How do you calculate theoretical net peptide content?

Theoretical net peptide content (calculated assuming that counterions are the only non-peptide components present in your peptide sample) can be estimated by dividing molecular weight of the peptide by a sum of this molecular weight and a number of trifluoroacetate counterions that are required to neutralize the peptide multiplied by the molecular weight of the TFA counterion (MW= 114). For example, a synthetic peptide of MW=1000 with a free N-terminal amino group and one Arg has theoretical net peptide content of 1000/(1000 + 2 x 114 ) = 1000/1228 =0.81 or 81%. In practice, counterions are not the only possible contaminants in the peptide sample. It can also contain water, absorbed solvents and traces of other substances. As a result, the actual net peptide content is usually determined by quantitative amino acid analysis.