Abstract: A method for determining a threat substance encountered by a time-of-flight mass spectrometer (TOF-MS) using a pre-computed threat library is described. The method comprising the steps of acquiring a spectrum of a test substance, wherein the acquired spectrum is an average of individual spectra acquired from a plurality of laser shots on the analyte; identifying mass/charge (m/z) values corresponding to each of a plurality of spectral peaks of the acquired spectrum; assigning a corresponding ranking code to the acquired spectrum based on the plurality of its spectral peaks and troughs, wherein a peak presence is indicated by a numeral 1, while peak absence is indicated by a numeral 0, relative to each of a set of substances in a threat library; comparing the assigned rankings of the acquired spectrum over all threat substances stored in the threat library; and identifying the threat substance as that which produced the highest ranking.