A mathematical study of sample modulation at a membrane inlet mass spectrometer-Potential application in analysis of mixtures.

An ANSI C program that simulates the diffusion profiles of sample modulation at a membrane inlet system has been developed to study the characteristics of modulated diffusion profiles. The program produces concentration profiles within the membrane and flux values at the exit side of the membrane as a function of time. Sample concentration on the inlet side can be switched between zero and an arbitrary value with a square or asymmetric cycle. Achievement of steady-state diffusion between alternations is not required. With this computer simulation, the flux profiles of analytes through a membrane inlet have been studied as a function of diffusion coefficient, modulation frequency, and concentration. The amplitude, shape, and time lag or phase angle of the flux profile are shown to be related directly to analyte concentration and diffusivity. A method that involves a set of linear equations is proposed to resolve mixtures of diffusing analytes based on differences in the time dependence of their flux profiles.