mass defect due to the modification is very small for spectrum filtering purposes one can use the assumption that that all mass defects follow the same distribution and add this extra mass defect to one side of the mass defect filtering window an occasion where the impact of a modification may become more significant occurs when the modification has one or more large mass defect elements such as br i or cs the mass defect distribution for the modified peptides is still normally distributed and possesses the same standard deviation as that of the unmodified ones in some applications a large mass defect has been added to peptides as a mass defect tag to efficiently track the desired tagged species the amount of defect introduced in the tagged peptide determines the amount of overlap between the two mass defect distributions one for untagged peptides the other for tagged and thus determines the probability of false positive identification in the overlapping region the tagged and untagged peptides can not be distinguished resulting in possible false positive identification low abundance proteins play very important roles in biological processes an active research area is the detection of biomarker proteins very often biomarkers are associated with low abundance proteins with mass peak intensities barely above background noise levels because of this and other factors reliably identifying biomarker patterns can be very challenging if mass spectra noise can be reduced without significantly affecting peptides peaks the chance of identifying low abundance proteins will likely be greatly improved using.