Chromatography is a laboratory technique used to separate mixtures into their individual components. It is particularly useful for analyzing colored compounds, such as pigments in plants or inks. While a mixture might appear as a single color, it often contains several distinct colored compounds, which chromatography helps identify and analyze.
The Chromatography Setup
Chromatography relies on two fundamental components: a stationary phase and a mobile phase.
The stationary phase is a material that remains fixed in place throughout the experiment. In paper chromatography, this is typically a strip of specialized absorbent paper. It acts as a surface where the components of the mixture can interact and temporarily adhere.
The mobile phase is a solvent, which can be a liquid or a gas, that moves through the stationary phase. For pigment separation, the mobile phase is usually a liquid solvent, such as water, alcohol, or a mixture of solvents. It carries the mixture’s components along with it as it travels through the stationary phase, often by capillary action, similar to how water soaks up a paper towel.
Molecular Properties Driving Separation
The separation of pigments in chromatography depends on their distinct molecular properties, primarily polarity and solubility. Polarity describes how electrical charges are distributed within a molecule: polar molecules have regions with slight positive and negative charges, while non-polar molecules have evenly distributed charges.
A common principle in chemistry is “like dissolves like,” meaning polar substances tend to dissolve well in polar solvents, and non-polar substances dissolve well in non-polar solvents. Different pigments possess varying degrees of polarity, which influences how strongly they interact with the stationary and mobile phases.
In many forms of chromatography, the stationary phase, such as paper, is polar due to the presence of water molecules attracted to its cellulose fibers. Polar pigments will have a stronger attraction to this polar stationary phase, causing them to “stick” to it more readily. Conversely, their solubility in the mobile phase also plays a role. Pigments more soluble in the mobile phase will be carried more effectively.
How Pigments Travel Differently
The interplay between a pigment’s affinity for the stationary phase and its solubility in the mobile phase dictates how far and fast it travels. Pigments highly soluble in the mobile phase and with weaker attraction to the stationary phase will spend more time moving with the solvent, traveling further up the stationary phase.
For instance, carotenoids, which are yellow to orange, are more soluble in common chromatography solvents and tend to move quickly. Conversely, pigments with stronger attraction to the stationary phase and less solubility in the mobile phase will spend more time adsorbed or “stuck” to the stationary material, slowing their movement.
Chlorophyll, a green pigment, moves more slowly than other pigments because it is less soluble in the solvent compared to other pigments like carotenoids. This difference in travel speeds results in pigments separating into distinct bands or spots along the stationary phase.