Sensors and Actuators B 150 (2010) 93–98 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Sensor array and stop-flow mode applied to discrimination and quantification of gas mixtures M. Maciejewska * , A. Szczurek, L. Bodzoj, B. Flisowska-Wiercik Institute of Air Conditioning and District Heating, Wroclaw University of Technology, Wyb. Wyspia´ nskiego 27, 50-370 Wroclaw, Poland article info Article history: Received 20 March 2010 Received in revised form 12 July 2010 Accepted 26 July 2010 Available online 1 August 2010 Keywords: Gas sensor Discrimination Quantification Mode abstract The presented work focused on the strategy which offers widening the scope of applicability of sen- sor systems. The strategy was based on: (1) employing sensor array, which consists of many sensors, (2) applying stop-flow as the mode of sensor array operation, (3) utilizing response values of selected sensors measured at the same time point, (4) using only small fraction of obtained results. The performance of the system was evaluated with reference to the problems of discrimination of gas mixtures and quantifica- tion of their components. A considerable experimental material was the basis for our study. It comprised measurement results of 250 two-component VOC mixtures in air. They represented 10 classes character- ized by fixed qualitative composition and variable quantitative one. The results of our work proved that sensor system based on big sensor array and operated in stop-flow mode has wider scope of application than the system utilizing small sensor array and one kind of exposure conditions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Commercial success of analytical instruments depends strongly on their scope of application. In case of systems using sensor arrays, e.g. e-noses, the applicability is determined by sensing elements and working parameters which result from an applied operation mode. In commercial instruments, sensors are chosen in accor- dance with criterion of applicability to recognition tasks [1–3]. Usually, all selected devices are used for the analysis of tested samples. Designers of these types of systems have to take into consideration two groups of problems. The large numbers of sens- ing elements can generate redundancy and calculation difficulties. The disadvantage of low dimensional arrays is a narrow scope of application since a small number of sensors determine the poor discrimination ability of instrument. Sensors are usually exploited in steady state, because this mode of operation can be easily real- ized in practice. Recently, this situation has changed. It is caused by a fact that transient responses provide much more informa- tion than response values obtained in stationary conditions [4–7]. After considering these advantages and shortcomings, we decided to propose another strategy. It is based on four assumptions. (1) Sensor array consists of relatively large number of sensing ele- ments. In this way, it is possible to get diversification in sensor array response to target gases. (2) Stop-flow mode of operation is used in this approach, because it provides a great diversity of * Corresponding author. Tel.: +48 713203599; fax: +48 713203599. E-mail address: monika.maciejewska@pwr.wroc.pl (M. Maciejewska). working conditions and transient responses, which include much information about target gas. (3) The discrimination and quantifi- cation of gaseous sample is performed using response values of a number of individual sensors measured at a given time point. Each of these points represents some working conditions. (4) The ana- lytical tasks are realized with high effectiveness on the basis of only small part of obtained results. It reduces calculation difficulties. The aim of this study is to reveal that this strategy is well justified by the experimental results. In order to display the advantages of the proposed strategy the performance of sensor system, regarding discrimination and quan- tification of measured gases, was evaluated for all time points of exposure in stop-flow mode. 2. Experimental 2.1. Materials The following substances were used in measurements: hexane, heptane, octane, cyclohexane, benzene, toluene, xylene, ethyloben- zene. Chemicals were purchased from Sigma–Aldrich. Gas mixtures composed of two volatile organic compounds (VOCs) in pure air were studied. 10 pairs of VOCs were considered. Hexane (128–817 ppm) was examined together with: benzene (11–255 ppm), toluene (13–302 ppm), heptane (8–183 ppm), octane (7–165 ppm) or cyclohexane (10–249 ppm). Toluene (159–1019 ppm) was investigated together with hexane (9–204 ppm), heptane (8–183 ppm), benzene (13–302 ppm), ethy- lobenzene (9–220 ppm) or xylene (9–222 ppm). In case of each pair 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.07.037