Mass spectrometric estimation of gas permeation coefficients for thin polymer membranes

We have developed a new method to estimate the permeation coefficient of gases through polymer membranes. A fixed volume of gas is put in a gas cell and introduced into ultrahigh vacuum. After positioning the gas cell to face a quadrupole mass spectrometer, the partial pressure of the gases are measured as function of time. In a simple model, the partial pressure as function of time obeys an exponential law. A formula is derived to calculate the permeation coefficient with the time constant of the partial pressure decay and geometric parameters of the gas cell as input. Using these parameters the method gives absolute permeation values without calibration. If the time constant is difficult to establish (this may happen for membranes with a low permeation coefficient), the permeation coefficient can be estimated by extrapolating the partial pressure to t=0. The method can be used to study the permeation behavior of individual components of gas mixtures. The sample size can be about two orders of magnitude smaller than usually used in conventional permeation measurements. The method is illustrated with oriented polypropylene and polyethylene terephtalate membranes of different thickness. The estimated permeation coefficients are in reasonable agreement with values obtained from a control experiment using a gas chromatograph and with values from the literature.