This paper presents the optimization of a micro gas preconcentrator applied to environmental monitoring. Different types of adsorbent materials introduced in silicon microchannels with various designs have been investigated and a special focus has been dedicated to the modes of operation of the preconcentrator. The optimization of the device and its operation were driven by its future application in outdoor environments. Parameters such as the preconcentration factor, cycle time and the influence of the humidity were considered along the optimization process. As a result of this study, a preconcentrator with a total cycle time of 10 minutes and the use of single wall carbon nanotubes (SWCNTs) as adsorbent exhibits a good preconcentration factor for VOCs with a limited influence of the humidity.