How can you analyze food for contaminants using chromatography?

1 Principles of chromatography

The key concept of chromatography is that different substances have different affinities for the stationary phase, which is a solid or a liquid that does not move, and the mobile phase, which is a liquid or a gas that flows through the stationary phase. The mixture to be analyzed is injected or applied onto the stationary phase, and the mobile phase carries it along. As the mixture moves, the components separate according to their relative retention times, which depend on how strongly they bind to the stationary phase or how easily they dissolve in the mobile phase. The separated components can then be detected, identified, and quantified by various methods.

2 Types of chromatography

There are many types of chromatography, but the most common ones for food analysis are gas chromatography (GC), liquid chromatography (LC), and thin-layer chromatography (TLC). GC uses a gas, usually helium or nitrogen, as the mobile phase and a column coated with a thin layer of liquid or solid as the stationary phase. GC is suitable for analyzing volatile and semi-volatile compounds, such as pesticides, flavor compounds, or fatty acids. LC uses a liquid, usually water or organic solvents, as the mobile phase and a column packed with small particles of silica or other materials as the stationary phase. LC is suitable for analyzing polar and non-polar compounds, such as vitamins, sugars, or proteins. TLC uses a thin layer of silica or other material coated on a glass or plastic plate as the stationary phase and a solvent as the mobile phase. TLC is suitable for analyzing simple mixtures, such as food dyes, or for screening samples before using other chromatographic methods.

3 Sample preparation and analysis

The first step in chromatographic analysis is to prepare the sample, which may involve extraction, filtration, concentration, or derivatization. Extraction is the process of transferring the analytes from the food matrix to a suitable solvent. Filtration is the process of removing solid particles or impurities from the sample. Concentration is the process of reducing the volume of the sample to increase the analyte concentration. Derivatization is the process of modifying the analytes to improve their chromatographic properties, such as volatility, polarity, or detectability. The sample preparation method depends on the type of food, the type of analyte, and the type of chromatography. The second step is to inject or apply the sample onto the chromatographic system and run the analysis. The system consists of an injector or applicator, a column or plate, a detector, and a recorder or computer. The injector or applicator introduces the sample onto the stationary phase. The column or plate separates the sample components as they travel with the mobile phase. The detector measures the signal from the components as they elute from the column or plate. The recorder or computer records and displays the chromatogram, which is a plot of the detector signal versus the retention time or distance. The chromatogram shows the peaks corresponding to the separated components, which can be identified by comparing their retention times or distances with those of known standards. The peak areas or heights can be used to quantify the components by using calibration curves or internal standards.