AMS offers the only ultrasensitive technique for 41Ca biological tracing studies. 41Ca AMS method using CaH2 target had been set up and used for biological problems with the HI-13 tandem at China Institute of Atomic Energy (CIAE). Recently, we have developed the AMS method using the easier-to-prepare CaF2 as target material to fulfill the requirement of large number of biological samples measurement. 41Ca labeled biological samples were digested with 4:1 (V/V) HNO3 and H2O2 mixtures. One ml of 5 mg/ml CaCl2 solution was spiked before digestion while the Ca content of the sample was lower than 1 mg. Then the solutions were chemically separated and purified and finally prepared to CaF2 targets by a procedure similar to that reported by Freeman etc. In the cation exchange step, 15 ml of 0.8 M HNO3 were adopted as eluant of K+, which was more effective than 0.08 M HNO3 for eluting adsorbed K+ and without influencing the quantitative recovery of Ca2+. Several milligrams of CaF2 mixed with about 1.5 times weight of silver powder were pressed into aluminum cathodes for subsequent AMS measurement with the HI-13 tandem. CaF3- negative ion beam with intensity in a range of 40-100 nA was extracted from the MC-SNICS ion resource, the terminal voltage was set at 8.50 MV and 7+ charge state of particles was chosen after carbon foil stripping for 41Ca ion analysis. The total transferring efficiency was only about 0.3%. 41Ca ions were finally recorded by the multi-anode gas ionization chamber filled to 140 mbar with P10. The results showed that 41Ca and the isobar, 41K were clearly identified with the &DeltaE1/&DeltaE2 two-dimensional spectra, the ratios of 41K counted in the spectra to the 40Ca recorded by the AMS Faraday Cup were no more than 10-12. The measured absolute values of 41Ca/40Ca of 4 standard samples prepared gravimetrically in our laboratory using neutron activated 41CaO and natural CaCO3 (41Ca/40Ca range 1.785x10-8 – 1.750x10-10) were typically only 0.6 of the theoretical values, probably because of incomplete detection of 41Ca by the ionization chamber. However, the normalized measured values of these samples were in good agreement with the nominal values. The 41Ca/40Ca blanks of the biological samples were estimated below 8.2x10-13.
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