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 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 and . It has a linear range of about but is somewhat unstable and requires great care to ensure the helium purity and to eliminate all leaks in the system.