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What is amino acid analysis of peptides? What it can count
Amino acid analysis hydrolyses a peptide and counts the residues. What that proves on a COA, what acid destroys, and how net peptide content is measured.
Amino acid analysis of peptides breaks a peptide into its free amino acids with strong acid, classically 6 M hydrochloric acid at 110 °C, then separates and measures each one. The result gives the peptide’s composition, which supports identity, and its absolute quantity, summed residue by residue from the known sequence. It does not give sequence order.
This is an evergreen explainer: the method is decades old and no news hook is claimed. This desk read four documents on 19 October 2026: a 2023 paper on amino acid analysis by LC-MS, the harmonized USP <1052> chapter text (official May 2018), Bachem’s quality-control guide and Biosynth’s analytics page.
What amino acid analysis of peptides establishes on a COA
| Question about the lot | Amino acid analysis | HPLC purity | MS identity |
|---|---|---|---|
| Which residues, in what ratio | Yes, by composition | No | No |
| How much peptide is in the powder | Yes, summed from residues against weight | No | No |
| Share of the main chromatographic peak | No | Yes | No |
| Whether the molecular mass matches the target | No | No | Yes |
| The order of the residues | No; Bachem says it gives no sequence information | No | Not this table’s claim |
USP <1052> lists the uses: quantifying proteins and peptides, identity based on amino acid composition, supporting structure analysis, evaluating fragmentation for peptide mapping, and detecting atypical amino acids. The site’s notes on what MS confirms that HPLC purity cannot and how to read a COA line by line cover the other two columns.
How is amino acid analysis performed?
The 2023 paper, from a group that built an LC-MS method for peptide hydrolysates, credits Moore and Stein with introducing acid hydrolysis in the early 1950s and gives the classical conditions as 6 M HCl at 110 °C for 18 to 24 hours. Its own samples ran at 100 to 120 °C. USP <1052> Method 1 specifies 6 N hydrochloric acid with 0.1% to 1.0% phenol, which prevents halogenation of tyrosine; samples are typically hydrolysed at 110° for 24 hours in vacuum or inert atmosphere, with 48 and 72 hours investigated if hydrolysis may be incomplete.
Most free amino acids lack a chromophore, so the paper notes derivatisation is usually needed for UV or fluorescence detection. Biosynth’s page says it uses the Waters AccQ-Tag pre-column fluorescent system.
What acid hydrolysis cannot count
The chapter is blunt about what the acid does to the sample.
| Residue or bond | Effect of acid hydrolysis, per USP <1052> |
|---|---|
| Tryptophan | Destroyed |
| Serine, threonine | Partially destroyed |
| Methionine | Might undergo oxidation |
| Cysteine | Typically recovered as cystine, recovery usually poor |
| Asparagine, glutamine | Deamidated to aspartic and glutamic acid |
| Ile-Ile, Val-Val, Ile-Val, Val-Ile | Partially cleaved |
USP states that the loss of tryptophan, asparagine and glutamine limits quantitation to 17 amino acids. A peptide rich in those residues is counted through what survives, which is a limit on the method, not a defect in any one certificate.
Where net peptide content comes from
The two manufacturer pages this desk read do not describe the same route. Bachem files amino acid analysis under identity and names nitrogen content from elemental analysis as a measure of peptide content. Biosynth says net peptide content is measured by elemental analysis or quantitative amino acid analysis, and that water and counterions make the net amount roughly 60-80% of gross weight. Both positions are published; a COA that prints a net content figure should say which method produced it.
That figure is why 99% purity is not 99% peptide by weight.
Questions readers ask
What does amino acid analysis tell you about a peptide?
Two things. It gives composition, the ratio of residues, which USP general chapter 1052 lists as a basis for identity. It also gives quantity: the 2023 paper summed the individual amino acid concentrations from the known sequence. It does not give sequence order, which Bachem states plainly, and it says nothing about the share of the main HPLC peak.
How is amino acid analysis performed?
The peptide is hydrolysed in strong acid until its bonds break, then the free amino acids are separated by chromatography and detected. Most lack a chromophore, so the 2023 paper notes a derivatisation step is usually needed for UV or fluorescence detection; that paper’s own method used LC-MS instead. Biosynth describes pre-column fluorescent tagging with the Waters AccQ-Tag system.
Is net peptide content the same as HPLC purity?
No. Bachem states that net peptide content and purity are not equivalent, because net content includes peptidic contaminants. Purity is a peak share; net content is peptide as a share of the powder’s weight, with counterions and moisture making up much of the rest. A pure peptide rich in basic residues can still show a low net content.
None of this concerns what any peptide does. The material on these certificates is supplied for laboratory research use only, and this desk reads amino acid analysis for what it documents about the powder in the vial.