Peptide purity is the share of a sample's UV-absorbing material that is the intended peptide, measured by reversed-phase HPLC with detection at 210 to 220 nm, where the peptide bond absorbs. It is reported as the area of the principal peak divided by the total integrated area of the chromatogram, as a percentage. A figure of 98% means 98% of what the detector saw was the target sequence. It does not mean 98% of the powder in the vial is peptide.
That distinction, between purity and content, is where most confusion about peptide quality starts. This post explains how the measurement works, what it catches, what it cannot see, and how to read a purity figure on a certificate.
Why purity is measured at all
Research peptides are made by solid-phase synthesis: amino acids are added one at a time to a growing chain anchored on a resin, then the finished chain is cut free and its protecting groups removed. Every step runs slightly short of 100%, so the crude product is a family of closely related molecules, not one. The introduction to research peptides covers why that matters to anyone using a reagent in the lab: an impurity that differs from the target by a single residue can bind, compete or interfere in an assay.
A review of related impurities in synthetic peptides by D'Hondt and colleagues groups them by origin:
| Impurity type | How it arises |
|---|---|
| Deletion sequence | A coupling or deprotection step fails, so one residue is missing |
| Insertion sequence | Excess amino acid reagent adds a residue twice |
| Diastereomer | Racemization of a residue during synthesis (an L residue becomes D) |
| Protection adduct | A side-chain protecting group is not fully removed |
| Oxidation product | A side chain, most often methionine, is oxidized |
| Dimer or oligomer | Two or more chains link, for example through cysteine |
| Degradation product | Later chemistry: deamidation, pyroglutamate or succinimide formation |
Purification by preparative reversed-phase chromatography removes most of these. Analytical HPLC is how the remainder is measured.
How reversed-phase HPLC separates a peptide
HPLC is liquid chromatography run at high pressure through a column of very fine particles. For peptides, the reversed-phase mode is the standard; Mant and Hodges describe it as the most widely used HPLC mode for peptide separations.
The column. A steel tube packed with silica particles whose surface carries C18 (octadecyl) chains. The surface is hydrophobic.
The mobile phase. Two solvents: water with a small amount of trifluoroacetic acid (TFA), and acetonitrile with the same acid. The run starts mostly aqueous and the acetonitrile share rises over time, which is called a gradient.
Separation. A peptide sticks to the C18 surface by its hydrophobic residues and lets go when the acetonitrile share is high enough. Peptides therefore elute roughly in order of increasing overall hydrophobicity. A deletion sequence missing a leucine is usually less hydrophobic than the full sequence and elutes a little earlier.
The acid. TFA is an ion-pairing reagent. It pairs with the protonated basic groups on a peptide (lysine, arginine, histidine and the N-terminus), which sharpens the bands and makes retention reproducible. Conventional methods use about 0.05% to 0.1% TFA; Mant and Hodges found somewhat higher concentrations improved some separations.
Detection. A UV detector reads the column outflow continuously. The amide bonds of the peptide backbone absorb strongly in the far UV, around 220 nm, so detection is usually set between 210 and 220 nm. Because every residue contributes a backbone amide, nearly every peptide-related impurity is visible at that wavelength, whatever its sequence.
How the purity number is calculated
The detector output is a chromatogram: signal against time. Each separated component appears as a band, and software integrates the area under each one.
Purity is the area of the principal peak as a percentage of the summed area of all integrated bands, after the solvent front and blank signals are excluded.
A worked example with invented numbers:
| Component | Retention time (min) | Area (mAU·s) | Area % |
|---|---|---|---|
| Deletion sequence | 11.8 | 14 | 0.7 |
| Target peptide | 12.4 | 1,962 | 98.1 |
| Oxidized form | 12.9 | 16 | 0.8 |
| Unknown | 14.1 | 8 | 0.4 |
| Total | 2,000 | 100.0 |
The certificate for this lot would state purity as 98.1% (HPLC, 214 nm), and a full report would include the chromatogram and this integration table.
What HPLC purity does not tell you
The method is powerful, and its limits follow directly from how it works.
It does not count what is not peptide. Water and inorganic salts give no signal at 214 nm, and counter-ions such as TFA and acetate are not part of the peptide calculation at all. A lyophilized peptide can contain 10% or more water and counter-ion by mass and still show 99% purity. Bachem's quality control guide makes the point directly: net peptide content and purity are not equivalent.
It assumes equal response. Area percent treats every milligram of every component as producing the same signal. That is approximately true for peptides of similar length, because the signal comes mostly from backbone amides, but it is not exact. Preston and Phillips note that the assumption that area is uniformly proportional to mass for all components "is not always valid". Aromatic residues (tryptophan, tyrosine, phenylalanine) add absorbance, so an impurity rich in them reads larger than its mass.
It can miss co-eluting impurities. Two molecules with nearly the same hydrophobicity can leave the column together and be integrated as one band. An FDA laboratory study of peptide drug quality found an amino acid deletion and an insertion in a synthetic peptide that the submitted HPLC-UV methods could not resolve; mass spectrometry separated them by mass. A single chromatogram is only as selective as its column and gradient.
It is not identity. A clean, single band says the sample is homogeneous. It does not say the band is the right sequence. Identity needs mass spectrometry, which measures the molecular weight of the main component and compares it with the value calculated from the sequence.
It depends on the method. Column chemistry, gradient slope, wavelength and integration settings all shift the number. A figure of 99% on a steep five-minute gradient and 97% on a shallow thirty-minute gradient can describe the same material. That is why a purity figure means little without the method beside it.
Purity versus net peptide content
Two numbers answer two different questions.
| HPLC purity | Net peptide content | |
|---|---|---|
| Question answered | Of the peptide-like material, how much is the target? | Of the total powder mass, how much is peptide? |
| Method | Reversed-phase HPLC, UV 210 to 220 nm | Amino acid analysis or nitrogen (elemental) analysis |
| Blind to | Water, salts, counter-ions | Which peptide the peptide mass belongs to |
| Typical figure | Stated per lot; purified research material is often 95% or more | Often well below 100%, because of counter-ions and water |
Bachem notes that peptides with many basic residues carry more counter-ion and so show lower net content, and that hydrophilic peptides can absorb considerable moisture. For a lyophilized TFA salt, counter-ion alone can be a substantial fraction of the mass: one 2025 study measured roughly 25% TFA by weight in its peptides before exchange.
For in-vitro work, both matter. Purity tells you how much of the signal in your assay can come from something other than the target. Net content tells you how much target is actually in the tube when you make a stock solution by weight.
Purity grades and what they are used for
Custom synthesis suppliers sell peptides at several purity levels. The ranges below follow GenScript's published guidance on matching purity to application; other suppliers draw the lines slightly differently.
| Purity | Typical laboratory uses |
|---|---|
| Crude | Initial screening, sequence optimization |
| 75% or more | ELISA, peptide arrays, antibody generation |
| 85% or more | In-vitro bioassays, epitope mapping, blocking studies |
| 95% or more | Quantitative receptor-ligand studies, competitive inhibition assays, NMR |
| 98% or more | Crystallography and other structural work |
Higher purity costs more because each extra point comes from narrower cuts in preparative chromatography, which discard more product.
How to read a purity figure on a certificate
When a certificate states a purity, check that it also states:
- The method. "HPLC" at minimum, ideally the column type, gradient and run time.
- The wavelength. 214 nm or 220 nm is standard. A figure at 254 or 280 nm mostly reflects aromatic residues and can hide impurities that lack them.
- The chromatogram. A figure without the trace behind it cannot be checked. Look for a flat baseline, a sharp main band and a listed integration table.
- The lot number. Purity belongs to one lot. It is not a property of the compound.
- Identity. A mass spectrum confirming the molecular weight of the main component.
Storage also affects purity over time. Oxidation and deamidation continue slowly in the dry state and much faster in solution, which is why the lyophilized form and frozen storage matter. The storage and handling page sets out how Anhydrolabs recommends keeping material: frozen at −4 °F (−20 °C), away from light.
Anhydrolabs supplies each compound as a lyophilized powder in a vacuum-sealed vial and as 10-vial kits, with the lot number on every vial. The certificate for a lot, for example a lot of BPC-157, is available by writing to [email protected] with the lot number.