Application of flow injection analysis adsorption–elution protocols for aluminium fractionation†

FIA (flow injection analysis) is a widely used technique for trace element analysis, but has a number of inherent problems. When used to determine the ‘free’ metal concentration, the ‘normal’ reaction time in the manifold, 15–30 s, promotes a significant sequestration of metals from labile and pseudo-labile complexes. Also there is potential for matrix components other than the target analyte to affect the rate of the analyte–reagent reaction. This may lead to an overor under-estimate of the amount of labile metal relative to calibrations based on simple aqueous standards. These problems can be minimised by use of flow systems with much shorter ‘reaction times’ and by separating the analyte fractionation step from the analyte–reagent reaction step. This can be achieved by use of on-line ‘adsorbents’ from which the captured analyte is eluted prior to the analyte–reagent reaction. This is illustrated for the fractionation of Al by use of ca. 1 s contact time with an oxine-derivatised gel. Real-time analysis of non-retained fractions coupled with selective elution of retained species provides the concentrations of three fractions: ‘free Al’ [Al3+ + Al(OH)2+ + Al(OH)2], ‘labile organic Al’ and ‘Al13’ hydroxy polymers. Quantitative separation of Al and Fe is achieved. For ‘free Al’ the linear working range is 0.3–16 mM, the LOD 70 nM and the RSD at 2 mM Al is 3.7%. The method is compared to conventional FIA for determination of ‘reactive Al’ in soil solutions and is applied to the Al-complexation capacity and pHdependent Al binding of a fulvic acid, and in the correlation of plant growth with Al fractions in soil solutions.