Nitric Oxide Analysis

Nitric oxide (NO) has been administered therapeutically in concentrations of 1 to 100 parts per million (ppm) as a pulmonary vasodilator. Continuous monitoring of the inhaled concentration can be accomplished using relatively inexpensive but specific and stable electrochemical detectors.^{[164] [165]} There has been interest in monitoring endogenously produced NO in exhaled gas.^[166] Analysis of exhaled NO waveforms, in which end-tidal NO concentrations are typically on the order of 1 to 100 parts per billion (ppb), can be satisfactorily performed only by using mass spectrometry or rapid-response chemiluminescence analyzers,^[167] which typically have a resolution of 0.1 ppb or less. Gaseous chemiluminescence analyzers usually rely on the chemical reaction of NO with ozone (O₃ ) to produce nitrogen dioxide (NO₂ ). Some NO₂ molecules are produced in an excited state (NO₂ *).^[168] When NO₂ * then spontaneously reverts to its ground state, a photon is emitted. This emitted chemiluminescence signal, which has a wavelength at maximum intensity at approximately 1200 nm, can be detected using a photomultiplier.

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Figure 36-13 Paramagnetic oxygen analyzer. Two sealed spheres filled with nitrogen are suspended in a magnetic field. Nitrogen (N₂ ) is slightly diamagnetic, and the resting position of the beam is such that the spheres are displaced away from the strongest portion of the field. If the surrounding gas contains oxygen, the spheres are pushed further out of the field by the relatively paramagnetic oxygen. The magnitude of the torque is related to the paramagnetism of the gas mixture and is proportional to the partial pressure of oxygen (PO₂ ). Movement of the dumbbell is detected by photocells, and a feedback current is applied to the coil encircling the spheres, returning the dumbbell to the zero position. The restoring current and output voltage are proportional to the PO₂ . (Courtesy of Servomex Co., Norwood, MA.)