Precision and accuracy of chemical analysis of silicate rocks

Abstract

Replicate analyses by 35 laboratories of a granite and a diabase, especially prepared as standards, give means and standard deviations which should be invaluable for calibration purposes in spectrochemical and chemical laboratories. It is clearly shown that relative error varies inversely with concentration of the constituent (e.g. 0.1% for SiO 2, 10% for P 2O 5). A test of accuracy has also been made by 11 laboratories, each analyzing a synthetic glass of the proportions found in granite. The composition of the glass is known within +-0.02% for SiO 2, +-0.01% for Al 2O 3 and is without significant error for the remaining constituents.The means computed from the chemical analyses showed satisfactory agreement for MgO, CaO, Na 2O, K 2O (<0.1%). For SiO 2 and Al 2O 3 a reciprocal systematic error appeared whereby SiO 2 was 0.4% low and Al 2O 3 was 0.6% high; total SiO 2 and Al 2O 3 agreed almost exactly (<0.1%) with the glass standard. As a result of these tests attention is directed anew to the practical but complex problem of evaluating error in a single analysis made by conventional methods. The advantages for many purposes of the more rapid photometric and spectrochemical methods and (for many igneous rocks) of micrometric methods are outlined.