Guide
HPLC vs Mass Spectrometry: What Each Test Actually Proves
Almost every peptide COA leads with a purity percentage. Far fewer show an identity confirmation. That difference matters more than most buyers realise, because purity and identity are not the same measurement — and a sample can score beautifully on one while failing the other entirely.
Two different questions
HPLC answers “how clean is this sample?” Mass spectrometry answers “is this actually the molecule on the label?” Those are separate questions, and no single test answers both.
A report showing only purity tells you the sample is consistent. It does not tell you what the sample is. A report showing only identity tells you the right molecule is present, but not how much of the material is something else.
What HPLC measures
High-Performance Liquid Chromatography pushes a dissolved sample through a column. Different components travel at different speeds and come out separately, producing a chromatogram with peaks. The size of the main peak relative to everything else is reported as a purity percentage.
That percentage describes proportion, not identity. HPLC can tell you that 99.5% of the material is one single compound. On its own, it cannot tell you which compound that is — only that the sample is overwhelmingly one thing.
What mass spectrometry measures
Mass spectrometry ionises the sample and measures mass-to-charge ratio, producing a molecular weight. Because a peptide’s sequence determines its mass, that measurement is a fingerprint: the observed mass either matches the theoretical mass of the expected sequence or it does not.
This is the test that confirms identity. It is why an MS result on a COA is worth more scrutiny than the purity figure everyone looks at first.
Why you need both: 99% pure of the wrong molecule
The failure mode that purity alone cannot catch: a sample that is extremely clean, extremely consistent, and simply not the compound named on the label. Every HPLC number looks excellent. The material is still wrong.
A single amino-acid substitution changes the molecule while leaving a chromatogram that can look nearly identical. Mass spectrometry catches the mass difference. Purity testing does not look for it.
This is the entire reason both tests appear on a serious COA. Purity says how clean. Identity says what it is. Either one alone leaves a question open.
How they appear on a COA
On a well-built report you will see them as separate rows with separate methods: an identity row citing MS (or HPLC/MS), and a purity row citing HPLC with a percentage. Each should name the method, not just the outcome.
If a report lists a purity percentage with no method, you cannot tell what produced the number. If it lists no identity row at all, that absence is the finding.
Phrases that sound like tests but are not
“Double-purified” and similar phrases describe a process someone says happened upstream. They are not measurements and they carry no document.
“Lab tested” names no lab, no method, and no result. It is a claim about testing, not the testing itself.
“Pharmaceutical grade” has no agreed definition in this market. It is marketing language, not an analytical standard.
The test is the row on the report with a method beside it. Everything else is a phrase.
The bottom line
Ask two questions of any COA: which method produced the purity number, and where is the identity confirmation. A report that answers both is describing the material. A report that answers one is describing half of it.
For a field-by-field walkthrough of everything else on the document, see our guide on how to read a peptide COA.