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

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.