Two Spectrophotometric Assays for Dopamine Derivatives in Pharmaceutical Products and in Biological Samples of Schizophrenic Patients Using Copper Tetramine Complex and Triiodide Reagent

Two simple, rapid, and sensitive spectrophotometric methods are proposed for the determination of levodopa (LD). The first method is based on coupling of 4-aminoantipyrine (4-AAP) with one of the dopamine derivatives (LD, CD) to give a new ligand that reacts with copper tetramine complex to give intensely colored chelates. The colored products are quantified spectrophotometrically at 525 and 520 nm for LD and CD, respectively. The optimization of the experimental conditions is described. The method has been used for the determination of $19.7$ - $69.0$ and $18.1$ - $54.3$ $\mu$ g mL ${}^{-1}$ of LD and CD, respectively. The accuracy of the method is achieved by the values of recovery ( $100\pm 0.2$ %) and the precision is supported by the low standard deviation (SD $=0.17$ - $0.59$ ) and relative standard deviation (CV $=0.4$ %- $1.54$ %) values. The second method is based on the formation of ion-pair iodinated inner sphere or outer sphere colored complexes between the LD and triiodide ions at pH 5 and room temperature ( $23\pm 3^\circ$ C). This method has been used for the determination of LD within the concentration range $39.44$ - $78.88$ $\mu$ g mL ${}^{-1}$ with SD $=0.22$ - $0.24$ and recovery percent $=100 \pm 0.3$ %. The sensitivity of the two methods is indicated by Sandell's sensitivity of $0.014$ - $0.019$ g cm ${}^{-2}$ . The results of the two methods are compared with those of the official method. The interference of common drug additives, degradation products, and excipients was also studied. The proposed methods were applied successfully to the determination of the LD-CD synthetic mixture and Levocare drug. The determination of LD in urine of some schizophrenic patients was applied with good precision and accuracy. The reliability of the methods was established by parallel determinations against the official British pharmacopoeia method.