Adsorption in gas mass spectrometry II. Effects on the measurement of isotope amount ratios

Gas adsorption in the mass spectrometer inlet system (equipped with molecular leak) causes a deviation from linearity of the measured InRi/l data versus time, which affects the extrapolation to time t = 0 required to obtain the isotope amount ratios of a sample. The model first presented in (R. Gonfiantini, S. Valkiers, P.D.P. Taylor, P. De Bievre, Adsorption in gas mass spectrometry. I. Effects on the measurement of individual isotopic species, Int. J. Mass Spectrom. Ion Proc. (1997) in press) is further developed in order to show the effects of adsorption on the data obtained in long measurements of SiF4 samples and improve the extrapolation of InRi/l values to t = 0. The model enables to estimate the isotope fractionation factors for the processes of gas effusion through the molecular leak, and for adsorption and desorption on the inlet system walls. The fractionation factors obtained for gas effusion and adsorption are close to the expected value of (M-l/M-i)(1/2), where M-l and M-i are the molar masses of the isotopic species considered. The isotope fractionation factor for desorption, which can be evaluated only indirectly from the data fitting, indicates that the heavy isotopes are preferentially retained in the adsorbate. Model extrapolation of data obtained during the first 10 hours of measurement gives values of R-i/l(0) which are slightly but significantly smaller than those given by linear extrapolation of the first two hour data. The difference is, in relative terms, -(1.88 +/- 0.23) x 10(-4) for the Si-29/Si-28 ratio and -(3.25 +/- 0.47) x 10(-4) for the Si-30/Si-28 ratio. The resulting molar mass for silicon is (1.0 +/- 0.1) x 10(-6) parts lower than that obtained with the isotope amount ratios of the linear extrapolation. This effect is not negligible but it cancels by using synthetic isotope mixtures for the measurement calibration. The model does not match the data after 15 h of measurement. This may imply that more than one adsorption mode occurs. Effects related to molecule fragmentation and the space charge distribution in the ion source are discussed. (C) 1997 Elsevier Science B.V.