Mercury in seawater

Seawater and estuarine water CRMs certified for their contents of trace elements (see sections 8.7 and 8.8) were not analysed for mercury. Due to the high volatility of mercury, water to be analysed for Hg content can not be stored in polythene bottles and a higher acidification is necessary. So far, no CRM existed for this element, and hence a separate reference material had to be produced. 

When selecting a site for the collection of the material, it was argued that most monitoring studies would be performed in coastal areas, and as a consequence a coastal seawater was preferred over an open ocean sample which, due to its lower Hg contents would be of use for only a limited number of users. A compromise was found in a sample with a content typically in the lower range of coastal waters. A feasibility study for the preparation of large batches of seawater for this element and results of interlaboratory studies demonstrated that a certification campaign could be contemplated [28], and a candidate reference material was prepared for this purpose [29,30]. This material (CRM 579) was certified for its content of Hg. 

Previous exercises have shown the difficulties of determining Hg in seawater. As an example, coefficients of variation of 26.7 and 11.6% between eleven laboratories (CV of the mean of laboratory means) were found for Hg levels of respectively 6.0 and 24.2 ng L [31]. Therefore, it was chosen to consider seawater samples containing much higher Hg concentrations (coastal seawater samples spiked with mercury) in a first method performance study and to use natural coastal seawater, along with a spiked sample, in the second interlaboratory study. 

As mentioned before, two interlaboratory studies were organised prior to certification, involving ca. 15 laboratories using techniques such as cold vapour atomic absorption spectrometry, direct current plasma atomic emission spectrometry (DCP-AES), differential pulse anodic stripping voltammetry (DPASV), microwave plasma atomic emission spectrometry (MIP-AES), electrothermal atomic absorption spectrometry (ETAAS) and neutron activation analysis with radiochemical separation (RNAA). 

In the first interlaboratory study, the examination of the raw data (14 sets of results of which 12 involved CVAAS, one RNAA and one MIP-AES) revealed a high spread of results due to two outliers. The mean obtained was 12.6 pg L of Hg with a coefficient of variation (CV) between laboratories of 33%. The two high results were attributed to a laboratory contamination. The accepted values showed a picture which was found more acceptable, i.e. the mean obtained was 10.8 pg L with a CV between laboratories of 6.6 /n [8]. At this stage, the agreement between the laboratories was found to be satisfactory however, the Hg content in this (spiked) sample was considered much too high for being representative of natural samples which justified the organization of a second interlaboratory exercise for which results are described below. 

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Olaffson J (1980) A preliminary report on ICES intercalibration of mercury in seawater for the joint monitoring group of the Oslo and Paris commissions, submitted to the Marine Chemistry Working Group of OCS, February 1980... 

Olafsson [478] has reported on the results obtained in an international intercalibration for mercury in seawater. Sixteen countries participated in this exercise, which involved analysis of a seawater and seawater spiked with 15.4 and 143 ng/1 mercury. The results show good accuracy and precision in the recovery of mercury for the majority of calibrations, but serious errors in the low-level determinations on the seawater. 

Gill and Fitzgerald [481] determined picomolar quantities of mercury in seawater using stannous chloride reduction and two-stage amalgamation with gas-phase detection. The gas flow system used two gold-coated bead columns (the collection and the analytical columns) to transfer mercury into the gas cell of an atomic absorption spectrometer. By careful control and estimation of the blank, a detection limit of 0.21 pM was achieved using 21 of seawater. The accuracy and precision of this method were checked by comparison with aqueous laboratory and National Bureau of Standards (NBS) reference materials spiked into acidified natural water samples at picomolar levels. Further studies showed that at least 88% of mercury in open ocean and coastal seawater consisted of labile species which could be reduced by stannous chloride under acidic conditions. 

Bloxam et al. [482] used liquid chromatography with an inductively coupled plasma mass spectrometric detector in speciation studies on ppt levels of mercury in seawater. 

Turyan and Mandler [483] determined ppt levels of mercury in seawater by first converting mercury salts to elemental mercury using stannous chloride, the mercury was then trapped on gold deposited on platinum gauze and released by heating prior to determination by inductively coupled plasma mass spectrometry. 

Wrembel [485] gives details of a procedure for the determination of mercury in seawater by low-pressure ring-discharge atomic emission spectrometry with electrolytic preconcentration on copper and platinum mesh electrodes. Between 40 5 (open sea) and 50 8 (shore area) xg/l mercury was found in Baltic sea waters. 

Wrembel and Pajak [486] evaporated mercury from natural water samples with argon and amalgamated the mercury with a gold foil. The mercury was excited in a ring-discharge plasma and determined by atomic emission spectroscopy. The method was applied to the determination of mercury in seawater in the range 0.01-1.0 xg/l. 

Voyce and Zeitlin [487] have used adsorption colloid flotation to determine mercury in seawater. The sample 500 ml is treated with concentrated hy-... 

Stroh and Voellkopf [746] utilised flow injection analysis coupled to ICP-MS to determine down to 0.6 ppt of antimony, arsenic, and mercury in seawater. 

Sipos et al. [789] have described a procedure for the simultaneous determination of copper and mercury in seawater down to the ng/1 range using differential pulse ASV at a gold electrode. Pretreatment is necessary, and comprises UV irradiation to release the trace metal bound to dissolved organic matter. 

Various workers have reported on the levels of total mercury in seawater. Generally, the levels are less than 0.2 p,g/l with the exception of some parts of the Mediterranean where additional contributions due to man-made pollution are found [49-52],... 

Yamamoto et al. [60] determined picogram quantities of methyl mercury and total mercury in seawater by gold amalgamation and atomic absorption spectrometry. Methyl mercury was extracted with benzene and concentrated by a succession of three partitions between benzene and cysteine solution. Total mercury was extracted by wet combustion of the sample with sulfuric acid and potassium permanganate. The proportion of methyl mercury to total mercury in the coastal seawater sampled was around 1%. 

Graphite furnace atomic absorption spectrophotometry has been used for the determination down to 5 ng/1 inorganic and organic mercury in seawater [61]. The method used a preliminary preconcentration of mercury using the ammonium pyrrolidine dithiocarbamate-chloroform system. A recovery of 85 - 86% of mercury was reproducibly obtained in the first chloroform extract and consequently it was possible to calibrate the method on this basis. A standard deviation of 2.6% was obtained on a seawater sample containing 529 ng/1 mercury. 

Cossa [12] has used this approach to monitor the levels of mercury in seawater samples. 

A cold-trap pre-concentration procedure, which is incorporated into a standard jlameless atomic absorption analysis of mercury in environmental samples, has been used for both shipboard and laboratory analyses of mercury in seawater, The coefficient of variation for seawater containing 25 ng Hg/l, is 15%, and a detection limit of approximately 0,2 ng Hg is attainable. In surface seawaters of coastal and open regions of the northwest Atlantic Ocean mercury concentrations appear to decrease with increasing distance from terrestrial sources. In the open ocean samples they are less than 10 ng/l. and rather uniformly distributed. The amounts of mercury in inshore samples can approach 50 ng/l, A significant mercury fraction characterized by a stable association with organic material may be present in coastal waters. 

This section presents an overview of the cold-trap pre-concentration gas phase detection method for the determination of mercury in seawater. A more thorough and complete discussion of the analytical details (e.g., cleaning procedures, reagent preparation, analytical manipulations) can be found in Fitzgerald et al (21). 

Analytical Procedure. The cold-trap gas phase mercury detection system was designed and used for both laboratory and shipboard measurements of mercury in seawater. The Coleman Instruments mercury analyzer (MAS-50) was incorporated into the analytical system because of its portable and convenient design. However, the effective use of this simple one-element atomic absorption unit requires scrupulous attention to blank determinations for each seawater sample. For example, the undetected presence of either naturally occurring or sampling induced volatile organics which may absorb at the mercury wavelength in the seawater sample can be a serious error. Such artifacts were observed when acidifled seawater samples were stored in low density polyethylene bottles (21), Therefore, the analytical procedure used to determine the mercury concentration in a seawater sample consists of the following steps ... 

Figure 2. A composite calibration curve for mercury in seawater samples measured over a three-week period. The mercury spike quantities are in ng and the mercury absorption in arbitrary units (21).

The reported concentrations for mercury in seawater range from non-detectable to 364 ng/1. (11, 12, 15-17, 28-30). In several of these studies, the concentrations of mercury were greater than 100 ng/1. in open ocean waters (12, 28, 29, 30). The data from our investigations of the northwest Atlantic Ocean (Table II) and other studies (11, 15, 16, 17) indicate that the mercury concentrations should be closer to 10 ng/1. in these open ocean waters. 

This variability for the reported concentrations of mercury in open ocean waters may indicate that there are significant analytical diflBculties associated with the proper sample collection and the accurate measurement of mercury in seawater. These problems tend to override and preclude precise geochemical calculations and marine geochemical interpretations regarding the sources, sinks, and interactions of mercury in the oceans. These observational discrepancies for trace seawater constituents such as mercury can be resolved only through intercalibration programs and the use of standardized seawater samples. Such standards are not presently available for mercury concentrations at 100 ng Hg/1. or less in seawater. 

Figure 4.1 FI manifold for the determination of inorganic and organic mercury in seawater using CVG detection. (From Bloxham, S. J. et al., J. Anal. At. Spectrom., 11, 511, 1996. With permission.)...

Preconcentration methods have generally been used in studies involving the determination of mercury in seawater but there has been little standardization of the techniques and many different preconcentrating methods have been used. Most recent studies (27)... 

Preconcentration Method for the Determination of Mercury in Seawater and in other Natural Materials, Anal, Chem. 

For the determination of particulate mercury, add-cleaned Teflon and quartz fibre filters, the latter combusted at 500 °C (Coquery and Cossa, 1995), are recommended. A significant fraction of the mercury in seawater is present in colloidal forms and separated with the aid of add-cleaned ultrafiltration cartridges (Stordal et oL, 19%). 

Gadner, M. and Gunn, A. (1997) Stability of mercury in seawater samples. Anal Common., 35, 245. ... 

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