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

Abstract

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.

Keywords:

• Associative ionization
• monoterpenes
• online detection
• photoionization
• time-of-flight mass spectrometry (TOFMS)

Disclosure statement

The authors report there are no competing interests to declare.

Author contributions

Zuojian Zhang: investigation, data curation, writing-original draft, and writing-review and editing. Haijie Wang: writing-review and editing. Bo Yang: conceptualization, methodology, writing-review and editing, supervision, and funding acquisition. Jinian Shu: conceptualization, methodology, supervision, writing-review and editing, and funding acquisition. Zhangqi Yu: writing-review and editing. Zhiyang Wei: writing-review and editing. Jingyun Huang: investigation and data curation. Kui Jiang: investigation and data curation. Yedong Guo: investigation and data curation. Zhen Li: resources and funding acquisition.

Additional information

Funding

This work was supported by the National Key Research and Development Program of China under Grant 2019YFC1806105; the Research Equipment Development Project of Chinese Academy of Sciences under Grant YJKYYQ20180072; the National Natural Science Foundation of China under Grant 21777170 and 22076184; the Beijing Municipal Science and Technology Commission under Grant Z181100003818008; and the Fundamental Research Funds for the Central Universities under Grant Y954021.