SIMULTANEOUS DETECTION OF DOPAMINE AND ASCORBIC ACID IN URINE USING THE H-POINT STANDARD ADDITION METHOD E. Y. Hashem * and Ahmed M. El-Zohry UDC 547.994:577.164.2 A highly sensitive H-point standard addition method (HPSAM) was investigated for identification of dopamine and ascorbic acid in some synthetic and pharmaceutical samples in an aqueous medium at pH 9.2 using a universal buffer. The most suitable wavelengths for dopamine and ascorbic acid detection are 260:271 nm and 248:270 nm respectively. Recovery values are between 99.0–101%. Also, the effect of most common in- terferents was studied. Detection ranges for dopamine and ascorbic acid are 2⋅10 –6 –5⋅10 –5 M and 6⋅10 –6 – 3⋅10 –5 M respectively with an RSD range between 1.3 and 2.0%. The method was used to determine both reagents in real and synthesized samples. Keywords: dopamine, ascorbic acid, HPSAM, UV, pharmaceutical samples, urine samples. Introduction. Dopamine (DA) and ascorbic acid (AA) are compounds of great biomedical interest, playing significant roles in human metabolism and the central nervous and renal systems [1]. For instance, AA is an essential vitamin for humans, and has been investigated for the prevention and treatment of the common cold, mental illness, infertility, and cancer [2]. Similarly, DA is an important neurotransmitter in the brain’s neural circuits [3] and its de- pletion leads to Parkinson’s disease [4]. Additionally, abnormal dopaminergic transmission has also been implicated in Huntington’s disease and neuroendocrine disorders [5]. Since these two species usually coexist in real biological ma- trixes, the development of a selective and sensitive method for the simultaneous determination of DA and AA is highly desirable for analytical applications and diagnostic research. In recent years, voltammetric techniques for the de- tection of DA and AA attracted considerable interest due to their fast response and high sensitivity. However, a major problem encountered is the overlap of peak potentials for these two at conventional electrodes with a pronounced foul- ing effect, resulting in poor selectivity and reproducibility [6, 7]. Also, one of the major problems encountered in the electrochemical determination of DA is the intervention of AA, which has an oxidation potential close to DA at most solid electrodes, resulting in great difficulty of their si- multaneous determination due to overlapped signals. Moreover, the bare solid electrodes often suffer from the fouling effect due to the accumulation of oxidized products on the electrode surface, leading to rather poor selectivity and sensitivity [8]. Up to now, the ability to detect DA with high selectivity and sensitivity is still a major target of elec- troanalytical research [9, 10]. At unmodified electrode surfaces, both compounds are oxidized at the same potential (ca. 0.2 V), and in the subsequent cycles the peak current decreases due to fouling of the electrode surface by the products of ascorbate oxidation. Additional complications connected with chemical reactions appear in the presence of AA where oxidized dopamine would catalyze the oxidation of ascorbate, which results in a single and broad peak. The mechanism for the latter has been explained by Dayton et al. and Domenech et al. [11, 12], which involves a reaction of AA and DOQ with consecutive regeneration of initial DA. A modification of the standard addition method is the "H-point standard addition method (HPSAM)" [13, 14], which is used where the error resulting from a direct interference in the presence of an analyte is transformed into a systematic error. This error can then be evaluated and eliminated. This method also permits both proportional and con- stant errors produced by the matrix of the sample to be corrected. In 1991, Campins-Falco and coworkers [15] applied HPSAM to kinetic data for the simultaneous determination of binary mixtures and the calculation of the analyte con- Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt; e-mail: elham_hashem@ yahoo.com, amfzohry@yahoo.com. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 79, No. 3, pp. 443–449, May– June, 2012. Original article submitted September 28, 2011. 0021-9037/12/7903-0424 ©2012 Springer Science+Business Media, Inc. 424 Journal of Applied Spectroscopy, Vol. 79, No. 3, July, 2012 (Russian Original Vol. 79, No. 3, May–June, 2012) ∗ To whom correspondence should be addressed.