On-line I−/Te− separation for the AMS analysis of 125I

The isobar separator for anions (ISA) was used together with a 3 MV tandem accelerator mass spectrometer (AMS) to demonstrate the real time (on-line) separation of Te− from I−. Following the ion source mass spectrometry and major retardation to tens of eV, the ISA uses a radiofrequency quadrupole (RFQ) ion guide to confine and direct I− and associated Te− isobar anions through a gas-reaction cell, where chemical reactions occur at eV energies with the electronegative gas NO2. Anions are subsequently reaccelerated out of the ISA to near original ion source extraction energies for AMS analysis. At 5 mTorr NO2 in the ISA gas-reaction cell, 125Te− was observed to be attenuated by a factor of ∼107 as compared to 127I− that did not experience significant (<50%) losses. A comparative test using 37Cl− and 32S− (having similar chemical properties to iodine and tellurium) showed a 32S− attenuation of >107 relative to 37Cl− under the same ISA-AMS conditions. The preferential destruction of Te− (and S−) at eV energies in the ISA is likely due to a larger favorable destruction cross-section with NO2. This study is the first demonstration of I-Te anion separation for AMS, and makes possible the use of 125I, free of the contaminant 125Te isobar after suitable sample purification, for future 129I/125I carrier-free analyses of natural samples at ultra-low trace levels.