Isobutane Made Practical as a Reagent Gas for Chemical Ionization Mass Spectrometry

Abstract

Abstract. As a reagent gas for positive-and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C 4 H 10 ) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C 4 H 10 in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C 4 H 10 in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/ argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance.

Mapping Glyceride Species in Biodiesel by High-Temperature Gas Chromatography Combined with Chemical Ionization Mass Spectrometry

“…Signal stability is another important parameter to consider in the practical application of CI for high-throughput analyses, as some previous studies reported rapid fouling of the CI source and filament in analyses employing pure isobutane gas . In our experiments, we did not observe signal deterioration over the two-week period of continuous CI analysis (see Figures S5 and S6, Supporting Information).…”

Exploring the Use of Gas Chromatography Coupled to Chemical Ionization Mass Spectrometry (GC-CI-MS) for Stable Isotope Labeling in Metabolomics

“…To minimize fragmentation, different analytical strategies have been implemented, including chemical derivatization with N -methyl N -( tert -butylsilyl)­trifluoroacetamide (MTBSTFA), EI ionization at 15 eV, or the use of chemical ionization . The latter has been traditionally implemented using methane as the reagent gas; however, isobutane produces superior intact analyte signal abundance to methane, resulting also in a predominant protonated adduction and less fragmentation. Here, we have used this previous knowledge to demonstrate that CI-isobutane outperforms CI-methane, EI at 70 eV and EI at 15 eV for isotopologue analysis by GC-MS. We have also shown that the metabolic coverage of CI-isobutane allows analysis of the most relevant metabolites in central carbon metabolism with good ionization efficiency, including intermediates of glycolysis, TCA cycle, pentose phosphate pathway, amino acids, urea cycle, and polyamines.…”

Exploring the Use of Gas Chromatography Coupled to Chemical Ionization Mass Spectrometry (GC-CI-MS) for Stable Isotope Labelling in Metabolomics

“…To minimize fragmentation, different analytical strategies have been implemented, including chemical derivatization with N-methyl N-(tertbutylsilyl)trifluoroacetamide (MTBSTFA) 15 , EI ionization at 15 eV 31 or the use of chemical ionization 37 . The latter has been traditionally implemented using methane as reagent gas 31 , however, isobutane produces superior intact analyte signal abundance to methane 45 , resulting also in a predominant protonated adduct ion and less fragmentation.…”

“…31 or the use of chemical ionization 37 . The latter has been traditionally implemented using methane as reagent gas 31 , however, isobutane produces superior intact analyte signal abundance to methane 45 , resulting also in a predominant protonated adduct ion and less fragmentation.…”