HPLC purity is the main peak's share of detected peak area, measured at one wavelength after separation by hydrophobicity. It says how much of the sample is one thing. It cannot say what that thing is, because a clean chromatogram of the wrong peptide looks like a clean chromatogram of the right one. Identity needs mass spectrometry.
- HPLC purity is the main peak's share of detected area at one wavelength; it measures how much of a sample is one thing, not what that thing is.
- Water, salts and the counterion do not absorb at 214 nm and are absent from the purity calculation, so purity is not a mass figure.
- A different peptide of similar hydrophobicity, or a deletion sequence, can produce a clean chromatogram; only a mass measurement reveals it.
- Identity is established by mass spectrometry comparing observed mass to the sequence's theoretical mass, read against the monoisotopic value for small peptides.
- Mass cannot separate Leu from Ile, D from L, or sequence order; a confident identity claim uses orthogonal evidence from a second method.
One number that gets asked two questions
A research peptide certificate almost always leads with a purity figure, and a buyer reading 99.1 percent tends to hear two things at once: that the vial is nearly all one compound, and that the compound is the one on the label. The first is what the number says. The second is not, and the gap between them is the single most common misreading of peptide documentation.
The distinction is not pedantic. Purity and identity are separate tests in every specification framework that exists for these materials. The ICH guidance on specifications for biotechnological products, Q6B, treats identity as its own category of test, one that must be specific enough to distinguish the product from other materials, and treats purity as a separate category concerned with the product's impurities. A specification that reports purity without identity has answered half the question, and the half it has answered is the less important one.
This page explains what a reversed-phase HPLC purity measurement actually measures, what kinds of wrong material it cannot see, why identity requires a method that reads mass rather than retention, and how the two results should be read together on a certificate.
What the chromatogram measures
Reversed-phase HPLC separates a sample by how strongly each component interacts with a hydrophobic stationary phase as a gradient of organic solvent moves it through the column. Components elute in order of increasing hydrophobicity, each as a peak, and a detector records absorbance as they pass. For peptides the detector is nearly always ultraviolet, set near 214 nanometres where the peptide bond absorbs, or at 280 where tryptophan and tyrosine absorb.
The purity figure is the area of the main peak divided by the total area of all detected peaks, expressed as a percentage. That is the entire calculation. It rests on three assumptions worth saying aloud: that everything present absorbs at the detection wavelength, that everything present absorbs about equally per unit mass, and that every component actually separated into its own peak. Each assumption fails in a specific, predictable way, and each failure produces a number that looks fine.
The first failure is invisibility. Water, salts, the trifluoroacetate or acetate counterion, and most residual solvents do not absorb at 214 nanometres, so they contribute nothing to the total area and nothing to the denominator. A sample that is 70 percent peptide by mass and 30 percent water and counterion can show 99 percent purity, because the 30 percent is not in the picture. This is why purity is an area figure and not a mass figure, and why net peptide content is a separate measurement covered in the net content guide.
What slips past a purity measurement
The second and third assumptions fail in ways that go to identity, which is the point of this page.
Unequal absorbance means that an impurity absorbing weakly at the detection wavelength is under-counted. A peptide-related impurity absorbs roughly like the peptide and is counted fairly. An unrelated contaminant without peptide bonds or aromatic residues may hardly register, and a sample carrying a meaningful mass of such a contaminant reports a purity higher than the truth. The chromatogram is honest about what it sees; it simply does not see everything.
Co-elution is the more serious failure. Two components that interact with the column almost identically emerge together as one peak, and the integration counts them as one compound. For peptides the components most likely to co-elute with the target are the ones most similar to it: a sequence missing one residue, a sequence with one residue in the wrong place, a sequence with one residue in the D-configuration instead of L, or a version with a modified side chain. Some of these separate under a good method; some do not; none is guaranteed to.
| What is actually in the vial | What the chromatogram shows | What would reveal it |
|---|---|---|
| A different peptide of similar hydrophobicity | One clean main peak | Mass spectrometry: the observed mass is wrong |
| The right sequence missing one residue | One main peak, possibly a shoulder | Mass spectrometry: mass short by one residue |
| The right sequence with a D-residue | Often a single peak | Chiral amino acid analysis; mass alone cannot |
| The right peptide with 30 percent water and salt | 99 percent purity | Karl Fischer, ion chromatography, amino acid analysis |
| The right peptide as the wrong salt | Identical chromatogram | Ion chromatography; the counterion is invisible to UV |
Read the first row again. A supplier who filled a vial with an entirely different peptide of comparable hydrophobicity, whether by error or by intent, would produce a certificate with a clean chromatogram and a high purity figure. Nothing about the HPLC result would flag it. That is not a weakness of HPLC. It is what the method is for: it measures how much of the sample is one thing, and it was never designed to say which thing.
What identity requires
Identity is established by a measurement that depends on what the molecule is rather than on how it behaves on a column, and for peptides that measurement is mass. Mass spectrometry ionises the sample, usually by electrospray, and reports the mass-to-charge ratio of the ions; from the charge states the neutral mass of the molecule is recovered and compared to the mass predicted from the sequence. A match within the instrument's accuracy is strong evidence that the molecule has the composition the sequence implies. A mismatch is decisive the other way.
The comparison has to be made against the right theoretical value. A small peptide is read against its monoisotopic mass, the mass of the molecule with only the lightest isotopes, because that is the tallest peak in its isotope cluster; a larger one is read against the stated peak of the cluster. The formula and weight guide works through the arithmetic and the difference between average and monoisotopic mass. What matters here is that the certificate should print the theoretical mass, the observed mass, and the difference, and that the difference should sit inside what the instrument can resolve.
Mass has its own blind spots, and an honest account names them. Leucine and isoleucine have identical mass, so a swap between them is invisible to a mass measurement and needs tandem mass spectrometry or sequencing to catch. A D-residue has the same mass as its L-form. Two sequences with the same residues in a different order have the same mass. These cases are why the ICH validation guidance, Q2(R1), asks that an identity test demonstrate specificity for the analyte, and why orthogonal evidence, meaning evidence from a second method that works on a different principle, is the standard for a confident identity claim. HPLC retention against a reference standard is one such orthogonal check; it is a weak identity test on its own and a useful one alongside mass.
Reading the two results together
Put the two measurements side by side and each covers the other's gap. The mass result says the main component has the right composition; the chromatogram says the main component is most of the sample. Neither alone establishes that a vial contains mostly the right peptide. Together they do, subject to the blind spots above and to the sample having come from the lot in question, which the independent testing guide covers.
A certificate that carries both results should also let a reader see that they describe the same sample. The chromatogram should show a main peak; the mass spectrum should have been taken on that peak, or on the whole sample with the main species identified. A report that shows an HPLC trace from one date and a mass result from another, or from a different lot, has combined two measurements that may not describe the same material. The certificate reading guide covers the fields that tie a result to a sample.
The last thing to read is the method. A purity figure with its column, gradient, wavelength and integration stated can be repeated by another laboratory and disputed if it is wrong. A mass result with its instrument, ionisation mode, resolution and mass accuracy stated can be judged. A certificate that prints two numbers and no methods has asked to be trusted, which is the one thing a certificate exists to make unnecessary.
What a defensible purity method looks like
Since the purity figure depends entirely on the method, a reader should know what a sound one contains, if only to recognise its absence on a certificate.
The separation runs on a reversed-phase column, usually a C18 phase, with a gradient of acetonitrile in water and a small amount of trifluoroacetic acid in both solvents to sharpen the peaks. The gradient's slope decides how well neighbouring peptides separate: a shallow gradient spreads them out and takes longer, a steep one is quick and merges them. A laboratory choosing a method for a specific peptide runs it shallow enough that the known related impurities, the deletion and truncation sequences the synthesis is likely to have produced, resolve from the main peak. A generic fast gradient applied to every peptide in a catalog will not, and its purity figures run high for that reason.
Detection wavelength is the second choice. At 214 nanometres the peptide bond itself absorbs, so every peptide and peptide fragment in the sample registers, which is what a purity measurement wants. At 280 only tryptophan and tyrosine absorb, so a peptide without them is invisible and its impurities without them are invisible too. A certificate that reports purity at 280 for a sequence lacking both residues has measured almost nothing.
Around the separation sit the controls that make it trustworthy. A blank injection shows the baseline is clean and nothing carried over from the previous sample. A system suitability check, the ICH validation guidance's term for confirming the instrument performs as expected on the day, establishes that the column still resolves and the detector still responds. Integration settings, the threshold below which a bump is not counted as a peak and the way the baseline is drawn, decide the final percentage, and a laboratory that states them has told the reader how the number was made.
None of this needs to be on a product page. All of it should be available from the laboratory for a given report, and the certificate should carry enough of it, at minimum the column, gradient and wavelength, that another laboratory could repeat the run. The certificate reading guide lists the fields.
Why suppliers lead with purity
There is a commercial reason the purity figure sits at the top of nearly every certificate, and a buyer is better off knowing it. Purity is a single number that is easy to compare across suppliers, easy to print large, and reliably high for any competently purified peptide, because reversed-phase purification is good at removing the things HPLC can see. A market that competes on that number rewards suppliers for reporting it prominently and punishes nobody for leaving identity as an afterthought.
The consequence is a category in which purity claims are ubiquitous and identity claims are thin. Many certificates report a mass result in a corner without the theoretical value beside it; some report none. A buyer who has read this far knows which number to look for first. The question to ask of any certificate is not whether the purity is high but whether the mass matches the sequence, and a supplier that cannot answer that has not established what it is selling, however clean the chromatogram.
None of this is an argument against HPLC. The separation is the right tool for its question, and a lot with a poor chromatogram has a real problem. It is an argument for asking both questions and expecting both answers, on every lot, from a laboratory that can be named.
How the two results appear on our records
Each LabFirst product record links the certificate for the lot currently shipping where that certificate has landed, and the record's own fields state the sequence, formula and theoretical mass the certificate's identity result should be read against. Purity is shown with its method where the laboratory reported one, and a purity figure without an accompanying identity result is described as exactly that. The Verify a COA page explains how to confirm either result with the issuing laboratory, and the purity testing guide goes deeper into what the detector sees.
Purity and identity describe what is in a vial. They describe nothing about what it is for. FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. Analytical documentation exists so that a laboratory can confirm what it received, and for no other purpose.
Common questions
Can a peptide be 99 percent pure and still be the wrong peptide?
Why is HPLC purity reported as a percentage of area rather than mass?
What can mass spectrometry not tell me about a peptide's identity?
Is a matching HPLC retention time proof of identity?
What should a certificate show for identity and purity to be read together?
Sources
- FDA / ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Identity and purity as separate categories of test, and the requirement that an identity test be specific enough to distinguish the product from other materials.
- FDA / ICH Q2(R1), Validation of Analytical Procedures: Text and Methodology. Specificity as a validation characteristic, and the basis for the orthogonal-evidence standard for identity.
- FDA / ICH Q3A(R), Impurities in New Drug Substances. The framework for reporting and identifying impurities, cited for what a purity measurement is meant to characterise.
- PubChem compound record: BPC-157. The reference for the sequence-derived formula and mass a certificate's identity result is read against.
- American Society for Mass Spectrometry, About Mass Spectrometry. Background on what a mass spectrometer measures and how a neutral mass is recovered from ions.