I see a lot of supplier comparisons built on one number: the HPLC purity percentage. It’s useful, but it’s only one piece of the analysis, and it can mislead if you read it in isolation. Here’s how I think about it:
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HPLC measures relative purity, not identity. A chromatogram shows how much of the sample elutes as the main peak. It doesn’t confirm that peak is the peptide you ordered. That’s what mass spec is for, by matching the observed molecular weight to the expected one.
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Integration choices change the number. Peak integration settings, detection wavelength, and gradient conditions all affect the reported percentage. Two labs can run the same sample and report different purities. Ask for the method details and the raw chromatogram, not just the summary.
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Co-eluting impurities can hide under the main peak. Truncated sequences, deletion products, and other closely related species may not fully separate under a standard gradient. That’s one reason an orthogonal method like MS adds real value.
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Purity is not net peptide content. A sample can be 98% pure by HPLC and still contain a large share of water, counterions (often TFA or acetate), and salts by weight. If you’re doing quantitative work, that difference affects your calculations.
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Endotoxin and residual solvents are separate questions. Standard purity testing doesn’t cover them. If your research depends on those, check whether they were tested at all.
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Batch-specific and independently verifiable. The report should match the lot number, name the lab, and carry a date. Third-party results are easier to trust than in-house summaries.
Practically, if I were comparing suppliers, I’d ask each for the same four things: chromatogram, MS spectrum, method details, and net content. The ones that can’t or won’t provide them tell you something.
This applies to research-grade material for laboratory use only. I can’t speak to human use, and many of these compounds aren’t approved for it.
Disclosure: I’m affiliated with Patriot Peptides. We have a COA explainer if you want a visual reference, but the points above apply to any supplier’s report.
Curious how others here handle it: do you request raw chromatograms, or rely on the summary COA?