Speciation of the immediately mobilisable As(III), As(V), MMA and DMA in river sediments by high performance liquid chromatography-hydride generation-atomic fluorescence spectrometry following ultrasonic extraction

In this work, a fast method is developed for the speciation of As(III), As(V), MMA and DMA in the immediately mobilisable fraction of river sediments (i.e. water-soluble and phosphate-exchangeable) by high performance liquid chromatography-hydride generation-atomic fluorescence detection (HPLC-HG-AFD) after extraction using focused ultrasound. The influence of relevant parameters influencing an ion-pairing chromatographic separation following isocratic elution (i.e. amount of MeOH in the mobile phase, ion pair reagent concentration, pH, flow rate) was studied. Focused ultrasound transmitted from an ultrasonic probe provided the same extractable contents as conventional extraction with no changes in the species distribution. The effect of the drying step over extraction of As species was investigated. The following drying procedures were compared: freeze-, oven-, microwave- and air-drying. No influence of the drying operation on the water-extractable fraction was observed. However, freeze- and air-drying yielded significantly higher phosphate-extractable amounts of As(III) and As(V) as compared to oven and microwaves. Detection limits for the As species were in the range 1.3-4.1 ng/g for the water-soluble fraction and 1.6-4.8 ng/g for the phosphate buffer exchangeable fraction. The method was applied to the speciation of immediately mobilisable As(III), As(V), DMA and MMA in 11 sediment samples collected along the beds of the Louro River (southern Galicia, Spain).