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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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SAQ 3.1
A series of 10 samples was taken at random from a site. The following results were obtained on the samples for lead. Using the above equations, calculate the magnitude of error that can be tolerated.
Sample 1 2 3 4 5 6 7 8 9 10
Pb (xi) (ppm) 91 95 104 82 95 103 97 89 85 89
X (mean) 93 93 93 93 93 93 93 93 93 93
(Xi - x) -2 2 11 -11 2 10 4 -4 -8 -4
(x,-x)2 4 4 121 121 4 100 16 16 64 16
SAQ 3.2
If the level of precision obtained from SAQ 3.1 is not acceptable and a maximum error of 2 ppm is only allowed, how many samples would need to be taken?
3.4 Sampling Soil and Sediment
Soil is an heterogenous material and significant variations (chemical and physical) can occur over even a small site, e.g. a field.
DQ 3.2
What might lead to different chemical and physical variations over the chosen site?
Answer
Variations can occur due to different topography, farming procedures, soil type, drainage and the underlying geology.
Obtaining a representative sample is therefore important (see Section 3.2 above). The tools required for sampling include an auger, a spade and a trowel. For shallow sampling of disturbed material, a trowel is sufficient to gather
32
Methods for Environmental Trace Analysis
material. Once obtained, it should be placed in a polythene bag, sealed and clearly labelled with a permanent marker pen. Deep sampling can be carried out either by using augers or spades, by utilizing trenches and road banks, or by digging soil pits and exposing soil profiles.
An auger is a device that can be screwed into the ground to remove soil. The commonest types are the twin blade and the corkscrew (Figure 3.3). Both return disturbed soil to the surface. Augers are useful for pilot surveys of sites. The general procedure for using an auger is as follows:
1. Identify the site to be sampled.
2. Bang into the ground, using a mallet, a piece of heavy-walled PVC tubing. This will act as a guide for the auger.
3. Place the auger in the PVC tubing and turn the handle. This should allow the auger to collect soil. Once the auger is filled, remove from the ground and place the collected soil in a plastic bag. A sub-sample of the soil is then transferred into a pre-labelled and pre-cleaned glass jar. The lid is then placed on the jar and the sample stored on ice prior to returning to the laboratory.
4. Repeat Step 3, until the required depth has been achieved. (Note that care is needed with the 0-15 cm soil layer, as this can often be compacted.)
(a) (b)
Figure 3.3 Types of augers used for soil sampling: (a) twin blade; (b) corkscrew.
Sampling
33
Soil dug from a pit should be transferred to a plastic sheet placed on the ground - not directly onto the surrounding grassland. Always keep one side of the pit ‘clean’ by trimming with a trowel. This will allow the soil stratigraphy to be observed and recorded. In the vertical section, soils can be characterized by various layers of differing composition, known as a soil profile. A typical profile may consist of the following:
• The L layer: the litter layer, which is composed of the debris from plants and animals.
• The A horizon (top soil): the uppermost horizon, which contains mineral matter and some organic matter from the L layer.
• The B horizon (sub-soil): this lies below the A horizon and has a lower organic matter content.
• The C horizon: this is part of the underlying parent rock from which the soil above was derived.
Use a tape measure to allow the recording of depths and hence the thicknesses of the soil horizons in the profile. Colour is used to identify the different horizons within the profile. Standardized descriptions of colour can be obtained by the use of the Munsell Soil Colour Chart System (see Chapter 1).
When soil sampling has been completed, replace any unwanted soil and cover with grass sods (if appropriate). After sampling, it is common, after transportation of the sample back to the laboratory, to let the sample air-dry (at <30°C).
DQ 3.3
As you are trying to remove soil moisture, why not dry the soil in an oven?
Answer
The use of elevated temperatures can lead to loss of both volatile metals, e.g. mercury, and volatile organic species, e.g. BTEX (benzene-toluene-ethylbenzene-xylene(s)) compounds.
The air-dried sample is then sieved (2 mm diameter holes) to remove stones, large roots, etc., prior to sub-sampling to obtain a small representative sample of finely ground soil (the sample will often be sieved again to reduce the particle size). The purpose of this process is to obtain a sample suitable for the analytical technique and yet still remain representative of the original bulk sample. One of the most popular methods of sub-sampling the air-dried soil to obtain a representative sample for subsequent extraction/digestion and analysis is coning and quartering. In this procedure, the dried soil is thoroughly mixed and poured onto a clean sheet of polythene to form a cone. The latter is then divided into four quarters, using a cross made of sheet aluminium,
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