The Model 410 Nitric Oxide Monitor™ is designed for the measurement of atmospheric nitric oxide (NO) in the concentrtion range 0-2,000 ppb (0-2 ppm) with a precison of ±1.5 ppb. When used in combination with the Model 401 NO2 Converter, NOx and NO2 (by difference) are also measured. The detection principle of the Model 410 is based on the selective reaction of NO with ozone. The resulting ozone depletion is measured using the absolute method of UV absorbance and thus requires only infrequent calibration. By comparison, chemiluminescence NOx instruments require nearly continuous calibration using a standard gas.
Theory of Operation
The NOzone™ technology employed by the Model 410 Nitric Oxide Monitor™ is based on the quantitative reaction of nitric oxide (NO) with ozone (O3):
NO + O3 → NO2 + O2 + light (1)
This reaction has long been used as a gas phase titration for the measurement of either NO or O3 in laboratory kinetics experiments, and the reaction is stoichiometric; i.e., one O3 molecule is consumed for every NO molecule oxidized to NO2 in the reaction. In the Model 410 Nitric Oxide Monitor™, a small concentration of ozone (~4 ppm) is added to the gas sample stream and the resulting decrease in concentration of ozone is measured by the absolute method of UV absorption. By providing adequate time for the reaction to go to completion, the decrease in ozone concentration is equal to the original concentration of NO in the gas stream.
Reaction 1 also is used in conventional chemiluminescence analyzers. Instead of measuring the change in ozone concentration, chemiluminescence detects the small amount of light produced in the reaction. That light is emitted by electronically excited NO2 molecules formed in reaction 1. Chemiluminescence instruments are highly sensitive and have a very fast response time, but require frequent calibration using a gas standard.
Figure 1 is a schematic diagram of the Model 410 Nitric Oxide Monitor. Here we trace the gas flow through the instrument beginning at the Air Inlet near the bottom of the diagram