Countercurrent extraction of sparingly soluble gases for membrane introduction mass spectrometry

Membrane introduction mass spectrometry has been applied to inert gas measurements in blood and tissue, but gases with low blood solubility are associated with reduced sensitivity. Countercurrent extraction of inert gases from a blood sample into a water carrier phase has the potential to extract most of the gas sample while avoiding dependence of signal on blood solubility. We present the design of a membrane countercurrent exchange (CCE) device coupled with a conventional direct insertion membrane probe to measure partial pressure of low solubility inert gases in aqueous samples. A mathematical model of steady-state membrane CCB predicts that countercurrent extraction with appropriate selection of carrier and sample flow rates can provide a mass spectrometer signal nearly independent of variations in solubility over a specified range, while retaining a linear response to changes in gas partial pressure over several orders of magnitude. Experimental data are presented for sulfur hexafluoride and krypton in water samples. Optimal performance is dependent on adequate equilibration between the sample and carrier streams, and the large resistance to diffusion in the aqueous phase for insoluble gases presents a substantial challenge to the application of this principle.