75AS AND 13C NMR OF SOLID ARSENIC COMPOUNDS: GROUNDWORK FOR SPECIATION OF ARSENIC IN ENVIRONMENTAL SAMPLES

Nuclear magnetic resonance spectroscopy (NMR) can aid in determining the speciation of As compounds in environmental samples. Total As levels in freshwater and marine organisms are reported in the literature. However, the toxicity and biological fate of As varies dramatically depending on the chemical form. For example, inorganic arsenite, As(III), is extremely poisonous while organic arsenic is less toxic. Fundamental information on the biogeochemistry of As is essential for the design of effective remediation procedures to deal with As environmental contamination.

Few ⁷⁵As NMR studies of As compounds in solution have been reported. Even less information is available on ⁷⁵As NMR of solids. NMR of As is complicated by the large quadrupole moment of the ⁷⁵As nucleus, which tends to produce broad lines.

We have obtained ⁷⁵As NMR spectra of solid As compounds at 9.4 T using magic-angle spinning (MAS). Spectra of solid LiAsF₆ at spinning speeds of 10 kHz and 16 kHz display a single peak, 2700 Hz wide, with minimal spinning sidebands. The transverse relaxation time is 0.3 ms. The spectrum of LiAsF₆ in solution displays a septet due to coupling with F, J=956 Hz.

⁷⁵As MAS NMR spectra of solid Na₃AsO₄ acquired at spinning speeds of 5 to 16 kHz show a central peak with a linewidth of 6800 Hz. Spinning sidebands are observed at all spinning speeds. The spectrum of AsO₄^-3 in solution at pH 13 displays a peak at 372 ppm (relative to LiAsF₆) with a linewidth of 2900 Hz.

The narrower linewidth observed for solid AsF₆^-1 compared to solid AsO₄^-3 reflects the higher degree of spatial symmetry of AsF₆^-1. At lower pH, partial protonation of AsO₄^-3 reduces the symmetry to the point that no NMR signal is observed. Likewise, no signal is observed for arsenite, AsO₃^-3. It is clear that the spatial symmetry of the compound under study will be a determining factor in the quality of speciation information gained from ⁷⁵As NMR.

Additional information can be obtained on organo-arsenic compounds using ¹³C MAS NMR. Spectra of solid dimethylarsinic acid at spinning speeds from 4 to 16 kHz show the partial overlap of two signals. The width of the non-spinning peak is 2000 Hz, while the width narrows to 700 Hz at a spinning speed of 4 kHz. ¹³C MAS NMR is a viable method for studying organo-arsenic compounds.