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Fluorophotometric method detects compounds in drug tablets

BioPhotonics
May 2007
Lauren I. Rugani

Researchers at Yangzhou University in China have employed fluorophotometry — a process used for quality control and pharmaceutical analysis — to determine phenothiazine drugs in commercial tablets. The technique offers a lower detection limit than spectrophotometric and electrochemical methods for drug determination and produces a good linear relationship between phenothiazine concentration and fluorescence intensity. Phenothiazines are used as antipsychotics, neuroleptics and antihistamines and include chlorpromazine hydrochloride, promethazine hydrochloride and trifluoperazine hydrochloride, the compounds employed in this fluorophotometric study. Tablets containing each of the samples were powdered and mixed with 0.5 ml of a 0.01-M Tris-(hydroxymethyl)aminomethane (Tris-HCl) buffer solution and 1.0 ml of a 0.1-M sodium dodecyl sulfate (SDS) solution, then diluted to the flask volume with bidistilled water.

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Researchers used a fluorescence technique to detect these three phenothiazine drugs in tablets. Reprinted with permission of the Journal of Fluorescence.

The researchers chose the Tris-HCl after investigating the effects of various buffer solutions, pH levels and solution concentrations on the fluorescence intensity of phenothiazine compounds. The intensity changed slightly for weak acid and weak base solutions, but did not vary noticeably in the pH range of 5.0 to ~8.5 for Tris-HCl. At a pH of 7.0, Tris-HCl concentrations between 1 and 10 mM did not exhibit any significant differences on the fluorescence intensity of the compounds.

Given the increase in molar absorption of a fluorophore with the addition of a surface active agent above its critical micellar concentration, the researchers studied the fluorescence properties of the phenothiazine compounds using (n-hexadecyl) triethylammonium bromide (CTAB) and SDS. Using a Shimadzu spectrofluorimeter, the researchers found that the emission wavelengths for each of the phenothiazine compounds increased in intensity and experienced a slight blueshift compared with the emission spectra in aqueous solution.

The anionic SDS medium demonstrated the highest increase in fluorescence intensity and was thus employed further in the study. Increasing the SDS concentration up to 8 mM correlated with a rapid increase in fluorescence intensity in each of the three phenothiazine compounds. For concentrations beyond 8 mM, the intensities remained relatively constant.

The researchers investigated the effect of the equilibrium time to achieve a stable fluorescence intensity of the phenothiazine compounds in the micellar medium. The micelle-enhanced fluorescence intensity increased during equilibrium times below 20 minutes and remained stable for equilibrium times in the range of 20 to 240 minutes. The phase equilibrium between the micellar medium and the phenothiazine compounds remained stable for at least three hours, allowing the researchers to determine the phenothiazine compounds fully.

Under the determined optimal conditions, the researchers reviewed the fluorescence intensity dependence on the phenothiazine compound concentration. The dependence remained linear for concentrations in the ranges of 9.0 × 10–8 to 5.0 × 10–5 M; 1.0 × 10–7 to 5.0 × 10–5 M; and 5.0 × 10–8 to 7.0 × 10–5 M for chlorpromazine, promethazine and trifluoperazine, respectively, with detection limits of 3.0 × 10–8 M, 2.5 × 10–8 M and 1.5 × 10–8 M.

Journal of Fluorescence, published online Feb. 6, 2007.


GLOSSARY
fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
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