Purity is not content
The most expensive misreading in peptide testing is a one-word substitution: reading a purity percentage as though it described how much is in the vial. It never has.
A client sends in a vial labelled 10 mg. The report comes back at 98.7% purity, identity confirmed. Everything reads well. Six weeks later the same client is asking why a second laboratory says the vial contains 3.4 mg.
Both laboratories are right. They measured different things, and only one of them measured the thing the client actually cared about.
For a comparison between reports, first check the units and reporting basis. See why two results can differ before comparing percentages with milligrams or combining findings from separate vials.
What area purity is counting
A reversed-phase chromatogram is a record of detector response against time. Components leave the column at different times, each producing a peak, and the area under each peak is proportional to how much of that component passed the detector. Area purity is a ratio: the principal peak’s area divided by the total integrated area, expressed as a percentage.
Read that denominator carefully. It is the total detected response: everything the detector responded to at the stated wavelength. It is not the total mass in the vial. Anything that does not absorb at that wavelength contributes weight to the powder and nothing at all to the calculation.
Things that are in the vial and are not in the denominator:
- Counter-ion: acetate or trifluoroacetate paired with the charged peptide, and real mass in the powder
- Residual moisture retained in the lyophilized cake, which grows every time the vial is opened
- Inorganic salts and buffer residue from the final processing steps
- Bulking agents such as mannitol, where one has been used
- Simply less peptide than the label says, with the vial not filled to claim
This is why a purity figure cannot go down when a vial is underfilled. Underfilling removes material from both the numerator and the denominator in the same proportion. The ratio does not move.
What content measures instead
Content is an absolute measurement. The analyte response from the sample is compared against a calibration curve built from a reference standard of known concentration, and the result is back-calculated through the dilution scheme to an amount for the vial as submitted. The output is milligrams, not a percentage.
That difference in kind is why the two tests are priced differently. Purity compares parts of one sample against each other and needs no external anchor. Content needs a reference standard of assigned value, a demonstrated linear range, standards bracketing the sample response, replicate preparations, and a recovery check to show the preparation is actually releasing the analyte. The cost sits in the calibration infrastructure.
The basis problem
Once you are measuring milligrams, a second question appears that purity never has to answer: milligrams of what, exactly?
A peptide supplied as an acetate salt is a peptide plus its counter-ion. Content expressed as peptide base excludes the counter-ion and residual moisture. Content expressed as gross salt includes them. The same vial, measured on the same instrument in the same run, produces two different numbers depending on which basis is used, and the gap is not small. On a short peptide, counter-ion and moisture together can account for a meaningful fraction of vial weight.
A content figure with no stated basis is therefore not a complete result. Canada Peptide Testing settles the basis at scope review and prints it on the certificate, because a reader comparing two reports needs to know whether they are comparing like with like.
A third thing neither of them is
Purity and content are both chemical measurements. Neither is potency. Potency is a biological activity, established by a bioassay against a reference preparation, and it is expressed in international units. A product labelled in units is making an activity claim, and no amount of chromatography converts to it.
Where a submission arrives labelled in international units, this laboratory reports mass-based content and says explicitly on the certificate that the figure is not a potency result. That is a narrower answer than the label asks for, and stating the narrowness is the point.
How to decide what to order
- Is it the right molecule? That is identity: the sample compared against reference data for the compound named at scope review. Nothing else answers it, and a pure sample of the wrong compound is still the wrong compound.
- Is it clean? That is purity: area percentage under stated conditions, with the resolved related substances reported alongside.
- Is there as much of it as the label says? That is content, and only content. If this is your question and you order a purity panel, you will receive a technically correct report that does not answer it.
The standard peptide analysis answers the first two, because identity and purity are reported together. The third is an addition: quantitative content is not run by default, it has to be requested, and it depends on a suitable reference standard being obtainable for the compound. Where the label claim is not in question (a research group checking a synthesis, a formulator confirming a raw material is what the supplier says), the standard analysis on its own is the right scope and the cheaper one.
Reading someone else’s certificate
Applying this to a COA you were handed rather than one you commissioned: find the results table and check what unit each row is in. A row in percent is a purity row. A row in milligrams or a concentration is a content row. A certificate carrying only percentages is telling you about composition and is silent on quantity, however impressive the number is.
Then check that the content row, if there is one, states its calculation basis and the reference standard used. A milligram figure with neither is a number without a foundation.
Further reference
Sources and related guidance for this article:
- ICH Q2(R2): Validation of Analytical Procedures
Framework for evaluating analytical procedure performance; this reference does not certify CPT or an individual result.
- CPT testing methods and reporting scope
Published service scope and technique-specific reporting information.
Common questions
Can a peptide be 99% pure and still underdosed?
Yes, and it is common. Area purity describes the composition of the material the detector saw, expressed as the principal peak’s share of total detected response. Content describes how many milligrams of target peptide are in the vial. A vial holding 3 mg of a 10 mg label claim can report 99% purity, because the 7 mg that is missing was never in the denominator of the purity calculation.
Which test do I need: purity or content?
If the question is "is this the right compound and is it clean", the standard peptide analysis answers it, since purity and identity are reported together. If the question is "am I getting what the label says", you need content, which is a separate measurement and has to be requested. Where a label claim is in play, the usual recommendation is the peptide analysis with quantitative content added.
Why is content testing more expensive than purity testing?
Purity is a relative measurement: the instrument compares parts of one sample against each other. Content is an absolute measurement and requires a reference standard of known assigned value, a calibration curve across a working range, bracketing standards, replicate preparations, and a recovery check. The extra cost is the calibration infrastructure, not extra instrument time.
What is the difference between peptide base and gross salt?
Two different bases for expressing the same measurement. Peptide base excludes counter-ion and residual moisture; gross salt includes them. The same vial gives two different numbers depending on which is used, which is why the basis is agreed at scope review and printed on the certificate rather than left for the reader to guess.