Atomic absorption spectrometry

For the quantitative analysis of heavy metals such as lead, cadmium, mercury and chromium according to the RoHS directive, atomic absorption spectrometry (AAS) is the method of choice. AAS is a relative measuring method for quantitation of element concentrations down to the ppt level of liquid or solid samples, using the relationship between concentration and absorbance according to Lambert-Beer's law. The measurement of unknown samples is performed against a calibration curve of aqueous standard samples. Unfortunately problems can emerge when the composition of standards and samples is different. Problems related to background absorption are classified as spectral interferences. Interferences which can be solved by background compensation methods include molecular absorption, particulate caused scattering and absorption line overlapping. During determination of heavy metals according to RoHS guideline, the spectral interferences by direct and indirect absorption line overlapping are to be expected for cadmium, lead and chromium [4]. Since the deuterium lamp method is not able to compensate for these interferences, the high speed self reversal method has been selected here as a universal technique covering the entire wavelength range from 190 nm to 900 nm in flame and furnace atomization and compensating interferences for both, direct and indirect line overlapping. All determinations have been performed using the Shimadzu atomic absorption spectrometer AA-6800 which is equipped with both deuterium lamp and high speed self reversal background compensation methods as a standard configuration, enabling proper compensation of all interferences.

For electrothermal atomization the GFA-EX7 high sensitivity graphite furnace with digital control has been used in all cases [5]. In fully automatic measurement sequences the calibration curves have been prepared from multi-element stock standard solutions using the autosampler ASC-6100 in combination with an ASK-6100 autodiluter. The experimental results have been obtained from standard solutions, diluted samples and certified reference material. In contrast to the EDX screening method which does not require any sample preparation, the determination of heavy metals using AAS needs the digestion procedure to bring all samples into solution. The recommended sample preparation for polymers is a dry ashing method or a microwave acid digestion using nitric acid with hydrogen peroxide and hydrofluoric acid.