Cathode-Region Method for the Direct Spectrochemical Determination of 50 Trace Elements in Rock Phosphate

Abstract

A direct spectrochemical method for 50 trace elements in rock phosphate was developed, exposing only the cathode region (0.5 and 0.8 mm below the upper electrode) instead of the whole dc arc plasma. In the 0.5-mm region 31 elements were analyzed, while the 0.8-mm region was used for Sc, Y, U, Th, Zr, and the rare earths. The axial gradients of the dc arc, burning in air under atmospheric pressure, were studied in order to find the suitable matrix for the trace element standards. The behavior of the temperature, voltage, electron density, line intensity, and particle concentration indicate calcium metaphosphate as suitable matrix for rock phosphate. When 20% by weight graphite was added to both, standards and samples, the detection limits of the traces was in the range of 0.1–10 ppm. An average precision of 20% was calculated for concentrations up to 400 ppm.