Development of Membrane Inlet Based on Ultra Violet Ionization Time of Flight Mass Spectrometer and Its Application in VOCs Analysis at Workplace

Due to an increasing application requirement of rapid and on-line analysis of volatile organic compounds (VOCs), various VOCs on-line analysis mass spectrometer have been developed in recent years. The design principle and performance characterization of a membrane inlet based small ultra violet ionization time-of-flight mass spectrometer and its application of VOCs detection in workplace was described. The instrument was equipped with polydimethylsiloxane (PDMS) as the inlet interface, which not only ensured the vacuum of the instrument, but also played the role of enriching VOCs and reducing the detection limit. A krypton lamp of emitting 10.6 eV photons was used as the light source, which was designed for ionization source. VOCs with an ionization energy lower than 10.6 eV could be ionized through photon ionization. The principle of orthogonal acceleration was adopted to reduce the kinetic energy dispersion of the ion beam in the acceleration direction and thus improved the resolution of the instrument. The double pulse region was designed in the pulse acceleration zone to reduce the pulse interference and improve the signal-to-noise ratio. Reflective second-order focusing could effectively improve the flight path in order to improve the time-of-flight difference of ions with different mass-charge ratios. At the same time, second-order focusing reduced the time-of-flight difference of ions with the same mass-charge ratio, improving the instrument resolution. Molecular ion peak was produced by this instrument, which was convenient for spectral recognition and high through-put analysis. A miniaturized design of the time-of-flight mass analyzer was adopted with a total length of less than 360 mm. Experimental results showed that the full width half maximum (FWHM) resolution of the instrument is 2 000 and the detection limits of benzene, toluene and xylene range from 0.5×10^-9 to 3×10^-9 mol/mol. The instrument was successfully applied to analyze VOCs in the workplace. Target analysis of benzene in furniture work place air had also been achieved. The purpose of the study was to build an in situ and fast method for VOCs analysis. Single stage vacuum was used with a small flight tube to keep the small size and low cost. Although the developed ultra violet ionization time-of-flight mass spectrometry (UVI-TOF MS) sulfuring from the ppbv level detect limit and isomer confusion, UV photoionization mass spectrometry still has a certain opportunity to show its ability in most applications where the requirements of qualitative analysis are not high. Further works will be concentrated on characterization of the instrument stabilization.