EFFECT OF SAMPLE PRESSURE ON MEMBRANE INLET MASS-SPECTROMETRY

The effect of sample pressures up to 6 bar on the signal of a membrane inlet mass spectrometer was studied with supported and unsupported me mbranes of silicone rubber. In both cases we found undesirable pressur e effects. With a supported membrane increasing sample pressure causes a reduction of the signal. This is because lateral diffusion of analy te in the interstitial space between the membrane and the membrane sup port contributes to the transport of analyte to the mass spectrometer, and the space available for this diffusion is increasingly compressed by increasing sample pressure. With an unsupported membrane increasin g sample pressure causes an increasing signal, which is caused by the fact that an unsupported membrane bulges increasingly with increasing sample pressure. The consequently increased area and decreased thickne ss of the membrane results in increased flux of analyte. Bulging depen ds strongly on the diameter of the unsuppported membrane area and on t he thickness of the membrane. With a 0.185 mm thick membrane spanning a circular hole of 1 mm diameter the bulging is minimal and there is n o significant aberration due to sample pressure up to 2 bar. With a la rger diameter, a larger pressure or a thinner membrane, significant ab errations occur including change of relative selectivity to polar and unpolar analytes and hysteresis when the pressure is changed up and do wn. These effects are due to non-linear elastic deformation of the mem brane. The effects described should be taken into account in the desig n of membrane probes for systems where pressure changes may occur.