Hydrogen Bond Acidic Polymers for Surface Acoustic Wave Vapor

Anal. Chem. 1999, 71, 1033-1040

Hydrogen Bond Acidic Polymers for Surface Acoustic Wave Vapor Sensors and Arrays Jay W. Grate,* Samuel J. Patrash, and Steven N. Kaganove

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Barry M. Wise

Eigenvector Research, Inc., P.O. Box 483, Manson, Washington 98831

Four hydrogen bond acidic polymers are examined as sorbent layers on acoustic wave devices for the detection of basic vapors. A polysiloxane polymer with pendant hexafluoro-2-propanol groups and polymers with hexafluorobisphenol groups linked by oligosiloxane spacers yield sensors that respond more rapidly and with greater sensitivity than fluoropolyol, a material used in previous SAW sensor studies. Sensors coated with the new materials all reach 90% of full response within 6 s of the first indication of a response. Unsupervised learning techniques applied to pattern-normalized sensor array data were used to examine the spread of vapor data in feature space when the array does or does not contain hydrogen bond acidic polymers. The radial distance in degrees between pattern-normalized data points was utilized to obtain quantifiable distances that could be compared as the number and chemical diversity of the polymers in the array were varied. The hydrogen bond acidic polymers significantly increase the distances between basic vapors and nonpolar vapors when included in the array. The use of an acoustic wave device as a chemical vapor sensor typically requires the application of a sorbent material as a thin film on the surface.1-6 Ideally, this material will strongly and selectively sorb the vapor of interest to provide a sensitive sensor. Thus, the design of such a material should optimize particular interactions with the target vapor. Where a single sensor does not provide adequate selectivity, the use of sensor arrays with (1) Grate, J. W.; Frye, G. C. In Sensors Update; Baltes, H., Goepel, W., Hesse, J., Eds.; VSH: Weinheim, 1996; Vol. 2, pp 37-83. (2) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A948A. (3) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 987A996A. (4) Grate, J. W.; Abraham, M. H.; McGill, R. A. In Handbook of Biosensors: Medicine, Food, and the Environment; Kress-Rogers, E., Nicklin, S., Eds.; CRC Press: Boca Raton, FL, 1996; pp 593-612. (5) Frye, G. C.; Martin, S. J. Appl. Spectrosc. Rev. 1991, 26, 73-149. (6) Ricco, A. J.; Crooks, R. M.; Xu, C.; Allred, R. E. In Interfacial Design and Chemical Sensing; Mallouk, T. E., Harrison, D. J., Eds.; ACS Symposium Series 561; American Chemical Society: Washington, DC, 1994; pp 264279. 10.1021/ac9810011 CCC: $18.00 Published on Web 01/27/1999

© 1999 American Chemical Society

pattern recognition is advantageous.7-25 Compared to a single sensor, an array of sensors collects more chemical information about a sample so that potentially interfering vapors can be distinguished from the analyte or analytes of interest. A sensor array should be designed to include materials collecting chemical information about the full spectrum of potentially interfering vapors, as well as including one or more sensors optimized for the analyte of interest. Thus, a sorbent material for a sensor array may be selected because its sensitivity and selectivity for the vapor of interest helps to differentiate this vapor from other vapors or because the material helps to differentiate other vapors from the target analyte through its sensitivity to other vapor types and properties. In the development of sorbent polymer materials for surface acoustic wave (SAW) chemical vapor sensors, solubility interac(7) Carey, W. P.; Beebe, K. R.; Kowalski, B. R. Anal. Chem. 1987, 59, 15291534. (8) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986, 58, 3077-3084. (9) Carey, W. P.; Beebe, K. R.; Kowalski, B. R.; Illman, D. L.; Hirschfeld, T. Anal. Chem. 1986, 58, 149-153. (10) Ballantine, D. S.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58, 3058-3066. (11) Grate, J. W.; Rose-Pehrsson, S. L.; Venezky, D. L.; Klusty, M.; Wohltjen, H. Anal. Chem. 1993, 65, 1868-1881. (12) Rose-Pehrsson, S. L.; Grate, J. W.; Ballantine, D. S.; Jurs, P. C. Anal. Chem. 1988, 60, 2801-2811. (13) Ema, K.; Yokoyama, M.; Nakamoto, T.; Moriizumi, T. Sens. Actuators 1989, 18, 291-296. (14) Zellers, E. T.; Pan, T.-S.; Patrash, S. J.; Han, M.; Batterman, S. A. Sens. Actuators B 1993, 12, 123-133. (15) Patrash, S. J. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 1994. (16) Patrash, S. J.; Zellers, E. T. Anal. Chim. Acta 1994, 288, 167-177. (17) Hierlemann, A.; Wiemar, U.; Kraus, G.; Schweizer-Berberich, M.; Goepel, W. Sens. Actuators B 1995, 26-27, 126-134. (18) Schweizer-Berberich, M.; Boeppert, J.; Hierlemann, A.; Mitrovics, J.; Wiemar, U.; Rosenstiel, W.; Goepel, W. Sens. Actuators B 1995, 27, 232-236. (19) Auge, J.; Hauptmann, P.; Hartmann, J.; Roesler, S.; Lucklum, R. Sens. Actuators B 1995, 26, 181-186. (20) Rapp, M.; Boss, B.; Voigt, A.; Bemmeke, H.; Ache, H. J. Fresenius J. Anal. Chem. 1995, 352, 699-704. (21) Nakamoto, T.; Fukuda, A.; Moriisumi, T. Sens. Actuators B 1993, 10, 8590. (22) Nakamoto, T.; Fukuda, A.; Moriisumi, T.; Asakura, Y. Sens. Actuators B 1991, 3, 221-226. (23) Amati, D.; Arn, D.; Blom, N.; Ehrat, M.; Saunois, J.; Widmer, H. M. Sens. Actuators B 1992, 7, 587-591. (24) Zellers, E. T.; Batterman, S. A.; Han, M.; Patrash, S. J. Anal. Chem. 1995, 67, 1092-1106. (25) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991, 3, 85-111.

Analytical Chemistry, Vol. 71, No. 5, March 1, 1999 1033

Table 1. Test Polymers abbrev PIB

description

characteristics

poly(isobutylene)a

nonpolar aliphatic hydrocarbon material OV215 an OV stationary dipolar nonbasic trifluoropropyl groups OV25 an OV stationary phasea polarizable phenyl groups PECH poly(epichlorohydrin)a slightly basic ether linkages and slightly dipolar chloromethyl groups b PEM poly(ethylene maleate) dipolar basic ester linkages a OV275 an OV stationary phase dipolar basic cyanoallyl groups PEI poly(ethylenimine)a basic amine linkages FPOL fluoropolyol,c see Figure 1 strong hydrogen bond acid SXFA see Figure 1 strong hydrogen bond acid BSP3 see Figure 1 strong hydrogen bond acid BSP6 see Figure 1 strong hydrogen bond acid phasea

Figure 1. Diagram of the repeat units of the structures of the four hydrogen bond acidic polymers used in this study and their abbreviations.

tions and linear solvation energy relationships (LSERs) have been used to systematically investigate vapor/polymer interactions and polymer properties.4,25-30 It has been proposed that a sorbent polymer-based SAW sensor array will collect the most chemical information if the polymers in the array cover the full range of solubility interactions, including dispersion, dipole/dipole, and hydrogen-bonding interactions.4,25,28,30 This approach leads to a requirement for polymers that are nonpolar, polarizable, dipolar, hydrogen bond basic, and hydrogen bond acidic. With one exception, polymers with all these properties can be obtained commercially. However, strongly hydrogen bond acidic polymers with low glass-to-rubber transition temperatures and other properties needed for effective use on a SAW vapor sensor are not readily available.31 The first study on polymer-coated SAW sensor arrays included a hydrogen bond acidic oligomeric epoxy dubbed “fluoropolyol”, whose chemical structure is shown in Figure 1.10 This material was originally developed at the Naval Research Laboratory (NRL) as a component of nonstick epoxy paints, and NRL has remained the sole source of this material for sensor studies. In the SAW sensor array study, fluoropolyol emerged as the material providing the most sensitivity for basic organophosphorus compounds,10 a result that can be attributed to hydrogen-bonding interactions.32 In this and a subsequent study, pattern recognition analysis also selected fluoropolyol for use in sensor arrays for classifying organophosphorus compounds,10,12 and fluoropolyol was included in a smart sensor array system for detecting these compounds.11 The use of fluoropolyol in these studies has established it as the de facto standard for hydrogen bond acidic SAW sensor coatings. (26) Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G.; Grate, J. W.; McGill, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 369-378. (27) Grate, J. W.; Snow, A.; Ballantine, D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988, 60, 869-875. (28) Grate, J. W.; Abraham, M. H.; McGill, R. A. In Polymer Films in Sensor Applications; Harsanyi, G., Ed.; Technomic Publishing Co.: Lancaster, PA, 1995; pp 136-149. (29) Grate, J. W.; Patrash, S. J.; Abraham, M. H. Anal. Chem. 1995, 67, 21622169. (30) McGill, R. A.; Abraham, M. H.; Grate, J. W. CHEMTECH 1994, 24 (9), 27-37. (31) Commercially available Fomblin ZDOL is a hydrogen bond acid due to terminal hydroxyl groups, as demonstrated in LSER studies, but it is unsuitable as a SAW sensor phase due to a finite volatility leading to drift. (32) Snow, A. W.; Sprague, L. G.; Soulen, R. L.; Grate, J. W.; Wohltjen, H. J. Appl. Polym. Sci. 1991, 43, 1659-1671.

1034 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

a Commercial material. b Material synthesized at PNNL. c Sample from NRL.

Since the original studies, several new strongly hydrogen bond acidic polymers have been synthesized and characterized in SAW sensor and/or LSER studies. Snow and co-workers synthesized hexafluoro-2-propanol (HFIP)-containing polymers based on polystyrene and poly(isoprene) polymer backbones.32 While these were good sorbents, the parent polymers and the HFIP-derivatized polymers had glass transition temperatures above room temperature. More recently, a polysiloxane with pendant HFIP groups was synthesized and described (SXFA in Figure 1).26 LSER studies demonstrated that this low glass transition temperature polymer was as good a hydrogen bond acid as fluoropolyol.26 There have been little or no SAW sensor results reported for this material; however, the use of LSER models to estimate SAW sensor behavior predicts that this polymer should be very good for the detection of a variety of basic vapors.29 Very recently, a method has been developed to synthesize a series of polymers incorporating hexafluorobisphenol A between poly(dimethylsiloxane) spacers (BSP3 and BSP6 in Figure 1).33 Preliminary sensor results have shown that these materials are good sorbents for basic compounds. In this paper, we compare a series of hydrogen bond acidic polymers as SAW sensor coatings for the detection of basic vapors. These studies demonstrate that the new materials are superior to fluoropolyol with respect to sensitivity and response time. In addition, we explore how the incorporation of strong hydrogen bond acidic polymers in SAW sensor arrays influences the spread of vapors in feature space, using unsupervised learning techniques to analyze a matrix of SAW sensor response data. This matrix includes the hydrogen bond acidic polymers and a variety of other sorbent polymer materials. EXPERIMENTAL SECTION Materials. The polymers used as SAW sensor coatings are listed in Table 1 along with their abbreviations. All were commercial materials except poly(ethylene maleate) and the hydrogen bond acidic polymers in Figure 1. Fluoropolyol was a kind gift from Arthur Snow of NRL. SXFA was first synthesized by one of (33) Grate, J. W.; Kaganove, S. N.; Patrash, S. J.; Craig, R.; Bliss, M. Chem. Mater. 1997, 9, 1201-1207.

Table 2. Organic Vapors and Abbreviations vapor name

code

typea

∑βH 2

concn rangeb (mg/m3)

n-hexane isooctane benzene toluene dichloromethane carbon tetrachloride trichloroethylene perchloroethylene tert-butyl methyl ether nitromethane acetonitrile methyl ethyl ketone methyl isobutyl ketone N,N-dimethylformamide dimethyl methylphosphonate 2-propanol 1-butanol trifluoroethanol

HEX IOC BZN TOL DCM CTC TCE PCE BME NME ACN MEK MIK DMF DMP IPR BTL TFE

LPV LPV LPV LPV LPV LPV LPV LPV PV PV PV PV PV PV PV PV PV PV

0 0 0.14 0.14 0.05 0 0.03 0 0.45 0.31 0.32 0.51 0.51 0.74 1.05 0.56 0.48 0.03

1210-110000 502-45800 681-62100 237-21600 3410-310000 1560-150000 912-83100 256-23300 2120-194000 185-16900 339-30900 613-55900 190-17300 21.8-1980 7.52-687 210-19100 38.3-3480 563-51200

a LPV, low-polarity vapor; PV, polar vapor. b Four concentrations were tested in ratio of approximately 1:3.9:18.5:91.

the present authors at the NRL according to the procedure in ref 26. The sample used in the present study was prepared at Pacific Northwest National Laboratory (PNNL) by the same procedure. The BSP3 and BSP6 polymers were prepared at PNNL according to ref 33. Poly(ethylene maleate) was synthesized according to ref 34. The liquid organic solvents used to generate vapor streams were commercial chemicals from Aldrich of 99% or greater purity, except nitromethane (ACS reagent grade, 95%) and dimethyl methylphosphonate (97%). The test vapors used are listed in Table 2. SAW Resonators, Oscillators, and Frequency Data Collection. SAW devices and oscillator circuitry to operate them were obtained from Femtometrics (Costa Mesa, CA). The SAW devices were 200-MHz two-port resonators described and used in previous studies.35-37 We obtained these devices packaged in two ways, each with different operating circuitry. Single SAW resonators were obtained mounted on headers (Femtometrics RS2-SIP single SAW package) and operated with the Femtometrics model 2001E SAW mass microbalance. Frequency measurements were made using a Hewlett-Packard 53131A high-performance universal counter with a medium-stability time base, transferring the data to a Macintosh computer using the IEEE-488 bus and collecting data with Labview software. Sensor array packages consisted of six SAW resonators mounted in a single header in two rows of three sensors each (Femtometrics RS6 multi-SAW array DIP package), with a large amount of space between the two rows. This array package was operated in a circuit (Femtometrics model 200-6TC Multi-SAW array instrument) with a socket for the sensor array package, a socket for a single-sealed reference device, oscillator circuits for each SAW device, and a mixing system to beat each of the sampling sensor frequencies against the reference device frequency. This circuit generates six difference frequencies. The Femtometrics model 200-6TC Multi-SAW array instrument also includes an on-board microcomputer with frequency-counting capabilities and data storage. Frequency data from the array (34) Snow, A.; Wohltjen, H. Anal. Chem. 1984, 56, 1411-1416.

system were collected on a Macintosh computer using Smartcom communications software. Due to occasional cross-talk problems between adjacent sensors in the package, we only coated the four sensors in the corners of the package, leaving the two sensors in the center of each row uncoated. With both experimental systems, we controlled sensor temperatures using a single brass heat sink clamped against the lids of both the sampling sensor (or sensor array) package and the reference device package. Water from a refrigerated circulating water bath circulated through the brass heat sink, and all experiments were conducted at 25 °C. Temperatures were monitored with a type K thermocouple (0.005-in.-diameter wire, Omega) in contact with the sensor or sensor array header; vapors were delivered to the sampling sensor or sensor array with gas inlet and outlet tubes on the sides of the lid. The lid for the array package was modified with a stainless steel bar running down the inside center of the lid. When placed on the package, this bar fit between the two rows of sensor devices and provided solid thermal contacts from the lid to the header for efficient heat transfer. The gas inlet to the lid delivered the gas to a manifold of passages machined in the stainless steel bar so that test gas was delivered to each sensor in parallel. The lid had a single gas outlet. Spray-coated polymer films were applied to the SAW resonators as described previously,35,36,38 using an airbrush supplied with compressed dry nitrogen and a dilute solution (0.2 wt %) of the polymer. The resonators were always cleaned in a Harrick plasma cleaner prior to film application.37 The solvent used was typically chloroform, except when OV215 and PEI were applied. The frequency was monitored during deposition, using the change in frequency as a measure of the amount of material applied. Spraycoated films were always examined by optical microscopy with a Nikon microscope using reflected light Nomarski differential interference contrast. The films were annealed at 50 °C overnight after application. Typical film thicknesses were between 150 and 250 kHz. Vapor Generation. Vapor streams were generated from bubbler sources that were maintained at 15 °C and diluted with a pulse-width modulation method described in detail in ref 39. The operation of this system has been described previously.33 The instrument output is either the diluted vapor stream or clean carrier gas, each at a flow rate of 100 mL/min. Typical sensor exposure times were 5 min, with the sensors reaching a steadystate response in this time. During testing with the array instrument, frequency data were collected every 10 s. When testing with single sensor packages to follow the sensor response times, frequency data were collected every 2 s. Chemometric Analysis. Chemometric analyses were carried out using MATLAB software (The Mathworks, Inc.) on a Macintosh and the PLS Toolbox (Eigenvector Research, Inc.). The data for all sensors were first normalized to a coating thickness of 250 kHz so that the analysis would consider a set of sensors in which (35) Grate, J. W.; Klusty, M. Anal. Chem. 1991, 63, 1719-1727. (36) Grate, J. W.; Klusty, M.; McGill, R. A.; Abraham, M. H.; Whiting, G.; Andonian-Haftvan, J. Anal. Chem. 1992, 64, 610-624. (37) Grate, J. W.; McGill, R. A. Anal. Chem. 1995, 67, 4015-4019. (38) Grate, J. W.; Wenzel, S. W.; White, R. M. Anal. Chem. 1991, 63, 15521561. (39) Grate, J. W.; Klusty, M. Vapor Stream Dilution by Pulse Width Modulation. Naval Research Laboratory, 6762, 1990.

Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

1035

Figure 2. Calibration curves for polymer-coated SAW sensors to methyl ethyl ketone and N,N-dimethylformamide. The sensor responses were first normalized to equivalent film “thicknesses” of 250 kHz.

each had the same amount of polymer. All data were then pattern normalized by dividing each sensor response in a pattern by the square root of the sum of the squares of the sensor responses in the pattern. This normalization puts all the data on a hypersphere with a radius of 1, prior to doing cluster analysis. For principal component plots, the data were also mean scaled. RESULTS AND DISCUSSION Comparisons of Hydrogen Bond Acidic Polymers as SAW Sensor Coatings. We first set out to investigate the relative merits of the four polymers shown in Figure 1 as SAW sensor coatings. These materials offer high sensitivities to basic vapors at trace concentrations. The calibration curves for methyl ethyl ketone and N,N-dimethylformamide for SAW sensors with these polymers are shown in Figure 2. These calibration curves over a concentration range of 2 orders of magnitude are nonlinear, reflecting the interaction of the basic vapor with a finite number of hydrogenbonding sites in the material. The curve for more basic N,Ndimethylformamide is more nonlinear than that for less basic methyl ethyl ketone. Nonpolar vapors produce linear calibration curves on these polymers. Additional data on the sensitivities of these polymers to both basic and nonbasic vapors are given in 1036 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

Table 3, along with data from two low-polarity polymers for comparison. Sensitivities are reported as the Hertz response per 100 mg/m3 of vapor for a sensor with 100 kHz of applied polymer. For basic vapors leading to nonlinear calibration curves on the hydrogen bond acidic polymers, the sensitivity was taken from the response at the lowest tested vapor concentration. These results show that the three new hydrogen bond acidic polymers, SXFA, BSP3, and BSP6, all offer superior sensitivity to basic vapors compared to fluoropolyol. The sensitivity of the BSP3 polymer to basic vapors is generally superior to that of the BSP6 material, which is most likely due to its greater concentration of hydrogen-bonding sites. (It has been observed that a material with even longer polysiloxane bridges between the bisphenol groups is still less sensitive.)33 The best polymers in terms of sensitivity are SXFA and BSP3. Compared to sensors coated with low-polarity polymers such as PIB and OV25, the strong hydrogen bond acidic polymers offer sensors with 10-20 times greater sensitivities for the basic vapors at the tested concentrations. Among the low-polarity vapors, the BSP3 polymer results in more sensitive sensors than the SXFA sensor. This likely results from the polarizable aromatic groups in BSP3 offering stronger interactions with these vapors than the dimethylsiloxane and fluoroalkyl groups of SXFA. In this regard, the SXFA is somewhat more selective for basic vapors over nonpolar vapors than BSP3. We also examined the response times of sensors coated with the hydrogen bond acidic polymers. It was observed that fluoropolyol-coated sensors were noticeably slower to respond than those with the other hydrogen bond acidic polymers in tests using array sensor packages and 10-s intervals between data points. We checked these results by testing polymer-coated SAW resonators in single sensor packages (to reduce dead volume relative to the array package) and collected frequency data every 2 s. The results for toluene are shown graphically in Figure 3. Table 4 reports sensor responses to toluene and methyl ethyl ketone in terms of the fraction of the full response as a function of time for the first 30 s of exposure. These data clearly show that the fluoropolyolcoated sensor is indeed slower to respond (noting that all sensors were tested under identical conditions with respect to temperature, sensor packaging, and gas flow) and that this is true for both a low-polarity and a basic vapor. From Table 2 it is apparent that sensors coated with SXFA, BSP3, and BSP6 all reach 90% of full response within 6 s of the first indication of a response, while the FPOL-coated sensor only registers 50% or less of full response in the same time frame. Thus, the three new polymers are superior to fluoropolyol in terms of response time. Use of Hydrogen Bond Acidic Polymers in Sensor Arrays. Having demonstrated that the new hydrogen bond acidic polymers are superior to FPOL as coatings on individual sensors, we then investigated the more general question of how these materials are useful in SAW sensor arrays. We began with the premise that the selection and diversity of the polymers in the array would influence the spread of the vapor response data in feature space. Unsupervised learning techniques were then used to examine the distribution and clustering of pattern-normalized SAW sensor response data.40 The data matrix for this analysis included data for 18 diverse organic vapors (see Table 2), each at 4 concentra(40) It is also possible to investigate sensor array data with a variety of other pattern-recognition and quantitative chemometric methods; these are beyond the scope of the present paper but will be reported in future papers.

Table 3. Sensitivitiesa to Selected Vapors

N,N-dimethylformamide (DMF) methyl ethyl ketone (MEK) tert-butyl methyl ether (BME) trichloroethylene (TCE) carbon tetrachloride (CTC) toluene (TOL) isooctane (IOC) a

SXFA

BSP3

BSP6

FPOL

OV25

PIB

20300 239 51.5 5.30 2.96 17.8 4.05

26200 249 50.6 10.4 5.88 35.3 6.66

16400 129 38.5 8.85 5.34 25.0 6.28

13400 82.1 9.42 3.70 2.03 13.7 1.48

1150 14.8 3.75 18.2 8.85 46.8 6.01

892 4.19 2.97 11.8 6.89 24.8 12.6

These sensitivities are in hertz shift observed per 100 mg/m3 vapor concentration per 100 kHz of applied polymer.

Table 4. Response Times as a Fraction of Steady-State Response time (s)

Figure 3. Response times of SAW resonators coated with four hydrogen bond acidic polymers to toluene at 4380 mg/m3. The amounts of polymer on each sensor are FPOL, 173 kHz; SXFA, 174 kHz, BSP6, 201 kHz; and BSP3, 196 kHz. These plots show the first minute of response following a valve operation that changes the output of the vapor generator from clean carrier gas to test vapor. Consecutive data points are 2 s apart.

tions, and a set of 11 polymers including low-polarity materials (such as PIB and OV25), more basic dipolar materials (such as Silar 10C and OV275), and the 4 hydrogen bond acidic polymers under investigation (see Table 1). The sensor response data were normalized as described in the Experimental Section. Pattern normalization removes the effect of vapor concentration on the variance in the data and thus helps to focus the analysis on the separation of vapors on the basis of differing vapor/polymer interactions.

BSP6

FPOL

BSP3

SXFA

0 2 4 6 8 10 20 30

Responses to Methyl Ethyl Ketone at 613 mg/m3 0.00 0.00 0.00 0.00 0.00 0.01 0.46 0.09 0.48 0.84 0.34 0.79 0.90 0.49 0.88 0.91 0.59 0.91 0.93 0.81 0.93 0.94 0.89 0.94

0.00 0.00 0.48 0.81 0.89 0.91 0.93 0.94

0 2 4 6 8 10 20 30

Responses to Toluene at 4380 mg/m3 0.00 0.00 0.00 0.02 0.00 0.00 0.65 0.04 0.38 0.87 0.26 0.82 0.92 0.41 0.93 0.93 0.51 0.95 0.95 0.75 0.98 0.95 0.86 0.98

0.00 0.00 0.29 0.79 0.91 0.95 0.98 0.98

To simplify the discussion, we will divide the vapors into two groups, defining all the aliphatic, aromatic, and chlorinated hydrocarbons as low-polarity vapors and all the remaining vapors as polar vapors, as indicated in Table 2. Except for trifluoroethanol, the polar vapors are all significantly basic, as indicated by Abraham’s ∑βΗ 2 parameter in Table 2. Though not very basic, trifluoroethanol is a strong hydrogen bond acidic vapor and it is also grouped with the polar vapors. The low-polarity vapors are all either nonbasic or only weakly basic. Two principal component plots are compared in Figure 4. The principal component plot using all 11 polymers (Figure 4a) revealed a clear distinction between the low-polarity vapors on one side of the plot and most of the polar vapors on the other side of the plot. Trifluoroethanol and nitromethane are distinguished from each of the two major groups. When the four hydrogen bond acidic polymers are removed from the array, the principal component plot (Figure 4b) no longer provides this clear distinction between the low-polarity and polar vapors. These results support the hypothesis that inclusion of the hydrogen bond acidic polymers provides better separation of low-polarity from basic vapors. We also examined distances between vapors in feature space. Because the data were pattern normalized as described in the Experimental Section, all data points are on a hypersphere of radius 1. We determined the distances between data points in terms of the radial distance in degrees between the points. These distances were determined for all pairs of data points and Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

1037

Figure 4. Two principal component plots: (a) using all 11 polymers in Table 1 (set A in Tables 5 and 6) and (b) with all four hydrogen bond acidic polymers removed from the array (set B in Tables 5 and 6). Basic vapors in (a) are not individually identified. In (b), lines are used to connect data points of the same vapor (four concentrations of each vapor were determined).

summarized in distance tables. We used this method to obtain quantifiable distances that could be compared as the number and identities of the polymers in the array were varied. The data were also clustered using the K-nearest neighbor (KNN) method and displayed as dendrograms. The same distance metric was used for the KNN clustering. Selected results from these analyses are presented in Tables 5 and 6. Figure 5 shows a hypothetical dendrogram to help explain the data in Table 5. The vapors divide into two major groups, consisting of the low-polarity vapors (LPV1-3) and the polar vapors (PV1-3). Each of these clusters is shown with three vapor members. Individual concentrations of each vapor are not shown in Figure 5, but they were considered in our treatment of the actual data. The largest distance indicated on the dendrogram, which is 12 deg between the two major clusters, indicates the distance between the pair of low-polarity and polar vapors that are the closest. The dendrogram itself does not tell which pair of vapors this is, nor does it tell the separation between the most distant points. The distance tables must be consulted to determine this information. Within the low-polarity vapor cluster, dichloromethane was always least similar to the other low-polarity vapors and thus occupied a position like that of LPV-3 in Figure 5. With some polymer combinations, some of the polar vapors moved from the 1038 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

major cluster of polar vapors into the cluster of low-polarity vapors, indicating that they were more similar to the other low-polarity vapors than dichloromethane. In Table 5, dendrogram results are shown for six arrays. We first considered an array A containing all 11 polymer-coated SAW sensors. The largest distance in the dendrogram is given as the “cluster separation”, and the two vapors determining that spread (found by examining distance tables) are given in the next column. With 11 polymers including the hydrogen bond acidic polymers, the dendrogram places all the low-polarity vapors in one major group and the polar vapors in a second major group, just as shown in the hypothetical Figure 5. The cluster separation is 27 deg. When the four hydrogen bond acidic polymers are removed from the set, the remaining 7 polymer array has a spread of only 19 deg between the two major clusters. In addition, the clustering no longer keeps the polar and nonpolar vapors in separate groups: methyl ethyl ketone, methyl isobutyl ketone, and tertbutyl methyl ether are clustered with the nonpolar vapors. When the set is further reduced by removing the most polar remaining polymers to give only a four-sensor array, there are again two major clusters separating the polar and nonpolar vapors; however, the cluster separation is only ∼8 deg. These results clearly show that the overall separation of polar and nonpolar vapors is best (27 compared to 19 deg) when hydrogen bond acidic polymers are included in the array, which is consistent with the results shown in the principal component plots. To be certain that this result was not simply due to the number of sensors, we then began making substitutions into set C, the four-sensor array containing only low-to moderate-polarity polymers. Substituting OV275 into set C to give set D increased the cluster separation from 8 to 13 deg. This set is comparable to set B because it includes both low-polarity polymers and basic polymers. Like set B, the nonpolar and polar vapors do not form distinct groups; in fact, all the polar vapors are more similar to the low-polarity vapors than dichloromethane. Substituting BSP3 into set D to give set E increased the cluster separation to 27 deg, the same result as the 11-polymer set A. In addition, polar and nonpolar vapors are clustered separately. Using SXFA instead of BSP3 as the hydrogen bond acidic polymer resulted in a cluster separation of 31 deg for set F. Thus, the spread of the vapor as determined from the maximum distance appearing on the dendrogram depends on polymer diversity rather than polymer number. Table 5 also examines the distance between dichloromethane and the next nearest vapor within the low-polarity cluster as determined from dendrograms and indicates the identity of the next nearest vapor. For example, this distance would be 8 deg in Figure 5 when LPV-3 is dichloromethane. For diverse polymer sets A, E, and F, this distance is 18-20 deg. For sets B and D, which lack the hydrogen bond acidic polymers, the distance is only 12-13 deg. In set C containing only low- to moderate-polarity polymers, this distance is reduced to 8 deg. Therefore, greater polymer diversity increases the spread among low-polarity vapors when that class is defined to include dichloromethane. For sets B and D, the vapor nearest to dichloromethane is methyl ethyl ketone rather than another low-polarity vapor. The last column in Table 5 summarizes the instances when a polar vapor clusters in the low-polarity class defined to include dichloromethane.

Table 5. Clustering Results Summarized from Dendrogramsa set/no. of polymers

polymers

cluster separationb/vapor pair determining cluster separation (deg)

separation within low-polarity clusterc/vapor pair (deg)

polar (basic) vapors in low-polarity group

A/11 B/7 C/4 D/4 E/4 F/4

Table 1 set A minus hb acids in Figure 1 PIB, OV215, OV25, PECH PIB, OV215, OV25, OV275 PIB, OV25, OV275, BSP3 PIB, OV25, OV275, SXFA

27/DCM-NME 19/TFE-ACN 8e/CTC-BME 13/DCM-MEK 27/DCM-NME 31/DCM-NME

20/DCM-BZN 12/DCM-MEK 8/DCM-BZN 13/DCM-MEK 18/DCM-BZN 18/DCM-BZN

none MEK, MIK, BMEd none all vaporsf none none

a Eighteen organic vapors, each at four concentrations; see Table 2. b Maximum distance on dendrogram. c Maximum distance on dendrogram from DCM to low-polarity vapor, indicating distance from DCM to nearest vapor within low-polarity cluster. d Lowest concentration of BTL also clustered among low-polarity vapors. e Largest spread is actually 9.6 deg due to one IPR outlier at lowest concentration. f The distance from DCM to any other vapor is the largest distance on the dendrogram.

Table 6. Selected Vapor-Vapor Distancesa largest vapor-vapor distanceb (deg)

selected vapor distancesb (deg)

set/no. of polymers

all vapors

excluding TFE

excluding TFE and NME

DCMIOC

TCEIOC

TCEBZN

TCEPCE

DCMMEK

TCEMEK

CTCBME

A/11 B/7 C/4 D/4 E/4 F/4

72/IOC-TFE 75/HEX-TFE 57/IOC-NME 76/IOC-TFE 76/BME-TFE 74/IOC-TFE

61/IOC-NME 71/IOC-NME 57/IOC-NME 69/IOC-NME 62/IOC-NME 68/IOC-DMP

56/DMP-PCE 68/IOC-ACN 55/IOC-ACN 68/IOC-ACN 59/IOC-DMP 68/IOC-DMP

48 55 47 51 47 49

28 33 31 30 27 30

9 9 8 8 8 9

11 12 10 10 9 10

49 12 14 13 49 54

46 24 23 25 47 57

44 9 8 8 43 55

a Eighteen organic vapors, each at four concentrations. b The most widely separated two vapors, based on the smallest distance between any combination of concentrations of those vapors.

Figure 5. Hypothetical dendrogram with the low-polarity vapors (LPV) and the polar vapors (PV) in separate clusters.

The distance results in Table 6 consider the same six sets of polymers. This table seeks to examine the most widely separated vapors and to look at the separation of particular vapor pairs. For example, for diverse polymer sets A and F, the largest separation is between nonpolar vapor isooctane and trifluoroethanol. If trifluoroethanol is removed from consideration, the second most distant pairs of vapors are isooctane and nitromethane for sets A-E. Eliminating both trifluoroethanol and nitromethane from consideration, the most distant vapors are a nonpolar vapor (typically still isooctane) and a basic dipolar vapor (acetonitrile or dimethyl methylphosphonate). The identities of the most widely separated vapors do not depend heavily on the particular set of polymers used. Hence, hydrogen bond acidic polymers are not critical to this criterion for the spread of vapors in feature space. The final columns in Table 6 consider particular vapor pairs. The presence or absence of hydrogen bond acidic polymers does

not have a significant effect on the distances between pairs of nonpolar vapors. By this metric, these polymers are not necessary for detection problems where distinctions must be made between such vapors. On the other hand, the hydrogen bond acidic polymers have a large effect on the separation of certain polar vapors from nonpolar vapors. These effects are seen in comparisons of distances between chlorinated hydrocarbons and methyl ethyl ketone, for example, and between carbon tetrachloride and tert-butyl methyl ether. The distance between dichloromethane and methyl ethyl ketone triples when a hydrogen bond acid polymer is included in the array, while the distance between trichloroethylene and methyl ethyl ketone nearly doubles. Without a hydrogen bond acid polymer to see the differences in basicity between methyl ethyl ketone and a chlorinated hydrocarbon, they simply look like dipolar vapors. The distance between carbon tetrachloride and tert-butyl methyl ether quadruples with the inclusion of a hydrogen bond acid in the array. These two vapors look like nondipolar vapors if they are not distinguished by their differing basicities using a hydrogen bond acidic polymer. DISCUSSION It has been recognized for many years that a hydrogen bond acidic polymer is advantageous for the sensitive detection of hydrogen bond basic vapors,27,32,41 and such a polymer was included in the first publication on a SAW sensor array.10 The present study shows that new polymers, which were deliberately designed and synthesized to obtain the desired hydrogen-bonding interactions and favorable physical properties (low glass transition temperature), offer advantages with respect to both sensitivity and (41) Barlow, J. W.; Cassidy, P. E.; Lloyd, D. R.; You, C. J.; Chang, Y.; Wong, P. C.; Noriyan, J. Polym. Eng. Sci. 1987, 27, 703-715.

Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

1039

response time relative to fluoropolyol. These new polymers are not very difficult to synthesize, and so there exists the opportunity for more widespread use of polymers with these properties. It has been proposed for several years that a hydrogen bond acidic polymer would be useful in a sensor array, but this issue has not been systematically investigated previously. Unsupervised learning techniques with a quantifiable distance between pattern-normalized data points in feature space show that the spread between polar and nonpolar vapors, both in general and between particular vapor pairs, is improved when a hydrogen bond acidic polymer is included in the array. The inclusion of the hydrogen bond acidic polymer in the sensor array increases the chemical information obtained from the sample being observed. Whether this information is analytically useful depends on the specifics of the detection problem to be solved. Others have shown that satisfactory discrimination between vapors can be obtained without such a polymer in a SAW sensor array.24,42,43 The hydrogen bond acidic polymers are (42) Zellers, E. T.; Han, M. Anal. Chem. 1996, 68, 2409-2418. (43) Patrash, S. J.; Zellers, E. T. Anal. Chem. 1993, 65, 2055-2066. (44) Zellers noted that DCM and MEK were difficult to analyze in a mixture, although detection limit issues may have been responsible for the difficulty in this case. The array had diversity similar to that of sets B and D in our paper, where we observed that DCM and MEK were not separated by a major division in clusters.

1040 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999

expected to be useful in the specific cases where sensitivity to basic vapors is critical and in samples where a nonbasic but somewhat dipolar vapor, such as some of the chlorinated hydrocarbons, may occur in combination with a moderately dipolar and basic polymer.44 The presence of the polymer will help to distinguish between these vapors. In a detection problem where unknown vapors may occur, the ability of an array that includes these polymers to spread the vapors in feature space has the potential to help prevent unexpected interferences from vapors that were not in the training set. ACKNOWLEDGMENT The authors thank Arthur Snow for the fluoropolyol sample. This authors are grateful for funding from the United States Department of Energy Office of Nonproliferation and National Security, NN-20, and from the Office of Environmental Science and Technology within the Department of Energy Office of Environmental Management. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the Department of Energy by Battelle Memorial Institute. Received for review December 4, 1998. AC9810011

September

9,

1998.

Accepted