Protonation enhancement by dichloromethane doping in low-pressure photoionization.

Jinian Shu,Yao Zou,Bo Yang,Peng Zhang,Pengkun Ma,Haixu Zhang,Wanqi Sun,Ce Xu,Zhen Li

doi:10.1038/srep36820

Abstract

Doping has been used to enhance the ionization efficiency of analytes in atmospheric pressure photoionization, which is based on charge exchange. Compounds with excellent ionization efficiencies are usually chosen as dopants. In this paper, we report a new phenomenon observed in low-pressure photoionization: Protonation enhancement by dichloromethane (CH2Cl2) doping. CH2Cl2 is not a common dopant due to its high ionization energy (11.33 eV). The low-pressure photoionization source was built using a krypton VUV lamp that emits photons with energies of 10.0 and 10.6 eV and was operated at ~500–1000 Pa. Protonation of water, methanol, ethanol, and acetaldehyde was respectively enhanced by 481.7 ± 122.4, 197.8 ± 18.8, 87.3 ± 7.8, and 93.5 ± 35.5 times after doping 291 ppmv CH2Cl2, meanwhile CH2Cl2 almost does not generate noticeable ions itself. This phenomenon has not been documented in the literature. A new protonation process involving in ion-pair and H-bond formations was proposed to expound the phenomenon. The observed phenomenon opens a new prospect for the improvement of the detection efficiency of VUV photoionization.

Highlights

Low-pressure photoionization (LPPI), defined as photoionization running under hundreds to thousands of Pa, has not been used as widely as Atmospheric pressure photoionization (APPI) and conventional vacuum PI
We report a new phenomenon: The detection efficiencies of the three small volatile organic compounds (VOCs) can be remarkably enhanced via
Protonation of acetonitrile (IE = 12.20 eV) was observed in APPI with a krypton lamp as the VUV light source by Marotta et al The authors speculate that photon irradiation leads first to the isomerization of acetonitrile molecules, affording species that exhibit ionization energy (IE)

Summary

Introduction

Low-pressure photoionization (LPPI), defined as photoionization running under hundreds to thousands of Pa, has not been used as widely as APPI and conventional vacuum PI. The use of dopants has been found to be very effective for enhancing the ionization efficiency of analytes[6,7,9,10,11] in APPI and LPPI12 via charge exchange: www.nature.com/scientificreports/. The resulting analyte ions may subsequently react with other molecules via proton transfer. The detection sensitivity could be enhanced by ~100 times via doping[22] These dopants cannot be applied to the detection of methanol (CH3OH, IE = 10.84 eV), ethanol (C2H5OH, IE = 10.48 eV), and acetaldehyde (C2H4O, IE = 10.23 eV) due to their higher IEs. Dichloromethane has been chosen as a dopant for characterizing the molecular structures of analytes via secondary ion–molecule reactions, rather than for enhancing ionization efficiency[23]. The results and experimental method are described

Methods

Results

Conclusion