Mercury determinations, open ocean

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. 

A mong the preferred analytical methods for determining mercury con-centrations in natural samples save been closed system reduction-aeration procedures using mercury detection by gas phase atomic absorption or atomic fluorescence spectrophotometry (I-I5). In studies in the oceanic regime, where the amount of mercury in a liter sample of open-ocean seawater can be as small as 10 ng (11,15,16,17), a. pre-concentration stage may be required. The lowered detection limits which accompany a preliminary concentration step are most desirable when the sample materials are rare or in limited quantities such as carefully collected open-ocean biota, open-ocean rain water, and deep-ocean seawater. 

Open Ocean Mercury Determinations. In our initial studies concerned with the marine geochemistry of mercury, we obtained open ocean smrface samples by hand from a small work boat away from any adverse influence of the oceanographic research vessel. The concentrations of mercury in the open-ocean surface waters (western Sargasso Sea) were small (ca. 10 ng/1.) and rather imiformly distributed (26). However, to collect seawater to determine the concentrations of mercury at other depths, we needed an artifact-free sampling procedure. 

In our previous investigations of the amounts and distribution of mercury in the surface waters of the northwest Atlantic Ocean, we found a mean total mercury concentration of 7 ng/1. and a range of 6-11 ng/1. (26). Also, we found in open ocean surface waters no significant difference between the mercury concentrations measured directly in pre-acidified seawater ( reactive mercury) and the total mercury determination in the organic free samples. In the work shown in Table II, we also found no significant difference between the reactive mercury determination and the total mercury measurement, which was carried out in approximately one third of the samples. The total mercury measurements appear in the square brackets for the results tabulated in Table II. 

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