A retardation device situated after the final collector slit of a tandem magnetic mass spectrometer has improved the abundance sensitivity measured on the low mass side of a Cs⁺ peak from 1· × 10⁶ to 4 × 10⁹. On the high mass side the abundance sensitivity of approximately 10⁹ is not affected significantly by the retardation technique. The abundance sensitivity extrapolated to the uranium mass region is improved from 5 × 10⁵ to 10⁹ on the low mass side and is 10⁸ on the high mass side.