Atmospheric Photoionization Detector with Improved Photon Efficiency: A Proof of Concept for Application of a Nanolayer Thin-Film Electrode.

Adelaide Miranda,Pieter A A De Beule

doi:10.3390/s21227738

Abstract

Atmospheric photoionization is a widely applied soft ionization mechanism in gas sensing devices for the detection of volatile organic compounds in ambient air. Photoionization is typically induced by low-pressure Vacuum Ultraviolet (VUV) lamps with MgF2 or LiF lamp surface windows depending on the gas fill and the required wavelength transmission window. These lamps are known to exhibit gradually reduced VUV transmission due to hydrocarbon contamination. LiF surface windows are known to be especially problematic due to their hygroscopic nature, reducing VUV lamp lifetime to a mere 100 h, approximately. Here, we present a new design for the electrode of a photoionization detector based on thin-film technology. By replacing the commonplace metal grid electrode’s VUV lamp surface window with a chromium/gold thin film we obtain a doubling of photon efficiency for photoionization. Replacing the hygroscopic LiF lamp window surface with a metallic layer additionally offers the possibility to vastly increase operational lifetime of low-pressure Argon VUV lamps.

Highlights

Atmospheric Pressure Photoionization (APPI) is a well-known soft ionization mechanism that induces the formation of electron-ion pairs from molecules upon the absorption of high energy photons, usually without disintegrating the molecules into multiple fragments
We deposited a thin film Cr (2.5 nm)/Au (2.5 nm) stack on a bare MgF2 glass window and compared the photoionization performance of a home-built photoionization detectors (PID) with a bare MgF2 glass window, providing an indirect measurement of the Vacuum Ultraviolet (VUV) transmission efficiency of the thin film at the VUV wavelength produced by a low-pressure Krypton lamp
Transmission efficiency, which is related to the 26% utilization of the VUV lamp emission in a state-of the art PID sensor

Summary

Introduction

Atmospheric Pressure Photoionization (APPI) is a well-known soft ionization mechanism that induces the formation of electron-ion pairs from molecules upon the absorption of high energy photons, usually without disintegrating the molecules into multiple fragments. Most frequently encountered gas fills are Xenon (Xe, 8.4 and 9.6 eV), Deuterium (D, 10.2 eV), Krypton (Kr, 10.6 eV) or Argon (Ar, 11.8 eV) At such photon energies light is commonly referred to as vacuum ultraviolet (VUV) because it is absorbed by air with an oxygen component The IPs of water (12.6 eV), acetonitrile (12.2 eV), nitrogen (14.5 eV) and helium (23 eV) are all considerably greater than the photon energies of VUV lamps, rendering them insensitive to light, while most organic molecules exhibit IPs between 7 and 10.5 eV. This makes VUV lamps ideal ionization tools for the detection of a wide variety of molecular compounds through the fact that the IPs of most carrier gasses and carrier liquids lie above the photon energy of VUV lamps

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Conclusion