Mass-change sensitivity

Campbell, G. A., Mutharasan, R. PEMC sensor s mass change sensitivity is 20 pg/ Hz under liquid immersion. Biosensors and Bio electronics 2006, 22 (1), 35—41... 

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. 

An example of the very high sensitivity of the EQCM to mass change at the surface of the crystal is provided in the work of Bruckenstein and Shay (1985) on the electro-oxidation of gold in perchloric acid. 

The quartz balance uses a thin quartz crystal, a few hundred /xm thick, with thin, vapor-deposited gold films on the two sides. Such a crystal has a fundamental mode for shear waves with a frequency in the 1-15 MHz region, which can be excited by application of a corresponding alternating voltage on the two electrodes. The resonance frequency is very sensitive to small mass changes of the system. One... 

The circles in Fig. 16.6a show the calculated sensitivity, AX/Am for these different cases. As can be seen, the innermost holes are the most sensitive to any refractive index changes in the local environment as opposed to the holes that are further away from the cavity. These results can be explained by noting that the evanescent field is largest inside the innermost holes and decreases inside holes that are situated further away from the cavity. This is important to note because targeting only the inner most holes for functionalization allows for the lowest possible limit of mass detection for this device. In the case where only the inner two holes are functionalized we find that the resonance shifts by 3.5 nm when 1 fg of DNA binds to the resonator. Therefore, a mass change of 10 ag would result in a mass surface density of 0.84 ng cm 2 and an approximate shift of 0.01 nm, which can be experimentally detected. We therefore take this as the potential LOD of the device. 

The QCM is extremely sensitive and measures small changes in mass, that is, 1 ng mass change gives a 1 Hz frequency change (Lu and Czanderna, 1984). 

The method of neutron radiography is simple to implement. Since water has a large neutron attenuation coefficient, the technique of radiography has a high sensitivity for small water mass changes... 

First, the underlying principles upon which bulk acoustic wave (BAW) devices operate are described. When a voltage is applied to a piezoelectric crystal, several fundamental wave modes are obtained, namely, longitudinal, lateral and torsional, as well as various harmonics. Depending on the way in which the crystal is cut, one of these principal modes will predominate. In practice, the high-frequency thickness shear mode is often chosen since it is the most sensitive to mass changes. Figure 3.4 schematically illustrates the structure of a bulk acoustic wave device, i.e. the quartz crystal microbalance. 

There are other major problems with peak assignment on the basis of the areas. These problems relate to the reproducibility of peak area measurements under widely varying conditions. Ideally, the area of a peak remains constant even if its capacity factor varies. However, varying the conditions may affect the peak areas. If the column temperature is changed in GC, then the flow rate may be affected. Peak areas will change (by a constant factor) if concentration-sensitive detectors such as the hot wire detector (H WD katharo-meter) are used, but not with mass flow sensitive detectors (such as the flame ionization detector, FID). 

Quartz crystal microbalance — The quartz crystal microbalance (QCM) or nanobalance (QCN) is a thickness-shear-mode acoustic wave mass-sensitive detector based on the effect of an attached foreign mass on the resonant frequency of an oscillating quartz crystal. The QCM responds to any interfacial mass change. The response of a QCM is also extremely sensitive to the mass (density) and viscoelastic changes at the solid-solution interface [i-vi]. 

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