Detection of Volatile Organic Compounds with Secondary Electrospray Ionization and Proton Transfer Reaction High-Resolution Mass Spectrometry: A Feature Comparison

Abstract

The analysis of volatiles is of high relevance for a wide range of applications from environmental air sampling and security screening to potential medical applications. High-resolution mass spectrometry methods offer a particularly wide compound coverage, sensitivity, and selectivity. Online approaches allow direct analysis in real time without the need for sample preparation. For the first time, we systematically compared the analysis of volatile organic compounds with secondary electrospray ionization (SESI) and proton transfer reaction (PTR) high-resolution mass spectrometry. The selected instruments had comparable mass resolving powers with m/Δm 15000, which is particularly suitable for nontargeted analysis, for example, of exhaled breath. Exhalations from 14 healthy adults were analyzed simultaneously on both instruments. In addition, 97 reference standards from nine chemical classes were analyzed with a liquid evaporation system. Surprisingly, in breath, we found more complementary than overlapping features. A clear mass dependence was observed for each method with the highest number of detected m/z features for SESI in the high mass region (m/z = 150-250) and for PTR in the low mass region (m/z = 50-150). SESI yielded a significantly higher numbers of peaks (828) compared to PTR (491) among a total of 1304 unique breath m/z features. The number of signals observed by both methods was lower than expected (133 features) with 797 unique SESI features and 374 unique PTR features. Hypotheses to explain the observed mass-dependent differences are proposed.

Minimizing ion competition boosts volatile metabolome coverage by secondary electrospray ionization orbitrap mass spectrometry

“…[15] [16] We generated a spectral stitching method by using twenty-two Ion competition particularly suppresses analytes in the higher m/z region -presumably because the ion competition is mainly caused by high abundant low mass matrix compounds. In fact, the ion competition-reduced feature distribution more closely resembles the distribution reported on a SESI-quadrupole time-of-flight (QTOF) instrument [19], where ion competition is not expected because of the ion-trap free nature of a QTOF instrument.…”

“…In both polarities the method with 20 m/z windows outperform the full scan method in the higher mass region ( m/z ≥ 200 ), which matches exactly what we expected; that the dominating peaks in the low mass region are saturating the C-trap therefore reduce the overall detection sensitivity ( figure 2 (c)). Reducing the ion competition with the 20 m/z windows results in a similar feature distribution as observed on a SESI-quadrupole time-of-flight (QTOF) instrument [16] , where we assume that ion competition is less pronounced. Therefore, we conclude that a window splitting approach should be considered to make full use of the SESI's high mass ionization capabilities.…”

Hyphenated Mass Spectrometry versus Real-Time Mass Spectrometry Techniques for the Detection of Volatile Compounds from the Human Body

“…SESI-MS has been repeatedly reported as a robust tool for cancer identification based on breath analysis [ 73 , 74 , 75 ]. An interesting study was described by Bruderer et al [ 76 ], who compared the potential of SESI-MS and PTR-MS for VCs analysis in the breath. SESI-MS detected the highest number of compounds in the high mass region ( m / z = 150–250), while PTR-MS was more sensitive in the low mass region ( m / z = 50–150).…”

“…PTR-MS allowed for the detection of 491 peaks, whilst SESI-MS detected almost double that number (828). Interestingly, only 133 peaks were common for both techniques, and 797 and 374 unique features were detected for SESI and PTR, respectively [ 76 ].…”

Parallel secondary electrospray ionisation mass spectrometry and selected ion flow tube mass spectrometry quantification of trace amounts of volatile ketones

“…Note that PTR‐MS can be also used for quantification at sub‐pptv LOD. Recently it has been compared with SESI‐MS in parallel qualitative analyses, 4 which has shown that the major ions of the analytical mass spectra from both techniques are protonated molecules [M + H] + together with fragments and adducts. However, quantifications were not compared.…”