Peptide Testing: HPLC Purity vs. Mass Spectrometry Identity
Peptide testing usually means HPLC, mass spectrometry, or both. HPLC measures purity; mass spectrometry confirms identity. A COA needs both.
The short version
HPLC (high-performance liquid chromatography) and mass spectrometry answer two different questions about the same vial. HPLC tells you how clean the sample is: the purity percentage most COAs lead with. Mass spectrometry tells you what molecule is actually in there. A COA that only runs HPLC can report a very clean sample of the wrong substance, and the purity number alone cannot tell you that.
- HPLC measures purity - how much of the sample is one dominant substance.
- Mass spectrometry measures identity - which molecule is actually present.
- A high purity percentage can describe a very clean sample of the wrong peptide.
- A COA naming only HPLC has answered one question and left the other open.
A Certificate of Analysis, or COA, is a lab report on one production batch. See my page on how to read one for the full list of what it should contain. Most COAs lead with one number, a purity percentage, and it usually comes from one method: HPLC. It is a real measurement of a real thing. On its own, though, it does not answer the question most buyers actually have. That question is not “how clean is this sample” but “what is actually in this vial.”
Those are two different questions, answered by two different kinds of test. This page explains both in plain terms, and why a COA that only answers the first one has not answered the second.
HPLC: how clean is the sample
HPLC stands for high-performance liquid chromatography. Chromatography is a family of techniques that separate a mixed sample into its individual component substances. In HPLC, a liquid sample is pushed under pressure through a narrow tube packed with a material (the column). Different molecules travel through it at different speeds, depending on their chemical properties. Substances that interact more with the column material move more slowly, and substances that interact less move faster. By the time the sample exits, its components have been separated in time rather than all arriving together.
A detector at the end of the column measures how much material passes it at each moment. The result is plotted as a chromatogram, a graph with time on one axis and detector signal on the other. Each separated substance shows up as a peak: a rise and fall in the signal at the moment that substance exits the column. A sample that is mostly one substance, with only trace amounts of anything else, produces one large peak and little else. A sample with meaningful contamination or a mix of related substances produces several peaks.
Purity, on a COA, is typically reported as peak area. That is the area under the largest peak divided by the total area under all the peaks, as a percentage. If the main peak accounts for 98% of the total area detected, the COA reports “98% purity.”
That is a useful measurement, but it measures cleanliness relative to whatever the method detected, not what the dominant substance is. HPLC on its own cannot report whether the large peak is the intended peptide. It could be something else with similar chemical behaviour that travels through the column at a similar speed.
Mass spectrometry: what is actually in the vial
Mass spectrometry is a different kind of measurement. It does not separate substances by how they move through a column. It measures the mass of molecules directly, specifically the mass-to-charge ratio of ionised molecule fragments. A lab uses that to work out the molecular weight of what is in the sample. Molecular weight is how heavy one molecule of a substance is, measured in a standard unit (daltons). Different peptides are, in general, built from different sequences of amino acids and so have different molecular weights.
A mass spectrometry result that matches the expected molecular weight of the peptide named on the label is meaningful evidence that the sample contains that molecule. It is not a look-alike, a fragment or a different peptide entirely. That makes it an identity test. It is not, by itself, a purity test: matching the right molecular weight does not say what percentage of the total sample is that molecule versus something else.
Why a COA needs both
Purity and identity are separate properties, measured by separate methods. A result on one tells you almost nothing about the other:
| What it answers | What it misses | |
|---|---|---|
| HPLC (purity) | How much of the sample is one dominant substance | What that dominant substance actually is |
| Mass spectrometry (identity) | Which molecule is present, by matching molecular weight | What percentage of the sample is that molecule |
This is not a hypothetical technicality. FDA’s regulation for approved-drug manufacturing, 21 CFR 211.165(a), requires laboratories to determine both the identity and the strength of each active ingredient before a batch can be released. It treats them as two separate, separately verified questions. Peptides sold as research chemicals outside the approved drug supply are not manufactured under that rule. A buyer therefore cannot assume both questions were even asked, let alone answered.
Peptides are also not simple to characterise. FDA’s own review of peptide bulk drug substances describes individual peptides as carrying “complexities with regard to peptide-related impurities and active pharmaceutical ingredient characterization.” Confirming exactly what is and is not present in a peptide sample is a real analytical challenge, not a box any lab can tick by default.
What this means when you are reading a COA
- A COA that names only HPLC, or reports a purity number with no method at all, has addressed cleanliness and left identity unconfirmed. See my page on reading a COA for the complete list of fields a genuine one should carry.
- A COA that reports both a named purity method and a named identity-confirming method answers a more complete question. The identity method can be mass spectrometry or another method that establishes molecular identity. The answer still covers only the specific batch the document describes.
- Neither test, alone or together, is a safety or efficacy claim. They describe what is in the vial and how clean it is, not whether that substance is safe to take, at what amount, or with what effects.
Related
- How to read a peptide certificate of analysis (COA)
- Lab-tested peptides: what “third-party tested”, “99% pure” and “USA-made” prove
- Are peptides safe?
- Research peptides and US law
- Research-peptide access
Common questions
What does HPLC actually measure?
HPLC, or high-performance liquid chromatography, separates a sample into its component substances. It pushes the sample through a column that different molecules travel through at different speeds, then measures how much of each substance passes a detector over time. The result is a chromatogram, a graph with one peak per substance. Purity is usually reported as the area of the largest peak divided by the total area of all peaks, as a percentage. That tells you how clean the sample is, not what the main substance is.
What does mass spectrometry actually measure?
Mass spectrometry measures the mass of molecules in a sample. A lab can then work out the molecular weight of whatever is present and compare it with the expected molecular weight of the peptide named on the label. Different peptides generally have different molecular weights, so a match is evidence the sample contains the intended molecule. It is an identity test, not a purity test. It does not by itself tell you what percentage of the sample is that molecule.
Can a sample be highly pure by HPLC but still be the wrong peptide?
Yes. HPLC purity describes how much of the sample is one dominant substance relative to everything else the method detected. It does not identify that substance. A vial could contain a molecule with a similar chemical structure to the intended peptide. It would appear as a single clean peak and report a very high purity percentage, while not being the compound on the label. Only an identity test such as mass spectrometry can catch that.
Why would a certificate of analysis report only HPLC and not mass spectrometry?
Cost and speed are common reasons. HPLC purity testing is generally simpler and cheaper to run than mass spectrometry identity confirmation. Whatever the reason, a certificate that reports only a purity percentage has answered how clean the sample is and left open what it actually is. My companion page on reading a certificate of analysis covers what a complete document should name.
Does running both tests guarantee a peptide is safe?
No. HPLC and mass spectrometry together can support a claim that a sample is a clean, correctly identified batch of a named molecule. Neither test says whether that molecule is safe for a person to take, at what amount, or with what effects over time. Purity and identity are quality questions. Safety and efficacy are separate questions with their own, generally much larger, evidence requirements.
Are peptides a drug or a steroid?
Peptides are not steroids. The two are chemically distinct classes of molecule. A peptide is a chain of amino acids. Under FDA regulation, an amino acid polymer of 40 or fewer amino acids is treated as a peptide, while a longer chain with a defined sequence is a protein. An anabolic steroid is a synthetic substance similar to testosterone, built on a hormone structure with no amino acid chain at all. Whether a given peptide counts as a 'drug' depends on the molecule. Some peptides are FDA-approved prescription drugs. Many peptides sold as research chemicals are unapproved and have not been reviewed by FDA for safety or efficacy in humans.
How do I test my peptides, and where can I get them tested?
Send a sample to an analytical lab that runs HPLC for purity and mass spectrometry for identity, and ask for both. My vendor reviews reference independent testers such as Janoshik Analytical and Finnrick. Janoshik's certificates can be checked on its own public portal. Finnrick publishes its own purity results by vendor. I have not assessed any lab's accuracy, and neither test says whether a peptide is safe to take.
Can you be tested for using peptides?
In sport, yes. The World Anti-Doping Agency's Prohibited List bans named peptides under categories S0 and S2, at all times, in and out of competition. Standard workplace drug panels are different: they target illicit drugs, and I could not verify a general practice of employers testing for peptides. The page on whether peptides show up on a drug test has the detail.
References (4 sources)
Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.
- 1.21 CFR 211.165(a) - the FDA regulation requiring, for each batch of a drug product, 'appropriate laboratory determination of satisfactory conformance to final specifications for the drug product, including the identity and strength of each active ingredient' prior to release, treating identity and strength as separate required determinations.- Govinfo, US Government Publishing Officestatute
- 2.FDA briefing document on peptide bulk drug substances - describes individual peptides as having 'complexities with regard to peptide-related impurities and active pharmaceutical ingredient characterization,' the analytical difficulty that makes naming a specific identity-confirming method meaningful.- US Food and Drug Administrationregulatory
- 3.21 CFR 600.3(h)(6) - the FDA regulation defining a protein as 'any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size,' the threshold that separates a peptide from a protein for regulatory purposes and shows peptides are defined by amino acid structure, not by any steroid classification.- Govinfo, US Government Publishing Officestatute
- 4.National Institute on Drug Abuse overview of anabolic steroids, defining them as 'synthetic substances similar to the male sex hormone testosterone' - the contrasting hormone-based structure that distinguishes a steroid from an amino-acid-chain peptide.- National Institute on Drug Abuse, National Institutes of Healthother