Aqueous measurement of nitric oxide using membrane inlet membrane inlet mass spectrometry

Membrane inlet mass spectrometry for the measurement of nitric oxide in aqueous solution provides a direct, continuous, and quantitative determination over long periods of time. The method uses a membrane that is permeable to nitric oxide and separates solution or cell suspension from a partial vacuum leading to the ionization source of a mass spectrometer. The construction of the probe varies depending on use; this report describes an inlet probe comprising a 1.0 cm segment of silicon rubber tubing attached to the vacuum inlet of the mass spectrometer. The probe is immersed in solution or suspension and in the system described here has a response time of 5-7 s and a lower detection limit of 0.5 nM nitric oxide. This apparatus was used to measure the generation of nitric oxide in solutions of NONOates and from the reactions of nitrite with hemoglobin. The usefulness of such an inlet in measuring nitric oxide in physiological systems is discussed.