Why three frequently confused terms require different analytical approaches.
Identity
An identity test asks whether the principal signal is consistent with the expected substance. It may use mass, retention time or another characteristic compared with a standard. Identity is a categorical question and does not answer how much material is present.
Purity
Purity often describes the relative distribution of signals in a particular method. It depends on what the method detects and how components respond. A high relative percentage does not exclude substances that are not visible to the selected detector.
Content
Content is a quantitative determination relative to a known standard and stated basis, such as mass or dry substance. It requires suitable calibration and accuracy control. HPLC area percentage should therefore not automatically be presented as the amount of material.