Response surface methodology for the optimisation of flow-injection analysis with in situ solvent extraction and fluorimetric assay of tricyclic antidepressants
Introduction
The tricyclic antidepressants (TCAs) are one of the largest groups of drugs for the treatment of psychiatric disorders such as depression, mainly endogenous major depression [1]. For this group of drugs, distinct ranges of optimum plasma concentration for therapy are required (100–300 μg L−1 for the most of the TCAs). At high concentrations, severe adverse effects and toxicity can appear. Therefore, the analysis of these compounds is important for obtaining optimum therapeutic concentrations and for quality assurance in pharmaceutical preparations.
A variety of techniques have been used to determine TCAs in diverse biological fluids and pharmaceuticals, including radioimmunoassay, gas chromatography, liquid chromatography (LC), capillary electrophoresis, spectrofluorimetry, spectrophotometry, chemiluminiscence, titrations, etc. [2], [3], [4], [5], [6], [7], [8], [9].
Some of the TCAs are not fluorescent and different reagents have been used for their derivatization. Reactions with oxidants [8], methyl orange [10], erythrosine B [11], OPA (o-phthalaldehyde) and NAC (N-acetylcysteine) [13], etc., have been used. Subsequent extraction in an organic solvent improves both sensitivity and selectivity of the methods. Conventional batch solvent extraction process is a tedious and time-consuming procedure, and many efforts have been directed towards the automation of this procedure. Flow-injection technique has advantages that makes it suitable also for this purpose [10], [11], [14].
In this paper we investigate the formation and extraction of the ion pairs formed between TCAs (dialkyl and trialkylamines) with the fluorophore 9,10-dimethoxyanthracene-2-sulfonate (DMAS), in order to develop a sensitive and automatic fluorimetric method for the determination of six TCAs in pharmaceuticals. In the bibliography, papers referred to FIA procedures for the analysis of TCAs describe methods to determine, separately, three or less of them [7], [8], [10], [11], [12], [13], [14], [15].
The DMAS have been yet used as counter-ion in analysis [16], [17]. It presents native fluorescence and forms hydrophobic ion pairs with amines extractable in organic solvents. Westerlund and Borg [16] studied the extraction of the ion pair formed between DMAS and the amitriptyline and protriptyline allowing quantitative extraction in dichloromethane. The TCAs studied here have been: imipramine (IM), amitriptyline (AM), doxepin (DO) (as tertiary amines) and desipramine (DE), nortriptyline (NO) and clomipramine (CLO) (as secondary amines).
Chemometrics have been shown to be successful for the optimisation of variables in flow-injection procedures [12], [13], [18], [19]. In the development of this FI system we have used experimental design approaches (fractional factorial design and central composite design) and the response surface analysis methodology to elucidate the manner in which the FI and chemical variables affect the response function [20], [21]. For the latter, we have considered separately the peak height and the sampling rate or a simple combination of both.
Section snippets
Instrumentation and software
Fluorescence measurements were made on a SLM Aminco-Bowman, Series 2, spectrofluorimeter, equipped with a 150 W continuous Xenon lamp, and with an Hellma 176.752QS (Z: 8.5 mm; light path 1.5 mm) flow through compact cell. The spectrofluorimeter was interfaced by a GPIB card and driver with a Pentium PC microcomputer. Data acquisition and data analysis were performed by the use of AB2 software, Version 5.00, running under Windows 98. The excitation and emission slits were 4 nm for both.
Two Gilson
Results and discussion
Some preliminary studies about the reaction between the TCAs and the reagent DMAS were realized, in batch conditions. From the studies of Westerlund and Borg [16] and selecting one of the antidepressant, the amitriptyline, we checked the optimal conditions for the formation of the ion pair and its extraction. It was found that the formation of the ion pair is favoured in strongly acid medium and does not occur at pH values higher than 6 (pKa of amitriptyline 9.42). A rate of concentration
Conclusions
It has been demonstrated that non-fluorescent tricyclic antidepressant drugs display fluorescence by means of the formation of an ion pair with DMAS. It is interesting to develop a FI method for the determination of the tricyclic antidepressant drugs in routine analysis. The optimization of the FI and chemicals variables by an experimental design (Central Composite Design and Response Surface Methodology) gives good results. The accuracy of the proposed method has been validated by comparison
Acknowledgements
The authors wish to thank to Ministerio de Ciencia y Tecnología Español for the financial support (CTQ2004-02142). Antonio Silva Rodríguez acknowledges a fellowship from the Consejería de Educación, Ciencia y Tecnología-Junta de Extremadura.
References (27)
- et al.
J. Chromatogr. B
(1985) - et al.
J. Electroanal. Chem.
(2002) - et al.
Anal. Chim. Acta
(2002) - et al.
J. Pharm. Sci.
(1998) - et al.
Talanta
(1994) - et al.
Anal. Chim. Acta
(2002) - et al.
J. Pharm. Biomed. Anal.
(2002) - et al.
Anal. Chim. Acta
(1973) - et al.
J. Chromatogr. A
(1997) - et al.
Talanta
(1998)