Real-time monitoring of trace-level VOCs by an ultrasensitive lamp-based VUV photoionization mass spectrometer

Abstract

Abstract. In this study, we report on the development of a lamp-based vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) in our laboratory; it is composed of a radio-frequency-powered VUV lamp, a VUV photoionizer, an ion-migration lens assembly, and a reflection timeof-flight mass spectrometer. By utilizing the novel photoionizer consisting of a photoionization cavity and a VUV light baffle, the baselines of the mass spectra decreased from 263.6 ± 15.7 counts to 4.1 ± 1.8 counts. A detection limit (2σ ) of 3 pptv was achieved for benzene after an acquisition time of 10 s. To examine its potential for real-time monitoring applications of samples, the developed VUV-PIMS was employed for the continuous measurement of urban air for 6 days in Beijing, China. Strong signals of trace-level volatile organic compounds, such as benzene and its alkylated derivatives, were observed in the mass spectra. These initial experimental results reveal that the instrument can be used for the online monitoring of trace-level species in the atmosphere.

Ultrasensitive detection of explosives and chemical warfare agents by low-pressure photoionization mass spectrometry

“…The mass peak at m/z 182 corresponds to the molecular ion of 2,4-DNT (ionization energy, IE ¼10 eV). Under 10 eV VUV radiation in the LPPI source, [26] the ionization of 2,4-DNT is reasonable. The mass peak at m/z 183 corresponds to protonated 2,4-DNT, which resulted from the interaction between the molecular ions and neutral species in matrix gases [24].…”

“…The daughter ion with 165 Th was the dominant mass peak of 2,4-DNT photoionized by VUV photons emitted from a krypton VUV lamp and was identified by MS/MS [20]. The formation of daughter ions with 135 and 119 Th may have resulted from the unique photoionizer, which enhances fragmentation during or after PI [26]. The LOD at a signal-to-noise ratio of 2 was estimated by LOD ¼2sc/h, [27] where s is the standard deviation of the noise, c is the sample concentration, and h is the ion signal intensity.…”

“…However, the first four peaks were assigned to protonated H 2 O, (H 2 O) 2 , (H 2 O) 3 , and (H 2 O) 4 , respectively. Protonated water and water clusters originate from the background gases in the chamber [26]. The others were assigned to organic residuals in the chamber.…”

“…Molecular ions and protonated molecules were observed in LPPI mass spectra [24,25]. A previous study revealed that LPPI has an excellent sensitivity and have the ability to on-line monitoring trace amount volatile organic compounds in the atmosphere [26].…”

“…The LPPI mass spectrometer was recently developed in our laboratory and has been described in detail elsewhere [26]. Briefly, it is composed of an LPPI source, an ambient sample inlet, and a short reflection time-of-flight mass spectrometer.…”