Abstract: The present invention provides methods for analyzing a peptide or peptides of interest in a protein sample using a combination of a relatively generic isotope tag with a decoupled selection process, allowing simplified customization of the application with a single reagent. These methods comprise providing a first and a second protein sample; labeling the first protein sample with a first Universal Peptide Isotope Tag (U-PIT) reagent and the second protein sample with a second U-PIT reagent; separating the peptide of interest from the combined first and second protein samples; and determining the relative amount of the first U-PIT reagent and the second U-PIT reagent bound to the peptide or peptides of interest. The U-PIT label of the present inventive methods has the following general formula A-B-C wherein A is a nucleophilic reactive group, B is a detectable moiety that can be isotopically labeled, and C is a charge replacement group.