High precision Ag isotope measurements in geologic materials by multiple-collector ICPMS: An evaluation of dry versus wet plasma

This paper presents an improved technique for the precise and accurate determination of Ag isotopic composition in geologic samples. Following the separation of Ag from the rock matrix with a three-stage ion exchange procedure, the isotopic composition is measured by MC-ICPMS. Instrumental mass bias is corrected externally using a Pd standard, which was added to each sample, and sample-standard bracketing. The use of wet plasma significantly improves the analytical precision by eliminating variations in fractionation between Pd and Ag. A virtually identical behavior of the mass bias for Pd and Ag during the analyses is a prerequisite for the external correction method. Replicate dissolutions of the carbonaceous chondrite Allende with <100 ng Ag yield an external reproducibility (2 S.D.) of 53 ppm for 107Ag/109Ag. Since the ion exchange procedure provides a good separation from matrix elements such as Ti and Fe, the method is suitable for the analysis of stony and iron meteorites as well as sulfide minerals and terrestrial basalts. The technique thus can be applied to investigate Ag isotopic variations in a large variety of solar system materials.