Quality
Identity, purity and content: three different questions
Why three frequently confused terms require different analytical approaches.
An identity test asks whether the principal signal is consistent with the expected substance. It may use mass, retention time or…
02PurityPurity often describes the relative distribution of signals in a particular method. It depends on what the method detects and how…
03ContentContent is a quantitative determination relative to a known standard and stated basis, such as mass or dry substance. It requires…
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.
LABORATORY APPLICATION
Turning the principle into an auditable record
In a technical report, use three separate lines and avoid the broad term ‘quality’ as the only result. For each line state method, unit, standard, criterion and uncertainty. This reveals whether the data package answers all three questions or only part of them.
PRIMARY & OFFICIAL SOURCES
Sources and further reading
COMMON QUESTIONS
Questions specific to this topic
Can identity be correct while purity is low?
Yes. Presence of the expected substance does not exclude additional signals or other attributes outside specification.
Is area percent the same as content?
Not automatically. It is relative response within the method and needs suitable calibration for a quantitative conclusion.
Why state the reporting basis?
To make clear whether the result refers to total mass, dry substance, a specified salt or another defined basis.