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 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.
