Sample application sweeping techniques

There is another characteristic that all these techniques present the current is sampled at a given moment during the application of any individual potential of the sequence (typically at the end of each applied potential), so the response is a discrete collection of pairs of data (potential-current). Conversely, in the case of techniques like Linear Sweep Voltammetry or Cyclic Voltammetry, the current is recorded continuously (see Sect. 5.1).11... 

Analyses for "copper, cadmium, and lead were carried out continually by DPASV. Zinc determinations were excluded to permit use of a lower electrolysis potential. The samples were analyzed at pH 4.9 by sparging with carbon dioxide. An 8-min. electrolysis at —1.0 V vs. silver/ silver chloride and a 25-mV pulse were used during the Seattle-Saanich portion of the trip (Leg 1) while a 10-min. electrolysis and a 50-mV pulse were used from Saanich to Seattle (Leg 2). Application of the DPASV technique resulted in greater sensitivity and thus shorter plating times for the low levels encountered. It also afforded better resolution for "copper than linear-sweep ASV. It should be pointed out, however, that DPASV does not result in shorter analyses times because the stripping portion of the analysis is very slow. Nevertheless, it is worthwhile to limit the time of electrolysis because this also reduces the concentrations of interfering metals accumulated in the mercury fllm. Under the... 

The mean surface concentrations enforced by depend on many factors (a) the way in which is varied (b) whether or not there is periodic renewal of the diffusion layer (c) the applicable current-potential characteristic and (d) homogeneous or heterogeneous chemical complications associated with the overall electrode reaction. For example, one could vary sequential potentiostatic manner with periodic renewal of the diffusion layer, as in sampled-current voltammetry. This is the technique that is actually used in ac polarography, which features a DME and effectively constant during the lifetime of each drop. Alternatively one could use a stationary electrode and a fairly fast sweep without renewal of the diffusion layer. Both techniques have been developed and are considered below. The effects of different kinds of charge-transfer kinetics will also be examined here, but the effects of homogeneous complications are deferred to Chapter... 

In DPV, the difference between two sampled currents is measured, registered just before the end of the pulse and just before pulse application. In the first instruments to offer this technique, the pulses were superimposed on a linear ramp of potential. However, in digital potentiostats, it is simpler to superimpose the pulses on a staircase waveform. Thus, the base potential is incremented in a staircase (Fig. 4) and the pulse, of constant height, is of width 10 or more times smaller than the period of the staircase waveform (Fig. 6a). If these conditions are followed, the staircase width is generally the same as for NPV, so that once again the effective sweep rate is of the order of 1 to 10 mV s . ... 

Another technique involving distillative concentration of sample constituents is sweep co-dis-tillalhn. Here the sample is treated with a highly volatile solvent introduced with the aid of a stream of carrier gas. The solvent in turn transports soluble components of the sample to a cooled distillation receiver. The method is useful for such applications as the isolation of volatile pesticides from animal and plant fats [237], [238]. 

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