The typical method for the identification of seized drugs is to analyze unknown samples using gas chromatography-mass spectrometry (GC-MS) and to perform a visual comparison of the resulting mass spectrum to a suitable reference spectrum. However, for spectra of structurally similar compounds, visual comparison of spectra for identification can be challenging. Previous work in our laboratory focused on the development of a statistical method to compare the mass spectrum of an unknown sample to a suitable reference spectrum using an unequal variance t-test. In this work, GC-MS was used to analyze two sets of fentanyl isomers which included the ortho-, meta-, and para- forms of fluoroisobutyryl fentanyl (FIBF) and the ortho-, meta-, and para- forms of fluorobutyryl fentanyl (FBF). All compounds were analyzed over three months and the resulting spectra within each month were statistically compared. The ability to maintain correct association and discrimination across the three-month time study as well as the effects of refining the model on the overall results were observed. Proper association and discrimination of the FIBF and FBF spectra were achieved in most cases at the 99.9% confidence level and the ability to maintain similar overall results across the time study was demonstrated. Refining the model resulted in the reversal of an incorrect association (false positive) and a greater number of discriminating ions in many comparisons. Ultimately, this research provides insight into the robustness of the previously developed statistical comparison method to differentiate between positional isomers using instrumentation readily available in a forensic laboratory.
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