A method of quantifying the concentration of an endogenous central nervous system biomolecule in a subject, wherein the labeled amino acids incorporate into the endogenous central nervous system biomolecule comprising: (a) providing a biological sample previously obtained from the subject having been previously administered at least one labeled amino acid, wherein the at least one labeled amino acid incorporates into the endogenous CNS biomolecule such that the biological sample comprises a metabolically labeled fraction of the endogenous CNS biomolecule and an unlabeled fraction of the endogenous CNS biomolecule, wherein the endogenous CNS biomolecule is an amyloid-beta (Aβ) or isoform, variant or digestion product thereof; (b) contacting the biological sample with a Quantitation Standard comprising a known concentration of a biomolecule labeled with at least one moiety such that the QS has a molecular weight that differs from the at least one labelled amino acid administered to the subject, wherein a trypsin digest of the QS comprises Aβ 17-28 and 29-40 peptides; (c) isolating the metabolically labeled, and unlabeled endogenous CNS biomolecule from the biological sample, wherein the QS is different from the metabolically labeled and unlabeled endogenous CNS biomolecule; and (d) calculating the concentration of the endogenous CNS biomolecule in the biological sample using a first and second standard curve, wherein the first standard curve is generated using Aβ peptides with known ratios of +6D leucine to unlabeled leucine or using cell media with known ratios of labeled to unlabeled Aβ 29-x peptides, and wherein the second standard curve is generated using known quantities of synthetic or biologically generated standard. (a) providing a biological sample previously obtained from the subject having been previously administered at least one labeled amino acid, wherein the at least one labeled amino acid incorporates into the endogenous CNS biomolecule such that the biological sample comprises a metabolically labeled fraction of the endogenous CNS biomolecule and an unlabeled fraction of the endogenous CNS biomolecule, wherein the endogenous CNS biomolecule is an amyloid-beta (Aβ) or isoform, variant or digestion product thereof; (b) contacting the biological sample with a Quantitation Standard comprising a known concentration of a biomolecule labeled with at least one moiety such that the QS has a molecular weight that differs from the at least one labelled amino acid administered to the subject, wherein a trypsin digest of the QS comprises Aβ 17-28 and 29-40 peptides; (c) isolating the metabolically labeled, and unlabeled endogenous CNS biomolecule from the biological sample, wherein the QS is different from the metabolically labeled and unlabeled endogenous CNS biomolecule; and (d) calculating the concentration of the endogenous CNS biomolecule in the biological sample using a first and second standard curve, wherein the first standard curve is generated using Aβ peptides with known ratios of +6D leucine to unlabeled leucine or using cell media with known ratios of labeled to unlabeled Aβ 29-x peptides, and wherein the second standard curve is generated using known quantities of synthetic or biologically generated standard.