Ultratrace analysis of calcium with high isotopic selectivity by diode laser resonance ionization mass spectrometry

Abstract

A refined diode laser based resonance ionization mass spectrometer for highly selective ultratrace analysis is presented, which combines coherent narrow-band multistep resonance excitation and ionization with a compact quadruple mass spectrometer. The widespread analytical potential and applicability of this system is demonstrated in the determination of calcium trace and ultratrace isotopes for cosmochemical studies, medical investigations and radiodating. For a detailed theoretical understanding of the coherent high resolution multistep excitation process a description in the density matrix formalism was worked out. For calcium optical isotopic selectivities of more than 1010 and efficiencies of up to 5 percent are predicted, which compare well to the analytical requirements for 41Ca-determination, which reach a maximum isotopic selectivity of more than 1015. Experimentally different ionization schemes, including single-, double- and triple- resonance excitation of calcium, have been investigated. The first-step excitation at 422,7 nm requires frequency doubling of a diode laser, while second and third steps are directly excited with extended cavity diode lasers. Analytical measurements cover meteorite and blood samples and demonstrate the feasibility of the predicted specifications. Continuing work will focus towards the application of the full triple-resonance scheme for ultra low-level measurements of 41Ca and shall establish resonance ionization mass spectrometry as a competitive technology to accelerator mass spectrometry.