How do you read a peptide COA? HPLC purity and mass spec explained
By the NexXGEN Research Team · Updated October 19, 2026 · 6 min read
Key takeaways
- A COA (certificate of analysis) reports test results for one specific lot, identified by its lot or batch number.
- Mass spectrometry confirms identity: the measured mass should match the sequence’s theoretical mass.
- HPLC reports purity as the main peak’s share of the detected peaks, not as the peptide’s share of the vial’s mass.
- Always match the lot number on your vial to the lot number on the COA.
- For in-vitro, laboratory research use only.
A peptide COA tells you two things about one specific lot: whether it is the right compound (identity, usually by mass spectrometry) and how clean it is (purity, usually by HPLC). Reading it well means checking that both results are present, that they make sense for the sequence, and that the lot number matches your vial.
This guide walks through each section of a typical COA and gives you a short checklist to use before you order.
What is on a peptide certificate of analysis?
A complete COA is tied to a single production lot. Expect to see the product name and sequence or formula, the lot or batch number, the name of the testing laboratory, the test date, the methods used, the specification for each test, and the measured result. Most COAs also include the raw evidence behind the numbers: an HPLC chromatogram and a mass spectrum. A signature or approval line shows who released the results. If a document lists a purity number but no lot number, no laboratory and no chromatogram, it is a summary, not a certificate of analysis.
What does HPLC purity actually measure?
High-performance liquid chromatography separates the components of a sample as they pass through a column. For peptides this is usually reversed-phase HPLC with UV detection at a wavelength where the peptide bond absorbs, typically around 214 to 220 nm. The software integrates the area under each peak, and purity is reported as the main peak’s area as a percentage of all integrated peaks.
Two points follow. First, a single sharp main peak with small, well-separated minor peaks is what a high-purity result looks like on the chromatogram. Second, HPLC purity only counts what the detector sees: water, salts and counterions such as acetate or trifluoroacetate do not show up as UV peaks. That is why HPLC purity and net peptide content are different numbers.
How does mass spectrometry confirm identity?
Mass spectrometry measures the mass-to-charge ratio of ionized molecules. Electrospray ionization (ESI) often produces several charged forms of the same peptide, while MALDI-TOF usually shows a singly charged ion. Either way, the COA should report an observed mass that agrees with the theoretical molecular mass calculated from the sequence, within the tolerance of the instrument. A purity result without an identity result tells you the sample is clean, but not what it is.
HPLC vs mass spec vs other tests: what each one tells you
| Test | What it answers | What a good result looks like | What it does not tell you |
|---|---|---|---|
| HPLC (UV) | How clean is the sample? | One dominant main peak; purity at or above the stated specification | Identity, water or salt content |
| Mass spectrometry | Is it the right compound? | Observed mass matches the theoretical mass of the sequence | How much impurity is present |
| Net peptide content | How much of the powder is peptide by mass? | A stated percentage, often by amino acid analysis or nitrogen content | Whether the peptide is the correct sequence |
| Appearance | Does the material look as expected? | Typically a white to off-white lyophilized powder | Purity or identity |
The COA for your lot is the source of truth. Every NexXGEN lot is tested by an independent third-party laboratory, and the COA for each lot is linked on its product page.
What should a researcher check?
- Lot number match. The lot or batch number on the vial label matches the COA exactly.
- Named laboratory and date. The testing laboratory is identified and the test date is consistent with the lot.
- Identity result. A mass spectrum with an observed mass that fits the sequence.
- Purity result with evidence. An HPLC purity figure backed by the chromatogram itself.
- Specification vs result. Each result is shown next to the specification it is judged against.
- One COA per product and lot. The same document should not appear for different products.
Related research guides
- Is retatrutide a GLP-3? The naming explained
- GHK-Cu: peptide chemistry and stability
- Research Library
- How NexXGEN tests every lot
- Research peptides catalog
Research-use note
All Nexxgen Peptides compounds are intended strictly for in-vitro research, laboratory and analytical purposes. They are not for human or animal use, are not approved by the U.S. Food and Drug Administration, and must be handled only by qualified researchers aged 21 or older. Nothing in this article is medical advice.
Frequently asked questions
What does COA stand for?
COA stands for certificate of analysis: a document from a testing laboratory that reports the results of specific tests on one specific lot of material.
Does 99% HPLC purity mean the vial is 99% peptide by mass?
No. HPLC purity is the share of the main peak among the UV-absorbing components the method detects. Water, salts and counterions are not counted, so net peptide content by mass is usually lower than the HPLC purity figure.
Which test confirms the peptide is the right compound?
Mass spectrometry. The observed mass should match the theoretical molecular mass of the sequence within the instrument’s stated tolerance. HPLC alone shows purity, not identity.
How do I know a COA belongs to my vial?
Match the lot or batch number printed on the vial label to the lot number on the COA. If the numbers differ, the COA does not describe that vial.
What is a red flag on a peptide COA?
Common red flags are a missing lot number, no named testing laboratory, no chromatogram or mass spectrum, a measured mass that does not match the sequence, or the same COA reused across different products.
