Quartz crystal mass

The rotatable reactor can also be used for reactions in fluids having suitably low (< 10"3 Torr) vapor pressure. In this mode, metal atoms are evaporated upwards into the cold liquid, which is spun as a thin band on the inner surface of the flask. Reactions with dissolved polymers can then be studied. Specially designed electron gun sources can be operated, without static discharge, under these potentially high organic vapor pressure conditions (6). Run-to-nin reproducibility is obtained by monitoring the metal atom deposition rate with a quartz crystal mass balance (thickness monitor). 

Curie J, Curie P (1880) An oscillating quartz crystal mass detector. Comp Rend 91 294—297... 

Besides the standard weighing methods, it has also been proposed to use the piezoelectric properties of quartz for mass determination. Change in the weight of a sample that has been deposited direcdy on the quartz surface, causes a change in the oscillation frequency of the quartz crystal. Mass changes as small as 10" g can be determined by this method. The changes in frequency with temperature must be established by calibration, or the experimentation must be done isothermally. 

The thickness of the deposited layer (deposited mass) should not be higher than ca. 2% of the quartz plate (quartz crystal mass) at higher mass loadings the simple linear Am-A f relationship becomes invahd, and at very high loadings the crystal stops functioning. 

Mecca VM (2005) From quartz crystal microbalance to fundamental principles of mass measurements. Artal Lett 38 753-767 Bucur RV, Carlsson J-O, Mecea VM (1996) Quartz-crystal mass sensors with glued foil electrodes. Sens Actuators B 37 91-95 Bucur RV, Mecea VM, Carlsson J-O (2003) EQCM with air-gap excitation electrode. Calibration tests with copper and oxygen coatings. Electrochim Acta 48 3431-3438 Mecea V, Bucur RV (1979) The mechanism of the interaction of thin films with resonating quartz crystal substrates the energy transfer model. Thin Solid Films 60 73-84 Mecea V, Bucur RV, Indrea E (1989) On the possibility of thin film structure study with a quartz crystal microbalance. Thin Solid Films 171 367-375... 

The active element of the electrochemical quartz crystal microbalance (EQCM) was a gold-coated quartz crystal whose oscillation frequency was equal to 5 or 6 MHz. It was plated in a similar way, but the metal coatings were thinner (0.2-0.4pm). The constant relating the variations in quartz crystal mass with its oscillation frequency was determined by special calibration using EQCM data obtained in acid CuSO solutions at a controlled current density. 

The measurement of mass using a quartz crystal microbalance is based on the piezoelectric effect.When a piezoelectric material, such as a quartz crystal, experiences a mechanical stress, it generates an electrical potential whose magnitude is proportional to the applied stress. Gonversely, when an alternating electrical field is... 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

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). 

Fig. 4. Schematic of a multisequence biosensor in which the target glucose is first converted to glucose-6-phosphate, G6P, in the test solution by hexokinase. G6P then reacts selectively with glucose-6-phosphate dehydrogenase immobilized on the quartz crystal surface. Electrons released in the reaction then chemically reduce the Pmssian blue film (see Fig. 3), forcing an uptake of potassium ions. The resulting mass increase is manifested as a...

The electrochemical quartz crystal microbalance (EQCM) is a powerful tool for elucidating interfacial reactions based on the simultaneous measurement of electrochemical parameters and mass changes at electrode surfaces. The microbalance is based on a quartz crystal wafer, which is sandwiched between two electrodes, used to induce an electric field (Figure 2-20). The field produces a mechanical oscillation... 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

The electrochemical quartz crystal microbalance (EQCM) is a very useful technique for detecting small mass changes at the electrode surface that accompany electrochemical processes. In 1880, Jacques and Pierre Curie discovered that when stress was applied to some crystals, such as quartz, it resulted in an electrical potential across the... 

The quartz crystal microbalance has a long history of application as a means of determining film thickness in vacuum deposition techniques and more recently as a means of detecting trace constituents in the gas phase. In essence, it is an extremely sensitive sensor capable of measuring mass changes in the nanogram range. 

The mass sensitivity of the quartz crystal is at a maximum at the centre and decreases to zero at the edge. In general, for the purpose of mass calculation, the experimental mass-sensitive region is chosen to be the geometric working electrode area plus any of the exposed patch. 

SJ. Martin, V.E. Granstaff, and G.C. Frye, Characterization of a quartz crystal microbalance with simultaneous mass and liquid loading. Anal. Chem. 63, 2272-2281 (1991). 

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