HELIUM IONIZATION DETECTOR (HID)

The Helium Ionization Detector ( HID ) is a "universal" detector which responds to all molecules. The HID requires helium carrier and make-up gas, and delivers sensitivity in the low ppm range. Unlike an FID however, the HID needs no hydrogen or air. It is especially useful in combination with a TCD detector because the TCD is not sensitive enough to detect low ppm concentrations, while the HID saturates in the low percent range, so together it is possible to cover 1ppm to 100%.

The HID is particularly useful for volatile inorganics like NOX, CO, CO2, O2, N2, and H2 which do not respond on FID or other detectors. Many labs are reluctant to store hydrogen fuel gas for safety reasons, so the fact that the HID needs only helium is a significant advantage. The HID incorporates a low cost, easily replaceable electrode which supports a low current arc through the helium make-up gas flow. This elevates the surrounding helium to a metastable state. When the metastable helium molecules collide with the sample molecules as they elute from the column, the sample molecules are ionized and attracted to a collector electrode, amplified and output to the data system.

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