Influence of Ambient Gases on Detection of Exhaled Gas in Secondary Electrospray Source by Ultra-High Resolution Mass Spectrometry

The secondary electrospray ionization mass spectrometry ( SESI-MS ) has great potential in acquiring breathomics by in vivo, real-time and direct breath analysis. However, as an ambient source, the ionization efficiency of exhaled volatile organic compounds ( VOCs ) can be disturbed by the occurrence of indoor VOCs ( background matrix) in SESI source. Therefore, we explored the effects of pure gases in the SESI source on the ionization efficiency of typical exhaled endogenous compounds , i. e. , acetone and indole , as well as typical background matrix , i. e. , phthalic anhydride ( PA ) and dibutyl phthalate ( DP) , by using a homemade SESI source coupled with an ultrahigh resolution mass spectrometer ( UHRMS) . The pure gases in this study included nitrogen ( N-2 , purity 99. 999% ) , carbon dioxide (CO2 , purity 99. 99% ) and Zero air ( VOCs free , 21% O-2 and 79% N-2 by volume) . The results showed that the presence of all three pure gases could significantly increase the signal intensity and signal-to-noise ratio of exhaled acetone and indole. In contrast, the intensities of matrix interferences PA and DA decrease, because the concentrations of these compounds were diluted by the pure gases. Among three pure gases , the effect of N-2 was most significant. In summary, the results of this study provided helpful information for SESI source design and parameter setting, which was an important prerequisite 14 the development of qualitative and quantitative methods for the detection of exhaled VOCs by using SESI-MS.