Centrifugal filter units are used to purify and concentrate DNA. When extracting DNA from small or degraded forensic samples, the final concentration of DNA may be too low for subsequent amplification. Although 1ng is a target quantity of DNA for amplification, if 1ng is suspended in 100µl of fluid, it would be impossible to transfer 100µl of this solution into an amplification reaction optimized for 50µl or less. Centrifugal filter units increase the concentration of DNA in solution by retaining the DNA while eliminating a portion of the fluid from the sample. Another benefit of the unit is the ability to secure DNA while contaminants (possibly PCR inhibitors) are washed from the sample.

Centrifugal filter units separate molecules by size through a series of washing and centrifugation steps. The Millipore Corporation produces two centrifugal filter units under the names Centricon^® and Microcon^®.

Attributes of Centricon^® and Microcon^® filter units:

• Both employ Amicon^® filters to retain the DNA.
• Filter porosity varies.
• Proteins flow through rather than sticking to the surface during centrifugation.
• Filters are anisotrophic, with increasingly smaller interstitial spaces in the direction of filtration allowing for better retention of smaller molecules.
• Filters are composed of regenerated cellulose, which can be sterilized.
• Filters exhibit sufficient strength when wet.

Textile dye molecules, such as indigo, are known PCR inhibitors and are readily washed through the filter. Salts introduced by buffers are removed from the sample during the process. Salts carried over from the process may interfere with capillary electrophoresis.

Read more about capillary electrophoresis in course: Amplified DNA Product Separation.

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