Everything you need to know
Purity is a ratio. Content is a quantity. They answer different questions, and the one almost every certificate reports is not the one most buyers think they are reading. A vial can be 99% pure and hold half the peptide its label claims.
What is the difference between peptide purity and content?
Purity is a ratio: what fraction of the peptide material present is the peptide you wanted. Content is a quantity: how many milligrams of that peptide are actually in the vial. They are independent. A vial can be 99% pure and hold 3 mg where the label says 5 mg, because purity says nothing about how much material was put in the vial. Purity is normally measured by reverse-phase HPLC with UV detection, reported as the target peak area over total integrated peak area. Content is measured against a reference standard of known concentration, and reported in mg. If you only have a purity number, you do not know what you bought.
Can a vial be 99% pure and still be underfilled?
Yes, and this is the most common failure mode in the research peptide market. Purity is blind to fill weight. If a vial is filled with 3 mg of 99% pure peptide and labelled 5 mg, the purity result is a completely accurate 99% and the customer is short 40% of what they paid for. Independent analyses of large numbers of vendor reports have repeatedly found that most out-of-spec vials fail on quantity rather than purity. The purity number is the one the market advertises; the content number is the one that determines what is in the vial.
How is peptide content actually measured?
Content is measured by quantitative HPLC against a reference standard of the same peptide at a known concentration. The sample response is compared to the standard response, and the result is scaled by the standard's assigned value. Two things determine whether the answer is trustworthy. First, the reference standard has to be characterised for how much peptide it actually contains, not just how pure it is; a standard certified only for chromatographic purity will bias every content result upward. Second, the whole vial has to be reconstituted and the reconstitution weighed, otherwise you are reporting a concentration rather than mg per vial.
Why do purity and content disagree with the label in different directions?
They fail for different reasons. Low purity means the synthesis or the purification left related substances behind: truncated sequences, deletion sequences, oxidised or deamidated forms, residual solvent-related species. Low content usually means a fill or a formulation problem, or that the mass in the vial includes counterion and water that the label counted as peptide. A peptide supplied as an acetate or trifluoroacetate salt carries counterion mass, and for some peptides that is a meaningful fraction of the total. If a certificate reports mass without saying what basis it is on, the number is ambiguous.
Which number should I ask a lab for?
Ask for both, and ask what basis the content number is reported on. A useful certificate states purity as a percentage of total peak area with the detection wavelength named, content as mg in the vial with the reference standard identified, and the basis for the content calculation. If a lab quotes content as an add-on, ask whether the reference standard is characterised for content or only for purity, because that single choice moves every result. At Prodigy Labs content is included rather than sold as an add-on, and the certificate states the basis.
Does a higher purity percentage mean a better product?
Not on its own. Purity above roughly 98% is common across the market and does not differentiate much. What differentiates a vial is whether the declared compound is actually the one present, whether the quantity matches the label, and whether an injectable is free of endotoxin. A certificate that reports 99.2% purity and nothing else has answered the easiest question and skipped the expensive ones. See what identity, purity and content cost.