129I has been routinely measured for many years at low-levels using charge-state Q = +5 and at 2MV terminal voltage because the ratio M/Q is then not an integer. However, even though the ratios for charge states +4 and +7 are also not integers strong interference of ions with mass m and charge q, associated with Mq - mQ = n = +/-1, is observed due to the inadequate electric analysis available at present. Significantly higher efficiencies for 129I would be possible with charge-state +3, but then the molecular fragments 86Sr+2 and 43Ca+1 would be expected to enter the detector with the full intensity, together with 129I+3, as in this case n = 0. In addition the possibility of the coincidence add up of the fragments from the anions CaSr- and Ca3- would then also be a problem. These fragments differ from the 129I+3 only by their mass defects. However, based on a recent actual performance assessment with charge-state +3, a background of 129I/127I ~ 3x10-14 was obtained. It now appears that as long as the prepared AMS samples are of reasonable chemical purity, the intensities of these molecular fragments can be expected to remain quite modest, unlike the situation for charge state +4. This is apparently because these fragile molecular anions have low electron affinities and binding energies. Thus, if equipped with an ionization detector using an appropriately thin window, and with both low and high-energy system having adequate mass and energy resolutions, low-level 129I samples can be measured more efficiently and more cost effectively at low MV terminal voltages, using charge-state +3. The background level for +3 is similar to that for the +5 case. Fortunately no triply charged molecular ion is abundant enough to be a problem and such a molecule would be in any case recognised by the lower total pulse height.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)