The parameters that can be extracted from experimental data depend upon the electrochemical technique (e.g., peak potential, peak current and peak area for cyclic voltammetry, and slope and intercept for chronoamperometry/chronocoulometry). Therefore, different measurement methods are required for different techniques. The methods available on the epsilon are as follows:
This function will measure and report the peak potential, current, and area under the peak for symmetric and tailing peaks, and the half-wave potential and limiting current for sigmoidal curves. The baselines for these measurements can either be determined automatically by the software or manually by the user. The operation of this function is best illustrated by an example (Fig1).
Figure 1The cyclic voltammogram to be analyzed is shown above. The peak parameter values are calculated using the default values for the peak finding function. These paramneters can be changed using Data - Change Peak Parameters from the pop-up menu. The Peak Parameters dialog box is shown in Fig2.
Note that there are two measurement options available (Auto Peak and Manual Peak), which are selected using check boxes. Let us first examine the Auto option. Clicking the Apply button will find peaks based on the Min Width and Min Height parameters, and will generate the graph shown in Fig1. The peak potentials, currents and areas are also listed.
The Manual option can be used if the user is not satisfied with the output from the Auto Peak measurement (e.g., if inappropriate baselines are used, or if there is an extraneous peak). To use the Manual option, first click the Manual Peak check box. Since the peaks must be analyzed individually when using this option, the first peak to be examined must be selected. This is done either by clicking the appropriate peak [#] in the listing or by selecting the appropriate # in the Number box (Fig3). Note that, once a peak has been selected, that segment is highlighted (Fig4).
If this peak is not required, it can be removed by clicking Delete. Note that the listing changes automatically upon deletion.
The baseline for measurements of a selected peak can be redefined by first clicking Update, and then redrawing the line using the left mouse button. The peak measurement values based on this new baseline will replace the old values. A new peak can be measured on the specified segment by clicking Insert, and then defining the baseline. Once all the peaks have been detected, and their baselines defined, clicking Redraw will show the baselines for all the peaks in the voltammogram. Fig5 shows the effect of checking the Paint Peaks box; that is, the calculated peak area is shown for each peak. The Peak Parameters dialog box can then be closed (by clicking the Close button).
The above description used a cyclic voltammogram as an example, but it also applies for data from other techniques that generate a peak-shaped or sigmoidal response (however, note that the baseline used for the Manual option for a Symmetrical peak is based on the x values of the line defined by the user). It is important to ensure that the correct Peak Shape is selected (e.g., Symmetrical for differential pulse voltammetry, Tailing for cyclic voltammetry with linear diffusion (planar disk electrodes on millimeter dimensions), and Sigmoidal for normal pulse voltammetry or cyclic voltammetry with radial diffusion (e.g., microelectrodes at slow scan rates)).
Note that all Peak baselines and peak lines are saved with the data file. If one reanalyzes saved data and manually redraws baselines, then be sure to resave the file.
Data from potential step experiments can be analyzed using the linear relationship between the response and some function of time. Clicking the Select Graph item in the pop-up menu produces a sub-menu of the available graph options, and the appropriate option is selected (e.g., Q vs sqrt(T)).
Once the correct graph has been selected, the dialog box required for the calculation is opened by selecting Data - Calculate CA-SIR from the pop-up menu (Fig6).
There are two parameters to be specified for the linear fit. R - Window specifies the percentage of data points to be used in the fit. This parameter is required since the finite time required to change the potential from the initial value to the step value means that the first few data points are not valid. The default condition is that the final 80 % of the data points are used for the calculation (i.e., the first 20 % are discarded). W is the weighting factor used in the calculation (x, 1/x, or 1/x2). On clicking CAL_SIR, the parameter values are calculated and displayed in the window (Fig7) (this window must be displayed if the results are to be printed with the graph). The graph now displays the lines used for the calculations (Fig8).