Isobar Attenuation Using Anion-Gas Reactions for Accelerator Mass Spectrometry and Application to 36Cl

Preliminary tests of a prototype radio-frequency quadrupole (RFQ) collision cell system, known as an isobar separator for anions (ISA), for the removal of isobaric interferences for accelerator mass spectrometry (AMS) and for studies of anion-gas interactions are reported. The ISA decelerated a mass-analysed beam of anions from an energy (similar to 20 keV) typically generated by an AMS ion source to < 10 eV. RFQs and electrostatic lenses then guided the ions through the collision cell where ion-gas collisions reduced both the energy and energy spread of the ion beam (cooled the ions) and ion-gas reactions attenuated most of the unwanted isobars. The anions were then re-accelerated to their original energy for injection into the rest of the AMS system. With the ISA installed on a full 3 MV AMS system, attenuations of S-32(-), C-12(3)- and K-39(-) by six, seven and greater than ten orders of magnitude, respectively were achieved using 0.7-1 Pa NO2 gas in the collision cell, while maintaining approximately 10-30% of the chlorine anion transmission. A further measurement of a Cl-36/Cl = 4.1 x 10(-11) RM is also described. The results suggest that the Cl-36/Cl lower detection limit of the current system was 10(-14)-10(-15) for samples that could be prepared with S/Cl ratios below 10 mu g g(-1).