Determination of Low Level 129I in Soil Samples Using Coprecipitation Separation of Carrier Free Iodine and Accelerator Mass Spectrometry Measurement

The accurate determination of 129I in samples at ultralow level is critical and essential for the application of natural 129I in geological dating and environmental tracer studies. In this study, iodine is first separated from soil through combustion at high temperature; the released iodine is collected in an alkali trap solution. AgI-AgCl coprecipitation is used to separate carrier-free iodine from the trap solution and to prepare 129I target. 129I is then measured using a 3.0-MV accelerator mass spectrometry (AMS) in the Xi'an Accelerator Mass Spectrometry Center. The recovery of iodine is higher than 95%, during combustion. An iodine recovery of about 75%–85% is obtained through coprecipitation separation method, and the total recovery of iodine is above 70%. The developed method has been successfully used to determine 129I in soil samples with low iodine content. An atomic ratio of 129I/127I as low as 10−11 in deep soil has been determined. With this method, the ratio of 129I/127I down to 10−12 can be determined in solid samples with low level of iodine.