Amino acid analysis for peptide content
Determining how much peptide is actually present, as distinct from how pure it is.
Purpose
Determining how much peptide is actually present, as distinct from how pure it is.
This page exists so that a discussion about a number can start from a shared understanding of what produced it. It is written for a reader who is technically literate but not a practising analyst.
How it works
Amino acid analysis hydrolyses the peptide into its constituent residues and quantifies them against calibrated standards. Because it measures the residues rather than the intact molecule, it gives a content figure that is independent of the chromatographic purity figure.
The distinction is the single most useful thing on this page. Purity answers "of the material detected, what proportion is the intended species". Content answers "how much of the intended species is in the container". A vial can be 99% pure and contain 80% of its label claim, because the remaining mass is water, counter-ion and excipient.
What determines the result
Counter-ion form matters here. Peptides purified by reversed-phase chromatography with trifluoroacetic acid are commonly isolated as TFA salts; acetate salts are also common. The salt contributes mass and the two differ by several per cent for a typical peptide.
Very few supplier certificates report content, and very few report counter-ion form. Both absences are worth noting when you assess documentation quality.
Typical criteria
| Parameter | Typical value or criterion |
|---|---|
| What it reports | Peptide content as a percentage of gross mass |
| Typical content for a TFA salt | Frequently 75 – 90% of gross mass |
| Alternative methods | Quantitative NMR, UV against a characterised standard |
These are typical values, not universal standards. The criteria that apply to a specific result are the ones in the validated method that produced it, which is why a report without a method identifier is harder to interpret than one with.
What it cannot tell you
Every analytical method answers one question. The most common error in this community's discussions is treating an answer to one question as an answer to another: a mass match read as a purity result, a purity figure read as a content determination, a sterility test read as a guarantee of safety.
When you read a report, write down which question was asked. The rest follows.
Related pages
Other method notes in this commons cover the neighbouring techniques, and the certificate interpretation page covers how these results get reported. The coa interpretation subcategory is where the discussion happens.
Revision history
| 2026-02-17 | v.szabo | Split an overlong section in two and gave the second a proper heading. |
| 2025-11-09 | Birkeland | Citation sweep: two references did not support the sentences attached to them and have been replaced. |
| 2025-07-03 | o.lindgren | Recorded a dissent from one maintainer rather than resolving it silently. |
| 2025-06-03 | bench_notes | Reverted an unsourced change and asked the editor to re-apply it with a citation. |
| 2025-04-06 | y.mensah | Added the table of acceptance criteria requested in doc review. |
| 2025-01-09 | forest_plot | Initial promotion from the source topic. Structure taken from the marked solution. |
Every edit to a maintained document records its author and a note explaining the change. An edit without a note may be reverted by any wiki editor under R9, and the revert is logged. Disagreements about content belong on the source topic rather than in the revision history (R10).