Ink is a homogenous mixture that is made up of dyes of several different colors. Each brand of ink consists of a different ratio and type of dyes. Paper chromatography can be used to separate the dyes that compose an ink. During paper chromatography, a dot of the dye is placed at the base of a piece of chromatography paper. The end of the paper that contains the dot is then dipped into water. The dyes that comprise an ink separate based on their polarities.  The ratio to which a dye in the ink moves in respect to the distance that the front of the solvent moves up the chromatography paper is called the Rf vale. The Rf value stands for “ratio-to-front”. By separating the colors of an ink that was used in a crime scene, and comparing the separated pattern to the inks of several other pens, police and other officials can determine what pen was used on a crime scene.