detection limit #00151OIML (1) The detection limit is sometimes defined as an output signal equal to some other multiple (e.g., two or ten) of the noise level [2]. The noise level is the random background output signal of an instrument when sampling a blank or clean filtered air. (2) In this Document, the concentration of an airborne pollutant in either a gas or vapor phase is most often expressed in terms of the volume fraction of the pollutant in air: parts- per-million (ppm) [v/v (10-6)]. Conditions of