Absorption acoustic mass sensors

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

The key feature of all acoustic wave sensors for detecting vapors is that measurable characteristics of the acoustic wave is altered as a result of adsorption on the surface of a receptive layer or absorption into the bulk of a thin layer (Figure 3). After sorption of the vapor by a thin film on top of the acoustic resonator equilibrium conditions are established and as a consequence of the increased mass or more accurately the change in the phase velocity of the acoustic wave a signal is created. Surface coatings generally enhance the sorption of vapors with the key properties of selectivity and sensitivity while affording reversibility. Typically, rubbery polymers were used on SAW devices such as polyisobutylene or substituted polysiloxanes but also self-assembled... 

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

Kg. 1.9 Schematic diagrams of mass-sensitive gas sensors (a, b) quartz crystal microbalance (QCM) device (c) surface acoustic wave (SAW) device (d, e) microcantilever - (d) dynamic mode absorption of analyte molecules in a sensor layer leads to shift in resonance frequency, and (e) static mode the cantilever bends owing to adsorption of analyte molecules and change of surface stress at the cantilever surface (Reprinted with permission from Battison et al. (2001). Copyright 2001 Elsevier)... 

Operation of a SAW device as a chemical sensor normally involves the deposition of some chemically selective material onto the surface of the SAW device (2-5). In this work thin polymer films are coated over the entire surface of the SAW device. When a vapor is exposed to the polymer-coated SAW device, it distributes between the gas, bulk polymer phase, and interfaces, and comes to rapid thermodynamic equilibrium. Changes in mass and modulus that result from vapor absorption poturb the propagation of the surface acoustic wave (6), and ordinarily result in a r uction of the observ resonant frequency. The ratio between the concentration of the vapor in the gas phase, Cy, and the concentration of vapor in the polymer phase, Cp, is known as the partition coefficient, Kp, given by equation 1. The portion of the... 

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