A sensor cell comprises receiving means, which may be in the form of an electrode 10 coupled to the gate electrode of a thin film transistor T1. In one form of the invention a voltage supplied to the gate electrode of the transistor T1 via a switching transistor T7 is controlled in dependence upon the value of capacitance C.sub.A arising at the electrode from receipt of a sample for identification. Thus, the operation of transistor T1 can be used to identify the sample received by the electrode 10.