Wave impedance

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

A single waveguide section between two partial sections is shown in Fig. 10.6. The sections are numbered 0 through 2 from left to right, and their wave impedances are... 

The Three-Multiply Normalized Scattering Junction. Figure 10.10 illustrates a three-multiply normalized scattering junction [Smith, 1986b], The one-multiply junction of Fig. 10.8 is normalized by a transformer. Since the impedance discontinuity is created locally by the transformer, all wave variables in the delay elements to the left and right of the overall junction are at the same wave impedance. Thus, using transformers, all waveguides can be normalized to the same impedance, e.gRi= 1. ... 

The wave impedance can be seen as the geometric mean of the two resistances to displacement tension (spring force) and mass (inertial force). 

Figure 10.6 A waveguide section between two partial sections, a) Physical picture indicating traveling waves in a continuous medium whose wave impedance changes from R0 to Ri to R2. b) Digital simulation diagram for the same situation. The section propagation delay is denoted asz- T. The behavior at an impedance discontinuity is characterized by a lossless splitting of an incoming wave into transmitted and reflected components.

In case of vacuum (e = e0) the impedance is real and its absolute value is 377 fl (wave impedance of vacuum). In case of a metal an incident wave decays rapidly from the surface, thus the impedance of a metal is called surface impedance Zs = Rs + iXs. The real part of Zs is called surface resistance Rs, the imaginary part surface reactance Xs. 

Type /4, shortened Frequency range 26.5-27.5 Mcps and 34.5-35.5 Mcps Wave impedance 50 ohms Operational voltage >10 V Power 1W Type of protection EEx ia I Certificate BVS 92.C.1193 Dimensions overall length 450 mm Weight approx. 0.7 kg. 

The equivalent circuit model of Figure 3.7 can be used to describe the near-resonant electrical characteristics of the quartz resonator coated by a viscoelastic film. The surface film causes an increase in the motional impedance, denoted by the complex element Zg. From Equation 3.19, this element is proportional to the ratio of the surface mechanical impedance Zj contributed by the film to the characteristic shear wave impedance Zq of the quartz. 

Part II of the book deals with lesser known aspects of US for the analytical chemists such as its use as an energy source for detection purposes. Thus, ultrasound-based detection techniques viz. US spectrometry in its various modes including ultrasound attenuation, ultrasonic velocity, resonant ultrasound, laser-generated, ultrasound reflection and acoustic wave impedance spectroscopies) are dealt with in Chapter 9. Finally, Chapter 10 is devoted to seleoted applioations of US spectrometry — mostly non-analytical applications from whioh, however, analytical chemists can derive new, interesting analytical uses for ultrasound-based deteotion techniques. 

Figure 10.9. Continuous system used by Su et al. to implement various anaiyticai methods inciuding a fiow-injection manifoid, a gas-diffusion unit and a buik acoustic impedance sensor. A acceptor, BAWIS buik acoustic wave impedance sensor, C — carrier, GDC — gas diffusion ceii, iV — injection vaive, PP — peristaitic pump, R — reagent, RC — reaction coii, SL — sampie ioop, 1/1/— waste, WB — water bath. (Reproduced with permission of Eisevier, Ref. [98].)...

SAW, surface acoustic wave (impedance) FET, field-effect transistor EIS, electrolyte-insulator-semiconductor GOx, glucose oxidase LDH, lactate dehydrogenase LOx, lactate oxidase. 

The correlations (4) above, result in the layer having the same wave impedance as air (120 tt), i.e., there is no reflection from the outer surface. The radiation passing through the layer is reflected back by the metallic base. If the dielectric and magnetic losses, represented by e and fi" respectively, are large, the greater part of the radiation can be absorbed by the layer even if it is thin. 

Figure 9.2 Dependence of wave impedance on distance from source normalized to XHn.

When an EM wave propagates through the material, the wave impedance approaches the intrinsic impedance of the material. For dielectric... 

Hence, the complex wave impedance Z in the conducting medium can be written as ... 

Given the surface wave impedances Zsw l and Zsw r and the generator impedance Zq. ... 

Grating Periodic variation in refractive index or wave impedance, causing wavelength-dependent coupling between propagating waves, and often used as an optical filter. 

Besides the fact that the medium resists the passage of a pressure wave (impedance), an elastic medium possesses another complicating characteristic. A purely sinusoidal pressure wave travels with a characteristic velocity in a medium, i.e., the phase velocity c (= f A). When any simple wave at any frequency travels through a medium at the same phase velocity, this medium is said to be non-dispersive. Actually, a medium is more or less dispersive so that when a multifrequency wave pulse travels in such a medium, the pulse spreads out. 

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