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We describe chemical separation and mass spectrometric procedures for the measurement of radium isotopes in geologic samples. These methods provide 226Ra/228Ra ratio measurements for 1 g or less of rock sample containing subpicogram amounts of radium with precision better than 1.5% (95% confidence level). Radium-226 concentrations were measured by isotope dilution for smaller sample sizes (100-500 mg) containing as little as 1-10 fg of total 226Ra with similar high precision.

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