ASSAY DEVELOPMENT
The assumption that the drug is stable may not be
valid as drugs are notoriously unstable, particularly
as hydrolysis is often the predominant cause. In
order to follow drug stability, in both solution and
solid phase, it is mandatory to have suitable stability-
indicating assays. In some cases UV spectroscopy
can be used, but in general chromatography
is required to separate the drug from its
degradation products and any excipients. Thin-layer
Fig. 8.5 The relationship between melting point and solubility for three polymorphs of riboflavine.
chromatography (TLC) is widely used in a semiquantitative
mode to estimate impurity levels, to
establish the number of impurities, and to collect
samples from the plate for subsequent HPLC (highperformance
(pressure) liquid chromatography.
HPLC is now acknowledged as the most versatile
and powerful technique in pharmaceutical analysis
and is the method of choice in preformulation
stability assessment.