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DNA Solid Phase Extraction Techniques

Solid phase extraction (SPE) is an important component of molecular biology. It allows the user to separate DNA and RNA, which are the genetic materials of life, from other components in a sample. This is particularly important because the practical application of genetics means that DNA and RNA do not occur in nature in a pure form, and SPE allows the user to purify it in order to study it. There are several different types of SPE.

The Terms
- Molecular biology has a unique vocabulary that is important to understanding how SPE works. Solid Phase Extraction is a general technique in which genetic material is dissolved in a liquid, called the "mobile phase," and passed through a filter, the "stationary phase." The filter is specifically designed to attract whatever type of molecule is of interest, keeping in mind that in chemistry, opposite charges attract each other. The user is left with a liquid that has passed through the filter and the filter itself, which has captured a certain particulate. It is important that the observer know whether the desired compound is going to be caught by the filter or excluded by it, so that he knows where to find the desired compound. The scientist must take in account pH, polarity and molecule size when choosing what kind of stationary and mobile phases to use to capture the molecule of interest.
Normal Phase SPE
- The most basic form of SPE, Normal Phase SPE works exactly like a filter in which the desired molecule is caught on the filter. The stationary phase is designed to attract polar particulates, so a nonpolar solution is used to rinse the stationary phase of impurities. When the observer is ready for the desired molecule to be washed off, a polar solution is used.
Reverse Phase SPE
- Reverse Phase SPE separates molecules by their polarity. This method is used for nonpolar or weak polarity molecules, since the stationary phase is hydrophobic. Reverse Phase SPE is the least selective of all the methods as it retains anything that is hydrophobic.
Anion Exchange SPE
- Anion Exchange SPE is one of two ion exchange types of SPE that exploit the differences in charges between the filter or "sorbant" and the molecule of interest. In the case of Anion Exchange, the molecule of interest is negatively charged, such as an acid. This molecule is attracted to the negative charge of the stationary phase and is caught there.
Cation Exchange SPE
- Cation Exchange SPE works exactly opposite from Anion Exchange SPE, in that the filter is negatively charged in order to catch the molecule of interest that is positively charged. Immunoglobins, for example, have a positive charge, and are often captured with Cation Exhange SPE.

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