Mathematics – Logic
Scientific paper
Apr 1969
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969gecoa..33..431n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 33, Issue 4, pp.431-453
Mathematics
Logic
180
Scientific paper
For accurate X-ray spectrographic analysis of geological samples of widely varying composition, a fusion with lithium borate containing lanthanum oxide is used to make a suitable glass disc. Mg, Al, Si, P, K, Ca, Ti, Mn and Fe are determined on this disc, using similar discs made with pure chemicals for calibration. "Nominal percentage" composition of the sample is obtained as a direct reading from the scaler, counting time adjusted to give 10 4 or 10 3 counts for 1 per cent. For normal silicates this value will be within ±3 per cent (relative) of the correct value. For more variable samples and for greater accuracy with silicates, matrix corrections are made using the "nominal percentage" in a first order equation. The matrix correction coefficients have been determined experimentally and are shown to agree well with theoretical values. They are presented in a form suitable for use in other laboratories. For Na, a separate pressed powder sample is prepared and matrix corrections are not necessary. The precision of the method has been studied and is largely controlled by counting errors. An estimate of the accuracy has been made by comparison of X-ray results with two sets of published data on five "standard" rock samples; observed differences could all be explained as due to precision errors in the X-ray and chemical results. The coefficient of variation between the X-ray and chemical results was less than 1 per cent where the concentration of an element, expressed as an oxide, was greater than 1 per cent. Results are given to show that when matrix corrections are applied, samples with extreme compositional variability, e.g. rocks, minerals or ores, may be analysed satisfactorily. With the borate discs, background determinations are not made on each sample so that determinations involve reading the K line only. Rapid analyses are therefore possible; one operator can prepare and analyse more than 12 samples per day for 10 elements.
Hutton J. T.
Norrish K.
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