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, by a gas detector processor from a detector, a detector signal, the detector signal being a spectrum of the illuminated gas sample, generating, by the gas detector processor, a background, the background defined by a mean and a covariance, converting, by the gas detector processor, the detector signal into spectrum data, screening, by the gas detector processor, the spectrum data for a detected chemical, wherein screening the spectrum data comprises screening the spectrum data using the mean and covariance of the background, adding, by the gas detector processor, the spectrum data to the background when the detected chemical is not a threat, and wherein the spectrum data is old spectrum data, the method further comprising the steps of, delaying the adding of the old spectrum data to the background, repeating the receiving, converting, and screening steps for generating new spectrum data, the screening step indicating the new spectrum data is a threat, and skipping the adding step.