Detection of Volatile Organic Compounds with Secondary Electrospray Ionization and Proton Transfer Reaction High-Resolution Mass Spectrometry: A Feature Comparison
Bruderer, T.; Gaugg, M.; Cappellin, L.; Lopez-Hilfiker, F.; Hutterli, M.; Perkins, N.; Zenobi, R.; and Moeller, M.
Journal of the American Society of Mass Spectrometry, 2020
This publication highlights groundbreaking joint research between ETH Zurich, TOFWERK, the University Children’s Hospital Zurich and the Children’s Research Center Zurich. The research focuses on disease diagnosis via detection of volatile organic compounds (VOCs) in breath using two cutting-edge mass spectrometry techniques.
Breath analysis is a very active field of research, especially in the medical sector, because it may provide an avenue to rapid, noninvasive, early detection of many diseases. However, it is extremely demanding in terms of the performance required of analytical instrumentation. This published research was a pilot study to understand the relative strengths of two high-performance techniques presently used in breath analysis research: Proton Transfer Reaction – Mass Spectrometry (PTR-MS) and Secondary Electrospray Ionization – Mass Spectrometry (SESI-MS).
A TOFWERK Vocus 2R PTR-TOF was deployed side by side with a SESI-TOF at the University Children’s Hospital Zurich. Breath samples from healthy adults were measured simultaneously by the two techniques. Interestingly, only about ten percent of all mass spectral features were common between the two methods, while the majority of the features were uniquely measured by a single instrument. This finding indicates that Vocus 2R PTR-TOF and SESI-TOF provide complementary information when attempting to assess the biological processes that contribute to the VOC profile of human breath.
The authors also evaluated so-called “matrix-effects”: breath contains a complex mixture of many chemicals at many different concentrations, and the presence of one species may affect the detection of others. The authors hypothesized that effects of ion suppression are present in SESI-TOF a derivative of the electrospray technique. For instance, insufficient charge is available to ionize some compounds (likely the ones with lower proton affinity), thus leading to matrix effects.
Matrix effects are instead not present in Vocus 2R, which has calibration factors independent from sample humidity- an important experimental consideration when measuring humid breath. Moreover, the sensitivity of Vocus 2R was improved by a factor of x5-x10 shortly after this study, thus opening further possibilities for breath analysis research.
More importantly, the semi-quantitative approach of the Vocus PTR allows for rough concentration estimation of most ion peaks in the Vocus spectra. Measurements of external standards, on the other hand, remain a prerequisite for SESI-TOF.