Calcium Isotope Effects in Atomic Absorption

Abstract

Measurements are reported for the absorption of the 422.7 nm line of a natural-abundance calcium hollow cathode lamp by solutions of ⁴⁰Ca, ⁴⁴Ca, and ⁴⁸Ca at 0.5 to 3.0 ppm levels in 0.06 M HCl (aqueous) when aspirated into air-acetylene and nitrous oxide-acetylene flames. In the case of the air-acetylene flame the relative absorptions per mole of calcium in solution were 1.00, 1.11, and 1.08 for the 40, 44, and 48 isotopes, respectively. With the nitrous oxide-acetylene flame similar experiments gave ratios of 1.00, 1.08, and 1.06 in the same order. These results may be rationalized with a model that considers the simultaneous effects of isotope shifting, Lorentz broadening and shifting, and Doppler broadening of the calcium resonance line in the flame. With the air-acetylene flame, solutions that also contained 5000 ppm of Sr as SrCl₂ gave relative absorbances for the 40, 44, and 48 calcium isotopes of 1.00, 1.04, and 1.10. Flame-atomic-distribution studies lead to the conclusion that in the presence of 5000 ppm of Sr, calcium kinetic isotope effects are also important.