Defines the maximal difference between an experimental mass and a theoretical one that is considered a match by the search. It reflects the spectrometer�s measurement and calibration errors. Mass tolerance may be entered as the absolute difference in Daltons, or as the fraction of mass in parts-per-million (ppm). The default value is in ppm units, since measurement and calibration errors in a spectrometer are proportional to the mass. Setting mass tolerance too high may compromise the quality of the search. Setting the tolerance too low may cause the search to miss peptides.