SciOly- Forensics
Chromatography refers to any technique used in the lab to separate a mixture of components. The substance being analyzed (the analyte) is dissolved in a solvent called the mobile phase. The stationary phase is a solid material which the mobile phase travels through. The stationary phase is used to separate the mixture but does not move with the components, while the mobile phase is used to separate the mixture and moves with the components. Different parts of the mixture will have more or less affinity for the stationary phase. If a part of the mixture interacts strongly with the stationary phase, it will separate out of the mixture more quickly. If a part of the mixture interacts weakly with the stationary phase, it will travel further from the point of origin. As a result, the mixture is separated based on a particular property.
In normal-phase chromatography, the stationary phase will be polar while the mobile phase will be non-polar. In reverse-phase chromatography, the stationary phase is non-polar while the mobile phase is polar. Paper chromatography is often normal-phase chromatography. The stationary phase is the cellulose in the paper, which is very polar. The mobile phase is usually a non-polar solvent like rubbing alcohol. Non-polar substances will interact strongly with the mobile phase and travel far up the paper, while polar substances will react with the mobile phase and stay close to the point of origin.
Most competitions will ask for Rf (retention/retardation factor) calculations. In some cases this can also be referred to as rate of flow. Rf will always be a value between 0 and 1. A value close to 1 means that the substance has a high affinity for the mobile phase, and a Rf value close to 0 means that the substance has a high affinity for the stationary phase. To calculate Rf, measure the distance that the analyte traveled from the point of origin as well as how far the solvent traveled. Divide the distance the analyte traveled by the distance the solvent traveled to find the Rf. For example, if the analyte traveled 4.3 cm while the solvent traveled 6.1 cm, the Rf for that substance would be 0.70. Always measure distance to the center of the band.
TLC and paper chromatography are both types of planar chromatography, where the mobile phase travels up a flat (planar) stationary phase. Another type of chromatography is column chromatography, where a bed of material is placed in a tube that the stationary phase drips through. This technique is commonly used to separate mixtures of proteins in a lab setting based on properties like size, charge, or polarity.
Other types of chromatography include gas chromatography (where gases like helium or nitrogen are used to move the gaseous mixture through absorbent material and is used to analyze volatile gases) and liquid chromatography (where liquids dissolve ions and molecules and which is used to analyze metal ions or organic compounds in solutions). Gas chromatography is often used in forensic analysis in combination with mass spectroscopy, where it is known as GC-MS. This technique is used to identify unknown substances in labs like debris or drug remnants.
The only chromatography techniques that competitors will be asked to perform are paper chromatography and thin layer chromatography (TLC). Paper chromatography utilizes a filter paper as a stationary phase and a solvent like water or rubbing alcohol as the mobile phase. TLC utilizes a thin plate made of a nonreactive substance like glass covered in an adsorbent material like silica (SiO2) or alumina (Al2O3). The silica serves as the stationary phase, adhering to the material being separated by the solvent. More information about performing these techniques is included in the section below.