Membrane mass spectrometer inlet for quantitation of nitric oxide

Nitric oxide (NO) is an important physiological and biochemical messenger that may be involved in endogenous carcinogenesis and cell toxicity via formation of N-nitroso compounds or direct DNA damage by nitrosating agents arising from the reaction of NO with O2. To study the reaction of NO with O2 in model systems and the formation and disappearance of NO in more physiological systems such as cell cultures, we adapted and optimized a membrane mass spectrometer inlet specifically for such analyses. The inlet consisted of Silastic tubing inserted into a Swagelok 'tee', which was attached to the mass spectrometer via the tuning probe. Kinetics of NO disappearance can be followed under electron impact conditions until NO2 interferes via the formation of NO+ during fragmentation of NO2+. The aqueous NO concentration for minimum detection was determined to be 1.4 microM. The inlet response time to step changes in aqueous NO concentrations was 7.0 s, fast enough to permit real-time measurements of aqueous NO changes upon addition of O2. Finally, the depletion of aqueous NO was observed in the presence of O2. The relative steady state responses of inlets designed for gas or aqueous samples, and their relative response times, are explained by an analysis based on mass transfer theory.