HPLC Purity Explained: What the Percentage Measures and What It Does Not

“99.2% purity by HPLC” is the most quoted number in research peptide listings. It is a real measurement with a precise meaning, and that meaning is narrower than most product pages imply. This guide explains what high-performance liquid chromatography measures, how the purity percentage is calculated, what the number cannot tell you, and how to read the peak table that a proper certificate includes. The scope is analytical only.

What HPLC does

High-performance liquid chromatography separates the components of a liquid sample by pushing it through a packed column under pressure. For peptides the standard mode is reversed-phase: the column packing is hydrophobic (a C18 or C8 silica is typical), and the mobile phase is a mixture of water and an organic solvent such as acetonitrile, usually with a small amount of trifluoroacetic acid or formic acid. The run starts with a mostly aqueous mobile phase and increases the organic fraction over time. This is the gradient.

Species that interact weakly with the column elute early. Species that interact more strongly elute later. Each one leaves the column at a characteristic retention time and passes a detector, which records a signal. The plot of signal against time is the chromatogram. A pure sample shows one dominant peak. A mixture shows several.

How the purity percentage is calculated

The detector in nearly every peptide COA is a UV absorbance detector set around 210 to 220 nm, where the amide bonds of the peptide backbone absorb. Software integrates the area under every peak above a threshold. Purity is then reported as area percent:

purity (%) = area of the main peak / sum of all integrated peak areas × 100

This is a relative measure. It compares the main peak with the other peaks in the same run. It says nothing about how much material was in the vial, and it only counts things that absorb light at the chosen wavelength.

Two integration decisions affect the number. The first is the threshold: small peaks below the threshold are ignored, which raises the reported purity. The second is how the baseline is drawn under a broad or tailing main peak. Credible reports state the integration parameters or attach the full peak table so a reader can see what was counted.

What HPLC purity does not measure

Identity

HPLC purity tells you how clean the main peak is. It does not tell you what the main peak is. Two different peptides of similar hydrophobicity can elute at nearly the same retention time. A sample that is 99% a single species is 99% pure in HPLC terms whether or not that species is the peptide named on the label. Identity requires a second method. The usual one is mass spectrometry, which measures the mass-to-charge ratio of the ions produced from the sample and reports an observed molecular mass. The certificate should show the observed mass next to the theoretical mass, and the two should agree within the instrument’s tolerance. A purity-only certificate has skipped this step.

Net peptide content

A lyophilized peptide vial contains the peptide, bound water, counter-ions from the purification (trifluoroacetate or acetate are common) and sometimes residual salts. None of those absorb at 214 nm the way the backbone does, so none of them appear as peaks. A sample can show 99% HPLC purity while the peptide makes up 75 to 85% of the vial’s weight. The absolute figure is called net peptide content and is measured by amino acid analysis, nitrogen determination or a quantitative HPLC against a reference standard. It is rarely shown on supplier certificates.

Non-absorbing impurities

Anything that does not absorb at the detection wavelength is invisible to the detector. Inorganic salts, many solvents and some process residues fall into that category. Endotoxin and microbial contamination are also invisible to HPLC; those require their own tests, covered in our guide to endotoxin and sterility testing.

Quantity

The purity percentage is independent of the amount in the vial. A vial labeled 5 mg could contain 3 mg or 7 mg of material and show the same purity. Quantity is a separate measurement.

Reading a chromatogram summary

Most certificates present a figure and a peak table. The table is the useful part.

Column	What it shows	What to check
Peak number	Order of elution	The main peak is usually not the first; early peaks are often solvent front
Retention time (min)	When the peak left the column	Should be stable across lots run on the same method
Area	Integrated area under the peak	Used in the purity calculation
Area %	That peak’s share of the total area	The main peak’s value is the reported purity
Height	Peak height	A secondary check on integration
Width or symmetry	Peak shape	Broad or split main peaks suggest co-elution or column problems

A summary that lists one row, the main peak, with a purity figure and no other peaks has been trimmed. Real samples produce minor peaks. A complete table shows them, even when they total well under one percent.

Also look at the method block near the table. It should state the column (dimensions and packing), the mobile phases A and B, the gradient program, the flow rate, the detection wavelength, the sample volume loaded and the sample concentration. A report that omits all of these cannot be reproduced and cannot be compared with another laboratory’s result for the same lot.

Why two certificates for the same lot can disagree

Different laboratories running different gradients will integrate different numbers of minor peaks. A short, steep gradient compresses the run and merges small impurities into the main peak, which raises purity. A long, shallow gradient resolves them and lowers it. Neither is wrong; they are different measurements. When a third-party aggregator reports a lower purity than the supplier’s own certificate, the gap is often method, not fraud, and the way to tell is to compare the two method blocks. Where our dataset includes aggregator results they are attributed as such and never presented as our own finding.

How this site records HPLC results

In each supplier’s record we note whether HPLC results are shown, whether mass spectrometry results are shown alongside them, and whether both are tied to a lot. Purity with identity scores higher than purity alone, because a purity figure without an identity result is an incomplete description of the sample. The point values are on the methodology page, and the general guide to the certificate document is how to read a certificate of analysis. We do not run our own HPLC and do not publish purity numbers of our own; the rankings reflect what each supplier publishes and whether it can be verified.

Summary

HPLC purity is a relative, wavelength-dependent measure of how much of the detectable material in one run eluted as a single peak. It is a necessary test and a poor standalone one. A credible certificate pairs it with an identity result, states the method in enough detail to be reproduced, and attaches the full peak table rather than a single number.