Chromatography and Instrumentation (Leaving Cert Chemistry): Revision Notes
Chromatography and Instrumentation
What is Chromatography?
Chromatography is a powerful separation technique used to analyse complex mixtures by isolating their individual components.
It works by moving a mobile phase (liquid or gas) containing the mixture across a stationary phase (solid or liquid on a solid support). Different components move at different rates depending on their interactions with the two phases, leading to their separation.
Principle of Chromatography
Chromatography separates substances based on their differing affinities for the mobile and stationary phases:
- Mobile phase: The solvent or gas that carries the mixture.
- Stationary phase: The material (such as silica gel or paper) that retains some components more strongly than others. The separation occurs because substances that interact strongly with the stationary phase move more slowly, while those that interact weakly with the stationary phase move faster with the mobile phase.
Thin-Layer Chromatography (TLC)
Thin-layer chromatography (TLC) is a simple, inexpensive method for separating small amounts of substances.
It is commonly used in forensic science and other fields for rapid separation and identification.
- Principle: A thin layer of silica or alumina on a plate serves as the stationary phase. The sample is spotted onto the plate, and a solvent (mobile phase) carries the components up the plate, separating them.
Example: TLC is used to separate dyes taken from fibres in forensic work, such as in cases where fibre evidence needs to be matched to a suspect's clothing.
Advanced Chromatographic Techniques
Gas Chromatography (GC)
Gas chromatography (GC) is a technique used to separate volatile compounds. It is particularly useful for environmental analysis and testing in the food and pharmaceutical industries.
- Principle: A gaseous mobile phase (usually helium or nitrogen) carries the sample through a column containing a liquid stationary phase. The components separate based on their boiling points and interactions with the stationary phase.
Example of Uses:
- Gases from waste dumps: GC can analyse the composition of gases, such as methane, released from landfills or waste treatment plants.
- Trace organic pollutants in water: GC detects low concentrations of harmful organic compounds in environmental water samples.
High-Performance Liquid Chromatography (HPLC)
High-performance liquid chromatography (HPLC) is used to separate and quantify non-volatile compounds in liquid samples. It is highly precise and widely used in food testing, pharmaceuticals, and agriculture.
- Principle: The mobile phase is a liquid under high pressure, and the stationary phase is a packed column. Separation is based on differences in solubility and interactions with the stationary phase.
Example of Uses:
- Growth-promoters in meat: HPLC can detect illegal additives or hormones used to promote growth in livestock.
- Vitamins in foods: HPLC is used to ensure that food products contain the correct amount of vitamins as part of quality control.
Instrumental Methods of Separation and Analysis
Chromatography is often coupled with other analytical techniques to provide more detailed information about the separated compounds.
Mass Spectrometry (MS)
Mass spectrometry (MS) is a technique used to identify compounds by measuring the mass-to-charge ratio of ionised molecules. It is often combined with chromatography (GC-MS or HPLC-MS) for high-precision analysis.
- Principle: After separation by chromatography, compounds are ionised and separated based on their mass-to-charge ratio, producing a unique spectrum for each substance.
Example of Uses:
- Drug tests on athletes: MS can detect trace amounts of performance-enhancing drugs in athletes.
- Blood alcohol tests: MS is used to accurately measure blood alcohol levels in legal and medical testing.
Infra-red Absorption Spectrometry (IR)
Infra-red (IR) spectrometry identifies organic compounds based on their absorption of infrared light, which causes molecular vibrations. Each type of bond absorbs at a specific wavelength, providing a "fingerprint" for the compound.
- Principle: Organic molecules absorb specific wavelengths of infrared radiation, resulting in a characteristic absorption spectrum.
Example of Uses:
- Identification of plastics and drugs: IR spectrometry is commonly used to identify materials in manufacturing or pharmaceutical production.
Ultraviolet Absorption Spectrometry (UV)
Ultraviolet (UV) spectrometry is a quantitative technique used to measure the concentration of compounds that absorb UV light, often in the analysis of plant pigments or drug metabolites.
- Principle: Compounds absorb ultraviolet light at specific wavelengths, and the amount of absorption is directly proportional to the concentration of the compound.
Example of Uses:
- Quantitative determination of drug metabolites: UV spectrometry is used in drug testing and research to determine the concentration of metabolites in biological samples.
- Plant pigments: UV spectrometry can quantify pigments like chlorophyll or carotenoids in plants.
Summary of Chromatography Applications
- Environmental Analysis: GC is used to analyse gases from waste dumps and trace organic pollutants in water.
- Forensic Science: TLC is employed to separate and identify dyes from fibres in crime scene investigations.
- Food and Agriculture: HPLC is used to detect vitamins in foods and illegal growth promoters in meat.
- Pharmaceuticals and Drugs: MS, IR, and UV spectrometry are used in the analysis of drugs, drug metabolites, and the identification of plastics.