Comparison of internal standard and standard additions calibration procedures for the determination of selected heavy metals in treated municipal effluent by MP-AES

We compared the internal standard calibration and standard additions calibration methods in the determination of Cd, Cr, Fe, Mn, Pb and Zn in municipal effluent using microwave-induced plasma atomic emission spectrometer. Comparison was based on validation parameters such as linearities, linear ranges, limits of detection (LOD), precisions and recoveries obtained using both calibration methods. Both methods gave good linearities applicable to a simple linear regression data fitting. Severe loss of linearity was observed at levels beyond 3 mg/L particularly for Zn. Considering this, the compromise linear range for all target analytes was established at LOD–3 mg/L. While the LODs for the target analytes were sufficiently low for both methods, the internal standard method was found to be more sensitive compared to standard additions method. Thus, the optimal linear ranges were accurately established at 0.24–0.96 mg/L for the internal standard and 1.10–1.96 mg/L for the standard additions method. Analyte recoveries including for the certified reference material averaged ∼ 100%. Therefore, all target analytes with the exception of Cd, which was possibly below the method detection limit were confidently detected and accurately quantified in municipal effluent samples. Overall, the excellent quantitative data obtained in this study inferred a high level of confidence in the developed methods in that they differed by less than 10%. Indisputably, the performance of the MP-AES operated in internal standard- and standard additions modes have proved to be a sufficiently sensitive technique especially for heavy metals occurring at sub-ppm levels.