Table IV.
19F Chemical Shifts for a Dowex 1 Resin QdV"
DVB,
1 2 8
'Z
Qwet'
(megig)
(megig)
2.8 4.3 3.6
0.4 0.8 1.3
ZHzOa 81.8 71.6 45.0
Shift? ( P P ~ ) -0.49
-0.83 -2.24
Values taken from label accompanying purchased resins. * Relative to 3 m KF (external).
crosslinking. The downfield shift with increasing crosslinking (thus increasing interior electrolyte concentation) possibly results from a deshielding of the fluoride ion by some electrostatic association with the fixed ion. At present, however, little is known about the fluoride chemical shifts in homogeneous electrolyte solutions (26, 27), and a more precise interpretation must await further studies and interpretations.
a
studies. These 23Naresults strongly suggest that sodium is associated electrostatically with the resinate ion and that the difference between line widths of the resin and the monomeric analog at comparable concentration levels are chiefly due to differences in re since it is expected that q would be approximately the same in both cases. The NMR resonance of a counterion in an anion exchange resin was also investigated. In Table IV are given the lQF chemical shifts for a fluoride resin (Dowex 1) as a function of
RECEIVED for review September 30,1969. Accepted February 16, 1970. One of the authors (RWC) wishes to acknowledge the financial assistance provided by the University of North Carolina Materials Research Center, Contract SD-100 with the Advanced Research Projects Agency and by National Institutes of Health Grant GM-12598. We would also like to acknowledge the National Science Foundation Grant GP-6880 for the purchase of the Varian HA-100 spectrometer used in these studies. (26) A. Carrington and T. Hines, J . Chem. Phys., 28,727 (1958). ( 2 7 ) R. E. Connick and R. E. Poulsen, J. Phys. Chem., 62, 1002 (1958).
-
Homogeneous Chemiluminescent Measurement of Nitric Oxide with Ozone Implications for Continuous Selective Monitoring of Gaseous Air Pollutants Arthur Fontijn,' Albert0 J. Sabadell, and Richard J. Ronco AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, N . J . 08540 The reactions of common air pollutants, such as NO, NOz, and CO, with certain second reactants, such as ozone or 0 atoms, are known to result in light emission. Measurements of the emission intensity could be used to determine the concentration of the pollutants. In a detector based on this principle, ambient air and the second reactant would be continuously flowed through and mixed in a reactor under moderate vacuum. After calibration a continuous record of pollutant concentration could be obtained. Specific sensitivity to a given pollutant would be obtained by a suitable choice of the second reactant and a light filter. To demonstrate the feasibility of the method, the detection of NO using 0, has been studied experimentally. A linear response from about 4 ppb (v/v) NO to at least 100 ppm NO is obtained. NOz, C02,CO, CzH4,NH,, SO2,and HzOin concentrations encountered in air quality control do not interfere with NO monitoring. Based on these results and experimental data for other chemiluminescent reactions, conclusions show that homogeneous chemiluminescence monitors can probably also be developed for at least 03, NO,(=NO NO2), and CO.
+
SOMEHOMOGENEOUS GAS phase reactions of common air pollutants, such as NO, NO*, and CO, with certain second reactants, such as ozone or 0 atoms, result in light emission. These reactions have usually been studied in continuous flow reactors. Measurement of the light intensity of the reactions occurring when the pollutants are mixed with a large excess of the second reactant should in principle be a suitable method for continuous monitoring of pollutants. A large 1
To whom all inquiries should be addressed.
excess of second reactant is needed so that its concentration is not measurably affected by the pollutants. A priori calculations based on the published spectral distribution of the light emitted, the rate constants for light emission, and the response characteristics of photomultiplier tubes, indicated that the light intensity would be quite adequate for monitoring of pollutants over the concentration ranges of interest. The suggested use of homogeneous gas phase chemiluminescent reactions for monitoring purposes appears attractive for a number of reasons, particularly the following: the emissions are specific for the pollutant being monitored; suitable choice of a light filter and the second reactant should allow interference-free measurements; the chemiluminescent light intensities from homogeneous gas-phase reactions in continuous flow systems are rather insensitive to changes in surface properties; and a family of chemiluminescence monitors may be constructed, each unit of which is specific for one pollutant, but all of which are similar in operation. The convenience in the operation of monitoring stations of families of instruments with similar manipulation and maintenance requirements would be considerable. A schematic design for a chemiluminescence detector is shown in Figure 1. The air to be monitored and the second reactant, e.g., ozone, enter the reaction vessel through separate inlets. Rapid mixing occurs and a chemiluminescent reaction takes place. A preset flow of the gases is maintained by a mechanical vacuum pump. The pressure in the reaction vessel is typically 1 torr and the size of the vessel, 1 liter. The intensity of the light emitted is measured by a photomultiplier tube and associated read-out devices (current meter ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
575
POLLUTANT INJECTION P O R T 7
I
EXPONENTIAL DILUTION FLASK
MECHANICAL VACUUM PUMP
MAGNETIC STIRRER THROTTLING VALVE/
TO VACUUM PUMP
Figure 1. Chemiluminescence detector Figure 2. Gas flow system and recorder). After calibration with samples of known concentration, a continuous record of the concentration of the pollutant in air can be obtained. To demonstrate the feasibility of the method and to determine optimum operation conditions, an experimental monitoring system has been built and incorporated in an apparatus which allows for convenient change in flow rates and pressure. Our studies with this device have concentrated on the detection of NO, using 0 3 as the second reactant; the lack of interference by other common air constituents and pollutants with the response of the NO/Oa system has been established. EXPERIMENTAL
A schematic of the borosilicate glass-stainless steel-copper flow system is shown in Figure 2. Oxygen (Linde Aviators Breathing Grade) and Nitrogen (Matheson, Prepurified), dried near atmospheric pressure by activated alumina, pass through flowmeters and needle valves into the low pressure part of the system. On the high pressure side, the oxygen is ozonated ( = O S v/v) by a photolytic ozonator. Nitrogen serves as the carrier gas for NO and the other pollutants for which possible interference with NO detection was investigated, The NO is introduced into a 3-liter spherical borosilicate glass exponential dilution flask, containing a comand pletely enclosed magnetically-driven stirrer. The 02/03 N,/pollutant streams are mixed in the 1-liter spherical borosilicate glass reactor. The flow is maintained with a 5 CFM Welch Duo-Seal vacuum pump. The gas streams enter the reaction flask through nozzles having small openings (1 to 2 mm in diameter) in the direction perpendicular to the neck of the flask. The reactor has a 7.5-cm diameter flat quartz window facing the photocathode of the photomultiplier tube used for light intensity measurements. The reactor is coated externally with Eastman white reflectance paint (BaS04) and is packed with MgO powder inside an aluminum box. The use of this powder further increases the available light intensity and provides for a lighttight detection system. The radiation is filtered througb a Corning CS 2-60 filter, cutting off radiation of X < 6100 A ; its intensity is measured with an EM1 9558 QA trialkali photomultiplier tube. The tube is contained in a thermoelectrically-cooled housing, maintained at about -20 OC, and operated at a cathode-toanode voltage of 1400 V. The tube output is measured by an electrometer with dc zero-offset and recorded. The experiments were performed at room temperature at the following reactant concentrations (in moles liter-l): = 2.7 x 10-5; [o,]= 2.7 x 10-5; [o,]= 1.1 x 10-7. Total reaction pressure was held at 1 torr ([MI = 5.4 X mole 1.-1). The flow rates of Oz and N, into the system
A , absorbing tower; B, flowmeter; C, needle valve; D,stopcock; E , manometer
were 1.25 rnl (STP) sec-I. The ozone flow was measured by iodometric titration. Nitric oxide concentrations were varied over a range of 4 ppb to 100 ppm of that of Nz (1 X to 3 x mole 1-l). The NO was purified by passage over activated alumina (for removal of HzO) and Ascarite (for removal of NO,). Atmospheric pressure samples were injected with a gastight syringe through the rubber stopper of a 5-liter borosilicate glass predilution flask, containing prepurified N2 at atmospheric pressure. After thorough mixing, samples containing NO in concentrations in the range of 10 to 105 ppm were taken from this flask and injected through the rubber stopper injection port of the exponential dilution flask of the main flow system, see Figure 2. Under these conditions the concentration of a sample injected in the flask decreases exponentially ( I ) :
C = C, exp(-Qt/V)
(1)
where, C, = initial concentration; Q = volume flow rate at the flask pressure; Y = effective volume of the dilution flask; f = time elapsed from start of dilution. The pressure in the flask was maintained at 50 torr, which gives a convenient time constant ( Y / Q on the order of 150 sec) for our measurements. The recorder gives a continuous record of the system response; the points shown on the dilution plots (see below) are from these continuous records. In a number of experiments, the effect of the presence of other air constituents and pollutants on the detector response to NO was investigated. Best regular grade gases were used. COz, SOn, CzH4, and NH3 were taken directly from cylinders. CO was first passed through a liquid Nz trap for removal of carbonyl compounds. NO, was taken from a blackened borosilicate glass flask in which it had been mixed with an equal portion of atmospheric air; this procedure was followed to oxidize trace amounts of NO to NOz and to prevent photodissociation of NO2. These interference gases were injected in the same exponential dilution flask as used for NO. Separate predilution flasks were used. Water vapor could be introduced with a second Nz flow system. Up to 75 of the total N2flow was diverted through this system, which was designed so that Nz passed either through a saturator or directly to the H 2 0 port (Figure 2). This setup allowed for rapid comparison between the detector response to NO with 7 5 z saturated and dry N2. The saturator was filled with H 2 0 at 45’ C. Complete saturation of
m21 576
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
(1) J. E. Lovelock, “Gas Chromatography 1960,” R. P. W. Scott, Ed., Butterworths, London, 1960, p 26.
Nz flowing through the H 2 0 port was achieved, as evidenced by the formation of water droplets in a room temperature trap, downstream from the saturator.
1
0
-
4
1
1
RESULTS AND DISCUSSION
The Reaction between NO and 03. Ozone was selected as a suitable second reactant for NO monitoring on the basis of the following a priori considerations. The reaction between NO and O3had been investigated extensively (2, 3) and it had been established that the chemiluminescence is due to NO
+
0 3
+
NOz*
+ Oz
(2) (3)
The presence of NO in air is accompanied by that of NOs (4, which reacts only slowly with 0 3 producing higher oxides (5, 6). Other reactants such as 0 and H atoms also produce a rather intense chemiluminescence with NO (7, 8). However, they react rapidly with NOs producing NO (9, 10). As a result, these atoms appear suitable for the measurement of [NO] [NO2],but not for [NO] in the presence of NOn. The light intensity of the NO/03reaction is given by ( 2 ) :
+
Z3 = 12{ exp( -41 80
=t300)/RT} { [NOI[031/[MI} sec-
(4)
for the 6000 to 8750 A region for M = air. The [MI appears in the denominator because the emitting species, NOz*, is quenched by M-i.e., any gaseous species present. The units of Z3 obtained from Equation 4 are, for example, einsteins 1.-l sec-l or quanta ml-l sec-I, depending on whether mole 1.-’ or number of particles ml-l concentration units are employed. The relative intensity distribution of the emission spectrum (3) shows that no light is emitted below about 6000 A. Comparison to the EM1 9558 QA phototube spectral response characteristic shows that the tube’s peak sensitivity falls at wavelengths shorter than 6000 A. Therefore, the use of the CS 2-60 filter does not appreciably interfere with the sensitivity of the monitor for NO detection by 03, but decreases the possibility of interference by other pollutants. The phototube response cuts off near 8750 A, therefor the I 3 value given by Equation 4 is roughly that for the useful emission region of our experiments. The rate constant for the overall reaction, as defined by
is 1 X IO7 1. mole-’ sec-l at room temperature (2). This number is sufficiently small to make consumption of NO by O3 in the reaction flask essentially negligible ( =6 %) under our operating conditions (residence time = O S sec; [O,] = 1.1 X mole 1.-’). The absence of an appreciable change in reactant concentration is a necessary prerequisite for a uniform light intensity in the reactor, needed to obtain a linear response from the detector. (2) M. A. A. Clyne, B. A. Thrush, and R. P. Wayne, Trans. Faraday SOC.,60, 359 (1964). (3) P. N. Clough and B. A. Thrush, ibid., 63,915 (1967). (4) B. D. Tebbens, “Air Pollution,” Vol. I, A. C. Stern, Ed., 2nd ed., Academic Press, New York, 1968, Chapter 2. (5) H. S. Johnston and H. J. Crosby, J. Chem. Phys. 22, 689 (1954). (6) H. W. Ford, G. J. Doyle, and N. Endow, ibid., 26, 1336 (1957). (7) A. Fontijn, C. B. Meyer, and H. I. Schiff,ibid., 40, 64 (1964). (8) M. A. A. Clyne and B. A. Thrush, Discuss. Faraday SOC.,33,
139 (1962). (9) F. Kaufman, Prog. React. Kinet., 1, l(1961). (10) F. Kaufman, Ann. Geophysique, 20, 106 (1964).
too
IO’
to*
to’
104
105
[NO], ppb
Figure 3. Dependence of response of chemiluminescence detector on nitric oxide concentration 0 Initial concentrations 0
Data from exponential dilution plots
Linearity of Response and Limit of Sensitivity for NO Detection. Figure 3 gives a composite plot of the detector response to NO. The light intensity varies linearly with NO concentration over the range of concentrations investigated (=4 ppb to 100 ppm), and 4 ppb represents the approximate limit of sensitivity for our operating conditions. Two types of data points may be distinguished: those obtained from the initial injection of NO into the dilution flask and points taken from exponential dilution plots, cy. Figure 4, on the assumption that a decrease in intensity by a factor x corresponds to a decrease in NO concentration by a factor x. In Figure 3, both types of points fall along the same line, thus confirming the linearity of response with respect to NO concentration. The scatter in the data of Figure 3 increases with decreasing NO concentration. This can reasonably be attributed to a decreasing accuracy of sample preparation. The range of concentrations of NO encountered in (and of interest to) air quality control falls within the limits (4) 10 ppb to 1 ppm. The linearity of response and sensitivity of the detector thus appear quite satisfactory for its use as a monitor of NO in air. Effect of Other Air Constituentson Detector Response to NO. The possibility of interference by other commonly encountered air pollutants/constituents was investigated by adding them to the Nz/NO flow. A typical test run is shown in Figure 4 for the case of NOZ. This figure shows points obtained in a normal NO dilution run and a NO run during which NOz was injected. Because the concentration of NOz decreased in the dilution process (presumably at the same rate as that of NO), it was desirable to repeat the NO2 injections several times in the course of one NO run. The presence of 9 ppm NOn does not influence the detector’s response to NO down to about 6 ppb (see Figure 4). The same procedure as for NO2 was followed for C 0 2 ,CO, CnH4, “3, and SO2. The results are summarized in Table I in terms of concentration of the constituent tested which was found not to interfere with the nitric oxide signal at WO] 5 10 ppb. These concentrations exceed-and in most cases are considerably higherthan typical high concentrations of these compounds in ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
577
MONlTOIUNC OF OTHER GASES
A major advantage accruing to the application of homogeneous chemiluminescent reactions for monitoring of air pollutants would be the use of a set of similar instruments for a number of pollutants. One can have an idea of the likelihood of obtaining such multifunctional detectors by discussing a few examples. This is done here by using the experimental data obtained in the present work and published spectral distributions and rate constants for light emission of a number of chemiluminescent reactions. 0 3 . The NO/Oa reaction could also be used to monitor 0 3 . In this case one would merely replace the 0 2 / 0 3 flow line (Figure 2) with a carrier gas/NO flow line. The sensitivity of the method at [NO] = 1.1 X lo-' mole 1.-' (equal to the O3 concentration used above) would be the same as that found above for NO, i.e., 4 ppb, and interference effects would also be the same as for N O detection by 03. The sensitivity could be increased by using higher N O concentrations. NO,(=NO NOa). Oxygen atoms react with NO to produce light emission via
+
TIME, minutes
Figure 4. Effect of NOz on detector response to NO 0
0
Arrows indicate points at which 9 ppm of NO2 WAS injected during exponential dilution process ~
~~
~
Table I. Lack of Interference of Other Air Constituents with Nitric Oxide Monitoring by Ozone Concentration Maximum used at concentration which no encountered interference in air quality was detected at [NO1 I lOppb, monitoring," Constituent PPmb ppm 3 9 NOz 500 650 COP CZH4 3"
so2
Hz0
100
300
10 3 3
5
100% saturation
9 25
75% saturation
Data from Tebbens (4). All concentrations given are on a molar (v/v) basis. c Private communication from R. K. Stevens of the National Air Pollution Control Administration, 3820 Merton Drive, Raleigh, N. C., June 1969. b
polluted air. By using concentrations of NOa,C2&, and NH about two times higher than given in the last column of Table I, signals in excess of those obtained a t 10 ppb of NO were observed. The cause of these signals was not further investigated. For the other compounds, this column gives the highest concentrations tested. The HzO data were obtained in a different manner. They pertain to a comparison between streams of N2jNO dried and 75% saturated with water; no difference was observed in signals from these streams. With the experimental setup employed, we could not use a 100% saturated stream (some of the NO would have been absorbed in the water of the saturator). However, it appears very unlikely that any major interference could occur due to the fractional increase in [ H 2 0 ]represented by the increased saturation. 578
+ NO+
NOz
+ hv
NO1 is rapidly converted to N O on a 1 :1 basis, (9), cia
NO
0 NO+NOn
co
0
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
+ NOn
-+
NO
+
0 2
(7)
while the oxidation of N O by 0 atoms and 02 are comparatively slow processes (9, ZZ). Hence, 0 atoms could be used to determine the sum of the concentrations of N O and NO2 as NO uiu Reaction 6. The rate constant and spectral distribution of Reaction 6 have been determined by Fontijn, Meyer, and Schiff (7). oThelight is emitted in a continuum, stretching from 3875 A well into the infrared. Over the wavelength region for which the 9558 QA tube is sensitive, the rate constant ksis about 2.5 X l o 41. mole-' sec-1. Oxygen atoms can be generated at the same pressure and flow conditions as used in our NO/OI experiments, by replacing the ozonator with a microwave discharge, placed on the downstream side of the stopcock D in the 0 2 line, Figure 2. Previous work, e.g., Reference 7 indicates that under these conditions [O]in the reactor would be about 1 X 10-6 mole ].-I. Thus, the light intensity ZE = kg [OllNOl ~ 2 . 5X 10-2 [NO] sec-1. Using Equation 4, we obtain 1 3 = 1.7 X 10-5 mO] sec-I, for our NO/03 experiments. Hence, the sensitivity of the detector as an NO, detector using 0 atoms would be some 3 orders of magnitude higher than it is as an N O detector using 03. It actually would be somewhat higher yet, since the phototube sensitivity decreases with increasing wavelength for the spectral region of interest in comparisons of NOjO to NO/Os emissions. It must be recognized that because of the possibility of interference by emissions of reactions of 0 atoms with other air pollutants, the full spectrum of the NOjO reaction may not be available. However, it appears that even if a relatively narrow wavelength region is selected by the use of an appropriate filter, the sensitivity of the detector for NO, by 0 would still exceed that of the NO/03detector. CO. Carbon monoxide could be monitored by using its reaction with oxygen atoms,
co f o+coz
+ hv
(8)
which results in emission of the carbon monoxide flame b:nds. This banded emission spectrum falls in the 3000-5000 A re(11) J. Heicklen and N. Cohen, Aduan. Photochem., 5,157 (1968).
gion, cf. Dixon (12) and Pearse and Gaydon (13). The rate constant for light emission, ks has been determined by Clyne and Thrush (14) as 1.2 X lo1 1. mole-1 sec-I. Hence at [O] = 1 x lop6mole l,-I, 18 = ks [O][CO] = 1.2 X [CO] sec-1. Comparison to our NO/O, results gives 6 ppb as the approximate limit of sensitivity. However, the phototube response curve shows that in the wavelength region of the COjO emission, the tube sensitivity is some 5 times higher than in the region of the N O / 0 3 emission. This then places the limit of sensitivity at about 1 ppb. Again, interference by other pollutants could in practice result in a higher useful limit. It must be noted that the rate constants for light emission of the NO/03 and CO/O reactions, quoted in this paper, were measured by comparison to the NOjO reaction in the same apparatus. The absolute values then were obtained (2, 14) by using the directly measured absolute value for the NOjO reaction (7). The fact that the intensity ratios used here depend on relative measurements made in one apparatus enhances their accuracy. SO*. This major air pollutant is also known to participate in chemiluminescent reactions, e.g., with 0 atoms (15). (12) R. N. Dixon, Proc. Roy. SOC.(London),A275,431 (1963). (13) R. W. B. Pearse and A. G. Gaydon, “The Identification of
Molecular Spectra,” 3rd ed., Chapman and Hall, London, 1963, p 123. (14) M. A. A. Clyne and B. A. Thrush, “Ninth Symposium (International) on Combustion,” Academic Press, New York, 1963, p 177. (15) M. F. R. Mulcahy, “Twelfth Symposium (International) on
Combustion,” The Combustion Institute, Pittsburgh, Pa., 1969, p 329.
However, no rate constants for light emission of SOz reactions are available; hence, the limit of sensitivity cannot presently be estimated. Application. The natural atmospheric background levels of 03,NO, and CO are, according to Tebbens (4), approximately 10, 20, and 100 ppb, respectively. Because the lightemitting reactions are of first order in the pollutant concentrations, linear responses can be predicted. It thus appears highly likely that the homogeneous chemiluminescent detector can be used for the monitoring of these pollutants over the full range of their concentrations in polluted air. Of course, experimental confirmation remains desirable. Many chemiluminescent gas reactions occur (16). Therefore, the method described probably can be extended to analysis of further gases and to other environments. ACKNOWLEDGMENT
We have benefited from discussion with J. Hodgeson, R . K. Stevens, and A. E. O’Keeffe of the National Air Pollution Control Administration. RECEIVED for review November 5,1969. Accepted February 12, 1970. This paper is based on a presentation given at the 158th National American Chemical Society Meeting, New York, September 1969. Work supported by the National Air Pollution Control Administration under Contract CPA-22-69-11. (16) T. Carrington and D. Garvin, “Comprehensive Chemical Kinetics,” Vol. 3, C. H. Bamford and C. F. H. Tipper, Ed., Elsevier, Amsterdam (in press).
Sensitive, Wide-Range, Temperature-Controlled Cell E. W. Owen’ Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550 A temperature-controlled cell has been developed in which the temperature can be changed rapidly over a wide range (-190 to 300 “C). This range can be extended downward by using a lower-temperature coolant, and upward to the temperature limitations imposed by the materials. The cell, thermally isolated from the environment, is cooled by liquid nitrogen and heated by an electric heater. The control system operates according to a minimum-time-control law while making large temperature changes and according to a proportional-control law while holding the temperature at a set point. The proportional controller is a pulse-frequency-modulated system and thus consists of discrete rather than analog elements. The circuits are built from integrated-circuit logic gates and comparators. The cell was developed for use as a chromatographic column in a gas-separation facility, and should be applicable to systems having similar temperature-control requirements.
THE CELL WAS designed with a number of thermal considerations in mind. While these were dictated by the needs of a gas-separation process (I), similar specifications are encountered in the design of chemical cells, reactors, and Present address, Department of Electrical Engineering, University of California, Davis, Calif. 95616. (1) F. F. Momyer, “The Radiochemistry of Rare Gases,” National Academy of Sciences, National Research Council, NAS-NS 3025, October 1960.
columns of other types. The thermal design of the cell is governed by several specifications. 1. A wide temperature range, extending both above and below ambient temperature, is required. Within this temperature range, the temperature must be controlled to within 1 or 2 “C. 2. Large changes of temperature must be made as quickly as possible. Since the dynamic response depends on the mass of material to be heated or cooled, this requirement implies that the weight of the cell must be small compared with the weight of its contents. In addition, the geometry of the cell must facilitate the rapid exchange of heat between the heat sources and the contents of the cell. 3. The active material of the cell must be maintained at uniform temperature throughout. In the gas-separation cell, the chromatographic process depends on the temperature of the adsorbant. In other cells, similar temperaturedependent reactions or processes are likely to take place. 4. The dynamic behavior of the cell should be simple so that a simple control system can be used; complex dynamic behavior requires a complex control system. Therefore, the thermal behavior of the cell should be capable of being described by a simple mathematical representation. 5. Economy of operation must be considered. In particular, the liquid coolant should be conserved. Consideration of these five factors led to rejection of most of the conventional designs in favor of a novel design conANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
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