Calcium homeostasis is maintained by interrelated regulatory processes, including skeletal bone building and resorption. Radioactive calcium has been used to study calcium dynamics in adults. Similar studies are not possible with pregnant women, children and at-risk new borns because radioactivity may deposit in the growing skeleton. Studies with stable calcium isotope tracers and thermal ionization mass spectrometry are limited by instrumental sensitivity and contamination from sample processing. Thus, determinations are not accessible for small serum samples, as in premature infants, nor for small numbers of calcium atoms in biological units approaching the cellular domain. Advances in laser ionization mass spectrometry using pulsed lasers have permitted the determination of ultra trace metals in biological matrices with minimal sample preparation. Dramatically improved sensitivities may be achieved using two-photon spectroscopy in concert with auto-ionizing energy levels in the calcium atom. Using this quantum mechanical manipulation, greatly enhanced photon absorption cross-sections for excitation and ionization permit efficient ionization with continuous wave lasers, thereby overcoming measurement limitations with low repetition rate pulsed lasers. The specific aims are: l) investigate autoionization schemes for calcium, 2) Evaluate alternative atomization approaches for calcium, 3) Evaluate overall ionization efficiency, 4) Determine the isotope ratios in a pilot set of serum samples, 5) Project characteristics for a prototype system based on the results of aims l-4.