Use spike-in controls for absolute quantitation.
For most biological applications, relative quantitation should
suffice. Sometimes, though, absolute numbers matter. The solution:
spike-in controls. Add a known quantity of an isotopically labeled form of
the desired peptide (or close surrogate), and use its abundance to
correlate peak height with molecular abundance. This peptide will behave
identically with the peptides in your samples during sample preparation
and ionization, but it can be distinguished in the instrument because of the
slight difference in size.
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Tips for Quantifying Mass Spec
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