A method of mass or mobility spectrometry comprising obtaining one or more sample spectra for a sample. The one or more sample spectra are subjected to pre-processing and then multivariate and/or library based analysis so as to classify the sample. Before the sample spectra are acquired, a library of background spectra, each background spectrum relating to a certain class of sample material, is constructed. The background spectra in this library are used to subtract the background from a sample spectrum during the pre-processing of this sample spectrum.