A coral 129I/127I measurement method using ICP-MS and AMS with carrier addition

Abstract

Iodine-129 in coral cores provides historical records of human nuclear activities, establishes or confirms coral age models, and traces environmental processes. At the time of this writing, the only published method (Biddulph et al., 2006) for measuring ¹²⁹I/^127(stable)I in corals required 10–30 g of sample, which can be too large for common practice. To reduce this sample size requirement, we modified the existing method in two aspects: (1) we used inductively coupled plasma mass spectrometry (ICP-MS) instead of an iodide selective electrode (ISE) for stable ¹²⁷I measurement; and (2) we added 0.66 mg of a low-ratio (¹²⁹I/¹²⁷I = 1.5 × 10⁻¹⁴) iodine carrier instead of using a carrier-free method for target preparation for ¹²⁹I measurement by accelerator mass spectrometry. For ¹²⁷I measurement, we obtained the detection limit and limit of quantitation of 0.089 and 0.27 ppb, respectively, using a ¹³³Cs internal standard, a 286× dilution factor, and drift correction. The average ¹²⁷I concentration in corals was 4.5 ppm. For ¹²⁹I/¹²⁷I measurement, we determined the average anthropogenic (post-1950) and natural (pre-1950) ratios of 20 × 10⁻¹² and 1.4 × 10⁻¹², respectively, in our coral samples. In addition, the average relative standard deviations were 11.5% and 23% for anthropogenic- and natural-age coral samples, respectively. Our method successfully measured ¹²⁹I/¹²⁷I in 1–4 g of corals with accuracy and precision comparable to the existing method, which required 10–30 g of corals.