Osmium contamination of seawater samples stored in polyethylene bottles

Abstract

A low blank—high yield procedure for the accurate determination of seawater osmium concentration and isotope composition has been developed. The improvement in the detection limit has revealed a subtle temporal increase in the concentration of samples obtained during the GEOTRACES expeditions that were stored in high density polyethylene bottles. This increase in Os concentration is accompanied by significant decrease in the 187Os/188Os ratio of the water indicating substantial contamination of waters in two out of three bottles that were investigated; the third bottle, which is not contaminated, shows evidence of Os removal. In comparison, analyses of another aliquot of water stored in a Teflon bottle show no contamination. Samples collected in low density polyethylene bottles during SAFe expedition have also been contaminated. Bottle cleaning with HNO3 has directly/indirectly contributed to the bottle blank. We also find that a snow‐melt sample stored in a Teflon bottle has remained uncontaminated for more than a year, despite having an Os concentration that is ~14 times lower than seawater. We conclude that the low level samples, such as seawater, need to be stored in Teflon bottles for accurate and precise estimate of Os concentration and isotope composition, although polypropylene bottles may be an acceptable alternative. A re‐evaluation of previously published data and data obtained in the present study still indicates that Os is not a conservative element in the seawater column and that small but significant variations in the seawater 187Os/188Os ratio may exist with depth and among different ocean basins.