How to read a peptide certificate of analysis

Both numbers can be honest at once. They describe different denominators. A certificate reporting only the first is not wrong; it is incomplete, and incomplete in the direction that flatters the material.

Solid-phase peptide synthesis uses trifluoroacetic acid for cleavage and deprotection, and reverse-phase purification runs in TFA buffer. TFA therefore ends up as the counterion on basic residues, and it is lyophilised into the vial along with the peptide.

An illustrative breakdown of a 10 mg gross-weight vial. Actual proportions vary substantially by sequence. The HPLC purity of this vial could legitimately read 99%, because purity only examines the peptide fraction — the first figure above.

Counterion load scales with basic residues

This is worth doing by hand, because it explains why net content differs so much between compounds. Count the arginines, lysines and histidines in the sequence: each one is a site a TFA molecule can pair with, so a polybasic peptide carries proportionally more counterion mass and reports a lower net content.

BPC-157 is a useful worked example. Its sequence — Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val — carries a single lysine and no arginine or histidine at all, so its counterion load sits at the low end and its net peptide content at the high end.

What mass spectrometry actually proves

MS confirms that the observed mass matches the theoretical mass of the claimed sequence. That is a real and valuable check — it catches the wrong compound outright. It does not confirm the order of the residues, because rearranging a sequence does not change its mass.

So “identity confirmed by MS” should be read as the mass is consistent with what was claimed — meaningfully weaker than the sequence has been verified.

What the HPLC wavelength tells you

Purity runs should be detected at 214 nm, where the peptide backbone amide bond absorbs. That makes detection near-universal across peptide impurities, rather than selective for the few residues that absorb at 280 nm.

It also explains the limit of the method: the detector responds to peptide bonds, so it does not see the counterion, the water or the residual salt at all. They are invisible to the calculation rather than counted as zero.

HPLC and mass spectrometry answer questions about identity and chemical purity. They are silent on everything below, and no combination of the two can substitute for the specific test.

• Sterility. Requires a sterility test. Chemical purity says nothing about microbial content.
• Bacterial endotoxin. Requires the LAL assay per USP <85>. Endotoxin is a lipopolysaccharide fragment; it does not register on a purity chromatogram, and it is heat-stable enough to survive sterilisation.
• Residual solvents. Requires headspace GC.
• Heavy metals. Requires ICP-MS.
• Biological activity. No analytical certificate speaks to what a compound does.

We publish HPLC and mass spectrometry results by lot, and we state plainly that these do not establish sterility or endotoxin status. Where a test has not been performed, the certificate says so rather than leaving the field blank. Our own certificates are in the certificate library.

For any quantitative in-vitro work, the figure that matters is the peptide mass, not the label mass:

Any molarity calculated from the label figure inherits the full error. Assuming 10 mg where 7.7 mg is present overstates every concentration in the work by about 30%, which shifts a dose–response curve wholesale and quietly breaks comparison against published values.

Can a peptide be 99% pure and still contain much less than the labelled mass?

Yes, and this is the normal case rather than an edge case. Purity describes only the peptide-related fraction of the sample. Counterion, bound water and residual salts routinely account for 15–25% of gross weight and are invisible to the purity calculation.

Which number should be used for concentration calculations?

Net peptide content. Effective peptide mass is the gross vial mass multiplied by the net content fraction; using label mass alone overstates concentration by whatever the non-peptide fraction happens to be.

How is net peptide content determined?

Amino acid analysis is the reference method — the peptide is hydrolysed to free amino acids which are then quantified, and the European Pharmacopoeia recognises AAA for peptide content determination.

What is the difference between TFA and acetate salt forms?

TFA is the counterion left by standard reverse-phase purification. Acetate is produced by ion exchange and generally gives a higher net peptide content. The relevance is analytical as well as practical: TFA is not inert in cell-based work.

Does a COA prove the compound is safe?

No. A certificate of analysis is a chemical characterisation of a specific lot. It speaks to identity, purity and content. It does not address sterility, endotoxin, biological activity or safety.

Why do so many certificates omit net peptide content?

Amino acid analysis is an additional test with an additional cost, and the resulting figure is always lower and less quotable than the purity percentage. “99% pure” makes a cleaner claim than “99% chromatographic purity at 78% net peptide content”.

• European Pharmacopoeia, general chapter 2.2.56 — amino acid analysis for peptide identification and content determination.
• United States Pharmacopeia <85> — bacterial endotoxins test (LAL).
• Standard references on solid-phase peptide synthesis and reverse-phase purification, for counterion formation and the characteristic impurity profile of SPPS.
• Manufacturer technical documentation on net versus gross peptide content, including AmbioPharm's published guidance on the 60–90% range.