Sampling for particulate trace element determination using water sampling bottles: methodology and comparison to in situ pumps

Procedures for routine collection and analysis of particulate trace elements from 5-10 L samples filtered from rosette-mounted GO-FLO bottles were evaluated during the GEOTRACES intercalibration cruises of 2008-09. Issues important in obtaining reliable and consistent distributions of particulate trace elements were investigated: the effect of particle settling in sampling bottles; type, performance, and elemental blanks of filters; filter digestion procedures; and ICP-MS analytical procedures. We determined that gentle mixing of sampling bottles just before filtration, and limiting filtration time to 1-2 h, minimizes settling artifacts. Among those tested, 0.45 mu m polysulfone filters had the best particle loading characteristics, blanks, and physical attributes for routine use at sea. Maximum filter loading, requiring use of 25 mm filters at open ocean stations, was critical to achieve low blank corrections; most elements had <10% correction for a process filter blank. Heated digestion in nitric and hydrofluoric acids was necessary to effect dissolution of all particulate elements. Reproducibility, evaluated through replicate sampling, is very good overall, and analytical reproducibility was < +/-4%. Using the optimized methods, GO-FLO-filtered particulate elemental profiles for Al, P, Mn, Fe, Co, Cu, Zn, Cd, and Ba compared well to those collected by in situ pumping at the SAFe station (eastern N. Pacific) and at Santa Barbara Basin (coastal California, USA), with no systematic bias for either sampling system. The methodologies evaluated here demonstrate that low-volume methods can be used to determine distributions of particulate trace elements on ocean basin-scale hydrographic cruises, with efficiency, precision, and high spatial resolution.