soelectric focusing (IEF) is an electrophoretic separation based on the isoelectric points of proteins. The pI is the point at which the protein has an overall net charge of zero. Differences of only a few hundredths of a pH-unit in isoelectric points are sufficient to resolve proteins from each other. IEF is used as an alternative electrophoresis format complementing the widely used SDS-PAGE electrophoresis, which is based on size of proteins. The combination of both principles is applied in D-electrophoresis as outlined below (see page 2). This technique has become crucial for the development of Proteomics. In many applications, closely related proteins have to be separated. Examples are the differentiation of protein isoforms or enantiomeres. Such problems have been successfully solved by isoelectric focusing (IEF) [1] In contrast to other electrophoretic techniques,pH is not kept constant throughout the whole system. Instead, the sample components migrate electrophoretically in a stationary pH-gradient. Proteins will migrate until they reach the pH-point in the gradient at which the charge of the protein equals zero (pH = pI) The protein is said to focus at this point. This focusing also results in a concentration of individual proteins (Figure 1)