Reading an HPLC chromatogram, what the purity number does not tell you.

Every research peptide certificate of analysis prints a single headline number. Purity, the reported percent. Most buyers read that number, accept it, and move on. The number is not wrong. The number is also not the full story.

The full story is the chromatogram itself. The purity percentage is a single integration value pulled from a chart that contains five or six other pieces of information about what is in the vial. If the lab gives you the certificate without the chart, the lab is asking you to trust them on the integration. If the lab gives you both, you can verify the integration yourself.

This piece walks through what those other pieces of information are, why each one matters, and what you should look at before you trust the headline number.

What the chromatogram actually shows

HPLC stands for high-performance liquid chromatography. The principle is straightforward. The peptide sample is dissolved, injected into a column packed with stationary phase material, and pushed through with a mobile-phase liquid. Different molecules in the sample interact with the stationary phase differently and emerge from the column at different times. A detector at the end of the column reads each molecule as it emerges, plotting absorbance over time.

The result is a chart. The x-axis is retention time in minutes. The y-axis is absorbance. Each peak on the chart is a different molecule that came out of the column at a different time. A pure peptide produces a single sharp peak. A peptide with related impurities or degradation products produces a main peak plus smaller secondary peaks at other retention times.

The purity percentage is calculated by integrating the area under the main peak and dividing by the total integrated area of all peaks combined. Ninety-nine point six two percent means the main peak accounts for that fraction of the total absorbance signal. Zero point three eight percent of the signal came from somewhere else.

What to look at first

The shape of the main peak

A clean peak is sharp, symmetric, and tall. A peak that is broad and flat suggests the column is not separating the analyte well, which means the integration is less reliable. A peak with a leading or trailing shoulder suggests a co-eluting impurity is partly hiding inside the main peak. The integration may report the reported percent but the actual purity could be lower because the impurity is being counted as part of the main peak.

Look for symmetry. Look for sharpness. If the main peak looks like a clean Gaussian curve, the integration is reliable. If it looks like a lump with a shoulder, ask the lab about peak deconvolution.

The retention time

Every peptide has a characteristic retention time on a given column with a given mobile phase gradient. BPC-157 on a C18 reversed-phase column with the standard water-acetonitrile gradient with 0.1 percent TFA modifier elutes at approximately 6.7 minutes. If the certificate reports the main peak at 4.2 minutes, something is wrong. The peak that integrated as the product is not the product.

Reputable labs publish their method conditions on the certificate. Column type, mobile phase composition, gradient program, flow rate, detection wavelength. With those parameters the retention time of the analyte is predictable. The retention time on the chart should match the retention time the method predicts. If it does not, the certificate is not certifying what it says it is certifying.

The minor peaks

Most chromatograms show one or two small peaks that are not the main analyte. These are typically related substances, which means molecules structurally similar to the target peptide that arose during synthesis or during storage. Common related substances for synthetic peptides include deamidated forms (where an asparagine or glutamine residue has lost its amide group), oxidized forms (where a methionine or tryptophan has gained an oxygen), and truncation products (where the synthesis fell short by one or two residues).

Each minor peak should be labeled or at least identified by retention time. The integration of each minor peak gives you the percentage of that specific impurity. If the chart shows three minor peaks each at 0.1 percent, you have a 99.7 percent pure peptide with three different related substances at the 0.1 percent level. That is normal and expected.

What is not normal: a chromatogram with five, six, or more minor peaks. That suggests either a poorly purified product or a degraded one. Each minor peak is a fragment of the synthesis or breakdown story. The more peaks, the more story.

The baseline

This is the part that most certificate readers miss entirely. The baseline is the flat line of absorbance signal between peaks. In a properly running system, the baseline is flat and close to zero. In a system with problems, the baseline drifts upward or downward over the course of the run.

Baseline drift is more telling than the purity percentage. Drift suggests that the column is shedding material, that the mobile phase is contaminated, that the detector is unstable, or that the sample contains a high-molecular-weight aggregate that elutes slowly across the entire run. Each of those scenarios changes how you should interpret the integration.

If the chromatogram shows a baseline that climbs from 0 mAU at the start to 50 mAU by the end of the run, the integration is including a continuous low-level signal that should not be counted. The actual purity is lower than the printed number. A reputable lab will report integration parameters that account for baseline drift, including the baseline-correction algorithm used. A less reputable lab will not.

What the purity number cannot tell you

Even a clean chromatogram with a sharp peak, the right retention time, and a flat baseline does not certify everything. Specifically, HPLC alone cannot certify identity. It can certify that one molecule eluted at the expected retention time, but it cannot prove that molecule has the structure you think it has. For identity, you need an orthogonal method. Mass spectrometry is the standard.

A complete COA pairs HPLC with LC-MS. The HPLC chart gives you purity. The mass spectrum gives you identity. The molecular ion peak should match the calculated mass of the target peptide to within 0.01 daltons on a high-resolution instrument or to within 1 dalton on a quadrupole. If the masses match, the molecule is the molecule. If they do not, the chromatogram was telling you about the wrong substance.

The other thing the purity percentage cannot tell you is anything about microbiological contamination, endotoxin load, residual solvents, or heavy metals. Each of those requires a separate test. A peptide can be 99.6 percent pure on HPLC and still contain 50 endotoxin units per milligram, which would make it unsuitable for any cell-culture work.

The short version

When you receive a certificate of analysis with a peptide order, do not just read the percentage. Read the chromatogram. Check that the main peak is sharp and symmetric. Check that the retention time matches what the method predicts. Look at the minor peaks and ask whether their count and size are reasonable for the synthesis chemistry. Look at the baseline. Confirm that mass spectrometry is reported alongside the HPLC, and that the masses match.

If any of these are missing or any of these look wrong, the percentage is not telling you what you think it is telling you.

The number on the certificate is a summary. The chart is the evidence.