Arsenic Speciation by X-Ray Spectroscopy using Resonant Raman Scattering

H.J. SÁNCHEZ; JUAN JOSÉ LEANI; C.A. PEREZ

Campinas

Congreso; RAU XXII; 2012

The toxicity of arsenic species is widely known. A realistic evaluation of the risk posed by As depends on accurate determination of As speciation, because its toxi- city and mobility varies with oxidation state and chemical environment. The most toxic species are inorganic As (III) and As (V) called respectively arsenite or triva- lent arsenic, and arsenate or pentavalent arsenic. Recently, x-ray Resonant Raman Scattering spectroscopy has been successfully employed to determine the oxidation state of metals. In this work we use RRS spectroscopy to perform arsenic speciation. The measurements were carried out in XRF station of the D09B-XRF beam- line at the Brazilian synchrotron facility (LNLS, Campinas). Mineral samples of As in di®erent oxidation states (As(III) and AS(V)), and two biological forms of arsenic (monomethylarsonic acid (MMA(V) and dimethylarsinic acid DMA(V)) were analysed. The samples were diluted, deposited on silicon wafers and allowed to dry. The amount of liquid deposited on the re°ector before evaporation was 20 microliters for all the specimens. These samples were irradiated with monochro- matic photons of 11816 eV, i.e., below the K-edge of arsenic in order to inspect the Raman emissions. The measuring livetime was 3600 sec for each sample. Spectra were analysed with speci¯c programs for spectrum analysis using non- conventional functions for data ¯tting, i.e., modi¯ed Voight functions (for Compton peaks), Gaussian functions for °uorescent and for low intensity peaks (such as es- cape peaks and other contributions), and polynomial functions for the background. Raman peaks were ¯tted using speci¯c functions. In this work we have shown that resonant Raman scattering spectroscopy can be used to analyse arsenic species. The method is very simple and reliable. The most important feature of this method relies in the possibility of using the same spectrometer of XRF analysis or TXRF analysis. In this way, practically in the same experiment of quantifying the traces of arsenic in a sample the species can also be obtained. This work is pioneer in the area and an important work of interpretation is necessary in the future. Nevertheless, this behaviour in the residuals of compounds could be used to identify the oxidation state of the elements under study, o®ering a possibility of chemical environment determination using RRS spectroscopy. Acknowledgements: This work was partially supported by the LNLS (Campinas. Brazil)