Quantifying micro-niche behaviour in sediments : Precise and accurate measurements of DGT gels by laser ablation high resolution laser ablation ICPMS

New developments in micron-scale measurements of trace metals and sulphur contents within sediments by DGT (diffusive gradients in thin-films) have revealed micro-niche structures. The origin of micro-niches is unclear and it is important to understand the relative roles played by inorganic and biological processes in the localised remobilisation of metals. We have undertaken a S and Fe isotope study of micro-niches, to better understand their mechanisms of formation. With their inbuilt pre-concentration, element selection and simple matrix, DGTs are an ideal medium for laser ablation ICPMS. In contrast to analysis of mineral phases, fully matrix-matched standards can be easily made. We have initially concentrated on the measurement of S isotopes and will also present experiments where Fe isotopes have been measured in DGT gels. S and Fe isotopes were measured on a Neptune at mass resolutions of >9000. A New-Wave UP-213 nm laser was used with Ar as the carrier gas. Instrumental mass bias (IMB) for S was corrected for using sample-standard bracketing. Ablation experiments on two S DGT gel standards, 10 Hz, 40% power, 170 μm wide line, scan speed 70 μms-1, show in-run δ34S precisions for DGT gels with 2.1 μmol/cm2 S (3 V of 32S) of <0.2‰ (2SE). There is no observed correlation between δ34S and 32S intensity. Reproducibility on δ34S for individual analytical sessions (4-8 standards) is better than 0.3‰ while the reproducibility over 4 sessions is 0.19‰. Further tests to mimic ablation over micro-niches (continuous ablation across a gel with two isotopic compositions) show similar internal precisions and only slightly worse reproducibility-0.48‰ (2SD; n = 6). The accuracy of the LA δ34S measurements on a BaSO4 gel relative to the reference value of the starting BaSO4, bracketed by a AgI gel of known isotopic composition, is better than 1‰, showing negligible S isotopic fractionation during gel deployment. δ34S variations in micro-niches can be easily resolved at the 1‰ level and 100 μm scale whereas variations in natural micro-niches are expected to be 10‰ or more. Fe measurements, using external normalisation to a 60Ni/61Ni ratio and an exponential law to correct for IMB, suggest internal precisions and external reproducibility of <0.15‰ (2SE) and 0.07‰ (2SD) should be possible.Godkänd; 2006; 20120912 (ysko)