Sampling and filtration of seawater

Like other chemicals, organochlorines occur in seawater in a continuum of dissolved, colloidal and discrete particulate forms Gustafsson and Gschwend, 1997). Operationally, dissolved and particulate suspended forms may be distinguished on the basis of separation techniques such as filtration or centrifugation. Ihe separation may depend on size and density of the particles it may also depend on the composition of the suspension. For instance, particles smaller than the nominal filter pore size are retained on a clogged filter. 

As organochlorine compounds usually occur at extremely low concentrations in seawater solution, they must be extracted into an organic medium and be concentrated over many orders of magnitude prior to gas chromatographic analysis with electron capture detection (GC-ECD). The required water volume depends on  

3) the contribution of contamination during sampUng and clean-up procedures. 

Even if contamination could be avoided entirely, sample volumes must approximate 1000 L to enable analysis, at an instrumental detection limit of 0.1 pg (ECD), of components at the 0.01 pg/L concentration range typical for open ocean waters. Sample contamination aggrevates the problem. This limitation seems to have been overlooked in several earlier studies (see review by Harding, 1986). 

Further difficulties originate in the presence of particles Duinker, 1986 Brownawell and Farrington, 1986). The transport mechanisms of water and particles are different. The environmental fate of a chemical, therefore, depends on its distribution between solution and suspension, which is determined not only by its liquid-solid distribution coefficient but also by the characteristics of particles and their concentrations. 

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