Obviously greater confidence in the results is afforded if at least one additional ( d o g DOP % penetration/ A V L - ~ / ~V, L ( ~ )point ~ / ~is )used in plotting Equation 14. This requires the use of at least one more pair (P,, V L , , , ) of penetration data. Literature Cited Collins, R. E . , “Flow of Fluids through Porous Materials,” pp 10-11, Reinhold, S e w York, N.Y., 1961. Dorman, R. G., “Aerodynamic Capture of Particles,” Permagon Press, Oxford, 1960a. Dorman. R. G., “Aerosol Science,” Academic Press, Xew York, N.Y., 1966. Dorman, R. G., Air Water Pollut., 3,112 (1960b). Hall, A. J.: “The Standard Handbook of Textiles,” p 94, Chemical Pub., New York, N.Y.. 1966. Jonas, L. A., Lochboehler. C. M., Magee, W. S., Enuiron. Sci. Technol., 6,821-6 (1972).
Spiegel, M. R., “Statistics,” p 243, Schaum Pub., New York, X.Y., 1961. Stechkina, I. B., Kirsch, A. A,, Fuchs, N. A,, Ann. Occup. Hyg., 1 2 , l (1969). Wang, C. s.,private communication (1973).
Received for reciew March 21, 1973. Accepted August 9, 2973. Supplementary Material Available. Four tables of data on fibrous filter mats including composition, pressure drop vs. linear flow velocity, DOP 70 penetration vs. linear flow velocity, and DOP 70 penetration vs. velocity slopes will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (106 X 148 mm, 20X reduction, negative) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington D.C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number ES&T-73-1131.
Simple Piezoelectric Probe for Detection and Measurement of SO, Michael W. Frechette and James L. Faschingl Department of Chemistry, University of Rhode Island, Kingston; R. I. 02881
A new system for the detection and measurement of sulfur dioxide using a coated piezoelectric crystal has been designed and evaluated. The device is rugged, portable, inexpensive, and should lend itself easily to automation. The detector response was measured as a function of sample size, weight of substrate applied to the crystal, concentration of sulfur dioxide, and sample volume. Sulfur dioxide is often used as an index of general air pollution because of its widespread sources and occurrences. The burning of coal, petroleum, and wood, all of which contain a significant amount of sulfur, is the most widespread source. Most common methods of sulfur dioxide analysis are based on a color change of a solution or paste, although conductometric methods, such as that of Martin and Grant (1965), have been used with some success. The most widely used method is that of West and Gaeke (1956), a colorimetric determination with formaldehyde and acid bleached pararosaniline. Most of these methods give useful results but are troubled by chemical interferences, or lack the ruggedness and portability for “in the field” measurements. The piezoelectric detector developed by King (1964) has been shown to be a promising device for the measurement of gas compositions. The detector employs an electronic oscillator and a vibrating quartz crystal which, when coated with an appropriate compound (substrate), can selectively “sorb” the gaseous component of interest. The change in frequency of the quartz plate can be measured and directly related to the concentration of the gaseous analate. The change in frequency of the vibrating quartz crystal according to the Sauerbrey Equation (1959) is:
AF
F
where P F = frequency change, Hz F = frequency of the quartz plate, MHz A = area of coated electrode, cm2 T = thickness of quartz plate, cm PW = weight of applied coating or “sorbed” substance, g The uniqueness of the piezoelectric sensor is that it can be used both as an integrating weighing device or as a dynamic partition weighing device. King has developed a detector for hydrocarbons using a squalene coated crystal, an integrating detector for hydrogen sulfide using lead acetate and a moisture detector using sulfonated polystyrene as the substrate. To date, the work with coated piezoelectric analyzers has been centered on their use as gas chromatographic detectors-i.e., in a flowing gas stream. The authors have developed and investigated a “static” system for the detection and measurement of sulfur dioxide using a coated piezoelectric crystal. Experimental
Figure 1 shows the schematic block diagram of the piezoelectric sulfur dioxide detection system. The sample chamber is an Erlenmeyer flask modified with glass sidearms to permit gas flushing. A set of these flasks with varying sample volumes was constructed. The crystal probe consists of a 15 X 2 cm glass tube. At one end is located a 9 MHz AT cut quartz crystal obtained from International Crystal Corp., Fort Lee, Okla. The quartz wafer is plated with l/4-in. circular electrodes consisting of nickel
AW A
= -0.38X106X~X--
PROBE
To whom correspondence should be addressed.
Figure 1 . Block
diagram of “static” analyzer system
Volume 7, N u m b e r 13, December 1973
1135
on gold. A coating is applied to the electrode by dissolving the substrate in an appropriate solvent and using a microliter syringe to apply a small droplet to the electrode surface. An infrared lamp may be used to hasten the evaporation of the solvent. Once coated, the crystal is encased by a small nonreactive cage to protect it from mechanical damage. Gas samples can be flushed into the sample chamber or alternatively gas-tight syringes can be used to inject known amounts of pure sulfur dioxide or known sample volumes into the chamber through a silicone rubber septum in the sidearm. Although silicone rubber tends to adsorb SOz, no significant amounts are lost after an initial conditioning of the system with sulfur dioxide. The probe can then be inserted into the sample chamber through a breakable plastic diaphragm. The sealed unit is allowed to stand until a stable frequency read out is attained. The probe can then be removed and warmed with an infrared lamp to desorb the sulfur dioxide. The piezoelectric crystal is electronically connected to an International Crystal Corp. Model OT-3 9 MHz oscillator, and a Monsanto Model 101-A Counter Timer, which gives a visual display of the crystal frequency. This counter allows one to display any five consecutive digits of the crystal frequency (1 Hz accuracy). The output signal is sent to a Monsanto Model 503-A Digital-Analog Converter which produces a signal for display on a Speedomax H Recorder. The quartz crystal is coated with styrene-dimethylaminopropyl maleimide 1 : l copolymer which Frechette et al. (1973) have shown to be a suitable substrate for SO2 detection.
Results and Discussion The response of the detector was measured as a function of the size of the sample chamber. A 300-ppm sulfur dioxide/air blend obtained from MG Scientific Gases was used in this study. The data from this experiment are listed in Table I. As can easily be seen, the detector response increases with an increase in the sample size. This response was linear in the range studied. The frequency change per microgram of sulfur dioxide was not constant and, in fact, decreased with increasing sample size. This suggests that a t low loadings the sensor picks up more SOz, indicating that active sites are being covered up as loading increases. The time for the system to produce a stable frequency readout also increases when larger samples are used. It appears that the more active sorption sites get covered first so that the sensor’s speed of pickup decreases with amount of sulfur dioxide. This indicates that for optimization of experimental conditions, a sample chamber should be chosen so that a determination can be made as rapidly as possible yet still produce an easily detectable response. The detector response was also measured as a function of the weight of substrate applied to the crystal. From the Sauerbrey Equation the weight of the substrate is directly related to the frequency decrease when the coating is applied. Different amounts of substrate were applied to the crystal, and the detector response was measured using either the 300-ppm SOz/air blend previously mentioned or a similar blend of 30 ppm of SOp/nitrogen. The volume of the sample chamber was 130 cm3 for all runs in this study. Data from this experiment are listed in Table 11. The response of the detector increases linearly with increasing substrate weight until the frequency change due to the applied coating (.IFsubs)reaches about 13,000 Hz. At greater substrate loadings, reproducibility becomes difficult to achieve and the detector response becomes unpredictable. It is felt that this phenomenon is due to the thickness of the coating a t these high loadings, as well as the fact that the crystal is approaching the point where its 1136
Environmental Science & Technology
vibration is completely damped by the weight of the substrate coating. Finally, the detector response was measured as a function of concentration of SOz. Data from this study are listed in Table 111. The response is essentially linear in the range studied, 20-300 ppm SOz. This was the case both for a 130-cm3and a 30-cm3sample chamber. The use of a “static” system has allowed much greater sensitivity to be achieved than with a “flowing” gas system previously investigated by the authors. Additionally, the portability of the instrument is increase.d since the use of bulky tanks of carrier gas is precluded. All that is needed is a sample chamber and a probe. Thedetection limit of the described experimental setup is approximately 0.1 Table I . Detector Response vs. Sample Size Sample size, crn3
AFso?,
32 75 113 150 196 230 265
HZ
Time, rnin
515 640 720 790 880 930 1330
8 13 18 21 23 25 38
Concentration of SO2 = 300 p p m b y volume. Weight of SDM polymer = AF,,,b, = 10,228, Hz. AFsol = frequency change d u e t o sorbed SOz, HZ.
Table II. Detector Response vs. Substrate Weight AFso:, Hz
AFlubet,
27 120 150 220 240 350 456 543 975 510 560 1287 1227
HZ
625 2800 3875 5112 5796 7774 9701 10,950 13,177 14,003 16,773 20,029 20,060
Concentration = 30 Ppm of S02/Nitrogen 275 421 545 1025
3875 5112 7774 8203
Concentration = 300 p p m of SO?/air. Sample size = 130 cm3. AF;o? = frequency change d u e t o SO?,Hz. AF,,b,t = frequency change d u e t o applied coating, Hz. ~
Table Ill. Detector Response vs. Concentration AF.o.,
Concentration, p p m in air
Hz
615 375 245 135 60 40
200 100 50 25 10 5
Sample size = 130 crn3. applied coating, Hz.
AFaub.t
165 50 30 25 20 Sample size = 30 cm3. plied coating. Hz.
= 5800 = frequency change d u e t o
300 100 50 25 10 AF.,b,t
= 5800 = frequency change d u e t o a p -
ppm. Modifications to the system and optimization of experimental parameters should allow this limit to be reduced even further. King has shown the “sorption” detector can experimentally detect gram. The only common interference found was nitrogen dioxide (NOz) which produced serious detector fatigue by deactivating the amine coating. The detector was essentially insensitive to air, nitrogen, oxygen, carbon monoxide, and carbon dioxide. Gas b!ends made of SO2 in carbon dioxide showed a no different response from a blend of sulfur dioxide in air or nitrogen. The Wesence of moisture has been shown to produce consider’able interference and fatigue. Water droplets may physically condense on the crystal effecting a weight increase on the electrode, and thus a corresponding drop in frequency, will be observed. Also when large amounts of water vapor are present, the sulfur dioxide may dissolve forming sulfurous acid. This acid may attack the amine coating resulting in the formation of a salt:
‘
-
-c-c-c-c4
O & AI O
‘
-c-c-c-c@
O&-!+O
I
I ”\
CHj
CHj
This formation not only deactivates the coating but also produces a change in frequency of the crystal. A study using infrared spectroscopy has produced evidence which supports this hypothesis. Despite these shortcomings, the simplicity of the crystal probe lends itself to easy changing of the quartz plate or recoating of the crystal should the coating be destroyed or damaged. With proper modifications, a device employing a piezoelectric crystal would be most useful in monitoring the sulfur dioxide content of stack gases, gas streams, or other gaseous effluents where the described interferences are not present in high concentrations. In many stack gases, NO2 is not present in significant amounts compared to sulfur dioxide. Moisture, on the other hand, is present in high concentrations and would have to be removed before useful measurements could be obtained. Present work is continuing to surmount this problem.
References Frechette, M. W., Fasching, J. L., Rosie, D. M., Ana/. Chem., 45, 1765 (1973). King, W. H., Jr., ibid., 36, 1735 (1964). King, W. H., Jr., U. S. Patent 3,164,004, January 5, 1965. King, W. H., Jr., ibid., 3,329,004, July 4, 1967. King, W. H., Jr., ibid., 3,266,291, August 16, 1966. Martin, R., Grant, J., Anal. Chem., 37,664 (1965). Sauerbrey, G., Physik, 155, 206 (1959). West, P., Gaeke, G., Anal. Chem., 28,1916 (1956).
Received for review August 20, 1972. Accepted May 4, 1973
Test for Anticholinesterase Materials in Water Robert M. Gamson,’ David W. Robinson, and Alan Goodman Development and Engineering Directorate, Edgewood Arsenal, Aberdeen Proving Ground, Md. 21 01 0
A simple device containing paper impregnated with cholinesterase is reported for detection of organophosphorus inhibitors in the ppb to ppm range in water. Optimum performance is obtained at 20°C and p H 8. Under these conditions, the enzyme is completely inhibited in 20 min or less by 10 ppb up to 1 ppm depending on the inhibitor. Comparison of inhibition data with rate constants indicates that the sensitivity of the device to any given inhibitor can be estimated if the rate constant value is known for that inhibitor with horse serum cholinesterase. A technique using a source of cholinesterase enzyme impregnated on paper has been developed for the rapid and simple detection of organophosphorus inhibitors in water in concentrations a t or about the level of acceptability for drinking contaminated water (Epstein, 1973). The papers are immersed in a suspect water and after a suitable incubation time, a chromogenic substrate is added to the paper. The appearance of a blue color, different from that of the substrate is evidence that the enzyme is active To whom correspondence should be addressed.
and that the water does not contain sufficient anticholinesteratic material to inhibit the enzyme. Conversely, no change in the color is evidence that an anticholinesteratic material is present in a concentration equal or greater than that acceptable for ingestion. This device, designed for use in kit form by relatively untrained military personnel to provide simple go/no-go guidance, has potential use for the detection and approximate estimation of organophosphorus anticholinesterases such as are used as pesticides. This paper describes the device and gives experimental data on studies with the organophosphorus esters isopropyl methylphosphonofluoridate (Sarin), 0-ethyl S(2-diisopropy1amino)ethyl methylphosphonothioate, and 0,O-diethyl 0-p-nitrophenyl phosphorothioate (parathion). The basic design is a polypropylene “ticket,” 1 in. wide, 2 in. long, and ?46 in. thick, round at one end, and square a t the other (Figure 1). Each half of the ticket contains a glass fiber disk (7hs-in. diameter) impregnated with horse serum cholinesterase, It is packaged in an envelope of mylar/polyethylene. The round end is wetted with buffer (pH 8) and substrate (2,6 dichloroindophenyl acetate in ligroine) is added from a dispenser. After a short waiting period, the round end is observed for blue color developVolume 7,
Number 13, December 1973
1137
