Three approaches to the analysis of zinc(II) in pharmaceutical formulations by means of different spectrometric methods

Abstract

The present paper constitutes three different spectral methods A, B and C developed with the purpose of determining precisely the concentrations of zinc(II), either in various ophthalmic solutions (collyria) or in insulin injections. Method A is a spectrophotometric one, based on the formation of the chelate complex of Zn(II) to be analysed with 4-(2-pyridylazo)-resorcinol(PAR) at pH 8.07±0.01 and in a micellar medium produced by the non-ionic surfactant Triton X-100. This chelate complex shows a λmax at 493 nm, an apparent molar absorptivity ϵ=77728 l/mol per cm and a corresponding Sandell’s sensitivity of Ss=0.84 ng cm−2. The concentration of Zn(II) examined is calculated from the regression line equation: A=1.143C+0.029 (r=0.9998, n=25) with an optimum concentration range of 0.18–2.0 μg/ml. Method B exploits the fluorescence intensity of the 8-hydroxyquinolate chelate complex of Zn(II) to be analysed, measured at λemission=510 nm (λexcitation=420 nm). The concentration of Zn(II) examined is calculated from the regression line equation: A=0.25C+0.17 (r=0.9996, n=18), with an optimum concentration range of 0.26–1.05 μg of Zn(II)/ml. The third method C is an AAS method, based on the following analytical parameters: λ=213.9 nm; hollow cathode lamp (HCL): L1788-30NB; HCL current 18 mV; flame temperature 2700°K. The concentration of Zn(II) analysed is calculated from the regression line equation: A=1.499C−7.409×10−4 (r=0.9996, n=25), with an optimum concentration range of 0.2–1.2 μg/ml. The accuracy and the precision of the proposed methods A, B and C was experimentally investigated and proved to be very satisfactory. On the other hand, the methods described were successfully applied for the determination of the Zn(II) included in four eye drop solutions and in one insulin injection both provided by the pharmaceutical market. The analytical results of the afore-cited formulations were summarised in a comparative table and were considered to be very satisfactory.