Research note
Reading a Peptide Certificate of Analysis the Way an Analytical Chemist Does
Read a peptide certificate of analysis the way an analyst does: chromatogram conditions, charge-state maths, net peptide content and the red flags that count.
A certificate of analysis is not a quality grade; it is a collection of measurements taken under specified conditions, and those conditions are where the real information lies. Two vials may both state 99% and still be completely different materials — one analysed on a shallow gradient at 214 nm with a well-resolved chromatogram, the other analysed in a way that conceals everything eluting close to the void. Reading a COA as a chemist would means looking at the method before looking at the number.
Begin with the header, not the percentage
Four fields in the header decide whether anything else in the document carries weight: the product name including its sequence, the lot number, the analysis date, and the instrument or method references. Without a lot number that matches the label on the vial, a COA is promotional material. Without a date, it cannot be placed within a stability window. Without a named column, gradient or detection wavelength, it states a result you have no way of assessing.
The sequence in the header is more important than many realise, since it is the reference the mass spectrum is compared with. If no sequence is given, the identity check has no declared target.
What the purity figure on the chromatogram really measures
On a COA, peptide purity is nearly always a reversed-phase HPLC area percentage: the area of the main peak divided by the total integrated area, given as a percentage. That definition has three consequences.
First, the figure is relative and says nothing about mass. It ignores water, salt and counter-ion, describing only what share of the UV-absorbing material elutes in the main peak. Second, it hinges completely on the detection wavelength. Peptide bonds absorb at about 214–220 nm, making this the standard analytical wavelength and the one that detects practically all peptide material. At 280 nm only tryptophan, tyrosine and, to a minor extent, phenylalanine are seen — a peptide without them barely registers, so a purity figure at 280 nm for such a sequence has no meaning. Third, it depends on the gradient. A steep gradient squeezes closely related impurities — deletion sequences, oxidised forms, diastereomers — under the main peak, whereas a shallow, longer gradient separates them and usually yields a lower, more truthful value.
The helpful question is therefore not "98 or 99?" but "what would this material give on a 1%/minute gradient at 214 nm on a C18 column?" The method section tells you. More detail sits in HPLC purity explained and the definition at purity.
Work out the charge states on the mass spectrum yourself
Electrospray ionisation generates ions carrying several charges, so peptides heavier than about 1,000 Da typically produce a series of peaks instead of one. For charge state z, the observed mass-to-charge ratio is roughly (M + 1.00794z) / z, where M is the neutral average molecular weight.
Take BPC-157 as an example: average molecular weight 1,419.55 Da, sequence Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val. The ion carrying one proton should show up around m/z 1,420.6; the ion with two protons around (1,419.55 + 2.02)/2 ≈ 710.8; the ion with three around 474.2. A COA peak at 1,420.6 on a vial marked BPC-157 means identity is consistent. A peak at 889 — the mass of the TB-500 heptapeptide — means label and contents do not match, and no purity figure can fix that.
Two further points distinguish a careful reader from a casual one. Average and monoisotopic masses differ: by about 1 Da for a 1.4 kDa peptide and by about 3 Da for a 4.7 kDa peptide, so a small discrepancy may simply reflect mismatched conventions rather than an error. And a deconvoluted spectrum tells you more than a raw one, because it gives the neutral mass directly and reveals whether adducts — sodium at +22, potassium at +38 — are pushing the apparent mass up. The full treatment is in mass spectrometry and peptide identity.
Net peptide content changes every calculation
No field matters more, and none is left out more often. A vial marked 5 mg normally holds 5 mg of gross freeze-dried solid, which comprises the peptide plus counter-ion plus residual water plus any excipient. A peptide purified by reversed-phase chromatography with trifluoroacetic acid in the mobile phase is isolated as a TFA salt, and the TFA share is far from negligible: every basic residue carries a counter-ion, so a sequence rich in lysine and arginine can have a considerable part of its mass present as trifluoroacetate counter-ion. Add moisture absorbed from the air, and the true peptide content of a nominal 5 mg vial may be noticeably under 5 mg.
Net peptide content is properly determined by amino acid analysis or by quantitative nitrogen measurement. When a COA reports it, your molar calculations can be relied on; when it does not, regard concentrations as approximate and state this in your methods.
Getting independent confirmation
The best position is not relying on a nicer-looking COA but obtaining a second opinion on the identical lot. Independent laboratories will test identity and purity on a sample you send, and a supplier that publishes third-party reports next to its own is making a claim that can be checked rather than a bare assertion. How the process works and what to request is explained in third-party testing explained, with the step-by-step reading guide in how to read a peptide COA and the term itself at certificate of analysis. Recurring questions are answered in the purity, COA and testing FAQ.
Questions
Why is peptide purity measured at 214 nm and not 280 nm?
The peptide bond absorbs strongly at around 214–220 nm, so detecting there captures practically all peptide material, deletion sequences and oxidised variants included. Absorbance at 280 nm relies on tryptophan and tyrosine, which many sequences do not contain at all. For such sequences a purity value at 280 nm is almost worthless.
Does 99% purity mean 99% of the vial’s weight is peptide?
No. Chromatographic purity is the area share of UV-absorbing material relative to the total integrated peak area, and it leaves out water, salts and counter-ions completely. Net peptide content by weight is measured separately, usually by amino acid analysis, and normally comes out lower than the purity figure.
How do I compare a mass spectrum with the stated molecular weight?
With electrospray data, the observed m/z at charge state z is roughly (M + 1.008z)/z. For a peptide of 1,419.55 Da you would expect about 1,420.6 at charge 1 and 710.8 at charge 2. A deconvoluted spectrum shows the neutral mass directly, which makes comparison simpler and reveals any sodium or potassium adducts.
What is a TFA salt, and why does it affect my calculations?
Reversed-phase purification usually has trifluoroacetic acid in the mobile phase, so the peptide is recovered as its trifluoroacetate salt. Every basic residue may carry a counter-ion, which adds mass that is not peptide. In sequences rich in lysine or arginine this share is considerable, which is why net peptide content is reported on its own.
Can one material show different purity values on two COAs?
Yes, and quite legitimately. Gradient slope, column chemistry, run time and integration settings all influence how cleanly closely related impurities separate from the main peak. A shallower, longer gradient usually resolves more of them and gives a lower figure — which is why the method section matters as much as the result.
Which COA red flag can be checked fastest?
Check the measured mass against the molecular weight of the declared sequence. It takes a few seconds and the answer is yes or no. If the mass does not fit the labelled peptide, nothing else in the document deserves attention, however good the chromatogram appears.