Determination of elemental compositions by gas chromatography/time‐of‐flight mass spectrometry using chemical and electron ionization

Abstract

Many metabolomic applications use gas chromatography/mass spectrometry (GC/MS) under standard 70 eV electron ionization (EI) parameters. However, the abundance of molecular ions is often extremely low, impeding the calculation of elemental compositions for the identification of unknown compounds. On changing the beam-steering voltage of the ion source, the relative abundances of molecular ions at 70 eV EI were increased up to ten-fold for alkanes, fatty acid methyl esters and trimethylsilylated metabolites, concomitant with 2-fold absolute increases in ion intensities. We have compared the abundance, mass accuracy and isotope ratio accuracy of molecular species in EI with those in chemical ionization (CI) with methane as reagent gas under high-mass tuning. Thirty-three peaks of a diverse set of trimethylsilylated metabolites were analyzed in triplicate, resulting in 342 ion species ([M+H](+), [M-CH(3)](+) for CI and [M](+.), [M-CH(3)](+.) for EI). On average, CI yielded 8-fold more intense molecular species than EI. Using internal recalibration, average mass errors of 1.8 +/- 1.6 mm/z units and isotope ratio errors of 2.3 +/- 2.0% (A+1/A ratio) and 1.7 +/- 1.8% (A+2/A ratio) were obtained. When constraining lists of calculated elemental compositions by chemical and heuristic rules using the Seven Golden Rules algorithm and PubChem queries, the correct formula was retrieved as top hit in 60% of the cases and within the top-3 hits in 80% of the cases.

Metabolite Measurement: Pitfalls to Avoid and Practices to Follow

“…This type of ionization yields many highly reproducible fragments and rearrangement ions, making identification of known compounds straightforward, based on comparison to mass spectra and retention indices of authenticated standards that have been deposited in databases. However, the intact molecular ions are often absent from the mass spectra (84). An alternative ionization approach for GC-MS is chemical ionization, which is softer and thus more commonly yields a molecular ion peak (74).…”

Mass Spectrometry—based Metabolomics, Analysis of Metabolite-Protein Interactions, and imaging

“…Decision tree (DT)–based prediction algorithm makes use of mass spectral fragmentation and retention index (RI) information to enable sensitive and precise detection of compound substructures. It has also been shown that higher intensities of molecular and total ions (≤10-fold) are achieved by modulating a beam-steering voltage of the ion source at 70 eV EI of GC-TOF (e.g., alkanes, fatty acid methyl esters, and trimethylsilylated metabolites) (49). In this application, the accurate masses and isotopic data allowed more precise calculations of elemental compositions and facilitated metabolite identification when combined with fragmentation patterns from EI data.…”

Current applications of high-resolution mass spectrometry in drug metabolism studies

“…If the dynamic range is exceeded (e.g., in high analyte concentrations), the MA will dramatically decrease. This was described by Abate et al [60] acquiring data in dynamic range enhancement (DRE) mode to allow the accurate mass analysis of very abundant GC-MS peaks that would otherwise saturate the detector and become unusable for accurate mass analysis.…”