Fluorescence detection of nitric oxide in nitrogen

An accurate, rapid, and simple technique is described for the measurement of nitric oxide (NO) in standard reference mixtures of NO in N/sub 2/. It is based on the fluorescence intensity emitted by NO when it absorbs 213.8 nm radiation from a Zn discharge lamp. This intensity increases linearly with NO concentration in the 0.015-7 ppM range to within 1 percent error. The application of this method to detect NO and sulfur dioxide (SO/sub 2/) in automotive exhausts is discussed. By the method described, fluorescence signals from several NO-in-air mixtures were measured. The responses were almost linear with concentration. Small amounts of SO/sub 2/ interfere with the detection, but this may be minimized by using a filter transmitting in the region of 230-300 nm, corresponding to the NO fluorescence spectrum. Because of large concentrations of quenching gases, mainly H/sub 2/O and carbon dioxide, in automobile exhausts, the fluorescence signal of NO would be considerably less than the signal from the same concentration of NO present in pure N/sub 2/. An equation is given to calculate this effect when the composition changes within a stated range. Diluting the exhaust gas 1:100 with N/sub 2/ provides an almost linear calibration curve for measuring NO in the range of 30-3,000 ppM. If the exhaust gas is diluted by air or O/sub 2/, and a filter transmitting light of wave lengths 300-420 nm is used, it is possible to measure SO/sub 2/.