HPLC Purity: What the Chromatogram Actually Tells You

A Certificate of Analysis that prints “Purity: >99%” with no chromatogram is one of the most common — and least useful — claims in research-peptide procurement. The number is meaningless without the chromatogram it’s calculated from. Here’s how to read the actual data.

What HPLC measures

High-Performance Liquid Chromatography separates the molecules in a sample by their interaction with a stationary phase (the column packing) and a mobile phase (the solvent gradient). Each distinct molecular species elutes at a characteristic retention time, producing a peak on the chromatogram.

For research peptides, reverse-phase HPLC with a C18 column is the standard. UV detection at 220 nm (peptide-bond absorbance) is the default — sometimes also 280 nm if the peptide contains aromatic residues (Trp, Tyr, Phe).

What you should see on the chromatogram

A reference-grade peptide chromatogram should show:

• One dominant peak — the target peptide
• Minimal smaller peaks — synthesis byproducts and impurities
• Defined baseline — no significant drift between peaks
• Symmetric peak shape — no severe tailing or fronting

The “purity %” figure comes from integrating the peak areas: target peak area divided by total peak area, expressed as a percentage. It’s an objective number — but only if you can see the integration.

What “purity %” doesn’t tell you

A 98% pure sample with one impurity at 2% is different from a 98% pure sample with twenty impurities at 0.1% each. Both report identically on the COA. The chromatogram is the only place you’d see the difference.

Some impurity classes matter more than others:

• Truncation products (target peptide minus one or more residues) — usually elute earlier and are typically inert
• Deletion products (target peptide with a residue dropped from the middle) — can have unintended biological activity
• Racemization byproducts (target with one residue inverted) — concerning for assay work
• Aggregates / dimers — elute later; matter for solubility and assay reproducibility

A chromatogram with one major peak and a few small early-eluting peaks suggests truncation byproducts — generally fine. A chromatogram with multiple peaks of similar height suggests synthesis problems and warrants caution.

Wavelength matters

Most COAs report purity at 220 nm. This is the appropriate default for general peptides. If you see purity reported at 280 nm (aromatic absorbance) but the peptide doesn’t contain aromatic residues, that’s a flag — the measurement may be misleading.

For peptides with tryptophan, tyrosine, or phenylalanine, dual-wavelength reporting (220 nm and 280 nm) gives a more complete picture.

What chromatogram quality looks like

A good chromatogram has:

• Time axis labeled in minutes
• Absorbance axis labeled in mAU (milli-Absorbance Units)
• Retention time of the target peak called out
• Peak-area integration shown
• Method conditions noted (column type, flow rate, gradient)

A “stock-photo chromatogram” — same image reused across lots — is a serious red flag. The chromatogram should be lot-specific.

When to test independently

For research that depends on assay reproducibility, independent HPLC verification of a vial against the supplier’s reported chromatogram is a reasonable precaution — especially for new suppliers or new lot numbers. Most academic core facilities will run an analytical HPLC for a modest fee.

For routine reference work where you’re trusting the supplier, a published per-lot chromatogram with sensible-looking purity figures is enough.