in the FAB spectrum. Figure 16 shows that this is not the case. The base peak in the mass Spectrum in Figure 16 is for the m/z 208 ion of lead, and the potassium-glycerol background is almost entirely suppressed. It is possible that there is a 51 Figure 13 FAB Mass Spectrum of Lead Nitrate/Glycerol 93 131 r'x Is I 185! I 207’ L I A In A. A v I“ A JIL VJ fifi v.1 1I ' Y Y .L j ‘ ' rvfrlIicixj¥ujrl§hJiir TTT"I'”F" TTTT“ ' "TT’I" "l'_ ' l ‘ I jTFFT. I ' L_ L I__ -l. 90 1w. 110 128 136 14v 150 toe 170 lsu 1%) 3H) '.--.'1La 223 I 29s . ”C as: 2?? 3,9 3:- 1 ”111111 Illllxllllllll 111111114111I.1.11]IIIIIIIIAJIIIIIIMIIIITIJIITI . !I IIILJIIIrlI-J'Il triulrlniirlulllruiuli ... ...- v.9. -Afifiw‘erffiyw. .Y—r- ...v-... .fi.'.... ...V——fi.ww l I 2313 240 25E! 26E! 278 2813 296 300 3151 3'20 330 134? 52 Figure 1“ FAB Mass Spectrun of PMDEUTCE /Glycerol ‘ 288 1 .I ‘ 115 131 1 — 79 101 189 YTMYJI 1:]:li.jl LUJy 1541111111]; 11311715199; :lgnglJl rvlflf I'ngllenvlfx‘llnl.¢l¢1 :.1!r1;.j.ILIVLAV Ingli I ‘JiQ'v I I T, L- l. I_ I I” I I I_ I_, ,I__ 70 88 9B 180 “U 163.4 1J9 14v 159 16b 17“) 188 1‘38 20v 4133 an .I -‘ 299 q 333 2:1 -I u T"? 1 41.1 1.1 l .llLulluLul. 1".141111 ‘AIIILIIIIII 1 Annunl I ILLMHLLLIJHA A. L'nl 'J u x-l 1 "'fiT'vrvT'j'ijT‘l” "1""1'."'W'f'fif'n'ff'qn'j 'H' ‘f1.[.. ""1 226 230 245.3 258 29*: 279 28*.) 29B 308 310 320 338 340 35-0 368 3753 53 Figure 15 FAB Pass Spectrum of PbOe/Glycerol/HQO2 q j 95 114 44.4 1 L A I u 4 . J 1 1 1 1 4 Q. - 1’- I T_ T T _ F: *T. _ T. 78 8v 91 11.10 1m 1213 1;, 1 141.1 151.1 but 1.0 15»: 1‘“? v'n N n_l1l1 (A) C 1‘- (a) "' 11 . .1 1 14.1 I 11. 1 “J” 11 111. 1 11 . _ I_ r_ '_I. l_ ,1- _L- -l__ _1- _I._ E- _I -T. .I. 2U 2:0 240 251.1 24:0 2211 :21; 5:11.: sun 2:11.: 3:0 3w 2.74:3 .15». 3:. .I _ 413 d 4 456 '1 _ d A: A “A l.[!. A A A 1 A41 l V V Yj I V Y r?.7.' V V I’V-V V V I Y-' Y Y LY‘VY Y I. .fi 7 7T V- Y fl T-V Y ..V-fV V L - f'-V-W VT- - 378 338 52".: 41.11.! 411.1 441.1 4:1: 441: 451.1 4*.» 4.1.1 41:1; 43”.! 54 Figure 16 FAB Mass Spectrum of Lead Acetate/G1ycerol/KC1 d ‘2‘) E .1 g - fl 3 ‘ l ‘4' A A 1 1f 1 A ‘IYf‘V 17" IVVVI‘Y—v—rIvvvrlvyv—v’lrwv‘vlrrvvv fo Vfirr‘v fY—V’Y ‘rUY—VTT . I I . 70 88 9B 100 119 120 130 14B 188 169 170 130 1?3 289 210 22$ ‘ 251 q [All A Aug! lgLAlTAillly ''''' 111 A l vvvv YT‘IrVT 11v 218 220 230 248 256 266 278 230 298 300 3 0 0) no ‘0 (.1) w ‘ Q (a) > ‘— vI wT 55 competition between the potassium adducts and the lead adducts of glycerol in the FAB process and that the Pb adducts are ionized preferentially. It is also possible that there is a chemical reaction occurring in the glycerol under FAB conditions because the KCl/lead acetate/glycerol solution turns grey-black after a few’ minutes in the FAB bean. Althmgh the chemistry of this phenomenon remains unresolved, the reduction in chemical background was important from an analytical point of view. 8. Isotope Ratios Using Lead Acetate/Glycerol/KCl It was possible using FAB to determine the isotope ratios of a lead sample in the fbrm of the acetate salt by means of accelerating voltage switching as described in the experimental section of this paper. The results of these initial isotope ratio experiments were both enlightening and discouraging. Using magnetic scanning and MSSIN data collection, it had been typical fbr isotope ratios to vary by as much as 50% between repetitive scans. Under MSSAVA data collection by accelerating voltage switching it was possible to determine lead isotope ratios to within 10%. Although this was a major improvement over magnetic scanning, this was not precise enough fbr archaeological work by a full two orders of'magnitude. FUrthermore, the isotope ratios that were obtained simply were not reasonable for common lead. This lack of precision and accuracy was baffling at the time, but subsequent investigations have shown that there were three major sources of error in the initial isotope ratio experiments. First, there was indeed an 56 isorbaric interfering ion cluster that had not been selected against. Second, there were a wide range of instrumental parameters which were not adequately controlled. Third, the ionization process of lead in glycerol was not as stable as had been supposed. EVen though organic ions such as glycerol or polyethylene glycol give extremely smooth ion current profiles, the strange chemistry and ionization process of lead in the KCl/glycerol matrix caused it to desorb in spurts giving rise to a slightly jagged ion current profile that greatly limits the precision of lead isotope measurements. All three of these sources of error have since been significantly reduced, and each will be discussed in greater detail later in this thesis. B. FAB Mass Spectra of Lead Glasses and Glazes 1. Lead Class by FAB All of the FABImass spectra in this next section were obtained directly from glasses and glazes without a glycerol matrix. A general feature of these spectra is that ion intensities are somewhat lower than are seen from glycerol, but the lead glasses that were prepared often gave useful spectra for several weeks even when used daily. Another general feature of these spectra is that atomic ions dominate the Spectra althoigh occasionally molecular ions are seen as well. Figure 17 is the FABImass spectrum of a glass prepared from NBS isotopic standard 981 "common" lead which had been converted to lead 57 Figure 17 FAB Mass Spectrum of Lead Glass (Major Ions) 2? 39 m ID 0‘) AA .AAA - A fiv] vrv VYV Tvrv frrv V’Y‘YV vuv‘r wva—rv ‘1'] W—Vj T 'l l I l 69 80 100 120 146 169 130 280 229 243 #ul vvrl ‘Y—TY fi—Y‘lrvvvvIvxvvlvarlvvverT‘lIITW‘vv vv . 'I 289 380 320 348 366 330 480 426 440 466 58 nitrate and melted together with one part alumina and two parts silica. The major peaks in the spectrum belong to potassium at m/z 39 and M1, and lead at m/z 20“, 206, 207, and 208. The two other large peaks are at m/z uu and HS. These peaks in all likelihood correspond to the protonated molecular species A10H+and SiOH+. Under FAB conditions protonated molecular ions are the dominant mode of ionization in glycerol matrix and so one might expect protonated species are also produced in the glass. Figure 18 is the same mass spectrum as in Figure 16 but the intensity of the ions above m/z 200 has been multiplied by twenty times to reveal the less intense ions in the spectrum. Three of the lead isotope peaks are off scale, but the expanded scale indicates that there are two other peaks in the Pb+cluster at m/z 205 and 209. Under high resolution the 209 peak was peaktmatched to the known mass of Pb+isotope 208. The result agreed to within 5 ppm of the exact mass for Pb HT This striking result explained why the isotope ratios of lead measured in the glycerol/KCl matrix were inaccurate. The PbH+ion in glycerol is even larger than in the glass, and so because the isotopes of Pb+ and PbH+ at m/z 207 are unresolved at low resolution, ratios involving these isotopes are noticeably skewed. In retrospect one can look at the Spectrum in Figure 16 and observe the PbH+peaks at m/z 205 and 209. In principle, one could measure the ratio between the m/z 20“ peak of Pb+ and 205 peak of PbH+and correct the measured lead isotope ratios fbr presence of PbH+at m/z 207 and 208. In practice, it is very difficult to measure the intensity of the m/z 20M peak precisely, and frequently the m/z 205 peak cannot be measured at all. Clearly this isotopic interference limits the accuracy of the isotope measurements 59 Figure 1B FAB Pass Spectrum of Lead Glass (Minor Ions) . 2? ‘ 39 A l ' ‘ 1 V ' I Y Y V V I V V T V V V Y Y ' "V U i V V V I v ‘ I ' ' 1 T Tj 20 4B 63 80 160 128 140 168 180 280 228 240 _ 416 d 111 111 I]! J_ J ll 7 1 Y ' V V ' Yfi WT—ff VYT TVV T’YY. fi'vt *‘ I T I ' I 2413 2613 231:4 3913 328 340 360 v v v 1' 'fifT I " 1 380 408 420 440 460 60 that one can make with this method. Fortunately, in the glass the PbH+intensity is often very low and the most important ratio from an archaeological standpoint, the Pb*20u/206 ratio, is unaffected by the PbH*interference. The PbH+problem will be dealt with in greater detail later in the thesis. Returning to the mass spectrum in Figure 18, one can also see clusters of peaks at masses higher than lead. The clusters whose highest mass peaks are at m/z 225 and m/z "16 are PbOH+and qurespectively. Metal clusters of this kind have been reported previously in SIMS (35) but not in FAB-MS. Table 5 is a list of the ions in the mass spectrum from Figures 17 and 18 along with tentative identifications of the ions. lhfbrtunately it is nearly' impossible to identify most of these ions according to their exact mass, due to a lack of ions in the glass that could be used as peak matching standards except for potassium and lead. 2. Twp—Sided Tape by FAB Most of the ceramic samples were mounted on the FAB probe tip with two-sided cellOphane tape. When the sample was fairly large (3 mm x 6 mm) such as the lead glass in Figures 17 and 18, the tape was hidden entirely behind the sample, but many of the ceramic glaze chips were very small (approximately 1 mm in diameter) and the tape extended beyond the edge of the chip. In these cases the FAB beam produced ions from the tape. Figure 19 is a spectrum of two-sided tape. Fortunately, the 61 Table 5 Ions in the FAB Mass Spectrum of Lead Glass M/Z Fbrmula + 23 Na + 27 Al 4. 28,29 Si 4.. 39,u1 K 4. HO Ca + an AlOH + u5 SiOH 4. 20M,206,207,208 Pb 4. 205(207)(208)209 PbH + 221,223,22M,225 PbOH 4. 1112-1116 Pb 62 Figure 19 FAB Mass Spectrum of Twp-Sided Tape 141 113 l Al 1AA A L ”111' IIAA A ”11. AAJAlA A AIA A I ll AA 1 ' ‘ ' v ' ' V v V 1 v v v V Ll. A ALA - IL A AAA TV‘TfiIY rvvv IYvfi VVT v vvv‘v rfv Vvvr vrfifia vv—rv I I " ‘I I'fi*Ifi"I ' I 313 «I so 69 70 86 ea me 113 120 139 14c 150 use 1?»: 1:30 190 1 1+ 4 in g A vvvv rrvv YT—V'j 1T1 YfVV vvvv v1‘vv vvv Ivvv VTY’V rer u. I . 'l 1....i....[ee 190 260 210 239 239 248 253 260 2 B 286 299 300 318 328 vv A WYYY 7771] 33B 34B 350 63 intensity of ions from the tape diminishes quickly with time, whereas the signal from the ceramic glazes stays nearly constant or even grows slightly with time. Even so, it is possible to see some of the more persistent ions of the tape such as m/z 113 and 115 in some of the ceramic mass spectra. In no case, however, did ions from the tape interfere with any of the lead isotopes. 3. Lead-Glazed Redware by FAB Figure 20 is the mass spectrum of sample HL122HB, which is a piece of lead glazed redware. The color of the glaze is deep red. The Pb+and Pb0H+clusters are apparent at m/z 208 and m/z 225 respectively. At low mass, one can also see Na+, Mg", Al" , Si+ , and K+. A full list of peaks is given in Table 6. There is also possibly Ca+at m/z 110, and Fe+at m/z 56. There are a large number of very small peaks between m/z 50 and m/z 120. These may be ions from the two-sided tape, elemental species or compounds in the glaze. For this reason it is difficult to identify any peaks in this area for most of the ceramics, except where certain peaks are much larger than the rest. In general, the elements below potassium do not vary much in relative intensity among the glazes and are not shown in most of the following mass spectra. Figure 21 is the mass spectrum of sample HL12211A. Like Figure 20 it is a piece of lead glazed redware, and we observe the characteristic clusters for Pb+and PbOH*3t m/z 208 and 225. The other major peaks are m/z 39 and m/z 56, indicating K+and Fe+. 64 Figure 20 FAB Pass Spectrum of Lead—Glazed Redware (HL122uB) q 39 ‘ 23 X 6 d .4 56 . d V w v V Y 5 Y 197w ' ' ll'Ailf 1.11111 1A11£1LLAI¢1AIVAIj TAWA. 1'1 1 A V r A J r rj 1 r Y Y ' 1 1 1 f7 1 1 I 1 1 l ' T "T 10 20 30 48 58 68 7G 86 9B 188 116 120 136 A m 29% fl - V‘v‘v—f va A VYV‘I’VV‘VVIVVVYI'Y"IVVVYIVV' VYfiIIVIVj w I '1' ., 1' .. .1 130 146 150 160 1 8 18B 196 200 210 228 23B 240 250 65 Table 6 Major Ions in the FAB Mass Spectrum of Ceramic Glaze Sample HL122HB M/Z 23 2u,25 27 28,29 39,81 HO UN HS 56 20u,206,207,208 205(207)(208)209 221,223,223,225 Fbrmula + Na + M8 4. A1 4. Si Ca AlOH SiOH Fe PbH PbOH 66 Figure 21 FAB Mass Spectrum of Lead-Glazed Redware (HL12211A) 39 " 3 q q "‘ [61]" 8? L7 Vi 4 [111111 1 sllll!l“r1 I All! 1 lllllA WfiJAlAlvleltA A. ff'A AAI!J_fAf ATLYIY '1' v v A‘ 'I 316 4'0 60 7B 80 9B 1013 110 120 138 140 " 288 ' H 225 AAL . . . .1 . III "F‘ Ti" I""I""I'*7* i"'l“'jfi"'T' '1‘7 l 140 150 1213 188 1913 2‘36 210 2.10 2313 240 25-13 67 Samples HL122UA and HL122MB are similar in physical appearance and have similar mass spectra. u. Lead-Glazed Slipware by FAB Sample HL790 which is a piece of lead-glazed Slipware. This pottery has a red glaze on it with a decorative green glaze pattern called a slip. Figure 22 is the mass spectrum of the red section and Figure 23 is the mass spectrum of the green section. Surprisingly, both mass spectra contain lead. The red section mass spectrum has a prominent peaks at m/z 56, HO and 39, which are Fe+, Ca+and K’respectively. The green section of the glaze contains in addition to K+, Fb+and PbOH+a number of other peaks, the first of which are at m/z 63 and m/z 65. The mass and the approximate isotope ratio of this pair indicates Cu+. The other peaks in the spectrum are copper compounds or cOpper clusters such as Cu0H+and ng. Table 7 is a list of all of the species in Figure 23. It seems likely that copper present in the glaze is the cause of its green coloration. Since lead was fbund in both the red and green sections of the glaze, it apparently has little effect on the color of the glaze. The red coloration, therefbre, is probably due to the iron present in the glaze. 5. Pearlware, Creamware and Manganese-Glazed Redware by FAB we were also sent some artifacts that were considered to contain no lead. Among these were some samples of white pottery called pearlware, 68 Figure 22 FAB Mass Spectrum of Lead-Glazed Slipware (HL790(RED)) 39 o 4- :— fi—YV 1. 1 1.1 w G «’4 m : o.-_1 0 : 4 \‘I__: 0:. 4 to; 0]] J 0% (n VfiY‘V v. *3 11.5- 11011151202351.3413 Yr Y—v—v _w h) .31 Cu .- 111 vvvv v—vvv 1381351 vvvv v v 7W1 vvl 4B 145 151:1 155' 16-121 vvv VYTY‘I va|vvv 165 170175 13131351913 1952:1113 Eu- v—Yvw—~v—rv—v Y—‘r‘vvfiw v. 210215220 335 2;:U 69 Figure 23 FAB Mass Spectrum of Lead-Glazed Slipware (HL790(GREEN)) 63 80 _L vrv J I 129 d v ‘ Y 11VA$1 ¢ ngll 11 A A l A L +k rl£llrlv AA j ‘ !I!IA¥ 7 rr' vlfw—Yv 'Y—VvlljfiijIVT—Vfi 11v‘v 1 9B 108 110 120 130 140 (A) O b O m C) G\ O N O m 3' 208 .I 145 191 1H 1A1AALAA VAAAllYlvlr'lj‘Afilll 14B 150 1&0 170 180 1 ' ”Lu- Y V V J .111'1' 1.17 . . . . . A 1' . - . . :1 v I fifY V Y I V erI V V Major Ions in the FAB Mass Spectrun of Ceramic Glaze Sample HL790(GREEN) M/Z 39"11 HO NM 115 56 63,65 80,82 126,128,130 1N3,1U5,1u7 189.191.193.195 20u,206,2o7,208 Formul a + K 4. Ca AlOH SiOH Fe 71 and creanware, and some manganese-glazed redware. Figure 214 is the mass Spectrun of sample HL537A which is a piece of pearlware body sherd. Pearlware, indeed, contains no lead, or any other major ions above m/z 50. Figure 214 shows all of the typical elements lower in mass than potassiun but the Al+pea1< is very large in comparison to K+which is usually the base peak. Figure 25 is the mass spectrum of sample HL537B which is called creanware. The creanware spectrun has a relatively large peak at m/z 140 indicating Ca+, two peaks at m/z 63 and 65 indicating Cu+ and strangely it also contains lead as one of the major constituents. Figure 26 is the mass spectrun of‘ sample HL539 which is a piece of manganese-glazed redware. It has major peaks at m/z 52, 55 and 56 indicating Cr+, Mn+, and Fe+, but lead at m/z 208 is also present. It appears then that lead is a common element in these pottery glazes, even in the pottery that archaeologists generally suppose contains little lead. By the nineteenth-century, people began to realize that lead was poisonous (116) but substantial amounts of lead apparently were still present in their pottery according to the FAB-MS data. An extensive survey of lead-glazed and nonlead-glazed pottery from the period might have interesting historical implications if lead was in fact present in a wide variety of‘ pottery as recently as a century ago. At this point, FAB-MS of the pottery sherds is non-quantitative and considerable effort would have to be expended to 72 Figure 211 FAB Vass Spectrun of Pearlware Body Sherd (HL537A) . 39 ..1 1 A l 11 ' j I wfifinl' f" 4‘ 1] W1 Y'T ' fi ‘j ' 'Vfifiir 13 28 30 48 $3 60 70 86 90 100 110 120 130 IT .4 ‘ J 'rfir ""T""l"1'l'"YIVfY—'—I"j'1"'YIYfT'I'T' I "'T'fififr 13B 14B 150 160 170 183 193 200 210 220 230 240 250 73 Figure 25 FAB Vass Spectrun of Oreamware (HL537B) .. 39 -* X 6 .. 81 v U llll IJLIHL [1:171r xliiififilf. wLivl. %;v w 1 v Y .LLviw ' . . v 1 T 7 I_ I l T ' ' ' ' l SIB 48 50 653 7U 88 90 1‘39 110 120 130 140 ‘ 29.3 _ 22.: ‘ l x.l+ elf- e..- fi.esl-.j.]...1- -Lllfifi *ll'L'I‘*"'l"'l 148 150 16-0 1.70 16“ 195.1 200 216 258 236 248 25-0 74 Figure 26 FAB Pass Spectrun of manganese—Glazed Redware (H1539) .. 52 q 2 81 184 Ill L Jll l J vIll 114114 In lLlfiA #114 A I . . n . AAA; . 1111 vvvvrTvvvl vr—r] v—r—“rI—YT aTV+w I‘Vvvvlvvvv—lr—r .1. .. rrfj - 30 4B 50 63 70 83 9B 109 119 139 133 14D .4 2‘33 -‘ I 223 A All A [L l 1 All 1' l Hill'é ‘4 L 'rffi " ' vavj.fi..l.-- I rvjfif1V1" [fi'j I rfi 140 15‘) 169 170 1 B 190 ZUU 219 ’39 230 240 250 75 establish it as a quantitative tool. In principle, this is made possible by constructing a calibration table for each element from standard NBS glasses with certified elemental compositions. This kind of study, however, is outside the scope of this thesis. 6. ICP Results for Ceramic Pottery Glazes In order to have a more quantitative evaluation of the elements in the glazes, small portions of the glazes were sent to another laboratory for analysis by inductively coupled plasma emission spectrometry. The results of the ICP analyses are given in Table 8. The ICP data is parallel to the FAB-MS data in a qualitative sense except for Nb, which was not detected by ICP in any of the samples. The ICP data also show that lead and iron are by far the most common extractable elements from the glazes, and copper, indeed, is a constituent of the green slip glaze. Presumably alumina and silica, which, of course, did not dissolve, compose the bulk of the glaze. Lead is seen to be present in large quantities in all the glazes except the pearlware. These results showed that lead isotope analysis of these samples might be a profitable undertaking. i ii iii '76 Table 8 ICP Multielement Analysis of Ceramic Glazes ** Sample. Dilution Factor HL122UB 2550 HL122UA USUO HL539 U510 HL790(RED) 9800 HL537 1100 HL790(GREEN) 7810 Not detectable in sample by ICP 5*! Concentration (ppm) Pb Fe 3700(Y) 3800(Y) ”200(Y) 3800(Y) 1900(Y) 3000(Y) U700(Y) 7100(Y) *(N) *(N) 5600(Y) 6800(Y) Cu *(N) *(N) *(N) *(N) *(N) 200(Y) 1.00 ml nitric acid diluted to 5.00 ml final volume, e.g. fOr HL122NB - 1.96 mg chip: 5.00 / 0.00196 : 2550 Y or N in parentheses indicates if the element was found by FAB-MS in the same sample Nh *(N) *(Y) *(Y) *(N) *(N) *(N) Al 2700(Y) 2900(Y) 1700(Y) 5M00(Y) 900(Y) 3800(Y) 77 C. Isotope Ratio Mass Spectrometry of Lead Class by FAB 1. NBS Isotopic Standard 981 "Common" Lead The glass containing NBS standard 981 "common" lead, whose Spectrum appears in Figure 18, was used to study the precision and accuracy with which it is possible to measure lead isotope ratios by FABAMSW TWo methods of collecting isotope ratio data were available: accelerating voltage switching with data averaging on the peak top, and moderate speed magnetic scanning over a short range with very fast data accumulation. Table 9 is a list of the certified NBS values flor the lead isotope ratios in the 981 standard common lead (1“). These results are for three trials on different days, by two different operators on three different but identical isotOpe ratio instruments. 2. Lead Isotope Ratio Precision and Accuracy by FAB Table 10 is a list of the lead isotOpe ratios of the NBS lead glass obtained by accelerating voltage switching fbr three trials done within a period of ten minutes by one operator without any change in instrumental conditions, except that the detector was turned on and off three times. The fifth column is the relative accuracy of the mean as compared to the NBS certified value. Table 11 is the list of lead isotope ratios of the NBS lead glass obtained by magnetic scanning, for three scans done within a period of Pb Isotope Ratio 20U/206 206/207 207/208 '78 Table 9 NBS Isoptopic Standard 981 "common" Lead Standard Mean Deviation 0.059ou2 0.000037 1.0933 0.00033 0.36123 0.0008 Percent Relative Standard Deviation 0.063% 0.036% 0.17% 79 Table 10 Lead Isotope Ratio Internal Precision fbr Accelerating Voltage Switching Pb Isotope Standard Percent Relative Percent Relative Ratio Mean Deviation Standard Deviation Accuracy 20u/206 0.0M92 0.00033 0.66% 17% 206/207 1.09 0.00u2 0.US% 3.0% 207/208 0.N53 0.0016 0.35% 1.7% 80 Table 11 Lead Isotope Ratio Internal Precision for Magnetic Scanning Wlmwm Ratio 20H/206 206/207 207/208 Standard Percent Relative Percent Relative Mean Deviation Standard Deviation Accuracy 0.0575 0.00015 0.27% 2.6% 1.07 0.0016 0.17% 2.u% o.uuo 0.0017 0.39% 3.7% 81 ten minutes, by the same Operator on a different day from that used to collect the data in Table 10. There were no changes in instrumental conditions during the run. As one can see, the precision of the FAB isotope ratio analyses of lead by either mode are five to ten tflmes less precise than the precision reported by the NBS under much more stringent experimental conditions. Table 12 is the list of isotope ratios of the same NBS lead glass obtained by magnetic scanning of six scans taken over the course of several hours during which the sample probe was removed from the high vacuum between scans, and the FAB gun accelerating voltage and multiplier were turned off between scans. Table 12 represents much more realistic data—acquisition conditions because the sample probe must be retracted in order to change samples, and the high voltages are usually turned off as a safety precaution while the probe is removed. NOtice in Table 12 that the precision and accuracy of the isotope measurements are, as expected, much worse than those in Table 11. There are also a number of sources of error which contribute to the lack of accuracy that can be observed, in the lead isotope ratios measured by FAB—MS in Table 12. These sources of error include the interference of the PbH+ isotopes and possible isotope discrimination effects. Both of these sources of error will be discussed at length later in the thesis, but it was discovered that it was possible to increase the accuracy of the measurement by the following simple strategy. All of the ratios that contributed to the mean values in 82 Table 12 Lead Isotope Ratio External Precision for Pagnetic Scanning Pb Isotope Standard Percent Relative Percent Relative Ratio Mean Deviation Standard Deviation Accuracy 20A/206 0.0569 0.0007 1.2% 3.6% 206/207 1.06 0.0059 0.62% 3.1% 207/208 0.U35 0.0063 1.u% 5.6% 83 Figure 12 were multiplied an arbitrary factor that brought all of the ratios closer to the certified values. The correction factor that was used fbr all three lead isotope ratios was 1.03337. Table 13 is a list of the lead isotope ratios corrected for systematic experimental errors. The important features to notice in Table 13 are that while precision stays the same, the relative accuracy of all three isotope ratios increase dramatically over those in Table 12. One could go a step further and determine three correction factors, one for each ratio so that the corrected experimental means would correspond exactly to the certified values. The reason for making this correction, however, is to be able subsequently to correct the isotope ratios of lead from samples of unknown composition, and since the Pb+/PbH+ratio is frequently different between the standard and the unknowns, it seemed likely that the triple correction factor would have as much error associated with it as an single average correction factor. Therefore, at a given set of experimental conditions, one can apply a single correction factor which should improve the accuracy of the FAB—MS lead isotope ratio measurement for any glass or glaze sample despite PbH+ isotope problems. The relative accuracy of isotope ratios, however, will never be better than the relative accuracies in Table 13. There are obviously some experimental errors that make the FAB isotope ratio measurements at least an order of'magnitude less precise and accurate than the NBS 'method. Among the possible sources of inaccuracy and imprecision are probe positioning errors; small changes in voltage due to turning on and off the high voltage supplies for the 84 Table 13 Lead IsotOpe Ratios Corrected for Interferences and Isotope Discrimination Pb Isotope Standard Percent Relative Percent Relative Ratio Mean Deviation Standard Deviation Accuracy 20u/206 0.0588 0.00055 0.9u% 0.U1% 206/207 1.10 0.0057 0.63% 0.27% 207/208 0.U51 0.006” 1.“% 2.3% 85 accelerating voltage, FAB gun and electron multiplier; sequential rather than simultaneous detection of isotope intensities; moderate to low' ion intensities for the lead isotOpes; and isotopic effects of the FAB ionization process. 3. Xenon Isotope Ratio Precision by El It was impossible to investigate a multiple detector system for simultaneous ion measurements, but all of the other sources of error mentioned have been investigated. In order to investigate a system which was free of any detrimental effects due to imprecise probe positioning, surface sputtering phenomena or lack of ion statistics, an experiment was devised for examining the isotope ratios of xenon. The xenon was allowed to enter the mass spectrometer through the FAB gun with the high voltage turned off, and allowed to come to a stable pressure of five microtorr. The electron beam was turned on to 70 eV. Table 1“ lists the isotope ratios for three of the many stable xenon isotopes. The data are for three trials with all of the high voltages turned off and on between trials. The data were collected by the magnetic scanning method over the course of one hour. As expected, the isotope ratios of xenon are much more precise than the isotope ratios of lead in Table 12. It seems then that probe positioning and the FAB ionization process are significant causes of error. Under typical operating conditions, the xenon isotope ratio data in Table 1“ probably represent the best precision attainable on the varian CH—SDF in our laboratory fer isotope ratio determinations on three aliquots of a 86 Table 1“ Xenon Isotope Ratios External Precision for Magnetic Scanning Xe Isotope Standard Percent Relative Ratio Mean Deviation Standard Deviation 130/129 0.152 0.00059 0.39% 131/129 0.803 0.0032 0.37% 130/131 0.189 0.0015 0.78% IIL 87 single sample during the course of a day with a single operator. The only ways that this precision could be improved would be by the addition of dual detectors and more stable high voltage power supplies. A. Sample Probe Positioning Error An experiment was done to determine how sensitive the FAB ion signal was to probe position. A screw was placed in the sheath of the FAB probe so that the maximum total ion current was registered when the probe handle rested against the screw. Table 15 is a list of the intensities of the K+ion from KCl/Glycerol as the probe was moved back from the screw. One can see in Table 15 that after moving the probe only 0.127 mm, the peak height of the K+ion has dropped by almost ten percent, and that there is a smooth decline in signal intensity with probe position. Because the K+signal in KCl/Glycerol declines slowly with time, it was necessary to correct the values obtained for total ion drift. The major point to be learned from Table 15 is that the position of samples in the FAB ion source must be reproducible to 0.1 mm in order for signal intensities to be reproducible to one percent or better. Because real samples have a thickness which varies by much more than 0.1 mm, a probe stop does not reproducibly position the surface of different samples . Table 16 shows the effect of probe position on the 207/208 lead isotope ratio as determined by FAB of a glass containing NBS standard 981 "common" lead. The method of data collection for the data in Table 88 Table 15 Probe Position Versus Signal Intensity . mm from Probe Stop PVZ 39 Ion Intensity 0 135000 0 13H000 0 133000 0.102 131000 0.127 117000 0.152 67600 0.279 60000 0.305 30300 0.330 13H00 0.356 7520 0.906 2160 0.“57 992 0.508 736 0.559 608 0 117000 Intensity Corrected for Total Ion Drift 135000 13H000 133000 132000 121000 70500 63500 32200 1&800 8170 2370 1110 828 688 133000 89 Table 16 Probe Position versus Isotope Ratio mm from Probe Stop Pb Total Ion Intensity Pb Isotope Ratio 207/208 0 28000 0.U20 0.102 2u3oo O.HHO 0.20“ 23100 0.U50 0.U08 15100 0.U06 0.510 12300 0.811 0.612 6220 0.35” 0.71” 3190 0.333 9O 16 is the same as for Table 15 except that the probe was moved toward the instrument not away from it. Che can see that the isotope ratio is far less affected by probe positioning than the total ion current, but that the variation in the isotope ratio is much greater than the standard deviation of the measurements made without probe positioning shown in Table 11. In fact, moving the probe just 0.1 mm from its optimum position causes a change in isotOpe ratio which is too large to be acceptable fer archaeological work. A micrometer adjustment of probe position is therefere essential for doing isotope ratio work using FAB with a precision below the one percent level. One can obtain probes of this type, but such a probe was not available for this project. Fortunately, it is possible to attain optimal focus conditions despite the lack of sample position reproducibility by adjusting the accelerating voltage until the peak height of the ion of interest is maximized. This approach has been used in this research but it is not without difficulties as will be seen later in this thesis. 5. Isotope Ratios by SIMS IsotOpe ratio determinations have been done using SIMS, and a variety of isotopic effects due to experimental conditions have been documented (38). These other researchers found that in SIMS there is nearly always an isotope discrimination in favor of the lighter isotopes. The amount of this discrimination, however, varies as a function of the mass of the element being studied, of secondary ion 91 energy and of primary ion energy. Since SIMS and FAB-MS are very similar, we expected to observe isotope discrimination in FAB-MS. This section of the thesis describes the various isotope effects that were observed when conducting isotope ratio determinations on the lead glass containing NBS isotope standard 981 "common" lead. 6. PbH+Interference in Pb+Isotope Ratios The first attempts to measure isotope ratios. from lead glass by FAB-MS showed that there were a number of experimental variables that could drastically alter the lead isotope ratios, including sample probe positioning, the accelerating voltage and magnet settings, and the conditions affecting the fast atom beam. The presence of PbH+in the FAB mass spectrum of lead glass further complicates the isotope ratios. Attempts have been made to subtract the PbH+component from the m/z 207 and 208 peaks. Figure 27 shows the relationship of Pb+isotopes to the PbH+isotopes. Assuming that the isotope ratios of Pb+ and PbH+ are identical, it follows that one could determine the ratio of Pb+to PbH+by finding the ratio of the intensities of’m/z 20A and 205 as shown in Figure 27. This is not practical, however, because the minute intensity of‘m/z 205 is not measurable to a high degree of certainty. Let us define the ratio of P57Pbfi+as the constant R for a given sample. Then 205 4-206 -+ 206; Pb/ PbH also equals R. PbHT however, is not directly'measurable because it is isobaric wit11207Pbt Let us define the following peak intensities called I1 and I2: 92 Figure 27 P8 and PbH+Isotopes Ill Pb+ I _ .. Pol—1* Ill ‘ PbH*+Pb* I- I Isotope ratios of N88 sfmdord 981 "common" lead in gass form. ..._L._.l__L—L_L.._1_. 204 206 208 m/z 206 I1: Pb+ 207 206 12 = p6’. PbH+' These intensities follow directly from Figure 27. If we combine these two expressions, we obtain the following equation: 207 + 206 Pb * PbH I2 + 206 206 I1 96* Pb+ According to our previous definition: 207 Pb I2 1 +—=—— R I1 206 pa* Since it is possible to measure I1 and I2 experimentally, and since the Pb+ 207/206 ratio is known due to the fact that we are using an NBS isotopic standard, it is possible to calculate R fer a given sample. Knowing R, one can correct the observed experimental values to eliminate the PbH interference. It is important to notice that the Pb+ 209/206 ratio is unaffected by PbHT and the Pb+206/207 ratio is being used to make the corrections, so it is assumed to be correct. Therefore, the only ratio that is affected is the 207/208 ratio. If one makes the correction for PbHIinterferences on real data from FAB-MS of’ NBS lead glasses one observes that the 207/208 ratio is always significantly lower than the NBS certified value, for example: 0.1432 (corrected) versus 0.H6123 (NBS). One also notices that the observed value, 0.A38 +/- 0.0017, and the corrected value, o.u32, are very similar. The 94 similarity of the observed to the corrected value is due to the fact that I2/I1 is approximately equal to one. This means that 2361b}: and 337 PbH+ are nearly the same magnitude, and being small compared to the 207Pb+and208Pb*with which they are isobaric, the 207/208 ratio does not change much when corrected for PbH+interference. It is possible to neglect the PbHIinterference in the 207/208 ratio if the results need not be precise to more than one percent relative standard deviation. This is a fortunate circumstance that will allow us to explore various isotopic effects that are caused by experimental factors by measuring the 207/208 ratio. The 2011/206 ratio presumably could be used for the same purpose since it is not affected at all by the PbH’interference but the precison error in this measurement is frequently too large for one to derive useful information from it. 7. Isotope Ratio Dependence on Accelerating Voltage Table 17 is a list of the 207/208 isotope ratio of NBS lead glasses as a fUnction of accelerating voltage. These data were taken in the magnetic scanning with AVA data collection mode. The data in Table 17 shows that as accelerating voltage is increased, the heavier isotope becomes.more abundant with respect to the lighter isotope. This is the same trend that has been observed in SIMS (38). This effect is probably due to two instrumental biases. First, electron multiplier detectors are sensitive to the momentum as well as the energy of an ion, so they discriminate in favor of heavier ions, and they favor heavier ions more at higher accelerating voltages. Second, the ion collection efficiency 95 Table 17 Accelerating Voltage versus Isotope Ratio Accelerating Pb Isotope Ratio Voltage 207/208 2553 V 0.u59 2683 V o.uu8 2808 V 0.990 96 of a point focusing ion source, such as is used on the Varian CH-SDF, is greater for all ions ions at higher accelerating voltages. In a FAB experiment many sputtered ions are produced whose paths initially are not directed along the axis of the mass spectrometer toward the detector. The ion source collects the on-axis ions and a portion of the off-axis ions, but it tends to collect off-axis ions with lower momenta more easily. When the accelerating voltage is increased, the number of heavier ions collected increases relative to the number of lighter ions collected. These two relatively minor isotope effects are superimposed on each other so that isotope ratios may be measurably altered by changes in accelerating voltage. Clearly it is advisable to do all isotope ratio measurements for a particular element at the same accelerating voltage, or to prepare a calibration table of accelerating voltages versus isotope ratios. If the sample probe is properly positioned, then one can always use the same accelerating voltage. Since accelerating voltage can be measured accurately to four significant figures, probe positioning becomes the greatest single factor for error on the present mass spectrometer. 8. Isotope Ratio Dependence on FAB voltage It has been reported for SIMS that the magnitude of the primary ion energy also has a small but noticable effect on isotope ratios between secondary ions (38). This effect is presumably also present in FAB, but there is no direct measurement that one can make to determine the _v' 97 kinetic energy of the primary atom beam. The magnitude of the voltage which is displayed on the front of the Ion Tech FAB power supply actually performs a number of functions. It ionizes the argon or xenon gas molecules which are then accelerated toward an anode at ground potential. Before the ions leave the gun they encounter a saddle field of electrons trapped by the geometry of the voltages inside the gun. These electrons are captured by the positive fast ions and they are thereby converted into fast atoms. Because the ionization process at the "cold cathode" produces a wide variety of multiply charged gas species (67), the resulting fast atoms have a very broad distribution of energies as they leave the FAB gun. There are undoubtedly also some ions which escape from the FAB gun. Ion Tech claims that the number of these ions is negligible (68), but there have been reports to the contrary (67). Given this rather complex mixture of fast atoms and ions at widely varying energies, one might expect that the prflmary beam's effect on isotope ratios would be equally complex. Table 18 is a list of peak areas and isotope ratios as a function of FAB voltage. The isotope ratios were measured by the accelerating voltage switching method. The data in Table 18 were obtained by’ moving the potentiometer marked "current" on the FAB gun until the needle on the meter measuring voltage reached a appropriate value, at which point the electron multiplier was turned on and the intensities and isotope ratios were measured for 60 seconds. The voltage was increased stepwise from 8.0 kv to 8.6 kV and then back down. One can see from the data in Table 18 98 Table 18 FAB Voltage Versus Isotope Ratio FAB kV Peak Area m/z 208 Pb Isotope Ratio 207/208 5 8.0 8.308 X 10 0.999 5 8.2 9.971 X 10 0.996 5 8.9 10.77 X 10 0.951 5 8.6 12.96 X 10 0.993 5 8.9 10.93 X 10 0.953 5 8.2 10.60 X 10 0.997 5 8.0 8.097 X 10 0.998 99 that the intensity of the Pb+ions increases and decreases smoothly with voltage. The isotope ratios, however, stay almost constant throughout, but their highest values are consistently recorded at 8.9 kV. During this experiment it was impossible to record accurately changes in the current supplied to the FAB gun due to a poorly calibrated meter on the power supply, but the current was observed to increase and decrease with voltage. The current for this and all FAB experiments was approximately 1 mA. Table 19 is the isotOpe ratio of Pb+ 207/208 as a function of increasing FAB voltage due to decreasing xenon pressure. The data were obtained by the accelerating voltage switching method. In this case, the FAB voltage is slowly increasing due to a slow'drop in xenon gas pressure as the finite supply of gas is used up. The power supply is current-regulated, so the current supplied to the FAB gun stays steady even though the decreasing pressure increases the internal resistance in the gun, allowing the voltage to climb slowly. One can see from Table 19 that the isotope ratios increase smoothly with increasing voltage under these conditions. In SIMS, measured isotope ratios decline slighty with increasing primary' ion energy (38). Clearly the same effects are not observed in FAB according to Tables 18 and 19. In general, looking at Tables 18 and 19 one can say that under conditions where gas pressure is constant, one can expect reasonably constant isotope ratios throughout the usable range of FAB voltages. But if the gas pressure is not constant, one can expect anomolous behavior to result. It has been noticed that at voltages above 8.6 kv the glass 100 Table 19 Isotope Ratio Changes Due to Xenon Pressure Drop FAB kV Pb Isotope Ratio 207/208 8.20 0.962 8.25 0.965 8.28 0.968 8.30 0.970 8.33 0.972 8.90 o.u78 101 decomposed and turned black, causing the isotope ratios to fluctuate randomly. D. Lead Isotope Ratios from Archaeological Artifacts 1. Possible O'igins of the Highland's Artifacts It was decided to try to obtain isotope ratio measurements from archeological artifacts with the realization that only a limited amount of information could be derived from these isotope ratios due to expected precision of approximately one percent relative standard deviation. F1'om stylistic considerations, the Highland's ceramics are probably either of British origin, or are American copies of British-style pottery (59). Table 20 is a list of lead isotope ratios for B'itish ore deposits (1,2,3). Table 20 shows that lead isotope ratios do not vary much throlghout Great Britain. Isotope ratios of lead, however, vary enormously in North America. Because of the huge number of analyses, no attempt will be made in this paper to enumerate them. Table 21 lists a few of the mines in the United States which are typical of their regions (1,2,3). 2. Lead Isotope Ratios for Lead GLazed Redware Samples Table 22 is a list of the lead isotOpe ratios from lead-glazed redware samples HL1229A and HL1229B. These data were collected in the magnetic scanning mode, and the probe was moved only between samples. 102! Table 20 Selected Lead Isotope Ratios Location Chiverton Mine, Cornwall Rotherhope Fell Mine, CUmberland Leadhills Nfine, Lanarkshire Ballygrant Nine, Argyllshire Silver Ridge, Kirkendbrightshire Nutberry Hill, Lanarkshire Tyndrum Mine, Perthshire Strontian Mine, Argyllshire 209/206 0.05392 0.05327 0.05399 0.05987 0.05990 0.05200 0.05551 0.05307 from Great Britain Pb Isotope Ratios 206/207 1.177 1.161 1.169 1.160 1.166 1.211 1.197 1.199 207/208 0.9101 0.9096 0.9105 0.9127 0.9102 0.9123 0.9131 0.9126 P03 Table 21 Selected Lead Isotope Ratios from the United States Pb Isotope Ratios Location 209/206 206/207 207/208 Joplin, Nfissouri 0.09562 1.369 0.3890 0.09997 1.392 0.3911 Phoenixville, Pennsylvania 0.05393 1.199 0.9006 Balmut, New York 0.05953 1.077 0.9229 0.05899 1.092 0.9225 Minnie Moore Mine, Idaho 0.09719 1.297 0.3859 0.09772 1.293 0.3899 Friedenville, Pennsylvania 0.05196 1.228 0.3952 Pb Isotope Ratio 209/206 206/207 207/208 Pb Isotope Ratio 209/206 206/207 207/208 Lead 109- Table 22 Isotope Ratios of Lead-Glazed Redware Mean .0959 .39 .387 Mean .0968 .39 .387 HL1229B Standard Deviation 0.00093 0.0036 0.0022 HL1229A Standard Deviation 0.0016 0.018 0.0051 Percent Relative Standard Deviation 2.0% 0.50% 0.58% Percent Relative Standard Deviation 3-3% 2.6% 1.3% 1135 The total measured intensity of the lead ores in HL1229B was approximately twice that in HL1229A, and one can see the resulting increase in precision. The two remarkable aspects of data in Table 22 are that the two pottery samples have very similar isotope ratios, and that they have rather unusual lead isotope ratios, certainly very different from the NBS standard lead. The fact that the mean isotope ratios of HL1229A and HL1229B are so close to each other suggests that the lead for the glazes came from the same deposit and that the two sherds may actually be from the same vessel. It was determined earlier in this thesis that by applying a single arbitrary correction factor, one could bring the experimental values for the NBS standard lead into better agreement with the certified value. The same conditions were used with the ceramic glazes as with the standard lead glazes, so the ceramic lead isotope data were multiplied by the same arbitrary correction factor. The results of these corrections appear in Table 23. If one compares the data in Table 23 with the isotope ratios of lead ore (galenas mostly) from Great Britain and the United States, it is apparent that even given the relatively large errors involved in the FAB isotope ratio measurements, the lead in the Highland's ceramic glazes in all probability did not come from Great Britain, since the ceramics' lead isotope ratios fall considerably outside the range of British lead ore isotope ratios. This observation leads one to believe that the lead did indeed come from the United States, but it is impossible to determine from where without much better precision in measuring the lead isotope ratios. 106 Table 23 Corrected Lead Isotope Ratios for Lead-Glazed Redware HL1229B Pb Isotope Ratio Corrected Mean 209/206 0.0979 206/207 1.uu 207/208 0.900 HL1229A Pb Isotope Ratio Corrected Mean 209/206 0.0989 206/207 1.99 207/208 0.900 -~ SUMMARY The ability to measure quantitatively the amount of lead in a sample and to determine precisely the isotope ratios of that lead is of major importance to a number of disciplines including geology and archaeology. Therefore, elegant and highly precise methods involving thermal ionization isotope ratio mass spectrometry have been developed for analyzing samples containing lead. These methods have the disadvantages of being time-consuming, expensive and inflexible. Recent alternatives to thermal ionization mass spectrometry for lead isotope analysis include laser-assisted field desorption mass spectrometry, inductively coupled plasma mass spectrometry, secondary ion mass spectrometry, and in this thesis fast atom bombardment mass Spectrometry. All of the newer techniques are presently less precise than thermal ionization mass spectrometry, but they greatly decrease the analysis time for a given sample and increase the flexibility of the instrument for other analyses. 0f the four techniques mentioned, fast atom bombardment is by far the least expensive, and is probably the simplest to install on existing instrumentation. Because of instrumental constraints, a single detector rather than multiple detectors was employed and isotope ratio data were collected by a dual mini-computer data system using either magnetic scanning or accelerating voltage switching. Careful control over instrumental conditions and specialized data collection programs resulted in a dramatic increase in the precision of the Varian MAT CH-SDF mass 107 108 spectrometer for collecting isotope ratios. Isotope ratio precision was increased by two orders of magnitude over that obtained with this instrument when Operated in the rapid magnetic scanning mode usually employed for organic mass spectrometry. Attempts to measure lead isotope ratios precisely were generally unsuccessful when performed by electron impact-thermal desorption mass spectrometry of lead dioxide, electron impact mass spectrometry of a volatile lead complex, lead(II)diethyldithiocarbamate, and fast atom bombardment of lead salts in glycerol. These methods were plaued with ion signal instabilities, isotope fractionation effects and isobaric interferences due to the sample matrix. The lead compounds in glycerol, however, did exhibit some interesting chemistry which deserves to be investigated further . Fortunately, it was possible to measure lead isotope ratios quite successfully by insertion of glasses and ceramic glazes containing lead directly into the fast atom beam without the use of a liquid matrix. The lead ion signals produced by fast atom bombardment of the glazes were stable and exceptionally long-lived. Lead and a variety of other elements including iron and copper were detected in the glazes. The mass spectra of the glazes were composed almost exclusively of atomic ions, their hydroxides, and cluster ions of the various metals. The lead glazes that were analysed are samples of archaeological significance from the nineteenth century, but they are representative of a large category of lead-based ceramic glazes that have been employed by 1(39 humans for centuries and, therefore, this kind of glaze is of interest to archaeologists throughout the world. No attempt was made to quantify the metals in the glazes by fast atom bombardment mass spectrometry, but in principle this could be accomplished by using a standard glass of known elemental composition, such as those produced by the National Bureau of Standards. Quantitative elemental analyses were obtained by inductively coupled plasma emission spectroscopy, and these results correlate qualitatively with the ions found in the fast atom bombardment mass spectra. National Bureau of Standards isotopic standard 981 "common" lead was incorporated into a glass manufactured from 1:1:2 lead oxidezaluminazsilica. The alumina to silica ratio is approximately the same as that found in the archaeological samples. Using the isotopic standard it was possible to measure the precision and accuracy of the fast atom bombardment isotope ratio method for lead. Internal precision for a set of three isotope ratio measurements on the isotopic standard was approximately’ 0.5% relative standard deviation, while external precision for six different trials was approximately 1% relative standard deviation. Internal precision for the archaeological samples was worse due to smaller sample sizes and decreased ion statistics. The precision varied from 0.5 to 3 percent relative standard deviation. The resulting lead isotope ratios were compared to the National Bureau of Standards certified ratios to determine the accuracy of the —-—' 110 fast atom bombardment method. Inaccuracies in the ratios were determined to be mainly the result of a low intensity isobaric interference from PbH+ions, and the result of isotope discriminations. The major isotope discrimination was determined to be an increase in the observed intensity of the heavier isotopes relative to the lighter isotopes with increases in accelerating voltage. FAB exhibits both similarities and differences to the isotopic effects noted in SIMS. Based on the lead isotope ratio data that has been gathered for the archaeological samples from the Highland's site in Pennsylvania, it appears that the lead for the pottery glazes did not come from England but rather from America. Because of the lack of precision in determining isotope ratios by fast atom bombardment it is impossible to specify exactly from where the lead originated in the United States. It has been determined that the ultimate external precision currently possible on the mass spectrometer used for this work is probably about 0.9% relative standard deviation. This precision was measured for three intense xenon isotopes under electron impact conditions. 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