Why "≥99% HPLC" isn't the whole story

"≥99% HPLC purity" sounds definitive. It isn't. Here's what HPLC purity actually measures — and the four other tests you should be looking for.

Walk into a research peptide forum and you'll find HPLC purity treated as a binary: above 98%, "pure"; below, suspect. That framing flattens away the chemistry, the assay limits, and the difference between purity and identity. Below is a more useful frame.

What HPLC purity measures

HPLC integrates the area under the peak corresponding to your compound and divides by the total integrated peak area in the chromatogram. The result is a percentage: how much of what's in the vial — by detected absorbance — is your target compound, vs. everything else the column resolved.

Two things this calculation does not tell you:

• Whether the target compound is the molecule you intended to order. A truncation product or sequence isomer can co-elute or separately elute with similar purity to the parent.
• Whether anything else in the vial is invisible to the detector (e.g. salts, water, residual solvents, endotoxin).

The four tests that close the gap

ESI-MS — molecular weight confirmation

Pairs with HPLC. Confirms you have this peptide, not a near-isomer at the same purity. Without it, "≥99% pure" can mean "≥99% pure" of the wrong compound.

LAL — endotoxin

Lyophilized peptide that's chemically pure can still carry bacterial endotoxin from a contaminated manufacturing line. Endotoxin is invisible to HPLC. LAL quantifies it directly.

Residual solvents — GC-MS panel

SPPS and the cleavage cocktail leave traces of DMF, DCM, TFA, and others. These don't show on a UV chromatogram unless you run a method tuned for them. A residual-solvents panel reports them against ICH Q3C limits.

Karl Fischer — water content

Lyophilization removes most of the water, never all of it. Water-by-mass affects the actual peptide content per vial — a vial labeled "5 mg" with 10% w/w water actually contains 4.5 mg of peptide. Reputable suppliers report water content and use it to correct net peptide mass on the label.

Why the gap matters

If you're running a binding assay and your peptide is 99% pure but has 5% water, your dose-response curve is shifted. If you're testing in cell culture and your peptide carries undeclared endotoxin, your inflammatory readout is contaminated. None of these problems are detectable from a single HPLC number — and yet "≥99% HPLC" is what most suppliers offer as the entire spec.

What we publish

Every Pepcore Labs lot ships with the full panel: HPLC purity (with chromatogram), ESI-MS, LAL endotoxin, residual solvents, and Karl Fischer water content. The COA includes the test method, the lab, the lab's ISO 17025 accreditation, and the date of analysis. We re-test any lot held longer than 18 months and re-publish the COA.

The five-line panel is not a luxury. It's the floor for material going into a research workflow.

— Pepcore Labs · Verified to spec, every lot.