What is HPLC purity and why does 99% matter?
When a research peptide is advertised as "≥99% purity," that number almost always comes from an HPLC assay. Understanding what HPLC measures — and what it doesn't — lets you judge the quality of a batch instead of trusting a figure on its own.
What HPLC is
HPLC stands for High-Performance Liquid Chromatography. It's a technique that separates the components of a sample by pushing it, dissolved, through a column. Each component travels at a different speed depending on how it interacts with the column, and exits at a different moment — its retention time. A detector records each exit and draws a chromatogram: a series of peaks.
The compound you care about produces a large peak. Impurities produce smaller peaks at other retention times. HPLC doesn't guess what each peak is; it only separates them and measures the area of each one.
How purity is calculated
Purity is simple arithmetic on the chromatogram: the area of the main peak divided by the area of all the peaks, expressed as a percentage.
| Term | Meaning |
|---|---|
| Main peak | Your compound |
| Secondary peaks | Impurities or by-products |
| Purity % | Main area ÷ total area × 100 |
A purity of 99.4% means that 99.4% of the area under the curve corresponds to your peptide and the remaining 0.6% is spread across impurities. It doesn't measure toxicity or activity: it measures how much of the sample is the stated compound versus everything else.
Why the 99% threshold
Peptide synthesis always leaves by-products behind: truncated chains, sequences with a deletion, leftover reagents. Purification reduces those fractions, but each additional percentage point takes more work. The ≥99% figure became the reference standard in research because it marks the difference between barely acceptable material and something well characterized, where the impurities are minor noise rather than a variable that contaminates your results.
A 99% is not a "rounded-up" 90%. The difference between 90% and 99% is ten times less impurity. That's why the number matters, and why it's worth seeing it backed by its chromatogram.
HPLC doesn't tell the whole story
HPLC tells you what proportion of the sample is the main peak, but it doesn't confirm what that peak is. That's what mass spectrometry (MS) is for: it verifies identity by measuring molecular mass. A complete COA includes both: HPLC for purity, MS for identity. A perfectly pure peak of the wrong compound would still read 99%.
That's why purity and identity are read together. The guide how to read a COA step by step shows how to combine both pieces of data.
What to check in practice
- The purity number and the chromatogram that backs it — not one without the other.
- A symmetrical main peak, not broad or with shoulders.
- Few secondary peaks, with small areas.
- The method and the lab that ran the assay.
Every batch of our compounds — for example retatrutide or any product in the catalog — is verified by HPLC with its per-batch COA. If you're going to reconstitute after confirming purity, the reconstitution calculator gives you the exact concentration and units. And if this is your first encounter with the topic, start with what are research peptides?