Noise Measurement Setup for QCM

The detection of chemical substances dissolved in a gas or liquid strongly requires chemical sensors which able to adsorb reversibly the molecules of analyzed matter in amounts that depend on its concentration and on the specificity of adsorbing sites in a sensor. For the transformation of non-electric input quantity to electric signal, the sensors utilize various physical or chemical phenomena. One of universal transfer mechanisms is the change of the resonant frequency of quartz crystal.

Quartz crystal microbalance represents highly sensitive sensor for a detection of chemical substances. QCM-based gas sensors are widely utilized as a result of their robust nature, availability and affordable interface electronics. There are several mechanisms that are related to fluctuation phenomena. Generally, users require minimizing their impact on measurements in order to achieve maximal signal-to-noise ratio. However, fluctuation mechanisms can also give useful pieces of information about physical or chemical processes occurred in a sensor.

Noise measurement and its evaluation represent a new approach of extracting more selective response from resistive and surface acoustic wave sensors. Experimental results demonstrated that the noise spectral density of sensor's resistance fluctuations is modified by exposure to different gases as well as by exposure to different concentration of gases. Smulko et al. showed that sensibility and selectivity of gas detection in Taguchi gas sensors can be increased by observing resistance noise of sensors. The observed fluctuations give more information than a single DC resistance value and therefore lead to a reduction of the number of gas sensors necessary for detection of different gas mixtures. Gomri et al proposed a model of adsorption-desorption noise in metal oxide gas sensors, based on the free electron density fluctuation produced by the gas adsorption. Using this model for simulating the oxygen chemisorption - induced noise, they found that the contribution of oxygen adsorption-desorption noise to the noise spectra is a Lorentzian component having a corner frequency and low frequency magnitude which are specifics of the adsorbed gas. Thus, noise spectroscopy might be highly useful for improving gas sensors selectivity and sensitivity.

Our preliminary experimental results revealed, that absorption of detected matter affects fluctuations in QCM. Hence we proposed a new method based on noise measurements for evaluation of QCM measurements besides standard techniques. The paper describes our measurement setup which was designed to study small frequent fluctuation of QCM.