At my lab we test animal feed. We test for Ca, Na and Mn on the AA. For most digests we use the MARS with nitric. To digest the CaCO3 we use 1:1 solution of HCL. Our other Ca results are where they belong and our proficiency sample is sometimes a little high, but our CaCO3 should be at 41% conc and is anywhere from 32-35%. We don't know where the loss of Ca is happening. We are using lanthanum as our diluent.