Investigation On The Chemical Composition Of Exhalation By Using A Homemade Secondary Electrospray Ionization Source Coupled With A High Resolution Mass Spectrometer

Human exhalation is predominantly composed of alveolar gas and dead-space gas,the constituents from alveolar gas are highly interrelated with the blood metabolome.Secondary electrospray ionization?SESI?is an emerging ambient soft ionization technique,which can be coupled with various types of mass spectrometers?MS?to detect the exhaled volatile organic compounds?VOCs?in real time.However,identification of the signals turned out to be a critical challenge for the application of SESI-MS to the breathomics.This is largely because the peaks observed in the mass spectra are not only resulted from the compounds in the breath air but are also attributed to the compounds in the indoor air or those released from the sample delivery tubing,parts in the mass spectrometer,etc.Thus,in this study,based on a homemade SESI source coupled with a high resolution mass spectrometer?HRMS?,the effects of VOCs-free gases,including Nitrogen?N₂?,Carbon dioxide?CO₂?and Zero Air on the ionization efficiency of typical endogenous metabolites in breath?i.e.,indole and acetone?and background interferences?i.e.,phthalic anhydride and dibutyl phthalate?have been investigated.Furthermore,positive matrix factorization?PMF?was applied for the SESI-HRMS data for the first time.The following results have obtained:?1?The presence of all three VOCs-gases has significantly increased the signal intensity and signal-to-noise ratio of exhaled acetone and indole.Among the three VOCs-gases,the effects of N₂ are the most significant.This is presumably because N₂ is an inert gas;whereas the oxygen atom in CO₂ or O₂?zero air?is electronegative and may affect the charge distribution over the surface of ESI microdroplets,thus influencing the ionization efficiency.These results may provide helpful information for optimizing the method for the detection of exhaled VOCs by using SESI-HRMS.?2?Based on the two-dimensional matrix of time-m/z obtained by SESI-HRMS,three main sources have been found by using PMF analysis and each source is characterized by a time-series and a mass spectral fingerprint.The three sources were further assigned to the dead-space gas,alveolar gas and indoor air,respectively.The successful application of PMF for treating the data obtained by SESI-HRMS shows the potential of PMF as a data mining strategy for ambient MS methods.