Progress in 41Ca ultratrace determination by diode-laser-based RIMS

We report on progress in development and application of Ca-41 ultratrace determination by diode-laser-based RIMS. Applications include biomedical isotope-tracer studies of human calcium kinetics, cosmochemical investigations of meteorites, environmental dosimetry and radiodating. Depending on the application, 41Ca needs to be determined at isotopic abundance in the range of 10(-9) to 10(-15) relative to the major stable isotope Ca-40. We use either a double- or triple-resonance excitation scheme and subsequent non-resonant photoionization of calcium atoms in a collimated atomic beam. All resonant steps are excited with narrow bandwidth extended cavity diode lasers, non-resonant photo-ionization is attained with either an argon ion laser or a CO2 laser. The resulting photo-ions are detected with a quadrupole mass spectrometer. With double-resonance excitation, the optical isotopic selectivity for Ca-41 against Ca-40 is 2x10(4), while the triple-resonance scheme provides optical selectivity of more than 10(9). By adding the third resonant step, overall detection efficiency increases from 1x10(-6) to 5x10(-5) and the detection limit for relative 41Ca abundance improves from 5x10(-10) to 2x10(-13). Both schemes have been applied to various sample types and accuracy and reproducibility of the resulting Ca-41/Ca-40 isotope ratios have been determined to be better than 5%.