Amino acid analysis of peptides

Amino acid analysis breaks a peptide down into its amino acids and counts them. It answers two questions that no other routine method answers as directly: is the composition what the sequence says, and how much peptide is actually in the sample? It is the usual basis of net peptide content, and with isotope-labeled standards it is how reference materials are assigned their values. This article describes the method, its known blind spots and how its results are calculated.

Principle

A weighed or measured sample is hydrolyzed: every peptide bond is broken, freeing the constituent amino acids. These are then separated by chromatography, detected and quantified against standards of known concentration. The amount of each amino acid, divided by the number of times it appears in the sequence, gives the amount of peptide; the ratios between amino acids show whether the composition matches.

Hydrolysis

The classical conditions are 6 M hydrochloric acid at 110 °C for 24 hours, in a sealed tube under vacuum or nitrogen to keep oxygen out. A little phenol is usually added to protect tyrosine. Hydrolysis in the vapor phase, with the acid in the bottom of a vessel and the samples in small tubes above it, keeps the acid's own amino acid contamination away from the sample, which matters at the sub-microgram amounts typical of peptide work. Microwave heating can shorten the time to under an hour.

Acid hydrolysis is not gentle, and several amino acids do not survive it intact:

What acid hydrolysis does to individual amino acids

Amino acid	What happens	Usual remedy
Tryptophan	Largely destroyed	Hydrolysis with methanesulfonic acid, or alkaline hydrolysis; or measure by UV absorbance
Cysteine and cystine	Partly destroyed, recovered as a mixture	Oxidation with performic acid first, to measure as cysteic acid
Methionine	Partly oxidized	Performic acid oxidation, to measure as methionine sulfone
Asparagine and glutamine	Converted to aspartic and glutamic acid	Reported together as Asx and Glx
Serine and threonine	Partly destroyed, roughly in proportion to time	Hydrolyze for several times and extrapolate back to zero time
Isoleucine and valine	Bonds between them hydrolyze slowly, so release is incomplete at 24 hours	Longer hydrolysis (48–72 hours) and take the highest value
Tyrosine	Can be halogenated by traces of chlorine	Phenol in the acid

Non-standard residues need the same scrutiny. Pyroglutamate opens to glutamic acid. Peptide bonds next to α,α-disubstituted residues such as α-aminoisobutyric acid (Aib) resist hydrolysis. D-amino acids are hydrolyzed and counted like their L-forms, and the hot acid itself slowly racemizes a small fraction of every residue, so standard analysis cannot measure stereochemistry; chiral methods with deuterated acid are used when that is the question.

Separation and detection

Most amino acids have no useful chromophore, so they are made detectable by reaction with a reagent, either after or before separation:

• Post-column ninhydrin. The original automated method of Spackman, Moore and Stein (1958): amino acids are separated by ion-exchange chromatography, then react with ninhydrin to give a colored product read at 570 nm (440 nm for proline). Robust and well understood, still used as a reference.
• Pre-column derivatization. Amino acids are tagged before a reversed-phase separation with a fluorescent or UV-absorbing group, for example 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC), o-phthalaldehyde (OPA) with FMOC-Cl for secondary amines, or phenyl isothiocyanate (PITC). Faster and more sensitive, at picomole levels.
• Mass spectrometry with isotope dilution. A known amount of each amino acid labeled with stable isotopes (¹³C, ¹⁵N) is added to the sample, and the ratio of natural to labeled amino acid is measured by LC-MS. Losses during the method affect both equally and cancel, which makes it the highest-accuracy approach and the one national metrology institutes use for peptide and protein reference materials.

Calculating composition and peptide content

Each amino acid's amount is divided by its count in the sequence. For a well-behaved sample, the stable amino acids (typically alanine, leucine, phenylalanine, glycine, lysine and arginine) all give close to the same answer: the amount of peptide. The less stable ones are compared against it, with the corrections above.

A worked example: 100.0 µg of a powder containing a peptide of 1,500 g/mol is hydrolyzed. If it were all peptide it would contain 66.7 nmol. The analysis finds:

Amino acid analysis of 100.0 µg of a 1,500 g/mol peptide (illustrative)

Amino acid	Count in sequence	Measured (nmol)	Peptide (nmol)
Leucine	1	52.0	52.0
Alanine	2	105.2	52.6
Phenylalanine	1	51.5	51.5

The mean, 52.03 nmol, is 78.0 µg of peptide at 1,500 g/mol: a net peptide content of 78.0 percent. The other 22 percent is counter-ions, water and anything else the powder holds, as described in the net peptide content article. Agreement between the stable amino acids, here within about one percent, is itself a check that the composition matches the sequence.

What it shows, and what it cannot

• Shows: the amount of peptide in the sample, and whether the amino acid ratios match the stated sequence. A missing or extra residue type, or a wrong ratio, is a strong signal that the material is not what it is labeled.
• Cannot show: the order of the residues; modifications such as amidation, acetylation or disulfide bonds; asparagine versus aspartic acid; or stereochemistry. Related peptide impurities, such as deletion sequences, hydrolyze to nearly the same amino acids and are counted as peptide.

That is why amino acid analysis is read with mass spectrometry, which confirms the intact mass and modifications, and with HPLC, which separates related impurities.

Frequently asked questions

Why is tryptophan often missing from an amino acid analysis?

Standard hydrolysis in hydrochloric acid destroys most of it. It is measured with a different hydrolysis, such as methanesulfonic acid, or estimated from UV absorbance.

Why are results reported as Asx and Glx?

Acid hydrolysis converts asparagine to aspartic acid and glutamine to glutamic acid, so each pair is measured together.

Can amino acid analysis confirm the sequence?

No. It confirms the composition, the numbers of each amino acid, but not their order. Sequence is confirmed by tandem mass spectrometry or Edman degradation.

How much sample does it need?

With modern derivatization and detection, micrograms or less. The difficulty is not detection but contamination: amino acids from dust, skin and reagents are everywhere, so blanks are run alongside.

Is net peptide content from amino acid analysis the same as purity?

No. It measures how much of the weighed mass is peptide of any kind, including related impurities. HPLC purity measures how much of the peptide-like material is the intended sequence.

Sources & Further Reading

1. Spackman DH, Stein WH, Moore S. Automatic recording apparatus for use in chromatography of amino acids. Analytical Chemistry 1958;30(7):1190–1206. doi:10.1021/ac60139a006
2. Fountoulakis M, Lahm HW. Hydrolysis and amino acid composition analysis of proteins. Journal of Chromatography A 1998;826(2):109–134. doi:10.1016/S0021-9673(98)00721-3
3. Rutherfurd SM, Gilani GS. Amino acid analysis. Current Protocols in Protein Science 2009;58(1):Unit 11.9. doi:10.1002/0471140864.ps1109s58
4. Cohen SA, Michaud DP. Synthesis of a fluorescent derivatizing reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate, and its application for the analysis of hydrolysate amino acids via high-performance liquid chromatography. Analytical Biochemistry 1993;211(2):279–287. doi:10.1006/abio.1993.1270
5. Burkitt WI, Pritchard C, Arsene C, Henrion A, Bunk D, O'Connor G. Toward Système International d'Unité-traceable protein quantification: from amino acids to proteins. Analytical Biochemistry 2008;376(2):242–251. doi:10.1016/j.ab.2008.02.010
6. Stoppacher N, Josephs RD, Daireaux A, Choteau T, Westwood SW, Wielgosz RI. Impurity identification and determination for the peptide hormone angiotensin I by liquid chromatography–high-resolution tandem mass spectrometry and the metrological impact on value assignments by amino acid analysis. Analytical and Bioanalytical Chemistry 2013;405(25):8039–8051. doi:10.1007/s00216-013-6953-7

Cosmic Peptides. (2026, October 25). Amino acid analysis of peptides. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/methods/amino-acid-analysis

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  author       = {{Cosmic Peptides}},
  title        = {Amino acid analysis of peptides},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/methods/amino-acid-analysis},
  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
}

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