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 Pointed electrode Arc window Sample-laden carrier gas inlet Makeup gas inlet HID detector between TCD and FID detectors on an SRI GC Close-up of the same HID detector Detector body Flat electrode 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 17.7eV are ionized upon contact with photons from the helium cloud. The ionized component molecules are then attracted...

DETECTORS Helium Ionization Detector - HID filu tdil Yinw flcquirtion Melt? |Q s£B ■ g|||T 2 3 * HID noise run Columns: 1 m Mol. Sieve, 2m Hayesep-D Carrier: Helium @ 10mL/min HID gain = HIGH HID current = 70 HID temp = 200oC Temperature Program: Initial Hold Ramp Final Method: valve injection Column: 1 m (3’) Silica Gel Carrier: Helium @ 10mL/min HID gain = HIGH HID temp = 150oC HID make-up = 29psi @ 40mL/min

DETECTORS HID - Helium Ionization Detector General Operating Procedure 1. Set the HID amplifier gain switch to HIGH for most applications from the ppm level to 1%. Use the MEDIUM gain setting for slightly more concentrated samples. 2. Set the helium make-up gas flow to 40mL/min, and the helium carrier gas flow to 10mL/min. The make-up gas flow is critical to the HID’s performance. With insufficient make-up flow, the chlorinated peaks will be inverted on the chromatogram; see the chromatograms compared on the HID Make-up Gas Flow page. Clean, high purity helium is best; moisture, air, and other...

HID Make-up Gas Flow The following chromatograms were produced by an SRI HID equipped GC. Excepting the make-up gas flows, all run conditions are identical. The first chromatogram resulted from a make-up gas flow of 20mL/min. Drastically different in appearance from the first, the second chromatogram was produced with a make-up gas flow of 10mL/min. In the absence of sufficient make-up gas flow, the chlorinated peaks are negative. Not every HID has the same optimum make-up flow; experiment with different flow rates until you find the best range for your detector. Temperature program: Initial Hold Ramp...

DETECTORS HID - Helium Ionization Detector Cleaning the HID If your HID baseline seems noisy, try cleaning the electrodes following the steps below. Over time, the HID electrodes can develop a coating of soot, which can cause the arc to flicker or change position, resulting in sudden baseline jumps. Green wire 1. Unclip the amplifier lead and slide it off the collector electrode. Unclip and remove the leads from the pointed and flat electrodes (note that the green wire is connected to the pointed electrode, and the red wire is connected to the flat electrode). 2. Remove the the arc and ground...