Comparison of ultrasound-assisted extraction, slurry sampling and microwave-assisted digestion for cadmium, copper and lead determination in biological and sediment samples by electrothermal atomic absorption spectrometry

Abstract

Ultrasound-assisted extraction is compared with slurry sampling and microwave-assisted acid digestion for the determination of cadmium, lead and copper in biological samples and cadmium in sediment samples by electrothermal atomic absorption spectrometry (ETAAS). The main advantage of using an ultrasound-assisted extraction in comparison to slurry sampling is a doubling of the tube lifetime for all the chosen analytes, most likely because the sample matrix was not introduced into the atomiser, avoiding the build up of carbonaceous residues or silicates on the graphite platform. Consequently the analytical variable costs were remarkably diminished. In comparison to microwave-assisted digestion, ultrasound-assisted extraction resulted typically in a two-fold better precision and better detection limits. For the extraction, slurries in nitric acid were sonicated for 2–5 min using a titanium probe. Subsequently, the solid phase was separated by centrifugation, and 20 µl of the liquid phase was introduced into a graphite tube previously treated with a W–Rh permanent modifier. For slurry sampling, the slurries were sonicated for 20 s before being delivered to the W–Rh treated platform. Nitric acid concentration, sonication time, sonication power, sample mass and particle size were optimised for each analyte to obtain quantitative recoveries. For cadmium the optimum conditions (sediments and biological materials) were, 0.2–3.0% m/v slurry, 0.5% HNO3, 2 min sonication at 15 W. For copper and lead (biological materials only) the optimum conditions were 0.2–2.0% m/v slurry, 5.0% HNO3, 5 min 35 W. The particle size chosen for all analytes was <30 µm. For biological materials, the extraction recoveries were: Cd, 95–105%; Cu, 90–101%; and Pb, 88–98%. However, only cadmium was quantitatively extracted (based on the certified values of the reference materials) from sediment samples (recoveries 92–95%). For Pb and Cu the maximum recoveries attained were 54% and 60% respectively. Detection limits, based on integrated absorbance for the extracts of a 2.5% m/v biological material were: Cd, 3.0 ng g−1; Cu, 88 ng g−1; and Pb, 55 ng g−1. For 0.5% m/v sediment the cadmium detection limit was 15 ng g−1.