Ultrasonic extraction as a sample preparation technique for elemental analysis by atomic spectrometry

This work presents performance data after ultrasonic extraction (UE) for the elemental analysis of a number of metal species in samples of interest in environmental and occupational health. In this study, several National Institute of Standards and Technology (NIST) Standard Reference Materials® (SRMs) were subjected to UE in various acid solutions. The extraction solutions employed were 25% nitric acid (v/v), 25% nitric–hydrochloric acids (v/v) and concentrated nitric–hydrochloric acids (1∶1). NIST SRMs 1648, 1579a, 2583, 2704, 2710, 3087a, and 8074 were subjected to these acid conditions and ultrasonic energy (about 1 W cm−2), and elemental recoveries were determined following analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES). Observed recoveries were higher overall with HNO3–HCl mixtures than with nitric acid alone, and recoveries were generally higher for concentrated acid mixtures. Recoveries of >80% could be achieved for some elements (As, Cd, Cu, Mn, Pb, Zn) when using acidic ultrasonic treatment with no deliberately added heating, even under diluted acid conditions. However, several elements (Ba, Co, Cr, Fe, Mg, Ni, V) yielded <75% recoveries when using sonication without deliberate heating, even in concentrated HNO3–HCl. For comparison with UE, selected SRMs were subjected to acid leaching (no sonication) in the above acid solutions. Elemental recoveries from acid leaching without sonication were generally lower overall when compared to results obtained from UE, thereby demonstrating the effect of ultrasound for the dissolution of target analytes. Sonication of chromate-containing certified reference materials and certified filter samples (European Commission Certified Reference Material 545) in slightly basic buffer solutions was shown to be effective for the complete (>90%) dissolution of hexavalent chromium (CrVI) from both soluble (potassium chromate) and insoluble (lead chromate) CrVI reference sources. Sonication in HNO3–HF was used to extract Cd, Fe, Pb and Zn from aerosol filters, water rinsates and aerosol samplers alone; ICP-AES was used for elemental measurement. Sample losses were found for these four elements from rinsates and samples following removal of the aerosol filter, thereby demonstrating the utility of carrying out sonication directly within the aerosol sampler containing the filter. It is hypothesized that UE will be increasingly used for analytical sample preparation purposes.