The practical answer
Amino acid analysis hydrolyses a peptide into its amino acids, then separates and quantifies them. It answers two questions HPLC purity does not: how much peptide is in the weighed powder (net peptide content) and whether the residue ratios match the sequence. Acid hydrolysis destroys tryptophan, partly destroys serine and threonine and converts asparagine and glutamine, so results for those residues need care. When content is calculated from well-recovered residues, the harmonised pharmacopoeial text says a deviation of more than 5% from the mean is typically considered unacceptable. [1] [2]
Evidence, method & source dates ↓What amino acid analysis measures
Bachem describes amino acid analysis (AAA) as using a strong acid to break the peptide into individual amino acids, which are separated by chromatography and quantified; the method does not provide sequence information. [3] Its net peptide content FAQ adds that the peptide is hydrolysed with 6N hydrochloric acid, the free amino acids are derivatised before the column and separated by reversed-phase HPLC. [2]
The Pharmacopoeial Discussion Group text on amino acid determination, harmonised across the European, Japanese and United States pharmacopoeias, describes AAA as the method for determining the amino acid composition or content of proteins and peptides and sets out hydrolysis, calibration, repeatability and calculation. [1] AAA therefore sits next to HPLC and mass spectrometry rather than replacing them: HPLC estimates the share of peptidic material that is the target, MS confirms the mass, and AAA counts how much peptide material is there.
From AAA to net peptide content
Lyophilised peptide powder is not all peptide. Bachem defines net peptide content as the fraction of peptidic material relative to counter-ions and residual water, gives the relation net weight = gross weight x NPC / 100, and uses the example of an 85% NPC making 1 mg gross equal to 0.85 mg net. It notes that peptides rich in basic residues, especially arginine, can be expected to have relatively low content because of salt formation. [2]
GenScript describes net peptide as the target peptide plus peptidic impurities such as deletion peptides, lists amino acid analysis and peptide content (nitrogen) analysis as tests customers can request, and gives the amount of target peptide as gross weight x net peptide content x HPLC purity. [4] The two numbers measure different things, so a certificate showing 99% purity says nothing about how much of each milligram is counterion or water.
| Result | Question it answers | Typical method |
|---|---|---|
| HPLC purity | What share of peptidic material is the target? | Reversed-phase HPLC [5] |
| Identity | Is the main component the expected mass? | Mass spectrometry [5] |
| Net peptide content | What share of the powder is peptide? | AAA or nitrogen (elemental) analysis [2] [4] |
| Composition | Do residue ratios match the sequence? | AAA mole ratios against expected residues [1] |
| Water | How much is moisture? | Karl Fischer titration [3] |
Residues acid hydrolysis cannot recover reliably
The harmonised text states that during acid hydrolysis tryptophan is destroyed, serine and threonine are partially destroyed, methionine may oxidise, and cysteine is typically recovered as cystine, whose recovery is usually poor. Bonds between isoleucine and valine residues are only partly cleaved, and asparagine and glutamine are deamidated to aspartic and glutamic acid. The loss of tryptophan, asparagine and glutamine limits quantitation to 17 amino acids, and glycine and serine are common contaminants. [1]
Standard hydrolysis is about 110 degrees C for 24 hours under vacuum or inert gas. Alternative procedures exist for tryptophan and for cysteine-cystine, such as oxidation to cysteic acid, but they have to be requested. [1] So a composition report that shows Asx and Glx instead of Asn, Asp, Gln and Glu, or omits tryptophan, is expected behaviour, not a defect.
Tolerances you can expect from the method text
For a sample of known sequence, the harmonised text calculates content from well-recovered amino acids, typically aspartate-asparagine, glutamate-glutamine, alanine, leucine, phenylalanine, lysine and arginine. Each one's quantity is divided by its expected residue count, the results are averaged, and values deviating from the mean by more than 5% are typically considered unacceptable and discarded before recalculating. The average relative compositional error usually excludes tryptophan and cysteine. [1]
For repeatability, the text describes analysing six or more replicates of a standard and determining the relative standard deviation of retention time and peak area for each amino acid, with each laboratory establishing its own analytical limits. [1] It does not set a single pass range for a composition result, so a certificate that states one should say whose limit it is.
Worked example with fictional numbers
An invented 10-residue peptide contains Ala 2, Leu 2, Lys 2, Arg 1, Phe 1 plus one Gly and one Ser, with a free-base molecular weight of 1,100 g/mol. A 30 microgram gross aliquot is hydrolysed and analysed.
Dividing each well-recovered result by its residue count gives 20.5, 20.2, 19.8, 20.6 and 19.9 nmol per residue for Ala, Leu, Lys, Arg and Phe. The mean is 20.2 nmol, and the largest deviation is 2.0%, inside the 5% rule. Gly reads 1.14 residues (contamination is common) and Ser 0.83 (partial destruction is expected), so neither is used for content. 20.2 nmol x 1,100 g/mol = 22.2 micrograms of peptide in 30 micrograms of powder, a net peptide content of about 74%. With 98% HPLC purity, 1 mg gross contains about 1 x 0.74 x 0.98 = 0.73 mg of target peptide.
| Residue | Expected | Measured nmol | nmol per residue | Deviation from 20.2 |
|---|---|---|---|---|
| Ala | 2 | 41.0 | 20.5 | +1.5% |
| Leu | 2 | 40.4 | 20.2 | 0.0% |
| Lys | 2 | 39.6 | 19.8 | -2.0% |
| Arg | 1 | 20.6 | 20.6 | +2.0% |
| Phe | 1 | 19.9 | 19.9 | -1.5% |
| Gly (not used) | 1 | 23.1 | 23.1 | contaminant-prone |
| Ser (not used) | 1 | 16.8 | 16.8 | partly destroyed |
How often research certificates include AAA
Rarely, in the pages we checked. GenScript treats HPLC and MS as normally sufficient and lists AAA as a test you request. [4] Among research suppliers on this site, Peptides-EU describes HPLC purity and mass-spectrometry identity, and Evolve lists purity, quantity, sterility, endotoxin and heavy-metal testing without naming a method for quantity; neither page we checked mentions AAA or net peptide content. [5] [6]
If your work depends on the amount of peptide rather than the weight of powder, ask the supplier whether net peptide content was measured, by which method, and on which lot. If not, record content as unmeasured and consider an independent laboratory. Do not back-calculate content from a purity percentage.
Questions before you decide
Is AAA the same as purity?
No. Purity is the target's share of peptidic material by HPLC. AAA measures how much peptide is in the powder and its residue ratios.
Why does my AAA report show Asx and Glx?
Acid hydrolysis converts asparagine and glutamine to aspartic and glutamic acid, so they are reported together.
Can AAA confirm the sequence?
No. It gives composition, not order. Sequence identity needs mass spectrometry or sequencing.
Check the underlying evidence
Sources & review dates
- Pharmacopoeial Discussion Group B-01 Amino Acid Determination, harmonised EP/JP/USP text (Revision 1, Correction 1, November 2024), hosted by PMDA ↗Accessed 29 Oct 2026
- Bachem FAQ: What does net peptide content mean? ↗Accessed 29 Oct 2026
- Bachem Knowledge Center: Quality control of amino acids and peptides, a guide ↗Accessed 29 Oct 2026
- GenScript AccuPep peptide quality control services ↗Accessed 29 Oct 2026
- Peptides-EU FAQ: HPLC purity and mass-spectrometry identity testing ↗Accessed 29 Oct 2026
- Evolve Peptides homepage: per-batch tests listed ↗Accessed 29 Oct 2026
Sources are documentary evidence. They do not establish firsthand testing of a listed supplier.