Portable proton transfer reaction ion trap mass spectrometer with selectable buffer gas.

Proton transfer reaction mass spectrometry (PTR-MS) is a rapid and highly sensitive technique for the detection of volatile organic compounds and has been increasingly used in the fields of environmental monitoring, medicinal diagnosis and food industry. The large size and heavy weight of conventional PTR-MS devices limit their on-site application for real-time and rapid analysis. In this work, a miniaturized PTR-MS instrument with a total weight of 25 kg was developed on the basis of a portable ion trap mass spectrometer, which was characterized by the use of a small hollow-cathode discharge for hydroxonium ion generation and a compact ion funnel as the drift tube. For this instrument, mass resolution, ion isolation and fragmentation efficiencies, and sensitivity have been investigated in detail with helium and nitrogen as buffer gases. Compared with nitrogen buffer gas, when helium buffer gas was used, the mass resolution of protonated acetone ions doubled, whereas the ion fragmentation efficiency could be increased by a factor of 17. With helium buffer gas, the limits of detection for acetone, benzene, and toluene were found to be 1 ppbv, 0.5 ppbv, and 0.1 ppbv, respectively, and linear calibration curves for these compounds were established across concentration ranges spanning at least two orders. Under the optimal vacuum pressures of 1.5 mbar and 2.5 mbar, the gas consumption rates for helium and nitrogen were 81 mL min-1 and 60 mL min-1, respectively. In addition, the sample injection flow rate exerted a pronounced influence on the detection sensitivity and response time of the system.