Flow-through piezoelectric sensors

More recently, Yang and Thompson implemented this type of sensor in FI manifolds, which they consider ideal environments for relating the sensor s hydrodynamic response to the analyte s concentration-time profile produced by the dispersion behaviour of sample zones. Network analysis of the sensor generates multi-dimensional information on the bulk properties of the liquid sample and surface properties at the liquid/solid interface. The relationship between acoustic energy transmission and the interfacial structure, viscosity, density and dielectric constant of the analyte have been thoroughly studied by using this type of assembly [171]. 

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Low-pressure flow injection interfaces have been used as links between the extractor and either a photometric detector [118], a flow-through potentiometric sensor [119] or a piezoelectric sensor [120] in dynamic flow injection (FI) systems. Figure 7.18 depicts these unusual types of interface. In the first (Fig. 7.18A), a membrane phase separator (total fluid volume 50 pi) was used to remove CO, from the extract. In this way, interferences were suppressed while ensuring quantitative transfer of the solutes (viz. chloramphenicol and penicillin G) to the hydrodynamic system. 

Fig. 7.18. Low-pressure interfaces to detectors based on flow injection. (A) Interface to a photometric detector across a membrane. (Reproduced with permission of the American Chemical Society.) (B) Interface to a flow-through photometric sensor with prior derivatization by the modified Griess reaction. (Reproduced with permission of the American Chemical Society.) (C) Interface to a piezoelectric detector. P peristaltic pump, C collector, CUC clean-up column, DB debubbler, SA sulfamic acid, NEDD /V-( 1-naphthyl)ethylenediamine dihydrochloride, SV switching valve, W waste, DF displacement flask, IV injection valve, FC-PZ flow-cell-piezoelectric crystal, OC oscillator circuitry, F frequency counter, PC personal computer. (Reproduced with permission of Elsevier.)...

Thermal and mass flow-through sensors rely on differential measurements owing to the low selectivity of these types of detection. They use two flow-cells arranged in series (Fig. 2.9.B) or parallel (Fig. 2.9.C), each containing a sensitive microelement (a piezoelectric crystal or a thermistor). One of the cells houses the sensitive microzone, whereas the other is empty or accommodates an inert support containing no immobilized reagent (e.g. see [35]). 

Piezoelectric flow-through sensors based on a non-regenerable immobilized reagent... 

Most chemical flow-through sensors based on piezoelectric phenomena (measurements of gases or liquids) are of the regenerable type. 

Devices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signals reflected from discontinuities in the fluid stream to obtain flow measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam from a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow stream. Signals reflected off flow disturbances are detected by a second piezoelectric crystal located in the same sensor. Transmitted and reflected signals are compared in an electrical circuit, and the corresponding frequency shift is proportional to the flow velocity. 

Fig. 2. Measuring set-up (A) photograph of the piezoelectric device and flow system, the inset shows the cell holding the quartz sensor (B) sample QCM sensor with 10 MHz base frequency (as used throughout the described experiments) (C) cross-section through the piezo-cell showing the two rubber O-rings holding the quartz plate, only one side of the sensor is in contact with the fluid (D) cross-section of the cell used for combined piezoelectric and amperometric measurements, the lid also hold a titanium wire electrode and the Ag/AgCI reference electrode.

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