Patent ID: 6864099

Claim:
A method for detecting a difference in the concentration of a protein present in a first sample and in a second sample, each sample comprising a plurality of proteins, the method comprising: covalently attaching a first isotopic variant of a chemical moiety to a protein in the first sample to yield at least one first isotopically labeled protein; covalently attaching a second isotopic variant of the chemical moiety to a protein in the second sample to yield at least one second isotopically labeled protein, wherein the first and second isotopically labeled proteins are chemically equivalent yet isotopically distinct; mixing at least portions of the first and second samples to yield a combined sample; and subjecting the combined sample to mass spectrometric analysis to determine a normalized isotope ratio characterizing proteins whose concentration is the same in the first and second samples and an isotope ratio of the first and second isotopically labeled proteins, wherein a difference in the isotope ratio of the first and second isotopically labeled proteins and the normalized isotope ratio is indicative of a difference in concentration of the protein in the first and second samples.