Ohm’s Law Plot to Optimize your Separation can bring many benefits and is essential for good CE Methods.
- Select the optimal Voltage Setting for your Separation.
- Maximize the Efficiency of your Method.
- Learn the Upper Voltage Limits of your Method.
- Use to Validate your Capillaries and Columns in your Method.
Performing an Ohm’s Law Plot or Current/Voltage Plot is very simple. The optimal voltage is a function Capillary Diameter, Capillary Length, Background Electrolyte (BGE) Concentration, BGE Ionic Mobilities, and the Temperature of your System and Efficiency of your Cooling System.
The point at which there is a positive Deviation from Linearity of this plot is the Maximum Voltage you should use with the conditions you tested under.
To Perform and Ohm’s Law Plot:
- Fill your Capillary with a Buffer or BGE that you will use in your Method.
- Set the Instrument Temperature to the temperature in your Method.
- Set the voltage to 1-2kV.
- Activate and record the Current.
- Increase the Voltage in 1-5kV increments. Record the current once it stabilizes each time.
- Enter the Data in a spreadsheet program and perform a plot of Current V. Voltage
- Use Voltage as the X-axis.
- Determine when a 5% Positive Deviation from Linearity occurs.
- This is the OPTIMAL Voltage to run at.
If you change any experimental conditions you should re-run the above Ohm’s Law Plot.
This Ohm’s Law Plot shows the difference between two different instruments with two different cooling systems. Plot A, uses an air-cooled temperature control system and Plot B, uses a liquid cooled system. Capillaries and buffers were the same. It is obvious by this graph that a liquid cooled system is more efficient and provides a greater capability. It does bring in other factors such as cost and is a little less easy to use.