Surface waters sample handling

Surface-water samples are usually collected manually in precleaned polyethylene bottles (from a rubber or plastic boat) from the sea, lakes, and rivers. Sample collection is performed in the front of the bow of boats, against the wind. In the sea, or in larger inland lakes, sufficient distance (about 500 m) in an appropriate wind direction has to be kept between the boat and the research vessel to avoid contamination. The collection of surface water samples from the vessel itself is impossible, considering the heavy metal contamination plume surrounding each ship. Surface water samples are usually taken at 0.3-1 m depth, in order to be representive and to avoid interference by the air/water interfacial layer in which organics and consequently bound heavy metals accumulate. Usually, sample volumes between 0.5 and 21 are collected. Substantially larger volumes could not be handled in a sufficiently contamination-free manner in subsequent sample pretreatment steps. 

X-ray fluorescence spectrometry was the first non-destructive technique for analysing surfaces and produced some remarkable results. The Water Research Association, UK, has been investigating the application of X-ray fluorescence spectroscopy to solid samples. Some advantages of nondestructive methods are no risk of loss of elements during sample handling operations, the absence of contamination from reagents, etc. and the avoidance of capital outlay on expensive instruments and highly trained staff. 

Previous reports 13] emphasized the importance of sample handling, and indeed because of the very volatile nature of the compounds measured in this type of analysis, sample collection deserves special consideration. In general, narrow mouth glass vials with a total volume in excess of 50 ml are acceptable. The bottles need not be rinsed or cleaned with organic solvents, but simply cleaned with detergent and water, rinsed with distilled water, air dried, and dried in a 105°C oven for one hour. The vials are carefully filled with sample to overflowing (zero head space) and a Teflon faced silicone rubber septum is placed Teflon face down on the water sample surface. The septa may be cleaned in the same manner as the vials, but should not be heated more than one hour because the silicone layer slowly degrades at 105°C. 

About 10% of the sample was concentrated in a vacuum centrifuge to give an estimated concentration of 1 pmol/pl if possible. Samples were not dried down completely to avoid sample loss. The MALDI matrix was a saturated solution of a-cyano-4-hydroxy cinnamic acid dissolved in water/acetonitrile (7 3) [8]. A 0.5-pl aliquot of the matrix solution was placed on the stainless steel probe and mixed with an equal volume of sample solution. The mixture was left to dry at room temperature prior to introduction into the mass spectrometer. A new sample preparation method which decouples matrix surface preparation and sample handling was also used [9]. 

Immediately upon return to the ship, the hydrocast was initiated with the closed PVC sampler (containing the surface water collection) placed as the flrst bottle on the hydrowire followed by four open and cocked samplers set at various depths from 25 to 750 m. The hydrocast and sample handling were then carried out in the usual fashion. 

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