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Methods for Environmental Trace Analysis
Any sampling protocol involves the selection of the sample points, the size and shape of the sample area, and the number of sampling units in each sample. Before this can be done, information regarding the likely distribution of the contaminants under investigation is required. Contamination from inorganics or organics can be random, uniform (homogenous), patchy, stratified (homogenous within sub-areas) or present as a gradient (Figure 3.1). Preliminary testing of the site (a pilot study) is therefore beneficial to establish the likely distribution.
(b) Uniform (homogenous)
(d) Stratified (homogenous within sub-areas)
Figure 3.1 Different distributions of inorganic and organic contaminants: (a) random;
(b) uniform (homogeneous); (c) patchy; (d) stratified (homogeneous within sub-areas); (e) gradient.
The key questions to be asked before sampling begins include the following:
1. Have arrangements been made to obtain samples from the site (e.g. permission from the site owner)?
2. Is specialized sampling equipment required and available?
3. How many samples and how many replicates are required?
4. Are the samples required for qualitative or quantitative analyses?
5. What chemical or physical tests are required?
6. What analytical methods and equipment are needed?
7. What mass/volume of sample is required for the analytical techniques to be used?
8. Is there a quality assurance protocol in place?
9. What types of container are required to store the samples and do you have enough available?
10. Do the containers require any pre-treatment/cleaning prior to use and has this been carried out?
11. Is any sample preservation required and do you know what it is?
3.2 Sampling Methods
The sampling position can be determined randomly, systematically or in a stratified random manner (Figure 3.2). In random sampling, a two-dimensional coordinate grid is superimposed on the area to be investigated. The selection of samples is completely down to the luck of the draw without regard to the variation of the contaminant in the soil. It should be noted that the entire sample area is not sampled, but that every site on the grid has an equal chance of being selected for sampling. This type of sampling is ideal if the contaminant is homogeneous within the site.
Systematic sampling involves taking the position of the first sample at random and then taking further samples at fixed distances/directions from this. For example, samples may be taken at intervals of 5 m. This type of sampling has the potential to provide more accurate results than simple random sampling. However, if the soil contains a periodic (systematic) variation which coincides with this type of sampling, biased samples can result. An initial pilot study of the site can help prevent this.
Stratified sampling is commonly used in a location which is known to have contaminants heterogeneously distributed. This is therefore the most common approach to sampling. In this type of sampling, the site is sub-divided into smaller areas, each of which is fairly homogeneous, and thus more accurate sampling can take place. Each sub-area is then randomly sampled. The sub-dividing of the
Methods for Environmental Trace Analysis
(a) Random (b) Systematic
f . ,
(c) Stratified random (1) - (d) Stratified random (2) -
sub-divided into equal areas weight related to sub-area
Figure 3.2 Basic methods used for sampling: (a) random; (b) systematic; (c) stratified random (1) - sub-division into equal areas; (d) stratified random (2) - weight related to sub-area of habitat.
site can be carried out either to give equal areas, or be related to known features within the site (see Figures 3.2(c) and 3.2(d), respectively).
3.3 Number of Samples
If the sampling site is homogeneous, then more samples need to be taken in order to achieve a certain accuracy. In reality, economic considerations often restrict both the quantity of material removed and the number of samples. In this case, the question becomes how many samples should be taken in order to achieve an acceptable error. This can be done, by using the following example, after first deciding on the magnitude of the error, E, that can be tolerated :
E = ±t(Vf5 (3.1)
V = S 2In (3.2)
where t is the test value, V the variance, S2 the sum of the squares, and n the number of samples.
The sum of the squares, S2, is calculated by using the following equation:
S = J2 (x - x)2/(n - 1) (3.3)
The number of samples to be taken can then be calculated by using the following relationship:
n = 12S2/E2 (3.4)