helium ionization detector in gas chromatography

A weak beta source and a high potential raise the helium atom of the carrier gas to a metastable state. All other substances having an ionization potential lower than $18 eV$ are ionized, and the current that results is used to measure the components. The detector is usually employed to measure inorganic compounds at concentrations between $0.1$ and $10 ppmv$ . It has a linear range of about $10 2$ but is somewhat unstable and requires great care to ensure the helium purity and to eliminate all leaks in the system.

Source:

PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2191