Absolute expression using quantms

Absolute quantification values (e.g., how many micromoles of a peptide is contained in a sample) are inherently more complex to obtain. One reason is that different peptides have different physio-chemical properties and ionization efficiency. To obtain absolute quantities isotope-labeled standards of known concentrations can be added during sample processing. The relative quantity of a peptide can be matched to the calibration curve of its isotopelabeled version and converted into an absolute quantity.