Currently the QMB resonance frequency is determined with a read-out electronic oscillator. More accurate crystal parameters are provided by special impedance spectrometers.

For gas sensing purposes the QMBs are coated with sensing layers, which, ideally, are specific, sensitive, stable and fast responding to the target gases (Figure 1). If such a QMB is exposed to an analyte, the mass of the sensing layer and, therefore, the mass of the whole sensor will increase and the resonance frequency will respectively decrease.

Figure 1. The sketch of a gas sensing QMB and of the corresponding gas sensing mechanism.

Typical exposure protocol and the results obtained with a gas sensing QMB are presented in Figure 2. Only the resonance frequency shifts have been presented; the nominal resonance frequency of the resonator was 30 MHz.

Figure 2. The response to different n-propanol concentrations pulses of a polyurethane coated QMB.

Applications

Coated with polymers or other organic materials the QMBs are employed for the detection VOCs (volatile organic compounds) or/and humidity [2-5] (Figure 2). By using an array of gravimetric sensors with different sensitive layers it is possible to analyze the qualitative and quantitative composition of a gas mixture.

List of publications

[1 ] G. Sauerbray, Z.f.Phys. 155, 1959, 206

[2 ] Interdependence of Ammonia and Water Sorption in Polyacrylic Acid Layers, M. Sahm, A. Oprea, N. Barsan, U. Weimar, Sensors & Actuators B, 130, 2008, 502-507.

[3 ] Ammonia Detection Mechanism with Polyacrylic Acid Sensitive Layers: Field Effect Transduction, A. Oprea, N. Barsan, U. Weimar, Sensors and Actuators B, 111-112, 2005, 577-581.

[4 ] Gas sensing: basic understanding and devices. Weimar, U., N. Barsan, et al. (2004). Chemical Sensors 20(Suppl. B): 6-9.

[5 ] Influence of ammonia and water sorption on the chemical and electrochemical properties of polyacrylic acid and its derivatives, M. Hörter, Dissertation

Gravimetric measurements with the Quartz Micro Balance (QMB)

QMBs are devices that measure very small mass changes, typically 1010 Hz*g-1.

They are special devised quartz resonators, cut (usually AT cut) from quartz single crystals. Due to the piezoelectric properties of the material (electro-acoustical coupling), an alternating voltage will cause a resonating acoustic wave; the resonance frequency depends on the size geometry and mass load of the crystal slab. Thus a QMBs mass change results in a shift of its resonance frequency. The mass change can be calculated by using the Sauerbrey equation [1]: