Determination of lanthanides and of U and Th in geological samples

Abstract

A review is presented on analytical techniques which are frequently used to determine the lanthanides and U and Th in geological samples. Instrumental neutron activation analysis followed by high-resolution gamma-ray spectrometry using conventional coaxial and planar germanium detectors is applicable to most practical problems. Radiochemical neutron activation analysis allows the analysis of very small samples, or samples containing the lanthanides at the sub-ppm to ppb level; the latter elements are usually separated from the remainder as a group. Various types of mass spectrometry, combined or not with isotope dilution techniques (e.g., thermal ionization, spark source and inductively coupled plasma mass spectrometry) will be discussed with regard to sensitivity, accuracy, interferences and cost- effectiveness. Of the microbeam methods for the in situ determination of the lanthanides in individual mineral crystals, secondary ion mass spectrometry (ion microprobe mass analysis) is presently the only method with sufficient sensitivity. Delayed neutron counting is a suitable method for determining U and Th in large series of bulk samples; the distribution of these elements can be studied using the fission track technique.