Fig. 1. A schematic diagram showing the dual-inlet, single-outlet _HDID. The ana-lytes from a micromachined separation column are introduced at the top of the biaselectrode, bypassing the auxiliary channel fed helium microdischarge. Ionization ofanalytes in the region between the collector and bias electrodes results in the detec-tor response at the collector electrode. The parameters of interest, namely l, w, Vp,and Vbare denoted.larger than that found in microGC and a power dissipation on theorder of tens of watts [39].Commercially available _GC systems have adopted a hybridapproach wherein the detector is similar in style to conventionalultraviolet photoionization detectors (UV-PID). These systems offerexcellent detection sensitivity, but are somewhat restricted by thephotoionization energies available (<11.7 eV with argon lamps) aswell as incorporation into a _GC system. Micro-discharges or plas-mas have also been utilized in gas detectors since 1991. Eijkel et al.reported on a detector for _GC that fragmented the analytes in aDC microplasma to produce diatomic fragments from which emis-sion was detected spectrophotometrically [40]. Improvements onthis technique included an innovative electrode structure to gener-ate a pulsed plasma with drastically reduced power consumption[41,42]. Spectrophotometric detection is an intensive operationthat can consume power on the order of watts. An alternative isto monitor the current through the discharge itself as reported byFu et al. [43]. However, a common concern with these designs isthe fouling of the electrodes due to fragmentation of the analytes.Fragmentation also does not allow for the analytes to be subjectedto further analysis.We previously reported on a proof-of-concept micro-heliumdischarge detector [44] to address the need for a sensitive, low-power, easy-to-fabricate universal detector. This microdischargeutilizes high-energy photons and excited state helium metastablespecies to ionize the analytes and the resultant current monitoredon a remote collector electrode. Their lack of sensitivity to temper-ature makes them suitable for robust gas detection systems [45].A limit of detection (LOD) of 350 pg for octane was demonstrated.Additional design parameters for our _HDID are considered here.Specifically, the effect of the He discharge voltage, distance ofthe bias electrode from the He discharge, collector electrode frombias electrode, and the bias voltage are parametrically studied.The results were used to choose the best design to enhance theLOD.
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