What an Independent Test Is Actually Measuring
On its own the phrase “third-party tested” carries almost no information. It sits equally comfortably above a document that demonstrates a great deal and above one that demonstrates very little, and telling those apart is a matter of knowing which figures to go looking for on the page in front of you.
Find the wavelength before anything else
Of everything printed on a chromatogram, the detection wavelength is the most informative single item and the one readers skip most reliably. Peptide work is normally done at 214 nm or 220 nm, where the peptide bond itself absorbs, which means anything peptidic in the vial registers. A minority of certificates instead report 280 nm, and at that wavelength the only appreciable absorbers are tryptophan, tyrosine and, weakly, phenylalanine.
Follow the consequence through. A sequence containing none of those three is close to invisible at 280 nm, and so is every impurity that also lacks them. A 99% result produced at 280 nm on an aromatic-free sequence has not measured purity in any useful sense. The number is printed on the chromatogram. Go and find it first, because it determines how much the rest of the document is worth.
Identity and proportion: two questions, two instruments
Underneath every certificate are two separate enquiries. Is this the compound the label names, and what share of the material in the vial is that compound? The first belongs to mass spectrometry, the second to chromatography, and a report addressing only one of them has done half the job. In practice the half that goes missing is usually the second.
Electrospray ionization yields a measured mass that can be laid against the mass computed from the sequence. Agreement to within roughly a dalton is a strong identity finding, and the reason it is strong is that the ways a synthesis commonly goes wrong all move the mass by recognizable amounts.
| Discrepancy | What it corresponds to |
| About 1 Da | A C-terminal amide where a free acid was expected, or the reverse |
| 0.98 Da heavier | Deamidation at an asparagine or glutamine |
| 16 Da heavier | An oxidized methionine |
| 18 Da heavier | A cyclic peptide whose ring never closed, since closure expels a molecule of water |
Each of those is a different substance from the one named, and each shows up at its own predictable offset. The purity side works differently. An HPLC purity figure is the area of the main peak divided by the summed area of every peak the detector saw, which is not the weight fraction of the vial’s contents. Equal quantities of two compounds need not give equal areas, since how strongly something absorbs depends on its structure. Area percent is a serviceable stand-in and it is what the industry uses, but a stand-in is all it is.
How much of the vial is peptide at all
Purity compares the peptide against other peptide-like material in the same sample. It is silent on the question of how much of the powder is peptide in the first place, and the balance of that powder is counter-ion, residual water and any excipient present. Net peptide content is the figure that accounts for it, arrived at by amino acid analysis or by nitrogen determination.
Trifluoroacetate salts commonly come in somewhere between 70% and 85% on that measure. So a vial can be 98% pure and 78% peptide simultaneously, with neither figure dishonest. When a certificate gives purity and omits net content, what it has told you is how clean the peptide is, not how much of it you received; the arithmetic appears in net peptide content.
The counter-ion deserves a word of its own, because it is easy to misread as contamination. Reversed-phase purification uses trifluoroacetic acid, and TFA leaves the column alongside the peptide as its counter-ion. It is not a synthesis failure but an expected constituent of the isolated salt. What it does do is add mass, generally in the low tens of percent, and that is one reason the acetate and trifluoroacetate forms of a single peptide read differently on the same balance. The differences between them are set out in TFA and acetate counter-ions.
Water is the third occupant. Freeze-drying takes out most of it and never all of it, so a few percent typically remains, and that quantity is measured by Karl Fischer titration rather than assumed. It counts twice: it is mass in the vial that is not peptide, and the dried solid is hygroscopic enough that a vial opened cold and left standing will take on more. Water content in lyophilized peptides covers the determination.
Whose sample, and whose column
Any certificate is a statement about one particular sample that reached one particular laboratory. About every other vial it says nothing directly, only by inference, and the strength of that inference rests on who chose the sample and how it got there. A report generated from material the supplier selected and shipped is a different kind of evidence from one generated from material drawn out of stock by whoever paid for the test. Neither is worthless and the two are not interchangeable, and the document ordinarily records which of them occurred. Third-party versus in-house testing works the distinction through.
Column choice matters just as much. Two laboratories can analyze one material and report different purity figures without either being mistaken, because column chemistry, gradient shape, run length, wavelength and integration settings all push the result around; a shallow gradient will pull out a shoulder that a steep one tucks inside the main peak. Disagreement between two reports is therefore information rather than grounds for dismissal. The usual causes are collected in why two certificates can disagree, and what a shoulder looks like is shown in reading an HPLC chromatogram.
Where the document stops, and how to go further
A routine identity-and-purity certificate says nothing whatever about sterility, nothing about endotoxin, and nothing bearing on use in a person or an animal. Those require separate tests that form no part of a standard peptide report, and their absence reflects the fact that they were never run rather than any oversight. Treating a purity percentage as a safety statement is the single most frequent misreading of these documents; the extended treatment is endotoxin, sterility and the limits of research grade.
Anyone in the USA willing to pay for it can commission verification independently. Analytical laboratories take submitted samples for identity and purity work, and the version of that exercise worth doing looks like this:
- Pick the material yourself, out of your own stock.
- Specify the detection wavelength rather than leaving it to the laboratory.
- Ask for the chromatogram itself, not a summarized figure.
It is the one form of third-party testing in which every step is under your control, and it tends to cost less than people expect.
What we publish
The third-party reports we have obtained are published on our certificates of analysis page, reproduced as the laboratory issued them instead of being reduced to a single figure. Certificates do not travel in the box; a vial is matched to its report by comparing cap and crimp color against the photographs the report contains. What we will not do is attach a purity figure to a claim about what a compound does, in a person or anywhere else. The material is supplied for laboratory research use only.