Scanning Microscopy


Four laboratories co-operated to test repeatibility and reproducibility of the semi-quantitive Coal Mineral Analysis (CMA) method. CMA is an instrumented image analysis method which identifies mineral particles in coal by chemical composition (energy dispersive X-ray spectroscopy) and size (scanning electron microscopy). The repeatability of weight percent data was better than 0.2 relative standard deviation for most minerals constituting more than five percent of all coal minerals. The type of mineral had no effect on repeatability. Errors arising from counting statistics were shown to be the major source of bias at a given instrument setting.

Inter-laboratory data for the major minerals agreed to within 0.1 relative standard deviation in about 50% of the cases. For other major minerals the relative standard deviation from the inter-laboratory average varied between 0.1 and 0.3. The weight percentages of kaolinite and "mixed silicates" showed poorer reproducibility than those of quartz and illite. Differences in detector window thickness may have affected discrimination between light elements and, therefore, inter-laboratory agreement of clay minerals data. Means to compensate window thickness effects are suggested.

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