A gas detection system and method for analysis of infrared gas spectra is used for chemical threat detection, quantification and alarm, using a chemical library, a chemical threat list, and a background model that incorporates the data history, allows spectra containing interferent signals into the background model, the model being updated using delay buffering to prevent threat spectra incorporation and using exponential decays to preferentially represent recent background history, all computed in the logarithmic space for rapid detection and alarm.