ADSORPTION IN GAS MASS-SPECTROMETRY .1. EFFECTS ON THE MEASUREMENT OFINDIVIDUAL ISOTOPIC-SPECIES

The adsorption-desorption process of gas molecules on the walls of the mass spectrometer inlet system was studied in order to assess quantit atively its influence on measurement results. The effects on individua l isotopic species in SiF4 measurements required for the re-determinat ion of the Avogadro constant are discussed in this paper, while the ef fects on isotope amount ratio determinations will be discussed in a co mpanion paper. A model based on the Langmuir adsorption isotherm is de veloped, which fits well the experimental observations and provides th e means to investigate adsorption and desorption kinetics in the inlet system. A parameter called the 'apparent leak-rate coefficient' is in troduced; this represents the relative variation with time of any isot opic species in the inlet system. All the adsorption parameters appear ing in the balance equations are derived from the apparent leak-rate c oefficient. Application of the model to long mass-spectrometric measur ements of SiF4 yields a rate constant of 6.5 x 10(-5) s(-1) for SiF4 e ffusion through the molecular leak of the inlet system. Adsorption and desorption rate-constants are equal to 20-25% of the leak rate-consta nt, and the adsorption sites are about two orders of magnitude lower t han the number of Ni and Cu atoms present on the inlet system walls. ( C) 1997 Elsevier Science B.V.