高灵敏VOCs在线真空紫外单光子电离飞行时间质谱仪的研制

研制了一种高灵敏度在线膜进样真空紫外电离源飞行时间质谱仪（MI-SPI-TOF MS）用于检测低浓度挥发性有机物（VOCs）。仪器包括真空系统、膜进样接口、真空紫外单光子电离源、垂直加速反射式飞行时间质量分析器和数据采集系统等。该仪器使用聚二甲基硅氧烷（PDMS）膜作为大气压下直接进样的接口，膜的选择透过性能直接、快速地富集大气中VOCs，可实现快速在线进样检测。真空紫外单光子灯作为电离源，能将电离能低于10.6 eV的VOCs电离，形成分子离子峰。结果表明，该仪器的分辨率优于750 FWHM，对苯、甲苯、二甲苯和氯苯的检测限达10^-12 mol/mol级别，检测速度达秒级，可用于低浓度VOCs的实时在线检测。

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.