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portional to the sample volume. The analyte is effectively preconcentrated into the much smaller volume of the recirculating acceptor solution in which the analyte is subsequently determined by the amperometric detection. Since a single eight-port, two-position valve is used to control all the flow patterns in the preconcentrating gas diffusion manifold, the system may easily be fully automated and miniaturized. As a consequence of the preconcentrating/accumulation step, the preconcentrating method has a lower throughput than the conventional one. Registry No. Sulfide, 18496-25-8.
LITERATURE CITED Pacey, G. E.; Hollowell, D. A.; Miller, K. G.; Straka. M. R.; Gordon, G. Anal. Chlm. ACte 1988, 179, 259. Valdrcei, M.; Luque de Castro, M. D. J . chrometogr. 1987. 393, 3. Martin, G. B.; MeyemOff, M. E. Anal. C h h . Acta 1988, 186, 71. Zhu, 2.; Fang, 2. Kaxue Tm@o (Forelgn Le-. M.)1888, 3 1 , 1728. Burguera, J. L.; Townshend, A.; (Leenfleld, S. Anal. Chlm. Acta 1980, 174, 209. Duffleid, E. J.; Moody, G. J.; Thomas, J. D. R. Anal. plot. 1980, 17, 533.
L&Wtt, D. J.; Chen. N. H.; Mahadevappa, D. S. Anal. Chim. Acta 1981. 128. 163. Me, H.; Yan, H. Kexue Tongb80 (Forelgn leng. Ed.) 1982, 2 7 , 959. Bablker, M. 0.;Delzlel, J. A. W. AM/. Roc. 1983, 20, 609. Bwguera, J. L.; Burguera, M. Anal. Chh.Acta 1884, 157. 177. Rbs, A.; Luque de Cestro, M. D.; Vaicircel, M. Aneiysf (bndon) 1984, 109, 1487.
(12) Glaistef, M. G.; Moody, G. J.; Thomas, J. D. R. Analysf (London)1985, 110, 113. (13) Kurzawa, J. Anal. Chlm. Aeta 1885, 173, 343. (14) Johnson, K. S.; Beehler, C. L.; Sakamoto-Arnold, C. M. Anal. Chhn. Acta 1988, 179, 245. (15) Peterason. B. A.; Fang, 2.; RuFlEka, J.; Hansen, E. H. A M I . Chim. Acta 1888, 184, 165. (16) Dasgupta, P. K.; Yang. H. C. Anal. Chem. 1988, 56. 2839. (17) Fang, 2.; RElEka, J.; Hansen. E. H. Anal. Chlm. Acta 1984, 164, 23. (18) Rocklin, R. D.; Johnson. D. C. Anel. Chem. 1983, 55. 4. (19) BaadenhulJsen,H.; SewenJacobs, H. E. H. C h . Chem. (W/nsfon-Salem, N . C . ) 1979, 25, 443. (20) RiZiEka, J.; Hansen, E. H. Flow Injecfbn Analysk; Wliey: New York, 1981. (21) Van der Linden, W. E. Anal. CMm. Acta 1983, 151, 359. (22) Hollowell, D. A.; Pacey, G. E.; Gordon, 0. Anal. Chem. 1985, 5 7 , 2851. (23) MacKoul, D.; Johnson, D. C.; Schick, K. G. Anal. Chem. 1984, 5 6 , 436. (24) Granados, M.; Maspoch, S.; Blanco, M. Anal. chim. Acta 1988, 779, 445. (25) Analytical Methods Commltiee. Analyst (Lonrkn)1887. 712, 199. (26) RQiiEka, J.; Hansen, E. H. Anal. Chlm. Acta 1986, 780, 41.
RECEIVED for review September 14,1987. Resubmitted June 27,1988. Accepted September 15,1988. This research was supported by the United States Bureau of Mines under the Mining and Minerals Resources Research Institute Generic Center Program (Grant Number G1125132-3205, Mineral Industry Waste Treatment and Recovery Generic Center).
NanoHter Volume Sequential Differential Concentration Gradient Detector Janusz Pawliszyn Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl
A sknple low volume dlfterentlal refractlve index gradient detectlon scheme has been described. Thls method elbnlnates low-frequency ndse associated with refractive Index drMs produced by temperature hmtabllltles andlor gradient solvent progremmlng condnkne. The reduction of laser posItion nolse and mechanical vlbratlons was also achleved by anoveloptkalmangemecrt. lBedetectorcomlstsdadngle He-Ne laser, a slmple 0pUcal procebdng scheme,and a single photosensor. The sdgnal produced by thls 881uor Is proportknal to concentratlon by over 4 orders of magnitude with the rdractlve Index gradlent detectlon h i t of 2 X lo-' r e fractlve Index unltlm. TMs translated into a concentration ddectbn Unn of 7 X lo-' M sucrose (subpicogram level) for our detector design. The detectkn volume Is about 2 nL and can be made even smaller by focusing a laser beams dhectly Into the caplllary. This scheme can be used both as a unlversai method (slgnai proportional to the second derivatlve of the concentratlon In respect to distance) or as an optlcal absorption probe (through photothermal process, In which case the signal Is proporttonal to the second derivative of optical denrlty In respect to position). Thls sensor has been appbd In flow lnjectlon analysis and caplllary zone electrophoresls.
The introduction of capillaries to separation technology has resulted in the generation of high efficiency methods (I). They 0003-2700/86/0360-2796$01.50/0
are characterized by smaller sample volumes, higher resolution, and shorter separation times compared to conventional techniques. Capillary chromatography is already well accepted when gases (2) or supercritical fluids (3) are used as mobile phases. Recently, much research effort has been directed to designing systems involving liquids such as in capillary liquid chromatography (4) and capillary zone electrophoresis (5,6). These schemes require very small inside diameter capillaries to operate at optimum efficiencies, sometimes smaller than 10 Mm i.d. (7).This development necessitates the design of ultralow-volume detection methods that are able to monitor narrow bands within small bore capillaries. Many techniques developed for this purpose have been employing lasers as light sources (8). In most cases, well-accepted detection schemes have been used. Laser beams, with their collimated characteristics, allow fine focusing of high-intensity beams and therefore reduction of detection volume by several orders of magnitude. Capillary separation methods using the liquid mobile phase are able to achieve close to a million theoretical plates per meter of capillary (9). They are likely to separate even quite complex mixtures. Therefore, the ideal detector for capillary methods would be a nondestructive universal type that is able to monitor all eluting components. If full separation cannot be achieved or additional information about the sample is required, then simultaneous selective analysis can follow universal detection. One detector type, which fulfills these requirements, applies refractive index monitoring. Much effort has been recently placed on developing an ultrasmall 0 1958 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
volume scheme based on this principle. In one approach, a laser beam diffraction at the glass-liquid interface is measured ( 1 0 , I I ) . This method applied on-capillary scheme to achieve low detection volumes. However, the concentration detection limita produced by this method were about 2 orders of magnitude above the best refractive index detectors; also temperature fluctuations appear to limit its performance. Temperature fluctuations, and more importantly, gradient elution conditions are known to restrict the performance and application of refractive index detectors (12). To reduce this problem, detectors can be used in differential arrangement (13),preferably in sequential mode (14-16). This approach reduces the drifts associated with gradient elution by about 4 orders of magnitude. It also allows effective detection of sharp peaks located on the tailing band. A more direct application of the differential method, highly compatible with capillary technology, was the introduction of a concentration gradient detector based on Schlieren optics (17). In this approach a single light beam propagates through the low volume cell and undergoes a deflection proportional to the refractive index gradient in the detector cell. The signal is measured with the help of a light beam position sensor (18). Since this detector produced a signal proportional to the gradient of concentration rather than concentration directly, it possesses drift reduction properties similar to sequential differential detection. Due to its simplicity, it is therefore highly compatible with capillary separation technology (19). The signal associated with the eluting Gaussian peak is, as can be expected, a derivative with a linear response of over 3 orders of magnitude to the amount of sample injected. The detection limit of the method is 5 X lo4 M of sucrose solution injected for sharp peaks generated by high-efficiency methods (18). The limiting factor of this technique is long-term beam position drift (201, particularly severe when lasers are used as light sources. The other limiting factor, laser intensity fluctuations, can be effectively reduced by using an intensity reference sensor. In this paper a simple optical detection scheme is proposed that generates a sequential differential concentration gradient output. This method through its double differential nature is able to reduce substantially better effects produced by refractive index drifts and broad tailing peaks compared to the single differential step. It also reduces the impact of the pointing instability of light sources and mechanical vibrations on the detector performance.
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\
1 FREQUENCY
Flgure 1. I n Gaussian peaks: time (A) and frequency (B) domains: 1. narrow Gaussian: 2, broad Gaussian. T
THEORY Reduction of high-frequency noise when the signal is of low frequency can be accomplished by using low pass filters in analog or time domain (21). The more challenging task involves the reduction of low-frequency noise such as drift and broad peaks. These factors often limit performance of separation methods. The main reason for difficulty in reducing low frequencies is the fact that both broad drifts (Figure la-2) and sharp peaks (Figure la-1) contain low frequency information. In other words, it is impossible to filter out drifts without changing the shape of a Gaussian peak. Figure 1 shows broad and narrow peaks in time and frequency domains. It is clear from Figure I b that the frequency range that differentiates between sharp peaks from drifts are midrange frequencies. Broad drifts do not contain information in this region while (Figure lb-2) sharp peaks extend to these frequencies (Figure lb-1). Therefore any filter that emphasizes midrange will be able to eliminate drifts successfully. As it has been noted above, any change in low frequencies by applying such a fiter will result in a different signal shape since it will remove some information about the peak. It is important, therefore, to apply a filter that will produce a welldefined signal shape with magnitude directly related to the
FREQUENCY
Flgure 2. Frequency response curves for (A) first derivative and (B) second derivative filters.
concentration of the sample injected. It is well known that differentiation or derivitization can reduce drifts substantially. Differentiation in the time domain corresponds to the application of a low pass filter in the frequency domain (22)
where n is an order of the derivative and F(w) is a time output, f ( t ) ,described in the frequency domain. For example, the fmt derivative of the concentration signal in the time domain is equivalent to the linear fiter in the frequency domain (Figure 2a). Higher order derivatives, such as second order (Figure 2b), are even better able to filter low-frequency noise.
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T I
Flgure 4. Experimental optical arrangements: (A) two-beam optical system and sequentla1dlfferentlai detector arrangement: (B) differential detection using single photodiode. Key: 1, 3, lenses; 2, Wallaston prism; 4, flow detector cell; 5, photodiode: 6, razor blades; 7, laser
beams.
indicates that the magnitude of the gradient at the inflection points of the Gaussian peak increases faster than its height when the peak narrows. In other words, relative sensitivity enhancement of the gradient measurement increases with improvement in the efficiency of the separation process (17). This effect is more pronounced if a second derivative response detector is used
(3) (Figure 3a). Also, the second derivative of the signal resembles a peak much sharper than the original Gaussian which can result in a higher resolution of the chromatogram. The simplest way to generate a derivative response is to apply mathematical differentiation to the signal output. However, this approach is not very useful since high-frequency noise is multiplied simultaneously by this process. The best results can be achieved by designing detectors or systems that directly produce a derivative or a differential response.
D
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Flgure 3. Slgnal shapes associated wlth Gaussian: (A) second derivative (B) first derivative; (C) Gaussian; (D) first integral: (E) second
integral.
During differentiation, the Gaussian peak of a given height C, (Figure 3c) is converted into the derivative of magnitude (17)
(Figure 3b) where a is the standard deviation of the chromatographic peak in units of length. This relationship strongly
EXPERIMENTAL SECTION Figure 4a shows a schematicof the optical system that generates two focused beams propagating close to each other from a single laser beam. It consists of two lenses (1and 3) separated by the distance that determines the focal length of both beams, and the Wollaston prism (2), which splits the laser beam. The focus spot of the beam is about 50 pm. Changing the distance between lens 3 and the prism changes the separation distance between two beams at the focal plane. All three components are placed in a single modified lens positioner (Model LP-105-B, Newport Research Corp., Fountain Valley, CA). The detailed description of this optical system is provided in ref 23. The differential deflection detection of the laser beams was Hammamatsu, performed by using a single photodiode (51087-01, Middlesex, NJ) as shown in Figure 4b. Two pieces of a razor blade were used to limit the aperture of the photodiode. Each of the beams was intercepted by a blade which allowed only half of the beam to reach the photodiode. The system was optimized by ensuring that no photodiode current change occurs when the laser beam was moved about a micrometer upward or downward. This exercise moved both beams simultaneouslyon the detector plane. In this case, net loss of one beam intensity reaching the detector was compensated by an increase in the light from the other. The laser beam was produced with a He-Ne laser emitted at 633 nm (Model 1303, Uniphase, San Jose, CA) equipped with a dc power supply (Model 1201-2)and a 12-V car battery. In most experiments, part of the laser beam energy was split and directed into the reference photodiode, which was used to compensate for intensity fluctuations in the fmal output. In some cases, a simple compensation circuit was used based on direct laser current monitoring, shown in ref 18 (see Figure 3b there). The flow injection experiments were performed by using the cell and conditions described in detail in ref 18 and 24. The electrophoretic apparatus is shown in Figure 5. The separation was occurring in 50 cm of 50-pm fused silica capillary (Spectran Corp., Sturbridge, MA). A high-voltage power supply (Spellman, Plainview, NY)was used to provide a 10OOO kV potential for the electrophoresisseparations. Electrodes were made with platinum wire. The detector cell was identical with that used in flow injection analysis. The sheath flow was generated electrokinet-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER
15, 1988 2799
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Flguro 5. Schematic of capillary zone electrophoresis experimental
arrangement.
i d l y and regulated by using an appropriate length of 10-pm fused silica tubing, which worked as a voltage drop resistor. The injection was produced electrokinetically by applying 10 kV to a sample solution for a few seconds. The separation was performed in phosphate buffer at pH 7. The sample mixture and buffer were degassed by filtering through a 0.2 pm pore size cellulose nitrate membranes (Watman Limited, Maidstone, England).
RESULTS AND DISCUSSION The main noise component in low-frequency light beam position measurements is its pointing instability (20). One way to solve this problem is to use highly stable light sources such as LED’s(18). They are inexpensive and can be easily integrated onto a single silicon component; however, they cannot be very efficiently focused into a few micrometer inside diameter fused silica tubing, which is required if ultralowvolume on-column detection is required. In this case, a laser light source is preferred. One can w e differential measurement to eliminate quite large pointing noise of laser beams. Figure 4 shows the schematic of the optical system that generates two focused, closely propagating beams from a single laser source. The focal length of the system is determined by the relative distance between two lenses (23). This system can be used in a parallel differential arrangement. One of the beams propagates through the capillary that contains the sample and the other one through the reference capillary filled with the solvent. A single quadrant detector with associated electronics (23) can be used to monitor the difference in deflection of both beams related to the refractive index gradients produced by the sample components. This arrangement is very difficult to align and does not reduce drifta as well as the sequential differential method (14). In our experiments, we applied the sequential arrangement shown in Figure 4a. Both beams are focused into the detector volume close to each other along the capillary axis. The difference in deflection between two beams is measured by use of a single photodiode (see Figure 4b). Both of these irradiate only part of the photodiode active area. One of the beams is split in half by the upper edge of the photodiode while the other one is split by the lower edge. Any change of position of the laser beam affects both beams to the same degree, but the net signal (photocurrent) generated by the detector will not change. For example, if both beams move upward simultaneously the net change of light flux reaching the detector is unchanged because the loss produced by translation of the upper beam is compensated by the movement of the lower beam. This simple optical system compensates not only for the pointing instability of the laser but also for the mechanical vibrations that might occur in real detection devices. Experimental measurement of laser drift (20) indicates 2 orders of magnitude reduction for the differential sequential
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TlME IS1
Flgure 6. Slgnals produced by injecting about 4 mL of 6.4 X lod M solution of sucrose: (A) sequential differential signal, (B) reconstructed
concentration gradient response, (C) concentration response. system compared to a single laser beam method. The measurement was performed in a similar way as that described in detail in ref 20. This optical arrangement enables the long-term laser beam deflection to be monitored very close to the shot noise limit (about 10-8 rad). This allows the design of sensitive on-column concentration gradient detectors. Results related to application of this system for flow injection analysis and capillary zone electrophoresis are described below. The sequential differential detection can distinguish between high and low gradients. Figwe 6a shows a typical signal produced by injecting about 4 nL of 6.4 X lob M sucrae into the detector cell. The front of the injection produces a substantially higher differential concentrationgradient signal than the tail. Figure 6b shows a refractive index gradient signal reconstructed from differential response performed as described in ref 14. The dramatic difference between the front and tail, shown in Figure 6a, is not as easily noticeable in the reconstructed and integrated response (Figure 6c). Figure 6c is equivalent to the signal that would be generated if the refractive index detector is used. This comparison indicates the drift and tail effect reduction capability of the sequential differential concentration gradient detector. Also, one will notice that the second derivative signal is much sharper than
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
the original Gaussian peak (Figures 6 and 3). This enables resolution between peaks and therefore, for example, an increase of the throughput of flow injection analysis (25). The maximum sensitivity in this differential detection system is achieved when the distance between two beams is close to 2a of the sample peaks generated during the experiment. In this case, one beam is experiencing the highest negative concentrationgradient while the other one the highest positive. Therefore, the maximum change in light flux irradiating the photodetector is produced, which results in high signal amplitude. In flow injection experimentsdescribed here, this distance corresponds to about 200 pm. In that case the concentration detection limit of this method was close to 7 x lo-' M of sucrose solution (signal to noise ratio of about 3:l). This is an improvement of close to an order of magnitude compared to a single beam experiment (18). This corresponds to about 3 X lo4 refractive index (RI) units for this detector design when assuming dn/dc 5 X lo-* L/mol. This value was estimated from the refractive index table of aqueous solution of sucrose (26). The most universal way of expressing detection limit of any method based on schlieren optics is in refractive index gradient units (RI units). This approach eliminates contribution of the particular design of the detector cell. A modified eq 2
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t
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TIME lMiCl
(4)
can be used to estimate the maximum RI gradient (dnldx), produced when injecting the peak of height (An), in RI units. In this calculation we assume Gaussian peak shape. The RI gradient detection limit is about 2 x 10-4/m for sequential differential arrangement. A similar system can be used in selective concentration gradient mode (21). However, the differential scheme is not expected to improve the detection limit below what has been reported before. In this measurement, the narrow band high frequency measurement is applied. At this frequency range the position noise is shot noise limited and the differential system does not improve substantially its sensitivity. However, this approach will reduce background absorption effects and therefore will enhance precision of the measurement. The sequential differential concentration gradient scheme was used as a detector in the capillary zone electrophoresis experiment. Figure 7a shows the electrophorogram of three amino acids. All peaks appear to be well separated. But in fact they are not fully resolved, which is indicated in Figure 7b, which corresponds to an electropherogram produced by the detector with the signal proportional to concentration. The second-order derivative produces a peak with a width cloee to 40 while Gaussian width is close to 6a (Figure 3). It is also possible to notice large drifts of the base line in Figure 7b. The reconstruction and integration of the sequential differential signal converted the vertical axis into the refractive index. Now the drifts, due to temperature variation, are easily noticed. The information about this type of drift is dramatically reduced in the differential gradient signal due to filtering properties of this detector (Figure 2). An interesting application of the single-beam concentration gradient detector using frontal analysis (for example: frontal chromatography or moving boundary electrophoresis) results in higher sensitivities and the ability to quantify asymmetric peaks (19). In this case, the integral chromatogram (Figure 3d) is converted to peaks (Figure 3c) by the differential properties of the schlieren optics detector. One of the important applications of frontal analysis is in physicochemical studies. Sequential differential concentration gradient detection can fiid applications in that area as well. Introduction of a well-defined injection in the form of a slope (linear gradient) (Figure 3e) will not only generate the chromatogram
40
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Flgure 7. Capillary zone electrophorogram of three underivatired amino acid mixtures (about 2 mM for all): glycine (l), L-alanine (2), L-asparagine (3); sample volume, about 10 n k fused silica capillary, 50 mm X 50 cm; 0.01 M phosphate buffer, pH 7; voltage, 10 kV; (A) sequential differential signal, (B) reconstructed and Integrated response.
(Figure 3c) but also detect any changes in dynamics of s e p aration. This approach can replace multiple concentration frontal analysis approach. The simple optical arrangement described in this paper is only one example of the system that can produce an output corresponding to the second derivative with respect to distance. For example, an appertured single beam with a flat intensity profile will also produce this effect; however, it will not be as sensitive as the two separate beam system with intensities centered at the photodiode edges. Sequential differential gradient detection has very unique and powerful characteristics that make it very attractive for chromatographic analysis. It should be remembered,however, that careful reproduction of separation conditions is required if direct quantitative analysis is to be performed by measuring height of peaks (second derivatives). Any change in the conditions can result in a substantial change in peak heights. If stable conditions cannot be ensured, conversion to refractive index (Figure 6) followed by additional integration in order to obtain peak areas will provide proper quantification. However, this conversion will enhance drifts as shown in Figure 7. ACKNOWLEDGMENT I thank Beckman Instruments, Inc., for the high-voltage power supply and M. Dignam for some optical components. LITERATURE CITED (1) Golay, M. I n Oss Chromatography 1958; Desty, D.. Ed.; Butterworths: London, 1958; p 58.
Anal. Chem. 1988, 60, 2801-2811 (2) Recent Advances In cepi#erv @as Chrometogmphy;Bertsch. W., Jennings, W., Kalser. R., Eds.; Huthlng: Heldehrg, 1981. (3) White, C.; Houck, R. HRC CC. J . High Resohit. Chromatogr. Chromatogr. Commun. 1986. 9 , 4. (4) Knox, J.; Gilbert, M. J . chrometogr. 1879, 186, 405. (5) Mlkkers. F.; Everaerts. F.; Verheggen. Th. J . Ckomtogr. 1979, 169, 11. (6) Jorgenson. J.; Lukac, K. Sdence (Washhgton, D .C .) 1963,222,266. (7) Jorgenson. J. Science (Washington, D . C . ) 1984, 226, 254. (8) Yeung, E. In Advances in Chromatography; Glddlngs, J., Ed.; Marcel Dekker: New York, 1984 Voi. 23. (9) Meyer, V. J . Chmmatcgr. 1985, 334, 197. (10) BornhQp, D.; Dovlchi, N. Anal. Chem. 1986, 58, 504. (11) Bornhop, D.; Nolan, T.; Dovlchl, N. J . Chromatogr. 1987, 384, 181. (12) Munk. M. In LlquM Chrometographic Detectors; Vlckers, T., Ed.; Marcel Dekker: New York, 1983. (13) Yeung. Edward In LlquM Chromatography Detectors; Yeung, E., Ed.; Marcell Dekker: New York, 1987. (14) Woodruff, S.; Yeunp, E. J . Chromatcgr. 1983, 260, 363. (15) Bogoslovskll, Y.; Razln, V. Zh. F k . Khim. 1971, 40, 1410.
(16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
2801
Banerjee, S.; Pack, E., Jr. Anal. Chem. 1982, 54. 326. Pawllszyn, J. Anal. Chem. 1S86, 58. 243. Pawliszyn. J. Anal. Chem. 1988, 56, 3207. Pawliszyn, J. J . Llq. chnunatcg. 1987, 10, 3377. Pawllszyn, J. Rev. Sci. Instrum. 1987, 58, 245. Sa, A. Pnndpks of Elecbtmk InsbvmentaMon; Arnold: East Kilbrldge, Scotland. 1961; p 227. Lephardt, J. In Transform Techniques in Chem&try; Grlffiths, P., Ed.; Plenum: New York, 1979; p 294. Pawllszyn, J.; Weber, M.; Dignam, M. Rev. Sci. Instrum. 1985, 56, 1740. Pawllszyn, J. Anal. Chem. 1988, 60, 766. Ruzlcka, J.: Hansen, E. Fbw InJecttOn Analysis; Wlley: New York, 1981. CRC Handbook of Chemistry and Physics, 64th ed.;CRC Boa Raton, FL, 1985; p E 365.
RECEIVED for review June 13,1988. Accepted September 13, 1988.
Detection of Hazardous Vapors Including Mixtures Using Pattern Recognition Analysis of Responses from Surface Acoustic Wave Devices S u s a n L. Rose-Pehrsson* a n d J a y W . Grate
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 David S. Ballantine, Jr.
GEO-Centers, Inc., 10903 Indian Head Highway, Ft. Washington, Maryland 20744
Peter C. Jurs 152 Davey Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802 Surface acowrtk wave (SAW) devices coated with selecthrely sorbent materials are being investigated as monitors for personnel safety where detectlon of hazardous vapors Is requlred at wb-part-per-mlliion concentrations in ambient alr that can contain Interferences at much higher concentratbns. A set of ten SAW devices coated with polymer materials designed to interact wkh different vapor propertles has been used to detect hazardous vapors by producing unique fingerprhrts to represent a glven vapor. The coatlngs were exposed to nlne lndlvldual vapors and two-component mlxtures of the vapors representlng dlfferent chemical classes and concentratlons, and the resunlng data matrix was studied by uslng pattern recognkhm methods. Four of the coatings were common to a prevkus study, and their r to the slngle vapors were used as a prediction set t o x c l a a c a t i o n capacky of a llnear dlscrlmlnant developed In that study. Ail of the vapors were correctly identlfled, except water. Principal components analysls and clustering methods were a p pHed to the responses of the coatlngs to ail the vapors, inciudlng mixtures. The Individual vapors cluster into speclflc regions In space, and the mlxtures lie In the areas between the clusters. Supervlsed learning techniques were used to reduce to eight the number of sensors necessary to identify the hazardous vapors in the presence of mixtures.
INTRODUCTION Great demands have been placed on toxic vapor detectors because detection of sub-part-per-million concentrations is required in ambient conditions where interferences can be present a t much higher concentrations. In addition, field 0003-2700/86/0360-2801$01.50/0
instruments should be small, portable, and inexpensive. It would also be advantageous to have an instrument that could be adapted to many detection problems. Few instruments have the necessary sensitivity and selectivity, while maintaining the small size needed in the field. Microelectronic chemical sensors meet the size requirement, can be very sensitive, and can show 3 to 4 orders of magnitude selectivity between toxic vapors of interest and common interferenta. Nevertheless, these capabilities may be insufficient if the interferent concentrations are orders of magnitude greater than the vapor concentrations that must be detected. Pattern recognition techniques in conjunction with an array of sensors is a promising approach to this type of analytical problem. This approach has been applied to vapor response data from electrochemical sensors (l),to the selection of coatings for piezoelectric crystal aensors (21,and to the study of coatings and identification of vapors using surface acoustic wave (SAW) devices (3). In the latter study, the clustering of vapors demonstrated that vapor solubility properties, such as hydrogen bonding, were important in determining how the sensors respond. In a recent paper by Carey and co-workers, multiple linear regression and partial least-squares calibration techniques were used to measure the concentration of twoand three-component mixtures by an array of nine piezoelectric crystal sensors (4).The goal of our present work is the detection of a single vapor or class of vapors in a complex environment, rather than the identification of the different components of the large number of mixtures possible in the environment. Our specific objective is the detection of toxic organophosphorus compounds. Several research groups are investigating SAW devices for toxic vapor detection because of their smallsize and excellent sensitivity (3, 5-21). These planar microsensors consist of 0 1988 American Chemical Society
