Most of the underlying principles of phytochemical operations are traceable to the early nineteenth-century methods of plant drug isolations. Comprehensive knowledge of the physicochemical nature of targeted constituent is an essential prerequisite before its isolation may be attempted. Extraction refers to the physical separation of soluble active metabolite from the insoluble, inactive/inert plant cellular matrix. Depending on the nature of the active constituent, an appropriate method of extraction may be adopted from the innumerable extraction techniques that are available today.

Following is a brief listing of some basic principles of phytochemical isolations:

• Authenticated plant material of established botanical identity free from adulteration and other contaminants shall be the source material.
• Fresh plant parts are used for the direct isolation of active constituents as in the case of essential oil components.
• In general, however, the plant parts are appropriately dried and size reduced before further processing.
• To avoid unwanted chemical changes and enable long-term storage, an extract representative of the chemical constituents of the plant material is prepared.
• Maceration, percolation, hot continuous percolation, digestion, decoction, aqueous-alcoholic extraction by fermentation, and enfleurage are some of the conventional methods of extraction.
• Super critical fluid extraction, microwave-assisted extraction, ultrasonication-assisted extraction, accelerated pressure–assisted solvent extraction, pressurized counter–current liquid extraction, thermal desorption, and phytonic desorption with hydrocarbon solvents are some of the newer methods of extraction.
• Hydrodistillation, sublimation, solvent extraction, enfleurage, and expression are some of the methods applicable to volatile constituent isolations.
• Head space trapping, solid-phase micro-extraction, protoplast extraction, microdistillation, thermodistillation, and molecular distillation are the newer extraction techniques applicable to essential oil components.
• Selection of solvent and method of extraction are dependent on the nature of the plant material, purpose of extraction, and the properties of constituents targeted for isolation.
• When the classes of active constituents are unknown, a preliminary phytochemical screening using qualitative chemical tests will reveal the classes of compounds of relatively high yield.
• The prepared extract may be further fractionated with solvents of varying polarity to segregate the phytoconstituents.
• Further processing of the prepared extract or its fractions is the general approach to the isolation of individual active constituents.
• All extraction and isolation processes need careful monitoring at every stage since processing conditions are likely to bring about chemical degradation due to enzymatic or nonenzymatic hydrolysis, loss of volatile matter, molecular rearrangements, racemization of optically active compounds, proteolytic degradation, artifact formation, etc. These changes may lead to loss of activity.
• Chromatographic separation on a column is a conventional and still the only widely used method for the isolation of constituent compounds in large-scale extractions.
• GC and GLC are ideally suited for the detection and analysis of volatile compounds and volatile derivatives of nonvolatile compounds.
• Volatile mixtures of compounds and single constituents are isolated using simple or fractional distillation.
• Preparative separations of mixture of compounds from fractionated extracts may be undertaken by preparative TLC using adsorbents laid thicker on the supporting medium.
• Paper chromatography and thin-layer chromatography are indispensable tools guiding chromatographic processing as they enable preliminary identification of phytoconstituents in plant extracts by means of co-chromatography with authentic sample of the compound.
• High-speed counter-current chromatography and droplet counter-current chromatography are some of the advanced liquid–liquid chromatographic processing techniques used for the detection and isolation of a range of phytoconstituents.
• High-performance liquid chromatography is one of the most preferred automated separation techniques that gives much rapid and improved separations over conventional column chromatography. From a range of stationary phase columns available, it is possible to effectively use it for the efficient separation and analysis of a number of phytoconstituents of different polarities.
• The versatility of HPLC for the isolation of many classes of phytoconstituents (however, in smaller quantities relative to column chromatography) using preparative columns has made it a convenient separation technique in phytochemistry.
• Characterization of newly isolated constituents is now undertaken with relative ease on milligram quantities of samples using a combination of UV, IR, Mass, NMR, X-ray crystallography and optical dispersion methods.
• Quantitative determination of phytoconstituents is a significant aspect of phytochemical analysis. Newer analytical techniques available today enable accurate estimation of secondary metabolites in multicomponent plant extracts, galenicals, body fluids, and formulations.