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DIGESTION
Analytical packages under this heading are appropriate for sample types containing silicates and carbonates, such as rock, till, sediment, concrete, cement, porcelain etc. Small amounts of organic material are permissable in the samples.
Loss on ignition (LOI) is determined by ignition at 1000°C of part of the sample. The resulting weight reduction, LOI, usually reflects the content of organic material. However, some inorganic components may be driven off during ignition (such as carbon dioxide from carbonates) and contribute to the LOI.
After drying at 105°C, the sample is fused in a carbon crucible with a flux (lithium metaborate) at 1000°C. The "bead" which forms on cooling is then dissolved in dilute acid.
MAIN ELEMENTS
Elements that usually constitute the main components of rock (silicon, aluminium, calcium etc.) occur in such samples together with oxygen in specific proportions. Formally, this is expressed as the sample consisting of oxides of the different elements (SiO2, Al2O3 etc.). In typical silicate rock, the total of these main elements in oxide form plus the LOI should equal 100% (tolerance 1.5%). This verifies that a total analysis has been obtained.
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TRACE ELEMENTS
Trace elements (the concentrations of which are not expressed in oxide form) usually make a negligible contribution to the total content. Some of these are found in specific minerals which may be more or less poorly soluble. Most minerals in rock are effectively dissolved by the method we use. For example, monitoring over an extended period using certified reference materials shows good results (deviation less than 10%) for zirconium, a "difficult" element in this context.
The appropriateness of fusion for trace elements is also illustrated by the fact that results are obtained for rare earth elements (REE). The unique properties of these chemically similar elements make them useful for geological interpretations concerning formation conditions for geological materials.
CHOICE OF ANALYTICAL PACKAGE
Some elements (e.g. As, Cd and Pb) cannot be analysed by fusion since they are volatile and therefore lost in the fusion process. Another limitation applies to samples with a high sulphur content (>0.5%), which appears to induce loss of copper and zinc. The MG-1 package in the "Environment" section, in which fusion is supplemented by another technique, may be an alternative in such instances. However, sulfide minerals should not be fused but digested using a different method. Please discuss your needs with the laboratory. |
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