Ask a supplier how pure their peptide is and you will usually get a single number: 99%, 98%, sometimes an unqualified 99.9%. That number, on its own, does not tell you what you need to know. It does not say what was measured, by what method, or against what. And critically, it does not tell you how much peptide is actually in the vial.
Proper purity testing is a set of separate measurements, each answering a different question. This guide covers what those questions are.
Question one: is it the right molecule?
This is identity, and it is measured by mass spectrometry. Chromatography cannot answer it. A chromatogram showing a single sharp peak tells you the sample is homogeneous; it does not tell you that the homogeneous thing is what you ordered.
Electrospray ionisation mass spectrometry gives you the observed mass, which you compare against the theoretical mass calculated from the sequence. Agreement within the instrument's tolerance confirms identity. A discrepancy of, say, 42 Da tells you something specific — that is the mass of an acetyl group, and it means you have an acetylated variant rather than the free peptide.
Question two: how much of it is the target?
This is chromatographic purity, measured by reversed-phase HPLC and reported as area percent. The sample is separated on a stationary phase, the eluate monitored — usually by UV absorbance at 214 nm, where the peptide bond absorbs — and the area under the main peak expressed as a percentage of total peak area.
Two caveats matter. First, area percent assumes every species in the sample absorbs equally at the detection wavelength, which is an approximation. Second, and more importantly, it only counts what elutes and what absorbs. Anything that does not — inorganic salts, residual water, non-absorbing solvent — is invisible to the method entirely. This is the single most misunderstood point in peptide analysis, and it leads directly to the next question.
Question three: how much peptide is in the vial?
This is net peptide content, and it is a different number from chromatographic purity. Often a much lower one.
A lyophilised peptide vial contains the peptide, plus water the powder has taken up, plus counter-ions from the final purification step — usually acetate or trifluoroacetate. All of that contributes to the gross weight. None of it is peptide, and none of it appears in an area-percent figure.
| Component | Typical share | Counted in area %? |
|---|---|---|
| Target peptide | 78–90% | Yes |
| Water | 3–8% | No |
| Counter-ion (acetate/TFA) | 5–15% | No |
| Related substances | 0.5–2% | Yes |
A peptide can legitimately be 99.4% pure by HPLC and still be around 82% peptide by mass. Both figures are correct; they measure different things. If you are calculating concentration from vial weight and using the chromatographic figure to do it, your solutions will be systematically more dilute than you think — by fifteen to twenty percent, consistently, across every run.
Question four: how much water?
Measured by Karl Fischer titration, a coulometric method specific to water. Lyophilised peptides are hygroscopic to varying degrees, and some — NAD+ is a good example — are aggressively so.
Water content matters twice over. It affects the mass calculation described above, and it affects stability: water is a reactant in the hydrolysis and deamidation reactions that degrade peptides in storage. A vial at 8% water will not hold specification as long as the same peptide at 3%.
Question five: which counter-ion, and how much?
Peptides are purified by reversed-phase chromatography using an ion-pairing agent, and the peptide leaves that process as a salt. The two common counter-ions are trifluoroacetate and acetate, and the difference is not cosmetic.
Trifluoroacetate is cytotoxic at concentrations that appear in ordinary cell culture work. If your assay involves living cells, residual TFA is a variable you have to control for, and material supplied as the acetate salt avoids the problem. Whichever is present, the certificate should state which and at what level.
Question six: is anything else present?
Depending on the application, further tests apply. Endotoxin testing by LAL assay matters for anything approaching a cell-based system. Residual solvent analysis by gas chromatography matters where the synthesis route uses solvents that are difficult to remove. Bioburden testing matters where sterility is relevant.
These are not routine on every research-grade certificate, and their absence is not a failing — but if your work needs them, ask before ordering rather than after.
What a complete certificate covers
- Identity — observed mass against theoretical, by ESI-MS
- Chromatographic purity — area percent by RP-HPLC, with the method stated
- Net peptide content — the figure you calculate concentration from
- Water content — by Karl Fischer titration
- Counter-ion identity and level — acetate or TFA, quantified
- Appearance — against a reference description
- Batch code and release date — traceable to the vial in your hand
A certificate covering all of that is doing its job. One reporting a purity percentage and nothing else is a marketing document wearing the costume of an analytical one.
Where to go from here
The two methods doing most of the work above are chromatography and mass spectrometry, and each has enough detail to warrant its own treatment. The companion guides cover how RP-HPLC purity determination actually works and what its limits are, and what mass spectrometry can and cannot establish about a peptide.
This guide is written for laboratory practitioners and describes analytical and handling practice. It is not medical advice, and it contains no dosing or administration guidance. Material supplied by Ryzen Research Ltd is for research use only and is not for human or veterinary use.