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 $18\text{eV}$ 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 $10\text{ppmv}$. 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.