Research-use-only context. This is an analytical-chemistry reference for evaluating peptide Certificates of Analysis. It is not medical advice and not a usage guide. American Peptides products are sold strictly for in vitro laboratory research.
When you open a peptide Certificate of Analysis (COA), you typically see two test results: an HPLC purity percentage and a mass spectrometry confirmation. They look like redundant validation. They're not.
Understanding what each test actually measures — and what it misses — is the difference between an informed peptide researcher and someone who's been told "≥99% purity" and assumed that's the whole story.
The two questions a peptide COA needs to answer
For any peptide research compound, your COA needs to answer two distinct questions:
- "Is this molecule the right molecule?" (Identity)
- "How much of what's in this vial is the right molecule?" (Purity)
Mass spectrometry answers question 1. HPLC answers question 2. Both are needed because either one alone leaves a half-answered question.
What HPLC actually measures
High-Performance Liquid Chromatography (HPLC) separates the contents of a sample by their physical and chemical properties — typically by polarity (reverse-phase HPLC is most common for peptides) or charge (ion-exchange HPLC).
The sample is dissolved, injected into a column, and pushed through with a solvent gradient. Different molecules emerge from the column at different times based on how they interact with the column packing. A detector (usually UV at 215 nm for peptide bonds) records the concentration of each species as it elutes.
The result is a chromatogram — peaks plotted against time. The main peak (the target peptide) is integrated as a percentage of the total area under all peaks. That percentage is your purity number.
What HPLC tells you:
- The percentage of the sample that is the dominant species
- The presence and approximate quantity of impurities (any peak that isn't the main one)
- Whether the sample is consistent with previous lots
What HPLC does NOT tell you:
- Whether the dominant peak is actually the molecule you ordered
- The molecular weight of the main peak or any impurity
- Whether two peptides with similar elution times are co-eluting (appearing as a single peak)
A 99.0% pure HPLC chromatogram could, in principle, show 99% of the wrong peptide. HPLC alone never confirms identity.
What mass spectrometry actually measures
Mass spectrometry (MS) ionizes the sample and measures the mass-to-charge ratio (m/z) of each ion. For peptide identity confirmation, the typical workflow is:
- Ionize the sample (electrospray ionization is most common for peptides — it's gentle enough to preserve the intact molecule)
- Separate the resulting ions in a mass analyzer (quadrupole, time-of-flight, or orbitrap depending on the lab)
- Detect each ion and report the spectrum
A peptide produces a characteristic m/z pattern based on its molecular weight and ionization state. For example, BPC-157 (molecular weight ≈ 1419.5 Da) appears at m/z 710.7 in [M+2H]²⁺ form (doubly protonated). That signature is essentially a fingerprint.
What MS tells you:
- The molecular weight of the dominant species (= identity confirmation)
- Whether common synthesis byproducts are present (deletion sequences, oxidation products, aggregates)
- Confirmation that the peptide hasn't degraded during storage
What MS does NOT tell you:
- The percentage purity
- Whether the sample contains buffer, salts, or non-ionizable contaminants
- The exact stereochemistry or sequence ordering (this requires tandem MS / MS/MS for definitive sequencing)
A pure MS spectrum tells you the molecule is the right molecule, but it doesn't tell you the molecule is the only thing in the vial.
Why both together is the standard
HPLC says "99% of the sample is one peak." Mass spec says "the dominant peak is the right molecule." Together: "99% of the sample is the molecule you ordered." That's the complete answer a researcher needs before running an experiment. Either test alone leaves a critical gap.
What a complete peptide COA should include
- Lot number (matched to vial label)
- HPLC purity percentage (≥99% is the gold standard)
- HPLC chromatogram image (so you can verify the integration yourself)
- Mass spectrum with calculated and observed molecular weight
- Test date (peptides degrade; tests > 6 months old are stale)
- Independent lab name (in-house COAs are not third-party verification)
- Sequence (so you can sanity-check the molecular weight)
- Storage and handling recommendations
Some COAs add water content (Karl Fischer titration), elemental analysis, or endotoxin testing — these are extras, not table stakes.
Red flags on a peptide COA
- HPLC only, no mass spec. Half the question is unanswered.
- Mass spec only, no HPLC. The purity is unverified.
- In-house testing claimed but no lab name. "Third-party verified" without naming a specific external lab is a marketing claim, not a fact.
- A single COA used for multiple lots. Every batch should have its own batch-specific COA.
- No test date, or test date > 12 months old.
- Chromatogram images replaced with a percentage only. No image means no way to verify integration.
- Numbers that look implausibly clean. 100.00% purity is essentially impossible at the detection limits of HPLC; ≥99.0% is the realistic ceiling.
What we do at American Peptides
Every batch we ship has both HPLC and mass spec analysis from an independent third-party lab. We require ≥99.0% HPLC purity and matching molecular weight on MS before any vial leaves the facility. The chromatogram image and mass spectrum are linked to each batch on our COA library. Our standard test panel is run within 90 days of shipment date, not at synthesis date — so the COA reflects the actual material in your vial.
Frequently Asked Questions
Does HPLC confirm the peptide's identity?
No. HPLC measures purity (how much of the sample is the dominant species). Identity confirmation requires mass spectrometry, which measures molecular weight.
Is a percentage-only COA acceptable?
No. Without the chromatogram image and mass spectrum, the numbers can't be independently verified. A research-use COA includes the underlying data, not just a figure.
What does the HPLC purity percentage on a COA actually represent?
It represents the area of the main chromatographic peak as a fraction of the total integrated peak area, typically by reversed-phase HPLC with UV detection. It quantifies how much of the detectable material is the dominant species, but it does not by itself confirm that the dominant species is the intended peptide — that requires mass spectrometry.
Why are HPLC and mass spectrometry used together rather than separately?
The two techniques answer different questions: HPLC measures relative purity, while mass spectrometry measures molecular weight to confirm identity. A research-use COA pairs them so a lab can verify both that the material is predominantly one species and that the species matches the expected monoisotopic or average mass.
This article is for laboratory research reference only. American Peptides products are sold strictly for in vitro research. Not for human consumption.
Compliance Notice: American Peptides products are sold strictly for laboratory and academic research purposes only. They are not intended for human or veterinary consumption, diagnosis, treatment, or prevention of any disease. All content on this page is educational in nature and does not constitute medical advice or product claims. Researchers are responsible for handling these compounds in accordance with their institutions safety protocols and applicable laws.
