How to Interpret Purity Percentages on a Peptide COA
How to interpret purity percentages on a peptide COA: what the HPLC number measures, what it leaves out, and how to read it against the full certificate.
Diana Walsh, PharmD, BCPS is a Board-Certified Pharmacotherapy Specialist with a Doctor of Pharmacy from the University of Minnesota and pharmacy residency training at Mayo Clinic.
How to interpret purity percentages on a peptide COA starts with knowing what the number is actually a result of: a chromatography run, not a general quality score. A certificate showing 98.6% purity is reporting the proportion of detected material that eluted as the target compound under one specific test method, on one specific instrument, on one specific date. It is not a certification that the vial is 98.6% “good” in some broader sense, and reading it that way is the most common misstep in evaluating a COA.
What the Purity Percentage Actually Measures
Most peptide certificates report purity from high-performance liquid chromatography, commonly shortened to HPLC. The instrument separates a sample into components based on how they travel through a column, then measures the size of each resulting peak. The purity percentage is the target peptide’s peak area divided by the total peak area of everything detected, expressed as a percentage. A 98% figure means the target compound accounted for 98% of the detected peak area, with the remainder split among impurities, truncated sequences, or degradation products that eluted separately.
This matters because HPLC purity is a relative measurement, not an absolute one. It says nothing on its own about how much total material is in the vial, whether that material is actually the peptide it claims to be, or how it was stored between testing and shipping. Those are separate questions, and a COA that only states a percentage without the underlying method and date is giving a partial answer to all of them.
HPLC Purity vs. Mass Spectrometry Identity
A complete certificate usually reports two different things side by side, and conflating them is where most misreadings happen.
| Test | What it answers | What it does not answer |
|---|---|---|
| HPLC purity | What proportion of detected material matches the target peak | Whether that peak is actually the correct peptide |
| Mass spectrometry (MS) | Whether the detected mass matches the expected molecular weight of the peptide | How much of the sample is that peptide versus impurities |
| Combined HPLC + MS | Both proportion and identity, read together | Nothing about storage, stability, or shipping conditions |
A high HPLC purity number next to no MS data confirms that most of the sample is one consistent substance, without confirming that substance is the correct sequence. A COA that reports both figures from the same test run gives a more complete picture than either number alone, and a certificate that reports only a purity percentage with no identity confirmation is answering a narrower question than it might first appear to.
Reading the Purity Number in Context
A purity percentage only means what it claims to mean when it is read alongside the rest of the certificate, not in isolation.
The test method matters. HPLC purity figures are not directly comparable across different column types, gradient methods, or detection wavelengths, so a 99% result from one lab and a 97% result from another are not automatically ranked in that order unless the methods are comparable. The date matters too, since a certificate generated at the time of manufacture describes the peptide as it was then, not necessarily as it is after months in transit or storage. And the lot number matters, because a purity figure is only relevant to the specific batch it was measured from. A certificate with no lot number attached is, in practical terms, describing an unspecified batch rather than the vial in hand.
A Worked Example: Purity and Effective Mass
Purity percentage also affects the practical relationship between a vial’s labeled mass and the mass of actual peptide it contains, which is worth working through with real numbers rather than treating as an abstraction.
Take a vial labeled 5 mg with a COA purity result of 97%. The mass of the labeled compound present, discounting the roughly 3% of detected material that is not the target peptide, is:
5 mg × 0.97 = 4.85 mg
If that vial is reconstituted with 2 mL of bacteriostatic water, the nominal concentration calculated from the label alone would be 5 mg ÷ 2 mL = 2.5 mg/mL. Using the purity-adjusted mass instead gives:
4.85 mg ÷ 2 mL = 2.425 mg/mL
The difference, 0.075 mg/mL, is small at this purity level but illustrates the general relationship: lower purity results widen the gap between the number printed on the label and the mass actually present, and that gap compounds with any imprecision in the reconstitution volume itself. A calculator like peptcalc.com can run this kind of purity-adjusted concentration math directly, which is a faster way to check the arithmetic than doing it by hand each time.
What a High Purity Number Doesn’t Tell You
A purity figure in the high 90s is often treated as the single number that matters on a certificate, but it leaves several things unaddressed. It does not confirm identity by itself, which is why pairing it with mass spectrometry data matters. It does not describe residual solvents, counterions, or salts left over from synthesis, which some certificates report separately and others omit entirely. It does not describe endotoxin levels, which are a distinct test with its own separate result. And it does not describe how the compound was handled between the test date and the point of sale, which is outside what any single certificate can capture.
None of this makes a high purity percentage meaningless. It is a meaningful, checkable result when it comes from a named method, a stated date, and a matching lot number. The point is that it answers one specific question well, and a recipient evaluating a certificate is better served treating it as one data point among several rather than the entire verdict on the vial.
Comparing Certificates Across Vendors
Because purity percentages are only comparable when the underlying method is comparable, evaluating a certificate for something like retatrutide means checking that the test method, date, and lot number are all present and internally consistent, not just scanning for the highest number on the page. A certificate that states a specific column and gradient method alongside its result is giving more to verify than one that states only a percentage with no supporting detail.
Where two certificates report different purity figures for what is nominally the same compound, the more useful comparison is not which number is higher, but which certificate discloses enough about its method to make the number meaningful in the first place.
Summary
A purity percentage on a peptide COA reports the proportion of a chromatography peak matched to the target compound under one specific test, not a general grade of quality. Reading it accurately means checking it against the test method, the date, the lot number, and, where available, a separate identity confirmation from mass spectrometry. Purity also has a direct, calculable effect on the actual mass of peptide in a vial relative to its label, which is worth working out numerically rather than assuming the two figures match. Taken together, these checks turn a single percentage into a certificate that can actually be verified against the product it describes.