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Amino acid analysis on a peptide COA: what residue composition confirms

Published 9 Nov 2026 · By The QLD Analytical Team

Informational guide for research samples — not medical advice. QLD Analytical arranges independent testing; we do not sell or endorse any product.

Some research-peptide certificates of analysis include amino acid analysis (AAA) or residue composition. This guide explains what that table measures, how to read residue ratios after acid hydrolysis, what AAA cannot prove on its own, and how it sits beside HPLC purity and mass spectrometry identity.

What amino acid analysis measures

Amino acid analysis (AAA) hydrolyses a peptide into its free amino acids, then quantifies each residue type. On a research-peptide certificate of analysis it is used as a composition check: do the molar ratios of residues match the declared sequence, and (when calibrated carefully) how much peptide is present by weight?

AAA answers a different question from HPLC purity or mass spectrometry. HPLC reports peak area for related species. MS confirms molecular mass and often charge-state pattern. AAA asks whether the residue inventory matches what the sequence predicts after accounting for residues that are destroyed or only partly recovered in acid hydrolysis.

Related reading: what a COA should show, how to read an HPLC chromatogram and how to read a mass spec result.

How AAA is usually run on peptides

A common path is acid hydrolysis (often 6 M HCl at elevated temperature), optional derivatisation, then chromatography with UV or fluorescence detection. Some labs use modern UHPLC or LC–MS variants. Whatever the instrument, the report should name the method and the hydrolysis conditions.

Residues that need careful interpretation include:

• Tryptophan — often destroyed or poorly recovered in standard acid hydrolysis unless a protective protocol is used.
• Cysteine / cystine — may need oxidation or specialised handling before reliable quantification.
• Serine and threonine — can show partial loss during harsh hydrolysis.
• Asparagine and glutamine — typically convert to Asp and Glu, so Asn/Gln are not reported separately from Asp/Glu unless a different method is used.

Those caveats are normal chemistry, not automatic red flags. They do mean you should not treat every residue ratio as exact to two decimal places without reading the method notes.

Reading an AAA result on a COA

Look for:

1. A clear statement that amino acid analysis (or residue composition) was performed.
2. Hydrolysis and detection method named, not only a table of numbers.
3. Residue ratios normalised to the expected sequence composition, or absolute nmol values with a stated reference.
4. Notes on residues that were not quantified or were corrected (Trp, Cys, Ser/Thr).
5. Whether AAA was also used to support net peptide content, and if so how that calculation was done.

AAA can support content estimates when recovery and calibration are documented, but it is not a substitute for a dedicated net peptide content discussion when salts and moisture dominate the powder mass. Moisture lines such as Karl Fischer water and salt notes such as residual TFA and counter-ions still matter.

What AAA can and cannot prove

AAA can support sequence composition and, with good calibration, peptide content. It cannot by itself prove that the residues are in the correct order, that termini are acetylated or amidated, or that disulfide bridges are correctly paired. Sequence order needs orthogonal identity evidence (usually MS of the intact peptide or fragments). Terminal form and bridge state need explicit declaration and matching mass / chromatographic behaviour.

A clean AAA table with a failed MS identity check is still a failed identity package. A strong MS result without AAA is common and often acceptable when HPLC purity and mass match the specification — AAA is complementary, not mandatory on every research COA.

A practical checklist

• Does the COA name AAA (or residue composition) and the hydrolysis method?
• Do reported ratios match the declared sequence after Asn→Asp / Gln→Glu conventions?
• Are Trp, Cys and Ser/Thr caveats explained if those residues are present?
• Is AAA being used only for composition, or also for content — and is that calculation shown?
• Do HPLC, MS and AAA tell a consistent story for the same batch?

AAA and testing through QLD Analytical

QLD Analytical is a testing intermediary. Samples are analysed offsite by Apex Labs Pty Ltd. Our standard panel covers mass spectrometry identity, HPLC purity and net content. Amino acid analysis is not automatically included in every identity-and-purity order. If you need residue composition, check the available tests at order time or ask before you ship the sample.

Enter the compound name, labelled strength and salt form exactly as printed on the vial so the laboratory can interpret composition and content correctly. As with every test we arrange, the result applies only to the sample sent in. Packing and postage details are in our customer guide and FAQ.