Identity testing answers “what is this compound?” — typically via mass spectrometry, comparing the observed mass to the theoretical mass of the expected sequence or structure. Purity testing answers “how clean is this sample?” — typically via HPLC, reported as a percentage of the detected material that matches the main compound.
Assay, or content, testing is a related but distinct question: “how much of the compound is actually present,” often expressed as a percentage of the labeled amount. Qualitative testing establishes whether something is present at all; quantitative testing measures how much. Not every certificate necessarily reports every one of these — what a given lot’s certificate actually states is the authoritative answer for that lot, not a general assumption.
What impurities actually are. A sample can have confirmed identity and still carry impurities — related substances, byproducts of synthesis, or residual solvents. Purity testing is what quantifies them. Confirmed identity and high purity are two separate findings, and neither implies the other.
Reference standards. A reference standard is a well-characterised sample of a compound, used to calibrate an analytical method and confirm it is measuring correctly — a known quantity that a laboratory compares an unknown sample against. Without one, a method can be precise and still be wrong.
Why they are reported separately. Purity and identity being different measurements is exactly why a certificate of analysis reports them as separate lines rather than collapsing them into a single figure. A single “quality” number would hide which question had actually been answered.