An ultrasensitive SPI/PAI ion source based on a high-flux VUV lamp and its applications for the online mass spectrometric detection of sub-pptv sulfur ethers.

Single-photon ionization mass spectrometry (SPI-MS) is an attractive analytical technique for the online detection of volatile organic compounds; however, the low photon flux of the vacuum ultraviolet (VUV) lamp commonly used in the SPI ion source and the corresponding low detection sensitivity remain a constraint to its wide field applications. In this study, a new VUV lamp-based SPI ion source was developed. By increasing the discharging volume and optimizing the configuration of the lens and ionizer, the photon flux of the VUV lamp and the sensitivity of the ion source were significantly improved. The VUV lamp output was 2.8 × 10 photons s, which was focused into the small ionization zone, and the ion intensity of gaseous benzene under SPI achieved 1.7 × 10 counts per second per pptv (cps pptv). This ion source can also function in photoinduced associative ionization (PAI) mode by introducing gaseous CHCl to initiate an associative ionization reaction. Because of the high efficiency of the ion source, the amount of doped CHCl needed for PAI was greatly reduced (∼2.5% of that used in previous experiments). PAI exhibited an outstanding protonation effect on monosulfur ethers (CHS). The signal intensities of the protonated molecular ions (MH) were 46-128 times higher than those of the molecular ions (M) produced by SPI. Since monosulfur ethers generally have lower SPI cross-sections than polysulfur ethers (CHS or CHS), the PAI and SPI modes were selected for the online measurement of a series of mono- and polysulfur ethers, respectively. The obtained detection sensitivity of the mono- and polysulfur ethers reached 476.5 ± 1.72-2835.1 ± 99.5 and 47.9 ± 0.4-105.1 ± 2.3 counts pptv, respectively, in 10 s of sampling time. The corresponding 3σ limits of detection (LODs) were 0.12-0.71 and 0.06-0.14 pptv, respectively. This study provides advances in the development of a high-flux VUV lamp and a highly efficient SPI/PAI ion source, as well as an ultrasensitive analytical method for detecting sulfur ethers.