Determination of Ultralow Level 129I/127I in Natural Samples by Separation of Microgram Carrier Free Iodine and Accelerator Mass Spectrometry Detection

Separation of carrier free iodine from low iodine level samples and accurate measurement of ultralow level I-129 in micrograms of iodine target are essential but a bottleneck in geological dating of terrestrial system and tracer research using naturally produced I-129. In this work, we present a carrier free method using coprecipitation of AgI with AgCl for preparing micrograms of iodine target, associated with combustion using a tube furnace for separating iodine from solid samples and anion exchange chromatography for preconcentrating iodine from a large volume of water. An accelerator mass spectrometry was used to measure ultralow level I-129 in micrograms of iodine target. The recovery of iodine in the entire separation procedure is higher than 80% and 65% for solid and water samples, respectively. One microgram iodine in the target (AgI-AgCl) can produce a stable I-127 signal for AMS measurement of I-129/I-127, and a detection limit of this method for I-129 is calculated to be 10(5) atoms. This will allow us to accurately determine I-129 in prenuclear geological samples of low iodine concentration with I-129/I-127 of 10(-12), such as loess, soil, coral, rock, sediment, and groundwater. Some samples with low iodine content have been successfully analyzed, and the lowest value of the I-129/I-127 ratio of 2 x 10(-11) was observed in 23.5 and 63.5 m loess samples collected in the Loess Plateau, China. The developed method sheds light on a wide application in earth science.