Assessing bias in total mercury results after removing a subsample from the bottle

<p>U.S. EPA Method 1631 for total mercury (THg) analysis in water recommends that bromine monochloride (BrCl) be added to the original bottle in which the sample was collected, to draw into solution any Hg that may have adsorbed to the bottle walls. The method also allows for the removal of a subsample of water from the sample bottle for methylmercury (MeHg) analysis prior to adding BrCl. We have demonstrated that the removal of a subsample from the sample bottle prior to THg analysis can result in a positive concentration bias. The proposed mechanism for the bias is that ‘excess’ inorganic Hg, derived from the subsample that was removed from the bottle, adsorbs to the bottle walls and is then drawn into solution when BrCl is added. To test for this bias, we conducted an interlaboratory comparison study in which nine laboratories analysed water samples in fluorinated polyethylene (FLPE) bottles for THg after removing a subsample from the sample bottle, and analysed a replicate sample bottle from which no subsample was removed. We received seven complete data sets, or 63 unique sample pairs. The positive concentration bias between the bottles was significant when comparing all samples in aggregate (1.76 ± 0.53 ng/L after subsample removal, 1.57 ± 0.58 ng/L with no subsample removal, <i>P</i> < 0.05), however when comparing each of the three samples individually, the only significant bias was in the saline sample (Site UJ; 1.51 ± 0.31 ng/L after subsample removal, 1.32 ± 0.47 ng/L with no subsample removal, <i>P</i> < 0.05). Based on the findings presented here, we conclude that water chemistry, volume of water poured off, and the sample storage temperature explain some but not all of the observed bias, and we recommend collecting THg and MeHg samples in separate bottles whenever possible.</p

Bottom distribution of geophysical and biogeochemical parameters in the Eastern Mediterranean Sea.

<p>(<b>a</b>) Bottom distributions of θ, S, DO, THg, BPC and its constituents PRT, CHO and LIP, as well as enzymes GLU collected in 21 sites in the southern Adriatic basin and Ionian abyssal plain during MSM 13–2. (<b>b</b>) Bottom distributions of θ, S, and δ¹⁸Ο observed in the same region during POS 414. Data refer to the bottom-deep casts at depth within ~10–12 m above the seabed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145299#sec002" target="_blank">methods</a>). Interpolation was performed with the weighted-average gridding algorithm of the software Ocean Data View.</p

Grouping of the Eastern Mediterranean Sea bottom waters according to bacterial diversity.

<p>(a) Cluster analysis of the ARISA fingerprints. (b) Map of part of the Eastern Mediterranean Sea, where the stations for which ARISA was determined are delineated. Each color indicates stations belonging to the same group.</p

Grouping of the Eastern Mediterranean Sea bottom waters according to geophysical and biogeochemical parameters.

<p>Data are standardized Potential Temperature, S and DO data (<b>a</b>) and standardized CHO, LIP, BPC, GLU, AMPT and THg data (<b>b</b>). The grouping is based on a K-means clustering algorithm with four clusters. In each panel, the different colours identify the stations that belong to the same group.</p

Bathymetry encompassing the Euro-African Mediterranean Sea where the data used in our investigation were collected.

<p>Samples from the different cruises are marked by different dot colours (red: POS414; blue MSM 13–2; green: MSM 15–4). Only the positions of the stations actually considered are indicated in the map.</p

Grouping of parameters and stations in the Eastern Mediterranean abyss.

<p>The analysis is performed on parameters (a) and stations (b) of MSM 13–2 based on all available standardized physical and biogeochemical parameters. The grouping is based on a tree clustering algorithm with single linkage amalgamation rule.</p

Vertical distribution of geophysical quantities in the abyssal Ionian Sea.

<p>Potential Temperature (θ,) Salinity (S), Dissolved Oxygen (DO), and Potential Density Anomaly (σ_θ) were measured in the abyssal plain of the Ionian Sea in October 2009, July 2010 and June 2011. The data refers to depths > 3500 m. Colours individuate the different cruises (red: POS414; blue MSM 13–2; green: MSM 15–4). The vertical profiles refer to the locations mapped in the left panels (colour code is the same as for the right panels).</p