The Chemical Analysis of Plants

The chemical analysis of plants has provided a substantial amount of information about the components of plant material. Plant cells contain an extensive range of compounds, each with their own individual properties and methods of extraction for chemical analysis. Several techniques have been developed to examine and exploit the chemical constituents of plants.

Plant Components

• The main components of plants are proteins, carbohydrates and fatty acids. Secondary compounds include tannins, lignins, terpenoids, phenols and alkaloids. These compounds have different chemical structures and functions within the plant and different therapeutic and industrial benefits for humans. Lignins provide strength and structure to plant stems and tree trunks, and tannins can precipitate proteins into insoluble complexes that can be used to tan animal hides. Analysis techniques depend on the type of plant and the compound being sought.

Steam Distillation

• Steam distillation is used to isolate aromatic compounds like essential oils. This is a relatively simple process that uses steam to vaporize the oil contained in the plant material. The vapor then passes through a condenser containing cold water, where it liquefies, and the plant oils, which are lighter and float on top of the water, are separated. The remaining water, which normally contains dissolved plant material, is called floral water or the hydrolat.

Gas Chromatography

• Gas chromatography (GC) is mainly used to analyze fatty acids and cholesterols, but its ideal application is for heat-stable, volatile substances such as plant alcohols. Compounds can be extracted from the plant material and directly injected into the capillary column of the gas chromatograph. Heat stability is necessary as the samples are exposed to high temperatures within the gas chromatograph, which causes them to vaporize. This vapor is transported through the column via an inert gas that will not react with the sample, such as hydrogen or helium.

Detection

• For analysis, a range of detectors can be used, but flame ionization detection (FID) is commonly applied for organic substances. This works by burning the sample to produce an electrical response, which can then be measured. Although the sample cannot be recovered after burning, this remains an efficient method for detection in gas chromatography.

High Performance Liquid Chromatography (HPLC)

• High performance liquid chromatography (HPLC) is a common method for plant analysis that is used to analyze vitamins, amino acids, sugars and certain plant toxins. HPLC is similar to gas chromatography, except the compounds need not be volatile or gaseous. The sample, however, must be in solution form before being injected into the column and be free of particles that may interfere with analysis. Detection involves measuring the ultraviolet absorbance of the separated compounds.