Adsorption in gas mass spectrometry. I. Effects on the measurement of individual isotopic species

Abstract

The adsorption-desorption process of gas molecules on the walls of the mass spectrometer inlet system was studied in order to assess quantitatively its influence on measurement results. The effects on individual isotopic species in SiF 4 measurements required for the re-determination of the Avogadro constant are discussed in this paper, while the effects on isotope amount ratio determinations will be discussed in a companion paper. A model based on the Langmuir adsorption isotherm is developed, which fits well the experimental observations and provides the means to investigate adsorption and desorption kinetics in the inlet system. A parameter called the ‘apparent leak-rate coefficient’ is introduced; this represents the relative variation with time of any isotopic species in the inlet system. All the adsorption parameters appearing in the balance equations are derived from the apparent leak-rate coefficient. Application of the model to long mass-spectrometric measurements of SiF 4 yields a rate constant of 6.5 × 10 −5 s −1 for SiF 4 effusion through the molecular leak of the inlet system. Adsorption and desorption rate-constants are equal to 20–25% of the leak rate-constant, and the adsorption sites are about two orders of magnitude lower than the number of Ni and Cu atoms present on the inlet system walls.