PAPER CHROMATOGRAPHY • The separation technique known as chromatography was first used in the early part of the twentieth century. The word chromatography comes from two Greek words that mean ‘‘colored graph,’’ a good description of the earliest applications of the technique to separate colored plant pigments. • All chromatographic methods involve the use of the same general principle. • A mixture of dissolved solutes that is to be separated is passedover a stationary material. The moving phase in which the solutes are dissolved is called the mobile phase, and the stationary material is called the stationary phase. PAPER CHROMATOGRAPHY • Separation occurs when the dissolved solutes have different affinities for the mobile and stationary phases. A solute that is strongly attracted to the stationary phase spends more time attached to that phase than a solute that is strongly attracted to the mobile phase. • As a result, the solute with a stronger affinity(more polar) for the stationary phase spends less time in the moving mobile phase and does not move along as rapidly as the solute that is attracted more strongly to the mobile phase. • Thus, the solutes move different distances and are separated during the time the mobile phase is moving through the stationary phase. PAPER CHROMATOGRAPHY • The extent of separation of solutes in a chromatogram is expressed by the Rf value of each solute. • The Rf (retention factor) value is calculated by dividing the distance traveled by the mobile phase into the distance traveled by the solute: PAPER CHROMATOGRAPHY • The distances used in Equation 1 must be measured from the same starting point, which is usually the point at which the solute sample was applied to the paper.