The Helium Ionization
Detector is a "universal" detector which responds to all molecules
except neon. The HID is particularly useful for volatile inorganics like
NOx, CO, CO2, O2,
N2 and H2
which do not respond on the FID or other detectors.
Unlike an FID, the HID needs no hydrogen or air. Requiring only helium
carier and make-up gas, the HID delivers sensitivity in the low ppm range.
For labs that are reluctant to store hydrogen fuel gas for safety reasons,
the helium-only HID is a significant advantage.
The HID is especially
useful in combination with a TCD detector. The
TCD is not sensitive enough to detect low ppm concentrations, while the
HID saturates in the low percent range. When using both detectors in series,
it is possible to cover 10ppm to 100%.
Unlike other HID designs,
the SRI HID can be easily disassembled for cleaning. The HID incorporates
robust, easily servicable electrodes which support a low current arc through
the helium make-up gas flow. This elevates the helium to a metastable
state. When the metastable helium molecules collide with 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. Our
HID features a window through which the low current arc is visible, so
it is easy for the operator to verify that the detector is functioning.