Peptide purity and identity testing explained: what 99% actually means
The short answer. Two different tests are involved and they answer different questions. Mass spectrometry answers "is this the compound on the label", by measuring the molecule's mass. High performance liquid chromatography answers "how much of this sample is that compound", expressed as a percentage. A purity figure without a confirmed identity is a number describing an unknown substance, which is why the two are only meaningful together.
For research purposes only. Not for human consumption. 18+. UK only. This article describes analytical testing methods for laboratory research materials. It makes no claim about what any compound does.
The two questions a certificate answers
Suppliers tend to quote one number, usually a purity percentage, as though it settled everything. It does not, because it only ever answers half the question.
Identity, by mass spectrometry
Mass spectrometry ionises the sample and measures the mass to charge ratio of what comes through. Every peptide has an expected mass determined by its amino acid sequence. If the measured mass matches, the vial contains the molecule named on the label.
This is the question that has to be answered first. Everything else assumes it.
Purity, by high performance liquid chromatography
HPLC pushes the dissolved sample through a column that separates its components by how strongly each interacts with the packing material. Different substances emerge at different times, and a detector records each as a peak.
Purity is the area of the target compound's peak as a proportion of the total peak area. A result of 99% means the target accounts for 99% of what the detector saw.
Why the order matters
Consider a vial reported at 99.5% purity, with no identity test.
That figure is internally consistent and completely uninformative. It says one substance made up 99.5% of the sample. It does not say which substance. A vial of something else entirely, if it were uniform, would also report high purity.
This is why a report that quotes purity and never confirms identity should be treated as incomplete rather than impressive. The two tests are cheap to run together, and a laboratory doing one is capable of doing both.
What "99%" does not tell you
Even with identity confirmed, a purity percentage has limits that are rarely stated.
It describes what the detector saw, not everything in the vial. HPLC with ultraviolet detection responds to compounds that absorb at the wavelength used. Substances that do not absorb there, including some salts and residual solvents, may not appear in the calculation at all. This is normal and expected, but it means "99% pure" is not the same claim as "99% of the vial's mass is peptide".
It says nothing about content. Purity is a proportion, not a quantity. A vial can be 99% pure and substantially underfilled. That is why a content by mass figure matters separately, and why an underfilled vial is a common quality failure that a purity number alone will not reveal.
It is specific to one batch. A certificate describes the material tested on the day it was tested. It says nothing about a different production run, which is why a batch number tying the report to the vial in your hand is not administrative detail but the whole point.
It is not a safety or sterility statement. Purity and identity are analytical results. Sterility, endotoxin content and microbial limits are separate tests that only appear on a report if they were performed and named.
The three figures worth reading together
| Figure | Method | The question it answers |
|---|---|---|
| Identity | Mass spectrometry | Is this the compound named on the label? |
| Purity | HPLC | What proportion of the sample is that compound? |
| Content by mass | Quantitative assay | How much is actually in the vial? |
A report carrying all three, tied to a batch number and issued by a named independent laboratory, is a complete picture. A report carrying one of them is a marketing figure.
What to ask a supplier for
If you are comparing suppliers on testing, these are the questions that separate them:
- Is the report batch specific? A single certificate reused across every batch describes one production run and nothing else.
- Is the laboratory named, and independent of the supplier? A report produced in house is the supplier assessing themselves.
- Can the result be confirmed at the laboratory? A report number that resolves on the laboratory's own website removes the supplier from the chain of trust entirely.
- Is identity confirmed, not just purity? See above.
- Is content by mass reported? This is where underfilling shows up.
- What is the date? A report predating the batch it is attached to cannot describe it.
There is a walkthrough of running that check at verify a certificate, and a field by field explanation of a report at how to read a Certificate of Analysis.
A note on blends
A vial containing several intended peptides cannot be given a single purity percentage. Purity is a proportion of one target compound, and a blend has no single target. Multi-peptide vials are instead tested for the identity of each peptide present and the content by mass of each. Any supplier quoting one purity figure for a blend is quoting a number that has no meaning. There is a worked example in what is in a peptide stack.
Common questions
What lab tests confirm a peptide's identity and purity? Mass spectrometry confirms identity by measuring the molecule's mass against the mass expected from its sequence. High performance liquid chromatography reports purity as the proportion of the sample that is the target compound. A complete report carries both, plus a content by mass figure, tied to a specific batch.
What does 99% purity mean for a peptide? That the target compound accounted for 99% of the total peak area the detector recorded during chromatography. It is a proportion of what was detected, not a statement about the vial's total mass, and it is only meaningful once identity has been separately confirmed.
Is a purity percentage enough on its own? No. Without a confirmed identity it describes an unknown substance, and without a content figure it says nothing about how much material is in the vial.
What capabilities matter when comparing testing providers? Identity confirmation and impurity profiling are separate capabilities. A provider offering both, issuing batch specific reports, and publishing a verification route so a result can be checked independently, covers the ground. Independence from the supplier is the part that cannot be substituted.
How much does peptide purity testing cost in the UK? Costs vary by laboratory, by the tests requested and by turnaround, so any figure quoted here would be out of date quickly. The more useful question is whether a supplier already publishes independent batch reports, since that removes the need to commission testing at all.
Does a certificate prove the vial I received came from that batch? No. It proves what the laboratory found in the sample it tested. Whether your vial belongs to that batch depends on the supplier attaching the right report to the right stock, which is why the batch number on the vial should match the batch number on the report.
Where to look next
About the author
Sebastian Reuters is a science and health writer working with BioHack London on research-orientation content. He covers analytical methodology, regulatory landscape, and supplier-evaluation topics for the UK research community.