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Methods for Environmental Trace Analysis
and the two opposite quarters combined and mixed to form a sub-sample of approximately half the original weight. This half sample is again coned and quartered and the process repeated until a sub-sample of the desired sample weight is obtained.
3.5 Sampling Water
Water is a common substance to sample considering that the earths surface is composed of ca. 70% water.
How many different types or classifications of water can you think of?
The different types of water can be classified as follows: surface waters (rivers, lakes, run-off, etc.), groundwaters and spring waters, waste waters (mine drainage, landfill leachate, industrial effluent, etc.), saline waters, estuarine waters and brines, waters resulting from atmospheric precipitation and condensation (rain, snow, fog, dew, etc.), process waters, potable (drinking) waters, glacial melt waters, steam, water for sub-surface injections, and water discharges, including water-borne materials and water-formed deposits.
While water would appear to be homogeneous, this is not in fact the case. Water is heterogeneous, both spatially and temporally, thus making it extremely difficult to obtain representative samples. Stratification within oceans, lakes and rivers is common with variations in flow, chemical composition and temperature all occurring. Variation with respect to time (temporal) can occur, for example, because of heavy precipitation (snow, rain, etc.) and seasonal changes.
Water samplers can be either automatic or manually operated. Automatic samplers are used to collect samples at either fixed time-intervals or in proportion to the flow and then to retain the water sample in a separate container. These are commonly used, for example, in rivers or from a point source (effluent outfall). In addition, automatic samplers can be used to allow the collection of time-averaged samples or precipitation. In the case of the latter, the onset of rainfall triggers the collection mechanism. Manually operated samplers are essentially open tubes of known volume (typically 1 to 30 l) fitted with a closure mechanism at each end. They are constructed of stainless-steel or PVC. Manually operated samplers are particularly useful when sampling from open waters (e.g. oceans, seas, lakes, etc.) at specific depths. The sampling device is lowered on a calibrated line to the specific sampling depth, the sample is taken and then the top and bottom lids are closed and sealed.
A good manual water sampler should possess the following attributes:
provide a rapid descent in the water;
be substantial enough to prevent minimal drift from the vertical;
have a suitable closing and sealing mechanism to retain the sample but not allow the ingress of the surrounding water;
be inert and hence not contaminate the sample;
be easy to use and maintain;
have an appropriate sample capacity.
A typical manual water-sampling device is shown in Figure 3.4.
Figure 3.4 A typical manual device used for water sampling . From Jones, A., Duck, R., Reed, R. and Weyers, J., Practical Skills in Environmental Analysis, Prentice Hall, Harlow, UK, 2000. © Pearson Education Limited 2000, reprinted by permission of Pearson Education Limited.
Upper spring-operated lid
Methods for Environmental Trace Analysis
Lockspring holds sorbent layers securely
Foam separator Sealed tips
Figure 3.5 Air sampling: (a) a typical sorbent tube; (b) the system used to carry out measurements . From Jones, A., Duck, R., Reed, R. and Weyers, J., Practical Skills in Environmental Analysis, Prentice Hall, Harlow, UK, 2000. © Pearson Education Limited 2000, reprinted by permission of Pearson Education Limited.
3.6 Sampling Air
The sampling of air can be classified into two groups, namely particulate sampling, in which particles are collected on filters, and vapour/gas sampling, in which air-borne compounds are trapped on a sorbent. Air sampling can take two forms. In the first, i.e. passive sampling, air-borne material diffuses onto filters and is retained. In the second type of sampling, the air-borne material is actively pumped through a filter or sorbent and then retained. A filter simply presents a physical barrier, while a sorbent provides an active site for chemical/physical retention of the material. The filters used can range in composition from fibre glass to cellulose fibres, while sorbents range from ion-exchange resins to polymeric substrates.
In sorbent tube sampling (Figure 3.5), volatile and semi-volatile compounds are pumped from the air and trapped on the surface of the sorbent. By sampling a measured quantity of air (typical volumes of 10-500 m3), quantitative sampling is possible. The sorbent tube is then sealed and transported back to the laboratory for analysis. Desorption of volatile and semi-volatile compounds takes place either by the use of organic solvents (solvent extraction) or heat (thermal desorption), followed by analysis using gas chromatography (see later).