Avaliação de diferentes sistemas de extração assistida por ultrassom para determinação de As, Cd, Cr, Ni e Pb em solo e sedimento

Abstract

In this study a sample preparation method using ultrasound-assisted extraction (UAE) for further determination of As, Cd, Cr, Ni and Pb in soil and sediment by inductively coupled plasma mass spectrometry (ICP-MS) was proposed. Elemental characterization was performed after microwave-assisted digestion (MAD), as recommended by methods 3051A and 3052 of United States Environmental Protection Agency (EPA). The digest was used for determination of Cd, Cr, Ni and Pb by ICP-MS and As was determined by graphite furnace atomic absorption spectrometry (GF AAS). These results were used as reference values during optimization of proposed UAE method. For UAE, three different ultrasound systems (bath, cup horn and probes) were evaluated and the optimization of analytes extraction was carried out using the soil sample. Some variables such as extraction time, temperature, amplitude, frequency, sample mass and extraction solutions were investigated. Initially, univariate experiments were applied in order to determine which variables presents higher influence on extraction efficiency. However, due to the high number of variables and the difficulty to found a compromise extraction condition for all analytes, the application of a multivariate factorial design was required. In this sense, three types of factorial design were applied: 25-1 fractional factorial design, 23 full factorial design and the central composite design (CCD), varying temperature, sample mass, extraction solution, sonication time and amplitude as variables. These experiments were applied in cup horn systems (130 and 750 W). For bath systems, a mapping with aluminum foil was performed. The frequency of baths was also evaluated for all equipments and the baths with 100 and 200 W were selected to apply the 23 factorial design and CCD. After the optimization, the experiments were carried out for analytes extraction in order to evaluate the precision and accuracy of the proposed method. Thus, using the ultrasonic bath of 200 W and 35 kHz, the method was applied to soil and sediment samples and four certified reference materials (CRMs). For determination of As, Cd, Cr, Ni and Pb in soil, sediment and CRMs (except Cr in sediment ant CRMs) by ICP-MS, no statistical difference (Student t-test, p < 0,05) was observed comparing to reference method. The UAE method was also compared with extraction by mechanical stirring and better extraction efficiency was observed for UAE method. In order to check the effects of ultrasound on the surface of samples, they were also characterized by scanning electron microscopy before and after UAE. The limits of detection of the proposed method for the determination of As, Cd, Cr, Ni and Pb by ICP-MS were 75, 2, 174, 156 and 24 ng g-1, respectively.