A fluid sensor is constructed to have a pair of electrodes whereas between electrodes there are not additional materials designated to adsorb analytes if their concentrations are high, or there are adsorbents if the analyte concentrations are low. An alternating current voltage of varying frequencies is applied to the electrodes of the sensor by an alternating current device. In return, it detects electrical properties such as impedance and its components, reactance, resistance, and phase angles of the sensor with analytes whereas the analytes reside in or pass through the electrodes at each frequency. Thus a spectrum of electrical properties of the analyte can be established at various applied frequencies. The electrical properties are analyzed by a pattern recognition process, and compared with those of the known fluids. Therefore, the fluid can be detected and identified. A reference sensor is provided with the same configuration of the fluid sensor. With combining electrical signals from the fluid sensor and reference sensor, the present invention brings a number of advantages, including elimination of humidity influence, polymer film aging effect, and effect of temperature variations. In addition, a temperature programming is suggested in the present invention to better control processes of adsorption and desorption for analytes thus the analytes can be better detected and identified.