Model 400 Nitric Oxide Monitor™
The Model 400 Nitric Oxide Monitor™ is designed for the measurement of atmospheric nitric oxide (NO) in the range of 0-2000 ppbv (0-2 ppmv). In combination with the Model 401 NO2 Converter, NOx and NO2 (by difference) are also measured.
Note: Although we still support the Model 400 NO Monitor, this instrument has been replaced by the Model 410 NO Monitor. The Model 410 has a factor of two better precision and better baseline stabiity.
Comparison of Models 400 and 410 NO Monitors
The NOzone™ technology employed by the Model 400 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 400 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 Nitric Oxide Monitor. Here we trace the gas flow through the instrument beginning at the Air Inlet near the bottom of the diagram.
A miniature air pump, Air Pump #1, draws sample air into the instrument at a flow rate of 0.8-1.2 L/min. The gas sample then passes through a miniature solenoid valve which alternately directs the flow through a NO scrubber or bypass and then into an overflow tee which vents part of the air sample. A fraction of the sample air is drawn from the overflow tee by Air Pump #2, which pumps the scrubbed or unscrubbed sample air through a Nafion tube and into the reference absorption cell. The Nafion tube equilibrates the humidity of the sample air with that inside the instrument case, assuring that the humidity is the same in scrubbed and unscrubbed air samples. In the Reference Cell, UV light intensity, Iref, is measured at 254 nm using a low pressure mercury lamp and photodiode. The purpose of the reference cell is to correct for ambient ozone and fluctuations in the lamp intensity. Needle Valve #1 is used to adjust the flow through the reference cell in the range 550 to 650 cc/min. Following Air Pump #2, a small flow of ozonized air is added to the sample air stream. The flow rate of ozone/air is regulated by Needle Valve #2, and is set to produce an ozone concentration in the range of 3-5 ppm (3,000-5,000 ppb). This flow constitutes typically 3-5% of the total flow rate.
The sample air containing added ozone next passes through a coiled reactor, which provides adequate reaction time (typically 3.5-4.5 s) for nearly all NO in the sample to react with ozone via reaction 3 above. Correction is made in the software for lack of complete reaction. That correction is typically less than 3%. The sample air, now depleted in ozone by the amount of NO originally present in the air sample, next passes through the Detection Cell where UV light intensity is again measured. From there the air stream passes through an Ozone Scrubber where O3 is catalytically converted to O2, through a Mass Flow Meter where the flow rate is measured, and to the instrument case.
Ozone is produced by pumping ambient air through a Pyrex tube containing a low pressure mercury lamp. The lamp has a fused silica window that passes highly energetic atomic emission near 185 nm in addition to the resonant emission at 254-nm. The wavelengths near 185-nm are absorbed by molecular oxygen (O2) to produce oxygen atoms (O). Those oxygen atoms rapidly attach to O2 via a termolecular reaction to form O3:
O2 + photon → 2 O (2)
O + O2 + M → O3 + M (3)
Here, the photon has a wavelength near 185 nm, and M is any molecule, principally N2, O2, Ar and H2O in air. The molecule M catalyzes the combination of O and O2 by removing excess relative translational energy. The flow rate of ozone/air and thus the ozone concentration is controlled by needle valve #2. Flow rates and the actual ozone concentration produced are measured in a cycle at the beginning of an analysis by modulating solenoid valve #2. This valve either allows the ozone/air mixture to enter the stream of gas being analyzed or diverts it to an ozone scrubber.
NO is measured by modulating solenoid valve #1. A low-pressure mercury lamp is located on one side of the absorption cells, and photodiodes are located on the opposite side. The photodiodes have built-in interference filters centered on 254 nm, the principal wavelength of light emitted by the mercury lamp. Shorter wavelengths of light that could produce ozone are absorbed by the low-grade quartz envelope of the lamp itself and by the window of the detection cell, which passes 254-nm but not 185-nm light. The state of Solenoid Valves #1 is switched every 10 seconds in order to measure light intensities for sample air, Iref and I, and scrubbed sample air, Iref,o and Io. The values of Iref, I, Iref,o, and Io are used to calculate the concentration of NO in the original air sample. In addition to a small correction for incomplete reaction, a correction is made for the small amount of dilution of the NO concentration by the added ozonized air.