Chromatography

Chromatography is a technique that can be used to separate mixtures of chemicals

Mixtures are two or more substances that have been physically combined Compounds are two or more elements that have been chemically combined

Chromatography is a method for analyzing complex mixtures (such as ink) by separating them into the chemicals from which they are made Chromatography is used to separate and identify all sorts of substances in police work. Drugs, from narcotics to aspirin, can be identified in urine and blood samples, often with the aid of chromatography

Two Phases There are two phases in paper chromatography Stationary phase Mobile phase

Stationary Phase The chromatography paper is the stationary phase The paper acts like a wick, drawing the solvent upward by capillary action and dissolving the mixture as it passes over it

Mobile Phase The solvent is the mobile phase The mixture (pen, lipstick) dissolves in the solvent

Different components of the mixture interact differently with the two phases Some components will be more strongly attracted to the stationary paper and absorb to the filter paper

Other components will be more attracted to the mobile phase and will migrate with the solvent As the mobile phase passes over the stationary phase, the mixture’s components that are more strongly adsorbed to the stationary phase will travel slower than those soluble in the mobile phase

Components of the sample will separate on the stationary phase according to how strongly they absorb to the stationary phase versus how much they dissolve in the mobile phase

Because molecules in ink and other mixtures have different characteristics (such as size and solubility), they travel at different speeds when pulled along a piece of paper by a solvent

Many common inks are water soluble and spread apart into the component dyes using water as a solvent If the ink you are testing does not spread out using water, it may be “permanent” ink - you will have to use a different solvent such as rubbing alcohol

The final result is that different pigments in the mixture show up as colored streaks or bands on the paper The pattern formed is called a chromatogram

Quantify Calculate the Rf (relative rate of flow) of each pigment Rf value represents the ratio of the distance a pigment traveled relative to the distance the solvent traveled

Process A drop of mixture is placed in one corner of a rectangular piece of absorbent paper. One edge of the paper is immersed in a solvent. The solvent migrates up the sheet by capillary attraction. As it does so, the substances in the drop are carried along at different rates. Each compound migrates at a rate that reflects the size of its molecule and its solubility in the solvent.

How can chromatography be used to solve crimes? Lipsticks and inks are mixtures of several different colored chemicals Companies that produce these products use their own unique mixtures of colors If the Rf of the unknown compound is close to or the same as the Rf of the known compound then the two compounds are most likely similar or identical