Ž . Sensors and Actuators B 62 2000 121–130 www.elsevier.nlrlocatersensorb Vapor recognition with an integrated array of polymer-coated flexural plate wave sensors Qing-Yun Cai a , Jeongim Park a , Dylan Heldsinger a , Meng-Da Hsieh a , Edward T. Zellers a,b, ) a Department of EnÕironmental Health Sciences, UniÕersity of Michigan, Ann Arbor, MI 48109-2029, USA b Department of Chemistry, UniÕersity of Michigan, Ann Arbor, MI 48109-2029, USA Received 23 July 1999; received in revised form 24 September 1999; accepted 24 September 1999 Abstract Ž . Preliminary testing of a prototype instrument employing an integrated array of six polymer-coated flexural plate wave FPW sensors and an adsorbent preconcentrator is described. Responses to thermally desorbed samples of individual organic solvent vapors and binary and ternary vapor mixtures are linear with concentration, and mixture responses are equivalent to the sums of the responses of the component vapors, which co-elute from the preconcentrator in most cases. Limits of detection as low as 0.3 ppm are achieved from a 60-s Ž 3 . 34 cm air sample and peak widths at half-maximum range from 1 to 4 s. Tests at different flow rates suggest that the kinetics of vapor sorption in the sensor coating films may limit responses at higher flow rates, however, low data acquisition rates may also be contributory. Assessments of array performance using independent test data and Monte Carlo simulations with pattern recognition indicate that individual vapors and certain binary and ternary mixtures can be recognizedrdiscriminated with very low error. More complex mixtures, and those containing homologous vapors, are problematic. This is the first report demonstrating multi-vapor analysis with an integrated FPW sensor array. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Vapor sensor; Sensor array; Acoustic-wave sensor; Pattern recognition; Vapor recognition; Flexural plate wave 1. Introduction Arrays of partially selective sensors of various types have been used to analyze multiple individual vapors and w x simple vapor mixtures 1–10 . In most cases, the sensors in the array are overlaid with interfacial films consisting of polymers of diverse structures into which vapors rapidly and reversibly partition to varying degrees. The differential partitioning gives rise to the characteristic response pat- terns used for vapor recognition. Where measurement of low concentrations of organic vapors in the ambient environment are required, precon- centration via a bed of a hydrophobic granular porous polymer can be useful. In addition to increasing sensitivity Ž . and reducing limits of detection LOD , preconcentration ) Corresponding author. Department of Environmental Health Sci- ences, University of Michigan, 109 S. Observatory St., Ann Arbor, MI 48109-2029, USA. Tel.: q1-734-936-0766; fax: q1-734-763-8095; e-mail: ezellers@umich.edu can provide immunity from baseline drift as well as a w x degree of water-vapor compensation 2,11,12 . The prototype instrument described here employs an Ž . array of flexural plate wave FPW sensors, which are similar in many respects to the more common surface Ž . w x acoustic wave SAW sensors 1–4,13–18 . In both sen- Ž . sors, radio-frequency mechanical acoustic waves are gen- erated within a piezoelectric substrate that has been coated Ž . with a chemically sensitive e.g., polymer film. The acoustic waves are launched and received by a pair of Ž . interdigital transducers IDTs on the device surface, and a feedback amplifier connecting the IDTs permits sustained oscillation at a frequency determined by the device struc- ture. Small changes in the mass or viscoelastic properties of the coating film caused by interactions with gases or vapors result in a change of the acoustic wave velocity, which can be measured indirectly as a change in wave frequency using digital frequency counting electronics. In contrast to the SAW device, the active region on which the acoustic waves travel in the FPW device is a membrane whose thickness is much smaller than the w x acoustic wavelength 13–18 . As a result, wave energy is 0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0925-4005 99 00381-0