Polarography with phase-sensitive

Phase-sensitive AC with natural time 5 x 10"7 1 x 10 6 AE = 10 mV frequency = 100 Hz recommended far superior to nonphase-sensitive AC polarography... 

In the more advanced kinetic measurements, which were carried out by using chronopotentiometry [118], chronocoulometry [124, 139], linear [146] and convolution [18, 147] potential sweep voltammetry, or phase-sensitive ac polarography [142, 143], the ohmic drop was either numerically subtracted [118], or compensated [124, 139, 142, 143, 146, 147] with the help of the positive feedback. The feedback adjustment was based either on the assumption that the separation of the current peaks measured by the slow potential sweep voltammetry should reach the value of (59/z)mV [124, 139, 146, 147], or on the value of the solution resistance obtained by an ac bridge technique [142, 143]. However, the former adjustment is not very sensitive, whereas the estimated accuracy of 10 Q [142] in the latter case may not be... 

Both Faradaic components are proportional to bulk concentration and Yp is a signal completely without charging components, with a zero line ideally suited to analytical applications. Furthermore, phase-sensitive AC polarography gives instantaneous information about reversibility at a given co by comparing the ratio of Yp and Yp. 

The segments of dh DNA with distorted conformation desorb from the mercury electrode at the potentials by c. 0.2-0.3 V more negative than are the potentials of the peak 1 [102]. This desorption results in an appearance of the peak 2 on a.c. polarograms, which can be detected only for carefully isolated high-molecular mass nucleic acids and with the aid of in-phase component of phase-sensitive a.c. polarography. Peak 2 occurs at still more positive potentials than the peak 3 yielded by the corresponding ss nucleic acids [102]. 

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

By using phase selective measurement of a higher-harmonic alternating current (e.g., 2nd harmonic with a frequency 2j) increased sensitivity (lO mol/L) is obtained because of the marked reduction in the capacitive current component of the higher harmonic (AC2 or second harmonic wave polarography). 

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