Identity, Purity and Quantity: Three Separate Measurements

A common mistake in evaluating research materials is treating a single number — usually an HPLC purity percentage — as a complete quality statement. In reality, identity, purity and quantity are three independent measurements, each answering a different question.

Identity: is it the right compound?

Identity asks whether the material is what it is labeled to be. It is typically confirmed by mass spectrometry (often LC-MS), which measures molecular mass against the expected value. A purity percentage says nothing about identity on its own — a sample can be highly "pure" and still be the wrong compound.

Purity: what proportion is the target?

Purity, usually measured by HPLC, reports the relative proportion of the intended compound among the peaks the method detects. It is method-dependent: what counts toward "purity" depends on the analytical conditions. A high purity result does not establish sterility, endotoxin status, or suitability for any particular use.

Quantity: how much is actually present?

Quantity — the actual mass of compound in the vial — is a separate measurement again. A purity percentage describes a proportion, not an absolute amount, so it cannot confirm the labeled quantity by itself.

Reading a COA with this in mind

When you review a Certificate of Analysis, look for each measurement separately rather than collapsing them into one figure. Our guide to peptide purity testing covers the HPLC and mass-spectrometry methods in more detail. TagPep documents identity and reported purity for qualifying batches; all materials are supplied for laboratory research use only.

For laboratory research use only. Not for human or veterinary use. Not for human consumption. Not for diagnostic or therapeutic use.