Quantitative Detection of Nitrogen Dioxide in Nitrogen Using Laser Magnetic Resonance at 1616 cm-I S.
M. Freund’
Optical Physics Division, Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234
D. M. Sweger
and J. C. Travis
Analytical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
The selective, quantitative detection of nitrogen dioxide in part-per-million concentrations in nitrogen using magnetic field modulation of the molecular absorption of 1616 cm-’ carbon monoxide laser radlation is reported. A magnetic fleld of less than 500 gauss is sufficient to shift the Infrared transition of interest into coincidence with the laser. The Zeeman-modulated absorption of the laser light is synchronously detected, and is found to vary linearly with NOn concentration over the two orders of magnitude investlgated. The synchronous detection scheme dlscrimlnates against nonparamagnetic species, such as water and ethylene, which could interfere wlth direct absorptlon measurements.
We report the quantitative detection of nitrogen dioxide in part-per-million by volume (ppmv) concentrations in nitrogen using magnetic field modulation of the molecular absorption of 1616 cm-l carbon monoxide laser radiation. The method relies on the near coincidence between the lower frequency spin component of the 616 615 transition of the v 3 band in NO2 (1)and the P(11) transition of the 20 19 band of the CO laser ( V N O ~ uco GZ 0.01 cm-l) (2).It is an obvious extension of the recent laser magnetic resonance investigation of this molecule in the 1600 cm-l and 120 cm-l regions of the infrared ( 3 , 4 ) ,and of similar detection schemes demonstrated for nitric oxide (5, 6). The small spin splitting for the upper state and the factor of 4 greater splitting, approximately 0.04 cm-l, of the lower state produces a large difference in Zeeman effect for the two levels. This permits sufficient alteration of the transition wavelength by small magnetic fields to sweep a Zeeman component of the infrared transition of interest in and out of coincidence with the laser. Consequently, a modulated absorption of the laser light can be observed. The sample to be analyzed is subjected simultaneously to a high frequency magnetic field varying sinusoidally and a slowly varying square-wave magnetic field. The sinusoidal field modulates the absorption which is then amplified and synchronously detected. The 0.03 Hz square-wave magnetic field permits alternate baseline and maximum modulated absorption signal measurements to be made. The difference is proportional to the NO2 concentration. With an extra-cavity absorption cell, we have observed the linear dependence of modulated signal for NO2 concentrations ranging from 1.5 ppmv to 170 ppmv. The method allows the possibility of sequential analyses for mixtures of NO and NO2 using the same apparatus and with no coupling of the measurements ( U N O - UNO^ > 250 cm-l). Further, since both species are paramagnetic and only the Zeeman modulated absorption is detected, substances such as water and ethylene which interfere with direct ab-
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Present address, Los Alamos Scientific Laboratory, P.O. Box 1663, Mail Stop 565, Los Alamos, N.M. 87545. 1944
sorption measurements no longer present a problem. The method bears a strong resemblance to a recent modulated absorption scheme for the quantitative detection of vinyl chloride (7) in which the electric field analogue of the Zeeman effect, the Stark effect, was employed.
EXPERIMENTAL The apparatus is shown schematically in Figure 1. An 18-cm long continuous flow absorption cell, sealed with polished BaFz windows a t Brewster’s angle is situated inside a 19-cm long solenoid. Since it is difficult to simultaneously impress a high frequency modulation and a dc offset field onto a single coil without interference between power sources, the solenoid consists of two concentric coils. One coil, with a measured inductance of 141 mH, is made to be resonant a t 3 kHz by the addition of series capacitance, and is driven by a 300-watt audio power amplifier. Such an arrangement is sufficient to produce a modulation field of 150 gauss peak-to-peak. A 0.03-Hz, zero-based square-wave produces a 0-200 gauss square-wave “tuning field” on the second, 52-mH coil. Carbon monoxide laser light passes axially through the cell and is detected by either a PbSnTe or a HgCdTe detector, the former giving better signal-to-noise ratio and greater freedom from magnetic field pickup. The laser is described elsewhere ( 3 ) .In the present apparatus, however, the laser is locked to the peak of its gain profile in the conventional manner. The laser power entering the cell was approximately 0.5 watts. Using NBS SRM 1629 NO2 permeation tubes (8),the concentration of nitrogen dioxide in nitrogen can be adjusted in a flowing system to any value in the range 1-200 ppmv by setting the nitrogen flow rate. The concentration range was limited a t the high end by available flowmeters and lengthy equilibration times. Nitrogen of an assumed purity of 99.999 mol % was used for all of the measurements. T h e flowmeters were calibrated with nitrogen by volumetric expansion to maintain an overall system accuracy of approximately 5%. Measurements were made from low to high concentrations to avoid effects of NO2 adsorption on the walls. Mixing was achieved in a chamber which contained the permeation tube, upstream from the absorption cell. The pressure in the cell was maintained a t 27 Torr (1 Torr = 133.3 Pa). The experimental parameters were optimized a t the very beginning of our experiments on a high concentration sample a t reduced pressure. A slowly varying ramp of 0-300 gauss was applied to the “tuning coil” of the solenoid to scan the modulated absorption response. A peak a t -200 gauss was found to be the lowest field where a maximum in modulated signal occurred reflecting the region of maximum slope of the absorption profile. The much smaller modulated absorption a t zero field, on the wing of the absorption profile, was taken as the baseline, or reference measurement. The difference between the two was used as the analytical signal. The peak-to-peak modulation field, a t the 200 gauss tuning field, was varied to maximize the difference between peak and baseline. The optimum value of 150 gauss should correspond to a frequency modulation of about one third of the full width a t half maximum of the Zeeman component ( 5 ) . Finally, the pressure was increased until deterioration of the absorption cross-section by collisional broadening balanced the increase in signal from increased NO2 density, a t about 27 Torr. The output of the lock-in amplifier was recorded in one of two ways. For the first method, the analog output was recorded on an X-Y recorder (3-s lock-in time constant). The recording took 4-5 min and several cycles of the square-wave were averaged in obtaining the readings. The second method utilized voltage-to-frequency conversion of the lock-in output, the resulting frequency being counted by a
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
M i x i n g Chamber
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Figure 1. Apparatus
multichannel analyzer multiscaling at 1 s per channel, allowing digital averaging of the results. A 30-ms lock-in time constant assured that the averaging time was determined by the channel advance clock. The two methods gave identical results. The square-wave shape (Figure 2) resulted from the low frequency switching of the tuning field from 0 to 200 gauss. The difference in the two signal levels, used as the measure of concentration, was found to be insensitive to small drifts in the square-wave amplitude. All measurements were normalized for changes in laser power and variations in the temperature of the permeation tube. A reference beam was split from the main laser beam, mechanically chopped, and synchronously detected to provide a continuous monitor of laser power. The temperature of the permeation tube was measured to f O . l "C with a liquid-in-glass thermometer situated in the mixing chamber. Nitrogen dioxide permeation tubes change their permeation rate by about +lo% per "C and the data were appropriately corrected (8). During the course of the measurements, three permeation tubes with different rates were employed as a check for systematic errors.
RESULTS Table I lists the results from experiments run with three different NO2 permeation tubes. These values are compiled for NO2 concentrations ranging from 0.8 to 16 ppmv from sets of data resembling those of Figure 2 from which the values of the difference between signal and reference were determined. The slopes as derived from the least-squares fits to the data agree to within the expected measurement accuracy of the system. Figure 3a shows a plot of NO2 concentrations ranging from 1.5 to 2 1 ppmv. The data points fall on a straight line passing through the origin. The test for linearity of the system has been extended to 170 ppmv and the result is shown in Figure 3b. The linear dependence of the detected modulated signal on the NO2 concentration at these low concentrations,
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Figure 2. Multichannel analyzer recordings of lock-in amplifier output
at several concentrations and the fact that the line goes through the origin, allows calibration of the apparatus by measuring a single reference point. A detailed investigation of the lower concentration (
