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Compare the data presented in Figure 2.3 above with those shown in Table 5.10.
In both cases, the results are presented as the ‘certified values’ with an ‘uncertainty’ or ‘±’value. This uncertainty is determined as one standard deviation of the mean value.
Finally, it is recommended that for inductively coupled plasma (ICP) analysis a final filtration (0.45 ^m) is carried out in order to prevent nebulizer blockages. If graphite-furnace atomic absorption spectroscopy (GFAAS) is the method of analysis, it is recommended that the standard additions method of calibration is used (see Chapter 1).
An alternative approach using diethylenetriaminepentaacetic acid (DTPA) has also been evaluated (Figure 5.19). The DTPA solution is prepared as follows: dissolve 149.2 g triethanolamine (0.01 moll-1), 19.67 g DTPA (0.005 moll-1)
Methods for Environmental Trace Analysis
Table 5.10 Extractable metal contents of two certified reference materials (CRM 483 and CRM 484) used in the analysis of soils or sediments 
Certified reference material/extract Certified value (mg kg 1) Uncertainty (mg kg 1)
CRM 483 (Sewage sludge-amended soil) EDTA extracts Cd 24.3 1.3
Cr 28.6 2.6
Cu 215.0 11.0
Ni 28.7 1.7
Pb 229.0 8.0
Zn 612.0 19.0
Acetic acid extracts
Cd 18.3 0.6
Cr 18.7 1.0
Cu 33.5 1.6
Ni 25.8 1.0
Pb 2.10 0.25
Zn 620.0 24.0
CRM 484 (terra rossa soil) EDTA extracts Cd 0.51 0.03
Cu 88.1 3.8
Ni 1.39 0.11
Pb 47.9 2.6
Zn 152.0 7.0
Acetic acid extracts
Cd 0.48 0.04
Cu 33.9 1.4
Ni 1.69 0.15
Pb 1.17 0.16
Zn 193.0 7.0
CRM 484 (terra rossa soil) EDTA extracts Cd 2.68 0.09
Cr 0.205 0.022
Cu 57.3 2.5
Ni 4.52 0.25
Pb 59.7 1.8
Zn 383.0 12.0
Acetic acid extracts
Cd 1.34 0.04
Cr 0.014 0.003
Cu 32.3 1.0
Ni 3.31 0.13
Pb 15.0 0.5
Zn 142.0 6.0
Figure 5.19 Procedure adopted in the single extraction method for metals (employing DTPA), as applied to the analysis of soils and sediments.
and 14.7 g calcium chloride in approximately 200 ml of distilled water. Allow the DTPA to dissolve and then dilute to 9 l. Adjust the pH to 7.3 ± 0.5 with HCl while stirring and dilute to 10 l. This working solution should be stable for several months.
5.7.4 Sequential Extraction Procedure
The sequential extraction procedure consists of three (main) stages, plus a final (residual fraction) stage (Figure 5.20), as follows:
• Step 1. Metals extracted during this step are those which are exchangeable and in the acid-soluble fraction. These includes weakly absorbed metals retained on the sediment surface by relatively weak electrostatic interaction, metals that can be released by ion-exchange processes and metals that can be coprecipitated with the carbonates present in many sediments. Changes in the ionic composition, influencing adsorption-desorption reactions, or lowering of pH, could cause mobilization of metals from such fractions.
• Step 2. Metals bound to iron/manganese oxides are unstable under reducing conditions. Changes in the redox potential (Eh) could induce the dissolution of these oxides and could also release adsorbed trace metals.
84 Methods for Environmental Trace Analysis
Figure 5.20 Overview of the sequential extraction method for metals, as applied to the analysis of soils and sediments.
• Step 3. Degradation of organic matter under oxidizing conditions can lead to a release of soluble trace metals bound to this component. Amounts of trace metals bound to sulfides might be extracted during this step.
It is common to analyse for trace metals in the residual fraction. In this situation, the latter should contain naturally occurring minerals which may hold trace metals within their crystalline matrix. Such metals are not likely to be released under normal environmental conditions. The residual fraction is digested by using a ‘pseudo-total’ approach with aqua regia as most metal pollutants are not silicate-bound. However, for complete digestion, hydrofluoric acid is required.
Why is the term ‘pseudo-total’ used?
Aqua regia is a good acid mixture for the digestion of soil and sediment samples. However, it cannot liberate from the matrix metal pollutants that are silicate-bound, i.e. part of the silicate ‘backbone’. In this situation, if complete digestion is required then hydrofluoric acid must be used.
As it is unlikely that the silicate-bound metals will leach from the soil or sediment, the use of aqua regia to give a ‘pseudo-total’ analysis is perfectly acceptable in this situation.
For the sequential extraction procedure (see details given in Figures 5.21-5.23), it is important to note the following:
• All laboratory ware should be made of borosilicate glass, polypropylene, polyethylene or PTFE, except for the centrifuge tubes which should be made of either borosilicate glass or PTFE.
Figure 5.21 Details of Step 1 of the sequential extraction method (cf. Figure 5.20).
Figure 5.22 Details of Step 2 of the sequential extraction method (cf. Figure 5.20).