On the other hand, the electronic absorption
spectrum of diclofenac at pH 2 provides information
as to the maximum concentration at which
fluorescence linearity may be expected, i.e. that
for which A0.05. This limit was estimated at
approximately 1.8 mg l1 however, in subsequent
fluorescence experiments, the calibration plot was
found to be linear in the range 0.2–5.0 mg l1
(see Section 2). Notice that the absorption maximum
of diclofenac in acid solution lies at 275 nm,
whereas the excitation maximum appears at 287
nm (Fig. 1).
Unknown aqueous samples of diclofenac were
studied by the above procedure (Table 2). The
method was extended to several pharmaceutical
preparations (five tablets and one ointment), with
the results summarised in Table 3. It may be
noticed that in some of the cases shown in Table
3, diclofenac is combined with other drugs, yet no
interference is observed. Spectra of mixtures of
pure diclofenac with paracetamol, cianocobalamine
or betametasone are indistinguishable from
that corresponding to the pure drug at the same
concentration. This is especially important in the
case of cianocobalamine, which has been reported
to be fluorescent in acid media [25]. Likewise, no
interference from the preparation excipients were
observed. All recoveries are within the limits recommended
by Pharmacopeia (i.e. between 90 and
110% of the declared amount) [26].
In conclusion, it has been shown that diclofenac
can be determined in pharmaceutical preparations
using a rapid, sensitive and selective spectrofluorometric
method.