Mass discrimination in dynamic reaction cell (DRC)-ICP-mass spectrometry

The possibility of overcoming spectral interferences by means of selective ion-molecule chemistry in a dynamic reaction cell (DRC) leads to an extension of the application range of ICP-MS for isotopic analysis. In this work, the effect of various instrumental parameters and the matrix composition on the mass discrimination (defined as the deviation between the experimental result and the corresponding 'true' isotopic ratio) was systematically studied. It is demonstrated that various DRC parameters-collision gas flow rate, reaction gas flow rate and bandpass settings-affect the mass discrimination. This observation is probably to be attributed to in-cell fractionation effects, occurring as a result of collisional losses, space-charge effects and kinetic effects in the ion-molecule chemistry. As the isotopic standard and the sample are always measured under identical conditions, external correction for mass discrimination assures accurate results. Under all conditions tested, internal correction for mass discrimination on the other hand-a strategy often used in Sr isotopic analysis did not provide accurate results. Also the matrix composition is demonstrated to have a significant influence on the mass discrimination, especially when using condensation product ions for isotope ratio measurement. This disadvantage can be overcome by using a matrix-matched isotopic standard for external correction or by isolation of the target element from the concomitant matrix, or at least its separation from the dominant matrix components