Development of Small Sized High Performance Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF MS) and Its Application in VOCs Analysis in Exhaled Gas

Nowadays, trend of novel mass spectrometer development concentrates on fast, on-line analysis of samples with minor or without pretreatment. An important branch of modern mass spectrometer development is to build a fast and on-line instrument for volatile organic compounds (VOCs) analysis. In this study, a high-performance small size proton transfer reaction-time of flight mass spectrometer (PTR-TOF MS) was developed. The structure of the instrument was described in detail as well as the performance test and applications in exhaled gas analysis. The ion source of the described PTR-TOF MS utilized a simple structure hollow cathode discharge to produce hydronium ion. A linear drift tube was designed as the ion molecule reactor. The mass spectrum of reagent ion showed that the base peak of reagent ion was H₃O⁺ (m/z 19). Intensity of (H₂O)_nH₃O⁺ (n=1, m/z 37) was 5% of H₃O⁺ (m/z 19). The main pollution ion NO⁺ (m/z 30) was 0.5% of H₃O⁺ (m/z 19) which indicated that the purity of the ion source was good for proton transfer reaction. A direct current was arranged evenly across the whole reaction tube to maintain a consistent reaction E/N. The interface between ion source and TOF MS was a RF-only quadruple. The frequency and voltage of the RF-only quadruple were adjusted for the best transferring of VOCs ions. A set of focus lenses and collimation lenses was followed for ion beam refining before entering mass analyzer. The mass analyzer used in this instrument was a small sized time of flight mass spectrometer. The total length of the mass analyzer was 360 mm which ensured the small size of the instrument. The time of flight mass spectrometer used orthogonal acceleration, twin pulse, reflect second-order focusing was used to maximum the resolution. Experiment result showed that the full width half maximum m/z 115 is better than 2 500. The detect limit for acetone, benzene and toluene is from 6×10^-12 mol/mol to 9.6×10^-11 mol/mol. The instrument also shows good linearity between concentration 1×10^-8 mol/mol and 1×10^-6 mol/mol. The instrument was successfully applied to component analysis of human exhaled gas. Further works will be concentrated on the quantitative analysis of components of real samples. The developed high-performance small size PTR-TOF MS has potential in applications in environment, biology, petrochemical industry etc.