Impurity

Fixed at manufacture, or growing in storage

A deletion sequence present at release is still there years later in the same proportion, so it is a property of the lot. Oxidation products, hydrolysed fragments and aggregates grow with time, warmth, moisture and light.

That distinction decides what to do next. An unfavourable synthesis profile is a supplier and specification question; a rising degradation profile points at storage or repeated warming, and the storage guide sets out which conditions drive which route.

What a chromatogram hides

A UV trace records only species that absorb at the detection wavelength. Salt, water and many small residues pass without a peak, so they cannot enter the area-percent arithmetic behind a purity figure.

Quantity and identity are also different asks. Separating an oxidation product from a deletion sequence that elutes nearby needs mass data, and whether that was done is stated on the lot's Certificate of Analysis.

Frequently asked questions

Does one unknown peak make a lot unusable?

Not by itself. Acceptance depends on the limit your specification sets for individual and total unspecified peaks. Record the peak, its relative area and its retention, then judge it against written criteria.

Why does total impurity not equal a hundred minus purity?

Within the chromatogram it usually does, since both come from the same total area. It will not match the vial's mass balance, because anything the detector cannot see is excluded from both.

Can impurities be reduced after receipt?

Not in a routine workflow. Removing related substances means preparative separation and fresh characterisation, producing a new material with its own documentation. What a laboratory controls is degradation.