When you buy research peptides, the single most important number on the Certificate of Analysis is purity. But "purity" can mean different things depending on how it was measured.
This article walks through the two methods you'll see most often — High-Performance Liquid Chromatography (HPLC) and Mass Spectrometry (MS) — and explains what they each tell you.
What "purity" actually means
Purity is the percentage of your sample that is the target peptide, versus everything else. The "everything else" can include:
- Truncated sequences (the peptide is missing one or more amino acids)
- Deletion sequences (an internal residue is missing)
- Oxidized or deamidated forms
- Trifluoroacetic acid (TFA) salt content from synthesis
- Water content
- Inorganic salts and solvent residues
A peptide labeled "≥99% pure" does not mean the entire vial is the peptide. It means 99% of the analyzed peptide-related material is the target peptide. Net peptide content — how much actual peptide you get versus the gross weight of the vial — is a separate number, and it matters.
HPLC: the industry standard for purity
Reverse-phase HPLC (RP-HPLC) is the workhorse of peptide purity analysis. The principle is straightforward: dissolve the peptide, push it through a column with a controlled solvent gradient, and watch what comes out the other end. Different molecules elute at different times based on their hydrophobicity. The detector (usually UV at 214 or 280 nm) records each compound as a peak on a chromatogram.
- Percentage of the main peak vs. all peaks
- Retention time (a fingerprint matching reference)
- Close-eluting impurities (truncated/deletion sequences)
- Molecular weight — cannot confirm what you actually have
- Identity — 99% pure of something isn't 99% pure of the right thing
This is why HPLC purity claims should be paired with at least one orthogonal identity test.
Mass spectrometry: identity confirmation
Electrospray Ionization Mass Spectrometry (ESI-MS) and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) are the two MS techniques most often applied to peptides. Both ionize the peptide and measure its mass-to-charge ratio. The result is the molecular weight of the molecule — which, compared against the calculated theoretical mass for the peptide sequence, tells you whether you have the right peptide.
- Molecular weight of the dominant species
- Whether mass matches the expected sequence
- Mass-related impurities (+16 Da = oxidation; -1 Da = deamidation)
- Quantitative purity in the HPLC sense — ionization efficiency varies between species
When each is enough — and when it isn't
For most laboratory research applications, the gold standard is:
HPLC purity ≥98–99% paired with MS identity confirmation matching theoretical mass within ±1 Da.
A vendor that publishes only one of the two is leaving you with an incomplete picture. HPLC alone could show 99% purity of the wrong molecule. MS alone could confirm identity but miss low-level impurities that affect downstream assays.
What to look for on a Certificate of Analysis
A complete CoA for a research peptide should show:
- Sequence — full amino acid sequence in single-letter code
- Theoretical molecular weight — calculated from the sequence
- Observed molecular weight — measured by MS
- HPLC chromatogram — showing the main peak and impurity profile
- Calculated purity % — from the HPLC area integration
- Lot/batch number — for traceability
- Test date — when the analysis was performed
- Net peptide content — ideally measured by amino acid analysis or quantitative NMR
If any of these are missing, ask the vendor before purchasing.
Why this matters for laboratory research
In a research context, batch-to-batch variability can ruin reproducibility. A peptide that's 99% pure in one lot and 95% pure in another isn't just a quality issue — it's an experimental confound. Specifying purity, identity, and net peptide content in your method section makes your work reproducible. Demanding test data on the lot you actually received makes your supplier accountable.
How we apply this in practice
Our products are sourced to a target purity of ≥99% via HPLC analysis. We personally inspect every batch in-house for visual and packaging integrity before it enters our inventory, and reference batches across our catalog are submitted to independent third-party laboratories for HPLC verification. Test documentation is available on request.