The separation of 40Ca+ ions from 40Ar+ ions, at eV energy by NH3 vapour in a radio-frequency quadrupole (RFQ) reaction cell has been reported. The 40Ar+ ions were attenuated by ~108 leaving the rare 40Ca+ clearly visible. This remarkable result had implications for AMS and so a survey was made of many other ionic reactions. A reaction, which separates 81Kr+ and 81Br+ ions in C2H2, will be mentioned briefly as an illustration. However, anions are needed for AMS analysis and although it is possible to convert cations to anions or fast neutral atoms for AMS, starting with anions would be preferable. The anions used in AMS are in general; however, more fragile than cations and manipulation in a RFQ reaction cell is consequently much more difficult. Fortunately Cl- has the largest binding energy of the negative atomic ions and it was known in 1972 that Cl- and S- could also be separated in NO2 at eV ion energies. Here it should be noted that the eV deep potential well in an RFQ combined with a small electric field gradient along the RFQ can be used to suppress the effects of multiple scattering that are a problem for isobar separation at keV energies. The ions in an RFQ at eV energy are also cooled by their collisions and can be transported with a significant energy spread combined with good mass selection. A negative ion source has been designed that incorporates the suppression of mass separated 36S- and is being constructed for 36Cl- analysis at IsoTrace. The negative ions of the super-halide and in general the super-anion molecules have even larger electron affinities than Cl- and open the way to a possibly general way of isobar separation with or without AMS.
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