The invention provides methods for quantifying biomolecules, such as polypeptides in mass spectrometric analysis. The methods include use of a biomolecule standard having at least one atomic isotope different than that of the naturally occurring isotopes in the biomolecule of interest. Methods of the present invention also include methods for quantifying biomolecules where the copy biomolecule standard is made by expressing the biomolecule using a recombinant cell. Further included are the biomolecule standards themselves, method for making such standards, kits, systems, reagents, and engineered cells relating to the use of biomolecule standards in mass spectrometric analysis.