Reaction of water at hot filament interferes with measurements of dissolved gases by membrane inlet mass spectrometry

Abstract

The measurement by membrane inlet mass spectrometry of gases dissolved in water is hampered by high and unstable background signals caused by residual gases whose origins have not been thoroughly investigated. We propose that significant amounts of gases are created by chemical reactions at the hot filament in the electron impact ion source. By the help of isotopically labelled water we show that water entering the mass spectrometer through the membrane disintegrates or reacts with impurities to form hydrogen, oxygen and carbon dioxide. The partial pressures of hydrogen, oxygen and carbon dioxide respond to changes in the water activity of the sample with half times of about 1, 2 and 5 min, respectively. Possible ways to limit the formation of gases from water in the membrane inlet mass spectrometer are discussed. © 1997 John Wiley & Sons, Ltd.