Anal. Chem. 1980, 52, 1489-1492 (20) EDstein. J.: Kaminski. J. J.: Bodor, N.: Enever. R.: Sowa. J.; Higuchi, T. J: Org. Chem. 1878, 43, 2816. (21) Guilbault, G. G. Anal. Chim. Acta 1867, 39, 260. ( 2 2 ) Karasek, F. W.; Guy, P.; Hill, H. H.;Tiernay, J. M. J. Chromatogr. 1876, 124, 179.
RECEIVEDfor review December 3, 1979. Accepted April 21,
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1980. This work was suDDorted bv the DeDartment of Defense. Air Force Systems Command, Aerospace Medical Division, School of Aerospace Medicine, Crew Technology Division, Brooks AFB, Texas, under contract F33615-78-D-0617, and by a grant from the Army Research Office DAAG 29-77-G0226. * I
Continuous Detection of Toluene in Ambient A r w th a Coated Piezoelectric Crystal Mat H. Ho and George G. Guilbault" Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 722
Bernd Rietz National Institute of Working Environment, DK-2900 Hellerup, Denmark
A coated piezoelectrlc quartz crystal for detectlon and assay of toluene In the worklng place has been developed. Carbowax 550 was used as a coating substrate and toluene vapor can be detected In the bear range 30-300 ppm wlth a relative standard devlatlon better than 4%. The response time was 30 s and a complete reverslbillty was obtalned in less than 40 8. No Interferences were observed at a 5 % (v/v) level and water vapor can be removed selectively uslng a Naflon membrane. The ilfetlme of the detector Is more than 2 months. Also a portable monitoring devlce for toluene, whlch is 20 X 14.7 X 9 cm In dimenslon and less than 3 Ibs In welght, has been developed.
Toluene and other alkylbenzenes in ambient air are known to be reactive photochemically (I), and can have harmful effects upon long-term exposure a t moderate levels ( 2 ) . Toluene is widely used as a solvent in a large number of chemical industries and in printing plants. Also, i t is used as a solvent and a thinner in paints, lacquers, adhesives, and cleaners. T h e common method of measurement of toluene in ambient air is gas chromatography ( 3 , 4 ) . The application of photoionization detectors has also been reported (5). In recent years, coated piezoelectric crystal detectors have become of increasing interest for detection of traces of toxic atmosphere pollutants, not only as highly senstitive and selective detectors ( 6 ) , but also as simple, inexpensive, and portable devices, which are even small enough to be carried in a worker's pocket (7). King (8) developed a sensitive piezoelectric crystal detector for monitoring hydrocarbons in the atmosphere. Frechette and Fasching (9) have proposed their use in a static system for the detection of sulfur dioxide. Karasek applied them as detectors for gas chromatography (10-12). Guilbault et al. (13-20) developed sensitive and selective detectors for organophosphorus pesticides, sulfur dioxide, ammonia, nitrogen dioxide, hydrogen chloride, hydrogen sulfide, and explosives in the atmosphere. T h e principle of t h e detector is that the frequency of vibration of an oscillating crystal is decreased by the adsorption of a foreign material on its surface. A gaseous pollutant is selectively adsorbed by a coating on the crystal surface which is specific for that substance, thereby increasing the mass on the crystal a n d decreasing the frequency. The decrease in 0003-2700/80/0352-1489$0 1 .OO/O
frequency is proportional to the increase in mass owing to the presence of gas adsorbed on the coating, according to the Sauerbrey equation (18):
,.IF = K-AC
(1)
where AF is the frequency change in Hz, K is a constant which refers to the basic frequency of the quartz plate (MHz), area coated (cm2),and a factor to convert the mass adsorbed into concentration (ppm) of sample gas (AC). The theoretical limit of detection is about g (21)and the mass sensitivity is about 400 Hz/pg for a 9-MHz crystal and 2600 Hz/pg for a 15-MHz crystal. By coating the surface of crystal with a substance which will selectively adsorb a particular gas, the concentration of that gas can be determined quantitatively. Many compounds have been tested as the coating substrate for the detection of toluene. Among these Carbowax 550 shows a very sensitive and selective response to this pollutant. In this paper, we describe an evaluation of Carbowax 550, as a coating material for the detection of toluene and also other parameters that affect the detector.
EXPERIMENTAL Apparatus. The experimental setup is shown schematically in Figure 1. The detector cell design is largely the same as reported previously (20). The piezoelectric crystals used in these studies are 9-MHz, AT cut quartz crystals with gold plated electrodes on both sides (International Crystal Mfg. Co., Oklahoma City, Okla). The crystal oscillator was built from an OX transistor oscillator kit, Model OT-13 (International Crystal Mfg. Co.) and powered by a regulated power supply (Heathkit, Model IP-28). The applied voltage was kept constant at 9 V dc. The frequency output from the oscillator was measured by a Systron-Donnor Model 8050 frequency counter, which was modified by a digital-to-analog converter so that the frequency could be recorded on a Bristol Model 570 Dynamaster recorder. The frequency change could be read on either the frequency counter or the recorder as a peak maximum. Portable Monitoring Device. A portable detector which is 20 X 14.7 X 9 cm in dimension and less than 3 lbs in weight was developed for field use, and the schematical arrangement is shown in Figure 2. The detector included a piezoelectric crystal monitor, a miniature pump, a sampling valve, and batteries. The piezoelectric crystal monitor was build by Environmetric, Inc., St. Louis, Mo., and included reference and sensor oscillators, a frequency mixer, and solid-state display of the readout. The readout is the frequency difference between a sensor crystal, which is 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL.
52, NO. 9,AUGUST 1980 T O DCTECTOR CELL
h
llY
COATED
SAVPLE
I
1
5
TC3EiECT3XCELL
F L O U METER
Flgure 1. Experimental apparatus. (1) Recorder, (2) digital to analog converter, (3) frequency counter, (4)oscillator, (5) power supply
I
I
i
i
L. Flgure 2. Portable monitoring device for detection of toluene in the wckphce. (1) Sensor crystal, (2)reference crystal, (3) sensor oscillator, (4)reference oscillator, (5) frequency mixer, (6)solid-state readout, (7)batteries, (8)miniature pump, (9) sampling valve. (A, B) precolumn for purification of the air: (A) activated carbon, (B) silica gel
coated with Carbowax 550, and a reference uncoated crystal. A miniature pump (Spin personal pump) was used to sample contaminated air into the cell. The reversibility of the detector was achieved by turning a sample valve to let clean air, which was purified by activated carbon and silica gel, pass through the detector cell, thus desorbing the sample off the surface of the coating. The detector operates on NiCd batteries which can be recharged after 8 h of operation. Diffusion Cell. Toluene vapor was generated using a diffusion cell as shown schematically in Figure 3. The diffusion tube, D, was constructed from Pyrex tubing, 5.0-mm i.d. and 3.4 cm long. The liquid reservoir, R, 1.5-cm 0.d. and 4 cm long, which contained about 3 mL of toluene, was connected to the diffusion tube by a Pyrex 14/35 ground joint. The flow rate of dilute gas, N2 or clean air, is monitored by a calibrated flow meter of the Precision Calibration System, Model 570 (Kin-Tek Lab., Inc., Texas); the gas enters the mixing chamber, M, through a capillary, C. The flow rate is adjusted by a valve regulator. The standard vapor mixture was sampled into the detector cell by a sampling valve. The diffusion cell was kept at constant temperature by using an oven of Precision Calibration System, Model 570, (Kin-Tex Lab., Inc., Texas). The temperature was maintained within *0.03 "C. A heating tape is used to help maintain all the tubing lines of the system at 40 "C in order to minimize the condensation and absorption on the walls. All the system was constructued of either glass or stainless steel. The diluent was passed through a preheated coiled copper tube in order to equalize the diluent gas temperature with that of the diffusion cell. Following 3 to 4 h of thermal equilibration, the reservior is removed and sealed with a cork stopper. The stopped reservior is weighed to the nearest hundredth of a milligram, then it is immediately inserted into the cell. After a certain period of time, which was 180 min, the reservoir is immediately removed, stopped, and weighed again. The diffusion rate of toluene is determined from the weight loss. The standard vapor concentrations can be calculated from the known flow rate and the diffusion rate (22, 23). Method of Coating. Carbowax 550 is dissolved in acetone and the solution is applied onto the entire surface of the electrode on both sides with a microsyringe. The crystal was then placed in an oven at 80 "C for several hours, so that the solvent evap-
Figure 3. Generation of standard vapor concentrations of toluene by the diffusion method. M, mixing chamber; C, capillary: D, diffusion tube; R, liquid reservoir
orated, leaving a uniform, thin film of substrate on the crystal. The amount of coating was 40 pg on both sides of the crystal, calculated from the frequency change, 16 KHz, due to the weight of the coating. Reagents. Carbowax 550 was obtained from Applied Science Laboratories, State College, Pa. Toluene, benzene, p-xylene, ethylbenzene, mesitylene, n-heptane, and n-hexane were reagent grade from Matheson Coleman & Bell and used without further purification. Acetone was of technical grade. Inorganic gases tested (NH3,SOz, HzS, and CO) were used from lecture bottles and were obtained from Matheson Co, Inc.
RESULTS AND DISCUSSION Carbowax 550 when coated on the electrode surface of a quartz piezoelectric crystal, showed an interaction with toluene. The adsorption of toluene was shown by a decrease in frequency of the crystal; the amount of decrease was related to the concentration of toluene. In order to determine the optimum condition for the detection of toluene, several important parameters were studied. Preparation of Standard Vapor Concentration. Syringe dilution and injection methods (20) give poor reproducibility a t low concentrations, caused by loss of sample due to two reasons. First, the adsorption on glasswalls and second, the condensation caused by pressure buildup in the syringe during injection. Therefore, it is desirable to have a continuous sample gas generator, both to minimize these errors and to provide large quantities of gaseous mixtures. A diffusion cell was used to generate t h e standard vapor concentrations of toluene over the ppm range, and introduce these directly to the detector cell by a sampling valve. T h e method for calibration and calculation of the concentration has been shown elsewhere (22,23). A diffusion tube was calibrated by weight loss over a certain period of time. Fingerprints or Rdditional dirt on the outside of t h e reservior can seriously affect the accuracy of the calibration and the weighing must be carried out with extra care. Temperature and flow rate must be held constant and the cell calibrated after about 3 to 4 h, in order to allow the system to attain thermal equilibrium. Various standard concentrations of toluene vapor were obtained by either changing the temperature or the flow rate. If the total flow of the diffusion cell is in excess of the flow rate required by the detector, the excess flow is vented with a needle valve. T o prepare multiple component standards for interference studies, we use one diffusion tube for each material. T h e reproducibility of the diffusion rate was better than f 2 % . Effect of Flow Rate. Figure 4 shows the effect of flow rate on the sensitivity a t 100 ppm of toluene. T h e flow rate was varied from 40 to 120 mL/min. The change in frequency observed increased with flow rates up to 100 mL/min. LOSS of sample due to condensation and adsorption along the line can be avoided by increasing the flow rate. However, a t flow rates higher than 100 mL/min, sensitivity begins to decrease owing to incomplete adsorption of toluene vapor on the surface of the coating. The flow rate not only affects t h e sensitivity but also the response time, A , and the recovery time, C, as
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
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~-------1-1
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,
,
,
,
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Figure 4. Effect of flow rate on sensitivity. Conditions: sample, 100 ppm; cell temperature, room temperature; 40 pg of coating
33
,
,
i
,
,
,
,
' i ,
,
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flow rate ml/mln
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23
Figure 6. Isothermal adsorption-desorption curves of the detector. Conditions: sample, 100 ppm; cell temperature, room temperature; flow rate, 100 mL/min.; 40 pg of coating ~
-
_
_
-
Table I. Response to Organic Interferences at 5% (v/v) Level
t
1
interferences 100 ppm 100 ppm 1 0 0 ppm 100 ppm 1 0 0 ppm 100 ppm 100 ppm
--
2'/
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20
I -
42
60
k T D , 'c
Figure 5. Effect of temperature on sensitivity. Conditions: sample, 100 ppm; flow rate, 100 mL/min.; 40 pg of coating
defined in Figure 6. T h e response time and recovery time become quite slow, as the flow rate decreased below 40 mL/min. As the flow rate increased from 40 to 120 mL/min, the response time decreased from 60 to 25 s and the recovery time decreased from 50 to 20 s, respectively. On the other hand, the sensitivity begins to decrease if the flow rate increased above 100 mL/min. The optimum flow rate is about 100 mL/min. Effect of Detector Cell Temperature. The adsorption of gases on the surface of coating is considered to be highly dependent upon temperature. When the temperature of the detector cell was increased from room temperature up to 60 "C, a significant decrease in sensitivity was observed as shown in Figure 5. U p to 40 "C, the sensitivity was decreased slightly. However, a t temperatures above 40 "C, a marked effect on sensitivity was observed. These data indicated that a lower temperature is better for sensitivity. T h e cell temperature also affects the response time and recovery time of the detector. The effect on response time was not significant. As the cell temperature increased, a decreasing recovery time was observed. For 100 ppm toluene, the recovery times were 35 and 25 s a t room temperature and a t 40 "C, respectively. T h e improvement in the recovery time was not worth the sacrifice in sensitivity. Also, for convenience of operation of the portable device, we choose room temperature for this study. Amount of Coating. The sensitivity was affected not only by flow rate and cell temperature, but also by the amount of coating substrate. Our earlier study ( 1 4 ) has shown that the sensitivity increased with increasing amounts of coating u p t o a maximum, at which the crystal was overloaded. This maximum would seem to provide the maximum sensitivity.
toluene toluene toluene toluene toluene toluene toluene
+ + +
+ i-
+
5 5 5 5 5 5
ppm benzene ppm p-xylene ppm ethylbenzene ppm mesitylene ppm n-heptane ppm n-hexane
A F , Hz 33 34 34 35 34 32 33
By measuring the change in frequency (and hence the sensitivity) due to adsorption of sample gas a t constant concentration, i.e. 100 ppm, vs. the amount of coating, we found that about 40 pg of Carbowax 550 on both sides of the crystal appeared optimum. Response and Reversibility. Toluene vapor adsorbed on the coated piezoelectric crystal and caused a decrease in frequency. Usually i t is a considerably important factor that the coating should adsorb the sample gas selectively a n d sensitively. However, this interaction must be reversible, and the adsorbed toluene vapor must be removable within a n acceptable time since the following measurement cannot be done until the signal returns to the original base line. Figure 6 shows a typical isothermal adsorption-desorption plot of response, AF,vs. time under optimum conditions as indicated in the legend of the figure. When the sample gas was introduced into the detector cell by the sampling valve, which is in position 1 (Figure 3), adsorption occurred and caused a n increase in the response. This response was very small in the first 10 s, due to the dead volume of the detector cell and the line which connects the sampling valve and the cell. After 10 s, the response becomes very rapid, then approaches a maximum adsorption. Region B indicates that saturation has been achieved. The sampling valve is then turned to position 2 (Figure 3) to let N2 or clean air pass through the detector cell, and desorption occurs in region C. As described earlier, the response time and recovery time were affected by the flow rate and cell temperature. At optimum conditions of flow rate and temperature (100 mL/min and room temperature), the response time was about 30-40 s, and the recovery time was 30-50 s, depending on the concentration of sample. At higher flow rates, the response time and recovery time become faster, but, on the other hand, the sensitivity was decreased. Interferences. For selective detection of toluene in the workplace, various inorganic and organic inteferences which would be expected to exist were tested. With Carbowax 550, no interferences were observed from any inorganic gases, such as CO, SOz, NH3, or NOz at 1000 ppm. The organic vapors, especially benzene and alkylbenzenes, give some interference.
Anal. Chem. 1980, 52, 1492-1496
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Table I shows the effect of the organic vapors studied a t the 5% v/v (100 ppm toluene 5 ppm of interferences) level. Since in the real case, where the toluene is being used as a solvent, such as in printing plants, the presence of benzene and other alkylbenzenes is less than 5% v/v, no interference is anticipated as shown in Table I. However, a t the 100% v/v (100 ppm of toluene + 100 ppm of interferences) level these solvents may cause significant interferences. T h e interference from water vapor could be completely eliminated by using Nafion tubing (24, 25). Water is selectively removed by diffusion through a permaselective membrane of Nafion tubing (Du Pont, type 811) without any loss of the sample. As the water vapor is adsorbed and permeates across the walls of the Nafion tubing, it is removed by either a countercurrent flow of dry nitrogen gas sweeping the outer surface of the tubing (24) or using desiccants such as magnesium perchlorate or 13X molecular sieve. For the field portable monitoring device, it is more convenient to use a desiccant for drying the outer surface of Nafion tube as described by Foulger et al. (25). Calibration Curve. The change of frequency, 1F,ranged from 4 Hz for 30 ppm to 110 Hz for 300 ppm and fit the general equation AF = 0.39 [toluene] - 7.0. 1F was taken as the frequency difference between the base line before sampling and the steady state, which occurs when the saturation between toluene vapor a n d the coating substrate is achieved. The relative standard deivation of the detector was about 4%. Lifetime of the Detector. With Carbowax 550 as a coating, t h e detector was tested for the lifetime of operation by comparing the base-line shift and the responses after two months. The base line was shifted due to some loss of coating substrate. However, this loss was only about 0.2 pg or 0.5% of coating substrate calculated from the shift of base line of 80 Hz. T h e signals, AF,observed were 110 and 108 Hz, initially and after two months, respectively, for 300 ppm toluene. T h e coating technique seems to critically effect a shift of the base line, and hence the lifetime of the detector. The coated Carbowax 550 should strongly adhere to the surface of electrodes in order to minimize the loss of coating. We observed
+
that if the crystal was placed on an oven a t 80 "C for 24 h or more after coating, the base line was more stable. This may be due to an increased adhesion of coating on the electrode.
LITERATURE CITED Pitts, J. N., Jr.; Winer, A. M.; Darnell. K. R . ; Lloyd, A. C.; Doyl, G. J. "Hydrocarbon Reactivity and the Role of Hydrocarbons, Oxides of Nitrogen, and Aged Smog in the Production of Photochemical Oxidants", International Conference on Photochemical Oxidant Pollution and Its Control, Proceedings, Vol. 11. EPA 600/3-77-001b, Jan. 1977. Cohr, K. H.; Stokholm, J. S c a d . J . Work. Environ. Heatth 1979, 5 , 71. NIOSH Manual of Analytical Methods, "Organic Solvents in Air", P & CAM 127, DHEW, NIOSH Publication number 77-157-A, 1977. Grizzle, P. L.; Coleman, H. J. Anal. Chem. 1979, 51, 602. Hester, N. E.; Meyer, R. A. Environ. Sci. Techno/. 1979, 73, 107. Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977, 49, 1890. Scheide, E. P.; Warner, R . 0 . J. Am. Ind. Hyg. Assoc., J . 1978, 3 9 , 745. King. W. H. Jr.; Environ. Sci. Technol. 1970. 4 , 1136. Frechette, M. W.; Fasching, J. L. Environ. Sci. Techno/. 1973, 7 , 1135. Karasek. F. W.; Gibbins, K. R. J , Chromatogr. Sci. 1971, 9 , 535. Karasek. F. W.; Guy, P.; Hill, H. H.; Tiernay. J. M. J . Chromatogr. 1976, 724, 179. Karasek. F. W.; Tiernay, J. M. J . Chromatogr. 1974, 89, 31. Karmarkar, K . H.; Guilbault. G. G. Anal. Chim. Acta 1974, 77,419. Scheide, E. P.; Guilbault, G. G. Anal. Chem. 1972, 44, 1764. Karmarkar, K. H.; Webber, L. M.; Guilbault, G. G. Environ. Lett. 1975, 8 , 345. Karmarkar, K. H.; Guilbault. G. G. Anal. Chim. Acta 1975, 75, 111. Hlavay, J.; Guilbault, G. G. A n d . Chem. 1978, 50, 1044. Hlavay, J.; Guilbault, G. G. Anal. Chem. 1978, 50, 965. Webber, L. M.; Kamarkar, K. H., Guilbautt, G. G. Anal. Chim. Acta 1978, 9 7 , 29. Tomita, Y . ; Ho, M. H.; Guilbault, G. G. Anal. Chem. 1979, 57, 1475. King, W. H.. Jr.; Anal. Chem. 1964, 3 6 , 1735. Miguel, A. H.; Natusch, D. F. S. Anal. Chem. 1975, 47, 1705. Calibration Standards, Analytical Instrument Development, Inc.. Avondale, Pa. Simmonds, P. G.; Kerns, E. J . Chromatogr. 1979, 186, 785. Foulger, 6 . E.; Simmonds, P. G. Anal. Chem. 1979, 57, 1089.
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RECEIVED for review March 7,1980. Accepted April 30,1980. This work was conducted with the financial assistance of the Army Research Office (Grand number DAAG29-77-GO226) and a travel grant from the North American Treaty Organization (Grant Number NATORG 1719) which permitted a joint research effort between United States and Danish Laboratories.
Ultrasonic Extraction of Polychlorinated Dibenzo-p-dioxins and Other Organic Compounds from Fly Ash from Municipal Incinerators G. A. Eiceman, A. C. Viau, and F. W. Karasek" Guelph- Waterloo Centre for Graduate Work in Chemistry, Waterloo Campus, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Polychlorinated dibenzo-p-dioxins (PCDDs) and other organic compounds are solvent extracted from 10- to 20-9 samples of fly ash from municipal incinerators with 200 mL of benzene using ultrasonic agitation for 1 h. A convenient filtering device is used to separate fly ash and solvent which is then concentrated to 100 pL using a rotary evaporator. Extracts are analyzed directly by gas chromatography/mass spectrometry, including selected Ion monitoring for PCDDs. Results from five replicate analyses of a fly ash sample yielded averages and standard deviations (ng/g) for the tetra- to octachlorinated dibenzo-p-dioxins of 8.6 f 2.2, 15.0 f 4.0, 13.0 f 3.4, 3.2 f 1.0, and 0.4 f 0.1, respectively. 0003-2700/80/0352-1492$01 O O / O
Fly ash which is produced during the incineration of municipal wastes is 7 5 to 90% inorganic matter but may also contain a complex mixture of extractable organic compounds. Approximately 100 organic compounds have been identified by gas chromatography/mass spectrometry (GC/MS) in benzene extracts of some fly ash samples. These compounds include polycyclic aromatic hydrocarbons (PAHs) and numerous chlorinated compounds which include polychlorinated phenols, polychlorinated benzenes, polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs) (I). Polychlorinated dibenzo-p-dioxins are a series of tricyclic C 1980 American Chemical Society
