ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
s. The activity observed in the film levels off after about 5000 s has elapsed. Further research will be necessary before the nature of the observed behavior can be completely understood. The presence of this unexposed photoresist on the surface of the SAW device provided a convenient opportunity to study the effects of photo cross-linking of the resist. When polymers are cross-linked the effect is to raise the Tg(Le., make the polymer more brittle). This is seen in Figure 13. After illumination with UV light, the attenuation of the wave is reduced which is precisely the response expected. The signal-to-noise ratio obtained was poor. I t should be noted that there was no attempt made to filter the signal either by analog or digital means in both experiments. The important fact is t h a t it was possible to observe the effect of UV induced cross-linking in a very small photoresist film. Improvements in instrumentation could provide significantly greater signal-to-noise ratios. ACKNOWLEDGMENT T h e authors thank A. J. Wnuk and T. C. Ward for the
1475
polymer film samples and valuable discussions.
LITERATURE CITED Wohltjen, H., Dessy, R . E. Anal. Chem., preceding paper in this issue. Ward, I.M. "Mechanical hoperties of Soli Polymers", Wiley Interscience: New York, 1971; p 98. Rosen, S.L. "Fundamental hinciples of Polymeric Materials for Practicing Engineers"; Barnes & Noble, New York, 1971; p 229. Wendlandt, W. W. "Thermal Methods of Analysis", 2nd ed., Wiley Interscience: New York, 1974. Haldon, R . A,; Schell, W. J.; Simha, R. "Transitions in Glasses at Low Temperatures," "Cryogenic Properties of Polymers", Koenig and Serafini, Ed., Marcel Dekker: New York, 1968; p 152. Haldon. R . A.; Schell, W. J.; Simha, R. Ref. 5 , p 148. Dill, F. H.; Shaw, J. M. "Thermal Effects on the Photoresist AZ1350J," IBM J . Res. Dev. 1977, 21, 210. Hatzakis, M., Thomas J. Watson Research Laboratories, P.O. Box 218, Yorktown Heights, N.Y. 10598, private communication, April 25, 1978.
RECEIVED for review January 22,1979. Accepted May 1,1979. The authors thank the Gillette Charitable and Educational Foundation whose funds helped support this research. All plots were made on a Benson Lehner Plotter, a gift from Corning.
Detection of Explos ves with a Coated Piezoelectr c Quartz Crystal Yutaka Tomita, Mat H. Ho, and George G. Guilbault" Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 722
A coated pierolelectric quartz crystal, which has potential use as a simple device for assay of explosives, is used for the detection of mononitrotoluenes (MNT). The detector can indicate the presence of trinitrotoluene, the less volatile parent molecule. Carbowax 1000 was found to be useful as a coating for the Sensitive and selective detection. With the coating, MNT vapor in the ppb-ppm range can be detected without serious interferences. The response time observed was only 10 s and a complete reversibility of response was obtained in less than 50 s. Some parameters that affect the efficiency of the detector (amount of coating, interferences, flow rate, temperature) were also investigated.
A highly sensitive and selective method for detection of explosives is in great need. For example, airport surveillance is only able to check for the presence of metallic materials but cannot detect explosives strapped to an individual's body. Some of the recent skyjackings have been caused by this threat. A number of techniques for detection of explosives have been reported using gas chromatography ( I , Z ) , mass spectrometry (3),NMR ( 4 ) ,plasma chromatography ( 5 , 61, thin-layer chromatography (71, and visible spectrometry (8). While some of these laboratory techniques are capable of ppb detection and may be satisfactory for a specific purpose, the detection systems need elaborate techniques for operation and are usually not portable and simple, thus are not useful for field use. In recent years there has been a growing interest in coated piezoelectric quartz crystals, not only as a highly sensitive and selective detector of various air pollutants (9) but also as a simple, inexpensive, and portable device which is even small enough to be carried in a worker's pocket (10). King ( I I ) , 0003-2700/79/0351-1475$01.00/0
developed a sensitive piezoelectric-sorption detector for monitoring hydrocarbons in the atmosphere. Janghorbani and Freund (12) have described the use of these crystals as digital sensors for sulfur compounds commonly found in pulp mill effluents. Frechette and Fasching (13)have proposed their use in a static system for the detection of sulfur dioxide. Cheney et al. (14) have described several coatings suitable for sulfur dioxide detection. Guilbault et al. (15-21) developed a sensitive crystal cell design and coatings for organophosphorus pesticides, and inorganic gases such as SO2,NO2, NH3, HCl, and H2S in the atmosphere. The 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, thereby increasing the weight on the crystal and decreasing the frequency of vibration. The decrease in the frequency in proportional to the increase in weight due to the presence of gas adsorbed on the coating, according to the following equation (20).
AF = K-LC
(1)
where AF is the frequency change (Hz), K is a constant which refers to the basic frequency of the quartz plate, area coated, and a factor to convert the weight of injected gas (9) into concentration (ppm), and I C is concentration (ppm), of sample gas. Previous works (22, 23) have shown t h a t the theoretical limit of detection for a coated crystal is about g and that a commercially available 9-MHz crystal would have a mass sensitivity of about 400 Hz/pg. We examined many commercially available materials as coatings for the detection of mononitrotoluene (MNT), which is a volatile substance that can serve as a reliable indicator for the presence of its less volatile parent, trinitrotoluene C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
-UT-Detecror Cel;
.
Flowmete.
i
.... ......
I
Carrier Gas
T
Injection
Flgure 1. Experimental apparatus with the piezoelectric crystal detector. (1) Power supply, (2) oscillator, (3)frequency counter, (4) digital-teanalog converter, (5)recorder
10
5 00
20
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FLOW
RATE
40
,
50
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ml/rnin
Figure 2. Effect of gas flow rate on sensitivity. Conditions: Sample size, 7.5 ppm X 10 mL; cell temperature, 50 "C; carrier gas, nitrogen
(TNT). Among these materials Carbowax 1000 shows a very sensitive and selective response to M N T vapor. In this paper we describe a n evaluation of Carbowax 1000 as a coating material, and also other parameters that affect the efficiency of t h e detector.
EXPERIMENTAL 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 (20). An important feature of this design is that the injected sample gas is split into two equal streams which directly and simultaneously fall on the opposite faces of a coated crystal. The two streams of the sample gas are brought very close to the electrodes of the crystal, which are the most weight-sensitive positions. Because this system reduces the possibility of an undetected escape of the sample gas, the sensitivity of the detector becomes much greater (15).
The piezoelectric crystals used in this study 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., Florida). The instrumentation consisted of a low frquency OX transistor oscillator (International Crystal Mfg. Co.) powered by a regulated power supply (Heath Kit model IP-28). The applied voltage was kept constant at 9 V-dc. The frequency output from the oscillator was measured by a frequency counter with a resolution of 0.1 Hz (Heath-Schlumberger model SM-4100), which was modified by a digital-to-analog converter (Z), so that the frequency could be recorded. The frequency could be read on either the frequency counter or the recorder; the data resulting from the injection of a sample were recorded as a frequency change and as a peak on the recorder. Carrier gas, air or nitrogen gas, was supplied by a vibrating diaphragm air pump or from a cylinder of nitrogen gas. Temperatures of the detector cell and lines were controlled by heating tapes and voltage regulators. A 10-mL sample was injected into the carrier gas stream at the injection port by using a gas syringe. The carrier gas stream with sample flowed into the detector cell where the sample was adsorbed on the coated crystal; the mixture of the carrier and sample gas came out from the bottom of the detector cell. Carbowax 1000 (Applied Science Laboratories, Inc.) was used as the coating materials. The coated crystals were prepared by dropping a solution of the substrate in a volatile solvent (chloroform or ethylacetate) on each electrode face, using a micro syringe. The crystals were then placed in an oven at 80 "C for several hours so that the solvent evaporated leaving a thin coating of substrate on the surface. When the air was used as carrier gas, about an hour was required for equilibration of the crystal after it was placed in the air stream of the cell. The amount of coating applied to the crystal was determined by the frequency change of the crystal due to the weight of the coating. The purity of o-MNT liquid (ICN Pharmaceuticals Inc.) used was checked by UV (25) and GC (26) using a column packed with UC-W98 silicone gum rubber or Dexil300 GC. Inorganic gases tested were from lecture bottles (Matheson Co., Inc.) and organic chemicals used were reagent grade. The tested gases and vapors
100
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1
C
10
20
30
40
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, OC Figure 3. Effectof cell temperature on sensitivity. Conditions: Sample size, 7.5 ppm X 10 mL; flow rate, 30 mL/min; carrier gas, nitrogen CELL TEMPERATURE
were prepared by the syringe dilution method as earlier described by Karmarkar and Guilbault ( 2 4 , 2 7 )and also by Karasek and Tiernay (28). The concentration of MNT vapor, which was generated a t 50.0 f 0.1 "C by using this method, was calculated from vapor pressure and also measured by UV (25).
RESULTS AND DISCUSSION While many workers have reported coated piezoelectric crystals for various purposes, few (14,29) have described in detail the parameters which affect the efficiency of the detector. In order to determine the optimum condition for the detection of MNT, several important parameters were examined using Carbowax 1000 as a coating. Sensitivity. T h e important parameters to be considered in the sensitivity are mainly the flow rate of carrier gas, temperature of the detector cell, and amount of coating. T h e effect of flow rate on the sensitivity was determined by varing the flow rate of carrier gas from 10 t o 60 mL/min through the detector cell, a t 50 "C. T h e results shown in Figure 2 indicate that as the flow rate was increased above 30 mL/min, the change in frequency (and hence the sensitivity) decreased, owing to the incomplete adsorption of M N T vapor on the surface of the coating. On the other hand, as the flow rate decreased below 30 mL/min, the diffusion of M N T vapor became more significant resulting in a broader peak and lower sensitivity. The optimum flow rate indicated in Figure 2 is about 30 mL/min. T h e sensitivity was affected not only by flow rate but also by the temperature of the detector cell, as shown in Figure 3. While a n insignificant decrease in the sensitivity was observed u p t o 50 "C, the temperature above 50 "C had a marked effect on sensitivity. T h e change in frequency was 74 Hz a t 80 "C and 186 Hz a t 50 "C for 7 . 5 ppm MNT. T h e data indicate that a lower temperature is better for detection,
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979 r
15C
Table 1. Response of Carbowax 1000 Coating t o Organic Interferences interferant concn, ppm A F , Hz ethyl alcohol 1000 10 benzene 1000 5 toluene 1000 20
- I
\
acetone chloroform
500 1000 1000 500 200
0
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10
20
30
40
50
CELL TEMPERATURE
60
,
70
80
'C
Figure 4. Effect of cell temperature on speed of response. Conditions: Sample size, 7.5 ppm X 10 mL; flow rate 30 mL/min; carrier gas, nitrogen
because of a higher sensitivity. However, the temperature also affects the response time of the detector, since the adsorption and desorption of M N T vapor on the surface of the coating are dependent on temperature. The determination of the optimum temperature for the detection of M N T will be discussed later with the data for speed of response. Our earlier study (16) has shown that the sensitivity is highly dependent on the amount of coating applied to the crystal. The optimum amount of Carbowax 1000 was then determined by measuring the frequency vs. the amount of coating according to the Sauerbrey equation (22),until the crystal was overloaded provided the maximum sensitivity. In this study, the maximum amount was 30 pg, calculated from a 12-KHz decrease in the basic frequency of the crystal. Speed of Response. For rapid determination of MNT, the speed of response was studied as a function of cell temperature and flow rate. The effect of temperature on speed of response is shown in Figure 4. Curve A represents the response time from the injection of sample to the maximum height of a signal peak. The curve indicated that the effect of temperature on the response time was not significant. Curve B represents the total time which is required until the signal resulting from a sample returns to an original base line, as shown in Figure 4. This total time, a reversibility of response, is rather practical for the kind of detector, since the following measurement should be done after the signal returns to the original base line. The reversibility of response was observed to increase as cell temperature increased, indicating that desorption of M N T on the surface of the coating was highly dependent on temperature. Although a t higher temperatures the response is faster, there is a significant decrease in the sensitivity above 50 "C as mentioned before. Therefore, the cell temperature should be below 50 "C. Considering the large increase in the reversibility, and also the prevention of condensation of MNT vapor and atmospheric moisture on the cell wall, a cell temperature of 50 "C would be practical for both sensitivity and reversibility. The flow rate of carrier gas had a direct effect on both response time and reversibility. Increasing the flow rate from 30 to 60 mL/min nearly doubled the speed of response. However, as described before, this increase in flow rate reduced the sensitivity by approximately 20%. For this reason, 30 mL/min was chosen as the optimum flow rate with respect to both sensitivity and speed of response. Calibration Curve. The response curve for M N T vapor was obtained under optimum conditions: a flow rate of 30 mL/min, a cell temperature of 50 "C, and 30 pg of coating substrate. T o plot the change in frequency vs. concentration over a large range, Le. from the ppb-level to ppm-level, on one
11 10 50 21 9
Table 11. Interferences of Perfumes Frequency Change, Hz high low c0ncn.a concn.6 perfume 20 0 Gambit Chant illy 25 8 Eau de Toilette 21 6 a Perfume concentration saturated at room temperature One-tenth concentration of in a round bottom flask. the saturated perfume at room temperature. calibration curve, the logarithm of both sides of Equation 1 was taken as reported in our previous work (19): log AF = log K + log hC was plotted. This plot was linear over the range of concentrations 3 ppb to 7 . 5 ppm (flow rate 30 mL/min; 50 "C cell temperature; 30 pg coating; Nz carrier gas). The results fit the general equation AF = 11.4 MN7?.319. The change in frequency ranged from 193 Hz for 7.5 ppm to 17 Hz for 3 ppb. A detection limit of 1ppb was observed, with less reproducibility, perhaps due to the dilution method used (20) (i.e., difficulty in reproducibly diluting a stock M N T gaseous solution many times to a very low concentration). Interferences. With Carbowax 1000, no serious interferences were observed from any inorganic gases, such as CO, SOz, NH3, or NOz, a t 100-ppm concentration. Most of the organic vapors tested also gave no interferences, except for some perfumes and high concentrations of organic solvents. Interferences from organic vapors tested are listed in Table I. High concentrations of organic solvents, such as chloroform, interfere owing to a dissolution of the coating. The vapors of perfumes, which were saturated a t room temperature in round bottom flasks, also gave some response on the coating, as shown in Table 11. However, these concentrations of perfumes used are much higher than those usually found in ambient air. When the concentrations of these vapors were diluted only 10-fold with air, the frequency changes due to the perfumes were reduced from 20-25 Hz to 0-8 Hz. Since the actual concentrations of perfumes in the air to be detected can be considered much less than one-tenth of the samples which were saturated a t room temperature, no interference would be expected. Thus, unlike M N T to which the crystal shows greater sensitivity a t low concentration, and low sensitivity a t high concentrations (ideal behavior), the crystal gives little or no response a t low concentrations of interferences, and low sensitivity a t higher amounts. The coating was also active toward atmospheric humidity. An injection of room air produced a small frequency change, when nitrogen was used as carrier gas. However, since the humidity of room air is relatively constant during measurement, this interference can be compensated by using air of the same humidity as the carrier gas. Actually, no interference from humidity and no significant change in the sensitivity were observed when room air was used as a carrier gas and also as diluent for M N T vapor. The use of ambient air instead of nitrogen is more practical for field work; however,
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
less stability in the base line due to humidity changes might become a problem in long-term continuous measurement. Life Time. T h e Carbowax 1000 coating showed a reasonably long life time. T h e coated crystal gave almost the same sensitivity for the M N T vapor even after a month of use. It is concluded that this piezoelectric quartz crystal coated with Carbowax 1000 is useful for the detection of M N T with good selectivity and fast response, which are required for a T N T monitor, for example, in airports.
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
LITERATURE CITED R. W. Dalton, J. A. Kohlbeck, and W. T. Bolleter, J . Chromatogr., 50, 219 (1970). D. G. Gehring and J. E. Shirk, Anal. Chem., 39, 1315 (1967). J. Yinon. H. G. Boettger, and W. T. Weber, Anal. Chem., 44, 2235 (1972). D. G. Gehring, Anal. Chem., 42, 898 (1970). G. E. Spangler and P. A. Lawless, Anal. Chem., 50, 884 (1978). F. W. Karasek, Anal. Chem., 46, 710A (1974). C. D. Chandler, J. A. Kohlbeck, and W. T. Boileter, J . Chromatogr., 64, 123 (1972). J. C. Hoffsommer and D. J. Glover, J . Chromatogr., 62, 417 (1971). J. Hlavay and G. G. Guilbault. Anal. Chem., 49, 1890 (1977). E. P. Scheide and R. B. J. Warnar, Am. Ind. Hyg. Assoc. J . , 39, 745 (1978).
(27) (28) (29)
W. H. King, Jr., Environ. Sci. Technol., 4, 1136 (1970). M. Janghorbani and H. Freund, Anal. Chem., 45, 325 (1973). M. W. Frechette and J. L. Fasching. Environ. Sci. Technol.,7, 1135 (1973). J. L. Cheney and J. B. Homoiya, Anal. Left., 8, 175 (1975). K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71, 419 (1974). E. P. Scheide and G. G. Guilbault, Anal. Chem., 44, 1764 (1972). K . H. Karmarkar, L. M. Webber, and G. G. Guilbault, Environ. Lett., 8, 345 (1975). K. H. Karmarkar and G. G. Guilbault, Anal. Chlm. Acta, 75, 111 (1975). J. Hlavay and G. G. Guilbault, Anal. Chem., 50, 1044 (1978). J. Hlavay and G. G. Guilbault, Anal. Chem., 50, 965 (1978). L. M. Webber, K. H. Karmarkar, and G. G. Guilbault, Anal. Chlm. Acta, 97, 29 (1978). G. Z. Sauerbrey, Z.Phys., 178, 457 (1964). W. H. King, Jr., Anal. Chem., 36, 1735 (1964). L. M. Webber and G. G. Guilbault, Anal. Chem., 48, 2244 (1976). C. P. Conduit, J . Chem. SOC.,3273 (1959). N. B. Jurinski, G. E. Podolak, and T. L. Hess, Am. Ind. Hyg. Assoc. J., 36,497 (1975). K. H. Karmarkar and G. G. Guilbault, Environ. Left., 10, 237 (1975). F. W. Karasek and J. M. Tiernay, J . Chromatogr.. 89, 31 (1974). J. Cheney. T. Norwood, and J. Homolya, Anal. Lett., 9, 361 (1976).
RECEIVED for review February 12,1979. Accepted May 7,1979. The authors gratefully acknowledge the financial support of the Army Research Office, in the form of Grant No. DAAG-77-G-0266, in carrying out this research project.
Assay for Arsenic Trioxide in Air Carroll A. Snyder* and Daniel A. k o l a New York University Medical Center, Department of Environmental Medicine, A . J. Lanza Laboratories, Long Meadow Road, Tuxedo, New York 10987
A quick, safe, and inexpensive method for the assay of arsenic trioxlde In air has been developed. The procedure utilizes an ultraviolet absorbing moiety that develops when As,O, is dissolved in alkaline solutions. Arsenic trioxide dust is removed from air by filtration and dissolved in 1 N sodium hydroxide solution. The resultant solution is read in an ultraviolet spectrophotometer at 222 nm against a sodium hydroxide blank. The calibration curve gives a linear response from 11 pg to 44 pg As,O, per mL NaOH. Absorptivity data and some concepts on the nature of the bonding responsible for the observed spectral data are also presented.
arsenic and oxygen atoms. Alkaline solutions were used to accelerate the dissolution of As203 and to minimize interference from As" compounds which will not absorb a t the chosen wavelengths in basic solutions above certain strengths. Comparisons of the absorptivities of As"' and AsVcompounds in solutions of varying base strengths were used to determine the optimal base concentration for the procedure and to shed some light on the nature of the ultraviolet absorbing moiety. For our purposes, a sample for analysis could be collected in 3 min. Sample preparation required only a n additional 2 min and, therefore, airborne As203levels could be determined in 5 m i n intervals.
EXPERIMENTAL T h e most ubiquitous of the arsenic compounds found in industrial environments seems to be arsenic trioxide and there are firm data linking exposure to this compound with excessive lung and skin cancer ( I ) . In spite of the evidence for arsenic-induced cancer in humans, attempts to reproduce cancer in animals by arsenic exposure have been largely unsuccessful ( I ) . In this laboratory efforts have been focused on the experimental induction of lung cancer by treatment of animals with controlled atmospheres of As203dust. These exposures require frequent monitorings of the test atmospheres in order to make real-time corrections in the generating systems so that constant exposure levels can be maintained. The current procedures for the assay of arsenic in air, while accurate and precise (2-5), are too time consuming and, therefore, make frequent, routine monitoring difficult. As part of the study investigating the effects of exposure to As203dust, a new procedure for determining concentration of airborne As203was developed. The procedure utilizes the ultraviolet absorbance of the da-pa bonding between adjacent 0003-2700/79/0351-1478$01 .OO/O
Instrumentation and Equipment. Ultraviolet spectra scans and absorptivities were determined on a UV-visible double-beam scanning spectrophotometer equipped with a strip chart recorder (Perkin-Elmer Model 525, Oak Brook, Ill. 60521). Calibration curves and routine atmosphere monitorings were performed on a UV-visible, single-beam spectrophotometer (Hitachi Model 100-40, Tokyo, Japan). Air samples were taken with a sample train consisting of: (1) a modified end-of-line Swinney filter holder containing a 0.5-inch (1.27 cm) Whatman 41 filter paper, (2) a standard diaphragm pump with a 10 L/min capacity, and (3) a wet test meter (GCA/Precision Scientific, Chicago, Ill.) to determine air sample volumes. Arsenic trioxide dust atmospheres were generated in a 128-L stainless steel exposure chamber (6) using a Wright dust feed (L. Adams, Ltd., London) with an in-line cyclone to remove large particles, thereby maintaining a flow of respirable-sized particles (mass median diameter 2.83.2 pm). The atmosphere generating system and exposure chamber were completely enclosed in a glove box hood. Procedures. The optimal absorptivities of the arsenic compounds As203,?;aAs02, and Na2HAs0,.iH20 were determined as a function of base concentration by adding appropriate amounts c 1979 American Chemical Society
