A NEW MEMBRANE INLET FOR ONLINE MONITORING OF DISSOLVED, VOLATILE ORGANIC-COMPOUNDS WITH MASS-SPECTROMETRY

A conventional membrane inlet for the mass spectrometric monitoring of gases has a considerable length of connection tube between the membrane and the ion source. This design is unsuitable for the measurement of organic compounds as they absorb strongly to the walls of the connection tube, giving rise to long response times. In this paper, we describe a new inlet design, where the membrane is mounted close to the ion source. The initial transients in the response to various alcohols were measured and compared with the transients found with a conventional inlet. Faster response times were achieved with the new inlet. The transients were found to be composed of two components, one from the membrane and one from adsorption/desorption processes on the vacuum walls in the connection tube. Good agreement was found between the measured data and a simple model of adsorption/desorption processes in a vacuum system. © 1990.