Liquid Sampling
Sampling a liquid is a relatively simple procedure but, problems can arise if the sample is not homogeneous. Solid particles are removed by filtration or centrifugation. If trace components are important, any solids that are removed may contain some of those components adsorbed on their surface. Consequently, the separated solids should be extracted with solvents and the solution added to the filtrate. If the sample is an emulsion, then the mixture must be separated by centrifugation. The two liquids can be analyzed separately. If one liquid (usually the lower) also contains solids these must also be removed by filtration. Samples should be sealed in dark glass vials for storage to prevent photochemical decomposition and refrigerated to prevent possible bacterial decomposition.
Sampling bottles or drums containing liquid is simple as the material to be sampled is likely to be homogeneous. If heterogeneous, the bulk material must be well mixed before a sample is taken. Large masses of moving material such as sea-water from the sea shore and water from the banks of a swiftly flowing river can be difficult to sample and such samples are common in pollution monitoring. The sample composition will certainly differ with the distance from the shore or bank, with water velocity and the depth at which the sample is taken. It is advisable to take a number of samples at different depths and at different distances from the water edge. Some effort should also be made to assess the water velocity at each sampling point to relate the analytical results to water movement. Even when sampling 'still' water such as ponds and lakes, multiple samples need to be taken at different positions in the lake or pond and at different depths.
Water samples should be stored in dark glass bottles or stainless steel containers to reduce photochemical reactions and refrigerated at about 2°C to retard any bacteria or algae growth. Oxygen may change the amount, or nature, of the contaminants, so containers should be completely filled so there is a minimum of air between the water surface and the stopper. Where biological activity may occur, the analysis must be completed as soon as possible.
When delivered, petroleum products, milk etc. often arrive in large containers such as drums or tanks. To obtain a representative sample the volume taken from each container should be a fixed proportion of the container volume and then the samples are then bulked together. Samples can be stored in bottles or cans made from inert material such as glass, stainless steel, plastic etc. Chemically and thermally stable samples can be transported under ambient conditions, but if biological or biochemical change is possible (e.g. milk, whey, corn syrup, etc.) then the sample must be refrigerated at about 2°C until assayed. Even at 2°C, liquids of biological origin should not be stored for more than 24 hrs. before analysis. Heterogeneous liquids, such as paint, emulsions etc. must be well mixed before a sample is taken. If all the cans of a particular delivery are from the same batch, the contents of one or two cans may be representative of the total batch. Corrosive liquids should be handled according to published protocol and samples can be stored in glass, stainless steel or, for very corrosive materials, in Teflon containers. The containers must be protected against mechanical shock during transport and appropriately labeled. Due to the hazardous nature of such materials the volume or mass of material sampled should be kept to the necessary minimum.
