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22.214.171.124 Analysis by X-Ray Fluorescence Spectroscopy
The standards and samples were analysed by using a Spectro X-Lab 2000 X-ray fluorescence spectrometer operating with a Pd tube. The operating voltages ranged from 15 to 55 kV. The standards and samples of soil (4 g) were prepared as pressed pellets (pressure, 10 tonne) in the ratio 1:4 (wt/wt) of polymer (HWC, Hochst Wax C):soil. Calibration of standards and samples was achieved by using the ‘Geology Analytical Program’.
Methods for Environmental Trace Analysis
Cu Fe Pb Mn Ni Zn — Cu Fe Pb Mn Ni Zn
Figure 5.28 Comparison of the methods used in the analysis of (a) ‘Soil A’, and (b) ‘Soil CONTEST 32.3A’: ?, flame atomic absorption spectroscopy; H X-ray fluorescence spectroscopy  (cf. DQ 5.10).
126.96.36.199 Typical Results
These are shown in Figure 5.28 .
Comment on the results obtained in this study (see Figure 5.28).
It can be seen that for both soil samples the XRF results obtained produce similar concentration values to those obtained after acid digestion using the US EPA Method 3050B, followed by FAAS. The use of XRF spectroscopy, which analyses the soil directly, provides faster analysis than that achieved when using the acid digestion/FAAS procedure.
5.8.3 Example 5.3: Sequential Metal Analysis of Soils, followed by Flame Atomic Absorption Spectroscopy
188.8.131.52 Sequential Extraction Conditions
In each case, an accurately weighed 1.0000 g sample was used. The sequential extraction method has been described in Figures 5.21-5.23.
Comments Samples and sample blanks were filtered, cooled and made up to 100 ml with distilled water. Further dilutions were made, as appropriate. All glassware was soaked overnight in 10% nitric acid and then rinsed at least three times with distilled water.
184.108.40.206 Analysis by Flame Atomic Absorption Spectroscopy
The standards and samples were analysed by using a Perkin Elmer 100 FAAS system with an air-acetylene flame. The metals were determined at the following wavelengths: Cu, 324.8 nm; Fe, 248.3 nm; Pb, 217.0 nm; Mn, 279.5 nm; Ni, 232.0 nm; Zn, 213.9 nm. Calibrations were produced by serial dilution of
1000 ^gml-1 stock solutions in the range 0-10 ^gml-1. All calibration graphs exhibited linear relationships for each metal, except zinc, i.e. y = mx + c. For zinc, a curved relationship was obtained, i.e. y = ax2 + bx + c. All correlation coefficients were >0.96.
220.127.116.11 Typical Results
These are shown in Figure 5.29 .
Comment on the results obtained in this study (see Figure 5.29).
It can be seen that the two soils behaved differently to the sequential extraction process. On average (for both soils) ca. 50% of the metal content was in the residual fraction, i.e. unlikely to be released. The majority of iron was always in the residual fraction (54% for ‘Soil A’ and 92% for the ‘CONTEST’ soil).
The metals most available for plant uptake (Step 1 - see Figure 5.20 above) are as follows:
? D) 8 ?
104 ? 103 ? 100 10
1 j—“i r
Cu Fe Pb Mn Ni Zn Metal
106 -104 -100
1 I I r Cu Fe PbMn Ni Zn Metal (b)
.St' S .S’
106 ? 104 ? 100 1
Cu Fe PbMn Ni Zn Metal
.St' S .S’ g g>
106 -104 ? 100
Cu Fe PbMn Ni Zn Metal (d)
Figure 5.29 Sequential extraction results obtained for (a) ‘Soil A’, and (b) ‘Soil CONTEST 32.3A’: ?, Step 1; ?, Step 2; S, Step 3; ?, residual fraction. Comparisons of the sequential extraction/flame atomic absorption spectroscopy (FAAS) total metal analysis and acid digestion/FAAS approaches for (c) ‘Soil A’, and (d) ‘Soil CONTEST 32.3A’: ?, sequential total; H, FAAS total  (cf. DQ 5.11).
Methods for Environmental Trace Analysis
• Soil A: 72% Mn; 18% Pb; 10% Zn; 2% Fe (0% for Cu and Ni).
• Soil CONTEST 32.3A: 23% Mn; 4% Pb; 38% Zn and 0% Fe (2% for Cu and 28% for Ni).
This information then needs to be related to the risk associated with the ‘available’ metals.
A variety of methods for the determination of total metals and metal species/fractions have been described. While the most common method of sample decomposition of environmental samples is probably acid digestion, other approaches, most notably the use of fusion, have some specialist application areas, e.g. geological samples. The area of metal speciation is an ever expanding one, with new approaches being developed and evolving on a regular basis. The major limitation to this area of activity is the development of robust and reliable approaches to analysis. Some selected examples have been provided to assist the reader to evaluate some of the procedures that have already been developed. However, after reading this chapter you will have quickly realized that the methods are limited to only a few metal species.
1. United States Environmental Protection Agency, ‘Acid digestion of sediments, sludges and soils’, EPA Method 350B, National Technical Information Service, Springfield, VA, 1996.