Development of High Sensitive Online Vacuum Ultraviolet Single-Photon Ionization Time-of-Flight Mass Spectrometer for VOCs

In recent years, PM2.5 pollutions have occurred in most parts of China, attracting more and more attention. According to the literatures, volatile organic compounds (VOCs) are certain important precursors of secondary organic aerosols (SOA). For the requirement of detecting VOCs at low level, a highly sensitive membrane inlet vacuum ultraviolet single photon ionization time-of-flight mass spectrometer (MI-SPI-TOF MS) was developed, which included a vacuum system, a membrane sampling inlet interface, a vacuum ultraviolet single photon ionization source, a orthogonal acceleration reflection time-of-flight mass analyzer, and a data acquisition system. The polydimethylsiloxane (PDMS) membrane was used as an interface for sample directly-injection at atmospheric pressure. It could realize enrichment of VOCs quickly from the atmosphere. A vacuum ultraviolet lamp was used as the ion source. Single photon ionization with vacuum ultraviolet light was a particularly soft ionization technique. VOCs with ionization energy less than photon energy (10.6 eV) could be ionized into molecular ions. It was nearly fragmentation free compared with electron ionization, and very suitable for qualitative analysis. The mass analyzer included repeller, double pulsed acceleration region, field-free drift tube, reflector and ion detecter. The vacuum system achieved the pressure of 3×10^-5 Pa measured by a gauge. The size of the instrument was (64×64×55) cm³ and the weight was 77 kg. The performances of the instrument were tested by standard gas mixture of benzene, toluene, xylene and chlorobenzene, and PAMS standard gas sample. The mass resolution was higher than 750 FWHM and the mass accuracy was 10^-5. The limits of detection (LODs) of benzene, toluene, xylene and chlorobenzene could achieve 10^-12 mol/mol level for several seconds rapid measurement. The linear ranges covered 3 orders of magnitude from 50×10^-12 to 500×10^-9 mol/mol. 19 of the 25 kinds of sample with different m/z were detected in the PAMS standard gas, and R values of detected samples were better than 0.99 in the concentrations range from 1×10^-9 to 200×10^-9 mol/mol except N-dodecane. The instrument was applied for testing the air quality inside the laboratory, and 21 kinds of VOCs with different m/z were detected, whose concentrations were below 10^-6 mol/mol level after preliminary quantitative analysis. The average concentrations of toluene and xylene were 111×10^-12 mol/mol and 42×10^-12 mol/mol respectively, lower than the national standard of air quality. Overall, MI-SPI-TOF MS was suitable for various applications, such as on-site monitoring of trace VOCs, industrial process analysis and research of environmental sciences.