The Charge Signal

The charge detector responds in two ways. The primary response is from the ionic analyte constituents injected into the cell and their transport into the respective electrode compartments. When NaN03 is injected, the primary signal 

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Fig. 7.2. (a) a pressure sensitive array which uses OFETs to switch an otherwise linear pressure sensitive resistor sheet, (b) shows a piezoelectric sheet film (typically PVDF or one of its derivatives) locally amplified using an OFET. The OFET converts the charge signal at its gate into a current signal which can be transmitted through a significant capacitance. 

Lateral spreading of signal can be reduced in direct conversion detectors when compared with phosphors because the charge signal is quickly swept toward the collection electrode by the electric field before the charge has much opportunity to spread (Zhao 1997). This offers the possibility of excellent spatial resolution with a detector that is thick enough to obtain a high value of q. 

The chemical shifts of in natural abundance have been measured for thiazole and many derivatives (257,258). They are given in Tables 1-37 and T38. These chemical shifts are strongly dependent on the nature of the substituent CNDO/2 calculations have shown (184) that they correlate well with the ((t+tt) net charge of the atom considered. As a consequence, the order of the resonance signals is the same for protons and for carbon atoms. 

Although there are only three principal sources for the analytical signal—potential, current, and charge—a wide variety of experimental designs are possible too many, in fact, to cover adequately in an introductory textbook. The simplest division is between bulk methods, which measure properties of the whole solution, and interfacial methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode. The measurement of a solution s conductivity, which is proportional to the total concentration of dissolved ions, is one example of a bulk electrochemical method. A determination of pH using a pH electrode is one example of an interfacial electrochemical method. Only interfacial electrochemical methods receive further consideration in this text. 

A very simple equation exists that relates spontaneous polarization to the intensity of the second harmonic signal. Spontaneous polarization is conceived as the sum of the products of each of the charges in a dielectric material by each of their displacements from the centrosymmetric positions. Hence, the relationship between spontaneous polarization and the second harmonic signal value can be presented as follows ... 

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

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