In plain English
A peptide powder is never all peptide. It also holds water and counter-ions (small charged molecules, such as acetate, that pair with the peptide). Amino acid analysis, or AAA, is one way to find out how much of the powder really is peptide [1, 2].
Hot, strong acid snaps the peptide into its building blocks, the amino acids. An instrument then separates and counts each one. If the sequence is known, the counts tell the analyst how many peptide molecules were in the sample, and so how many micrograms of peptide it held [1].
The method has blind spots. The acid destroys tryptophan, damages a few other amino acids, and turns asparagine and glutamine into aspartic and glutamic acid [1, 2]. AAA also cannot show the order of the amino acids, or whether the peptide is the right sequence rather than a near copy.
The short version
- AAA measures how much peptide is present, not how pure it is.
- Acid breaks the peptide into amino acids, which are then counted [1].
- Only amino acids that survive the acid well are used in the calculation [1].
- Water and counter-ions are measured by other tests.
Why peptide content differs from purity
HPLC purity compares the target peptide with related peptide impurities. It says nothing about the non-peptide part of a powder. Pharmacopoeial value assignment keeps the two apart: peptide impurities are measured as a percentage of chromatographic peak area, while counter-ions, residual solvents, inorganic residue and water are measured as percentages by mass [3].
The non-peptide fraction can be large. In a candidate certified reference material of angiotensin II, the trifluoroacetate counter-ion made up nearly 25% of the mass, and the assigned purity was 691 ± 9 mg of peptide per gram [4]. A sample can therefore be highly pure by HPLC and still be well below 100% peptide by mass. See Purity vs Peptide Content and Counter-Ions in Peptides.
European guidance on synthetic peptides lists amino acid analysis among accepted assay approaches, alongside liquid chromatography against a reference standard, elemental analysis, Kjeldahl nitrogen analysis and quantitative NMR (qNMR) [5].
Step 1: acid hydrolysis, and what it changes
The harmonised pharmacopoeial chapter describes acid hydrolysis with 6 mol/L hydrochloric acid containing phenol, at 110 °C for 24 hours, as the most common method [1]. Hydrolysis can be done in the liquid or vapour phase, and a vacuum or an argon headspace reduces oxidative losses [1]. One methods review recommends vapour-phase hydrolysis for 20–24 hours [2].
Every set of conditions is a compromise. Peptide bonds differ in how easily they break and amino acids differ in how well they survive, so no single condition cleaves every bond without destroying any amino acid [6].
| Amino acid | What standard acid hydrolysis does | Common remedy |
|---|---|---|
| Tryptophan | Destroyed [1] | A separate hydrolysis, for example in methanesulfonic acid with tryptamine added [7] |
| Cysteine / cystine | Poorly measured: partly destroyed or reduced [1] | Oxidation with performic acid first, giving cysteic acid; or alkylation before hydrolysis [1, 8] |
| Methionine | May be oxidised [1] | Performic acid oxidation to methionine sulfone [8] |
| Asparagine, glutamine | Deamidated to aspartic and glutamic acid [1] | Reported as combined values (often written Asx and Glx) |
| Serine, threonine | Partly destroyed [1] | Hydrolyse for 24, 48 and 72 hours and extrapolate back to zero time [1] |
| Isoleucine, valine | Bonds such as Ile–Ile, Val–Val, Ile–Val and Val–Ile break slowly [1] | Use the plateau value reached at longer hydrolysis times [1] |
These losses are measurable. In one study of milk powders, a single 24-hour hydrolysis gave threonine, isoleucine, valine and serine values 3.7–5.6% lower than a model built from several hydrolysis times [9]. Because tryptophan, asparagine and glutamine are lost or converted, standard acid hydrolysis limits quantitation to 17 amino acids [1].
Contamination is the other risk. Low-purity hydrochloric acid can add glycine, and dust can add glycine, serine and alanine, so the chapter recommends clean glassware, high-purity reagents and few openings of the sample vial [1].
Step 2: separating and detecting the amino acids
Most amino acids absorb light weakly, so they are converted into detectable derivatives. There are two families of methods: separation by ion-exchange chromatography followed by post-column derivatisation, and pre-column derivatisation followed by reversed-phase HPLC [1].
| Method | How it works | Points to know |
|---|---|---|
| Ninhydrin (post-column) | Amino acids react after the column. Primary amino acids give a purple product read at 570 nm; imino acids such as proline give a yellow product read at 440 nm [1, 10] | One of the most common quantitative methods. Detection limit about 10 pmol (50 pmol for proline); less affected by buffer salts than pre-column methods [1] |
| OPA (o-phthalaldehyde) | Reacts with primary amines in the presence of a thiol to give fluorescent products [1] | Does not detect secondary amines such as proline, so it is often paired with FMOC-Cl [1, 11] |
| FMOC-Cl | Reacts with primary and secondary amino acids [1] | Detection limits in the low femtomole range [1] |
| AQC | Forms stable urea derivatives read by fluorescence (excitation 250 nm, emission 395 nm) [12] | Detection limits from 40 fmol (phenylalanine) to 800 fmol (cystine); useful composition data from about 30 ng of hydrolysed protein [12] |
| Isotope dilution mass spectrometry | Each amino acid is measured against an isotope-labelled version of itself | Used to assign values to peptide reference materials [4, 13] |
Step 3: calibration and internal standards
Calibration. The instrument is calibrated with an amino acid standard mixture of known concentrations, usually at four to six levels. This gives a response factor (peak area per nmol) for each amino acid and shows the range over which the response is close to linear. Peak area divided by the response factor gives nmol of each amino acid in the sample [1].
Repeatability. A laboratory typically analyses a standard solution at least six times and checks the relative standard deviation of retention time and peak area for each amino acid [1]. A well-characterised protein or peptide run alongside the samples checks the whole procedure, hydrolysis included [1].
Internal standards. An amino acid that does not occur in the sample, such as norleucine, nitrotyrosine or α-aminobutyric acid, can be added before hydrolysis to track losses, or afterwards to correct for differences in sample application [1]. In a test with a known amino acid mixture taken through hydrolysis, internal standardisation with norleucine generally improved both precision and accuracy compared with external standardisation [14]. There is a catch: a free amino acid does not break down at the same rate as one bound in a peptide, so correcting hydrolysis losses with an internal standard can mislead [1].
The calibration mixture is the anchor for the whole measurement. Its role is covered in Reference Standards Explained.
Worked example: net peptide content of angiotensin II
Angiotensin II is an eight-residue peptide, Asp-Arg-Val-Tyr-Ile-His-Pro-Phe, with formula C₅₀H₇₁N₁₃O₁₂ and molar mass 1,046.2 g/mol [15]. Each amino acid occurs once. The numbers below are illustrative.
Set-up. 1.000 mg of powder is dissolved in 1.000 mL of water, and 100 µL (100.0 µg of powder) is hydrolysed. If the powder were 100% peptide, it would release 100.0 µg ÷ 1,046.2 g/mol = 95.6 nmol of each amino acid.
| Amino acid | Measured (nmol) | Used for content? |
|---|---|---|
| Asp | 79.6 | Yes |
| Arg | 79.9 | Yes |
| Phe | 80.4 | Yes |
| Pro | 79.1 | No |
| His | 77.8 | No |
| Tyr | 76.5 | No |
| Val | 75.3 | No (slow to cleave) |
| Ile | 74.0 | No (slow to cleave) |
1. Choose the reliable residues. The harmonised chapter bases peptide or protein content on amino acids that are usually measured well, such as aspartic acid, glutamic acid, alanine, leucine, phenylalanine, lysine and arginine. Each value is divided by the number of times that residue occurs, the results are averaged, and any value more than 5% from the mean is dropped [1]. Here: (79.6 + 79.9 + 80.4) ÷ 3 = 80.0 nmol of peptide. All three are within 0.6% of the mean.
2. Convert to mass. 80.0 nmol × 1,046.2 ng/nmol = 83.7 µg of peptide.
3. Divide by the mass hydrolysed. 83.7 µg ÷ 100.0 µg = 83.7% net peptide content.
Averaging all eight amino acids instead gives 77.8 nmol and 81.4%, about 2.3 percentage points too low, because valine, isoleucine and tyrosine came out short. The remaining 16.3% of the powder is mostly water and counter-ion, which are measured separately (see Karl Fischer Titration). The molar mass used must be stated: the free peptide and its salt give different answers. See Units and Calculations in the Lab.
Alternatives, and how they compare
| Method | What it measures | Main limitation |
|---|---|---|
| Nitrogen analysis (Kjeldahl or combustion) | Total nitrogen, divided by the peptide’s known nitrogen fraction [16] | Any other nitrogen-containing substance in the sample is counted as peptide [16] |
| Quantitative NMR | The intact peptide against an internal standard of known purity, with no hydrolysis step [4, 17] | Related peptide impurities can interfere and must be accounted for [4] |
| HPLC assay against a reference standard | The main peak compared with a characterised standard [17] | Only as good as the standard’s assigned value |
| UV absorbance | Concentration from absorbance at 280 nm (needs tryptophan or tyrosine) or 205 nm [18] | Cannot tell the target peptide from related impurities |
For angiotensin II, nitrogen makes up 17.4% of the free peptide’s mass (13 × 14.007 ÷ 1,046.2). A powder containing 14.6% nitrogen would therefore be about 84% peptide, if nothing else in it contained nitrogen.
In a multi-laboratory comparison using a nine-residue peptide, an HPLC assay that used the same bulk material as its standard gave the lowest variability between laboratories. Proton qNMR was simpler and faster, and the authors proposed it for further evaluation as a primary method [17]. A methods review reports that AAA can usually quantify a peptide to better than 5% [2].
What AAA cannot show
- Sequence order. Hydrolysis breaks every peptide bond, so two peptides with the same amino acids in a different order give the same result. Identity needs mass spectrometry or another sequence-specific method.
- Purity. Deletion sequences and other related impurities release amino acids too. AAA-based values have to be corrected for related peptide impurities measured separately [4].
- Water and counter-ions. AAA gives the peptide fraction by difference only; it does not identify what the rest is.
- Chirality. Standard AAA does not separate D- and L-amino acids; chiral features need additional techniques [3]. Hydrolysis itself can cause some racemisation, more so with microwave heating in one comparison [19].
Reading an AAA result on a certificate of analysis
- Hydrolysis conditions, and any separate hydrolysis used for tryptophan or cysteine.
- Which amino acids were used for the calculation.
- The basis: free peptide or salt, and “as is” or water-corrected.
- The amount of sample hydrolysed, and whether replicates were run.
See How to Read a Peptide Certificate of Analysis.
What this means in Australia
- Pharmacopoeial methods. For medicines, the Therapeutic Goods Act 1989 defines monographs in the British Pharmacopoeia, European Pharmacopoeia and United States Pharmacopeia–National Formulary as standards [20]. The amino acid analysis chapter is harmonised between the European, Japanese and US pharmacopoeias [1].
- Australian participation. Laboratories of the Therapeutic Goods Administration (TGA) took part in the multi-laboratory study that compared HPLC assay, qNMR and AAA for peptide quantification [17].
- Accredited testing. NATA accredits Australian laboratories to ISO/IEC 17025 for the specific methods listed on their scope of accreditation [21].
- Labels. The TGA has stated that a “research use only” disclaimer does not make supply lawful, change regulatory status, permit importation, or remove advertising or supply obligations [22]. An AAA result describes one batch’s composition; it does not change any of this.
This is general information, not legal advice. Laws change; check the current Poisons Standard and TGA guidance.
Frequently asked questions
What is amino acid analysis used for?
It measures how much peptide or protein a sample contains and what its amino acid composition is. It can also support identity testing by comparing the measured composition with the expected one [1].
How is net peptide content calculated from AAA?
The nmol of well-measured amino acids are divided by their number in the sequence and averaged, giving nmol of peptide. Multiplying by the molar mass gives the peptide mass, which is divided by the mass of powder analysed [1].
Why is tryptophan missing from many AAA results?
Standard hydrolysis in hydrochloric acid destroys tryptophan. Measuring it needs a separate hydrolysis, such as one in methanesulfonic acid [1, 7].
Is amino acid analysis a purity test?
No. Related peptide impurities contain the same amino acids, so AAA counts them as peptide. Purity needs a separating method such as HPLC [4].
How accurate is amino acid analysis?
With well-controlled hydrolysis and calibration, quantitation better than 5% is usually achievable [2]. The result depends on hydrolysis conditions, contamination control and the choice of amino acids used for the calculation [1].
References
- The Japanese Pharmacopoeia, 18th edition (English version). General Information: Amino Acid Analysis
(harmonised with the European Pharmacopoeia and the US Pharmacopeia). mhlw.go.jp. Accessed 25 October 2026. - Højrup P. Analysis of polypeptides by amino acid analysis. Methods Mol Biol. 2024;2821:71–82. doi:10.1007/978-1-0716-3914-6_6. PMID: 38997481
- McCarthy D, Han Y, Carrick K, et al. Reference standards to support quality of synthetic peptide therapeutics. Pharm Res. 2023;40(6):1317–1328. doi:10.1007/s11095-023-03493-1. PMID: 36949371
- Melanson JE, Thibeault MP, Stocks BB, et al. Purity assignment for peptide certified reference materials by combining qNMR and LC-MS/MS amino acid analysis results: application to angiotensin II. Anal Bioanal Chem. 2018;410(26):6719–6731. doi:10.1007/s00216-018-1272-7. PMID: 30143839
- European Medicines Agency. Guideline on the development and manufacture of synthetic peptides. EMA/CHMP/CVMP/QWP/367182/2025. Adopted 4 December 2025; effective 1 June 2026. ema.europa.eu. Accessed 25 October 2026.
- Zumwalt RW, Absheer JS, Kaiser FE, Gehrke CW. Acid hydrolysis of proteins for chromatographic analysis of amino acids. J Assoc Off Anal Chem. 1987;70(1):147–151. PMID: 3104303
- Simpson RJ, Neuberger MR, Liu TY. Complete amino acid analysis of proteins from a single hydrolysate. J Biol Chem. 1976;251(7):1936–1940. PMID: 178649
- Gehrke CW, Rexroad PR, Schisla RM, Absheer JS, Zumwalt RW. Quantitative analysis of cystine, methionine, lysine, and nine other amino acids by a single oxidation–4 hour hydrolysis method. J Assoc Off Anal Chem. 1987;70(1):171–174. PMID: 3104304
- Rutherfurd SM, Moughan PJ, Lowry D, Prosser CG. Amino acid composition determined using multiple hydrolysis times for three goat milk formulations. Int J Food Sci Nutr. 2008;59(7–8):679–690. doi:10.1080/09637480701705424. PMID: 18608544
- Friedman M. Applications of the ninhydrin reaction for analysis of amino acids, peptides, and proteins to agricultural and biomedical sciences. J Agric Food Chem. 2004;52(3):385–406. doi:10.1021/jf030490p. PMID: 14759124
- Dubey A, Joshi S, Upadhyay K, Mahato A, Rathore AS. Automated method for quantification of 20 amino acids in cell culture media during biopharmaceutical development. Biotechniques. 2024;76(1):27–36. doi:10.2144/btn-2023-0068. PMID: 37997819
- 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. Anal Biochem. 1993;211(2):279–287. doi:10.1006/abio.1993.1270. PMID: 8317704
- Nicolás Carcelén J, Potes Rodríguez H, González-Gago A, et al. Evaluation of different isotope dilution mass spectrometry strategies for the characterization of naturally abundant and isotopically labelled peptide standards. Anal Bioanal Chem. 2024;416(7):1717–1731. doi:10.1007/s00216-024-05176-1. PMID: 38363304
- Gardner ML. A comparison of internal and external standardization in amino acid analysis. Anal Biochem. 1985;150(1):174–177. doi:10.1016/0003-2697(85)90457-9. PMID: 4083477
- National Center for Biotechnology Information. PubChem compound summary for CID 172198, angiotensin II. pubchem.ncbi.nlm.nih.gov. Accessed 25 October 2026.
- The Japanese Pharmacopoeia, 18th edition (English version). General Information: Total Protein Assay
, Method 7 (nitrogen method). mhlw.go.jp. Accessed 25 October 2026. - Li C, Bhavaraju S, Thibeault MP, et al. Survey of peptide quantification methods and comparison of their reproducibility: a case study using oxytocin. J Pharm Biomed Anal. 2019;166:105–112. doi:10.1016/j.jpba.2018.12.028. PMID: 30640042
- Anthis NJ, Clore GM. Sequence-specific determination of protein and peptide concentrations by absorbance at 205 nm. Protein Sci. 2013;22(6):851–858. doi:10.1002/pro.2253. PMID: 23526461
- Weiss M, Manneberg M, Juranville JF, Lahm HW, Fountoulakis M. Effect of the hydrolysis method on the determination of the amino acid composition of proteins. J Chromatogr A. 1998;795(2):263–275. doi:10.1016/s0021-9673(97)00983-7. PMID: 9528102
- Therapeutic Goods Act 1989 (Cth), section 3: definitions of “standard” and “default standard”. Federal Register of Legislation. legislation.gov.au. Accessed 25 October 2026.
- National Association of Testing Authorities, Australia (NATA). Laboratory accreditation: ISO/IEC 17025. nata.com.au. Accessed 25 October 2026.
- Therapeutic Goods Administration. Understanding your responsibilities when importing, compounding and supplying unapproved peptide products. Safety advisory, 13 April 2026. tga.gov.au. Accessed 25 October 2026.
