Anal. Chem. 1980, 52, 1484-1489
1484
the recovery decreases for Cd2+,Co2+,and Zn2+,in that order. Other investigations measuring the linearity of the XRF response vs. the metal concentration (16) yielded a selectivity order for the DEN filter of Hg2+> UOZ2+> Ca2+> Pb2+> Cr3+ > Zn2+ > Co2+> Ni2+ > Cd2+> Mn2+.
CONCLUSION The new ion-chelating filter made of 2,2'-diaminodiethylamine-cellulose has proved to offer an extremely simple and rapid preconcentration technique for the determination of trace metal ions from practically all water samples. T h e precision and the accuracy of the DEN filter enrichment appeared to be in the 10 to 15% range, as is shown in Table I. A volume of 1 L can be filtered within 1 h through a filtration area of 10 cm2 without depression of the collection efficiency due to insufficient contact time or elution of already collected ions. No pH adjustments are required for most water samples since the DEN filters collect at a pH of 6 and up, thus covering the natural pH range. The DEN filters have proved to be insensitive to practically all naturally occurring abundant substances, including humic matter. By using also a common membrane filter before the DEN filter, both the suspended and the dissolved trace metals can be enriched separately in one simple sample filtration step, providing ideal thin samples for analysis with X-ray fluorescence, for example.
ACKNOWLEDGMENT Grateful acknowledgment is made to Eric Ooms for his technical assistance.
LITERATURE CITED Campbell, W. J.; Spano, E. F.; Green,T. E. Anal. Chem. 1888, 38, 987. Van Grieken, R. E.; Bresseleers,C. M.; Vanderborght, B. M. Anal. C b m . 1877, 49, 1326. Gendre, 0; Haerdl, W.; Linder, H. R.; Schreiber, B.;Frei, R . W. Int. J . Envlron. Anal. Chem. 1977, 5 , 63. Burba, P.; Lieser, K. H.; Neitzert, V.; Rober, H. M. Fresenlus' Z . Anal. Chem. 1878, 291, 273. Rober, H. M. PhD. Thesis, Technische Hochschule, Darmstadt, West Germany, 1978. Schwarzenbach, G. Anal. Chem. 1880, 3 2 , 6. Dingman, J., Jr.; Slggia, S.; Barton, C.; Hkcock, K. B. Anal. Chem. 1972, 4 4 , 1351. Leyden, D. E.; Luttrell, G. H. Anal. Chem. 1875, 4 7 , 1612. Leyden, D. E.; Luttrell, G.H.; Sloan, A. E.; De Angelis, N. J Anal. Chlm. Acta 1876, 8 4 , 97. Okamoto, G. Y.; Chou. E. J. Sep. Sci. 1875, IO, 741. Okamoto, G. Y.; Chou, E. J. Sep. Sci. 1876, 1 7 , 79. Smits, J.; Van Grieken, R . Angew. Mekromol. Chem. 1878, 72, 105. Knight, C. S. Adv. Chromatogr. 1987, 4 , 61. Lieser, K. H.; Furster, M.; Burba, P. Fresenlus' 2. Anal. Chem. 1877, 284, 199. Prue, J. E.; Schwarzenbach, G. Helv. Chlm. Acta 1850, 33, 985. Smits, J.; Van Grieken, R . Unpublished resuits. Cotton, F. A.; Wllkinson, G. "Advanced Inorganic Chemistry"; Interscience Publishers: New York, 1972. Law, S. L. Int. Lab. 1873, Sept.-Oct., 53. Holynska, B. Radiochem. Radloanal. Lett. 1874, 17, 313. Smlts, J.; Nelissen J.; Van Grieken, R. Anal. Chlm. Acta 1979, 111,215. Schnltzer, M.; Kahn, S. U. "Humic Substances in the Environment"; Marcel Dekker: New York, 1972. Steelink, C. J. Chem. Educ. 1877, 5 4 , 599. Florence. T. M.; Batley, G. E. Talenta 1877, 24, 151.
RECEIVED for review January 25, 1980. Accepted March 25, 1980. T h e Belgian Instituut ter Aanmoediging van Wetenschappelijk Onderzoek in Nijverheid en Landbouw is acknowledged for research fellowships to J.S.
Coating for a Piezoelectric Crystal Sensitive to Organophosphorus Pesticides Yutaka Tomita' and George G. Guilbault" Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 722
A piezoelectric quartz crystal coated with 3-PAD, Triton X-100 and NaOH, was found to be a sensitlve detector for assay of organophosphorus compounds possessing properties of hlgh sensitivity, selectivity, response time in the order of seconds, and long lifetime. Several coatings were screened for response to organophosphorus compounds: 2- and 3-PAD, 2-PAM, histidine hydrochloride, and succinyl choline salts were the most reactive and selective. Of these, 3-PAD was the best coating for compounds with the G agent structure, and hlstldine hydrochloride for compounds of the malathion type.
I t has been only a few decades since organophosphorus insecticides were lauded as one of the great contributions to the welfare of mankind. However, pest control with these remarkable agents has inadvertently contributed to a remarkable proliferation of these chemicals in our environment, culminating in the ever increasing problems of environmental pollution. Great interest has arisen in recent years concerning the effects of organophosphorus compounds on the environment, especially those compounds developed for chemical warfare applications. Analytical methods for these compounds 'Present address: I n s t i t u t e of Ecotoxicology, Gakushuin U n i v ersity, Toshima-ku, T o k y o 171, Japan. 0003-2700/80/0352-1484$01.00/0
in water, soil, and food have been developed and are yielding important information. A gas chromatograph with electroncapture, flame-photometric, or alkali-flame detector has achieved particularly widespread use (1,2). Nevertheless, very few methods have been described for direct-monitoring of organophosphorus compounds in the atmosphere. A major fraction of such compounds on environmental surfaces enters the air through volatilization. It may evaporate from soil, even after it is applied and worked in, or fine spray particles may vaporize during application and never reach the soil. The concentration of these organophosphorus compounds in air is usually very low (ppb to ppt range), and very large volumes of air must be sampled to accumulate enough of the pesticides to be detectable (3). Direct-monitoring methods that possess the desired sensitivity and selectivity are urgently needed, especially to detect minute concentrations that might be present in the environment. Piezoelectric quartz crystals are currently used for frequency control in communications equipment, selective filters in electronic equipment, measurement of the temperature and the dew point of gases, and in very accurate clocks. King ( 4 ) demonstrated that quartz crystals could also be used as sorption detectors by coating the crystals with appropriate compounds. In recent years, coated piezoelectric quartz crystals have been developed, not only as a highly sensitive and selective detector of various air pollutants ( 5 ) ,but also 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
as a simple, inexpensive, and portable device which is even small enough to be carried in a worker's pocket (6). The sensitivity is observed as a change in the resonance frequency of the crystal; this change in frequency is proportional to the weight of the substance adsorbed on the crystal surface. By coating the surface of crystal with a substance which will selectively adsorb a particular gas, it is possible to quantitatively determine the concentration of that gas. A coated piezoelectric crystal has the ability to detect pollutants in the atmosphere a t ppm and ppb levels. T h e relationship between the weight of coating materials deposited on the crystal surface and the change in frequency is calculated from Sauerbrey's basic equation ( 7 ) :
AF = -2.3
X
lo6 X
F
(AMs/A)
where aF = the frequency change (Hz), F = the basic frequency of the quartz plate (MHz), AMs = the mass of the deposited material (g), and A = area coated (cm2). Previous works (8,9) have shown that the theoretical limit of detection for a coated crystal is about lo-'' g. Equation 1 can be simplified as follows (10): AF = K-AC (2) where K = constant which refers to the basic frequency of the quartz plate, the area coated, and a factor to convert the weight of injected gas (g) into concentration (ppm or ppb), and AC = concentration (ppm or ppb) of sample gas. Since the amount of air pollutants is mostly expressed as a volume ratio in the gaseous state (ppm, ppb, etc.), Equation 2 is preferred. So far, the coated crystals have been applied to detect air pollutants and others: HC1 (IO),SO2 (11, 12), NOz (13),NH, (14), H2S (15), hydrocarbons (16), organophosphorus compounds (17), and explosives (18). In previous studies concerning coated crystals for the detection of organophosphorus compounds, two kinds of materials were utilized as adsorptive coatings: the FeC13complex of pesticides (19), and the Co complex of isonitrilobenzoylacetate (17). Although useful as detectors, the FeCl, complex lacked the sensitivity desirable in many cases of assay of organophosphorus compounds in the atmosphere, and also showed a very slow response time (about 15 min). Use of the Co complex was limited by a short lifetime (about 1 week) and a slow response time, although the sensitivity was very good. We examined many materials as coatings for the detection of organophosphorus compounds. One of these materials, l-n-dodecyl-3-(hydroximinomethyl)pyridiniumiodide (3PAD), was found to be the most promising coating, showing excellent selectivity, sensitivity, and fast response to organophosphorus compounds. For organophosphorus pesticides, diisopropyl methylphosphonate (DIMP) was used as a model compound in this study, because of the following reasons: (1) the organophosphorus pesticides are all structurally related and undergo similar reactions, (2) almost all organophosphorus pesticides contain either phosphoryl or thiophosphoryl groups, and the thiophosphoryl pesticides readily undergo oxidation reactions to produce phosphoryl-containing compounds, (3) DIMP is similar in chemical reactivity to the G agents, and (4) it has a relatively low toxicity. DIMP, a phosphoryl-containing compound, has been shown to be a suitable model for this kind of study (19).
EXPERIMENTAL Apparatus. The experimental setup with the piezoelectric quartz crystal detector is shown schematically in Figure 1. The design of the detector cell is largely the same as described previously (18). The cell design is the most sensitive one for use in a flow system.
II
/I
-e
1485
L-
9
Flgure 1. Experimental apparatus. (1) Power supply, (2) oscillator, (3) frequency counter, (4) digital-to-analog converter, ( 5 ) recorder, (6) flow meter, (7) detector cell with piezoelectric crystal, (8) air pump,
(9) injection port
The piezoelectric cryst& used are 9-MHz quartz crystals with silver-plated metal electrodes on both sides. The crystals are AT-cut and mounted in HC 25/U holders (JAN Crystals Mfg. Co.). The instrumentation consisted of a low frequency OX transistor oscillator (International Crystal Mfg. Co., Oklahoma City) powered by a regulated power supply (Heath Kit model 1P-28). The applied voltage was kept constant at 9 V dc. The frequency output from the oscillator was measured by a frequency counter (Health-Schlumberger model SM-4100), which was modified by a digital-to-analog converter, so that the frequency could be recorded. The frequency (peak maximum) could be read on either the frequency counter or the recorder. Air as the carrier gas was supplied by a vibrating diaphragm air pump. An empty gas chromatographic column of stainless steel was placed between the pump and cell in order to minimize a change in flow rate. The flow rate of air was maintained at 30 f 2 mL/min, according to our previous works (10, 17, 18). For field measurements of organophosphorus compounds, the apparatus shown in Figure 1is inadequate. A self-containedsmall, portable detector was designed using the basic components shown in Figure 1, but with a 6-V battery, a mini pump, and digital readout (6). This work depends upon injection of a 2-mL sample of air containing the pesticide, not continuous monitoring. A drift of the detector could result as the weights of the organic compounds present in air which can adsorb, accumulate on the crystal. Reagents. 2-PAD iodide, and polymers (polyvinylacetate and polyvinylbutyrate) were kindly supplied by Don Owens, Tulane University, New Orleans, La.; 3-PAD was supplied by T. Higuchi, University of Kansas, Lawrence, Kan.; Triton X-100 was obtained from Sigma Chemical Co., Saint Louis, Mo.; DIMP and other organophosphorus compounds, analytical grade, were obtained from commercial sources and used without further purification. Inorganic gases tested were from lecture bottles, Matheson Co., Inc., East Rutherford, N.J., and the organic chemicals used were reagent grade. Crystal Coating. The method of coating the crystal has been shown to affect the sensitivity of the detector (10). A number of techniques for coating will be usable, depending on the substrates: vacuum deposition, dipping, spraying, electrodeposition, spreading with a tiny brush or cotton swab, and dropping with a micro syringe. Among those, the dropping method is suitable for a reproducible amount of coating applied to a crystal (1 7 ) . The coated crystals used in this study were prepared by dropping a solution of the substrate on each electrode face, using a micro syringe. About 20% n-propyl alcohol was used as a solvent to obtain a uniform coating, since an aqueous solution of 3-PAD was observed to be difficult to spread over the entire surface of the electrode, indicating less wettability of the solution on the electrode material. The crystals were then placed in an oven at 80 to 100 "C for several hours so that the solvent evaporated leaving a thin coating of substrate on the surface. The amount of coating applied to the crystal was determined by the frequency change of the crystal due to the weight of the coating, and was quite reproducible. Dilution Method of Sample Gases. In our previous works (10,14), a syringe dilution method has been used to obtain desired concentrations of the sample gases. This method, however, was
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 I
I I
Alr
r(
Table I. Response of Coatings to Organophosphorus Compounds
coating
Flgure 2. Apparatus for flask dilution method. (1) Rubber septum, (2) heating tape, (3) 500-mL flask, (4) stirring bar, (5) magnetic stirrer
DIMPa
aF.Hz Malathion
ParathionC
L-histidine 30 1414 126 hydrochloride DL-histidine 474 23 hydrochloride 55 519 76 succinyl choline chloride succinyl choline 41 490 44 iodide 2-PAD 290 40 3 64 24 3-PAD Sample concentration was 1 5 ppm DIMP. Sample concentration was 1 ppm Malathion. Sample concentration was 1.5 D D m Parathion.
found to have a difficulty in reproducibility at very low concentrations (18). The tested gases and vapors in this study were then prepared by a flask dilution method. The apparatus is shown schematically in Figure 2. A known volume of the sample was injected into the 500-mL flask which was filled with air. The mixture was then stirred sufficiently to obtain a homogeneously diluted sample. A heating tape was used to prevent the sample gas from adsorbing on the glass wall of the flask. The diluted sample was drawn into a 2-mL syringe and then injected into the carrier gas stream at the injection port of the detector.
RESULTS AND DISCUSSION Coatings. Our previous work (17) has shown that the oxime, 1-methyl-2-hydroximinomethylpyridiniumsalt (2PAM), is sensitive to organophosphorus compounds as a coating, but is unstable owing to its high volatility in a gas stream. Epstein et al. (20) have indicated that 3-PAD is a much more effective nucleophilic reagent for the reaction with organophosphates than is 3-PAM. This kind of hydrolysis reaction, however, usually takes place in solutions as the reaction media. There has been no report on the interaction between solid hydrolysis reagent and gaseous organophosphorus substrate. Many other compounds were screened as coatings, using DIMP, malathion, and parathion as organophosphorus samples. As shown in Table I, several coatings were found to be very sensitive to malathion, but insensitive to DIMP and parathion. One of these, L-histidine hydrochloride also has good selectivity, and is an excellent coating material for malathion and parathion. On the other hand, 3-PAD and the position isomer, 1-dodecyl-2-hydroximinomethylpyridinium iodide (2-PAD), showed relatively high sensitivities toward DIMP. This 2-PAD is also well-known as a hydrolysis reagent for organophosphorus compounds. However, since the sensitivity of the 2-PAD coating was about 30% less than that of 3-PAD, 3-PAD was chosen as the coating for detection of G type agents. 3-PAD. Next, we examined several important factors for the reaction of the gaseous substrate with solid 3-PAD, considering results which have been reported on studies of effective hydrolysis reactions for organophosphates in solution. Epstein et al. (20) reported that both surfactant and high p H accelerate the hydrolysis reaction between organophosphates and 3-PAD in solution. Our studies were mainly conducted to determine the effects of surfactants and pH on the sensitivity of the detector. Some other parameters, such as cell temperature, were also investigated. Pure 3-PAD was first applied over the crystal as a coating, without any additives. When room air was used as a carrier gas, the change in frequency was 403 Hz for 15 ppm of DIMP, while it was 325 Hz using nitrogen as the carrier gas under
0 0
50 TRITON X - 1 0 0 ,
100 Wt%
Flgure 3. Effect on sensitivity of Triton X-100 in the binary mixture with 3-PAD
the same conditions. As will be discussed later, moisture in air was observed to increase the sensitivity. Therefore, room air without a drying agent was used as the carrier gas in these studies. Effect of Surfactant. Although Epstein et al. (20) used cetyltrimethylammonium bromide (CTAB) as a surfactant, Triton X-100 was examined in our study since it was found to be sensitive to DIMP by itself (310 Hz for 15 ppm DIMP). Using air as a carrier gas, the effect of the surfactant was determined by varying the ratio of Triton X-100 mixed with 3-PAD, which was always kept a t the same amount on the crystal. As shown in Figure 3, the change in frequency (and hence the sensitivity) increased with increase in the ratio of Triton X-100 up to 65% while the sensitivity decreased above 65770, approaching the AF for reaction with Triton X-100 alone, about 300 Hz. At the ratio of 65% Triton X-100, the sensitivity of the mixed coating was almost 50% higher than that of pure 3-PAD. The optimum ratio indicated in Figure 3 is about 60 to 70% for Triton X-100 in the binary mixture. Effect of Base. For a hydrolysis in aqueous solution, it is obvious that the rate would be expected to increase with increasing p H of solution. Sodium hydroxide was then used as a pH mediator for the following reasons: (1) NaOH is very strongly basic, and (2) a solid of NaOH has high deliquescence which would be desirable for the coating, as will be discussed later. The effect of NaOH on the sensitivity was determined, using a similar procedure as described above for the surfactant.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
~
1
0
0
1487
7
-5
"4;
-3
LL Q
I
I-
200-
-2
-1
01
0
LL:
t -
"
1
'
"
50 SCOlUM HYDROXIDE,
"
'
1 100
wt%
Figure 4. Effect on sensitivity of NaOH in the binary mixture with %PAD
In this paper, since it would be meaningless to define the p H in the solid state, the weight percentage of NaOH in the binary mixtures was used to show the ratios of NaOH in mixtures, instead of the pH. T h e results shown in Figure 4 indicated that the sensitivity decreased as the ratio of NaOH was increased above 50%. At the ratio of 35% NaOH, the sensitivity of the mixture increased about 35%, as compared with pure 3-PAD. T h e optimum ratio of NaOH in the binary mixture ranges between 20 and 50% as can be seen in Figure 4. Optimum Mixture. According to the above results, Triton X-100 and NaOH were mixed with 3-PAD to obtain the ternary mixture: 56% Triton X-100, 13% NaOH, and 31% 3-PAD, incorporating the optimum ratios of 65% Triton X-100/35% 3-PAD and 30% NaOH/70% 3-PAD that were described as optimum ratios for binary mixtures. T h e detector with the ternary mixture was more sensitive to DIMP than were those formulated with binary mixtures. With the ternary mixture, the change in frequency was 612 Hz for 15 ppm, compared to 590 Hz with the Triton X-100 binary mixture and 560 Hz with NaOH binary mixture. Piezoelectric crystal detectors are usually classified into two types, according to mechanism of response: cumulative (mercury detector (21)and reversible (sulfur dioxide detector (12)). T h e present detector is the reversible type, in which physical and chemical adsorptions of substrates on coatings are usually the major interactions (14,19). For detectors of the reversible type, adsorption of moisture on a coating is generally considered to decrease the sensitivity toward a sample to be detected, because the moisture occupies active sites of the coating. The sensitivity of the detector coated with 3-PAD mixture, however, was observed to be increased by moisture in air, as described in this paper. It was found that the main factors affecting the rate of the hydrolysis reaction include the solubility of an organophosphorus substrate in the micellar 3-PAD, and the association constant of the substrate with the micelle (20). In the present case, there is not sufficient information to determine an interaction mechanism between 3-PAD coating and DIMP. However, such a dissolution and association of DIMP that are probably catalyzed by the surfactant (Triton X-100) and moisture in the carrier gas (air) could be considered to be one of the interactions, in addition to physical and chemical adsorptions. Effect of Cell T e m p e r a t u r e . Phenomena such as dissolution and adsorption of gases on the surface of the coating are considered to be highly dependent upon temperature. The effect of cell temperature on the speed of response and sensitivity was then studied, using the ternary mixture as the coating. T h e response time from the injection of 15 ppm of
10
20
30
50
40
,
CELL TEMPERATURE
5 B M
60
"C
Figure 5. Effect of cell temperature on sensitivity and recovery time. Sample, 15 ppm DIMP. (0) Response curve, (0)recovery time curve
DIMP to the maximum height of a signal peak ranged from 10 s for room temperature to 2 s for 50 "C. On the other hand, the temperature markedly affected the recovery time (the time required until the signal resulting from a sample returns to the original base line). The recovery time, the reversibility of response, is rather important for the kind of detector envisioned, since the following measurement should be done after the signal returns to the original base line. As shown in Figure 5, the recovery time becomes very fast at higher temperatures (change of 30 s / 5 "C change in temperature). The temperature was also observed to have a significant effect on the sensitivity, as shown in Figure 5 and discussed also in Ref. 22. The results indicate that lower temperatures are better for a more sensitive detection, while the response is faster a t higher temperatures. The detection 1s useful at any temperature. Calibration Curve. The response curve for DIMP was obtained under the following conditions: the ternary mixture as the coating, room temperature (22 "C), a flow rate of 30 mL/min (the flow rate was not critical between 30 f 5 mL/min), and about 40 r g of coating substrate calculated from Equation 1. In order to plot the change in frequency vs. concentration over a large range (Le., from the ppb-level to ppm-level) on one calibration curve, the logarithm of both sides of Equation 2, log 0= log K log AC, was taken as reported in our previous work (18). This plot was linear over the range of concentrations 1ppb to 15 ppm. The change in frequency ranged from 612 Hz for 15 ppm to 63 Hz for 1 ppb. T h e frequency change of 63 Hz for 1 ppb suggests that a detection at sub-ppb levels might be possible. However, some problems concerning reproducibility, such as a dilution method, adsorption of sample gas on the glass wall of an injection syringe, and the uniformity of a sample injection, would pose some difficulties at very low concentrations. In the present experiments, a standard deviation was 3% for 15 ppm and about 15% for 1 ppb, depending on the concentration of DIMP. Although the flask dilution method used was found to be better than the syringe dilution method which has been used in previous works (10, 12),more improvement is needed to obtain reproducible data, especially a t the sub-ppb level. Similarly, malathion can be assayed in the range of 1 ppb to 10 ppm using L-histidine hydrochloride as a coating. Interferences. With the ternary mixture, the various interferences which would be expected to exist in air as pollutants and organic vapors were studied. As shown in Table 11,no serious interferences from inorganic gases were observed, except for sulfur dioxide a t high concentration. Experiments
+
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Table 11. Response of 3-PAD Mixture Coating to Interferences interferant
SO 2 "3
co benzene toluene ethyl alcohol chloroform a
concn, ppm
A F , ~Hz
1000 100 1000 100 1000 100 100 100 100
70 42 15 4 2 11 25 9 71
LL
AF to 15 ppm of DIMP = 610 Hz.
01 0
Table 111. Selectivity of PHistidine Hydrochloride compound Malathion SO2
HZS
3
benzene toluene chloroform ethyl alcohol
concn, ppm
A F , Hz
1 100
1414
100 100 100 100 100 100 100
7 10 5 13
6
18
9 6
Table IV. Lifetime of 3-PAD Mixture Coatinga days 1
10 20 25 30 a
A F , Hz
610 580 59 5 590 532
Sample concentration: 1 5 ppm DIMP.
were performed to find which component of the ternary mixture was active toward SO2. It was found that NaOH gave a high response to SOz, due to an acid-alkaline reaction. Therefore, it would be possible to eliminate the interference from SO2 by using the binary mixture of 3-PAD and Triton X-100 as the coating when measurements of air samples containing high concentrations of SO2 must be performed. Most of the organic vapors, such as benzene, toluene, or ethanol, were observed to give no serious interferences a t 100-ppm concentrations, while 100-ppm chloroform interfered owing to a dissolution of the coating. Since this concentration of chloroform is much higher than that usually found in ambient air, no interference would be expected. Very high concentrations of organic compounds in air could be expected to interfere in the piezoelectric crystal detector. Finally the selectivity of L-histidine hydrochloride, which is an excellent coating for some organophosphorus compounds such as malathion, was investigated. The results shown in Table 111, demonstrate that this substrate coating is highly selective for malathion, which is one of the most widely used insecticides. If mixtures of organophosphorus compounds are present in the sample, some type of separation (i.e., gas chromatography) must be effected prior to measurement. Lifetime. A lifetime of the ternary mixture as the coating was measured under a continuous flow of carrier gas. As shown in Table IV, the coating was observed to have a reasonably long lifetime. Even after a month of use, loss of sensitivity was only 12%. Generally, the lifetime of the coating is limited by several factors: an oxidation of coating by air, a loss of coating by its evaporation, saturation of the active sites of the coating with sample, etc. Among those, the loss of coating is usually the main problem for the lifetime of the
u
200 -
a
" " " " ' I
100
50 POLYVINYL ACETATE
,
wt%
Flgure 6. Effect on sensitivity of polyvinylacetate in the binary mixture with 3-PAD
coating. A trial was then made to overcome this problem, using a polymer as an additive to the coating. This idea is based on the fact that a polymer mixed with a sensitive coating will lower the vapor pressure of coating, which will make the loss of coating less. Polyvinylacetate and polyvinylbutyrate (provided by Don Owens a t Tulane University, New Orleans, La.) were examined as polymer coatings. As shown in Figure 6, the results showed that the sensitivity of coating with polyvinylacetate decreased as the ratio of polymer in the mixtures increased. Probably the active sites of the sensitive materials were covered with the polymer films. Polyvinylbutyrate showed almost the same results. In these trials, addition of the polymers decreased the sensitivities of the detector. This idea, however, would be promising to obtain a coating with a long lifetime, since a short lifetime due to a loss of coating is occasionally a very serious problem for this kind of detector. CONCLUSIONS The piezoelectric quartz crystal coated with the ternary mixture, composed of 3-PAD, Triton X-100, and NaOH, was found to have outstanding advantages when compared with previously described coatings ( I 7,19): higher sensitivity, faster response, and longer lifetime. Some other coatings tested showed high sensitivities and selectivities to malathion. The high sensitivity and selectivity of the coating with the 3-PAD mixture toward DIMP predicts that this coating will be an excellent choice for detection and assay of G agents, since DIMP is the best model for such compounds (19). L-Histidine hydrochloride is, likewise, an excellent coating for assay of malathion. LITERATURE CITED Bevenue, A. In "Analytical Methods for Pesticides, Plant Growth Regulators, and Food Addiives", Zweig, G., Ed.; Academic Press: New York, 1963; Chapter 9. Fishbein, L. J. Chromatogr. 1974, 98, 175. Cooke, L. M., et al. "Cleaning Our Environment-The Chemical Basis for Action". American Chemlcai Society: Washington, D.C., 1969; p 193. King, W. H., Jr. ReslDev. 1989, 20(5), 28. Hlavay, J.; Guilbault, 0. G. Anal. Chem. 1977, 4 9 , 1890. Scheide, E. P.; Warnar, R. B. J. Am. Ind. Hyg. Assoc. J. 1978, 3 9 , 745. Sauerbrey, G. 2. 2. Phys. 1959, 755. 206. Sauerbrey, G. 2. 2.Phys. 1964, 778, 457. King, W. H., Jr. Anal. Chem. 1964, 36, 1735. Hlavay, J.; Guilbault, 0.G. Anal. Chem. 1978, 5 0 , 965. Cheney, J. L.; Homolya, J. G. Anal. Left. 1975, 8 , 175. Karmarkar, K. H.; Guilbautt, G. G. Anal. Chim. Acta 1974, 7 7 , 419. Karmarkar, K. H.; Guilbault, G. G. Anal. Chlm. Acta 1975, 75, 111. Hlavay, J.; Guilbault, G. G. Anal. Chem. 1978, 5 0 , 1044. Webber, L. M.; Karmarkar, K. H.; Guilbault, G. G. Anal. Chlm. Acta 1978, 9 7 , 29. Karmarkar, K. H.; Guilbault, G. G. Environ. Left. 1975, IO, 237. Shackelford, W. M.; Guilbault, G. G. Anal. Chlm. Acta 1974, 73, 383. Tomlta, Y.; Ho, M. H.; Guilbault, G. 0. Anal. Chem. 1979, 57, 1475. Scheide, E. P.; Guilbault. G. G. Anal. Chem. 1972, 4 4 , 1764.
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,
1489
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. The 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 the 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 and 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
