Photoinduced Associative Ionization Time-of-Flight Mass Spectrometry for the Sensitive Determination of Monoterpenes

Recently, photoinduced associative ionization (PAI) has been demonstrated to be extremely efficient for the determination of oxygenated volatile organic compounds. However, its application for the determination hydrocarbons has not yet been reported. In this study, associative ionization of nine typical monoterpenes induced by dichloromethane (CH2Cl2) under vacuum ultraviolet (VUV) light was investigated using a laboratory-constructed PAI time-of-flight mass spectrometer (PAI-TOFMS). Without doping CH2Cl2, VUV light generated ordinary photoionization mass spectra of monoterpenes that were dominated by fragment ions, while the molecular ions were inconspicuous. In contrast, doping with CH2Cl2 resulted in the appearance of mass peaks of the characteristic protonated molecular ions of the monoterpenes. The PAI method significantly enhanced the signals for the monoterpenes. The intensities of protonated molecular ions derived from PAI were amplified by 52 to 108 fold compared with those of molecular ions produced by VUV photoionization (VUV PI). With the high ionization efficiency of PAI toward monoterpenes, the sensitivities of PAI-TOFMS toward limonene, β-ocimene, β-myrcene, β-pinene, and α-pinene were from 291 ± 6 to 1338 ± 20 counts pptv−1 using an acquisition time of 30 s. The limits of detection were from 0.32 to 1.45 pptv, equivalent to between 1.92 and 8.83 ng m−3. The applicability of this ionization mode was further characterized by the determination of monoterpenes from potted Sabina chinensis. This study provides a new and highly efficient ionization method for the determination of plant monoterpenes using mass spectrometry.