Identification of 129I interferences in accelerator mass spectrometry

The identification of interferences to ¹²⁹Iⁿ⁺ (n = 3–6) was conducted with a 5 MV tandem accelerator mass spectrometer at SUERC. In addition to ¹²⁸Te, the detectable interferences to 12 MeV ¹²⁹I³⁺ ions include ⁴³Ca¹⁺ and ⁸⁶Sr²⁺ which are likely injected as ⁴³Ca⁸⁶Sr⁻, ⁴³Ca³²S¹⁸O₃⁻, etc. whereas those to 15 MeV ¹²⁹I⁴⁺ ions include ³²S¹⁺ injected as ⁴³Ca³²S¹⁸O₃⁻, ⁶⁵Cu²⁺ as ⁶³Cu⁶⁵Cu¹H⁻, ⁹⁷Mo³⁺ as ⁹⁷Mo¹⁶O₂⁻. The 18 MeV ¹²⁹I⁵⁺ ions might be interfered by ⁷⁷Se³⁺ and ¹⁰⁴Pd⁴⁺ (or ¹⁰⁴Ru⁴⁺) and the 21 MeV ¹²⁹I⁶⁺ ions by ⁴³Ca²⁺, ⁶⁵Cu³⁺, ⁸⁶Sr⁴⁺ and ¹⁰⁸Pd⁵⁺ (or ¹⁰⁷Ag⁵⁺, ¹⁰⁸Cd⁵⁺). Apart from ¹²⁸Te, these ions can be fully separated from ¹²⁹I in the gas ionization detector equipped with a thin SiN membrane window and propane gas, which contributes negligibly to the ¹²⁹I-AMS detection for samples after careful chemical preparation. Although the ¹²⁹I background is mainly controlled by Te content and the source memory, the rareness of Te in natural environmental samples results in our routine background ¹²⁹I/¹²⁷I level in range of ∼10⁻¹⁴. The precision and accuracy of 3% can be routinely achieved at ¹²⁹I/¹²⁷I > 10⁻¹² level at SUERC.