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Anal. Chem. 1980, 52, 1420-1424
(20) Milano, M.J., The Perkin-Elmer Corp., Main Ave., 124 Mail Station, Norwalk, Conn., personal communication, 1980.
Corporation. T h e computer system was acquired through grant number SER 77-06899 from the National Science Foundation. This work was presented in part as paper number 794 a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 14, 1980, Atlantic City, N.J.
RECEIVED for review February 15, 1980. Accepted May 15, 1980. This work was supported by a grant from the Research
Double Beam-in-Time Photoacoustic Spectrometer Michael F. Cox, Geoffrey N. Coleman," and Terry W. McCreary Department of Chemistry. University of Georgia, Athens, Georgia 30602
The system described uses a linear scanner to alternately illuminate sample and reference materials contained in a cell of unique design which employs a single microphone. Data acquisition requires only one preampllfler and one lock-In amplifier, keeping the cost of the system modest. The system operates in a ratio mode to permlt real-time compensation of wavelength variations In the source. However, overcompensation for the power spectrum of the source can occur which reduces the signal observed when the relative absorbances of the sample and reference greatly differ.
Interest in photoacoustic spectroscopy (PAS) has developed considerably in recent years principally because it is an excellent technique for studying the absorption spectra of solids a n d surfaces ( I ) . The signal arises due to the fact that when a molecule absorbs optical energy, it must lose an equivalent quantity of energy via photodecomposition, photoluminescence, or thermal relaxation. T h e last process is usually predominant; thus, if the energy lost is transferred to a gas which is in intimate contact with the sample, the gas will expand in direct proportion to the quantity of energy released. If the incident light flux is periodically interrupted, the energy released by the sample will also be periodic and, if contained in a cell having a constant volume, will give rise to pressure pulses. T h e magnitude of these pressure pulses (photoacoustic signal) is approximately given by ( 2 ) :
where PA = power of acoustic signal in watts, IOh= spectral irradiance of the source in watts cmd2nm-', o = monochromator slit width in cm, H = monochromator slit height in cm, T A = transmittance of the monochromator, s = spectral bandwidth in nm, Q = solid angle of sample illumination, t = molar absorptivity of analyte, c = concentration of analyte in sample matrix, 1 = thickness of absorbing layer, and p = conversion efficiency factor of radiationless transitions. I t is important to note t h a t the PAS signal is directly proportional t o both the incident photon flux (number photons/area/time) and the photon energy. T h e signal is also inversely related to the rate of modulation of the source radiation (2). For any continuum light source, the flux varies with time (drift and flicker noise ( 3 ) )as well as wavelength (as predicted by Planck's law of blackbody radiation (4)). Moreover, the energy of a photon varies according to the Planck relationship, E = hv = hc/X 0003-2700/80/0352-1420$01 .OO/O
where u is the frequency of the photon, c is the velocity of propagation of the photon in a particular medium, and h is the wavelength. In an analytical situation where reliable quantitative a n d qualitative information is desired, i t is necessary to minimize and/or account for all sources of variation in the measurement system. Compensation for wavelength variations and drift (slow changes in source flux) is achieved through the use of double beam systems where the sample signal is continuously compared to a reference measurement made a t the same wavelength. The effect of flicker noise is reduced by modulating the light source a t a frequency which is substantially greater than the flicker component and using frequency selective amplification ( 3 ) . At present, photoacoustic spectrometers are available commercially from Princeton Applied Research (PAR Model 6001), Rofin (Model OAS 400), and Gilford (Model R-1500). T h e first two are pseudo-double beam instruments which utilize pyroelectric detectors for real-time source compensation. In addition, the PAR system employs a microprocessor for digital storage of spectral information; thus references other than the pyroelectric detector may be compared but only in an off-line mode. T h e Gilford instrument is a double beamin-space design which has dual cells, microphones, preamplifiers, and lock-in amplifiers. Each design has its own advantages and limitations. For example, where pyroelectric detectors are used, only 5-10% of the total flux is deflected by a beamsplitter; thus more light impinges on the sample which yields greater sensitivity. However, the pyroelectric detectors do not exhibit a flat response over the spectral range of interest (UV-NIR), thus some additional compensation is necessary. T h e double beam-inspace configuration can provide true source compensation when carbon black is used as the reference and facilitates real-time compensation when other reference materials are employed. However, the double beam-in-space system is necessarily more complex than a single beam system and, because light must be split equally between the two cells, less flux is incident on the sample resulting in reduced sensitivity. A double beam-in-time photoacoustic spectrometer overcomes to a certain extent several of those disadvantages. By employing a rotating mirror which alternately illuminates sample and reference, the modulation and beamsplitting functions are combined. T h e resulting PAS signals from sample and reference differ in phase by 180'. If both sample and reference are contained within the same cell and detected by a single microphone, the use of synchronous amplification yields a signal which is due to a difference between them. Such a system will therefore permit real-time correction using references other than carbon black without a concurrent loss of sensitivity due to decreased illumination. Moreover, this C 2 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
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Figure 1. Schematic diagram of double beam-in-time photoacoustic spectrometer
m e 2. Diagram of optical train from monochromator to cell of double beam-in-time photoacoustic spectrometer
Table I. Commercial Components and Normal Operating Conditions of Double Beam-in-Time Photoacoustic Spectrometer source monochromator
optical modulator lenses microphone lock-in amplifier recorder
3 0 0 W Xe arc operated a t 20 A. Varian EIMAC VIX-3OOUV 0.25-m Ebert, f/3.5. Entrance aperture, 3.0 mm. Exit aperture, 1.5 mm. Scan rate, 25 nmimin. Jarrell-Ash 82-410 Galvanometer linear scanner driven by a square wave at 40 Hz. Bulova ALS-200 Fused silica, ESCO Electret, Realistic EM16 Sensitivity, 1-100 mV Time constant, 3 s Princeton Applied Research 186A Hewle t t-Packard 7 04 5A
system is a simpler arrangement which requires a single microphone, preamplifier, and lock-in amplifier. Consequently, analysis capabilities equivalent to those of commercial systems should be achieved at a substantially reduced cost.
EXPERIMENTAL Instrumentation. A schematic diagram of the instrument is shown in Figure 1. A list of commercial components and operating conditions is given in Table I. The output of the xenon arc lamp is focused through an IR filter to a 3.0-mm spot on the entrance aperture of the monochromator by a single fused silica lens (Ll). The IR filter consists of a cylindrical chamber capped with fused silica plates containing deionized, distilled water surrounded by a water jacket for cooling. Light of a selectable and narrow wavelength range (typically 5.0 nm) is imaged at the exit aperture of the monochromator. Because the cell is designed to hold liquid as well as solid samples, it is mounted vertically below the optical axis of the source-monochromator. In order to maintain the optical axes in the same plane, fused silica lenses L2 and L3 and the scanner mirror are mounted on a vertical steel optical bench by means of magnetic mounts (Capitol Tool and Supply, Los Angeles). The second lens (L2) transfers monochromatic radiation from the monochromator to the mirror which is attached to the shaft of the linear scanner. The oscillation of the mirror results in alternate illumination of the sample and reference which are contained within a single cell compartment. The composite acoustical signal is detected by the microphone, conditioned by the preamplifier and log amplifier (Analog Devices 759P), and selectively (in frequency and phase) amplified by the lock-in amplifier. The output is displayed on an X-Y recorder. The linear scanner is an analog device for which the angle of deflection of the shaft is proportional to the power input. Although it is resonant a t 200 Hz, frequencies of 10 to 500 Hz can be used. An external waveform generator provides a reference
Figure 3. Schematic diagram of celVpreamplifier housing of double beam-in-time photoacoustic spectrometer
square wave of a constant amplitude but of a variable frequency which is manually selectable. The function of the linear scanner is to move the monochromatic radiation, which is in fact an image of the source, from sample to reference and back, a distance of about 4 mm. The optical train from the monochromator to the cell is shown in Figure 2. The collimated source image which lies in the image plane of L2 must oscillate across 4 mm in the object plane of L3. To accomplish this, the scanner mirror must be moved slightly inside the focus of L2. The result is that an image of the source which is approximately 1.5 mm in diameter may oscillate between sample and reference with minimal distortion. A detailed view of the single cell compartment, which has separate holders for sample and reference, is given in Figure 3. The cell is constructed entirely of aluminum. The sample and reference holders are easily accessed at the sides of the cell. Each holder has a 4 X 8 mm depression which is 2 mm deep to contain sample and reference materials. Since a single detector is used in the double beam-in-time configuration, the acoustic paths from sample to microphone and from reference to microphone must be identical in length. This is mandatory in order to keep the sample and reference signals exactly 180' out of phase. Therefore, the channel leading t o the microphone is located between the sample and the reference holders which requires a distance of 2 mm between them. This produces an unavoidable increase in cell volume. However, as the total cell volume remains approximately 0.4 mL, the increase in internal dead volume is small in keeping with the criteria for negligible damping (5). The interiors of the sample holders are polished to reflect stray light. A right angle bend in the acoustic path was not deemed necessary, as the microphone diaphragm is highly reflective, precluding spurious signals from the microphone itself.
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
The cell is fitted with toggle valves for gas inlet and outlet, so that filler gases other than air may be used. The connections between the valves and the acoustic chamber are 0.020-inch holes drilled in the cell body. Excess volume inside the threaded portion of each valve is filled using a tightly fitting plastic plug having a 0.020-inch hole for passage of the filler gas. The volume of the valves themselves is relatively unimportant because of the small diameter of these passages. The joints between the cell body, quartz window, and cover plate are sealed with a thin layer of silicone rubber sealant. The joint between the microphone adapter and the cell body is sealed by a rubber gasket. The microphone is a condenser element from a low-cost, commercial unit. It is mounted in a Teflon insert which fits into the end of the preamplifier housing. The preamplifier circuitry is similar to that described by Eaton and Stuart (6) except that the output is capacitively-coupled to prevent dc offset. The preamplifier is contained within a brass cylinder and is attached directly to the cell. The proximity of the microphone and preamplifier and the use of the brass housing are necessary in order to minimize the pickup of electrical noise. The signal acquisition system consists of the preamplifier, a logarithmic amplifier, and a lock-in amplifier. The preamplifier gains the signal by a selectable factor of lo3 to lo6 prior to being conditioned by the logarithmic amplifier which has unity gain. The log of the signal is then conditioned by the lock-in amplifier. The lock-in unit amplifies only that portion of the small (0.1-10 mV) signal from the logarithmic amplifier which is in phase with the externally generated reference signal (which, in turn, is used to drive the linear scanner). O'Haver has shown that the lock-in is basically a switched inverter which has either a positive or negative gain depending on the phase angle (7,8). The resulting output is thus a dc level which corresponds to the difference between the signal at 0' phase angle and 180' phase angle. The signal is smoothed by an RC filter, the time constant of which is adjustable from 0.3 to 300 s. Since the level of the microphone signal oscillates from the sample signal (S) to a reference signal (R), the peak-to-peak amplitude of the input signal will be proportional to the difference, S - R. If the phase angle is adjusted so that the internal reference is in phase with R, the output will be proportional to R - S. By taking the log of the signal prior to lock-in conditioning, the output yields a ratio (log R/S) since the difference between two logs is the log of their ratio (e.g., log R - log S = log R/S). The instrument may also be operated in a differential mode by bypassing the log amplifier or in a single beam mode by simply masking either the sample or reference compartment. The lamp, lamp power supply, IR filter, and monochromator are mounted in a sturdy aluminum cabinet. The cell/preamplifier, linear scanner, and associated optics are contained in a separate aluminum/steel cabinet which is easily accessible, acoustically shielded, and optically aligned with the monochromator. The cell-preamplifer assembly is mounted magnetically to a steel plate which is placed on a bed of sand to aid in acoustical isolation. Both cabinets are placed on a 2-inch granite slab supported by rubber inner tubes to protect the system from spurious vibrations. Reagents. Carbon lampblack (Fisher) was used as a reference in most experiments. Holmium oxide and neodymium oxide (Alfa, Ventron Corp.) were used for characterization of the system. Magnesium oxide (Baker) was used to dilute samples when required owing to its highly reflective nature over the region of interest. All studies used approximately 5-mg samples of reagent materials.
RESULTS AND DISCUSSION T h e photoacoustic spectrometer can be operated in the single beam mode and in two double beam modes: differential (S-R) and ratio (S/R). Both double beam modes allow for correction of matrix effects by subtracting out absorption bands of the matrix or solvent which appear in both the sample and reference. Operating the system in a double beam ratio mode has the added advantage of correcting for flux variability since changes in the source flux do not affect the ratio of signal to reference. Consequently, all studies employed the double beam ratio mode.
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Figure 4. (A) Photoacoustic ratio spectrum of holmium oxide vs. 0.1 YO carbon black. (B) Same as (A) but with reference chamber blocked from illumination. (C) Same as (A) but with sample chamber blocked from illumination
Carbon black yielded a S/N ratio of -500 when the system was operated in the single beam mode under normal operating conditions with a modulation frequency of 40 Hz a t a wavelength of 550 nm. T h e signal was taken as the magnitude of the difference between the output with carbon black and the output with an empty cell. T h e noise was estimated as one fifth the average peak-to-peak variation in the signal measurement with carbon black. Under similar conditions Rosencwaig (9) obtained a S/N of -lo00 for a system of his o m design. Although direct comparisons are difficult because conditions between different investigations are not standardized, considering the inexpensive microphone employed in this study, the quality of design and construction of the photoacoustic cell appears to be more than satisfactory. The double beam, ratio mode spectrum of holmium oxide vs. 0.1% carbon black obtained under normal operating conditions is shown in Figure 4A. The same spectrum run under identical conditions with the reference channel blocked and again with the sample channel blocked yielded the single beam spectra shown in Figures 4B and 4C, respectively. The logarithm of the sample and reference signals, which are 180' out of phase, are taken prior to conditioning by t h e lock-in amplifier. Therefore, the double beam spectrum (Figure 4A) consists of the ratio of the single beam holmium oxide spectrum (Figure 4B) to the inverse of the single beam carbon black spectrum (Figure 4C). The power spectrum of the lamp is superimposed on both the reference and sample single beam spectra. Since the sample and reference signals have a 180' phase difference, the reference should negate the effect of the power spectrum on the sample spectrum. This assumes, however, t h a t the magnitude of the power spectrum superimposed on the sample and reference signals is equal; yet this depends on the individual absorbances of the sample and reference materials. If the reference greatly exceeds the sample absorbance, the magnitude of the power spectrum superimposed on the reference and sample signals will differ. Consequently, overcompensation by the reference will occur and the PAS ratio
ANALYTICAL CHEMISTRY, VOL. 52, NO. I),AUGUST 1980
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Figure 7. Photoacoustic spectra of neodymium oxide vs. 0.1 YO carbon
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Figure 5 . Photoacoustic ratio spectrum of holmium oxide vs (A) 1 YO, (B) 5 % carbon black
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Figure 6. Dependence of SIR ratio on the percentage of carbon bhck used as reference
spectrum will be distorted by the power spectrum of the source. In a double beam-in-space configuration where dual cells and microphones are employed, the gain of one of the microphone preamplifiers is usually adjusted until the power spectrum is suitably removed from the sample signal by the reference. However, in a double beam-in-time configuration where only one microphone is used, no such selectivity in gain exists between the sample and reference signals. Figures 5A and 5B show the effects of overcompensation due to the use of a reference containing 1% and 5% carbon black, respectively. The upward curving a t the extremes of the wavelength range increases with the percentage of carbon black in the magnesium oxide reference. Figure 6 shows the relative S/R ratios obtained for holmium oxide under normal operating conditions a t 450 nm as a function of the percentage of carbon black in magnesium oxide. As shown, the signal (S/R) magnitude approaches zero as t h e concentration (and absorbance) of carbon black increases. To prevent a significant reduction in the photoacoustic signal, and to minimize overcompensation, a reference must be used which approximates the sample base-line absorptivity. In practice for visible wavelengths, this is easily accomplished by using a mixture of carbon black and magnesium oxide which roughly matches the sample in opacity or darkness. For holmium oxide, 0.1% carbon black provides a n adequate reference, although some overcompensation still occurs as shown by the slight curvature of the base line in Figure 4A. A nearly flat base line was observed using 0.01 % carbon black as a reference. However, the spectra obtained
with the 0.1 7'0 carbon black reference is presented to illustrate better the differences between the spectra obtained with the double beam and single beam modes. When required, sample and reference base-line absorptivities can be more accurately matched using a trial and error method. T h e double beam ratio spectra of holmium oxide (Figure 4A) and neodymium oxide (Figure 7) are in good qualitative agreement with published spectra ( I O ) . Both spectra were taken under normal operating conditions. Although the. maxima a t 360,420,453, and 535 nm in the holmium oxide spectrum are clearly distinguishable, the 453-nm band is poorly resolved. Similarly, the resolution of the major bands in the neodymium oxide spectrum is less than desirable. Smaller apertures cannot be used in the monochromator t o increase resolution without a concurrent loss in signal due to decreased throughput. While a slower scan rate would reduce the band distortion which is due to the system response time, the rate used (25 nm/min) is the slowest possible with the present system. Smaller time constants ( < 3 s) generally yielded excessively noisy spectra. The signal magnitude was found to be inversely proportional to the modulation frequency. However, a maximum S/N of -330 for holmium oxide was observed when the system was operated in the single beam mode at 450 nm with a modulation frequency of 40 Hz. Even though the Electret microphone should show a flat response over the full acoustic range, some variations d o occur; when coupled with specific sources of noise characteristic of this particular system, a maximum S/Nwas obtained a t 40 Hz. An examination of the noise characteristic of the system revealed that the primary component was electrical noise from the microphone-preamplifier circuit. Since the preamplifier gains the signal from the microphone by a factor of lo3 to lo6, acoustic spikes and periodic (60 Hz) noise were also substantial. Shielding the preamplifier and attaching it directly under the cell reduced the periodic noise and improved the S/N by more than a factor of 2. When the double beam-in-space Configuration is employed for photoacoustic spectroscopy, the monochromatic radiation must be modulated by a chopper. Consequently, only half of the available light (corresponding to the period during which the chopper is open) may be utilized. T h e amount of the remaining light actually reaching the sample (and reference) is governed by the beam splitter. Thus, if the light is split equally, only 25% of the monochromatic light reaches the sample. In contrast, the double beam-in-time configuration described here allows 50% of the available light to reach the sample. T h e double beam-in-time system should therefore show increased sensitivity. With the present system, this was not achieved. The primary reasons are the comparatively low sensitivity of the microphone and noise in the preamplifier.
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Anal. Chem. 1980, 52. 1424-1426
CONCLUSIONS
for the loan of the lock-in amplifier used in this study.
T h e present double beam-in-time system is a viable configuration which yields good quality PAS spectra. A simple, yet workable cell design, coupled with a linear scanner allows the double beam-in-time mode to be utilized. The simplicity of design offers a substantial reduction in cost since only one microphone, one preamplifier, and one lock-in amplifier are used and construction is straightforward. The double beamin-time configuration permits the use of the ratio mode which provides compensation for variations in the source. Reference overcompensation can occur although this affects only the curvature of t h e base line and not the reproducibility or sensitivity. T h e system described is also easily converted to single beam and differential modes when required. Although the spectra observed are adequate, the acquisition of a more sensitive microphone is expected t o yield a substantial improvement in S/N.
LITERATURE CITED (1) Monroe, D. M.; Reichard, S. H. Anal. Chem. 1977, 4 9 , 119-131. (2) Adams, M. J.; King, A. A.; Kirkbright, G. F. Ana/ysf(London) 1978, 101, 73-85. (3) O'Haver, T. C. "Trace Analysis: Spectroscopic Methods for Elements", Winefordner, J. D., Ed.; W h y : New York, 1976; Chapter 2. (4) Jenkins, F. A.; Whie, H. E. "Fundamentals of Optics"; McGrawHill: New York, 1976; Chapter 21. (5) Rosencwaig, A.; Gersho, A. J. A@. Phys. 1978, 4 7 , 64-69. (6) Eaton, H. E.; Stuart, J. D. Anal. Chem. 1978, 50, 587-591. (7) O'Haver, T. C. J. Chem. Educ. 1972, 4 9 , A131-A221. (8) O'Haver, T. C.; Epstein, M. S.; Zander, A. T. Anal. Chem. 1977, 4 9 , 458-461. (9) Rosencwaig, A. Rev. Sci. Insfrum. 1977, 48, 1133-1137. (10) Blank, R. E.; Wakefield, T. W. Anal. Chem. 1979, 51, 50-54.
RECEIVED for review April 8, 1980. Accepted May 16, 1980. This work was presented in part at the Sixth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, Pa., September 1979.
ACKNOWLEDGMENT The authors thank Princeton Applied Research Corporation
Determination of Nitrogen Oxides and Nitric Acid Vapor by Infrared Spectrometry Jan B. Lefers"' and Pieter J. van den.Berg Laboratory of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
of nitrogen oxides with water vapor, nitric acid vapor and/or nitrous acid vapor can be formed ( 8 , 9 ) ,especially at higher concentrations of nitrogen oxides. Very little information, however, can be found in the literature concerning the quantitative analysis of NO, NO2, Nz04,and nitric acid vapor in such gas mixtures. Such information may be of importance for the manufacture of nitric acid and for pollution control purposes. Using infrared absorption Fontanella (10) studied the determination of NO a t a wavenumber of 1915 cm-', of NOz a t 1606 cm-', and nitric acid vapor a t 1326 cm-' in the stratosphere using the sun as source. This method is not applicable a t higher concentrations of NOz because of the strong overlap of the nitric acid vapor and the NOz absorption band. In this work, a method has been developed for t h e determination of NO, NOz, NZO4, and nitric acid vapor in gas mixtures a t concentrations which occur in the manufacture of nitric acid.
A quantitative analytical method for the determination of NO, NO2, N204,and nitric acid vapor In gas mixtures has been developed by infrared spectrometry. The foilowing peaks are used 1908 cm-' (NO), 2908 cm-I (NO,), 2980 cm-' (NZO,), 3160 cm-I (N204),and 895 cm-' ("0,). At rather high concentrationsof HN03,the 2980 cm-I Nz04absorption band shows a small overlap with a weak nitric acid band. I n that case, the N,O4 concentration should be determined with the 3160 cm-' peak or calculated from the equilibrium constant at known NO2 concentrations and temperature. In gas mixtures Containing nitrogen oxides and water, nitrous acid vapor could be detected, especially at high NO/NO, ratios.
For pollution control purposes, much attention has been paid to the determination of nitrogen oxides and several analysis methods have been developed (1-5). The disadvantage of most methods is that they are not applicable in the higher concentration range which occurs in the manufacture of nitric acid. Infrared spectrometry, however, can also be used for the determination of nitrogen oxides a t higher concentrations. Infrared absorption coefficients of NOz and NO for pollution control have been measured as a function of the optical path length and the temperature (6). Guttman (7) investigated integrated absorption intensities of pure NOz a n d N204 a t temperatures of 50 "C u p to 100 "C and a t pressures up to 2 MPa. The results of Guttman indicate that Beer's law is valid. Often water vapor is also present in gas mixtures containing nitrogen oxides. Owing to the reaction
EXPERIMENTAL All spectral measurements were carried out on a Perkin-Elmer Model 117 infrared spectrophotometer. The infrared absorption gas cell was constructed of glass with an inner diameter of 3.5 cm and a path length of 10.0 cm. Silver chloride windows were cemented on the gas cell which was kept in all experiments at a constant temperature of 25.0 *C by a thermostat. A gas loading system was used t o evacuate the gas cell and t o charge and make up gas mixtures of known composition. Details of the procedure have been published elsewhere (20). The points on the calibration curve for nitric oxide were obtained by blending a known amount of nitric oxide (Matheson Gas Products, purity: 99.2%) with dry nitrogen till a cell pressure of 0.1067 MPa (= 80 cm Hg) was attained. The small amounts of nitrous oxide and nitrogen dioxide which are present in com-
Present address: KEMA, Utrechtseweg 310, Arnhem. The Netherlands. 0003-2700/80/0352-1424$01 .OO/O
e
1980 American Chemical Society
