Comparison study of five analytical methods for the fractionation and subsequent determination of aluminium in natural water samples

Five methods for aluminium fractionation used in different laboratories in Norway and Finland were compared using six control, 75 soil water and 10 la ke water samples. Different fractionation principles [cation exchange, form ation of the Pyrocatechol Violet (PCV) or quinolin-8-ol (oxine) complex], t ypes of cation exchanger [Amberlite (Na/H) or Bond Elut (H)], reaction time (from 2.3 s), flow systems (flow injection analysis or segmented flow) and determination principles (molecular absorption spectrometry or ICP-AES) we re tested. Determination of the 'labile' fraction was strongly dependent on the method used and the largest differences were observed between the ICP- AES method with cation exchange (Bond Elut H form) and the 'quickly reactin g' method (oxine, 2.3 s). Different flow systems, both using cation exchang e and determination of the PCV complex but with different reaction times an d an extra acidification step, resulted in large differences in the 'reacti ve' and 'non-labile' fractions determined. However, the determination of th e labile fraction gave similar results with both these methods. The two dif ferent types of cation exchanger used (with and without pH buffering and wi th different counter ions) in the ICP-AES methods resulted in differences, mainly because of a smaller 'non-labile' fraction in the non-buffered syste m. The two flow injection systems (oxine and PCV complexation) showed commo n trends, which may be connected with the short reaction times used. Compar ison with theoretical equilibrium calculations using the model ALCHEMI sugg ested that the best correlation for the determination of the 'labile' fract ion were obtained with the ICP-AES method with an Amberlite column.