Assay versus purity: why the two numbers are not interchangeable
Two suppliers can both quote “98%” and mean different things. One means the assay by GC against a standard is 98%. The other means the material is 98% pure by area normalisation, with the remaining 2% split between a solvent, water and a structurally related impurity. For a route step that tolerates both, they are equivalent. For a coupling reaction where the impurity competes for catalyst, they are not.
Area normalisation has a blind spot
A GC trace normalised to 100% cannot see anything that does not elute: inorganic salts, water, polymeric material on the column, and anything that decomposes in the inlet. A material that is 98% by area can be 90% by mass. This is why an assay against an external standard is a stronger statement than an area percentage, and why the method should be named on the certificate.
What “purity” usually hides
For a catalogue entry the useful question is not “what is the purity” but “what is in the other 2%, and does it matter for my step”. The three cases that come up repeatedly:
- Residual solvent – matters if the next step is moisture or alcohol sensitive, or if the final product has a solvent limit.
- Isomeric impurity – matters if the entry is a defined geometric isomer; the other isomer usually behaves differently enough to change the step.
- Homologue – one methylene away; nearly impossible to separate and usually harmless, but it shifts the effective molecular weight used in the charge calculation.
How to write a specification that does not need rewriting
Name the method, name the impurities with limits, and state the water content if the material is hygroscopic. A specification written as a single percentage will be argued about at goods-in; one that names the method and the impurities will not.
Where an entry does not state an assay, none is implied. The value that applies is the one on the certificate for the lot, which is why the release documentation matters more than the catalogue text.