Speciation of Tin Compounds in Environmental Samples

Speciation, identification of the chemical states of elements in the environment, is indispensable in determining the environmental behavior, bioavailability, and the influence of elements on the quality of the environment. For speciation, X-ray absorption fine structure (XAFS) is a powerful tool, because it is a direct method by which we can avoid the change of the chemical state of target elements in environmental samples. XAFS is reasonably sensitive especially when conducted in the fluorescence mode using sensitive detection techniques such as the use of a multi-element semiconductor detector. The water solubility, bioavailability, and toxicity of an element depend on the oxidation state and the ligand coordinated to the element. We have studied the environmental behavior of arsenic, lead, iodine, and other elements [1-3] in terms of the redox reaction and the determination of tin species particularly organotin species [4,5]. Organotin compounds such as tributyltin (TBT) and triphenyltin (TPT) have been widely used as biocides in anti-fouling paints and fishing nets. However, it has been found that these compounds cause damage to marine organisms, such as imposex and deformity in mollusks. The speciation of organotin compounds has been of great interest due to the species-dependent toxicity and the widespread application of organotin compounds as biocides. In recent years, chromatographic techniques such as gas chromatography or liquid chromatography coupled with appropriate detectors have been widely used for the analysis of organotin compounds. These analytical methods essentially require many pretreatment procedures, such as decomposition, extraction, enrichment, and derivatization. In this study, we have tested the possibility of using an X-ray absorption near-edge structure (XANES) as a nondestructive method for identifying inorganic and organic tin species in solid environmental samples. A high-energy XANES for the Sn K-edge (29.19 keV ) was applied in this study to determine Sn speciation [4]. It is better to employ the K-edge rather than the Ledge (3.9 – 4.5 keV) for Sn, as this enables us to measure the XANES for environmental samples containing water, whereas the sensitivity would be lower using the L-edge, which is close to the soft X-ray region, as a result of X-ray scatterings. It is also expected that there would be no interference in the Kedge region from major elements such as Ca and K, which can interfere with the L-edge XANES for Sn. Tin K-edge XANES spectra of all reference materials of inorganic and organic tin compounds, an anti-fouling paint sample, and a sediment reference material (NIES CRM No. 12) were obtained at beamline BL01B1. In order to obtain the fluorescence XANES spectra, a 19-element solid-state detector (SSD) was employed to collect fluorescence X-rays from Sn in the samples. Figure 1 shows normalized XANES spectra at the Sn K -edge for the standard samples such as tetrabutyltin (TeBT), tributyltin chloride (TBTCl), dibutyltin chloride (DBTCl), monobutyltin chloride (MBTCl), and SnCl4. It is obvious that the first peak