Use of chemical ionization for GC–MS metabolite profiling

Abstract

Metabolite profiling is commonly performed by GC–MS of methoximated trimethylsilyl derivatives. The popularity of this technique owes much to the robust, library searchable spectra produced by electron ionization (EI). However, due to extensive fragmentation, EI spectra of trimethylsilyl derivatives are commonly dominated by trimethylsilyl fragments (e.g. m/z 73 and 147) and higher m/z fragment ions with structural information are at low abundance. Consequently different metabolites can have similar EI spectra, and this presents problems for identification of “unknowns” and the detection and deconvolution of overlapping peaks. The aim of this work is to explore use of positive chemical ionization (CI) as an adjunct to EI for GC–MS metabolite profiling. Two reagent gases differing in proton affinity (CH4 and NH3) were used to analyse 111 metabolite standards and extracts from plant samples. NH3-CI mass spectra were simple and generally dominated by [MH]+ and/or the adduct [M+NH4]+. For the 111 metabolite standards, m/z 73 and 147 were less than 3% of basepeak in NH3-CI and less than 30% of basepeak in CH4-CI. With CH4-CI, [MH]+ was generally present but at lower relative abundance than for NH3-CI. CH4-CI spectra were commonly dominated by losses of CH4 [M+1-16]+, 1–3 TMSOH [M+1-nx90]+, and combinations of CH4 and TMSOH losses [M+1-nx90-16]+. CH4-CI and NH3-CI mass spectra are presented for 111 common metabolites, and CI is used with real samples to help identify overlapping peaks and aid identification via determination of the pseudomolecular ion with NH3-CI and structural information with CH4-CI.