Determination of the aqua regia soluble content of rare earth elements in fertilizer, animal fodder phosphate and manure samples using inductively coupled plasma mass spectrometry

Abstract

The aqua regia soluble content of all rare earth elements (REE) was determined in manure, animal fodder phosphate and mineral fertilizer samples using a quadrupole-based inductively coupled plasma (ICP) mass spectrometer. In order to obtain accurate results, spectral overlap due to oxide formation had to be taken into account. The influence of the instrumental operation conditions on the oxide ratio was investigated, permitting systematic optimization. In order to further decrease the amount of oxide formation, the aerosol was desolvated by cooling the spray chamber to below 0°C. To ascertain the accuracy of the results obtained, two reference materials, USGS AGV-1 Andesite and USGS GSP-1 Granodiorite, were used as quality control samples. The REE data obtained for these materials show a good agreement with the corresponding reference values (within 10% for all REE in AGV-1, and for most of the REE in GSP-1). All REE results obtained were also “Chondrite-normalized” and the smoothness of the curve obtained was used as an additional indication for accuracy. The precision expressed as relative standard deviation for 3 independent measurements was observed to be better than 3%. Detection limits (3s-criterion) vary between 2 and 11 ng/l except for Sc, for which the detection limit is approximately 100 ng/l due to the occurrence of a spectral interference. Hence, when the Sc concentration was <1 μg/l, a magnetic sector ICP-mass spectrometer was used for its determination. The REE patterns of fertilizer and manure were found to be different and REE concentrations in the latter were lower. REE concentrations can therefore be used as an indication of the addition of fertilizer to manure, a practice which is speculated to be applied by some farmers to circumvent European regulations on manure disposal.