Analyses by Inductively Coupled Plasma Mass Spectrometry of Ba Concentrations in Water and Rock Samples. Comparison between Isotope Dilution and External Calibration with or without Internal Standard

Barium concentrations in various standards and natural samples are determined by inductively coupled plasma mass spectrometry (ICP-MS) using isotope dilution and using external calibration with or without internal standard. First, the measured Ba isotopic ratios confirm the existence of a mass bias, which is shown to follow a linear law and which is the same in unknown natural samples (rocks, river waters, rain waters and sea waters) and in standards (river water, rock samples and synthetic solutions) both in matrix-rich samples as well as in diluted samples. As it is not matrix dependent and it is stable on a daily time-scale, approximate correction can be made. Second, Ba concentrations obtained with isotope dilution and with external calibration with an internal standard show excellent agreement (within ± 2%) for geostandards as well as for unknown natural samples. This result implies that indium, as an internal standard for Ba analysis, is very effective for the correction of matrix effects, whatever the matrix and signal variations. Concentrations calculated by isotope dilution and by simple external calibration are, however, in agreement to within ± 6%, which indicates that matrix effects on Ba are small.