Microbalance configuration

Figure 2. Schemes for using piezoelectric quartz crystals. A. Quartz crystal microbalance configuration, standing shear wave between facing Au electrode contacts B. Surface acoustical mode configuration, surface undulation caused by bias between metal fingers C. Horizontal shear plate mode.

The characteristics of this kinetic superposition are strongly dependent on experimental parameters and will be different for both fixed bed reactors and the microbalance configuration. Any further interpretation of the data of Fig. 2.12 is therefore omitted. It is known, however, that under all circumstances the activation process will be controlled by a complicated interaction of kinetic influences and we should regard the curves shown in Fig. 2.12 only as a qualitative example. It is noted that line shapes, as shown in Fig. 2.12 for a heating rate of 60 Kh" have also been obtained as characteristic curves in model calculations of TPR profiles for three-dimensional phase boundary controlled reactions (topotactic reaction). ... 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Qualitative or quantitative mass spectrometric analysis can be made by one of two alternative configurations. Either the sample is decomposed in the high vacuum chamber of the mass spectrometer (MS) itself or reaction proceeds in an external system at higher pressure (e.g. a microbalance)... 

Daikhin, L. Urbakh, M., Influence of surface roughness on the quartz crystal microbalance response in a solution new configuration for qcm studies, Faraday Discuss. 1997,107, 27-38. 

There has been remarkable progress in the development and application of the quartz crystal microbalance (QCM) principle in sensitive devices for the detection and concentration measurement of specific molecules in gaseous and liquid media [1]. Since the behavior of quartz crystal resonator (QCR) sensors in gases is similar to quartz crystals technically used as frequency standards, a large set of circuit configurations is available, whose known properties can merely be adapted to particular applications [2-5]. In many cases quartz crystals used in electronic circuitry, sometimes even from mass production, are employed. 

Fig. 1. Schematic representation of vacuum furnace closed-cycle helium refrigeration system used for metal vapor microsolution optical spectroscopy, as well as conventional metal vapor-matrix isolation experiments. (A) NaCl or Suprasil optical window, horizontal configuration (B) stainless steel vacuum shroud (C) NaCl or Suprasil optical viewing ports (D) cajon-rubber septum, liquid or solution injection port (E) gas deposition ports (F) vacuum furnace quartz crystal microbalance assembly. With the optical window in a fixed horizontal configuration, liquid or solution sample injection onto the window at any desired temperature in the range 12-300 K is performed in position 1A, metal deposition is conducted in position IB, and optical spectra are recorded in position 1C see Procedure).

Urbakh M, Daikhin L (1997) Influence of siuface roughness rai the quartz crystal microbalance response in a sohitioiL A new configuration for QCM studies. Earaday Discuss 107 27—38... 

The quartz crystal microbalance (QCM) consists of a quartz crystal that is electrically driven into oscillation. The resonance frequency of the crystal is monitored. This frequency is highly dependent on any mass added to the crystal surface. Hence the mass dependence of the QCM resonance frequency can be, in air, used to weigh minute amounts of material with a sensitivity of the order of 1 ng/cm. QCM can also be coupled with electrochemistry here, the quartz crystal surface is coated with an appropriate electrode material, for example, thin film gold. This electrochemical QCM (EQCM) configuration can be used to monitor electrochemically triggered surface processes associated with the deposition (or loss) of material at the working electrode surface. However, in liquid medium the frequency shift of the QCM crystal is not solely sensitive to added mass but is also influenced by changes in the local property of the medium associated with the surface electrochemical process of interest. For example, density or viscosity variation of the medium in the electrode vicinity, in addition to variation in the viscoelastic properties of the deposited layer, can cause shifts in the resonant frequency of QCM. 

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