Almost every research peptide supplier states a purity figure, usually 98% or 99%. Far fewer state which method produced it, and the number means different things depending on the answer.
This explains how the figure is generated, what it covers, what it doesn’t, and what the remaining percent actually consists of — because the identity of the impurities usually matters more than their quantity.
Where impurities come from
Nearly all research peptides are made by solid-phase peptide synthesis (SPPS), developed by Merrifield in the 1960s. The peptide is built one residue at a time on a solid resin support: couple an amino acid, wash, deprotect, wash, couple the next.
Each coupling step is efficient but not perfect. At 99.5% efficiency per coupling — which is good — a 30-residue peptide finishes at roughly 86% of chains correct, purely from accumulated coupling losses. Longer sequences compound the problem.
That arithmetic produces a characteristic set of impurities:
Deletion sequences. A coupling fails and the chain continues without that residue. The result is a peptide one residue short, chemically similar to the target and often difficult to separate.
Truncated sequences. Synthesis terminates early, leaving a fragment.
Incompletely deprotected peptides. Side-chain protecting groups that should have been removed during cleavage remain attached.
Modified residues. Oxidation of methionine, cysteine or tryptophan; racemisation; side reactions during cleavage.
Non-peptide material. Residual scavengers, solvents, counter-ions and water.
Crude synthesis product is then purified, typically by preparative RP-HPLC. That removes most impurities but not all, and the ones it removes least effectively are the ones structurally closest to the target — deletion sequences especially.
How purity is measured
RP-HPLC — the standard method
Reverse-phase HPLC separates components by hydrophobicity. The sample is injected onto a column, an increasing gradient of organic solvent pushes components off in order of how strongly they bind, and a UV detector records the result.
Detection is normally at 214 nm, where the peptide bond itself absorbs, because it responds to essentially all peptide material. Some methods use 280 nm, which is specific to aromatic residues — tryptophan, tyrosine, phenylalanine — and will not see a peptide lacking them.
Purity is the main peak area as a percentage of total integrated peak area.
What affects the number
The same sample can produce different figures under different methods, which is why a stated purity without a stated method is weaker evidence:
| Variable | Effect |
|---|---|
| Gradient slope | Shallow gradients resolve closely related species better; steep gradients can merge them |
| Column chemistry | C18 is standard; C4 or C8 suit more hydrophobic sequences |
| Detection wavelength | 214 nm sees all peptide bonds; 280 nm only aromatics |
| Run length | Late-eluting impurities can be missed on a short run |
| Injection load | Overloading distorts peaks and can bury small impurities |
A related impurity that co-elutes with the main peak is counted as main peak. Purity figures are honest measurements under stated conditions, not absolute properties of the material.
Mass spectrometry — identity, not purity
MS confirms that the molecule present has the expected mass. It’s the right tool for identity, and the wrong one for quantifying purity, because ionisation efficiency varies between species — two compounds present in equal amounts can produce very different signal intensities.
Used together, the pair answers both questions: HPLC establishes how much of the detected material is one species, MS establishes that the species is the right one.
Amino acid analysis — how much is peptide
The sample is fully hydrolysed to free amino acids and their quantities measured. Comparing the observed ratios against the theoretical composition gives both a composition check and, importantly, net peptide content — the proportion of the material by mass that is actually peptide.
Why 99% pure does not mean 99% peptide
This is the most consistently misunderstood point in the category, and it’s worth being precise about.
HPLC purity answers: of the UV-absorbing material detected, what proportion is the target peptide?
Net peptide content answers: of the total mass in the vial, what proportion is peptide?
The gap between them is counter-ions and water.
After purification by RP-HPLC, peptides usually carry trifluoroacetate (TFA) counter-ions from the mobile phase, bound to basic residues. Lyophilised material also reabsorbs atmospheric water readily.
Typical composition of a lyophilised vial:
| Component | Rough proportion |
|---|---|
| Peptide | 70–90% |
| Counter-ions (TFA or acetate) | 5–20% |
| Water | 3–10% |
So a vial labelled 10 mg at 99% HPLC purity typically contains somewhere around 7–9 mg of actual peptide. That’s normal and expected — but if your work depends on absolute quantity, the net peptide content figure is the one you need, and many certificates omit it.
TFA is not always inert. Residual trifluoroacetate can interfere with some cell-based assays. Where that matters, salt exchange to acetate or hydrochloride is available, and it’s worth specifying.
What the missing 1% is
The number matters less than the composition. One percent of closely-related deletion sequences is a different situation from one percent of residual solvent.
Deletion and truncation sequences are structurally similar to the target and the hardest to remove. Whether they matter depends entirely on your assay.
Oxidised variants — methionine sulfoxide is the common one, and it forms on storage as well as during synthesis. Mass will read 16 Da higher.
Scavengers and residual solvent — from cleavage. Generally removed well, but present in trace amounts.
Aggregates — some sequences self-associate. These may not resolve on a standard HPLC method at all.
A COA that reports impurity peaks individually rather than only a total gives you meaningfully more information than one reporting a single figure — see what a certificate of analysis actually verifies for how to read one. Purity figures for our tissue repair research compounds and the rest of the catalog are batch-specific.
Reading a purity figure critically
A stated purity is more useful when accompanied by:
- The method. “99% by RP-HPLC at 214 nm” is a claim you can evaluate. “99% pure” is not.
- The chromatogram. The trace itself shows peak shape, baseline quality, and whether the main peak is symmetric or shouldering — a shoulder often indicates a co-eluting related species.
- The batch number, matching your vial.
- Net peptide content, where absolute quantity matters.
- The testing laboratory, named.
Consistent 99%+ figures across every batch and every product in a catalogue, with no supporting chromatograms, warrant more scrutiny than a supplier reporting honest per-batch variation.
What purity does not tell you
A purity figure is a chemical measurement. It does not speak to:
- Sterility — not assessed by HPLC or MS
- Endotoxin content — a separate LAL assay
- Correct folding or disulfide pairing — requires additional characterisation
- Biological activity — a functional question, not a chemical one
- Stability over time — the figure describes the material on the analysis date, under the stated storage conditions
References
- Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. Journal of the American Chemical Society, 1963;85(14):2149–2154. doi:10.1021/ja00897a025
- Mant CT, Chen Y, Yan Z, Popa TV, Kovacs JM, Mills JB, Tripet BP, Hodges RS. HPLC analysis and purification of peptides. Methods in Molecular Biology, 2007;386:3–55. doi:10.1007/978-1-59745-430-8_1
- Andrushchenko VV, Vogel HJ, Prenner EJ. Optimization of the hydrochloric acid concentration used for trifluoroacetate removal from synthetic peptides. Journal of Peptide Science, 2007;13(1):37–43. doi:10.1002/psc.793
- Rutherfurd SM, Gilani GS. Amino acid analysis. Current Protocols in Protein Science, 2009;Chapter 11:Unit 11.9. doi:10.1002/0471140864.ps1109s58
- Palmer M, Chan A, Dieckmann T, Honek J. Biochemical Pharmacology. Wiley, 2014. Chapter on peptide synthesis and characterisation.
Related reading
- What a certificate of analysis actually verifies
- Reconstituting and storing lyophilised peptides
- Sources & References
All VANTA products are supplied for laboratory research use only. They are not for human or veterinary use, not for use in diagnostic procedures, and have not been evaluated by the U.S. Food and Drug Administration.