Optimization of cold vapour atomic absorption spectrometric determination of mercury with and without amalgamation by subsequent use of complete and fractional factorial designs with univariate and modified simplex methods

Abstract

Experimental conditions for the determination of Hg with commercially available purpose-built apparatus were optimized by sequential use of two multiple parameter methods, based on a factorial design and a modified simplex procedure. The responses that were evaluated to determine the optimum conditions were the peak height and peak area of the Hg signal. The significance of the effects was tested using the analysis of variance (ANOVA), at a 99% level of significance. Interactions observed between the parameters were quantitatively evaluated and discussed. The flow-rate of air, volume of sample solution, use of a desiccant and the interactions between these parameters in the determination of Hg by cold vapour atomic absorption spectrometry (CVAAS) without amalgamation were optimized according to a complete 2³ factorial design and a univariate method. The experimental design was also considered for the determination of Hg after amalgamation on an Au–Pt gauze. The flow-rate of nitrogen, mass of the amalgamator, trapping time, releasing time and interactions between them were statistically evaluated, according to a fractional factorial design (half-replicate of a complete 2⁴ factorial design) and subsequent use of the modified simplex method. This approach for partial optimization of a commercial system is rapid and has many advantages over simple univariate methods. An absolute detection limit of 0.33 ng of Hg was achieved using the amalgamation technique for a total solution volume of 50 ml. This is comparable to the limits obtained by univariate methods of optimization. An approximately 10-fold improvement in the detection limit was observed with this technique in comparison with the direct method.