A Pulsed Discharge Photo-Electron Ionization Detector for Gas Chromatography

Abstract

A novel pulsed discharge photoelectron ionization detector (PDPEID) was developed and evaluated for gas chromatographic detection. The detector is derived from a previously developed pulsed discharge helium ionization detector (PDHID), which uses high-energy photons generated by a pulsed electrical discharge as the ionization source. In the PDPEID, electrons produced from the same discharge are additionally utilized to ionize analytes, thereby significantly enhancing detector response. Because the electron flux generated during discharge is substantially greater than the photon flux, the PDPEID produces a much larger detector signal, with an average response approximately 20–40× higher than that of the PDHID. Although the increased background current results in higher noise levels, the overall sensitivity for n-pentane improved by approximately 1 order of magnitude relative to the PDHID. The photoionization and electron-ionization signals can be temporally resolved because of the large difference in transport velocities between photons and electrons within the detector cell. Electrons generated through photoionization are collected more rapidly than those produced through electron-ionization processes. Consequently, the photoionization signal appears nearly simultaneously with the discharge pulse, whereas the electron-ionization signal is delayed by approximately 40 μs, depending on the applied bias potentials. Because electron-ionization involves lower-energy electrons and electron-capture interactions distinct from those of photoionization, the relative magnitudes of the two signals provide complementary chemical information. The ratio of photoionization to electron-ionization response may therefore provide a useful semiqualitative parameter for differentiating compounds based on ionization potential and electron affinity.