only helium carrier and make-up gas, and is sensitive to the low ppm range. . With insufficient make-up flow, the chlorinated peaks will be . | DETECTORS Helium Ionization Detector - HID Overview The Helium Ionization Detector is a universal detector, responding to all molecules except neon. It requires only helium carrier and make-up gas, and is sensitive to the low ppm range. The HID is particularly useful for volatile inorganics to which the FID and other selective detectors will not respond, like NOx, CO, CO2, O2, N2, H2S and H2. It is a robust detector that, unlike the TCD, has no filaments to burn out. The SRI HID consists of a detector body, a collector electrode, an arc electrode assembly, and a thermostatted heater block which can be heated to 375oC. In SRI GCs, the HID is mounted on the right-hand side of the Column Oven. HID detector removed from GC and heater block Collector electrode HID Pointed electrode Arc Sample-laden window carrier gas inlet Makeup gas inlet Arc Detector body HID detector between TCD and FID detectors cross on an SRI GC Flat electrode Close-up of the same HID detector Support brace DETECTORS HID - Helium Ionization Detector Theory of Operation The SRI HID detector uses two electrodes which support a low current arc through the helium make-up gas flow. The helium molecules between the electrodes are elevated from ground state to form a helium plasma cloud. As the helium molecules collapse back to ground state, they give off a photon. The sample molecules are ionized when they collide with these photons. All compounds having an ionization potential lower than are ionized upon contact with photons from the helium cloud. The ionized component molecules are then attracted to a collector electrode, amplified, and output to the PeakSimple data system. Collector Ground electrode electrode Photons in the make-up gas stream Helium plasma cloud Sample-laden carrier flow Makeup gas flow Ionization reaction: Purple arc photons colliding with sample molecules Arc electrode NOTE: If the arc electrode is covered with TeflonTM (translucent) insulation, it should leave 1mm of its .
