Increased Sensitivity in VOC Detection by Using a Novel Photoinduced Multiple Ionization Mass Spectrometry.

Proton transfer reaction mass spectrometry (PTR-MS) is an important online monitoring technique for volatile organic compounds (VOCs). VOCs have the characteristics of many types, rapid change, and low concentration. Enhancing the detection sensitivity and expanding the detection range of VOCs have been key challenges in PTR-MS research. In this work, we have developed and characterized a novel photoinduced multiple ionizations (PMI) source, which consists of four direct current (DC) vacuum ultraviolet Kr lamps and a V-shaped focusing quadrupole ion funnel (FQ-IF) drift tube. The novel PMI source has four ionization processes: proton transfer reaction, charge transfer reaction, single photon ionization, and photoelectron impact ionization. It is capable of detecting VOCs detectable by conventional PTR-MS via the PTR, as well as VOCs (carbon disulfide and acetylene) that are difficult to detect by conventional PTR-MS through other ionization processes, thus effectively expanding the detection range. In further comparative experiments, the improvements in sensitivity for the PMI-MS in FQ-IF mode range from 122.5 to 647.7 times compared to the PTR-MS in DC mode (conventional PTR-MS) for 12 test VOCs. Notably, the sensitivities of four BTEXs in 12 VOCs were more than 10,000 cps/ppb. Moreover, compared with PTR-MS in DC mode, the LOD of PMI-MS in FQ-IF mode for 12 test VOCs increased by 26-128.6 times. PMI-MS not only expands the detection range but also improves the sensitivity by 2 orders of magnitude, which would provide an important tool for the detection of ultratrace VOCs.