Laboratory guide

How to test peptides for purity

Peptide purity is a measurement, not a label. It comes from separating a sample, detecting what comes off the column, and reporting the principal peak as a proportion of everything detected — under conditions that have to be stated for the number to mean anything.

Published August 24, 2026 · Reviewed by the Canada Peptide Testing laboratory team

Decide which question you are asking

Most disappointing test results come from ordering the wrong measurement. Three different questions get collapsed into the word “tested”, and no single method answers all three:

A vial can pass one and fail another. Pure but wrong, correct but underfilled, and correctly labelled but contaminated are all ordinary outcomes, and each is invisible to the methods aimed at the other questions.

How a laboratory measures purity

Reversed-phase high-performance liquid chromatography with ultraviolet detection is the working method for peptide purity. A weighed portion of the sample is dissolved and injected onto a column, where components travel at different speeds according to how strongly they interact with the stationary phase. As each component leaves the column it passes a UV detector, and the resulting chromatogram shows a peak for every separated component the detector responded to.

Purity is then the area under the principal peak divided by the total area of all detected peaks, expressed as a percentage. That definition carries three consequences worth understanding before reading any certificate:

• The figure is relative. It describes proportions of what was detected, not an absolute amount of anything.
• The figure depends on the method. Column chemistry, gradient, mobile phase, and detection wavelength all change what separates and what the detector sees.
• The figure only counts what the detector responds to. A UV detector responds to compounds that absorb at the chosen wavelength; something that does not absorb there does not appear as a peak.

This is why a purity percentage without its method is not a result. A report should show the chromatogram, name the method and detection wavelength, and make the integration approach visible.

Why identity has to be confirmed separately

Chromatography separates; it does not identify. A single sharp peak tells you the sample is dominated by one component, not which component that is. Mass spectrometry supplies the missing evidence by comparing observed mass data with the theoretical molecular mass of the expected peptide.

Identity and purity are therefore normally ordered together, because each covers the other’s blind spot. A pure sample of the wrong compound is still the wrong compound, and a correctly identified sample can still carry significant impurities. Even combined, mass evidence has limits: isomers and closely related structures can require additional work to separate.

For a fuller side-by-side of the two methods, see HPLC vs. LC-MS for peptide testing.

Why home testing cannot answer this

Home reagent kits work by producing a colour change in the presence of some class of substance. That can indicate that something is present. It cannot separate a mixture, assign a percentage, confirm a molecular mass, or measure a quantity — and it leaves no record anyone else can check.

The gap is not one of care or technique. Purity is defined in terms of chromatographic separation and detection, so producing the number requires an instrument that separates and detects. There is no home equivalent of the measurement, only of the impression.

What a defensible result looks like

Whoever runs the analysis, a purity result that can be relied on should let a reader reconstruct how it was produced:

• A sample record: what arrived, when it arrived, and which identifier it was given.
• A named method, with column, mobile phase, gradient, and detection wavelength stated.
• The chromatogram itself, with axes labelled and the principal peak and integration visible.
• The reported figure with its basis — area purity is not content, and the report should say so.
• Stated limitations, including that the result applies to the sample received and not to untested units.
• A way to verify the document later, such as a report number that resolves to a public record.

Our guide to reading a peptide Certificate of Analysis works through each of these on a real report layout.

Getting a sample tested in Canada

The workflow is deliberately front-loaded, because most failed submissions fail before any analysis happens — wrong quantity, undeclared matrix, or a compound the method was never going to resolve.

1. Submit the compound, sample form, nominal amount, and the question you need answered.
2. A chemist reviews method fit and confirms scope, required quantity, turnaround, and the final price.
3. You receive a unique sample ID, a packing slip, and the Canadian intake address. Nothing should be shipped before this point.
4. Mail the sample by tracked carrier and follow receipt, preparation, analysis, and review from your private dashboard.
5. Download the released Certificate of Analysis, which carries a QR-verifiable public record.

Target turnaround is 5 to 7 business days from receipt of an accepted sample. Full mailing requirements are set out in mailing a peptide sample in Canada.

What purity testing does not establish

A purity figure is narrow on purpose. It does not establish sterility, bacterial endotoxin levels, elemental contamination, clinical safety, efficacy, or regulatory status, and it describes only the vial that was analysed. Contamination questions are separate measurements with their own methods — bacterial endotoxin, microbial quality, and heavy metals — and batch-level conclusions require a sampling plan rather than a single submitted unit.