Ohm’s law plot

Figure 12.4. Example of an Ohm s law plot where the dashed line shows the predicted behavior if there is effective heat dissipation.

Ohm s law States that the current, /, in a circuit is proportional to voltage, E, and inversely proportional to resistance, R I = E/R Ohm s law plot In capillary electrophoresis, a graph of current versus applied voltage. The graph deviates from a straight line when Joule heating becomes significant. 

Figure 4.9 Ohm s law plots showing the optimum voltages for different capillary temperatures using air circulation for temperature control. (Reprinted from Ref. 24 with permission.) (A) No control (B) 25°C (C) 10°C (D) 4°C.

The buffer concentration plays an important role in the separation. Typical buffer concentrations range from 20 to 150 mM. At the higher buffer concentrations, the production of heat may require the use of lower field strength or smaller-diameter capillaries (25 pm instead of 50 pm). An Ohm s law plot is used to select the appropriate voltage. The advantages of high-concentration buffers include improved peak shape, fewer wall effects, and increased sample stacking. 

To calculate the Joule heating of a buffer, one should run an Ohm s law plot as outlined in Section 1.3.3.5.2, under Buffers. To calculate watts, we use the following ... 

The later onset of deviation from Ohm s Law for the FS capillary as voltage increases in the Ohm s Law plot in Figure 18.1a demonstrates the superior heat dissipation of FS over FEP. This is confirmed by the smaller slope of G vs. P/L for the FS capillary in Figure 18.1b. The ratio of the gradient of the G vs. P/L graph to the intercept can be used to estimate the temperature increase of the electrolyte. A more detailed description of temperature calculation will follow in Section 18.4. In addition, the average internal diameter (di) of the capillary can be derived from the conductance at zero power (Go). The intercept. Go, is proportional to the cross-sectional area of the capillary. Provided that the lengths of the capillaries are equal, this allows one to use Equation 18.3 to calculate the internal diameter of an unknown capillary based on the internal diameter of another capillary [14]. 

The possibility of determining the electrolyte temperature and the capillary diameter make the G versus P/L graph a superior method to the Ohm s Law plot for characterizing Joule heating. 

Figure 8.6. Ohm s law plot of the run buffer composed of 50-mM hydroxypropyl-P-cyclodextrin, 100-mM SDS, 25-mM sodium tetraborate pH 9.0 at 25°C...

Optimize field strength for optimal buffer formulation Wash the capillary with base (0.1 N NaOH) for 2 min, followed by the separation buffer for 5 min at the highest hydrodynamic flow available for the system. Apply increasing voltages (5 to 30 kV) for a few seconds and record the current generated. Construct an Ohm s law plot to select optimal field strength (see above). 

An experimental method for determining an optimum value for the applied voltage for a given capillary and buffer system is the Ohm s Law Plot, a plot of current versus applied voltage. Because of the linear relationship between voltage and current. 

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