Effect of collisional damping in the dynamic reaction cell on the precision of isotope ratio measurements

It is shown that collisions broaden the residence time of ions in the Dynam ic Reaction Cell (DRC). Short-term fluctuations of the ion signal are dampe d which effectively damps plasma noise on the millisecond time scale. This in turn provides an improvement in the precision of isotope ratio measureme nts. At least a factor of 2 of improvement in internal precision is demonst rated for 40 ppb Ag, with Ag-109/Ag-107 internal precision of 0.023% which compares to the statistical limit of 0.012%. Internal precision of an 11-mi nute measurement of 40 ppb Pb is demonstrated at 0.05% for Pb-208/Pb-206, 0 .057% for Pb-207/Pb-206, and 0.14% for Pb-204/Pb-206. Corresponding values of the external precision measured for nine samples are 0.032%, 0.027% and 0.06%, respectively.