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The most used algorithms for the identification of electron-ionization mass spectra are INCOS and probability based matching (PBM). For unknown spectra of high purity, approximately 75% of rank 1 answers are correct for both algorithms, matched against the National Institute of Standards and Technology 62,235 spectrum database. With matching criteria that retrieve 50% of the possible correct answers from the Wiley 228,998 spectrum database, 54% of the PBM and 42% of the INCOS answers are correct; for 85% purity unknowns, 48% and 27% are correct. For an unknown spectrum of two compounds, neither was reported in the first three INCOS answers; eight of the first ten PBM answers identify both components.

Statistical evaluations of the effects of five matching parameters on the probability of retrieving a correct answer with the probability based matching (PBM) system have been made. Combining the resulting values found In matching an unknown spectrum makes It possible to rank retrieved reference spectra according to the predicted match reliability. This ranking substantially improves the performance of PBM, and the reliability value Is especially helpful In avoiding the assumption that the best matching spectrum represents the correct compound when Its spectrum Is actually not in the reference file. Quadratic scaling of the abundance values of the unknown compensates for spectral differences caused by Instrumental variations, a critical problem In matching reference spectra. Other improvements Include a more effective "flagging" technique to remove spurious reference peaks. Extensive applications with a commercial GC/MS system have demonstrated the Increased effectiveness made possible by these PBM modifications.

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