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Anal. Chem. 1993, 65,1868-1881
Smart Sensor System for Trace Organophosphorus and Organosulfur Vapor Detection Employing a Temperature-Controlled Array of Surface Acoustic Wave Sensors, Automated Sample Preconcentration, and Pattern Recognition Jay W. Grate,’*+ Susan L. Rose-Pehrsson, and David L. Venezky Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000
Mark Klusty and Hank Wohltjen Microsensor Systems, Inc., 62 Corporate Court, Bowling Green, Kentucky 42103
A smart sensor system for the detection of toxic organophosphorus and toxic organosulfur vapors at trace concentrations has been designed, fabricated, and tested against a wide variety of vapor challenges. The key features of the system are an array of four surface acoustic wave (SAW) vapor sensors, temperature control of the vapor sensors, the use of pattern recognition to analyze the sensor data, and an automated sampling system including thermally desorbed preconcentrator tubes (PCTs). All the electronics necessary to control and operate the various subsystems and to collect and process the data are included in the system. Organophosphorus analytes were detected at concentrations as low as 0.01 mg/m3in 2 min, and the organosulfur analyte was detected at 0.5 mg/m3in 2 min. Pattern recognition algorithms correctly classified these analytes at these concentrations and discriminated them for a variety of other organic vapors. INTRODUCTION The decade of the 1980s saw great increases in research on microelectronic chemical sensors. (See for example, refs 2-6.) Several factors contributed to bring this about, including (1) microfabrication and micromachining techniques to fabricate sensor structures, (2) increasing interest in chemical detection for occupational safety, environmental monitoring, and process control, and (3) the increasing sophistication and decreasing size of digital components and instrumentation capable of operating sensors or using the information they provide. In addition, molecular recognition emerged as a subdiscipline of chemistry. Chemical sensor research has focused on various aspects of sensor use and performance, including device structures, physical transduction mechanisms, chemically selective materials, fundamental interactions responsible for selectivity, sensors for particular applications, and use of pattern recognition techniques to + Present address: Pacific Northwest Laboratory,Battelle Boulevard, Richland, WA 99352. (1)Deleted in proof. (2) Wohltjen, H. Anal. Chem. 1984, 56, 87A-103A. (3) Haugen, G.; Hieftje, G. Anal. Chern. 1988, 60, 23A-31A. (4) Hughes, R. C.; Ricco, A. J.; Butler, M. A.; Martin, S. J. Science 1991,254, 74-80. (5) Gopel, W. Sens. Actuators B 1991,4, 7-21. (6) Janata, J. Principles of Chemical Sensors; Plenum Press: New York, 1989.
0003-2700/93/0365-1868$04.00/0
analyze data from arrays of sensors. The ultimate goal of this interdisciplinary research is to be able to produce useful and reliable detection instrumentation. This leads to the consideration of sensor systems, as opposed to individual sensors. For discussion purposes, we will define an individual sensor as consisting of a physical transducer and an applied chemically selective material. Sensor systems consist of one or more of these individual sensors, circuitry to power the sensor(s) and to produce analytical signals, signal measurement, data processing, and a final output, while packaging the sensors in a suitable microenvironment for reliable operation. The use of sensor arrays with pattern recognition has been investigated by a number of laboratories.’-21 Detectors using this approach are sometimes popularly referred to as “smart sensor systems”,“odor-sensing”systems, and/or “electronic noses”.22,23 In this paper, we describe a sensor system developed for the detection of certain highly toxic organic vapors a t trace concentrations. The key features of the system are an array of four surface acoustic wave (SAW) vapor sensors, temperature control on the vapor sensors, the use of pattern recognition to analyze the sensor data, and an automated sampling system including thermally-desorbed preconcentrator tubes (PCTs). SAW vapor sensors with absorbent polymer coatings respond rapidly, reversibly, and reproducibly to organic vapors and can be quite sensitive. The ( 7 ) Abe, H.; Yoshimura, T.;Kanaya, S.; Takahashi, Y.; Miyashita, Y.; Sasaki, S. Anal. Chirn. Acta 1987, 194, 1-9. (8) Abe, H.; Kanaya, S.; Takahashi, Y.; Sasaki, S. Anal. Chirn. Acta 1988,215, 155-168. (9) Carey, W.P.;Beebe,K.R.;Kowalski,B.R.;Illman,D.L.;Hirshfeld, T. Anal. Chem. 1986,58, 149-153. (10) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986,58,3077-3084. (11) Carey, W. P.; Beebe, K. R.; Sanchez, E.; Geladi, P.; Kowalski, B. R. Sew. Actuators 1986,9, 223-224. (12) Carey, W. P.; Beebe, K. R.;Kowalski, B. R. Anal. Chern. 1987,59, 1529-1534. (13) Ema, K.; Yokoyama, M.; Nakamoto, T.; Moriizumi, T. S e m . Actuators 1989, 18, 291-296. (14) Abe, H.; Kanaya, S.; Takahashi, Y.; Sasaki, S. Anal. Chern. Acta 1988, 215, 155-168. (15) Stetter, J. R.; Jurs, P. C.; Rose, S. L.Anal. Chern. 1986,58,860866. (16) Gardner, J. W. Sens. Actuators B 1991, 4, 109-115. (17) Muller, R.; Lang, E. Sens. Actuators 1986, 9, 39-48. (18) Muller, R. Sens. Actuators B 1991,4, 35-39. (19) Ballantine, D. S.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986,58, 3058-3066. (20) Rose-Pehrsson, S. L.;Grate, J. W.; Ballantine, D. S.; Jurs, P. C. Anal. Chem. 1988, 60, 2801-2811. (21) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991,3,85-111. (22) Newman, A. R. Anal. Chem. 1991,63, 585A-588A. (23) Amato, I. Science 1991, 251, 1431-1432. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 65, NO. 14,J l L Y 15, 1993
selectivitiesofthe individualsensorsin an array vary aeeordiig to the properties of the applied polymer material. (An extensiveliteratureon theuse of SAW chemicalsensorsexists; for reviews and selected papers see ref 19-21 and 24-36.) All the electronics necessary to control and operate the various subsystems and to collect and process the data are included in the system. I t is a stand alone unit (ca. shoebox size) powered by 115 VAC. The emphasis in this study was to evaluate the analytical approach in a complete system. The system was not ruggedized or miniaturized. The performance of a sensor or sensor system is usually considered intermsofsuchfactorsasreversibility, sensitivity, selectivity, response time, and reproducibility. I t is usually neceasarytomaketradeoffsamongthesevarious factom when designing a sensor or sensor system. While the performance of the individualsensor(s)isclearly critical totheperformance of the complete system, it is also true that the system itself makes a substantial contribution to the final resultsobtained. Sensitivity and selectivity, in particular. can he greatly enhanced by a systems approach. In this paper, we will focus on those features of the system that contribute to the latter two factors, with particular emphasis on the use of thermally desorbed PCTs in the sampling system. Our immediate applications are in the detection of toxic organophosphorus compounds, such as nerve agents (and nerve agent simulants), and toxic organosulfur compounds, such as blister agents. The system is expected to detect these vapors over a large concentration range, with or without the presence of other vapors in the background, and to avoid false alarms when the agent vapors are not present. The presence of an agent vapor must he reported as a low, medium, or high alarm level. Precise analytical quantitation of the agents was not a requirement. In this paper, we will refer to the toxic organophosphorusand toxic organosulfur vapors to he detected, and simulants of them, as hazards. This terminology will be used to distinguish them from other organic vapors that are also toxic hut do not represent an immediate source of danger a t moderate concentrations, and which the sensor system is not intended to detect or report.
Jl I ,
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Flgure 1. Schematic diagram of a smart sensor system. The system communicatesdata and resuns via a serlal pon. A small dbbi display (not shown) is also present on itm instrument. SENSOR ARRAY
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Flgum 2. Schematicdiagram of itm core sensor system. The sonsor away and itm separatereferencedevice are sandwichad between osciilatorcircuibyand me memWeleciriccooler. The cwler k i n c W
to maintain me sensors at a constant temperature.
two flatpacks, twosemrsperflatpack. Inletandoutlettubes were provided for gas delivery, and the two packages were connected in series. An uncoated 158-MHz single-delay line device was packaged in a separate sealed flatpack and served as a reference. Oscillator circuits operated each of the four SAW sensors and the reference device. Each sensor frequency was mixed with the constant reference frequency to generate four low frequency differences signals (in the range of a few hundred kHz) for analysis. Shiftsinthe differencefrequenciesdiredy reflect shifts in the signals of the sensors. Low frequency difference signals are desirable because they can be easily counted with simple TTL electronics. The four sensors, reference device, oscillators, and mixers were all located on one printed circuit board. The polymers on the sensors were fluoropolyol. polySYSTEM DESCRIPTION (ethylenimine), ethyl cellulose, and poly(epich1orohydrin) The functionalcomponentsof the sensor system are shown (ahhreviated as FPOL, PEI, ECEL, and PECH, respectiveschematically in Figure 1. The core sensing Dortion of the lvLm Thev were aDDlied to the sensors in films causing ca. system consisted of a temperaturecontroli2 SAW sensor 250 kHz oishift in the sensor frequency. The frequency shift array operated hy oscillator circuitry, and this is shown in observed upon application of a thin film of polymer material Figure 2. to a hare SAW device surface is caused primarily by the mass of the material.% (By contrast, when vapors are absorbed by The sewor array consisted of four 158-MHz SAW singlepolymer films, both the film mass and film modulus are delay line sensors, each on a separate quartz chip, and each perturbed, with the latter causingthe larger change in signal") coated with a different sorbent polymer. SAW delay lines of From the frequency shift observed, and the polymer density, 158 MHz have been described and utilized in refs 20 and it can be estimated that the films applied were ca. 4&80 nm 35-39. The four singledelay line sensors were packaged in thick, assuming the material was evenly distributed over the (24)Alder, J. F.;Meallurn, J. J. Amlyat (London) lo&p,108.1169sensor surface. Each film was applied hy spray-coating a 1189. dilutesolution (ea.1mg/mL)ofthepolymerinvolatileorganic (25)Ballantine. D.S.;Wohltjen, H.Awl.Chem. 1989,61.7MA-115A. solvent (typically chloroform or methanol) onto the SAW (26)Bastiaana,G.J.F'iezoeloetric Tmnsducera:Blackie:GLasgoaand device and monitoring the shift in frequency as the coating London, 1988:pp 296-319. (27) Fox, C. G.; Alder. J. F. Awlyrt (London) 1989,114,997-1004. was applied, asdescrihed in previous publication^.'^^"^*^^^^ (28) Frye. G.C.;Martin, S.J. Appl. Spectroac. Re". 1991,26,73-149. Final frequencyshifta weredeterminedafter completesolvent (29)Nieuwenthuizen. M.9.; Venemia, A. Sew. Mater. 1989.5.261evaporation (as indicated by the sensor frequency and 300. (30) Wohltjen, H.; Dsssy, R. E. Aml. Chem. 1979,51,1458-1464. typically within 5-15min, usingruhbery polymersandvolatile (31) Wohltjen, H.;Dessy, R. E. A w l . Chem. 1979,51,147C-1475. solvents). (32)Wahltjen, H.; Deeay, R. E. AMI. Chem. 1979,51,1465-1470. Temperature control of the sensor array was achieved by (33) Wohltjen, H. Sens.Actuator8 1984,5,307-325. (34)Wohltjen.H.;Snow.A. W.;Bqer,W.R.;Bdlantine.D.S.IEEE placing all the sensor packages in contact with a single 'Row. Ultrason., Ferroelec.. Freq. Contr. 1987. UFFC-34, 172-177. (35)Grate,J.W.;Klusty.M.;Mffiill,R.A.;Abraham,M.H.;Whitiag,thermoelectric cooler (Peltier device) and a thermocouple. G.;Andonian-Haftvan,J. A w l . Chem. 1992,64.610424. (36)Grate, J. W.; Klusty. M. AMI. Chem. 1991,63,1715%1727. (39)Grata, J. W.; Snow. A,; Ballanthe. D. S.;Wohltjen, J.; Abnrham, (37) Bowers. W.D.; Chum, R. L. Re". Sei. Imtrum. 1989.60. lmM. H.;Mffiill, R A.; Sasson. P. Anal. Chem. 1988.60,869-875. 1302.
(38) Rezgui. N.; Alder. J. F. Aml. h e . 1989,26,4648.
(40)Grate, J. W.; Wenzel, S. W.; White, R. M. A w l . Chem. 1991.63, 1552-1561.
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During system operation, a control circuit monitors the temperature and adjusts the cooler power to compensate for the heat generated by the electronics in the box. Once the system is warmed up, the sensor temperatures are maintained at a set point of 30 “C. Without active cooling,we have noted in previous studies that oscillator circuitry can generate enough heat to raise sensor temperatures to 35 or 40 OC.% Variations in sensor temperature are undesirable for two reasons. First, vapor sorption by the polymer coatings and, hence, sensor responses are highly temperature-dependent, such that sensitivity decreases as the temperature rises.98 Thereforeconstant sensortemperaturesare required toobtain reproducibleresponselevels.Second, temperaturevariations contribute to baseline drift. The packaged devices have an inherent temperature drift, and they are quite sensitive to thermal expansion of the polymer on the surface via the sensitivity of the surface waves to changes in film modulus.% Temperature compensation schemes such as the dual-delay line SAW configuration do not compensate for the temperature dependence of vapor sorption, nor do they compensate for baseline drift due to polymer thermal expansion. Consequently, it is best to actively thermostat the sensors as described. With active temperature control, there are no compelling reasons to fabricate sampling and reference devices on the same chip as in the dual-delay line configuration. Therefore, we separated and packaged the reference as described above. This prevents perturbation of the reference frequency by vapor adsorption on its surface, and there is no possibility of direct cross-talk between two delay lines fabricated on the same chip. Cross-talk can contribute to baseline instability with dual-delay line devices.3’ Our short-term stability in this system was ea. 15 Hz. Under laboratory conditions, frequency drift over 2 min was typically 50 Hz,although this wasvariable. This drift cornpares withtemperature-induced drift due to polymer thermal expansion of 500-1000 H z / T for these types of sensors36 We did not test the ability of these systems to control temperature and drift in response to changing environmental temperatures or changing input gas stream temperatures. Digital frequency counters, one for each sensor, operated bycountingthe peaksinthewaveformoveraprevise 1-sgate time. These results were transferred to a microcontroller. Duringtesting, the frequency data were then reported out on aserial port every 2 s, where they were collected and stored for analysis hy a separate microcomputer. With these components, pattern recognition algorithms can be implemented in real time by two methods. They can be programmed into the on-board microcontroller, or they can he programmed into a separate microcomputer receiving the data from the serial port in real time. The latter method provides more flexibility during developmental work and allows the data and analysis results to be displayed on the computer screen. This is the method that we have used thus far. If the system were operated in the former configuration with the pattern recognition algorithms operating on-hoard, the results could be reported over the serial line and/or on asmall digital display present on the front of the instrument. A microcontroller also operates the sompling system on a preprogrammed schedule of sampling operations. The sampling system consisted of a sampling manifold, a small pump to pull samples through the sensor array, two PCT tubes, two large pumps (one associated with each PCT tube), and two triple three-way solenoid valves., The system in shown schematically in Figure 3. The system was constructed with Teflon-wetted parts. Connecting tubing was 1/8-in. 0.d. by 1/16-in. i.d. throughout, with the exception that the tube
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Flgm 9. Schema* &gam of Wm sampling system. w h m PCT indicatespfewncemtor tubes, and drcles indicate pumps. Sample airflow dkectbns are lndlcated by Wm arrows. where the wlld line indicates direct sampling (wlth vapor simultaneouslyc o r n e d on both FCTs). the dashed line shows Mivery of the preconcenlrated sampk from ttm upper FCT (operated on a 2-mln cycle) to the senw array (wkhvapwsimultanecusly collectedontheothafE T ) . and the dash& dotted line shows delivery of preconcemted s m p k from the lower FCT (operated on a 16mln cycle).
connecting the outside of the instrument to the sampling manifold wan U4-h. 0.d. hy l/&in. id. This sampling system is capable of delivering three types of samples to the sensor array. With the three-way valves in their normally open positions, ‘direct” sampling occurs, meaning that the sample air is pulled in from the outside to the sensor array (by the small pump) without modification. Atthesametime,thetwolargepumpspdsampleairthrough the PCT tubes, collecting vapors on a bed of solid Tenax sorbent in each tube. Actuating a triple three-way valve reroutes the gas flow so that sample air is pulled to the sensor array through one of the PCTs. At the same time, the large pump associated with that PCT is switched off. Heating the PCT tube releases the sorbed vapor, which is then carried hy thesampleairtothesensor array fordetection. Thisapproach provides preconcentration in two dimensions: (1)desorption times are shorter than the collection time on the tube, and (2) flow rates through the tubes are greater during sample collection than during desorption. The large pumps pull ca. 800 mWmin through the tubes during sample collection, whereas the small pump pulls only ca. 75 mWmin through a t the onset of desorption. The flow is further reduced to ca. 50mWminas tbesolid sorbent undergoes thermal expansion. We operated one of the PCTs on a 2-min cycle (the upper tuheinFigure3),andtheotherona14-mincycle.Thebmin
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Figuro 4. Schematic dlagram Illustrating the operatlon of the 2-mln preconcentrator tube. While the tube Is coilectlng the sample, the sensor array Is exposed to direct sample alr. At 70 8, actuating a trlple three-wayvatve redirects gas flows so that sample air pulled through the tube goes to the sensor array. Thermal desorptlon beglns 10 s
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PCT provided sensitive detection with a reasonably short overall response time, while the 14-min PCT was intended to help detect still lower vapor concentrations, albeit with a much longer response time. The preconcentrator tubes (PCTs) consisted of a 1/4-in. 0.d. by 1/8in. i.d. glass tube packed with 40-60 mesh Tenax GC over approximately a 1/4-in. length of the tube. The Tenax was held in place with glass wool, and 1/8in. 0.d. Teflon tubing was press fitted into the ends of the glass tube up to the glass wool. A sleeve of Tygon tubing over the junction where the Teflon tube entered the glass tube prevented leaks. A coil of nichrome wire wrapped around the glass tube provided heat for thermal desorption. The wire coil and a thermocouple were held to the tube with a high-temperature adhesive. In addition, a band of wire was added to ensure that the thermocouple could not come loose. The two tubes in the system were identical in construction but operated on different timing cycles. As shown in Figure 4, the 2-min PCT collects sample for 70 s. During this time, the sensors are in direct sampling mode (i.e., exposed to sample air that is drawn into the system and delivered directly to the sensors without passing through a PCT). Then the valves are switched so that the sensors are exposed to air that is routed through the PCT, and the large pump associated with the 2-min PCT is turned off. A slight perturbation in sensor baselines may occur on valve switching. After 10 s for baseline stabilization, thermal desorption is begun and continues for 40 s. Then heating stops, the valves are switched back so that the sensors are again exposed to sample air directly, and the large pump is turned back on so that the Tenax again collects sample. The PCTs are located near a fan to speed cooling. The 14-min PCT was operated similarly, except that it collected sample for 13 min and 10 s, followed by 10 s for baseline stabilization after valve switching and 40 s for thermal desorption. The PCTs were operated by an on-off control circuit with a 200 "C set point. This temperature was reached in 12 s, overshooting to 230-235 OC ca. 5 s later, and settling at temperatures between 200 and 210 OC for the remainder of the desorption time. Since these measurements were made with a thermocouple (Type K precision fine-wire thermocouple, 0.005-in. diameter) inserted into the adhesive on the outside of the tube, the Tenax inside the tube may heat slightly more slowly and overshoot less. After heating, the tubes cooled to 100 OC in 20 s, 75 "C in 30 s, 50 "C in 45 s,and were at 35-40 OC when the next heating began. It was possible to operate the prototypes without the PCTs being used, so that the sensors were exposed to direct sample air a t all times. This capability was useful for examining the performance of individual sensors and during any troubleshooting that might be necessary after assembling all the subsystems. We refer to operation in this fashion as "PCTs-
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Figure 5. Time llnes for a 14-mln sampling period consistlng of seven consecuthre 2mln cycles. In the first six cycles, the 2-mln PCT is desorbed. In the seventh cycle, the l 4 m I n PCT Is desorbed. Direct sampllng Isperformed for 70 8 of each cycle (in between PCT desorptbn processes). The lower broken lines show when each PCT is collectlng the sample by sorptlon on the Tenax. These llnes correspond to the time when the correspondlng large pumps are on.
off" operational mode. Normal operation included all three types of samples discussed above,i.e., direct sampling,samples preconcentrated and delivered over 2 min, and samples preconcentrated and delivered over 14 min. We refer to operation in this fashion as "PCTs-on" operational mode. During normal operation, the sampling system followed a preprogrammed schedule of sampling modes that was constructed by defining seven 2-min cycles that comprised a 14-min sampling period. In this schedule, the sampling system delivers sample air directly to the sensors for a total of 70 s each 2-min cycle. For the other 50 s, one of the PCTs is desorbed as described above. During each of the first six 2-min cycles (in the 14-minsampling period), the 2-min PCT is desorbed. During the seventh 2-min cycle, the 14-minPCT is desorbed. During the 2 min prior to the desorption of the 14-min PCT, sample is not preconcentrated on the 2-min PCT. Sample preconcentration on the 2-min PCT resumes after the desorption of the 14-min PCT. A time line of these sensor system operations is shown in Figure 5, beginning 1 min before the completion of a desorption event on the 2-min PCT. Since the 14-min sampling period schedule was repeated indefinitely,the definition of the beginning and end of one period is arbitrary. Note that the cycles defined in Figure 6 do not coincide with the 2-min interval of time in Figure 4 they are offset by 1 min. To summarize, the system electronics including the oscillator and frequency mixing board for the four SAW sensors, frequency conters, control circuitry for the valves and pumps, a control circuit for the thermoelectric cooler which maintainedthe sensor temperatures, control circuitsfor the thermal desorption of the PCTs, serial data output, and a small digital display. Two generations of prototypes have been fabricated thus far. The fiist generation prototypes contained two microprocessor boards for system control and data analysis, whereas the second generation prototypes contained a single more powerful microcontroller board to perform these functions. The control circuitries of the second generation prototypes were also improved over those of the first generation prototypes, and a simpler, more flexible power supply was used. Either 115 VAC or 12-15 VDC can be used to power the second generation prototypes. The sensor and sampling system configurations,however, are identical on all prototypes. The data presented in this paper are all from the first generation prototypes. Polymer selections for these prototypes were made on the basis of a number of previous studies. We required individual sensors that would provide high sensitivity for each of the target compound classes. At the same time, the total sensor array must produce patterns that allow the target analytes to be distinguished from other vapors that might be present in the background, thus preventing false alarms. In two previous studies, large numbers of polymer coatings were investigated to determine which would provide sensors with high sensitivity to organophosphorus compounds.1Q.M Flu-
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oropolyolwas found to be sensitive and selective for detecting the nerve agent simulant dimethyl methylphosphonate, also known as DMMP. In the same studies, pattern recognition methods were used to identify subsets of these sensors that formed arrays that could discriminate between the organophosphorus compounds and a variety of other organic vapors tested a t much higher concentrations. Separate investigations identified coatings that provide sensitivity to actual nerve agents and the blister agent known as mustard gas. On this basis, ethyl cellulose and poly(epich1orohydrin)were selected for blister agent detection. We selected poly(ethy1enimine) as the fourth polymer primarily because it provides information about changes in humidity and was found to be useful in sensor arrays in one of the previous studies.20 In the present study, the polymer selection process was dominated by the need to select sensors for the particular vapors to be detected and relied primarily on previous testing results. Methods of selecting polymers on the basis of solubility properties imparted to the materials by particular organic functional groups have been reviewed elsewhere.21 A variety of hydrogen-bond acidic materials with the potential for nerve agent detection have been reported r e ~ e n t l y . ~ ~ , ~ ~ Physically, all the polymers are rubbery noncrystalline materials above their glass transition temperatures at our system operating temperature,except for ethyl cellulose. The latter is a glassy and crystalline polymer. Generally, rubbery polymers are preferable for more rapid vapor diffusion. EXPERIMENTAL SECTION
System Testing. Laboratory testing of the prototypes was carried out to confirm proper functioning of the various subsystems, to collect data for a training set to develop pattern recognition algorithms, and to examine the response characteristics of the sensor system as a whole. Laboratory testing against various vapors was conducted in both dry air and humidified air streams. In each case, it was necessary to be able to modulate the sample air at the system inlet between clean air and air containing the vapor. When working with humidified air, the objective was to work at a constant humidity while varying the organic vapor concentration. This simulates the field environment where organic vapor concentration variesagainst a relatively constant background humidity. Our approach to testing is shown in greatly simplified form in Figure 6. An airstream of known, constant humidity was generated and split into pathways to two manifolds. A calibrated organic vapor stream in dry nitrogen at 120 mL/min was added to the humidified air in one of these pathways. The flow rate of the humidified air was known, so that the diluted vapor concentration could be calculated. Mixing chambers in the bottom of each manifold assured uniform vapor concentrations in the main chambers of the manifold. A three-way valve located between the two manifolds selected the manifold from which the sensor system drew its sample air. Changing from clean air to air containing the organic vapor was simply a matter of actuating the three-way valve. Two first generation prototypes (Pr no. 1 and Pr no. 2) were tested simultaneously; therefore, two three-way valves were located between the two manifolds,one for each prototype system. Frequency data from each prototype were collected by a dedicated microcomputer, one microcomputer per prototype. With both prototypes collecting samples on each of their PCTs, the total draw on a manifold was ca. 3.4 L/min. Therefore, the total dry or humidified air generated was sufficient to deliver at least ca. 5 L/min through each manifold. The quantitative test results reported in this manuscript were all derived from a single prototype, Pr no. 1, except as noted otherwise. Both first generation prototypes produced similar results, so the data were only analyzed in detail for one of them. Part way through the
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(41)Abraham, M.H.; Hamerton.,-., I.: Rose.J. B.:, Grate. J. W. J. .. Chen - .. .1. SOC.Perkin Trans. 2 199:L, 1417-1423. (42)Snow,A. W.; Spragve,L.G.;Soulen,R.L.;Grate,J. W.; Wohltjen, H. J. Appl. Poly. Sci. 19I)1, 43, 1659-1671. ~
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test program, which was conducted over several months, wirebonds on some of the sensors failed, and all sensors were replaced.43 Vapor Generation. The prototypes were tested in two separate laboratories: the Naval Research Laboratory and at Systems Research Laboratories, a division of Arvin/Calspan. (All chemical agent, engine exhaust, and smoke tests were conducted at SRL. All data were analyzed at NRL.) Some of the experimental details varied from one lab to another. In both cases, however, organic vapor streams were generated from bubbler sources and diluted by a pulse width modulation technique described in detail in ref 44. The vapors generated by the bubbler sources were calibrated gravimetrically by adsorbing them quantitatively on activated charcoal and molecular sieve traps as described previously.% The humidified airstreams were generated by methods described in ref 45. Humidities were measured using a Hydrodynamics hygrometer with separate sensing elements for different humidity ranges, or a Cole-Parmer digital hygrometer. The latter hygrometer was calibrated against a Dewall automated dewpoint hygrometer. The concentrations of the test vapors were calculated from the bubbler source calibration, the dilution by the pulse-width modulation system, and the dilution into the humidified air of known flow rate. All nerve and blister agent concentrations, and some DMMP simulant concentrations, were verified using a MINICAMS automated gas chromatographic system (CMS Research Corp., Birmingham, AL). The similarities in test results from the two laboratories demonstrated the reproducibility of the sensor system responses and also validated the vapor stream generation and calibration methods. (43)The frequency shifts in kHz due to the polymers deposited on the sensors in the first-generation prototypes were as follows: Pr no. 1: PEI; 232,FPOL; 252,ECEL; 241,PECH; 245. Replacement sensors in Pr no. 1: PEI; 195,FPOL; 257,ECEL; 258,PECH; 264. Pr no. 2: PEI, 236, FPOL; 252,ECEL; 241,PECH; 257. Replacement sensors in PR no. 2: PEI; 180,FPOL; 250,ECEL; 258,PECH 268. (44)Grate, J. W.;Klusty,M. NavalResearchLaboratoryMemorandum Report 6762,1990. (45)Rose, S.L.; Holtzclaw, J. R. Naval Research Laboratory Report 8848,1985.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
Table 1. Summary of Vapor Tests for Pattern Recognition Algorithm Development agent and simulant vauora concn (mg/m3) humidities4 agenta and simulanta GD
vx
DMMP HD agenta in mixtures GDIJP-4 GDIgasoline GD/diesel GD/dichloroethane GD12-propanol HDIJP-4 HD/gasoline HDIdiesel HD/dichloroethane HD/2-propanol
0.01,0.05,0.5, 5 0.01,0.05,0.5 0.1,1,10 0.05,0.5, 2,lO
high, medium, low high, medium, low high, medium, low, dry high, medium, low
0.5150 0.5150 0.5150 0.5150 0.5/50 2.0150 2.0/50 2.0150 2.0150 2.0150
medium medium medium medium medium medium medium medium medium medium
organic vapors dichloroethane 2-propanol JP-4/JP-5 vapors gasoline vapors diesel vapors isooctane toluene aerosolsc gasoline exhaust diesel exhaust jet exhaust cigarette smoke
nonagent vapors concn (mg/ma)
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humidities4
50 50 50 50 50 50,120,500 50,500
low, medium low, medium low, medium low, medium low, medium medium* mediumb
NIA NIA N/A NIA
low low low low
4 In agent testing, humidities were as follows: low = 10-20% RH,medium = 4040% RH, and high = 70-80% RH. In DMMP testing, humidities were as follows: low = 10-20% RH, medium = 3540% RH, and high = 5580% RH. Testa as 500 mg/ms were conducted in dry air and high humidity. These are mixtures of aerosols and vapors generated by the engine or smoke. Each aerosol was tested once.
Gasolineengine and diesel engine exhausts were generated by introducing automobile exhausts into a 590-ma atmospheric chamber equipped with a mixing fan. Aerosols from these exhausts were monitored using an electrical aerosol analyzer (Thermo Systems, Inc.), a Gardner small particle analyzer, and a Bendix Model 8201 reactive hydrocarbon analyzer. A 1986 Ford Escort was exhausted into the chamber for 85 min at high idle to bring the concentration of carbon and hydrocarbons to a total concentration of ca. 0.008 mg/ma. A 1978 Volkswagen Diesel Rabbit was exhausted into the chamber for 5 min to bring the concentration of carbon and hydrocarbons to a total concentrationof ca. O.OOO1 mg/ma. Cigarettesmokewas generated in the atmospheric chamber by heating a crucible containing 20 crushed cigarettes (minus their filters) with a propane torch. This test was intended to producea cigarette smokeconcentration ca. 2.5 times higher than that expected in a 20-ma office where three cigarettes are smoked per hour. Jet exhaust was generated outdoors on an airstrip, placing the prototypes 175 feet behind and downwind from a Lockheed NT-33A jet trainer using JP-4 fuel. The background humidities prior to generation of the exhausts and smokes were low. In these tests, clean air was supplied from a cylinder of dry air; the humidities of the clean air and the test samples were not matched. Test Vapors. The vapor tests conducted for the development of pattern recognition algorithms are summarized in Table I. The tests are divided into two categories: those involving agent or simulant vapors, and those that do not. The latter category includes exhausts and smokes under the category of aerosols. It is recognized that these samples also contain vapors. It is pertinent to emphasize three points about the vapor testing: (1) the agents were tested at trace concentrations, (2) they were tested at these concentrations at low, medium, and high background humidities, and in mixtures with other organic vapors, and (3) the systems were tested against other organic vapors at much higher concentrations than the agent concentrations. The nerve agents in the test program were GD and VX.a GD is also known as soman, its chemical names are pinacolyl methylphosphorofluoridate or methylphosphorofluoridic acid, 1,2,2-trimethylpropyl ester; its structure can be represented by CHSP(O)(F)OCH(CH~)C(CH&.VX has no common name; its chemical name is ethyl S-2-diisopropyl aminoethyl methylphosphorothiolate, and its structure can be represented by CHaP(O)(OC2H&3CH2CH2N(CH(CH&)2. The blister agent in this test program was HD, also known as mustard gas.a Its chemical name is bis(2-chloroethyl)sulfide, and its chemical formulation can be represented by ClCH2CH2SCH2CH2C1. GD, VX, and HD are extremely toxic substances whose possession and use are strictly regulated in the United States. (46) Compton, J. A. F. Military Chemical and Biological Agents;
Telford Press: Caldwell, NJ, 1987.
We used dimethyl methylphosphonate (DMMP) as a much less toxic simulant for nerve agents, as in previous studies.*B.mWe do not have a good simulant for blister agent HD. We performed tests with dichloropentane to see if this might be a simulant for HD, but the sensors are not as sensitive to dichloropentane as they are to HD, and the PCTs desorbthese two vapors at different times. Therefore, dichloropentane is not a good simulant and will not be further considered here. We also briefly investigated responses to methyl salicylate and diethyl malonate, which are sometimes suggested as simulants. We found that these are not good simulants for either nerve or blister agents so far as SAW sensor responsesare concerned,and they will also not be furthered considered here.
RESULTS AND DISCUSSION Simulant DMMP. Detailed results from tests with DMMP are presented first to illustrate several points about the performance of the sensors and PCTs. The FPOL sensor is the best sensor for detecting DMMP, so this section will focus on the responses of this sensor alone. Direct sampling in PCTs-off operational mode allowed the inherent sensitivities of individual sensors to particular vapors to be determined. In this mode, the sensors can be given time to reach their steady-state (or equilibrium) responses to vapors. Typical vapor exposure times in these tests were 6 min. With dry air as the carrier gas, DMMP was easily detected at 1.1 mg/m3 with steady-state responses of 1300 Hz in PCTs-off operational mode. At the lowest test concentration, i.e., 0.12 mg/m3, small signals of ca. 200 Hz were seen. At the highest test concentration, 11 mg/m3, signals of loo00 Hz were observed. Results from DMMP testing in carrier gas of various humidities were similar to those from tests in dry air. At 0.1 mg/m3, the signals were in the 0-200-Hz range. At 10 mg/m3 in humid air (73%), the signal was 10 500 Hz. In Figure 7, we illustrate the response characteristics of the FPOL sensor with the system in PCTs-on operational mode, where the test vapor is DMMP at 1.1mg/m3 in 40% relative humidity air. Two consecutive 14-min periods are shown. DMMP was introduced to the sampling system inlet between the PCT desorption intervals of the third and fourth 2-min cycles, 20 s after the system switched from 2-min PCT sampling mode to direct sampling mode. The initial sensor response during direct sampling is clearly seen when the DMMP is introduced (note the profile of the vertically expanded inset), followed by the appearance of PCT desorption peaks. Because of the limited time intervals between PCT tube desorption operations, direct sampling responses
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
DMMP 1.1 mglm3
,
40%RH
300000 N
280000
G
0
260000 3
$
240000
L
220000
0
28
14
MINUTES Figure 7. Response of the FPOL sensor to DMMP at 1.1 mg/m3 in medium humidity air with the PCTs on. The response is followed through two complete 14-min periods. The vertically expanded inset on the left more clearly shows the response of the sensor in direct sampling mode to the initial exposure to DMMP. The Inset number of 13 700 and 65 900 indicate the peak heights in Hz of the 2-min and 14-min PCT mode responses. ~~~~
~~
Table 111. DMMP Detection Using the 2-Min Preconcentrators.
Table 11. DMMP Detection in Direct Sampling in Preconcentrators-On Operational Mode at Various Humidities. humidityb DMMP (mg/m3)
dry
low
medium
DMMP (mg/m3)
high
0.12
responses in the 0-100-Hz range 840 880 900 850 7800 8900 9300 9300
0.12 1.1 11.5
Responses in Hz after ca. 30 s of exposure from the FPOL sensor. b Humidities: low = 10-20% RH, medium = 35-40% RH, and high = 5545% RH. ~~
1.1 11.5
dry
low
humidity* medium
high
2530 (f125) 2530 (3~125) 2120 (f111) 2050 (f63) 13280 (f187) 13550 (t229) 13650 (f141) 12980 (f599) 50810 (f1493) 52930 (f1495) 51290 (3905) 46730 (f1415)
a Responses in hz from the FPOL sensor. Eachentry is the average of 12 responses, with the standard deviation given in parentheses. Humidities: low = 10-2096 RH, medium = 35-40% RH, and high = 5545% RH.
*
~~~
determined in this operational mode are not necessarily steady-state responses and were typically smaller than the responses determined with PCTs-off. We determined approximate frequency shifts occurring 30 s after the vapor was introduced to the system inlet. (All numerical data in this report for direct sampling in PCTs-on operational mode were determined in this fashion.) At 30 s of DMMP exposure in the experiment in Figure 7, the FPOL sensor has not quite reached a steady-state response. The response at this concentration begins to level off to a steady-state response a t 40-50 s. The direct sampling responses to DMMP a t various humidities in PCTs-on mode are given in Table 11. At 0.12 mg/m3 DMMP was not reliably detected. At 1.1 mg/m3 it was easily detected with signals of ca. 900 Hz. These signals were less than the 1300-Hzsteady-state signals seen in PCTsoff direct sampling, as expected. At 11 mg/m3, signals of 8000-9000 Hz were observed. The data in the Table I1 show that sensor response levels are approximately linear with DMMP concentration in this concentration range and that background humidity does not have a significant effect on response levels. The variation with humidity has a range of & 4 % a t 1.1 mg/m3 and &9% at 11 mg/m3. These ranges would not be severe with regard to quantitation and are inconsequentialwith regard to establishing high, medium, or low alarm levels for the presence of a hazardous vapor. The 2-min PCT sampling mode provides significantly greater responses than direct sampling mode. As shown in Figure 7, the signals produced when desorbing these PCTs were very repeatable. Many such experiments with DMMP were conducted throughout our tests. Results from tests in 2-min PCT mode are given in Table 111. Responses were taken as the height of the peak relative to the baseline, the latter being determined during the 10 s between switching to PCT sampling mode and turning on the heater of the PCT
(see Figure 4).47 Using the 2-min PCTs, signals of 2000-2500 Hz at 0.12 mg/m3 DMMP in humid air are at least 20 times greater than signals in the 0-100-Hz range in direct sampling. We can also compare the 2100-Hz signal a t 0.12 mg/m3using the PCT to signals of 1300 Hz at 1.1 mg/m3 and 10 000 Hz at 11 mg/m3 in direct sampling mode with PCTs-off. The last two responses are steady-state response levels. By interpolation, a signal of 2100 Hz would correspond to ca. 2 mg/m3 DMMP in direct sampling. This yields an effective concentration factor of ca. 20. The effective concentration factor thus calculated can be compared with the method of preconcentration. First, the desorption flow rate of 75 mL/ min compared to a PCT collection flow rate of 800 mL/min indicates that samples should be concentrated by a factor of at least 10. Second, the sample is collected over 70 s and desorbed within a 40-s interval. Therefore,our concentration factor of 20 times is quite reasonable. We emphasize, however, that the above calculation is only a simple empirical method of showing the magnitude of concentration. When the PCT is heated, it delivers a “pulse” of vapor whose concentration over the sensor rises and then falls as the pulse passes through the sensor array packages. The sensor signal rises in response to this concentration profile, but the response time is not instantaneous. Therefore, sensor response lags behind the concentration profile, and as a result, the peak sensor response observed during this dynamic process does not strictly reflect the peak concentration achieved. As the humidity varied a t any given concentration, the sizes of the responses in 2-min PCT mode were not greatly changed. The largest variation with humidity occurred a t (47)Thepeak height duetothepreconcentratedsampleismuchgreater than the signal level due to the unconcentrated vapor in direct sampling mode. The difference between calculating the peak height by subtracting a true zero air baseline, as opposed to subtracting the unconcentrated signal level observed prior to the thermaldesorption, would be ca. 5-10%.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 100000
80000
P
d5
60000
>
0 2
w
a
3
40000
E 200OC
0
_ . . . , . 500
1000
1500
2
10
CONCENTRATION, mg/mr
Flgure 8. Callbration curve for a fluoropolyol-coated 158-MHz SAW vapor sensor exposed to DMMP. This graph illustrates the nonlinearity of the fluoropolyolcoated sensor response wkh concentration.
0.12 mg/m3, where signals decreased by ca. 20% from dry air to high humidity (i.e., a variation range of *lo%). This variation is not significant in a semiquantitative determination of low, medium, or high alarm levels. As the DMMP concentration was increased, the response of the FPOL sensor observed in 2-min PCT mode also increased,but not in a linear fashion. For example,the 13 650Hz response at 1.1mg/m3 (in medium humidity) is less than 10 times the 2100-Hz response a t 0.12 mg/m3. Similarly the response of 51 300 Hz at 11mg/m3 is less than 10 times the response a t 1.1mg/m3 and less than 100 times the response at 0.12 mg/m3. This nonlinearity reflects the nonlinearity of the response of the FPOL sensor itself to DMMP at higher concentrations.19*mJ9 It does not necessarily indicate that there is any nonlinearity in the collection efficiency of the PCT. A typical calibration curve for a FPOL-coated sensor is shown in Figure 8. The responses on the curve are steadystate responses (no preconcentration of the samples). The observed nonlinearity in response with DMMP concentration using the PCTs was not due to saturation of the Tenax sorbent; this is demonstrated by the fact that responses continued to increase as the DMMP concentration increased. In addition, signals continued to increase with the collection time was increased. Typical peak heights observed on desorption of the 14-min PCT (which is identical in construction to the 2-min PCT) were 18 000,64 000, and 130 000 Hzat 0.12,1.1, and 11mg/msDMMP, respectively, compared to peak heights of 2100,13 650, and 51 300 Hz in 2-min PCT mode a t the same concentrations respectively. (Peak positions were 18-28 s after the onset of heating in 14-minPCT sampling mode.) Estimating the extent of preconcentration achieved with the 14-min PCT is subject to the same qualifications as those noted above in connection with the 2-min PCT, with the additional qualification that the nonlinearity of the FPOL sensor calibration curve must be carefully considered. Therefore, if we take the 18 000-Hz signal at 0.12 mg/m3 in 1Cmin PCT mode and compare it with the calibration curve in Figure 8, we find that 18 000 Hz corresponds to 24 mg/m3 in direct sampling. This indicates a concentration factor of ca. 200. A t the lowest DMMP concentration, the signals observed in 14-min PCT mode were largely independent of humidity. At the highest DMMP concentration, signals decreased with humidity. However, a t high concentrations the results from the 14-min PCT mode would not be used because the direct
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and 2-min PCT sampling modes would more rapidly detect and identify the hazardous vapor. One additional aspect of PCT operation is quite important. Although Tenax is not very efficient at collecting water, some is sorbed from humid air. This did not present a problem when detecting DMMP in humid air, however, because the desorption times of water and DMMP were significantly different. These effects are shown in Figures 9 and 10. Water desorbed as a sharp spike 4-6 s after heating begins. By contrast, DMMP desorption peaks occurred 16-32 s after the onset of heating. Thus, the signal due to the organic vapor of interest was completely separated from that due to water. We did not originally plan on this feature of PCT use; we had expected simply to get a single pulse of vapor that would be identified by pattern recognition. No efforts were made to vary the tubing type or lengths between the PCT and the sensor array in order to influence the observed separation of vapors or to determine if these are significant parameters determining the observed separations. The use of a thermally desorbed siliconeoil preconcentrator to enhance organicvapor sensitivity and to separate the organic vapor from water has been described previouslyin connection with quartz thickness shear-mode sensors.48 The response profile of the FPOL sensor to DMMP at 0.12 mg/m3 in 65% RH is shown in Figure 9. Two consecutive 14-min periods are shown, with DMMP introduced to the system approximately 6 min into the first period. A small spike due to water occurred at the beginningof each desorption interval. After DMMP was introduced, amuchlarger,broader peak occurred after each water spike. Figure 10 shows a vertically expanded view of a portion of this data and compares the response of the FPOL sensor to that of the PEI sensor. FPOL is very sensitive to DMMP but not very sensitive to water. So water appeared only as a small spike on this sensor. PEI is very sensitive to water and has little sensitivity to DMMP. A large spike for water was seen on PEI, and no peak for DMMP was apparent. Note that the large spike on PEI and the small spike on FPOL coincidein time, confirming that the small spike on FPOL is due to water. As noted above, the desorption peak for DMMP occurred 16-32 s after PCT heating was begun. Within this interval, the peak position decreased with increasing DMMP concentration and increased with increasing humidity. These results refer to prototype Pr no. 1. On Pr no. 2 the DMMP peaks appeared more rapidly, i.e., 10-16 s after heating commenced. These are still separated from the water peaks. On Pr no. 2, peak position again decreased with increasing DMMP concentration, but humidity had little effect. Over the course of the entire test program, PCTs were occasionally replaced, and their performance became more consistent with experience in fabricating them. DMMP desorption times from the replacement tubes were between the two extremes represented by the first tests on Pr no. 1 and Pr no. 2, i.e., between 12 and 28 s. The rapid desorption of water followed by the slower desorption of DMMP allowed us to define an interval, beginning after water desorption, where we looked for desorption peaks of vapors we wished to detect. This completely eliminated any possibility of an interference due to background humidity when the instruments were in a PCT sampling mode. When a peak was found within the specified interval on the FPOL sensor, then corresponding peaks on the other sensors were examined to generate a pattern for analysis by the nerve agent pattern recognition algorithm. It is worthwhile to consider the factors that influence the apparent desorption times, Le., the time between initiating (48) Kindlund, A.; H. Sundgren; Lundstrom,I. S e w Actuators 1984, 6,1-17.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
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65%RH
254500
249500
244500
239500
234500 0
28
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Response of the FPOL sensor in PCTs-on operational mode to DMMP at 0.12 mg/m3 in humid air. The response is followed through two complete 1Cmin sampling periods. The inset numbers of 2020 and 18 350 indicate the peak heights in Hz of the 2-min and 14-min PCT mode responses. Flgure 9,
65%RH
DMMP 0.1 mg/m3 239000
2500
1
I
I
0 months
I II
I FLUOROPOLYOL
33 1000 E
500
235000
PEI
FPOL
I POLYfETHYLENIMINE)
ECEL
PECH
6 months
z w
1000
500
595000! 0
I
I
2
I
r
4
I
t
6
I
I
I
I
I
I
1 4
8 1 0 1 2 1 4
MINUTES
Figure 10. Responses of the FPOL and PEI sensors in PCTs-on operational mode to DMMP at 0.12 mg/m3 in humid air. The sharp spikes on both response profiles are due to the desorption of water. The broad peaks on the FPOL response profile are due to DMMP.
the heating of the PCT and the observed peak on the sensor. First, the rate and profile of heating is clearly important, which is dependenton the precise characteristics of the heating coil, the power to the coil, and the control circuit for the heating process. Second, it is dependent on the characteristics of the vapor and its sorption by the Tenax sorbent. Third, it is dependent on the flow rate and on the length of connecting tubing between the PCT and the sensor. Fourth, it may depend on the sorption and desorption of vapor from the walls of these connecting tubes as the vapor travels to the sensor (i.e., similar to a chromatographic process if the vapor concentration is very low). Fifth, it may depend on the dead volume of the package containing the sensor and the mixing of the incoming vapor. Finally, it will depend on the speed with which the sensor responds as the pulse of vapor passes over it. All these factors should be taken into account in order to reproducibly fabricate sampling systems with PCTs. It is also possible that some of these factors may change with time. We compensate for this possibility by defining a large window (in time) where we look for peaks due to hazardous organic vapors of interest.
" Flgure 11.
PEI
FPOL
ECEL
PECH
PEI
FPOL
ECEL
PECH
Response patterns to 0.1 mg/m3 DMMP in medium humidity air taken over several months. Nevertheless, we stress that these systems can be fabricated reproducibly. Given the number of factors that influence performance (Le., individual sensors, PCTs, PCT heating rate, and pump flow rates), it was encouraging that replacement of various components yielded detectors with response characteristics that were similar to the originals. This point is illustrated in Figure 11. The responses in 2-min PCT sampling mode of all four sensors to DMMP at 0.1 mg/m3in medium humidity air are shown in tests over several months. During this time, various sampling system components were exchanged for new ones (primarily pumps and PCTs), and between the second and third response pattern shown (i.e., 5.5 and 6 months), the sampling systems were overhauled and new sensors were installed. With the exception of the anomalously high response of the PECH sensor in the first test, the response patterns and the magnitudes of the responses are similar throughout these tests. All the response patterns, including the first, could be correctly classified as organophosphorus responses by the pattern recognition algorithms later developed. Agent Sensitivities. Results of tests against nerve and blister agents are summarized in Tables IV and V. Results without sample preconcentration (Le., in direct sampling) are presented in Table IV in both PCTs-off and PCTs-on operational mode. As in the simulant testing above, agents
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
Table IV. Lowest Detectable Experimentally Tested Concentrations of Simulants and Agents in Direct Sampling vapor
test concn next lowest reliably detected' test concn4 (mg/m3) (mg/m3)
comments
Direct Sampling in PCTs-Off OperationalMode* 1 0.1 signals exceed 1000 Hz at 1 mg/m3 signals of up to 200 Hz at 0.1 mg/m3 are variable4 GDc 0.5 0.05 signals at 0.5 mg/m3 are 200-300 Hz at 5 mg/m3 signals are near 4000 Hz vxc 0.5 0.06 signals at 0.5 mg/m3 are ca. 900 Hz sometimes detected at 0.05 mg/ms HDd 2 0.5 signals at 2 mg/m3 are ca. 200-300 Hz Direct Sampling in PCTs-On Operational Mode' DMMPC 1 0.1 signals of ca. 900 Hz at 1 mg/mS small variable signals at 0.1 mg/m3 GDc 5 0.5 at 5 mg/m3signals are
DMMPC
1500-2500 Hz signals at 0.5 mg/m3 are
