Capillary Electrophoresis and CE Expert Lite

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By Harry Whatley, Beckman Coulter, Inc.

What's it all about?

Capillary electrophoresis is a small volume separation technique. It is difficult to conceptualize--much less to calculate--the volumes used to rinse capillaries and inject samples.

CE Expert Lite is an online tool intended to help answer such questions as:

• How much am I injecting?
• Is this capillary rinsed enough?
• How do I partially fill a capillary?
• What changes when I change capillary dimensions?

CE Expert Lite is best viewed as a specialized calculator. On entering known parameters such as capillary dimensions, pressure, and time, fluid delivery volumes and times are automatically calculated and displayed.

How does CE Expert Lite work?

The core of the program is the Poiseuille equation (first described by Jean Louis Marie Poiseuille, a French anatomist and physician). This equation describes how fluid under pressure will flow through a cylindrical vessel. The equation takes the form

(Equation No. 1)

where V is the volume delivered across the capillary, is the pressure drop across the capillary (Pascals), d is the capillary inside diameter (meters), t is time the pressure is applied (seconds), is the buffer viscosity (Pascal-seconds), and L is the total capillary length (meters). This equation has wide applicability in fluid dynamics. The same equation can be used to calculate fluid flow in a CE capillary, an artery, or a water main.

By using suitable conversion factors CE Expert Lite allows pressure to be input in psi and converts the delivery volumes to microliters or nanoliters. The program also calculates the total volume of the capillary, the volume to the window, and how long it will take to fill the capillary under the specified conditions.

Equation No. 1 shows that the capillary diameter is a very significant factor in fluid flow. Because volume delivered varies with the fourth power of the diameter, a very small change in capillary diameter can cause a very large change in fluid delivery. In contrast, the delivery volume varies linearly with pressure, time, viscosity, and length.

To simplify the calculations CE Expert Lite assumes that the fluid under consideration behaves like pure water. The program has a built-in algorithm to approximate the change in viscosity of water with change in temperature. The use of non-aqueous buffers or buffers with additives that change viscosity will lead to results that differ from actual fluid deliveries.

The first parameter is the type of CE system being considered. This is selected in the System Type area.

There are three choices: Beckman Coulter P/ACE™ 2000-5000, Beckman Coulter P/ACE™ System MDQ, and Generic CE (any other CE system). The selection of system type will change the options available elsewhere in the CE Expert Lite window. Selecting either of the Beckman Coulter P/ACE options eliminates the need to input the total length of the capillary (which is automatically calculated based on the length to the window). The Generic CE option allows all parameters to be input. In addition to being applicable to almost any CE system, this option is handy for experimenting with the Poiseuille equation itself.

The second input area is for capillary dimensions. The total length can only be input if the Generic CE option has been selected. Capillary lengths are input in centimeters and the internal diameter is input in micrometers.

The third input area is for the flow conditions under consideration.

Upon clicking the Calculate Button, the results are displayed on the right side of the window.

Entering invalid values (such as total length less than length to detector, negative values, or values out of range) will cause the program to prompt you for correct values.

Use with caution!

The CE Expert Lite window displays the following notice:

There are many factors that will cause the values calculated by CE Expert Lite to differ from the values observed experimentally. The difficulty in working with viscosity has already been discussed. Other factors to consider are:

• capillary diameter is not always as advertised. 25 µm capillary that is actually 26 µm in diameter will cause noticeable changes in fluid delivery
• temperature control and pressure delivery systems must be in calibration
• the time required for pressure to rise and fall is assumed to be zero; in practice this time has a finite value, even is a system delivers pressure from a reservoir by opening a valve
• at the small values used in CE calculations rounding error can be significant

CE Expert Lite should be used to estimate fluid deliveries. The answers presented should be taken as approximations, not as absolutes.

• Return to CE Expert Lite

• Go to P/ACE Series Capillary Electrophoresis Systems Home Page
• Go to Capillary Electrophoresis Systems Home Page

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