Continuous Water Infusion Enhances Atmospheric Pressure Chemical Ionization of Methyl Chloroformate Derivatives in Gas Chromatography Coupled to Time-of-Flight Mass Spectrometry-Based Metabolomics

Abstract

The effects of continuous water infusion on efficiency and repeatability of atmospheric pressure chemical ionization of both methyl chloroformate (MCF) and methoxime-trimethylsilyl (MO-TMS) derivatives of metabolites were evaluated using gas chromatography-time-of-flight mass spectrometry. Water infusion at a flow-rate of 0.4 mL/h yielded not only an average 16.6-fold increase in intensity of the quasimolecular ion for 20 MCF-derivatized metabolite standards through suppression of in-source fragmentation but also the most repeatable peak area integrals. The impact of water infusion was the greatest for dicarboxylic acids and the least for (hetero-) aromatic compounds. Water infusion also improved the ability to detect reliably fold changes as small as 1.33-fold for the same 20 MCF-derivatized metabolite standards spiked into a human serum extract. On the other hand, MO-TMS derivatives were not significantly affected by water infusion, neither in their fragmentation patterns nor with regard to the detection of differentially regulated compounds. As a proof of principle, we applied MCF derivatization and GC-APCI-TOFMS to the detection of changes in abundance of metabolites in pancreatic cancer cells upon treatment with 17-DMAG. Water infusion increased not only the number of metabolites identified via their quasimolecular ion but also the reproducibility of peak areas, thereby almost doubling the number of significantly regulated metabolites (false discovery rate < 0.05) to a total of 23.

Identification of substances migrating from plastic baby bottles using a combination of low‐resolution and high‐resolution mass spectrometric analysers coupled to gas and liquid chromatography

“…The respective masses m / z 215.1469, 177.0947 and 147.0844 could be explained as the adduction of a water molecule to these fragments. The inclusion of a small amount of water in the APCI source to promote the formation of the [M + H] + could explain this phenomenon as already described by Wachsmuth et al . Therefore, dicyclopentyl‐dimethoxysilane was retained as the probably identified migrant.…”

Gas chromatography coupled to atmospheric pressure ionization mass spectrometry (GC-API-MS): Review

“…The highest suppression was approximately 40% for most compounds in apple [4]. Secondly, GC-APCI is less reproducible in comparison with GC-EI, but the work of Wachsmuth et al [37] shows a nice increase in reproducibility by water infusion into the APCI ion source.…”

“…A higher concentration of water into the APCI source by a syringe pump significantly increases the sensitivity and selectivity. Wachsmuth et al [37] reported not only an average 16.6-fold increase in intensity of the protonated molecule for 20 derivatized metabolite standards through the reduction of in-source fragmentation, but also the most repeatable peak area integrals by water infusion at a flow-rate of 0.4 mL/h. Ionization of analytes occurs not only by direct protonation but also by ligand switching, i.e., association of analytes in [H(H 2 O) n ] + water ion clusters followed by dissociation of water clusters [18,37].…”

Humidity Effects on Fragmentation in Plasma-Based Ambient Ionization Sources

“…It has been implied for direct analysis in real time (DART) ionization that under typical ambient conditions water cluster formation is sufficiently rapid [6] to have no effect on ambient mass spectra. However, several research groups have used supplementary water with corona discharge APCI [19][20][21][22] to increase analyte MH + abundance. Large effects on mass spectra were also observed with DART of one analyte in a range of humidity values [23].…”