Protonation enhancement by dichloromethane doping in low-pressure photoionization

TL;DR

This study reports a novel protonation enhancement in low-pressure photoionization using dichloromethane doping, significantly improving detection efficiency for certain analytes despite CH2Cl2's high ionization energy.

Contribution

The paper introduces a previously undocumented phenomenon where dichloromethane doping enhances protonation in low-pressure photoionization, proposing a new protonation mechanism involving ion-pair and hydrogen-bond formations.

Findings

Protonation of water, methanol, ethanol, and acetaldehyde increased by over 80 times.

Dichloromethane does not produce significant ions itself.

The phenomenon suggests new methods to improve VUV photoionization detection efficiency.

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.