Download (direct link):
Figure 5.24 The gastro-intestinal extraction procedure used to investigate the speciation of metals from ingested foodstuffs .
acid-soluble levels being generally higher than those at neutral pH; the processing or preparation of food of similar origin may alter the solubility (not shown in Table 5.12).
An alternative approach for determining the bioaccessibility of metals from foodstuffs (including soil) has been developed. This approach, termed the physiologically based extraction test (PBET) [29, 30], has been applied to study the dietary levels of metals in Uganda, Africa . It is postulated that endomyocardial fibrosis, a common cardiac disease in Africa, may be related to elevated levels of dietary cerium, and deficient levels of dietary magnesium. In Uganda, variation in dietary exposure to both Ce and Mg is probable due to variations in soil geochemistry and dietary habits, e.g. geophagia, the deliberate consumption of soil. In this instance, the soil was from termite nest soils and traditional
Table 5.12 Some selected results for the solubilization of trace metals in foodstuffs obtained by using an enzymolysis procedure  Foodstuff Metal
Cd Zn Fe Cu
Total Soluble portion Total Soluble portion Total Soluble portion Total Soluble portion of fresh (%)“ of fresh (%)a of fresh (%)“ of fresh (%)
weight weight weight weight
(mg kg^1) (mg kg-1) (mg kg-1) (mg kg-1)
Gb G + F G + F* G“ G + F G + F1 G“ G + I6 G + F G“ G + I6 G + F
Pig kidney 0.12 70 110 130 28 90 20 90 60 10 45 50 6.3 35 65 55
Crabmeat 1.26 120 50 120 83 90 25 95 64 25 5 45 21.0 75 80 85
100% beef — — — — 65 100 70 90 33 40 70 60 0.9 0 65 45
Wholemeal 0.04 100 50 50 20 105 0 95 29 30 35 55 2.9 65 90 55
Spinach 0.16 30 10 50 10 55 40 45 18 65 50 65 1.8 45 20 35
Tomato — — — — 2 90 100 75 6 60 80 55 0.5 80 55 50
^Levels quoted to nearest 10% (Cd) and nearest 5% (Zn, Fe, Cu).
* Gastric enzymes only, at pH 2.5.
^Gastric and intestinal enzymes, at pH 7.2-7.4.
dGastric and intestinal enzymes, after subsequent adjustment of pH to 2.5.
Methods for Environmental Trace Analysis
herb-soil remedies. The details of this physiologically based extraction test are shown in Figure 5.25. Samples were analysed by using either inductively coupled plasma-atomic emission spectroscopy (ICP-AES) or inductively coupled plasma-mass spectrometry (ICP-MS) at Stages 1-3 (inclusive).
Figure 5.25 The physiologically based extraction test (PBET) used to determine the bioaccessibility of metals from foodstuffs .
Methods for Environmental Trace Analysis
5.8 Case Studies on Total and Selective Methods of Metal Analysis
5.8.1 Example 5.1: Total Metal Analysis of Soil, followed by Flame Atomic Absorption Spectroscopy
220.127.116.11 Digestion Conditions
The US EPA Method 3050B and its optional method (see Figures 5.2 and 5.3, respectively, above) were used, plus an aqua regia digest method (Figure 5.26). In each case, an accurately weighed 1.0000 g sample was used.
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.
18.104.22.168 Analysis by Flame Atomic Absorption Spectroscopy
The standards and samples (n = 6) 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 Rgml-1 stock solutions in the range 0-10 Rgml-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.
22.214.171.124 Typical Results
These are shown in Figure 5.27 .
Figure 5.26 Procedure for the acid digestion of soils using aqua regia.
Cu Fe Pb Mn Ni Zn Metal
Figure 5.27 Typical results obtained for the total metal analysis of soil using flame atomic absorption spectroscopy. Digestion procedures: ?, aqua regia; S, US EPA Method 3050B; ?, US EPA Method 3050B (optional)  (cf. DQ 5.9).
Comment on the results obtained in this study (see Figure 5.27). Answer
It can be seen that all three digestion procedures have produced similar totals for each metal being analysed. Therefore, it can probably be concluded that on the basis of these results all three digestion procedures could be used for future studies.
5.8.2 Example 5.2: Total Metal Analysis of Soil Using X-Ray Fluorescence Spectroscopy - Comparison with Acid Digestion (Method 3050B), followed by Flame Atomic Absorption Spectroscopy
126.96.36.199 Digestion Conditions
No digestion procedures are necessary when using this analytical method.