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Models were developed for the adsorption and desorption processes of the isotopic gas molecules. Which take place in the mass spectrometer inlet system. These models show that memory effects persist for a long time. In particular, the measurement of a SIF4 sample with isotopic abundance ratios 28Si:29Si: 30Si approximately equal 1:1:1, carried out four months after measurements of SiF4 samples with natural isotope abundances, showed a memory of natural silicon of about 0.42%. Subsequent measurements on samples with natural isotopic composition showed that the memory contribution was gradually reduced.
As the model fitting was unsatisfactory, the hypothesis was tested of anotherdesorption mechanism, in which the stripping of the adsorbed gas molecules from the walls is catalyzed by the strikes of free gas molecules. This desorption mechanism follows a second order reaction path, and coexists and competes with that following a first order reaction path. The model, modified according to this hypothesis, produces a good fit of the experimental data. Memory in the ion source and the flight tube seems to be much less important than that in the inlet system.
The measurements reported provide a better insight in memory effects. The adsorption model enables in principle to correct for memory contribution, and improve the strategy for removing or reducing memory effects. .

Additional information

Authors: VALKIERS S, Institute for Reference Materials and Measurements, JRC Geel (BE);TAYLOR P D P, Institute for Reference Materials and Measurements, JRC Geel (BE);DE BIEVRE P, Institute for Reference Materials and Measurements, JRC Geel (BE);GONFIANTINI R, Istituto di Geocronologica e Geochimica Isotopica del CNR, Piza (IT)
Bibliographic Reference: Article: Adsorption in gas mass spectrometry - III Memory effects, 1998
Record Number: 199910446 / Last updated on: 1999-03-19
Original language: en
Available languages: en