DISCUSSIONS The error due to the drift of the X-Ray Spectrometer within a period of one hour was measured to be 0.045%. This value seemed to be in agreement with the value of 0.05% given in the Norelco instrument manual (3). For longer preset counting times, although the relative error due to the X-ray counting will decrease, long term instability which usually has several times the magitude of short term instability will start to exert its influence. The report of the ASTM task group on X-ray Fluorescence Spectroscopy on the first X-ray fluorescence roundrobin test ( 4 ) showed a value of the order of 0.5% for the voltage and current reset error. For each of our measurements, instead of simply lowering the high voltage and tube current and resetting back to the specified values for the measurements, the instrument was turned on, allowed to warm up, set to the specified voltage and current reading, and then turned off after the reading was taken. This was done for six consecutive days. In spite of this, the error of 0.43% thus obtained seemed to be in good agreement with ASTM's value of 0.5%. The ASTM task group also reported the value for the error of the spectrometer repositioning to be 0.11% ( 4 ) . Our
value of 0.030% seemed considerably smaller. The higher ASTM value could be attributed to the differences between the instruments and operators of the ASTM task force. The errors due to inhomogeneity of the sample, the placement of the sample in the sample holder, and the sample preparation are the most disturbing errors. It appears that pellets prepared from a sample which had been ground to pass a 200-mesh screen and carefully blended still would produce deviations of 0.68%. Still more disturbing was the 0.12% deviation encountered when the same sample was removed and reinserted in the sample holder. Finally, a deviation of 0.15%which resulted from the orientation of the sample in the X-ray beam is extremely difficult to understand; particularly, if the pellet had a smooth face and no measurable difference in thickness from one side to the other. As expected ( 5 ) ,the variations due to sample preparation contribute significantly to the total variation of our method in X-ray fluorescent analysis. A day-to-day reset of the voltage and current reading contributed to an error of 0.43%. This value seemed to be well within the accepted range, thus allowing the construction of a single calibration curve which would be good for routine determinations on different days.
RECEIVEDfor review March 14, 1974. Accepted June 26, (3) Norelco Electronic Circuit Panel Instruction Manual, p 6, Philips Electronic Industries Corp., 750 South Fulton Ave., Mount Vernon, N.Y. 10550. (4) ASTM Task Group on X-ray Fluorescence Spectroscopy, Appl. Spectrosc., 13, 3 (1959).
1974. (5) E. F. Kaelbe, "Handbook of X-rays," McGraw-Hill, New York. N.Y., 1967, Chap. 33, p 4.
Optimization of the Operating Parameters of Chemiluminescent Nitric Oxide Detectors D. M. Steffenson'
and
D. H. Stedman
Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48 104
The parameters that affect the sensitivity of a typical chemiluminescent NO detector have been investigated. Using a 54.8 ppm NO in N2 calibration gas as the NO source and maintaining a constant flow ratio of cal gas:ozonized oxygen of 2:l into the reactor, the detector signal was measured as a function of reactor pressure at several different pumping speeds in different reactors. The results are consistent with an analysis of the kinetic limitations on the chemiluminescent intensity, which shows that the Detector Signal = (Reactor Gas Flow/Reactor Pressure) (G) ( 1 exp(-Treact/TNO)) where G is the geometry of photon collection of the reactor and exp(-T,eact/TNO) is the fraction of NO molecules whose residence lifetime in the reactor ( T , ~ is~ short ~ ~ compared ) to their reactive lifetime with O3 ( T ~ ~From ) . this equation, and results using different reactor designs and different photomultipliers, it is shown that the relevant parameters for optimizing the detector are pumping speed, reactor size, O3 flux, reactor design, and choice of photomultiplier. The choice of these parameters is discussed and they are somewhat different for continuous sampling than for sampling from finite sources.
* Present address, Department of Chemistry, Albion College, Albion, Mich. 49224. 1704
The use of chemiluminescent nitric oxide detectors has grown rapidly in the past few years, particularly as air pollution monitoring instruments for atmospheric concentrations of NO,. A recent review of chemiluminescent detectors used in the measurement of air pollutants ( I ) lists nine different manufacturers of commercial NO, detectors even though the feasibility of such detectors was established by Fontijn e t al. (2) only in 1970, and the prototype for a number of these commercial instruments was developed in 1972 (3). Besides their application as monitors for atmospheric NO, these detectors have been used as laboratory analytical instruments for measuring NO as a product or reactant in standard kinetic or photochemical experiments (4).
In spite of the growth and rapid development of commercial instruments, there has been little systematic study of the physical and kinetic parameters that determine the optimum sensitivity of the NO detector. The basic design and the components of the detector have been established, but there remains a wide latitude of choice of the operating capabilities of each component. These choices must also be (1) R. K. Stevensand J. A. Hodgeson, Anal. Chern., 45, 443A (1973). (2) A. Fontijn, A. J. Sabadell, and R. J. Ronco, Anal. Chem., 42, 575 (1970). (3) D. H. Stedrnan, E. E. Daby, F. Stuhl. and H. Niki. J. Air Pollut. Contr. Ass., 22, 260 (1972). (4) D.H. Stedrnan and H. Niki, Environ. Sci. Techno/.,7 , 735 (1973).
A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 12, OCTOBER 1974
compatible with the basic kinetic limitations of the chemiluminescent NO-03 reaction. Furthermore, operating conditions for use in atmospheric gas monitoring, where the supply of sample is virtually unlimited, are not necessarily ideal for use in the laboratory to detect NO in a limited amount of sample. In this work we have systematically studied the operating parameters and reactor design that affect the intensity of the chemiluminescence that reaches the photomultiplier tube of the detector. This intensity was found to be a function of pumping speed and the flow parameters of the 54.8 ppm NO calibration gas and ozonized oxygen into the reactor in a way that is quite consistent with the proposed kinetics and mechanism of the reaction. The intensity is also a function of physical parameters, particularly reactor design, that facilitate getting the light to the photocathode of the photomultipler. Finally, we made a few experiments with a different photomultiplier to demonstrate how important the choice of photomultiplier is to the ultimate sensitivity of the detector.
EXPERIMENTAL T h e schematic design of‘ t h e NO/O? chemiluminescent detector used in these experiments is identical to those published earlier ( I , 3),a n d is similar t o those of commercial instruments. T h e photomultiplier used in most of t h e experiments was an RCA 8852 with a n E R M A 111 response t h a t extended its range t o 900 nm. I t was Dry Ice-cooled and nitrogen gas from a tank was slowly passed through a small space between t h e reactor window and t h e photomultiplier window t o prevent frost formation. A Kodak Wratten gelatin filter was used to remove a n y emission below fi00 nm. A few experiments were made using an Amperex 150 CVP photomultipler with an S-1 response, and nitrogen gas vaporized from a Dewar of liquid nitrogen was used for cooling. T h e power supply for the photomuliplier was a Heath EU-42A variable high voltage power supply with a maximum output of 1500 volts. T h e photocurrent was measured with a Keithleg 414s picoammeter whose outp u t was displayed o n a Honeywell 10-mV strip-chart recorder. A Cenco Hyvac 14 p u m p was used t o maintain t h e flow of 54.8 ppni NO cal gas a n d ozonized oxygen through the reactor, a n d a bellows valve was installed in front of t h e p u m p t o vary t h e pumping speed. An ozone killer was placed in front of t h e bellows valve to protect t h e p u m p oil. This was a 20-cm piece of 17-mm o.d. Vycor tubing t h a t had been wound with asbestos tape and several feet of nichrome wire. T h e wire was connected directly t o t h e 110volt ac line, and it was turned on simultaneously with t h e ozonizer. A large piece of copper screen was rolled u p and placed in t h e Vycor tube to provide a hot surface for the ozone destruction. T h e flows of the two gases into the reactor were controlled with Teflon needle valves and the flow rates were measured with Gilmont No. 11 or No. 12 flow meters. T h e flow meters were calibrated with a wet test meter or by displacing water from a volumetric flask. For these investigations, t h e instrument was used as an NO detector with a tank of Linde 54.8 ppm NO in N2 as t h e NO source for all of the experiments. Oxygen was passed through a n ozonizer operated a t 9000 volts, through a flow meter, and then into the reactor. T h e pressure in the reactor was measured with either a mercury or oil manometer using a cathetometer to read t h e manometer levels to 0.0j mm. Four different reactors were used in these experiments t o test design features t h a t might increase the sensitivity of t h e detector. T h e design and dimensions of the two reactors used for most of t h e experiments are shown in Figure 1. T h e smallest reactor. referred to hereafter as t h e “brass reactor”, effective volume about 36 cm:’, was designed t o facilitate rapid and effective mixing of NO and 01 with t h e gases emerging from small jets interspersed with one a n other and surrounded by a glass cylinder t o guide t h e reactants toward the front window before being pumped down t h e sides and o u t t h e back. T h e other reactor, referred to hereafter as t h e ”glass reactor”, volume about 300 cm:’. was designed t o mix t h e reactant gases as close t o t h e window in front of t h e photomultiplier tube as possible. T h e other two reactors were also made of Pyrex glass. and were similar to t h e glass reactor in Figure 1. One was identical excepl’ t h a t t h e outlet t o t h e p u m p was two 10-mm tubes exiting a t the front of t h e reactor near t h e window. T h e other was larger.
i WINDOW
w GLASS REACTOR +3 8 ern-
SIDE VIEW
FRONT VIEW
BRASS
REACTOR
Figure 1. Cross-sectional designs of the two main reactors used in the NO detector Both the window and the internal glass cylinder of the brass reactor had a diameter of 3.8 cm. which limited the effective reaction volume open to the photomultiplier
Figure 2. NO detector signal as a function of reactor pressure at three different pumping speeds for the glass reactor A ,data at 136 I./min: 0, data at 63 I./min; and 0 . data at 30.5 I./min
about 20 cm long and about 400 cm“ volume, h u t the gas inlets had only one hole in t h e end and they pointed straight forward with a separation approximately four times t h a t of t h e glass reactor. This reactor provided t h e poorest mixing of NO and 0.4. With both t h e glass reactor and t h e brass reactor, a t t h e highest pumping speed, t h e maximum photocurrent was obtained with a cal gas:OZ flow ratio of 2:l (where 0:’ refers to the 0210.3 mixture from t h e ozonizer). This flow ratio was used for all of the experiments. T h e pumping speed was varied over a range of a factor of ten, so t h a t t h e total gas flow in t h e reactor. as measured by t h e calibrated flowmeters and corrected for t h e reactor pressure, varied from around 170,000 ml/min t o 17,000 ml/min. T h u s , the residence time of the gases in t h e reactor ranged from about 0.11 sec to 1.1sec for t h e glass reactor and from about 0.012 sec to 0.12 sec for the brass reactor. In almost all of these experiments, t h e high voltage power supply for t h e photomultiplier was set a t 1140 volts. A few measurements were made a t 1500 volts. Usually the photomultiplier was cooled with Dry Ice, but cooling was not critical since t h e cooled and uncooled signals were identical and only the dark current was reduced.
RESULTS The experiments were designed to measure the intensity, I , of the chemiluminescence for the 54.8 ppm NO sample as a function of reactor pressure. This pressure was varied by
A N A L Y T I C A L C H E M I S T R Y , V O L . 46. N O . 12. OCTOBER 1974
1705
Table I. Effect of Reactor Design and Photomultiplier Voltage o n the Signal, Dark Current, and Noise of a Chemiluminescent NO Detector RCA 8862, ERMA 111 Response 1140 volts
Reactor
Signal, A
1. Brass (36 em3) (see Fig. 1) 2. Glass (300 ema) (see Fig. 1) 3. Glass (300 cm3) (same as 2 with front pumping) 4. Glass (400 em3) (same as 2 with poor gas mixing) 5. Glass (300 em3) (same as 2 with reflective coating)
6.0 18.0 3.2 6.5 60.0
X
x x
lo-' 10-9 10-9
X X
1500 volts
Dark current and noise, A
Signal, A
2 i X lo-'* n.m. n.m. 8 i4 X 2 f 1 x 10-12
6.2 X 3 . 2 x 10-7 n.m. 1 . 4 X lo-' n.m.
Dark current and noise, A
4 i4 3 &
x
10-11
n.m. n.m. 2 x 10-11 n.m.
Amperex lBOCVP, S - 1 Response
6. Brass (same as above)
70 X
n.m.
9 i0 . 4 X
n.m,
The dark current is very dependent on the cooling efficiency and this represents the lowest value found over a four-day period. On other days, it was as much as 20 times higher. a
The effect of reactor designs on the sensitivity of the detector is shown in Table I along with data for the dark current and noise of the photomultiplier. Table I also shows the change in signal and dark current achieved by an S-1 response photomultiplier.
DISCUSSION
REACTOR
PRESSLRE,
The main parameters that can affect the signal intensity of the detector can be understood through the kinetics and mechanism of the NO-03 reaction, and the relationship of the residence lifetime of the reactants in the reactor to their kinetic lifetime. The mechanism of the reaction between NO and 0 3 has been previously established (5, 6 )
torr
Figure 3. NO detector signal as a function of reactor pressure at three different pumping speeds for the brass reactor
NO NO
A, data at 136 I./min; 0, data at 40.5 I./min; and 0 ,data at 16 I./min. The last 0 at the 16 I./min pumping speed represents experimental points at 71 and 108 Torr with that ordinate value I
t
O3
NOz*
---
+
(1)
(2)
1 2 ~
(3)
NO2 + M
(4)
NO2
M
+
O2 O2
NO2
+
where N02* is an excited electronic state which emits radiation between 600 and 3000 nm with a maximum at 1200 nm. The rate constants for Equations 1 and 2 were meacm3 molecules-' sec-l sured to be h , = 1.1 f 0.6 X and h 2 = 1.4 f 0.6X cm3 molecule-1 sec-l. If the only important reactions were 1 and 3, then all of the NO molecules entering the reactor would lead to emitted photons, thus
where I is the intensity of the signal in photons sec-' and {NO is the mass flow of NO into the reactor in molecules sec-'. In fact, the above kinetics impose three constraints on Equation 5 . 1) Only a fraction, hll(h1 h 2 ) = 0.073, of NO 0 3 encounters leads to excited N02*. 2) Quenching of N02* allows only a fraction, h 3 / ( h 3 h4[M]) = 8.3 X lo-" a t 1 Torr (61, of the possible photons to be emitted. 3) The rate of reaction of NO 0 3 is relatively slow, which allows a fraction of the entering molecules, exp to leave the reactor untouched, or allows only a fraction, (1 - exp (-7react/7NO)), of the entering NO molecules to react inside the reactor. Here TNO, the reactive lifetime of NO, is the inverse of the pseudo-first-order rate constant for the reaction of NO in the presence of excess 0 3 ,
+
+
PRESSURE, tor.
Figure 4. Pumping speed as a function of reactor pressure These data correspond to the photocurrent vs. reactor pressure curves of Figures 2 and 3. 0, data collected with the glass reactor; and A , data collected with the brass reactor. Pumping speed can be converted to F/P in units of cm3 sec-' torr-' by multiplying the numbers along the ordinate by 7.85
changing the total flow of reactants through the reactor. The results are shown in Figures 2 and 3 for the glass and brass reactors, each at three different pumping speeds. The pumping speeds were determined from the total flow and the pressure in the reactor, and they are plotted in Figure 4 as a function of pressure. 1706
+
NO2*
O3
NO2*
I
r
+ +
+
+
7No
= l / ( ( k l + k2)[031), sec
(6)
(5) M. A. A . Clyne, B. A . Thrush, and R. P. Wayne, Trans. Faraday Soc., 60, 359 (1964). (6) P. N. Clough and 8 . A. Thrush, Trans. Faraday SOC.,63,915 (1967).
A N A L Y T I C A L C H E M I S T R Y . VOL. 46. NO. 12, OCTOBER 1974
and TreaCt, the residence lifetime of the reactants in the reactor, is determined by reactor size and the flow rate in the reactor,
where A is the cross sectional area of the reactor in cm2, d is the reactor length in cm, P is the gas density in the reactor in molecules ~ m - and ~ , F is the total flow in molecules sec-I. The above expression for the fraction of unreacted NO molecules derives from the pseudo-first-order kinetics of Equations 1 and 2. The fraction of unreacted NO molecules present a t the end of the reactor is
where t = Treact. Therefore, with the above three constraints on Equation 5, the light emitted from the reaction zone becomes
photons per second. In practice, we cannot collect all of the photons emitted, so the signal from the photmultiplier is not Itot,but has been attenuated by a further factor G which includes the geometry of photon collection and photomultiplier characteristics. Since k4[M] >> k 3 and [MI is the total gas density in molecules cm-3, Equation 9 can be written
This is the signal measured by the picoammeter and has been plotted as a function of reactor pressure in Figures 2 and 3 . For the moment, assume Treact >> T N O , so that most of the NO molecules react inside the reactor, and we can neglect the exponential portion of Equation 10. In this case, the signal is directly proportional to f NO/P or Ft,,JP since the NO cal gas flow was a fixed fraction (%) of the total flow. One can enhance the signal by increasing NO, decreasing P, or increasing fNO/P. For NO detectors with a constant volume vacuum pump, f N O and P are not independent of one another. If one doubles the flow of NO molecules into the reactor, one simultaneously doubles the gas density. One can increase the signal only by increasing f NO/ P. Since f ~ o / P is in units of cm3 sec-1, it is a measure of pumping speed and it can be increased by increasing the speed of the pump. Figure 4 shows the different pumping speeds utilized in these experiments. Comparison of these data with the signal intensities in Figures 2 and 3 gives excellent qualitative agreement as predicted by the above analysis. Decreasing pumping speed does decrease the photomultiplier signal. The fall-off in pumping speed a t low pressures, which is matched by an identical fall-off in signal, deserves some comment. The speed pressure curve for the Cenco Hyvac 14 (140 I./min maximum pumping speed) indicates that the pumping speed should be almost constant in this pressure region, and such a fall-off should not occur until Torr. However, a t low pressures, pumping speed is often limited by the resistance of smaller tubing in part of the pumping
system ( 7 ) , and here the tubing from the reactor to the pump, including the ozone-killer section, filled with wire, in front of the pump, causes the early onset of the fall-off in pumping speed a t low pressures. Quantitatively, on the basis of Equation 10, one would expect a plot of signal us. pumping speed to be linear with a zero intercept and a slope that varies with G, the geometry of photon collection for a particular reactor. Such a plot, using the data from Figures 2 and 4 for the glass reactor, was indeed linear with a zero intercept. For the brass reactor, using the data in Figures 3 and 4, only the data for the lowest two pumping speeds falls on a straight line through the origin. As predicted, the slope of this line for the brass reactor was different from that of the glass reactor, but the signal for the highest pumping speed was 2.5 times lower than that predicted by this plot. Clearly, a large fraction of the possible signal had been lost. T o understand the strong attenuation in the signal in the brass reactor a t high pumping speeds, we must examine the (1 - exp(-Treact/TNO)) term previously neglected in Equation 10. If the residence lifetime of NO and 0 3 in the reactor gets short, compared to the reactive lifetime of NO, as one might find with high pumping speeds in a small reactor, then some fraction of the photons are emitted outside the reactor, and the signal is attenuated by the fact that only a fraction of NO molecules, (1 - exp ( - T ~ ~ J T N o ) ) will actually emit light within sight of the photomultiplier. Thus, one cannot increase the sensitivity of an NO detector without limit by just increasing the pumping speed. One must consider the limitations of T~~~~~ and T N O in optimizing the NO detector signal. From Equation 7 , one can see that TreaCt is inversely proportional to FIP and thus is a constant for a given pumping speed, except a t low pressures where it increases as pumping speed falls off. In the experimental section, it was noted that Treact was 0.11 sec for the glass reactor and 0.012 sec for the brass reactor a t the highest pumping speeds in the pressure region where FIP is constant. Equation 6 gives the expression for T N O , which can be put in a more useful form by expressing [ 0 3 ] as
where P is the gas density in the reactor in molecules cm-ci, % O3 is the mole fraction of 0 3 produced in the oxygen streaming through the ozonizer, and fo2IF is the fraction of the total flow in the reactor, that is 0 3 0 2 . Substitution of Equation 11 into Equation 6 gives
+
+
Since ( k 1 k 2 ) and FIP are constant, the important variable is the product of the oxygen flow in the reactor and the mole fraction of O3 produced in the oxygen. This, however, is also constant rather than variable. Using an ozonizer similar to the one we used in our NO detector, Jack Horvath a t the Space Physics Research Laboratory a t the University of Michigan measured the mole fraction of 0 3 in 0 2 as a function of foz a t several ozonizer voltages. He found that the mole % of 03 decreased as the flow of oxygen through the ozonizer increased, and the ozonizer produced a constant flux of 0:j (8).A t 9000 volts (the operational voltage of our ozonizer), the (% 03) ( 1 0 2 ) product was effectively constant a t 4.50 f 0.18 cmi3/min,over an 0 2 flow range of 60 to 2160 cm3/min as measured on the ex-
( 7 ) H. W. Melville and E. G. Gowenlock, "Experimental Methods in Gas Reactions," Macmiilan & Co. Lid., London, 1964, p 41 ( 8 ) J. Horvath. Space Physics Research Lab., University of Michigan, Ann Arbor, Mlch., personal communication, 1974.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 12, OCTOBER 1974
1707
ternal flow meter. This covers all but the lowest flows of oxygen used in our experiments. Thus T N O should be constant for a given pumping speed except a t low pressures. For the highest pumping speeds in the pressure region where F / P is constant in either the brass or glass reactor, T N O is 0.073 sec using the values of (% 0 3 ) ( f o 2 ) from the calibrated ozonizer. Since both rreaCtand TNO are constant when F / P is constant, their ratio is also constant:
The NO detector signal, Equation 10, will be optimized if rreact/TNO is large so that exp(-rreact/rNO) is negligible. This fraction is negligible in these results except for the brass reactor a t the highest pumping speed where the signal is severely attenuated compared to the glass reactor. According to Equations 10 and 13, this signal loss is due to the smaller volume of the brass reactor and the subsequent escape of unreacted NO from the reactor. The easiest way to keep Treact/TNO large in an NO detector is to make the reactor volume ( A d ) large. This will help compensate for the fact that an increased pumping speed, FIP, which has been shown to directly enhance the signal, can also decrease the signal by lowering this ratio by a factor of ( P / F ) 2 .One might further increase this ratio by increasing the efficiency of 0 3 production which would increase the % 0 3 produced for a given 0 2 flow. This would only be important if the detector is operated in a region where Equation 13 is important and the signal thus gained is significant. A t this point, one might be tempted to optimize an NO detector by using a very high speed pump and a large volume reactor, but there is a third important factor, G in Equation 10, that must also be considered. This factor represents the geometry of the photon collection system and the photomultiplier characteristics. These have been studied by varying the reactor design and by changing photomultiplier tubes. The goal of good reactor design is to mix the entering flows of NO and 0 3 very quickly and efficiently and then allow them to react as near the window to the photomultiplier as possible. This is because chemiluminescence is a diffuse source and cannot be focused by lenses or mirrors onto the photocathode. Also, as the reactants flow through the reactor, the intensity of the light reaching the window falls off with the square of the distance. This is the reason one cannot continue to compensate for high pumping speed with large reactors. The ratio of rreact/rxO may be kept large but some fraction of the reaction occurs far enough from the window to be effectively lost to detection. The results in Table I with four different reactors support the above analysis. The best reactor was the glass reactor that mixed the reactants well in front of the window and allowed them to flow down the length of the reactor. Pumping them forward and out the side was 5-6 times less effective, and using a larger reactor with less efficient mixing gave only a third of the glass reactor signal. As already indicated, the brass reactor was too small, but its efficient mixing system and forward pumping might work well in a larger reactor. The best improvement in signal was produced by coating the glass reactor with Eastman Kodak reflectance paint which increased the signal by a factor of three. The choice of photomultiplier tube is very important, but we did not have the resources to undertake any systematic study of this parameter. Ideally, one would like to have a photomultiplier with a spectral response curve that maximally overlaps the chemiluminescence spectrum and one 1708
with a low dark current and noise. On the basis of spectral overlap, most photomultipliers fall short of the NOz* emission which peaks a t 1200 nm. The ERMA-I11 response of the RCA 8852 photomultiplier extends about as far into the infrared as any (900 nm) except the S-1 response of the Amperex 150 CVP, which extends to 1080 nm. This is shown in Table I by an order of magnitude increase in signal using this latter tube with the brass reactor. However, a t its best, the S-1 photomultiplier has a dark current 45 times larger than the ERMA-111. For maximum detector sensitivity for NO samples a t ppb concentrations, one must be concerned about the background noise of the photomultiplier, and the important characteristic is the signal/noise ratio of the tube. The noise was such a strong function of cooling efficiency for the S-1 response photomultiplier that its signal/noise ratio varied from 1.7 X lo4 to less than lo3 for the 54.8 ppm NO on NS sample. The ERMA-I11 response photomultiplier has the best signalhoke ratio for this sample for the reflectance painted glass reactor 6 X lo4, and it seems to be the better of the two for use in an NO detector. It should be noted in Table I that one can increase the signal by increasing the operating voltage of the photomultiplier, but the noise is increased by the same factor and the signal/noise ratio remains about the same.
CONCLUSIONS We can now summarize the choice of parameters for the optimization of the signal in an NO detector. They will be somewhat different for a detector designed for continuous sampling of NO where there is an infinite sample of gas available than for a detector designed to measure the NO concentration in a kinetics experiment or in a smog chamber where the amount of sample is finite and one may want to disturb the gases as little as possible. First, one should maximize the pumping speed within the constraints of being able to supply enough reactants to keep the reactor pressure in the plateau region such as in Figures 2 and 3. For atmospheric sampling, one does not care if the fall-off in pumping efficiency of the system comes a t high pressures because one can supply enough sample to keep the pressure above this point. For laboratory sampling, however, one wants to have a high speed pumping system that falls off a t a very low pressure so only a minimum of sample is required to keep the reactor pressure in the plateau region. Second, one should choose a reactor that is large enough to keep the residence lifetime, rreact,large compared to the reactive lifetime, T N O . There is a compromise between maximizing pumping speed and minimizing Treact/TNO because the signal intensity decreases by a factor of the inverse of the distance squared of the excited molecules from the photomultiplier, and there may be an upper limit to the usable pumping speed unless one can lower T N O by increasing the ozone flux into the reactor. Third, reactor design should provide rapid, efficient mixing as close to the window as possible. Furthermore, the walls of the reactor should be made as reflective as possible by coating with reflectance paint. I t is difficult to put a mirror deposit on the inside of the reactor because of the large amount of ozone passing through it. Finally, choose a photomultiplier with an extended response into the infrared that has a low dark current and low noise. One might be tempted to choose a photomultiplier with a larger diameter photocathode, to collect more light, but the noise increases as the square of the radius and one does not necessarily improve the signalhoke ratio. Ultimately, the quality and characteristics of the photo-
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 12, OCTOBER 1974
multiplier may be the most important factor in extending the sensitivity of an NO detector to lower NO concentrations.
RECEIVEDfor review February 19, 1974. Accepted June 17, 1974. The financial support of Grant 802418 from the Environmental Protection Agency is gratefully acknowledged.
Analytical Ion Cyclotron Resonance Spectrometry. Stereochemical Effects in Some Cyclic Ketones Maurice M. Bursey,' Jar-lin Kao, John D. Henion, Carol E. Parker, and Tai-In S. Huang Venable and Kenan Chemical Laboratories, The Universify of North Carolina, Chapel Hill, N.C. 275 74
The relative rates of acylation by CH3COCOCH3.+ of 2-alkylcyclohexanones and decalones in the gas phase do not agree with predictions of the three currently used theories. An overriding parameter which correlates well with steric blocking of the carbonyl group exists. Because the a-decalones can be distinguished on this basis, ICR is superior to conventional mass spectrometry, where their differences cannot be predicted on an a priori basis.
The potential use of ion cyclotron resonance (ICR) spectrometry for analytical purposes has been explored in several laboratories (1-3). One advantage of ICR spectrometry over conventional mass spectrometry lies in its use of ionmolecule reactions (Equation 1) in the gas phase to distinA-B'
+
C-D +A
A-B+
+
C-D - A
+ B-C-D' + B-C' + D
(1)
and more recently have found that the acetylation of pyridines is likewise reduced when a 2-alkyl substituent is large ( 6 ) , even when the reaction is somewhat different (Equation 4).
NCOCH,
A'
OH 2
1
+B
A particularly striking example of the applicability of ICR spectrometry to analysis is the identification of olefinic and cyclic isomers of C5H10, whose conventional mass spectra are very similar (2). Our own interest in applications of this technique derives from the apparent dependence of some ion-molecule reactions on steric environment. Some ion-molecule reactions, observed in conventional chemical ionization mass spectrometry, lead t o ions whose further fragmentation is a strong function of stereochemistry ( 4 ) . We have, on the other hand, been interested in establishing differences between the rates of formation of chemically ionized molecules in the ion-molecule reaction. We have shown qualitatively that the acetylation of phenols is a function of the accessibility of the hydroxyl group (Equation 3) ( 5 ) .
On the other hand, a simple proton transfer from one species to another in the gas phase does not appear, so far a t least, to be susceptible t o steric effects. The transfer of a proton between alkyl halides does not become appreciably slower when the size of the alkyl group increases (8).Perhaps the results should not be compared directly, but a tentative conclusion that might be reached is that groups larger than a proton ought to be transferred in order to detect stereochemical differences. In our studies, we have used either CH3COf ( 3 )or NOz+ (9). Transfer of CH:$O+ to some 2-alkylcyclohexanones 3 has now been studied. We have also extended our results to the a-decalone isomers 4 and 5. These last compounds may
' Author to whom correspondence should be addressed. J. M. S. Henis, Anal. Chem., 4 1 (IO),22A (1969). M. L. Gross, P.-H. Lin, and S. J. Franklin, Anal. Chem., 44, 974 (1972). M. M. Bursey, T. A. Elwood, M. K. Hoffman, T. A . Lehman. and J. M. Tesarek, Anal. Chem., 42, 1370 (1970). T. J. Odiorne, D. J. Harvey, and P. Vouros, J. Org. Chem.. 38, 4275 (1973). S. A. Benezra and M. M. Bursey, J. Amer. Chem. SOC.. 94, 1024 (1972).
(4)
Finally, we have noted that stereochemical differences are observed when the two isomers 1 and 2 are acetylated by the gaseous ion (CH?C0)3+,the more hindered 1 reacting more slowly ( 7 ) .
guish between compounds whose unimolecular decompositions in a conventional mass spectrometer (Equation 2) are so similar as to preclude easy identification of isomers. A-B'-
+ CH CO
3 a.
R = H: b. R
5
4 =
CH,:
c.
R = CIH,; d. R
=
n.C H-;
e.
R
= n-C,H
(6) J. D. Henion, M. C. Sammons, C. E. Parker, and M. M. Bursey, Tetrahedron Lett., 4925 (1973). (7) M. K. Hoffman and M. M. Bursey, Can. J. Chem., 49, 3395 (1971). (8) T. Su and M. T. Bowers, J. Amer. Chem. SOC.,95, 7609 (1973). (9) M. M. Bursey and W. B. Nixon, Tetrahedron Lett., 4389 (1970).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 12, OCTOBER 1974
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