[Retracted] Ternary Complexation Process for New Spectrophotometric Assay of Levodopa using Ni(II) and 2,3-Diaminopyridine

Table 1

Comparative analytical results of the proposed approach with various previously described methods for determining levodopa.

Method of analysis	Principle	Linearity	Limitation
Spectrophotometry	The method was based on the reaction of levodopa with vanadium(V) which was reduced to vanadium(IV) and the formed complex with eriochrome cyanine R to give the product has λmax at 565 nm [11].	0.028–0.84 μg·mL^−1	Nonselective among catecholamines
	The method was based on the reaction of alizarin red with the primary amino group present in the levodopa in basic medium; a purple color product has λmax at 588 nm [12].	10–60 μg·mL^−1	Also, nonselective among catecholamines
HPLC coupled with chemiluminescence	The method depends on the improved chemiluminescence (CL) signal of the online gold particle-catalyzed luminol-H2O2 system by levodopa, and the HPLC-CL detection system consists of an HPLC system and a CL detection system. The sample separation was achieved on an ODS column at 40°C with gradient elution at a flow rate of 1.0 mL·min^-1. The mobile phase was composed of a mixture of methanol and 0.2% aqueous phosphoric acid (5 : 95, V/V) [7].	10–1000 ng·mL^−1	Expensive method
HPLC coupled with mass spectrometry	Separation is achieved on a C8 column set at 30°C, with a mobile phase consisting of a gradient of water and acetonitrile: methanol (90 : 10 v/v), both containing 0.1% formic acid, eluted at 700 μL·min^−1 [8].	20–1000 ng·mL^−1	Expensive method
Voltammetry	This electrochemical method is based on chloranil(CA) modified carbon paste electrode (CAMCPE) in the voltammetric determination of levodopa in an aqueous solution [9].	3–500 μmol·L^–1	Lack of reproducibility because the electrochemical activity of CAMCPE decreased during successive scans due to the removal of the mediator by dissolving it into an aqueous solution, so the surface of the electrode must be renewed before each determination
	This method is based on adsorption stripping voltammetry by using a multiwalled carbon nanotubes-Nafion modified glassy carbon electrode [10].	3.5 × 10^−7-1.5 × 10^−5 mol·L^–1	Some cations may have effect on the analysis of levodopa because they interact with anion sites in the Nafion film, which will reduce the signal of levodopa, so it needs certain conditions to get rid of this interfering as in Fe(III) which needs to add 1.0 × 10^−3 mol·L^–1 oxalic acid to eliminate it