Can breath components be quantified with secondary-electrospray ionization coupled to mass spectrometry?
Absolute quantification is desirable for any analytical technique, especially in the context of medical diagnosis. As an alternative to traditional blood analysis, on-line breath analysis using secondary-electrospray ionization (SESI) coupled to mass spectrometry (MS) could serve as a non-invasive diagnostics tool. To reach the full potential of this technique, a gas standard generation system based on the controlled evaporation of liquid analytes and their dilution in a carrier gas stream was developed to produce low-concentration (down to part-per-trillion) standards.1 This system can operate at the same flow rates as typical human exhaled breath and under humid conditions. To test the analytical capabilities of the system, short-chain fatty acids were used to test the limits of detection and quantification, as well as the linearity. To elucidate whether this system could be used to calibrate a SESI-MS system externally, ion suppression effects also had to be characterized. Ion suppression was postulated to be potentially present in SESI2, which was investigated using the gas generation system described above. It was shown that ion suppression in SESI is mainly a gas-phase phenomenon. For this purpose, gas standards of D6-acetone, D3-acetic acid and pyridine were generated and their impact on each other’s signal was determined. D3-acetic acid seemed to be most affected by rising levels of D6-acetone and pyridine, whereas pyridine was the least affected by increasing the concentrations of the other two compounds. This indicated a mechanistic rationale for signal suppression within SESI related to gas-phase acid-base chemistry. Pyridine with the highest gas-phase basicity was not affected by increasing concentrations of D3-acetic acid and D6-acetone, while the other two compounds that have lower basicity are more strongly affected when the concentration of pyridine was increased. Regarding quantification of breath metabolites with SESI-MS, the use of internal standards is advised.
(1) Wüthrich, C.; Fan, Z.; Vergères, G.; Wahl, F.; Zenobi, R.; Giannoukos, S. Analysis of Volatile Short-Chain Fatty Acids in the Gas Phase Using Secondary Electrospray Ionization Coupled to Mass Spectrometry. Analytical Methods 2023, 15 (5), 553–561. https://doi.org/10.1039/D2AY01778D.
(2) Bruderer, T.; Gaugg, M. T.; Cappellin, L.; Lopez-Hilfiker, F.; Hutterli, M.; Perkins, N.; Zenobi, R.; Moeller, A. Detection of Volatile Organic Compounds with Secondary Electrospray Ionization and Proton Transfer Reaction High-Resolution Mass Spectrometry: A Feature Comparison. J Am Soc Mass Spectrom 2020, 31 (8), 1632–1640. https://doi.org/10.1021/jasms.0c00059.