In chromatography the trend is to stay away from mixed interactions. In ion exchange we wish to have only ionic interactions between the analyte and the stationary phase. In size exclusion we do not want adsorption but only steric interactions. In reverse phase the silanol interactions are very often masked. Controlled mixed mode interactions can be very helpful in chromatographic separations. Multi-mode interactions can be especially useful in separating closely related classes of compounds. Sielc chromatographic columns make the most of these mixed mode interactions in their Primesep family of columns. The Primesep family is a line of silica based columns combining the advantages of hydrophobic and ion exchange properties. The choice of column depends on the pKa of the analyte, the complexity of the sample matrix, and the aromatic/aliphatic nature of the analyte. Using mixed mode columns provides more selectivity. Selectivity is not only determined by the organic modifier in the mobile phase but by the ionic modifier as well; both my be adjusted to achieve the necessary separation when using the Sielc Technologies Primesep columns.