Performance Study of Diffusive Gradients in Thin Films for 55 Elements

Abstract

The technique of diffusive gradients in thin films (DGT) is a fairly new and useful tool for in situ measurements of labile metal ions in water. The applicability of DGTs was investigated by comparing independently determined or estimated diffusion coefficients with DGT effective diffusion coefficients (D_DGT) for 55 elements. The DGTs were exposed at a controlled fluid velocity of 0.1 m s^-1 and a concentration of 1 ng mL^-1 at four pH levels between 4.7 and 6.0, and the D_DGT values were determined from the uptake by the sampler. The measured D_DGT values for the elements Co, Ni, Cu, Zn, Cd, Pb, Al, Mn, and Ga were close to previously published values with some deviations for Pb and Zn. The uptake of V, Cr, Fe, U, Mo, Ti, Ba, and Sr varied with pH, and there were some experimental problems that require further investigations. A novel set of D_DGT values for the lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Yb, Lu, Y) was established. The D_DGT values for these were about 10−15% lower than for free ions in water and indicate that diffusion coefficients of metal ions in the agarose polyacrylamide diffusive hydrogel are 10−15% lower than in water. The high consistency of the data for the lanthanides establishes these elements as new performance test metals for the DGT sampler. The accumulation of the elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, Th, and Ag was low (D_DGT lower than 10% of theoretical values). A more efficient elution procedure using concentrated nitric acid for the absorbent gel was established, with elution efficiencies between 95 and 100% for most metals. For deployment times of 24 h, detection limits from 0.001 to 1 ng mL^-1 were achieved with moderate precautions to prevent contamination.