The invention provides methods for identifying and quantifying polypeptides in a sample. The methods include the steps of labeling peptides in a polypeptide sample with an isotope tag; adding a plurality of peptide standards to the polypeptide sample, wherein the peptide standards are labeled with an isotopically distinct version of the isotope tag; resolving the labeled sample and standard peptides into a plurality of fractions; analyzing the resolved fractions using mass spectrometry; identifying an isotope-tagged sample peptide in an analyzed fraction; and determining the amount of the identified isotope-tagged sample peptide in the analyzed fraction by comparison to the amount of isotope tagged standard peptide in the same fraction.