Microwave-Induced Plasma Optical Emission Spectrometry (MIP OES) and Standard Dilution Analysis to Determine Trace Elements in Pharmaceutical Samples

Abstract

In this work, we evaluate the application of microwave-induced plasma optical emission spectrometry (MIP OES) to determine of Al, Cr, Co, Cu, Fe, Mn, Ni and Zn in children’s cough syrup, eye drops, and oral antiseptic using standard dilution analysis (SDA). The SDA method is simple, with only two calibration solutions prepared per sample. The first solution (S1), composed of 50% sample +50% of a standard solution, is introduced into the plasma and the analytical signals are monitored in a time-resolved fashion. Then, the second solution (S2), composed of 50% sample +50% blank, is poured into the vial containing S1. As the solutions mix, the analytical signals gradually drop to a stable baseline. The calibration curve is computed by plotting the ratio of the analyte signal (S_A) over the internal standard signal (which is also part of S1) (S_IS) on the y-axis, versus the inverse of the IS concentration on the x-axis (i.e., S_A/S_IS versus 1/C_IS). In this study, SDA results were compared with values obtained with the traditional methods of external calibration (EC), internal standardization (IS), and standard additions (SA) in MIP OES determinations. The precision (represented as percent RSD) for SDA showed values in the range of 2.50–8.00% for all samples, while conventional calibration methods showed RSDs in the range of 6.40–32.50% for EC, 8.30–21.80% for IS, and 5.20–17.40% for SA. The LODs calculated for SDA are below the maximum limits allowed by the major pharmaceutical regulatory agencies, and presents superior precision and accuracy compared to the traditional calibration methods. Considering its simplicity and efficiency, SDA is an important new tool for accurate analyses of pharmaceuticals.