1. A method for detecting a threat being a threat chemical in an illuminated gas sample containing an interferent being an interferent chemical, the threat being in a threat list, interferent and threat being listed in a library of spectra of threats and interferents, method comprising the steps of, receiving a detector signal, the detector signal being a spectrum of the illuminated gas sample, generating a background, the background defined by a mean and a covariance, converting the detector signal into spectrum data, screening the spectrum data for a detected chemical, and adding the spectrum data to the background when the detected chemical is not a threat.