Calibration of the Ion Microprobe for the Quantitative Determination of Strontium, Iron, Manganese, and Magnesium in Carbonate Minerals

The necessary operating conditions have been determined for the measurement of trace concentrations of Mg, Mn, Fe, and Sr In calcite and dolomite using a Cameca 3f Ion microprobe with the energy filtering method. With this technique, the presence of Interferences can be ascertained by examining the deviation of the measured Isotopic ratios of an element from their natural values as a function of the kinetic energy acquired during the sputtering process. With the energy filtering method It Is possible, using an energy offset of −50 V, to reduce Interferences from other species at masses 24, 54, and 88 for Mg, Fe, and Sr. Although Mn has only one stable Isotope, It Is believed that a similar voltage offset may be suitable for this element. By use of these conditions, calibration lines have been established for Mg, Fe, Mn, and Sr employing 11 calclte and dolomite standards. It Is estimated that for carbonate materials, concentrations of Mn, Mg, and Sr can be quantitatively measured as low as 1 ppm and Fe as low as 10 ppm. No significant differences in calibration were observed between calcite and dolomite. The slopes and intercepts of the relationships were found to remain constant within error over periods of 3 years.