Key takeaways
- HPLC area-% purity is the share of UV signal at one wavelength in the main peak, so it depends on the method and misses co-eluting or non-absorbing species.
- Net peptide content is usually well below 100% because counter-ions such as TFA, bound water and residual salts add weight without adding peptide.
- Mass spectrometry confirms identity by matching observed and expected mass, but it cannot detect isomers or measure purity on its own.
- When comparing batches or suppliers, check the method, wavelength, chromatogram, mass data, batch number and whether the report can be verified.
A certificate of analysis reading “HPLC purity ≥99%” looks definitive, but the number answers a narrower question than most people assume. It tells you how much of the UV-absorbing material in one chromatographic run eluted in the main peak. It does not tell you how much of the powder in the vial is peptide, and on its own it does not prove the peptide is the right one. Those questions need different measurements.
How reversed-phase HPLC separates a peptide from its impurities
Routine peptide purity testing almost always uses reversed-phase high-performance liquid chromatography (RP-HPLC). The column is packed with silica bonded to C18 alkyl chains, giving a hydrophobic stationary phase. The mobile phase starts mostly aqueous and becomes richer in acetonitrile over the run, so peptides adsorb early and elute as organic content rises, broadly in order of hydrophobicity.
Both solvents usually contain about 0.1% trifluoroacetic acid (TFA). It keeps the pH low, suppresses interactions with residual silanols and ion-pairs with positively charged groups on the peptide, giving sharper peaks and better resolution between closely related sequences. The review by Mant and colleagues listed below covers how these choices shift selectivity.
Detection at 214–220 nm
Detection is typically by UV absorbance at around 214 to 220 nm, where the peptide bond itself absorbs. Every peptide and nearly every peptide-related impurity has backbone amide bonds, so this wavelength sees them whether or not they contain aromatic residues. A 280 nm trace picks up tryptophan and tyrosine but would overlook sequences lacking both.
What an area-% purity figure actually tells you
Software integrates each peak in the chromatogram, and purity is the main peak area divided by the total integrated area, multiplied by 100. A result of 99.1% means 99.1% of the integrated UV signal, at that wavelength, came from the main peak. That definition has built-in limits:
- Co-elution. An impurity eluting with the main peak is counted as product. Diastereomers and some oxidised or deamidated forms can sit very close to the parent, particularly on a steep gradient.
- Detector response. Area-% assumes similar signal per unit mass. Related peptides are often comparable at 214 nm but not identical, and a non-peptide contaminant may respond very differently.
- Invisible components. Water and inorganic salts give essentially no signal, and TFA elutes near the void volume where it is normally excluded from integration.
- Method choices. Gradient slope, run length, column and integration thresholds all move the number. A short run can finish before a late-eluting impurity appears.
None of this makes HPLC unreliable; a purity figure simply needs its method. ICH Q2(R2) sets out how a procedure is shown to be fit for purpose, including specificity, accuracy and precision.
The impurities HPLC is usually looking for
Most research peptides are made by solid-phase peptide synthesis (see what research peptides are), and the typical impurities follow from where that chemistry falls short:
- Deletion sequences: a failed coupling leaves a chain missing one residue. A missing glycine shows up 57 Da below the target mass.
- Truncations: chain growth stops early, often because unreacted chains were deliberately capped.
- Incomplete deprotection: a side-chain protecting group survives cleavage, such as a tert-butyl group adding 56 Da.
- Oxidation: methionine oxidises readily to the sulfoxide (+16 Da); tryptophan and cysteine are also susceptible.
- Other modifications: deamidation of asparagine or glutamine (about +1 Da), aspartimide formation (−18 Da) and racemisation, which changes stereochemistry but not mass.
Some of these also form during storage or in solution, which is why purity can drift. See peptide stability in laboratory settings and how lyophilised peptides are stored.
Why HPLC purity is not net peptide content
A lyophilised peptide is not pure peptide by weight, however clean its chromatogram. The powder also holds:
- Counter-ions. The N-terminal amine and the side chains of arginine, lysine and histidine each carry one. After standard purification this is usually trifluoroacetate (TFA is about 114 g/mol). Acetate or chloride forms are lighter but still add mass.
- Bound water. Lyophilised peptides are hygroscopic, and residual water varies with sequence, drying and storage.
- Residual salts and solvents carried through from purification.
Net peptide content is the percentage of total weight that is actually peptide, and it is commonly well below 100%. A 1,200 Da peptide carrying four TFA counter-ions (about 456 g/mol in total) is roughly 72% peptide by weight before any water is counted. A batch can therefore read 99% by HPLC yet contain less peptide per milligram than a batch reported at 97%, if salt form or moisture differ. For anyone preparing solutions by weight, a concentration taken straight from the balance reading will overstate the true value.
Mass spectrometry: confirming identity
HPLC shows how many components are present and in what proportions, but not what they are. Mass spectrometry (MS) measures mass-to-charge ratio (m/z), which lets you check whether the main component has the mass expected for the intended sequence.
ESI and MALDI
Electrospray ionisation (ESI) typically produces ions carrying several protons, so a 3,000 Da peptide may appear near m/z 1,001 (3+) and 751 (4+). Software deconvolutes that charge-state series to a single neutral mass. Matrix-assisted laser desorption/ionisation (MALDI) mostly gives singly charged [M+H]+ ions, which are simpler to read.
Monoisotopic versus average mass
Monoisotopic mass uses only the most abundant isotope of each element (carbon-12, hydrogen-1, nitrogen-14, oxygen-16, sulfur-32). Average mass uses standard atomic weights, which include heavier isotopes such as carbon-13; NIST publishes these values. For peptides the average mass runs roughly 0.6 Da higher per 1,000 Da, close to 2 Da at 3,000 Da. High-resolution instruments usually report monoisotopic mass and lower-resolution ones average mass, so a report should state which it uses.
What MS can and cannot show
A match between observed and expected mass, within the instrument’s stated accuracy, is good evidence of correct composition, and characteristic mass shifts make MS the quickest way to assign impurity peaks. Its limits are just as specific. Intact-mass MS cannot distinguish isomers, so racemisation and swapped residues go unseen, and standard fragmentation does not separate leucine from isoleucine. Ionisation efficiency varies between species, so signal intensity is not a purity measure. Salts, water and counter-ions do not appear in a normal positive-ion peptide spectrum.
LC-MS and the supporting assays
LC-MS gives a mass for each chromatographic peak, so impurities are identified rather than just counted. De Groot and colleagues review how orthogonal methods like this are used to characterise impurities in synthetic peptide products. One wrinkle: TFA suppresses the electrospray signal, so LC-MS methods often use formic acid instead, which changes selectivity. The LC-MS profile may not line up peak for peak with a TFA-based UV method.
Amino acid analysis
Amino acid analysis (AAA) hydrolyses the peptide, conventionally in 6 M hydrochloric acid at about 110 °C, then quantifies the released amino acids. Ratios confirm composition and absolute amounts give net peptide content. Hydrolysis destroys tryptophan, converts asparagine and glutamine to their acids, and partly degrades cysteine and methionine.
Karl Fischer titration and counter-ion assays
Karl Fischer titration measures water specifically, rather than inferring it from loss on drying. Ion chromatography can quantify TFA or acetate. With AAA or nitrogen analysis, these account for the non-peptide mass in a vial.
HPLC, MS and net peptide content compared
| Measurement | Question answered | Typical output | Main limitation |
|---|---|---|---|
| RP-HPLC with UV detection | How much of the UV-detectable material is the main component? | Chromatogram and area-% purity at a stated wavelength | Misses co-eluting and non-absorbing species; method-dependent |
| Mass spectrometry | Is the main component the intended molecule? | Observed mass against expected monoisotopic or average mass | Blind to isomers; not quantitative for purity; ignores salts and water |
| Net peptide content | How much of the weighed powder is peptide? | Percentage peptide by weight (AAA or nitrogen analysis, with water and counter-ion data) | Needs separate assays; not reported for every batch |
Comparing batches and suppliers
Two certificates both showing 98.5% can reflect quite different analyses. Before treating them as equivalent, run through this checklist (a field-by-field walkthrough is in how to interpret a peptide certificate of analysis):
- Method described: column, gradient and run time. A 10-minute and a 40-minute gradient will not resolve the same impurities.
- Wavelength matches: purity at 214 nm and at 280 nm are not interchangeable.
- Chromatogram supplied: it shows peak shape, shoulders, baseline and integration limits that a bare number hides.
- Mass data included: observed and expected values, labelled monoisotopic or average.
- Batch number matches your vial, not a representative lot.
- Report verifiable: an identifiable laboratory, a test date and a way to confirm authenticity.
- Net content stated where solutions of known concentration are needed.
Across different methods, gaps of a few tenths of a percent rarely mean much. A new peak or a growing shoulder usually says more than the headline figure.
The same checklist applies to any supplier, Aussie Peptide Co included. Batch reports are listed on our certificates of analysis page as they are verified, and all products are supplied strictly for in-vitro research use.
References & further reading
- Validation of Analytical Procedures Q2(R2) International Council for Harmonisation (ICH)
- HPLC analysis and purification of peptides Mant CT et al., Methods in Molecular Biology (2007), via PubMed Central
- Immunogenicity of Generic Peptide Impurities: Current Orthogonal Approaches De Groot AS et al., Pharmaceutical Research (2025), via PubMed Central
- Atomic Weights and Isotopic Compositions with Relative Atomic Masses National Institute of Standards and Technology (NIST)
Frequently asked questions
No. HPLC purity is the share of UV signal in the main peak at one wavelength. The powder also contains counter-ions such as trifluoroacetate or acetate, bound water and residual salts, none of which register in that calculation. Net peptide content, the percentage of total weight that is actually peptide, is commonly well below 100% and is measured with separate assays such as amino acid analysis.
The peptide bond absorbs strongly around 214 to 220 nm, so nearly every peptide and peptide-related impurity is detected there, whatever its sequence. Absorbance at 280 nm comes mainly from tryptophan and tyrosine, so a purity figure at that wavelength can overlook impurities lacking those residues. Purity results reported at different wavelengths should not be treated as directly comparable.
Not reliably on its own. Different molecules ionise with different efficiency, so signal intensity in a mass spectrum does not reflect their true proportions. Mass spectrometry is best at confirming identity by matching observed and expected mass, and at assigning impurity peaks. It cannot distinguish isomers of the same mass, and it does not show salts, water or counter-ions. Purity comes from the HPLC trace.
The report and the calculation may use different conventions. Monoisotopic mass uses only the most abundant isotope of each element, while average mass uses standard atomic weights that include heavier isotopes such as carbon-13. For peptides the average value runs roughly 0.6 Da higher per 1,000 Da. ESI spectra also show multiply charged ions that must be deconvoluted before comparison.
Our in-house team writes laboratory-focused guides on peptide documentation, testing, storage and handling. Content is reviewed against published analytical practice and is provided strictly for research use.
