Purity vs assay on a lab report

"Purity" and "assay" both appear on COAs, sometimes on the same line, and sometimes measured by the same instrument. They answer different questions.

• Purity: how clean is it? Of the material the method detects, what share is the main component?
• Assay: how much is there? Against a reference of known amount, how much of the target is in the sample?

Purity is relative

An HPLC purity figure compares peaks within one run. The main peak's area is divided by the total area of all integrated peaks. No outside reference is needed, which is why purity is quick and widely reported. It also means the figure says nothing about the absolute amount of anything.

Assay is absolute

An assay compares the sample to a reference standard: a well-characterized batch of the same molecule whose content is known. The lab runs the standard and the sample under the same method, and works out the sample's content from the ratio of their responses.

Assay is reported in one of two ways:

Form	Example	Meaning
Percent by weight (w/w)	Assay 84.6%	84.6% of the sample mass is the target molecule
Amount per container	9.7 mg per vial	The measured amount of the target in the vial

One instrument, two numbers

HPLC can produce both. The same chromatogram gives an area-percent purity, and if a reference standard was run alongside it, an assay figure too. That is the source of a lot of confusion: a report that says "HPLC: 99.2%" could, in principle, be either. The method line or the column heading usually settles it. Area % or "relative purity" means purity. "Assay vs reference standard," "w/w," or an amount per vial means assay.

How they relate

For a typical lyophilized peptide:

• Purity can be close to 100% because the material is very clean.
• Assay is lower because the powder also holds counterions and water.

An assay figure is closely related to net peptide content, and on some reports the terms are used for the same measurement. Others use "peptide content" for a nitrogen or amino acid method and "assay" for HPLC against a standard. Read the method line.

What about "potency"?

In regulated quality work, potency usually means biological activity measured in a functional test, which is a different thing again. Research peptide COAs rarely report it. If the word appears on a report, the method line should say what was measured.

Why it matters for a lab

A purity figure tells a researcher whether impurities are likely to confound an experiment. An assay or content figure tells them how much target material is present by weight, which matters for preparing analytical standards and comparing lots. Both are useful; neither substitutes for the other.