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.