At the higher concentrations, response gradually deviates from linearity because of recombination of oppositely charged ions formed in close proximity and/or 2) absorption of UV light without ionization. The signal produced by a PID may be quenched when measuring in high humidity environments, or when a compound such as methane is present in high concentrations of ≥1% by volume This attenuation is due to the ability of water, methane, and other compounds with high ionization energies to absorb the photons emitted by the UV lamp without leading to the production of an ion current. This reduces the number of energetic photons available to ionize target analytes. References Measuring instruments Gas chromatography