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Net peptide content
Two different numbers get read as though they were the same one. Purity tells you what fraction of the material is the compound you ordered. Net peptide content tells you what fraction of the vial’s weight is peptide at all. A lot can be 99% pure and 70% peptide, and both figures can be perfectly honest.
- What purity measures
- The share of chromatographic peak area attributable to the main peak. It is a relative figure: it describes the composition of whatever is in the vial, and says nothing about how much of it there is.
- What net content measures
- The share of the lyophilised mass that is actually peptide. The balance is water, counter-ions left from synthesis, and residual solvent. This is an absolute figure, and it is the one that corresponds to what you weighed out.
- Why a gap exists at all
- Lyophilised peptides are hygroscopic, so they hold water. Purification leaves counter-ions bound to basic residues. Neither is contamination, and neither shows up as an impurity peak, because chromatography is measuring something else.
- Why it decides what you paid
- You buy by weight. A vial labelled 10 mg at 70% net content holds about 7 mg of peptide. Two suppliers quoting the same price per milligram are not quoting the same thing, and the one who never mentions net content is quoting the more flattering number.
What to ask for
Ask for net peptide content stated separately from purity on the certificate, and ask which method produced it — typically nitrogen determination, amino acid analysis, or calculation from measured counter-ion content. A supplier who reports purity to two decimal places and has no figure for net content has measured the easy thing.
Our certificate check asks for this figure and flags its absence, and it refuses the combination of a net content higher than the purity, which cannot describe the same lot. The mass arithmetic behind it is in counter-ions and salt forms.