ABSTRACT
Preconditioning systems (preconcentrators) today are highly required in trace detection, either for conventional analytical methods such as gas chromatography or for gas sensors (Littlewood 1970; Van Es 1992; Filho et al. 2006; Chiriac 2007; Pijolat et al. 2007; Camara et al. 2009, 2010, 2011; Seo 2012). It was established that the preconcentration is a useful way to advance the selectivity and detection limits of these devices versus certain analytes of interest. Preconcentrators are also needed for the miniaturization and the higher mobility of such detection devices. The preconcentrator is conventionally placed at the front end of the analytical system based not only on the gas sensors (Moseley and Tofield 1987; Gopel et al. 1991) but also on the ion-mobility spectrometer and gas chromatograph (GC) (Janata 1989; Madou and Morrison 1989; Moseley et al. 1991). It concentrates and purifies the analyte at the sorption material. As a rule, physisorption is the phenomenon governing the preconcentration process, where weak intermolecular interactions between analyte molecules and an adsorbent surface are caused by means of Van der Waals force. Physisorption processes can easily be reversed by heating that raises the internal energy and breaks the weak bonds. Therefore, the method based on physisorption is preferred for preconcentration due to its simplicity and reversibility. Ideally, the sorptive material must absorb selectively one or more chemical species of interest over a time period necessary to concentrate the chemical compound in the absorptive material (Figure 12.1a) (Filho et al. 2006). Then, the sorptive layer must be heated with a pulse of temperature for providing narrow desorption peaks with relatively high concentration (Figure 12.1b) to the connecting detectors (e.g., gas sensors, electronic noise, or conventional analytical detectors such as gas chromatograph/mass spectrometers, etc). This process must allow the analytes present in a large air volume to be purified and concentrated, so increasing the efficiency of detection.
