Research note
Reading a Peptide Certificate of Analysis the Way an Analytical Chemist Does
How an analytical chemist reads a peptide COA: HPLC method details, charge-state arithmetic on the mass spectrum, net peptide content and the warning signs.
A COA does not award a grade. It records measurements made under particular conditions, and it is those conditions that carry most of the information. Two vials can both claim 99% and contain very different material: one tested with a gentle gradient at 214 nm and cleanly separated peaks, the other tested so that anything eluting near the void volume disappears. The analyst’s habit is simple — read the method first, the result second.
Begin with the header, not the percentage
The top of the document holds four items that determine whether the rest deserves trust: the compound name with its sequence, the batch code, the test date, and references to the instrument or method. If the batch code does not match the label on the vial, the page is advertising. If there is no date, you cannot relate it to any stability period. If the column, gradient or wavelength goes unnamed, the result cannot be evaluated at all.
People often overlook the sequence line, yet it is what the MS result gets checked against. A certificate that omits it gives the identity test nothing to aim at.
What the purity figure on the chromatogram really measures
The purity on a peptide COA is almost always a reversed-phase HPLC area value — main-peak area over total integrated area, expressed in percent. Three things follow from that.
One: it is a ratio, not a weight. Moisture, salts and counter-ions are invisible to it; it only reports which fraction of the UV-active material sits in the main peak. Two: the wavelength controls everything. Peptide bonds absorb near 214–220 nm, which is why that range is the default and why it picks up virtually every peptide species. At 280 nm the detector sees only tyrosine, tryptophan and, weakly, phenylalanine — a sequence lacking these hardly appears, and a 280 nm purity for it is meaningless. Three: the gradient matters. A fast gradient piles closely related by-products — truncated chains, oxidation products, diastereomers — beneath the main peak, while a slower, longer one pulls them apart and generally gives a lower but more honest figure.
So the useful question is not "98 or 99?" but "what result would a C18 column, 214 nm and a gradient of 1%/minute give?" — and the method block is where you find out. For more depth, read HPLC purity explained and our glossary note on purity.
Work out the charge states on the mass spectrum yourself
ESI puts multiple charges on each molecule, so a peptide above roughly 1,000 Da usually appears as a ladder of peaks rather than as a single line. At charge z the expected m/z is approximately (M + 1.00794z) / z, with M the neutral average mass.
Worked through for BPC-157 (Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val, average mass 1,419.55 Da): singly protonated, look near m/z 1,420.6; doubly protonated, (1,419.55 + 2.02)/2 ≈ 710.8; triply protonated, about 474.2. Seeing 1,420.6 for a vial marked BPC-157 fits the expected identity. Seeing 889 instead — the TB-500 heptapeptide’s mass — means the contents are not what the label says, whatever the purity.
Two refinements mark an experienced reader. Monoisotopic and average mass are not the same thing: the gap is about 1 Da at 1.4 kDa and about 3 Da at 4.7 kDa, so a slight offset can come from comparing different conventions rather than from a wrong product. And a deconvoluted spectrum beats a raw one, since it reports the neutral mass outright and makes adducts obvious — sodium adds +22, potassium +38. Our guide to MS-based identity testing covers this fully.
Net peptide content changes every calculation
This value matters most and is omitted most often. The 5 mg on a label is usually the gross weight of dried powder: peptide, counter-ion, leftover moisture and any bulking agent together. Reversed-phase purification with trifluoroacetic acid in the eluent yields the peptide as its TFA salt, and that salt is not a trivial fraction — each basic residue binds a counter-ion, so in lysine- and arginine-rich sequences a large share of the weight can be trifluoroacetate counter-ion. Include water picked up from humid air and the real peptide in a nominal 5 mg vial can fall clearly below 5 mg.
The correct methods for measuring it are amino acid analysis or quantitative nitrogen determination. If the certificate gives it, trust your molar figures; if it does not, regard concentrations as estimates and say so in your methods section.
Getting independent confirmation
Rather than hoping for a better-looking certificate, get a second analysis of the very same batch. Independent labs will run identity and purity on a sample you submit, and a supplier that posts external reports beside its in-house ones is offering something verifiable instead of an unsupported statement. What the process involves and what to order is set out in third-party testing explained; the line-by-line reading guide and the glossary entry on the COA complement it, and common questions are collected in our FAQ on purity and testing.
Questions
Why is peptide purity measured at 214 nm and not 280 nm?
Peptide bonds absorb strongly near 214–220 nm, so that range detects essentially every peptide species, including truncated and oxidised forms. Signal at 280 nm comes from tyrosine and tryptophan, and plenty of sequences have neither. For those, a purity reading at 280 nm tells you very little.
Does 99% purity mean 99% of the vial’s weight is peptide?
No. HPLC purity compares peak areas of UV-active material and ignores moisture, salts and counter-ions entirely. The peptide share by weight — net peptide content — is a separate measurement, typically by amino acid analysis, and is usually lower than the purity value.
How do I compare a mass spectrum with the stated molecular weight?
For ESI data, expect m/z of about (M + 1.008z)/z at charge z. A 1,419.55 Da peptide should give roughly 1,420.6 at charge 1 and 710.8 at charge 2. Deconvoluted spectra report the neutral mass directly, which simplifies the check and exposes sodium or potassium adducts.
What is a TFA salt, and why does it affect my calculations?
Trifluoroacetic acid is the usual additive in reversed-phase purification, so the peptide ends up as a trifluoroacetate salt. Basic residues can each hold a counter-ion, adding weight that is not peptide. With many lysines or arginines that extra weight is large, which is why net peptide content is stated separately.
Can one material show different purity values on two COAs?
Yes, for perfectly valid reasons. The column, gradient, run length and integration parameters all affect how well near-identical impurities are split from the main peak. A slower, longer gradient typically separates more of them and reports a lower number, so the method details deserve as much attention as the headline figure.
Which COA red flag can be checked fastest?
Compare the reported mass with the molecular weight of the stated sequence — a matter of seconds, with a clear yes-or-no answer. When the mass does not belong to the labelled peptide, the rest of the certificate is irrelevant, no matter how clean the trace looks.