Fluorescence Detection of Nitric Oxide in Nitrogen Frederick
P. Schwarz and Hideo Okabe
Institute for Materials Research, National Bureau of Standards, Washington,D.C.20234
An accurate, rapid, and simple technique is described for the measurement of nitric oxide in standard reference mixtures of NO in N2. The technique investigated is based on the measurement of the fluorescence intensity emitted by NO when it absorbs 213.8-nm radiation from a Zn discharge lamp (the A2Z ( Y = 1) X2?rIl2 ( Y = 0) transitlon). The fluorescence Is In the 220- to 300-nm region and Its intensity Is proportional to the 213.8-nm llght intensity and the NO concentration. The fluorescence Intensity at a constant light Intensity level Increases linearly wlth NO concentration In the 0.015- to 7-ppm range to within 1% error. In the 7- to 932-ppm range, the intensity increases sublinearly with NO concentratlon because of efficient selfquenching of the fluorescence. The signal-to-noise ratio is 1.0 at 10 ppb for a 1-minute countlng time. The quenching half pressures of NO, H20, CO,O2, 02,C2H4, CBHB,and H2 are, respectively, 0.34 f 0.03, 0.65 f 0.06, 8.3 f 0.6, 0.65 f 0.05,05, 1.1 f 0.1, 5.6 f 0.4, 56 f 4, and 115 f 8 Torr. The appllcation of this method to detect NO and SO2 in automotlve exhaust is discussed.
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Atmospheric nitrogen oxides (NO NOz), produced mainly from t h e fuel combustion in stationary sources and from automotive exhaust, are one of t h e most common air pollutants. Although NO itself is only moderately toxic i t is subsequently converted t o more toxic NO*. The interaction of NO2 in the atmosphere with sunlight and unsaturated hydrocarbons produces photochemical oxidants ( I ) . Few reliable analytical methods are currently available for rapid, sensitive, and specific determination of NO in air, N2, a n d automotive exhaust. These a r e based either on t h e oxidation of NO t o NO2 and subsequent determination of NO2 or o n the chemiluminescent reaction of NO with 0 3 ( I ). Although the chemiluminescent reaction is commonly used for t h e NO analysis, it requires a n ozone generator and a well calibrated flow system. An alternative method, based on fluorescence detection, has been developed by us mainly from the need of rapid analysis of t h e NO concentration in s t a n d a r d NO-N2 gas mixtures used for calibrating analytical instruments. It has been known for some time that fluorescence is produced when NO is excited with t h e Zn 213.8-nm line ( 2 ) , corresponding t o t h e near coincident transition A2Z(u = 1, K = 29) X2al/2(u = 0, K = 29) ( 3 ) .In t h e presence of a t mospheric Nz, t h e overlap between t h e existing line and t h e NO absorption line increases because of pressure broadening a n d the resulting fluorescence signal is expected t o increase. T h e fluorescence spectrum from t h e NO AQ(u = 1) s t a t e lies in t h e region 213.8 t o 310 n m ( 4 ) . Also, N2 is remarkably inert i n quenching t h e NO A2Z s t a t e ( 5 ) . It is therefore possible t o use t h e fluorescence method for detecting low concentrations of NO i n Nz. T h e present report describes t h e results obtained for such mixtures and the quenching effect of oxygen a n d various gases present in a u tomotive exhaust. T h e method is capable of detecting 0.015 p p m (parts per million by volume) NO in Nz. T h e response
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curve is nonlinear a t higher NO concentrations because of t h e self quenching (3, 5 ) . T h e method can be used for t h e analysis of NO in automotive exhaust by diluting t h e exhaust gas with Nz and separating t h e NO fluorescence by an appropriate filter from that of SOz, t h e only other known component in automobile exhaust t o emit a fluorescence in this region. A similar technique has been developed for detecting SO2 in air (6) at the p p b level.
EXPERIMENTAL The detection system for measuring NO in pure N2 was identical with that used for detecting SO2 in air ( 6 ) .Briefly, the NO fluorescence was excited by the 213.8-nm line from a commercial Zn glow discharge lamp. A Corning CS 7-54 filter (transmission 230 to 420 nm) was used to isolate the NO fluorescence in the region 230 to 310 nm. The photon counting ratio mode was used in which one photomultiplier monitored the lamp intensity while the second photomultiplier monitored the fluorescence. Both photomultipliers had 13 stages and a S-13 spectral response. The photon counting integration time was of the order of 1 minute. The maximum counting rate was about 60,000 counts per sec which is well below the saturation rate of 20 MHz. The fluorescence spectrum of 100 ppm NO in N2 excited by the 213.8-nm line was taken by a 0.3-m scanning monochromator with a 2400 lines per mm grating. A slit width of 2 mm was used giving a resolution of about 3 nm. The detector consisted of the fluorescence monitoring photomultiplier and the Corning CS 7-54 filter. Sample Preparation. The majority of the NO in N2 measurements was performed in a static system. The outlet port of the fluorescence cell (200 ml in volume) was connected to a vacuum manifold and the inlet port to a gas mixing vessel (5 1. in volume). Both the cell and the mixing vessel could be evacuated separately down to 1-m Torr pressure. A 250-ppm NO in N2 mixture was the initial concentration used for all the measurements in this system. It was successively diluted with water-pumped dry N2 in the mixing vessel to yield progressively lower concentrations. The concentrations were calculated from the pressure readings of a calibrated mechanical pressure gauge prior to and after dilution with N2. Total pressure of the mixtures was approximately 760 Torr. For each measurement, the cell was evacuated and a sample of the NO in Na mixture was introduced into the cell from the mixing vessel. After measuring the NO fluorescence intensity of the sample, the cell was again evacuated. The quenching effect due to the presence of HLO vapor, CO2, CO, 0 2 , H2, C2H4, and C3H8 in the NO in N2 samples was determined in the following manner. The fluorescence intensity of an NO sample from the gas mixing vessel was measured and then the cell was evacuated. A quenching gas at several Torr was introduced into the evacuated cell through the outlet port. Then another sample of the same NO in N2 mixture was introduced into the cell from the mixing vessel. After allowing the gases to mix for about 10 minutes at a total pressure of approximately 760 Torr, a fluorescence measurement was recorded. A comparison between the two measurements yielded the quenching effect of the interfering gas. The gases were reagent grade and used without any further purification. Several measurements of NO in Na mixtures were performed in a flow system where a slow flow of an NO in N2 mixture was mixed with a fast flow of pure NP and then introduced into the cell. The cell was open to the atmosphere and the flow rates were typically 10 l./min and 1 ml/min. The flow rates were measured by a calibrated mass flow meter. The NO in N2 mixtures prior to dilution were primary standards of 1.917, 0.1067, 0.9913, 1.515, and 0.5136% NO in N2. The measurement of NO in air was performed in the same manner with dry air as the faster flowing diluent gas. Rotameters were used to measure the relative flow rates. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 . APRIL 1975
703
240
250
260
270
280
290
300
WAVELENGTH [NANOMETERS)
Figure 1. The fluorescence spectrum of 100 ppm NO in N2 as measured by a combination Corning CS 7-54 light filter and a 13 stage photomultiplier with S-13 spectral response. A 0.34 scanning monochromator with a 2400 lines per mm grating was used at a resolution of about 3 nm. The A, is 213.8nm
RESULTS AND DISCUSSION The fluorescence spectrum of 100 ppm NO in N2 excited by the 213.8-nm line is shown in Figure 1. It is apparent that the spectrum consists of two progressions of about equal intensity, one from u' = 1 and the other from u' = 0. Since the u' = 1 is the only level produced by initial light absorption, u' = 0 must be formed by vibrational relaxation in the presence of N2. From the steady state condition one obtains doZ[NO] = h,[NO*][X]
+ kN2[NO*][N2] -Ik~o[No*][No] + k,[NO*]
(1)
where [NO*] is the NO A2Z+ concentration, c is the extinction coefficient of NO a t 213.8 nm, IO is the intensity of light a t 213.8 nm, 1 is the cell length; k x , NO, kN2, are the quenching rate constants of, respectively, a gas X, NO, and N2, and k f is the decay rate of the NO A22+ state. Since the quenching constants for NO A22 u' = 1and for v' = 0 were identical in NO and COZ (5), it is reasonable to assume that k x for u' = 1 and u' = 0 would be the same for other quenching gases. The integrated fluorescence counts, F, for a period t is
F = tafkf[NO]* (2) where Lyf is a constant representing the fluorescence quantum yield and a geometrical factor. From Equations 1 and 2 F = tdoZaf[NO](l + ax[x] + a,o[NO] a~,[Nz])" (3) where ai = ki/kf ( i = X, NO, N2) is the Stern-Volmer quenching constant in units Torr-'. In the absence of the quenching gas X, Equation 3 becomes where = t t l o l a f . Rearranging Equation 4, one obtains Equation 5
F'= B[NO] - F A I N 0 1 + S ( 5) where E = p(1 a ~ , [ N ~ l ) - A ' , = aNo(1 -t a~,[Nz])-' and S is the background counts of scattered light. Equation 5 gives a nonlinear plot of F" at higher NO concentrations because of a large self-quenching constant (quenching in almost every collision) ( 5 ) .Equation 5 assumes that the absorption of the 213.8-nm light is linearly proportional to NO concentrations over the range studied. Since the quenching constant, a N O , obtained from Equation 5 agrees reasonably well with the literature values, as will be discussed later, the above assumption may be justified. A pro-
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A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 4 , A P R I L 1975
portionality constant B, given in units of counts per concentration, is dependent on instrumental and geometrical factors, while A, in units of the reciprocal concentration, is determined only by the quenching constants a N O , a N 2 and the pressure of Nz. In the five runs of.NO in Nz mixtures, the best least squares fits of the fluorescence measurements to the concentrations were determined by the NBS Univac 1108 computer. The measurements were fitted to Equation 5. The quenching constants ax in units of Torr-l were determined by comparing the fluorescence counts of the NO in Nz sample to that of an identical sample containing a small amount of X. From Equations 3 and 4
F"/F = 1 + U,[x](1 + UNO[NO] + a ~ ~ [ N 2 ] ) - ' (sa) From A, determined from Equation 5, and the known value of a N , (the reciprocal of the quenching half pressure (5) of 1900 Torr), a N O can be obtained. It is then possible to obtain ax from the slope of the plot FOIF as a function of [XI. The Stern-Volmer linearity is obeyed for each quenching gas tested. In the presence of more than one quenching gas, Equation 6a becomes
Error Analysis. The weights in the least squares fit for Equation 5 were determined from the observed standard deviation of F" and an assumed standard deviation UNO in [NO]. At least 10 fluorescence measurements of each concentration were recorded, and the standard deviation, uf, was determined. In the static system, the error in the concentration is accumulative due to successive number of dilutions, n. If the self-quenching effect on the signal is neglected and the reading error on the mechanical gauges is 1%,then the corresponding standard deviation in the signal is
uN0 = F ( 0 . 0 1 ) ( 2 ~ ) " ~
(7)
where F is the fluorescence signal at the concentration corresponding to n. The weight of each fluorescence countconcentration measurement is
After determining the value of A from the least squares fit, NO was corrected at each concentration as follows,
uNo(corr) = (1 + A[NO])'2F(0.01)(2n)''2
(9)
The weights w were correspondingly corrected and the best least squares fits redetermined. The A, E , and F values presented in this paper are from this latter fit. The magnitude of the residual standard deviation of each fit was used to evaluate the completeness of the error analysis. A residual standard deviation greater than 1 indicates that the standard deviations at each concentration were underestimated. A residual standard deviation less than 1 indicates that the standard deviations were overestimated. Furthermore, simulations of two sets of measurements assuming a random error in F and [NO], within the error bounds of each, were run on the computer. The least squares fit was identical to that of the actual measurements. In the flow system, the error in the concentration was determined by assuming an error of 0.5 or 1.0% in each of the mass flow meter readings until the residual standard deviation was -1.0. The corresponding error in the signal F" is
uNO = Fo2''of
(10)
where af = 0.005 or 0.01. After determining A in Equation
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3870
50
'00
'50
N O IN Y I T R O C - E Y
Figure 2. The signal,
200
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25s
PPU
F,as a function of the NO in N2 concentration
3039
>E38
0486
c
7 ' -
h O IN N ' - R C G E \ PPZ'
Figure 3. The signal, F, as a function of the NO concentration in the sub-ppm range
5 , C ~ N Owas corrected a t each concentration by the factor (1 A [NO])-2. The weights were correspondingly corrected and the best least squares fits redetermined. The standard deviations of the constants B, A, and S from the least squares fits reflect the precision of the technique. T o determine the accuracy, the concentrations were calculated by
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error analysis is complete, and the sublinear behavior a t high NO concentrations is not an artifact of the detector but due to self-quenching of the fluorescence signal. The observed standard deviation in the signal a t each concentration was less than 1.0% except in the 10-ppb range where it varied from 1 to 2%. This error is due to statistical fluctuation of the counts integrated over a 1-minute time interval. In the static system, the error in the concentration at 15.3 ppb can be as high as 5.3% according to Equation 7. The error in the 100-ppm range is a couple of percent in the error analysis. In the flow system, Run 5 , an error of 0.5% in the concentration yielded a RSD of 2.37 and an error of 1.0%yielded a RSD of 0.467, indicating that the concentration error in Run 5 is between 0.5 and 1.0%. The signal dependence on the concentration is mainly manifested in the B parameter in Equation 5 . In Runs 1 through 5 , the standard deviation of B varies from 0.4% (Run 5 ) to 3.2% (Run 1).The optimum S/N (signal-to-noise ratio) for a 1-minute integration time can be calculated from the results of Run 4. In Run 4, B = 6023 f 47 counts/ ppm and S = 1247 f 14 counts yielding a counting error of 61 counts a t 1 ppm, thus S/N = 1.0 a t 10 ppb. This lower limit of detection can be extended by minimizing S and maximizing B. The background signal, S , which is partially due to the dark photon counts, can be reduced by cooling the photomultiplier. Since B is proportional to the lamp intensity and the integration time (Equation 5), the error due
(11)
[NO]calcd = (F" - S ) / ( B - AF")
3339
0'89
where F" is the observed fluorescence counts without quenching gas and B, A , and S were from the least squares fits. The calculated [NO] values are compared to the known NO concentrations. NO Fluorescence in Pure Ng. A typical signal response curve as a function of NO concentration in pure N2 is shown in Figures 2 and 3. The sublinear dependence of F" on the NO concentration is apparent in Figure 2. At the lower concentration range shown in Figure 3, a linear dethe pendence is apparent. The intercept in Figure 3 is background signal. The results of the least squares fits for 4 runs in the static system and 1 run in the flow system are presented in Table I. The range of measurements extends from 932 ppm (Run 5 ) to 15.3 pbb (Runs 3 and 4). The photon counting integration time was of the order of a minute in all the runs. The residual standard deviations (RSD) of Runs 1 , 4 , and 5 , respectively 0.848, 0.761, and 0.467, indicate an overestimation of the error in each measurement. The RSD of Runs 2 and 3, however, indicate an underestimation of the error. Since the RSD is close to 1.00 in all the runs, the
s,
~~
Table I. Fluorescence Counts of NO in Ng Mixtures Approximate
Run
Corresp.
decrease
coiicn
of concn
\ i o . oi
rinqi, p p r
a t e a c h dilution
dllutxons
Re. std de\
1
2 50-1 8.6
77
9
0.848
2
149-1.27
77
18
1.26
3
2503.039
59
16
1.08
4
2 5 0 3 . 0 153
50
10 m e a s u r e ments a Flow system run consiting of 10 measurements 5"
932-18.6
B
A
14
0.761 0.467
-1 (PPm I with (std deb)
5.52 x lo-' (24.3%) 1.86 x 10-3 (12.0%) 1.42 x 10-3 (2.97) 1.48 X lo-' (2.9%) 1.03 x 10-3 (1.15)
S
-1
(
~ ppm 0 )
~
~
(counts)
with (std dev)
with (std d a ]
3193 i 103 (3.2%) 2006 + 41 (2.09) 5745 i 42
21,026 i 3448 (16.45) 754.9 + 181 124.05) 911.9 33 (3.7%) 1,247 + 14 (1.1%) 14,722 i 991 (6.7%)
(0.79) 6023 i 47 (0.8%) 3964 + 1 7 (0.4%)
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NO. 4, APRIL 1975
705
Table 11. Quenching Half Pressures of Some Automotive Exhaust Gases
Table 111. Composition of Automotive Exhaust Gas and the Quenching Effect on NO Fluorescence
Experimental Literature
quenching
quenching
half pressure Quenching gas
N2 HzO
co
COZ O? C2H4 C3H8
H2 NO
a x - 1 (Torr)
...
0.65 + 0.06 8.3 f 0.6 0.65 j: 0.05 1.1 0.1 5.6 i 0.4 56 i 4 1 1 5 80 0.34 It 0.03
*
*
half pressure (Torr)
1900 (5) 0.47 (10) 7.14 0.70 (5) 0.36 (5), 0.62 (10) 0.90 0.10 (9)
...
* *
Approximate composition range in % vol. of automotive exhaust gas (11)
S O iluarcscence
quenching gas
0.003 to 0.3 13.0 to 13.2
NO H,O
co
0.8 t o 5.2 9.5 to 10.2 0 0 to 0.2 0 to 0.2 0.2 to 1.7
CO2 02 C2H4 C3H8
H2
G 2 0 (10) 0.63 (5), 0.29 (10)
F'IF [NO]
PPI"
3000
Before dilution
After 1:lM) dilution with Nz
33.7-35.5 5% change 184-194 5% change
2 .a3-2.9 3 4% change 2.93-3.03 3% change
30 to fluctuation in B can be improved by longer integration time and/or higher lamp intensity (6). The background counts have a standard deviation from 1.1 (Run 4) to 24.0% (Run 2). In Run 3, the background interfere with the NO detection in air (8).This interference counts were determined by filling the cell with pure N1. I t was approximately equal to the S determined by the least may be minimized by using a filter transmitting the region 230 to 300 nm corresponding to the NO fluorescence specsquares fit. ppm-' to 1.42 The A parameter ranges from 1.86 X trum (Figure 1).The SO2 fluorescence maximum is in the X ppm-' in Runs 2-4 while, in Run 1, the A parame300- to 330-nm region (8). The possibility remains of using other light sources to ter is much lower because the range of measurements is in the sublinear region and over only one order of magnitude, excite the NO emission with a greater S/N than with the Zn lamp. Since the NO absorption consists of discrete lines, a and accordingly, S is much larger. In Run 5, one measurecontinuum light source such as a H2 lamp would excite ment was performed at 932 ppm, the rest were from 18.6 to more of the vibrational-rotational absorption lines. The tel316 ppm, again, a range of only one order of magnitude. In lurium resonance line a t 214.7 nm would be more in coinciRuns 2-4, the measurements were performed over a t least dence with the u' = 1 NO line than the Zn 213.8-nm line two orders of magnitude and should give more reliable A (9). Melton and Klemperer have found an increase in the values. Using an average value 1.59 f 0.18 X ppm-I fluorescence signal by a factor of 10 to 100 by shifting a = 1900 Torr ( 5 ) ,UNO-' = for A from Runs 2-4 and 0.34 f 0.03 Torr is obtained from A = a p ~ o / ( l a ~ ~ [ N 2 ] ) . Zeeman component of the Cd I1 226.5-nm or 214.4-nm lines more in coincidence with NO absorption maxima by a sevWith this A value the NO fluorescence signal becomes a eral kilogauss magnetic field (7). linear function of the concentration from 15 ppb to 7 ppm Quenching of NO Fluorescence a n d Detection of NO to within an error of 1%. i n Automotive Exhaust. In order to explore the possibiliSince the flow system Run 5 duplicates the results of the ty of measuring NO in automotive exhaust by the fluoresstatic system, the accuracy of the technique can be determined by the results of Run 5. The Run 5 values for F",s, cence method, the NO quenching efficiencies of several gases present in the exhaust have been measured. The B, and A were substituted into Equation 11 and the NO pressure of various gases that quench the NO fluorescence concentration was calculated. Out of 10 calculated concenintensity by 50% are presented in Table 11. They are calcutrations, 9 were within 1%of the actual concentrations and lated from Equation 6a. The values UNO-' = 0.34 f 0.03 one calculated concentration was 1.2% greater than the acTorr obtained in this work and = 1900 Torr from tual concentration. In the static runs, 65% of the calculated Callear and Pilling ( 5 )were used. concentrations were within 3% of the actual concentrations. The quenching half pressures of NO, HzO, COS, CO, 0 2 , The concentration error in the static runs is from 1 to 5.3% HS, and N2 are in reasonable agreement with the literature and, in the flow system, it is between 0.5 and 1.0%. values. It is apparent from the Table that the most efficient Fluorescence signals from several NO in air mixtures quenching gases are HzO, CO2, 0 2 , and NO, while CO and were also measured. They were 997 f 24 counts a t 19.3 C2H4 are only moderately efficient. Hydrogen and propane ppm, 597 iz 32 a t 11 ppm, and 254 f 17 at 6 ppm. The are poor quenchers. The values for CzH4 and C3Hs have apbackground signal which is already subtacted from the parently not been measured previously. counts was 145 f 5 counts. At 6 ppm, the total standard Because of the presence of large concentrations of deviation of the background plus fluorescence counts is 18, quenching gases, mainly HzO and C o n , in automobile exyielding a signal to noise ratio of 12, i.e., at 0.5 ppm, S/N = haust, the fluorescence signal, F, of NO measured in the 1. The integration time for the counts was 1 minute. The automobile exhaust would be considerably less than the signal response is almost linear with concentration. The signal, F", from the same concentration of NO present in linear response of the fluorescence counts in NO-air pure N2. The approximate composition of automotive exmixtures can be explained by the high quenching efficiency haust gas is given in Table 111. The effect of the main of the NO fluorescence by oxygen (7), that is, in Equation quenching gases on the fluorescence signal, when the com3, the dominating term is ao2[02]since ao2= 3 a ~ and o [02] >> [NO] in air, Therefore, the denominator is invariant position changes within the stated range, can be calculated using Equation 6b. The ratios, Fo/F,are given at two NO with respect to the NO concentration. Thus, the detection concentrations, 3000 and 30 ppm, in Table 111. If the exlimit is reduced to lij0 that of NO-N2 mixtures. Furtherhaust gas is not diluted by N2, the signal reduction is highly more, even a small amount of SO2(1/500 that of NO) would
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
dependent on NO concentration ranging from 2.8% a t 3000 ppm to 0.53% at 30 ppm although the ratio is not sensitive to the change of composition (within 5%). On the other hand if the exhaust gas is diluted by 1:lOO with Nz, the ratio F"/F, is not much dependent on the NO concentration, that is, the signal is reduced to 34 f 1%of F" in the exhaust gas, independent of the change in NO concentration and in composition. A signal decrease caused by dilution is compensated by an inciease of signal due to an increased N2 concentration. It is still possible to detect the fluorescence signal of 30-ppm NO in the exhaust after 1: 100 dilution where the signal becomes equivalent to 0.1ppm NO in pure N2. Thus the dilution of the exhaust gas by N2 provides an almost linear calibration curve for measuring NO in the exhaust in the range 30 to 3000 ppm. Dilution by N2 has the further advantage that the condensation of H2O can be avoided when the gas mixture is brought to near room temperature for the NO measurement. To minimize an interference by SO2 which may be present in the exhaust gas, a filter transmitting predominantly the NO fluorescence (230-300 nm) and rejecting most of the SO2 fluorescence (250-420 nm) may be required. Alternatively, if the exhaust gas is diluted by air or by oxygen, and a filter transmitting light of wavelengths 300 to 420 nm is used, it is possible to measure SO2 as well.
ACKNOWLEDGMENT The help of William H, Kirchhoff in the error analysis is gratefully acknowledged. The flow system run was performed by Ryna Joseph Marinenko of the Analytical Chemistry Division.
LITERATURE CITED (1) Air Quality Criteria for Nitrogen Oxides, EPA(US) Air Pollution Control Office, Washington, D.C., Publ. NO. AP-84, 1971. (2) D. R . Crosley and R. N. Zare, Bull. Am. Phys. Soc.,-12, 1147 (1967). (3) L. A. MeRon and W. Klemperer, J. Chem. Phys., 59, 1099 (1973). (4) R. W. B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra." John Wiley & Sons, New York, N.Y.. 1963. (5) A. B. Callear and M. J. Pilling, Trans. Faraday Soc., 66, 1618 (1970). (6) F. P. Schwarz. H. Okabe, and J. K. Whittaker, Anal. Chem., 46, 1024 (1974). (7) L. A. Melton and W. Klemperer, Planet. Space Sci.. 20, 157 (1972). ( 8 ) H. Okabe, P. J. Splitstone, and J. J. Ball, J. Air Poilut. Control Assoc., 23, 514 (1973). (9) Private communication from R. N. Zare of Columbia University to H. Okabe. (10) A. V. Kleinberg, and A. N. Terenin, Dokl. Akad. Nauk, SSR, 101, 1031 (1955). (11) "Outlook-Electric Vehicle Revival of the Fifty-Year-Old Memory," Environ. Sci. Techno/., 1, 192 (1967).
RECEIVEDfor review October 11, 1974. Accepted December 19,1974. This work was supported by the Measures for Air Quality Program at the National Bureau of Standards.
Automated Computer-Controlled Spectrophotometer System for Kinetic or Equilibrium Methods of Analysis K. R. O'Keefe and H. V. Malmstadt' School of Chemical Sciences, University of Illinois, Urbana, Ill. 6 180 1
An automated computer-controlled spectrophotometer system is described that is applicable for stopped-flow kinetic or equilibrium methods of analysis. The system provides rapid sequential analysis using rate measurements in the time range of milliseconds to minutes or accurate equilibrium absorbance measurements of stable sample constituents. Only small volumes of composite reagent and sample, about 0.17 ml of each, are required for each complete fill/ inject cycle of operation. Absorbance changes of 0.00004 A are readily measured in the 2-cm pathlength cell by the split-beam dual-detector system. The automated analyzer has been programmed for use in either a routine analytical mode or in a research investigative mode. New light source and beam splitter modules that are compatible with other commercial spectrometer modules are described. Experimental results with the complete system compare closely with the theoretical results calculated on the basis of photon statistics.
In recent years, the use of reaction rate methods of chemical analysis has become increasingly popular, especially in laboratories where advantages in speed and selectivity over equilibrium methods are important ( I , 2 ) . The current interest in rate methods is attested to by several recent reviews covering reactions that are useful for analytiSend reprint requests to this author.
cal purposes (3, 4 ) and instrumentation and methodology for rate measurements ( I , 5, 6). Routine reaction rate methods on time scales from several seconds to minutes have been available for many years (1-3), and rate methods on the millisecond-to-second time scales have been reported (7, 8). In addition, the introduction of automated systems for rate measurements has had considerable impact in this area (3,6). In this report, a new system for reaction rate measurements is described that is useful for both the fundamental characterization of chemical reactions and the application of both rate and equilibrium methods to chemical analysis. A dual-beam optical arrangement is used that eliminates source fluctuations as a source of photometric error in absorbance measurements. This allows the use of an unregulated high intensity source while still permitting high precision absorbance measurements. The measurement system for the spectrophotometer is described and is shown to introduce negligible error into the measurement. The sampling-mixing system is a new automatic stopped-flow head which requires no operator attention during normal use. All control functions are readily implemented, and are TTL-compatible. The stopped-flow head provides for thermostating of the mixing syringes and optical cell and thermistor measurement of the mixed solution temperature. Volumes of sample and reagent that are required for each sampling-mixing cycle are 0.17 ml. System dead time is 5 f 2 msec. The entire system is automated using a PDP-8/f miniANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
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707
P. Schwarz and Hideo Okabe
Institute for Materials Research, National Bureau of Standards, Washington,D.C.20234
An accurate, rapid, and simple technique is described for the measurement of nitric oxide in standard reference mixtures of NO in N2. The technique investigated is based on the measurement of the fluorescence intensity emitted by NO when it absorbs 213.8-nm radiation from a Zn discharge lamp (the A2Z ( Y = 1) X2?rIl2 ( Y = 0) transitlon). The fluorescence Is In the 220- to 300-nm region and Its intensity Is proportional to the 213.8-nm llght intensity and the NO concentration. The fluorescence Intensity at a constant light Intensity level Increases linearly wlth NO concentration In the 0.015- to 7-ppm range to within 1% error. In the 7- to 932-ppm range, the intensity increases sublinearly with NO concentratlon because of efficient selfquenching of the fluorescence. The signal-to-noise ratio is 1.0 at 10 ppb for a 1-minute countlng time. The quenching half pressures of NO, H20, CO,O2, 02,C2H4, CBHB,and H2 are, respectively, 0.34 f 0.03, 0.65 f 0.06, 8.3 f 0.6, 0.65 f 0.05,05, 1.1 f 0.1, 5.6 f 0.4, 56 f 4, and 115 f 8 Torr. The appllcation of this method to detect NO and SO2 in automotlve exhaust is discussed.
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Atmospheric nitrogen oxides (NO NOz), produced mainly from t h e fuel combustion in stationary sources and from automotive exhaust, are one of t h e most common air pollutants. Although NO itself is only moderately toxic i t is subsequently converted t o more toxic NO*. The interaction of NO2 in the atmosphere with sunlight and unsaturated hydrocarbons produces photochemical oxidants ( I ) . Few reliable analytical methods are currently available for rapid, sensitive, and specific determination of NO in air, N2, a n d automotive exhaust. These a r e based either on t h e oxidation of NO t o NO2 and subsequent determination of NO2 or o n the chemiluminescent reaction of NO with 0 3 ( I ). Although the chemiluminescent reaction is commonly used for t h e NO analysis, it requires a n ozone generator and a well calibrated flow system. An alternative method, based on fluorescence detection, has been developed by us mainly from the need of rapid analysis of t h e NO concentration in s t a n d a r d NO-N2 gas mixtures used for calibrating analytical instruments. It has been known for some time that fluorescence is produced when NO is excited with t h e Zn 213.8-nm line ( 2 ) , corresponding t o t h e near coincident transition A2Z(u = 1, K = 29) X2al/2(u = 0, K = 29) ( 3 ) .In t h e presence of a t mospheric Nz, t h e overlap between t h e existing line and t h e NO absorption line increases because of pressure broadening a n d the resulting fluorescence signal is expected t o increase. T h e fluorescence spectrum from t h e NO AQ(u = 1) s t a t e lies in t h e region 213.8 t o 310 n m ( 4 ) . Also, N2 is remarkably inert i n quenching t h e NO A2Z s t a t e ( 5 ) . It is therefore possible t o use t h e fluorescence method for detecting low concentrations of NO i n Nz. T h e present report describes t h e results obtained for such mixtures and the quenching effect of oxygen a n d various gases present in a u tomotive exhaust. T h e method is capable of detecting 0.015 p p m (parts per million by volume) NO in Nz. T h e response
-
curve is nonlinear a t higher NO concentrations because of t h e self quenching (3, 5 ) . T h e method can be used for t h e analysis of NO in automotive exhaust by diluting t h e exhaust gas with Nz and separating t h e NO fluorescence by an appropriate filter from that of SOz, t h e only other known component in automobile exhaust t o emit a fluorescence in this region. A similar technique has been developed for detecting SO2 in air (6) at the p p b level.
EXPERIMENTAL The detection system for measuring NO in pure N2 was identical with that used for detecting SO2 in air ( 6 ) .Briefly, the NO fluorescence was excited by the 213.8-nm line from a commercial Zn glow discharge lamp. A Corning CS 7-54 filter (transmission 230 to 420 nm) was used to isolate the NO fluorescence in the region 230 to 310 nm. The photon counting ratio mode was used in which one photomultiplier monitored the lamp intensity while the second photomultiplier monitored the fluorescence. Both photomultipliers had 13 stages and a S-13 spectral response. The photon counting integration time was of the order of 1 minute. The maximum counting rate was about 60,000 counts per sec which is well below the saturation rate of 20 MHz. The fluorescence spectrum of 100 ppm NO in N2 excited by the 213.8-nm line was taken by a 0.3-m scanning monochromator with a 2400 lines per mm grating. A slit width of 2 mm was used giving a resolution of about 3 nm. The detector consisted of the fluorescence monitoring photomultiplier and the Corning CS 7-54 filter. Sample Preparation. The majority of the NO in N2 measurements was performed in a static system. The outlet port of the fluorescence cell (200 ml in volume) was connected to a vacuum manifold and the inlet port to a gas mixing vessel (5 1. in volume). Both the cell and the mixing vessel could be evacuated separately down to 1-m Torr pressure. A 250-ppm NO in N2 mixture was the initial concentration used for all the measurements in this system. It was successively diluted with water-pumped dry N2 in the mixing vessel to yield progressively lower concentrations. The concentrations were calculated from the pressure readings of a calibrated mechanical pressure gauge prior to and after dilution with N2. Total pressure of the mixtures was approximately 760 Torr. For each measurement, the cell was evacuated and a sample of the NO in Na mixture was introduced into the cell from the mixing vessel. After measuring the NO fluorescence intensity of the sample, the cell was again evacuated. The quenching effect due to the presence of HLO vapor, CO2, CO, 0 2 , H2, C2H4, and C3H8 in the NO in N2 samples was determined in the following manner. The fluorescence intensity of an NO sample from the gas mixing vessel was measured and then the cell was evacuated. A quenching gas at several Torr was introduced into the evacuated cell through the outlet port. Then another sample of the same NO in N2 mixture was introduced into the cell from the mixing vessel. After allowing the gases to mix for about 10 minutes at a total pressure of approximately 760 Torr, a fluorescence measurement was recorded. A comparison between the two measurements yielded the quenching effect of the interfering gas. The gases were reagent grade and used without any further purification. Several measurements of NO in Na mixtures were performed in a flow system where a slow flow of an NO in N2 mixture was mixed with a fast flow of pure NP and then introduced into the cell. The cell was open to the atmosphere and the flow rates were typically 10 l./min and 1 ml/min. The flow rates were measured by a calibrated mass flow meter. The NO in N2 mixtures prior to dilution were primary standards of 1.917, 0.1067, 0.9913, 1.515, and 0.5136% NO in N2. The measurement of NO in air was performed in the same manner with dry air as the faster flowing diluent gas. Rotameters were used to measure the relative flow rates. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 . APRIL 1975
703
240
250
260
270
280
290
300
WAVELENGTH [NANOMETERS)
Figure 1. The fluorescence spectrum of 100 ppm NO in N2 as measured by a combination Corning CS 7-54 light filter and a 13 stage photomultiplier with S-13 spectral response. A 0.34 scanning monochromator with a 2400 lines per mm grating was used at a resolution of about 3 nm. The A, is 213.8nm
RESULTS AND DISCUSSION The fluorescence spectrum of 100 ppm NO in N2 excited by the 213.8-nm line is shown in Figure 1. It is apparent that the spectrum consists of two progressions of about equal intensity, one from u' = 1 and the other from u' = 0. Since the u' = 1 is the only level produced by initial light absorption, u' = 0 must be formed by vibrational relaxation in the presence of N2. From the steady state condition one obtains doZ[NO] = h,[NO*][X]
+ kN2[NO*][N2] -Ik~o[No*][No] + k,[NO*]
(1)
where [NO*] is the NO A2Z+ concentration, c is the extinction coefficient of NO a t 213.8 nm, IO is the intensity of light a t 213.8 nm, 1 is the cell length; k x , NO, kN2, are the quenching rate constants of, respectively, a gas X, NO, and N2, and k f is the decay rate of the NO A22+ state. Since the quenching constants for NO A22 u' = 1and for v' = 0 were identical in NO and COZ (5), it is reasonable to assume that k x for u' = 1 and u' = 0 would be the same for other quenching gases. The integrated fluorescence counts, F, for a period t is
F = tafkf[NO]* (2) where Lyf is a constant representing the fluorescence quantum yield and a geometrical factor. From Equations 1 and 2 F = tdoZaf[NO](l + ax[x] + a,o[NO] a~,[Nz])" (3) where ai = ki/kf ( i = X, NO, N2) is the Stern-Volmer quenching constant in units Torr-'. In the absence of the quenching gas X, Equation 3 becomes where = t t l o l a f . Rearranging Equation 4, one obtains Equation 5
F'= B[NO] - F A I N 0 1 + S ( 5) where E = p(1 a ~ , [ N ~ l ) - A ' , = aNo(1 -t a~,[Nz])-' and S is the background counts of scattered light. Equation 5 gives a nonlinear plot of F" at higher NO concentrations because of a large self-quenching constant (quenching in almost every collision) ( 5 ) .Equation 5 assumes that the absorption of the 213.8-nm light is linearly proportional to NO concentrations over the range studied. Since the quenching constant, a N O , obtained from Equation 5 agrees reasonably well with the literature values, as will be discussed later, the above assumption may be justified. A pro-
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A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 4 , A P R I L 1975
portionality constant B, given in units of counts per concentration, is dependent on instrumental and geometrical factors, while A, in units of the reciprocal concentration, is determined only by the quenching constants a N O , a N 2 and the pressure of Nz. In the five runs of.NO in Nz mixtures, the best least squares fits of the fluorescence measurements to the concentrations were determined by the NBS Univac 1108 computer. The measurements were fitted to Equation 5. The quenching constants ax in units of Torr-l were determined by comparing the fluorescence counts of the NO in Nz sample to that of an identical sample containing a small amount of X. From Equations 3 and 4
F"/F = 1 + U,[x](1 + UNO[NO] + a ~ ~ [ N 2 ] ) - ' (sa) From A, determined from Equation 5, and the known value of a N , (the reciprocal of the quenching half pressure (5) of 1900 Torr), a N O can be obtained. It is then possible to obtain ax from the slope of the plot FOIF as a function of [XI. The Stern-Volmer linearity is obeyed for each quenching gas tested. In the presence of more than one quenching gas, Equation 6a becomes
Error Analysis. The weights in the least squares fit for Equation 5 were determined from the observed standard deviation of F" and an assumed standard deviation UNO in [NO]. At least 10 fluorescence measurements of each concentration were recorded, and the standard deviation, uf, was determined. In the static system, the error in the concentration is accumulative due to successive number of dilutions, n. If the self-quenching effect on the signal is neglected and the reading error on the mechanical gauges is 1%,then the corresponding standard deviation in the signal is
uN0 = F ( 0 . 0 1 ) ( 2 ~ ) " ~
(7)
where F is the fluorescence signal at the concentration corresponding to n. The weight of each fluorescence countconcentration measurement is
After determining the value of A from the least squares fit, NO was corrected at each concentration as follows,
uNo(corr) = (1 + A[NO])'2F(0.01)(2n)''2
(9)
The weights w were correspondingly corrected and the best least squares fits redetermined. The A, E , and F values presented in this paper are from this latter fit. The magnitude of the residual standard deviation of each fit was used to evaluate the completeness of the error analysis. A residual standard deviation greater than 1 indicates that the standard deviations at each concentration were underestimated. A residual standard deviation less than 1 indicates that the standard deviations were overestimated. Furthermore, simulations of two sets of measurements assuming a random error in F and [NO], within the error bounds of each, were run on the computer. The least squares fit was identical to that of the actual measurements. In the flow system, the error in the concentration was determined by assuming an error of 0.5 or 1.0% in each of the mass flow meter readings until the residual standard deviation was -1.0. The corresponding error in the signal F" is
uNO = Fo2''of
(10)
where af = 0.005 or 0.01. After determining A in Equation
4802 -
3870
50
'00
'50
N O IN Y I T R O C - E Y
Figure 2. The signal,
200
-
25s
PPU
F,as a function of the NO in N2 concentration
3039
>E38
0486
c
7 ' -
h O IN N ' - R C G E \ PPZ'
Figure 3. The signal, F, as a function of the NO concentration in the sub-ppm range
5 , C ~ N Owas corrected a t each concentration by the factor (1 A [NO])-2. The weights were correspondingly corrected and the best least squares fits redetermined. The standard deviations of the constants B, A, and S from the least squares fits reflect the precision of the technique. T o determine the accuracy, the concentrations were calculated by
+
error analysis is complete, and the sublinear behavior a t high NO concentrations is not an artifact of the detector but due to self-quenching of the fluorescence signal. The observed standard deviation in the signal a t each concentration was less than 1.0% except in the 10-ppb range where it varied from 1 to 2%. This error is due to statistical fluctuation of the counts integrated over a 1-minute time interval. In the static system, the error in the concentration at 15.3 ppb can be as high as 5.3% according to Equation 7. The error in the 100-ppm range is a couple of percent in the error analysis. In the flow system, Run 5 , an error of 0.5% in the concentration yielded a RSD of 2.37 and an error of 1.0%yielded a RSD of 0.467, indicating that the concentration error in Run 5 is between 0.5 and 1.0%. The signal dependence on the concentration is mainly manifested in the B parameter in Equation 5 . In Runs 1 through 5 , the standard deviation of B varies from 0.4% (Run 5 ) to 3.2% (Run 1).The optimum S/N (signal-to-noise ratio) for a 1-minute integration time can be calculated from the results of Run 4. In Run 4, B = 6023 f 47 counts/ ppm and S = 1247 f 14 counts yielding a counting error of 61 counts a t 1 ppm, thus S/N = 1.0 a t 10 ppb. This lower limit of detection can be extended by minimizing S and maximizing B. The background signal, S , which is partially due to the dark photon counts, can be reduced by cooling the photomultiplier. Since B is proportional to the lamp intensity and the integration time (Equation 5), the error due
(11)
[NO]calcd = (F" - S ) / ( B - AF")
3339
0'89
where F" is the observed fluorescence counts without quenching gas and B, A , and S were from the least squares fits. The calculated [NO] values are compared to the known NO concentrations. NO Fluorescence in Pure Ng. A typical signal response curve as a function of NO concentration in pure N2 is shown in Figures 2 and 3. The sublinear dependence of F" on the NO concentration is apparent in Figure 2. At the lower concentration range shown in Figure 3, a linear dethe pendence is apparent. The intercept in Figure 3 is background signal. The results of the least squares fits for 4 runs in the static system and 1 run in the flow system are presented in Table I. The range of measurements extends from 932 ppm (Run 5 ) to 15.3 pbb (Runs 3 and 4). The photon counting integration time was of the order of a minute in all the runs. The residual standard deviations (RSD) of Runs 1 , 4 , and 5 , respectively 0.848, 0.761, and 0.467, indicate an overestimation of the error in each measurement. The RSD of Runs 2 and 3, however, indicate an underestimation of the error. Since the RSD is close to 1.00 in all the runs, the
s,
~~
Table I. Fluorescence Counts of NO in Ng Mixtures Approximate
Run
Corresp.
decrease
coiicn
of concn
\ i o . oi
rinqi, p p r
a t e a c h dilution
dllutxons
Re. std de\
1
2 50-1 8.6
77
9
0.848
2
149-1.27
77
18
1.26
3
2503.039
59
16
1.08
4
2 5 0 3 . 0 153
50
10 m e a s u r e ments a Flow system run consiting of 10 measurements 5"
932-18.6
B
A
14
0.761 0.467
-1 (PPm I with (std deb)
5.52 x lo-' (24.3%) 1.86 x 10-3 (12.0%) 1.42 x 10-3 (2.97) 1.48 X lo-' (2.9%) 1.03 x 10-3 (1.15)
S
-1
(
~ ppm 0 )
~
~
(counts)
with (std dev)
with (std d a ]
3193 i 103 (3.2%) 2006 + 41 (2.09) 5745 i 42
21,026 i 3448 (16.45) 754.9 + 181 124.05) 911.9 33 (3.7%) 1,247 + 14 (1.1%) 14,722 i 991 (6.7%)
(0.79) 6023 i 47 (0.8%) 3964 + 1 7 (0.4%)
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705
Table 11. Quenching Half Pressures of Some Automotive Exhaust Gases
Table 111. Composition of Automotive Exhaust Gas and the Quenching Effect on NO Fluorescence
Experimental Literature
quenching
quenching
half pressure Quenching gas
N2 HzO
co
COZ O? C2H4 C3H8
H2 NO
a x - 1 (Torr)
...
0.65 + 0.06 8.3 f 0.6 0.65 j: 0.05 1.1 0.1 5.6 i 0.4 56 i 4 1 1 5 80 0.34 It 0.03
*
*
half pressure (Torr)
1900 (5) 0.47 (10) 7.14 0.70 (5) 0.36 (5), 0.62 (10) 0.90 0.10 (9)
...
* *
Approximate composition range in % vol. of automotive exhaust gas (11)
S O iluarcscence
quenching gas
0.003 to 0.3 13.0 to 13.2
NO H,O
co
0.8 t o 5.2 9.5 to 10.2 0 0 to 0.2 0 to 0.2 0.2 to 1.7
CO2 02 C2H4 C3H8
H2
G 2 0 (10) 0.63 (5), 0.29 (10)
F'IF [NO]
PPI"
3000
Before dilution
After 1:lM) dilution with Nz
33.7-35.5 5% change 184-194 5% change
2 .a3-2.9 3 4% change 2.93-3.03 3% change
30 to fluctuation in B can be improved by longer integration time and/or higher lamp intensity (6). The background counts have a standard deviation from 1.1 (Run 4) to 24.0% (Run 2). In Run 3, the background interfere with the NO detection in air (8).This interference counts were determined by filling the cell with pure N1. I t was approximately equal to the S determined by the least may be minimized by using a filter transmitting the region 230 to 300 nm corresponding to the NO fluorescence specsquares fit. ppm-' to 1.42 The A parameter ranges from 1.86 X trum (Figure 1).The SO2 fluorescence maximum is in the X ppm-' in Runs 2-4 while, in Run 1, the A parame300- to 330-nm region (8). The possibility remains of using other light sources to ter is much lower because the range of measurements is in the sublinear region and over only one order of magnitude, excite the NO emission with a greater S/N than with the Zn lamp. Since the NO absorption consists of discrete lines, a and accordingly, S is much larger. In Run 5, one measurecontinuum light source such as a H2 lamp would excite ment was performed at 932 ppm, the rest were from 18.6 to more of the vibrational-rotational absorption lines. The tel316 ppm, again, a range of only one order of magnitude. In lurium resonance line a t 214.7 nm would be more in coinciRuns 2-4, the measurements were performed over a t least dence with the u' = 1 NO line than the Zn 213.8-nm line two orders of magnitude and should give more reliable A (9). Melton and Klemperer have found an increase in the values. Using an average value 1.59 f 0.18 X ppm-I fluorescence signal by a factor of 10 to 100 by shifting a = 1900 Torr ( 5 ) ,UNO-' = for A from Runs 2-4 and 0.34 f 0.03 Torr is obtained from A = a p ~ o / ( l a ~ ~ [ N 2 ] ) . Zeeman component of the Cd I1 226.5-nm or 214.4-nm lines more in coincidence with NO absorption maxima by a sevWith this A value the NO fluorescence signal becomes a eral kilogauss magnetic field (7). linear function of the concentration from 15 ppb to 7 ppm Quenching of NO Fluorescence a n d Detection of NO to within an error of 1%. i n Automotive Exhaust. In order to explore the possibiliSince the flow system Run 5 duplicates the results of the ty of measuring NO in automotive exhaust by the fluoresstatic system, the accuracy of the technique can be determined by the results of Run 5. The Run 5 values for F",s, cence method, the NO quenching efficiencies of several gases present in the exhaust have been measured. The B, and A were substituted into Equation 11 and the NO pressure of various gases that quench the NO fluorescence concentration was calculated. Out of 10 calculated concenintensity by 50% are presented in Table 11. They are calcutrations, 9 were within 1%of the actual concentrations and lated from Equation 6a. The values UNO-' = 0.34 f 0.03 one calculated concentration was 1.2% greater than the acTorr obtained in this work and = 1900 Torr from tual concentration. In the static runs, 65% of the calculated Callear and Pilling ( 5 )were used. concentrations were within 3% of the actual concentrations. The quenching half pressures of NO, HzO, COS, CO, 0 2 , The concentration error in the static runs is from 1 to 5.3% HS, and N2 are in reasonable agreement with the literature and, in the flow system, it is between 0.5 and 1.0%. values. It is apparent from the Table that the most efficient Fluorescence signals from several NO in air mixtures quenching gases are HzO, CO2, 0 2 , and NO, while CO and were also measured. They were 997 f 24 counts a t 19.3 C2H4 are only moderately efficient. Hydrogen and propane ppm, 597 iz 32 a t 11 ppm, and 254 f 17 at 6 ppm. The are poor quenchers. The values for CzH4 and C3Hs have apbackground signal which is already subtacted from the parently not been measured previously. counts was 145 f 5 counts. At 6 ppm, the total standard Because of the presence of large concentrations of deviation of the background plus fluorescence counts is 18, quenching gases, mainly HzO and C o n , in automobile exyielding a signal to noise ratio of 12, i.e., at 0.5 ppm, S/N = haust, the fluorescence signal, F, of NO measured in the 1. The integration time for the counts was 1 minute. The automobile exhaust would be considerably less than the signal response is almost linear with concentration. The signal, F", from the same concentration of NO present in linear response of the fluorescence counts in NO-air pure N2. The approximate composition of automotive exmixtures can be explained by the high quenching efficiency haust gas is given in Table 111. The effect of the main of the NO fluorescence by oxygen (7), that is, in Equation quenching gases on the fluorescence signal, when the com3, the dominating term is ao2[02]since ao2= 3 a ~ and o [02] >> [NO] in air, Therefore, the denominator is invariant position changes within the stated range, can be calculated using Equation 6b. The ratios, Fo/F,are given at two NO with respect to the NO concentration. Thus, the detection concentrations, 3000 and 30 ppm, in Table 111. If the exlimit is reduced to lij0 that of NO-N2 mixtures. Furtherhaust gas is not diluted by N2, the signal reduction is highly more, even a small amount of SO2(1/500 that of NO) would
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
dependent on NO concentration ranging from 2.8% a t 3000 ppm to 0.53% at 30 ppm although the ratio is not sensitive to the change of composition (within 5%). On the other hand if the exhaust gas is diluted by 1:lOO with Nz, the ratio F"/F, is not much dependent on the NO concentration, that is, the signal is reduced to 34 f 1%of F" in the exhaust gas, independent of the change in NO concentration and in composition. A signal decrease caused by dilution is compensated by an inciease of signal due to an increased N2 concentration. It is still possible to detect the fluorescence signal of 30-ppm NO in the exhaust after 1: 100 dilution where the signal becomes equivalent to 0.1ppm NO in pure N2. Thus the dilution of the exhaust gas by N2 provides an almost linear calibration curve for measuring NO in the exhaust in the range 30 to 3000 ppm. Dilution by N2 has the further advantage that the condensation of H2O can be avoided when the gas mixture is brought to near room temperature for the NO measurement. To minimize an interference by SO2 which may be present in the exhaust gas, a filter transmitting predominantly the NO fluorescence (230-300 nm) and rejecting most of the SO2 fluorescence (250-420 nm) may be required. Alternatively, if the exhaust gas is diluted by air or by oxygen, and a filter transmitting light of wavelengths 300 to 420 nm is used, it is possible to measure SO2 as well.
ACKNOWLEDGMENT The help of William H, Kirchhoff in the error analysis is gratefully acknowledged. The flow system run was performed by Ryna Joseph Marinenko of the Analytical Chemistry Division.
LITERATURE CITED (1) Air Quality Criteria for Nitrogen Oxides, EPA(US) Air Pollution Control Office, Washington, D.C., Publ. NO. AP-84, 1971. (2) D. R . Crosley and R. N. Zare, Bull. Am. Phys. Soc.,-12, 1147 (1967). (3) L. A. MeRon and W. Klemperer, J. Chem. Phys., 59, 1099 (1973). (4) R. W. B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra." John Wiley & Sons, New York, N.Y.. 1963. (5) A. B. Callear and M. J. Pilling, Trans. Faraday Soc., 66, 1618 (1970). (6) F. P. Schwarz. H. Okabe, and J. K. Whittaker, Anal. Chem., 46, 1024 (1974). (7) L. A. Melton and W. Klemperer, Planet. Space Sci.. 20, 157 (1972). ( 8 ) H. Okabe, P. J. Splitstone, and J. J. Ball, J. Air Poilut. Control Assoc., 23, 514 (1973). (9) Private communication from R. N. Zare of Columbia University to H. Okabe. (10) A. V. Kleinberg, and A. N. Terenin, Dokl. Akad. Nauk, SSR, 101, 1031 (1955). (11) "Outlook-Electric Vehicle Revival of the Fifty-Year-Old Memory," Environ. Sci. Techno/., 1, 192 (1967).
RECEIVEDfor review October 11, 1974. Accepted December 19,1974. This work was supported by the Measures for Air Quality Program at the National Bureau of Standards.
Automated Computer-Controlled Spectrophotometer System for Kinetic or Equilibrium Methods of Analysis K. R. O'Keefe and H. V. Malmstadt' School of Chemical Sciences, University of Illinois, Urbana, Ill. 6 180 1
An automated computer-controlled spectrophotometer system is described that is applicable for stopped-flow kinetic or equilibrium methods of analysis. The system provides rapid sequential analysis using rate measurements in the time range of milliseconds to minutes or accurate equilibrium absorbance measurements of stable sample constituents. Only small volumes of composite reagent and sample, about 0.17 ml of each, are required for each complete fill/ inject cycle of operation. Absorbance changes of 0.00004 A are readily measured in the 2-cm pathlength cell by the split-beam dual-detector system. The automated analyzer has been programmed for use in either a routine analytical mode or in a research investigative mode. New light source and beam splitter modules that are compatible with other commercial spectrometer modules are described. Experimental results with the complete system compare closely with the theoretical results calculated on the basis of photon statistics.
In recent years, the use of reaction rate methods of chemical analysis has become increasingly popular, especially in laboratories where advantages in speed and selectivity over equilibrium methods are important ( I , 2 ) . The current interest in rate methods is attested to by several recent reviews covering reactions that are useful for analytiSend reprint requests to this author.
cal purposes (3, 4 ) and instrumentation and methodology for rate measurements ( I , 5, 6). Routine reaction rate methods on time scales from several seconds to minutes have been available for many years (1-3), and rate methods on the millisecond-to-second time scales have been reported (7, 8). In addition, the introduction of automated systems for rate measurements has had considerable impact in this area (3,6). In this report, a new system for reaction rate measurements is described that is useful for both the fundamental characterization of chemical reactions and the application of both rate and equilibrium methods to chemical analysis. A dual-beam optical arrangement is used that eliminates source fluctuations as a source of photometric error in absorbance measurements. This allows the use of an unregulated high intensity source while still permitting high precision absorbance measurements. The measurement system for the spectrophotometer is described and is shown to introduce negligible error into the measurement. The sampling-mixing system is a new automatic stopped-flow head which requires no operator attention during normal use. All control functions are readily implemented, and are TTL-compatible. The stopped-flow head provides for thermostating of the mixing syringes and optical cell and thermistor measurement of the mixed solution temperature. Volumes of sample and reagent that are required for each sampling-mixing cycle are 0.17 ml. System dead time is 5 f 2 msec. The entire system is automated using a PDP-8/f miniANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
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