Chemiluminescent Method for NO and NO, (NO + NO,) Analysis Francis M. Black' and John E. Sigsby Chemistry and Physics Laboratory, Environmental Protection Agency, National Environmental Research Center, Research Triangle Park, N.C. 2771 1
An instrumental method has been developed which will permit rapid. sensitive measurement of nitric oxide (NO) a n d oxides of nitrogen (NO, = NO NO2) by chemiluminescent techniques. NO is measured by photoelectric amplification of a signal produced by t h e chemiluminescent reaction of KO a n d ozone. NO, is measured by photoelectric amplification of a signal produced by t h e chemiluminescent reaction of NO, a n d atomic oxygen. T h e atomic oxygen is produced by gas phase thermal decomposition of ozone.
+
Oxides of nitrogen (SO,), namely nitric oxide (NO) and nitrogen dioxide (KOZ). are significant atmospheric pollutants. NO is a combustion product and NO2 is a n air oxi2N02 (Bufalini dation product of NO-i.e., 2x0 + 0 2 and Stevens, 1965). A rapid, sensitive method for S O analysis utilizing t h e chemiluminescent reaction of ozone ( 0 3 ) a n d S O has been described (Fontijn et al., 1970) and adapted (Kiki et al.. 1971) to a u t o exhaust analysis. T h e method was expanded to permit NO, analysis by development of a thermal catalytic NO2 NO converter compatible with t h e NO/Os chemiluminescent system (Sigsby et al., 1973) T h i s paper will describe a n alternative to the SO2 NO thermal catal-tic conversion for NO, chemiluminescent analysis. T h e method involves a n alteration of t h e S O j O s system to permit atomic oxygen (0)chemiluminescence. NO2 reacts with 0 a t o m s to yield NO (Kaufm a n , 1958a) a n d NO will produce a chemiluminescent glow when reacted with 0 a t o m s (Halstead a n d T h r u s h , 1964). Thus. by observing t h e N O chemiluminescent glow, a signal proportional t o NO, concentration is obtained.
-
-+
-+
+0 +0 + KO + 0
NO,
NO
-- + + + 0,
NO
M
--t
NO,
NO,
hr
(1)
M
(2) (3)
At rnom temperature
K1 = 109.58 = 0.2 1. mol-] set-1 K 2 = 1 0 1 0 . 4 6 r 0.1 1.2 mol-2 sec-l K 3 = 104.58 0 . 3 1. mol-1 sec-1
as a source for oxygen ( K a u f m a n . 1958b). This background signal c a n be very significant when attempting to observe part per billion or p a r t per million concentrations of sample NO. T h e mechanism of t h e gas phase thermal decomposition of ozone is given by t h e following reactions:
0,+ M
4 7 0 + O? + M 0, + M 0+ 0+M 0 + 02 + 0 3
For M =
-
(4)( 5 )
( 6 )( 7 )
0 2
(8)
0 2
K4 = 106.86* 0 . 2 exp [-lo00 (-1.05)/n 1.2 molk2 s e c - l K 5 = 1011.6s= O.lexp [-lo00 (11.43)/T]1. m o l - l sec-I K 6 = 1012.16 T-1.0 1.2 mol-1 sec-1 K7 = i016.48exp [-lo00 ( 5 9 , 7 4 ) / q 1. mo1-I sec-I K8 = 1010.08= 0 . 2 exp [-lo00 ( 2 . 4 1 ) / q 1. m o l - l s e c - ~ l (Johnston, 1968)
Experimental Description of Apparatus. Figure 1 illustrates the syst e m . Figure 2 illustrates t h e atomic oxygen generator in greater detail a n d its configuration with respect to t h e chemiluminescent cell. T h e system shown in Figure 2 was built from Pyrex glass with t h e exception of t h e 0 - a t o m generator which was quartz t o permit high-temperature operation. T h e generator is maintained a t temperature with a nickel-chromium wire furnace. A silent discharge ozonator producing a 2-370 ozone stream in 0 2 and a n EM1 9659 (red extended) P M t u b e with a -20°C cooled housing were used. Vacuum was mdintained with a d u oseal vacuum p u m p (free air displacement 21 l./min a n d ultimate pressure torr); pressure was monitored by a thermistor vacuum gauge. T h e sample and 0 3 flows were maintained a t 40 cc/min each with stainless steel needle valves. T h i s results in a reactor pressure of 1 torr. T h e configuration shown in Figure 2, including t h e cobalt glass insert, was necesary to prevent t h e quartz glow at 1000°C
zt
(Baulch et al., 1970:) Reaction 3 is definitive of t h e production of the radiative KO2 specie resultant in t h e glow used in t h e analysis. In t h e system described, gas phase thermal decomposition of ozone is used as a source of atomic oxygen. A product of t h e use of thermal decomposition of ozone as t h e 0 - a t o m source is t h e elimination of t h e NO background signal which results from t h e N2 impurity in cylinder O2 when electrodeless discharges (microwave, R F ) are used To whom correspondence should be addressed.
I
hLET
Figure 1. N 0 / 0 3 - N O x / 0 chemiluminescent system
Volume 8, Number 2, February 1974
149
5,
I
I
I
I
I
I
I
I 5
%
1 /:-
I I
1
I
Figure 3. Spectral response of EM1 9659 photomultiplier t u b e (Manufacturer's Specifications)
Figure 2. Glass 0-atom generator and chemiluminescent cell from being significant a t t h e chemiluminescent cell. T h e system was designed to permit flow of t h e ozone directly t o the chemiluminescent cell or through t h e atomic oxygen generator. This permitted observation of ozone as well as atomic oxygen chemiluminescence. T h e sample inlet was designed to permit entry a t t h e inlet or outlet of the oxygen atom generator.
Results T h e atomic oxygen generator was maintained at 1000°C. T h k r e s u l t s in K4= 1 0 7 . 2 2 = 0 . 2 1.2 mol-2 set-1 K5 = 107.78 0.1 1. mol-1sec-l K6 = 109.06* 0.3 1.2 mol-2 sec-1 K , = 1 0 - 3 3 1. mol-' sec-l K 8 = 109.26 0.2 1, mol-1 set-1
IC
**+ELCb",,
jnlrlts
Figure 4. Spectral distribution of NO/Os glow Relative intensities are normalized to I = 100 at h = 1200 n m
zt
I
in t h e generator. In t h e cool zone (room temperature) K4=
108.38
i 0.2
K 5= 1 0 - 4 . 8 6
J
1.2
0.1
mol-2 set-1
1, mol-1 Set-1
K6 = 109.68 * 0.3 1.2 mol-2 set-1 K7 = 1. mol-1 sec-l K 8 = 106.59 i 0 . 2 1, mol--l c p - 1 By directing t h e ozone-oxygen mixture to the atomic oxygen generator a n d t h e sample either to t h e generator inlet or outlet (Figure 1). a n SO,/O chemiluminescent glow was observed. Two experiments were conducted to assure atomic oxygen chemiluminescence was actually being observed. T h e ozone/oxygen flow was directed to the chemiluminescent reactor so as to produce a n NO/Os glow with a sample stream of 32 p p m NO in Sz.T h e glow produced was observed with a n EM1 9659 PM tube (spectral response given in Figure 3) with and without a 600n m sharp cut filter in t h e optical p a t h . T h e spectral distribution of t h e NO/O3 glow is given in Figure 4 (Clough and Thrush, 1966). T h e filter, a Corning CS 3-66. transmits 90% at wavelengths greater t h a n 618 n m . About 9070 of :he glow intensity observed without the filter should be observed with t h e filter. T h e experimentally observed intensity with the filter in place was 89% of t h e intensity without the filter. T h e same procedure was followed a second time except that t h e ozone was directed to t h e atomic oxygen generator. T h e spectral distribution of the SO,/O glow, given in Figure 5 (Fontijn et al., 1964) when considered with the 150
Environmental Science & Technology
"IYLLrlG-"
.,.11cm~11
Figure 5. Spectral distribution of N O x / O glow. Relative intensities are normalized to I = 100 at h = 600 n m phototube spectral response, would dictate a n intensity reduction of about 50% with the filter in place. T h e experimentally observed intensity with t h e filter in place was 5770 of t h e intensity without t h e filter. An additional observation which supported t h e conclusion t h a t atomic oxygen chemiluminescence was being produced was t h a t t h e signal produced by a 32-ppm S O sample, as monitored by t h e EM1 tube. was two orders of magnitude greater t h a n the ozone chemiluminescent signal, This would be predicted by t h e spectral distribution of the glows and q u a n t u m efficiency of the phototube in t h e spectral region of t h e glows. D a t a for the second experiment conducted to confirm atomic oxygen chemiluminescence are given in Table I. Assuming a large excess of atomic oxygen, t h e chemiluminescent glow intensity should be linear with NO, concentration. As indicated, the system response was linear with NO, concentration, 2.68 f 0.17 a m p X 10-lO/ppm SO,.
Applicatzon There are two major areas of concern in applying the method to real samples. Chemical species which chemilu-
Table I. System NO, Response Sample
NO, ppm
NO:, ppm
NO,, ppm
Instrument response amp X 1 0 - 1 0
1 2 3 4 5 6
0.1 5.5
5.2 5.5 4.3 3.4
5.3 11.0 12.3 16.5 34.0 48.2
15.4 26.9 34.6 45.4 89.6 122.6
8.0
13.1 34.0 23.0
0
25.2
minesce when reacted with 0 - a t o m s and emit in t h e spectral region being monitored offer potential positive interference. Chemical species which have high rates of reaction with G-atoms offer a potential negative interference through depletion of the 0 - a t o m s . A large excess of 0atoms is necessary to assure linearity of response. Chemical species other t h a n T O . which chemiluminesce in t h e 400-900 n m spectral region include carbon monoxide and olefinic hydrocarbons ( M a h a n a n d Solo. 1962: Krieger et al.. 1952). However. spectral distribution of t h e glows a n d reaction rate considerations permit photoelectric observation of t h e NO,y glow without significant interference from a simultaneous C O or H C glow. T h e C O j O chemiluminescent glow has a spectral distribution from 320--600 nm. with a maxima a t 400 n m ( M a h a n and Solo. 1962; Clyne and Thrush, 1962). T h e CO glow can therefore be segregated from t h e NO, with a s h a r p c u t filter a t 600 n m to permit observation of t h e KO, glow a t wavelengths greater t h a n 600 n m . Additionally. reaction rate d a t a indicate t h a t with properly controlled reactor residence time a n d temperature, the interference caused by t h e overlap of t h e CO glow when observing t h e total NO, glow can be made negligible for most NO,, sources (relative CO to NO, concentrations would dictate required conditions).
0
+ co
-
CO?
+
hr
(9)
At room temperature K3 = 1O4.'j I. m o l - ' s e c - l K 9 = i01.05 1. m o l - I s e c - l
(Clyne a n d Thrush. 1962) Therefore. it can be seen t h a t for a given residence time a t room temperature. the molar glow d u e to t h e radiative combination of NO a n d 0 is about 3300 times as intense as t h a t due to t h e radiative combination of CO a n d 0. OH Meinel band emissions from H C j O reactions also overlap the spectral region used. However. experimental results utilizing a u t o exhaust samples indicate this is not a significant interference. Krieger's work would indicate t h a t this emission probably occurs before t h e chemiluminescent cell in t h e configuration described in Figure 2. Probably more limiting in t h e systems application is t h e depletion of available 0 - a t o m s through reaction with other sample species and through recombination. NO2 titration indicated a system effective 0 - a t o m concentration of 1200 p p m . This was obtained with t h e 0 - a t o m generator at 1000°C a n d a system total pressure of l torr as described. T h e N O 2 3 2 titration is definitive of losses due to recombination a n d losses on t h e walls of t h e flow vessel. However. in real samples. available atomic oxygen is depleted further by reaction with sample species other t h a n SO,. Reaction 4 a n d t h e associated temperature-dependent rate constants predict t h a t molecular oxygen could be a significant competitor for t h e atomic oxygen. As was indicated earlier with t h e 0 - a t o m generator a t 1000°C. t h e equilibrium does favor t h e production of atomic oxygen:
b u t when t h e stream departs t h e heated zone. t h e equilibrium is shifted, and Reaction 5 is no longer significant. Only depletion reactions play a significant role in t h e cool zone. With the sample entered 28 in. from t h e outlet of t h e 0 - a t o m generator a n d with t h e ozone stream passed through t h e generator, neither ozone nor atomic oxygen chemiluminescence could be observed to any significant extent. With a n NO,/air stream entered a t the outlet of the oxygen a t o m generator. atomic oxygen ChemiJuminescence was observed. b u t the signal was only about 50% of t h a t observed when t h e sample was entered a t t h e 0 - a t o m generator inlet. As a result of these observations. a study was made to determine the effect of molecular oxygen concentration in the sample stream on instrument span. NO, samples with varying levels of 0 2 (Nzbalance) were run. T h e results are depicted in Figure 6. T h e effect is increased by operating the 0 - a t o m generator at lower t e m peratures. This effect may also be due to molecular oxygen quenching of t h e radiative specie; however, quenching is not considered to be of significance a t 1torr. CO is another Significant competitor. Although the formation of the radiative species by t h e reaction of C 0 with 0 is not significant at room temperature ( K g ) . the total reaction to CO2 is significant in depleting t h e available oxygen atoms when a n O3 source. such as described in this system. is used.
CO
+
K,,
-+ M -
0
CO?
+
M
10i.~l.2mol-2se-1
(IO)
(Clyne, 1962) Similarly. hydrocarbons a n d other sample species reactive with 0 atoms will limit the linear range of t h e system. T h e linear range could be extended by using a higher O3 source with resultant higher oxygen atom production. Seven percent ozone in oxygen is about the highest level attainable from commercial water-cooled discharge ozonators. Ozone may be concentrated in cold traps. but is very hazardous to handle when concentrated. Figure 7 depicts t h e system response with the 0 - a t o m generator operated a t 916°C. a n d the sample entered at generator inlet. T h e responses indicated in Figure 'iare with a Corning C S 3-66 filter (600-nm sharp cut) in the optical p a t h . T h e above results were with S O , samples in Nz.T h e following procedures were used to permit analysis of real samples. T h e instrument is spanned with a n NO.y standard in S Z .T h e oxygen level of t h e sample is then determined and a span correction factor established utilizing Figure 6. T h e sample is then entered and t h e NO, level determined by comparing sample glow intensity with the corrected s p a n . T h e linear range must be determined. however. to assure applicability to samples of interest. Table I1 gives d a t a from air-diluted a u t o exhaust samples. T h e system demonstrates linearity through about 50 p p m NO,. Samples with lower relative H C and CO concentrations may have extended linearity.
-1
c;
W!C11
Figure 6. Molecular oxygen effect. Dependence of molecular oxygen effect on 0-atom generator temperature. Plot A with generator at 650"C, plot B with generator at 916°C
Volume 8, Number 2, February 1974
151
the ozone directly to t h e chemiluminescent reactor and following procedures for NO/Os chemiluminescence. When we use the method for NO, analysis of real samples, care must be given to response linearity. T h e method should have feasibility with ambient samples and samples taken from mobile sources with emission control. With line voltage regulation (100 mV rms). system stability was excellent. Zero drift was effectively nonexistent.
