Impact of storage temperature and filtration method on dissolved trace metal concentrations in coastal water samples

Abstract

Trace elements play a major role in biogeochemical cycles and oceanographic processes. To determine trace element concentrations, the dissolved and particulate phase are usually separated by filtration. However, the frequently used membrane filtration as well as sample storage can bias the dissolved elemental concentrations by adsorption or desorption/contamination. We present a comparison of two filtration methods for coastal and estuarine water samples (pressure filtration with Nuclepore™ polycarbonate filters, vacuum filtration with DigiFILTER™s) applied to aliquots of a large‐volume coastal water sample that were stored at ‐18 °C or 4 °C for up to nine weeks. The filtrates were analyzed by seaFAST‐ICP‐MS for dissolved Cd, Ce, Co, Cu, Dy, Er, Eu, Fe, Ho, La, Mn, Mo, Nd, Pb, Pr, Sm, Tb, U, V, W, Y and Zn. The filtration blanks of DigiFILTER™s (0.0006 ± 0.0010 ng L‐1 for Ho to 110 ± 180 ng L‐1 for Zn) were sufficiently low for quantification of all analyzed elements with good repeatability, enabling a fast and reliable filtration of large sample sets of coastal water. However, the findings also highlight the need to measure procedural blanks including the filtration instead of only the instrument blanks to validate results. Measured concentrations of both filtration methods did not differ significantly for Cd, Cu, Mo, U, V, W, Zn but for other investigated elements, the ratio between both methods was up to 1.8 for Ce and 4.1 for Fe. Within nine weeks of storage, the elemental concentrations decreased significantly, resulting in losses of 20 % Mn in frozen samples and 63 % Pb, 64 % Co and 93 % Mn in cooled samples.

Dissolved trace elements and nutrients in the North Sea—a current baseline

“…The samples were thawed the night before filtration. Under clean room conditions, triplicates were filtered with Digi FILTER TM s (0.45 µm, 50 mL, SCP Science, Canada) into Digi TUBE TM s from which they were measured (Przibilla et al, 2023 ). Digi FILTER TM s and Digi TUBE TM s were cleaned for 1 week with diluted acid (0.1% ( w / w ) HCl and 1.3% ( w / w ) HNO 3 ), respectively; Przibilla et al, 2023 ).…”

“…Under clean room conditions, triplicates were filtered with Digi FILTER TM s (0.45 µm, 50 mL, SCP Science, Canada) into Digi TUBE TM s from which they were measured (Przibilla et al, 2023 ). Digi FILTER TM s and Digi TUBE TM s were cleaned for 1 week with diluted acid (0.1% ( w / w ) HCl and 1.3% ( w / w ) HNO 3 ), respectively; Przibilla et al, 2023 ). The sampling bottles (1 L, LDPE, Brand, Germany; 0.5 L, LDPE, Nalgene TM , Thermo Fisher Scientific, USA) were leached twice with HCl ( w = 0.3%) for 1 week and 2 weeks with HNO 3 ( w =0.3%) and stored with HNO 3 ( w = 0.1%) until use.…”

“…For most trace elements, all measured concentrations were above the LOQ, while Ce, europium (Eu), gallium (Ga), Gd, lanthanum (La), Mn, neodymium (Nd), praseodymium (Pr), and samarium (Sm) showed concentrations between the LOD and LOQ for some stations. Only for Fe and lead (Pb), a large share of the samples exhibited concentrations below the LOD, as we followed our previously described conservative approach (Przibilla et al, 2023 ) for the determination of the LOD and LOQ. However, to calculate an unbiased median, also, values below the LOD and LOQ were included.…”

Multi-element analysis of unfiltered samples in river water monitoring—digestion and single-run analyses of 67 elements

“…Moreover, the separation process of SPM from the water is accompanied by a small analyte loss since it is not possible to recover 100% of all SPM even after making use of the best-practice. For the dissolved fraction, water samples have to be filtered and acidified immediately after sampling, to avoid fractionation, changes during storage, and thereby analyte partitioning artefacts between SPM and water [ 12 ]. In this case, analysing the whole water sample, containing all fractions, is a unique opportunity, without timely preservation steps, that allows to draw conclusions on element transportation processes even at low SPM concentration.…”