Abstract: Improved micro machined (MEMs scale) heaters, which are particularly suitable for use in MEMs scale preconcentrators. Preferably the heater possess a trapping medium, in particular a polymer of intrinsic microporosity (PIMs). There is further provided devices comprising the preconcentrator, and methods of preparation and use. There is particular benefit directed to the use of a MEMs scale heater coated with the PIMs for use in hand-held or field portable chemical detection devices. The heater comprises a number of electrically conducting paths which have been engineered so that the electrical resistance of all the electrically conducting paths are substantially equal, to provide a more uniform heat distribution.