Suspected controlled substances are typically analyzed by gas chromatography-mass spectrometry (GC-MS) and the mass spectrum is compared to that of a reference material or library database for identification purposes. However, the identification is typically based on visual comparison of the spectra. With the increased prevalence of novel psychoactive substances, including positional isomers, identification in this manner is challenging due to high structural similarity among compounds.Previous work in our laboratory developed a statistical comparison method to compare two mass spectra (e.g., spectrum of the submitted sample compared to spectrum of the reference material). The work presented here demonstrates application of the statistical comparison method specifically for the differentiation of two sets of positional isomers. Each set of isomers was analyzed by GC-MS using two different instruments and with different injection parameters (e.g., different split ratios).Ultimately, association and discrimination of both sets of isomers was achieved. with one set discriminated at the 99.9% confidence level, while the other set was discriminated at the 99% confidence level. Varying split ratios and concentrations did have an effect on the number of discriminating ions reported, but discrimination was still possible. Overall, this research provides a method to identify positional isomers using mass spectral data routinely collected in a forensic laboratory.
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