A compact resonance ionization mass spectrometer is presented. It is presen
tly applied for sensitive and highly selective ultratrace determination of
the long-lived radioisotope Ca-41 for environmental, biological, and fundam
ental investigations. The development of coherent multistep resonance ioniz
ation enables the realization of experimental detection limits as low as 10
(6) atoms per sample and very high isotopic selectivity above 10(12).