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Anal. Chem. 1990, 62, 182-185
Gas Sensor and Permeation Apparatus for the Determination of Chlorinated Hydrocarbons in Water Joseph R. Stetter* and Zhuang Cao Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 A solid-state gas sensor with a selective response to chiorinated hydrocarbons has been combined wlth a simple dRcone rubber p e m a t b n apparatus. The apparatus has been tested In a way that simulates the on-line analysis of chlorinated hydrocarbons In a llquld process stream. The system can provide information on whether or not the sampled stream contains chlorinated hydrocarbons as well as quantitatlon of the Chlorinated hydrocarbon In the sample. No sensor response was observed for 1000 ppm hexane or phenol while concentrations of a few parts per million of chlorobenzene were easily detected. The permeation apparatus offers a new and convenlent method to anatyze the contents of an aqueous sample but allows use of a gas sensor. Slnce many more types of gas sensors are convenient and available than are llquld sensors, this approach may be more generally useful If other gas sensors are interfaced to the liquid sampling system by means of semipermeable membrane technology.
INTRODUCTION Chemical sensing techniques with continuous real-time analysis have attracted intense interest in the last decade ( I ) . The on-line determination of specific chlorinated hydrocarbons in wastewater is an important analytical problem. Water often contains more than a single pollutant and the most frequently applied methods for analysis include separation methods, especially gas chromatography. The Federal Environmental Protection Agency (EPA) approved method of collection and analysis of volatile organic priority pollutants involves obtaining a grab water sample from an effluent stream, transporting the sample to a laboratory, and analyzing the sample by a procedure called the purge and trap technique which includes analysis by GC/MS (2). While GC/MS methods are extremely versatile, sensitive, and selective, they are not real-time (continuous) or inexpensive. On-line process monitoring or screening methods that could be applied in the field would benefit from a low-cost alternative for the determination of chlorinated hydrocarbons. There have been significant advances in the field of membrane technology. Hellgeth and Taylor (3)have described a method for on-line aqueous based reversed-phase high-performance liquid chromatography/Fourier transform infrared spectrometry. This method involved the detection of the organics in an aqueous/organic segmented stream through a flow cell which is constructed by using multiple layers of Teflon membranes. Blanchard and Hardy (2) introduced a separation method based on the permeation of volatile organic compounds through a silicone polycarbonate membrane from an aqueous sample matrix into an inert gas stream. A portion of this stream was then injected into a capillary gas chromatograph. Recently, Melcher ( 4 ) developed a silicone membrane/flow injection system for the determination of trace organic compounds in aqueous samples. Organic compounds in the injected sample permeate a tubular siliconerubber membrane and are collected in a gaseous extractant external to the flowing liquid. The extractant containing the permeated compounds flows through a detector producing a peak response. No examples of the use of permeation mem-
branes with simple chemical sensors have been reported. In contrast to the above methods, our efforts have centered upon the development of an interface that employs a tubular silicone-rubber membrane to extract volatile chlorinated hydrocarbon compounds from a water matrix for subsequent presentation to a gas sensor. The purpose of this paper is to illustrate that the use of a rugged permeation membrane in combination with a low cost, small, selective gas sensor can provide a tool for on-line and near real-time, selective analysis of chlorinated hydrocarbons in an aqueous sample matrix. EXPERIMENTAL SECTION Sensor. The specific sensor for chlorinated hydrocarbon vapors used in this work consiss of an inner (negative) and an outer (positive) electrode separated by a rare-earth-doped ionic semiconductor as shown in Figure 1. Typical materials of construction include mixtures of lanthanum oxide and lanthanum fluoride (5). The outer electrode serves as a heater as well as a sensor terminal. The temperature of the sensor element is maintained at approximately 500 "C. A dc potential of about 4 V across the electrodes is maintained and the resistance is measured using Ohm's law and the voltage drop across a known resistor (see Figure 1). In the absence of a chlorinated hydrocarbon vapor, a high resistance is observed and the conduction between the terminals is very small. But in the presence of a chlorinated hydrocarbon vapor, the resistance decreases significantly and this increases the current flow between the terminals. The magnitude of the sensor background current at constant dc bias voltage is very sensitive to the presence of chlorinated organic vapors in the atmosphere surrounding the sensor since these gases alter the resistance of the semiconductor surface. This resistance or impedance change in the presence of chlorinated organic vapor is the analytical "signal" from this sensor, i.e., the impedance is a function of the concentration of the chlorinated vapor present. The sensor is relatively insensitive to many hydrocarbon contaminants found in industrial situations. The enhanced reactivity toward compounds with high electron affinity makes the sensor selective to chlorinated organic vapors even in the presence of hydrocarbons. Electronic Apparatus. An in-house electronic circuit was built for the control of the sensor heater voltage and for the detection of the analytical signal. A separate battery is used to apply a fixed voltage to the sensor at pin 1 (Figure 1) and this battery is in series with a fixed resistor. To measure the current through the sensor, the voltage drop a c r w the fiied (1kQ) resistor is measured. Once the current is known, the impedance of the sensor is calculated by dividing the bias voltage by the current. The conductance of the sensor is the inverse of the impedance. Permeation Apparatus. Since the chlorinated hydrocarbon sensor is a gas sensor, it is necessary to convert the aqueous sample containing the analyte into a vapor sample suitable for analysis by the gas sensor. After several permeable membrana and several geometries were tried, a permeation apparatus as illustrated in Figure 2 was assembled for our initial evaluation of the sensor. The permeable tubing is Silastic Medical-Grade Tubing (0.012 in. i.d. X 0.025 in. 0.d.) manufactured by Dow Corning. Silicone materials preferentially allow organic compounds to permeate while rejecting water and other highly polar molecules. Many strands of permeable tubing are used to increase the total transport of analyte at low concentrations and increase the total flow rate of vapor to the sensor at low applied pressures. Since chlorobenzene is almost insoluble in water, the water was stirred during analysis to provide mixing and homogeneous contact of the solution with the silicone tubing. Both "wrist-action shaker"
0003-2700/90/0362-0 182$02.50/0 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 2, JANUARY 15, 1990
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and “bar-stirrer” methods were attempted but the best mixing was accomplished with a simple magnetic stirrer. The gauze (see Figure 2) separates the permeable tubing and magnetic stirrer to prevent the stirring bar from inflicting damage to the tubing. Procedure. Figure 3 presents a block diagram of the experimental apparatus for measurement of the concentration of a chlorinated hydrocarbons in a water. The carrier gas (typically air) is pushed through the tubing with a small air pump and then directed to the sensor. When the permeable tubing is submerged in the water, organic compounds permeate through the silicone wall and are picked up in the carrier gas while polar liquids (and especially water vapor) are preferentially rejected. The gas sensor can detect the chlorinated organic vapors and the sample flow is maintained constant during analysis. The output of the sensor is continuously monitored and is a function of the concentration of chlorinated hydrocarbon in the vapor phase or in the water.
Gas in
Rare earth semiconductor
n
(Bias voltage)
U
Figure 1. Chlorinated organic vapor sensor.
RESULTS AND DISCUSSION Sensor Characterization. It has long been known that adsorption of a foreign species on a semiconductor surface will provide surface states (6) and that the electrical properties of semiconductor oxide catalysts change when adsorption or reactions occur on their surfaces (7). Consider the nature of the semiconductor material. If the conductance of the sensor can be interpreted as resulting from a single surface state, then 0
Carrier gas io
= uo[a/aol expHE, - E,)/kT1
(1)
Carrier gas
out
where u is the conductance, [a/ao]is the ratio of occupied to unoccupied states, E, is the energy of the conduction band, and E, is the energy of the surface state (8). At constant temperature the conductance of the sensor bead changes with the partial pressure of the reacting gas due to a change in the ratio of the densities of states. This pressure dependence of conductance often takes the form = kP”’
(2)
U / U ~
for chemisorption on a transition-metal-oxide system (6, 9, 10). This is consistent with the effect produced by a modified Langmuir adsorption isotherm equation (11)for a condensible vapor (such as the volatile chlorinated hydrocarbons used in this study) chemisorbing on the sensor surface. We can now express eq 1 as % I
- - - -
(3)
~
Figure 2. Permeation apparatus used to expose aqueous samples to permeable tubing.
Flow meter #I
u = u o b P exp[-(E, - E , ) / k T ]
Equation 3 may also be interpreted a t constant P” as In u = In k” - (E, - E , ) / k T
Flow meter #2
(5)
-
184
ANALYTICAL CHEMISTRY, VOL. 62, NO. 2, JANUARY 15, 1990
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Table 11. Sensor Response toThlorobenzene in Air at Different Concentrations conc P,ppm cond 0,’ 6.25 6.25 12.50 12.50 25.00 25.00 50.00 50.00 100.00 100.00 I 2.50
2.75
I 3.25
3.00 neater Volc.gc
I
11
1.50
(Valra)
1.325 X 1.350 X 2.200 x 2.250 X 4.100 X 4.125 X 8.500 X 8.625 X 1.502 X 1.496 X
51-l
log conc (In P)
10“ 10“ 10” 10“ 10” 10” 10” 10“
1.833 1.833 2.526 2.526 3.219 3.219 3.912 3.912 4.605 4.605
log cond (In u
) ~
-13.534 -13.515 -13.027 -13.005 -12.405 -12.398 -11.675 -11.660 -11.106 -1 1.110
a u = (5.3854X + (1.4727X 10-7)P;correlation coefficient, 0.9955. u = -15.2148 + 0.8920 In P; correlation coefficient, 0.9966.
Flgure 4. Sensor signal to 1000 ppm chlorobenzene in air (140
cm3/mln) at different heater voltages (Le. operating temperature). Table I. Relationship between Sensor Signal and Flow Ratea flow rate, cm3/min 110 150 175 200 310 360 516
conductance ( u ) , Q-’ 1.48 x 1.15 X 9.63 X 8.12 X 3.76 X 2.96 X 1.16 X
10-5 lo6 10” 10” 10” 10” 10”
log cond In u -11.12 -11.37 -11.55 -11.72 -12.49 -12.73 -13.67
Table 111. Sensor Responses to Chlorobenzene in Water at Different Concentrations conc P,ppm 500 lo00 2000 5000 loo00 5oo00
cond u, Q-’ 8.325 X 1.288X 2.088 X 3.125 X 3.700 x 4.825 X
10“ 10” 10” 10“ 10”
log conc In P
log cond In fl
6.21 6.91 7.60 8.52 9.21 10.82
-14.00 -13.56 -13.08 -12.68 -12.51 -12.24
aln u = -16.1562 + 0.3829 In P; correlation coefficient, 0.9565.
aRegressionof flow rate and log yield: constant, -10.4566,R2, 0.9980:sloDe. -0.0063.
This type of model for the gas sensor response has been wed to describe the response of p-type cobalt oxide to carbon monoxide (12)and can be used to explain the response of n-type semiconductors to various gases (13, 14). The sensitivity of the chlorinated hydrocarbon sensor is highly dependent upon the heater voltage which controls the temperature of the sensor. The sensor signals at the different heater voltages at constant concentration of 1000 ppm chlorobenzene in air are given in Figure 4. The sensitivity increases as the heater voltage increases (i.e., as the temperature of the sensor is increased) but after a voltage of 3.365 V, the signal amplitude increases slowly. In order to prolong the lifetime of chlorinated hydrocarbon sensor and to maximize the sensitivity, a heater voltage of 3.365 V was used. At high gas sample flow rate, the sensor signal decreases with the increasing flow rate. Since more heat is lost from the bead as the flow rate increases, the temperature of bead is inversely proportional to the carrier gas flow rate. This is consistent with eq 5, which predicts that the conductance of the bead should increase with a decrease in flow rate. Table I shows the relationship between sensor signal and flow rate. These data support the interpretation that simple dilution of the sample in the air carrier gas causes the loss in sensor signal at higher flow rates. The least-squares regression result (correlation coefficient, 0.998) supports eq 5 and our hypothesis. Considering both the analyte transport efficiency and sensor signal decrease with flow rate, an optimum flow rate of about 170 cm3/min was chosen for this particular sensor system geometry. Table I1 illustrates the concentration dependence of the chlorinated hydrocarbon sensor response and these data support the conductance theory described by eq 4 above. The five gas concentrations (6.25, 12.5, 25, 50, and 100 ppm) of chlorobenzene in air were detected by the sensor at constant heater temperature and constant flow rate with an amplifier of gain about 10. The linear relationship of the log conduc-
Flgwe 5. Responses of gas sensor to the air Row from the permeation apparatus during 10-min exposures of the permeation apparatus to aqueous chlorobenzene samples.
tasnce and log partial pressure (i.e. the gas concentration) of chlorobenzene vapor is consistent with the results predicted by our model of sensor response at concentrations below about 500 ppm. A series of different concentrations of chlorobenzene in water were prepared. The signals produced when the silicone tubing is immersed in the aqueous samples are shown in Figure 5. The high concentration requires more time to completely purge from the system than the low concentration. The reason for this can be explained because a longer time is required for cleaning the contamination from the silicone tubing after saturation with the analyte. The signal magnitude at high concentrations (500-5oooO ppm) of chlorobenzene in water (Table 111)is clearly not linear. However, the response is linear at low concentrations and low concentrations are of greater interest here (see Figure 6). The linear regression line for chlorobenzene in Figure 6 has a correlation coefficient of 0.9993 confirming linear response. The nonlinear results above about 500 ppm may be attributed to the low solubility of chlorobenzene in’water. Studies have confirmed the ability
ANALYTICAL CHEMISTRY, VOL. 62, NO. 2, JANUARY 15, 1990
ppm) were recorded by using an amplifier with a gain of 220 to boost the signal. There is a clear relationship between the response for all three chlorinated compounds and the concentration. This response was more clearly linear for the less volatile compound. With the same apparatus, no response could be detected for 1000 ppm of either hexane and phenol in the water. Figure 7 shows response characteristics for the sensor to (1) 100 ppm aqueous phenol, (2) 100 ppm chlorobenzene in water, and (3) 100 ppm chlorobenzene in 100 ppm aqueous phenol. It illustrates that the apparatus can be used for monitoring chlorinated hydrocarbons in water even in the presence of phenol.
/'
1
>
-
1 2 5
185
, I
I
I,
,
1
1
. i
(I
5
I
I
I
i i
6
6 5
1'
I? P
Figure 6. Responses of sensor to chlorobenzene, l,l, l-trichioroethane, and chloroform at several concentrations in aqueous samples.
CONCLUSIONS There are several major conclusions from this study. Using a semipermeable membrane allows the application of relatively inexpensive gas sensors to the analysis of liquid samples. The use of inexpensive gas sensors offers an opportunity to design relatively low cost, small, and rugged equipment with the potential for continuous on-line analysis of aqueous streams. The simple silicone permeation tube is rugged, resistant to many chemicals, and easily inserted into the water stream and can provide the analyte to the gas sensor in a form suitable for quantitative analysis. The selectivity of gas sensors can be exploited by using the approach described herein. There is a linear relation between the sensor response and the sample concentration for chlorinated hydrocarbons at low concentrations. The fact that the system operates with dilute samples under constant conditions simplifies stability problems associated with the use of semiconductor sensors. Future work will be focused upon understanding the sensor mechanism so that sensors with greater sensitivity can be fabricated. In addition, optimum geometry for the permeation system will be investigated to allow the design of practical field sampling systems for continuous analysis of chlorinated hydrocarbons in water with more rapid response time. ACKNOWLEDGMENT We wish to acknowledge the inciteful comments of Robert Bredeweg on this work, especially for suggesting the silicone membrane technology for the permeator. LITERATURE CITED
t
f
cn
off
10
0 -ime
(Tin
)
Figure 7. Response characteristics for the sensor to (1) 100 ppm aqueous phenol, (2) 100 ppm chlorobenzenein water, and (3) 100 ppm chlorobenzene in 100 ppm aqueous phenol.
to use silicone membranes beyond the solubility point (1.9, but a precise model of this behavior has not yet been published. Three different chlorinated hydrocarbons, chlorobenzene, l,l,l-trichloroethane, and chloroform, in water were (individually) analyzed by using the sensor-permeation system. The responses at different aqueous concentrations (10-500
(1) Callis, J. B.; Illman, D. L.; Kowalski, B. R. Anal. Chem. 1087, 5 9 , 624A-637A. (2) Blanchard, R. D.; Hardy, J. K. Anal. Chem. 1086, 5 8 , 1529-1532. (3) Hellegeth, J. W.; Taylor, L. T. Anal. Chem. 1087, 59, 295-300. (4) Melcher, R. G. Anal. Chlm. Acta 1088, 214, 299-313. ( 5 ) Loh, Jack C.; Lu, Chih-shun S o l i State Sensor, US Patent 3,751,966, 197.1
(6) Baideen, J. Phys. Rev. 1047, 71, 717. (7) Weller. S.W.; Voltz, S.E. Adv. Catal. 1057, 9 , 215-222. 18) Morrison. Sci. I W 4 . 45. 20. . .. , S. . R. Surf. -i 9 j Barry, T. I.; Stone, F.S. R o c . k . S&. London, A 1960, 355, 124. (10) Logothetis, E. M.; Park, K.; Meitzler, A. H.; Laud, K. R . Appl. Phys. Left. 1075, 2 6 , 209. (11) Thomas, J. M.; Thomas, W. J. Introduction to the Principles of Heferogeneous Cataiysls; Academic Press: New York, 1967; p 44. (12) Stetter, J. R. J. Collok? Interface Sci. 1078, 65, 432-443. (13) Heiland, G. Sens. Actuators 1082, 2 , 343-361. (14) Sieyama, T.; Kato, A.; Fujiishi, K.; Nagatani, M. Anal. Chem. 1082, 34, 1502-1503. (15) Coyne, 6.; et al. Determining Organic Compounds Using a Membrane, US Patent 4,715,217, 1987.
RECEIVED for review July 31, 1989. Accepted November 7 , 1989. The support of the Dow Chemical Co. is gratefully acknowledged.
