Phytochemical investigation of herbal products

phytochemical investigation of herbal products : extraction of the plant material separation and isolation of constituents characterization of the isolated compounds

the choice of extraction procedure depends on the nature of the plant material and the components to be isolated .

dried materials are usually powdered before extraction , whereas fresh plants ( leaves, flowers ….etc. ) can be homogenized or macerated with a solvent such as alcohol . ( dropingingping fresh leaves into the boiling solvent ). alcohol is a general solvent for many plant constituents ( most fixed oils excepted ) and as much may give problems in the subsequent elimination of pigments , resins , etc.

water immiscible solvents are widely used : –                                                                                    light petroleum - for   ( essential & fixed oils , steroids ) .

ether & chloroform – for ( alkaloids , quinines ).

in case of organic bases e.g. ( alkaloids ) usually necessitates basification of the plant material if a water – immiscible solvent is to be used .

aromatic acids & phenols acidification may be required .

extraction itself may be performed by repeated maceration with agitation, percolation or by continuous extraction e.g. ( in a soxhlet extraction ) .

special methods for volatile oils , such as the ( enfleurage ) process , are considered in chapter ( 22 ) .

ultrasound may enhance the extraction process for some plant materials and the bp used this in the preparation of a 50 % ethanolic solution of opium for the assay of alkaloids & in the assay procedure of agnus cactus . its use has been studied for the extraction of ( atropine ) from          ( hyoscyamus muticus ) using various solvent systems .

the most difficult operation in phytochemical research becomes that of the isolation & purification of plant constituents .although the chemical properties of functional groups and moieties contained in compounds  , such as , acids , aldehydes , phenols & alkaloids can be exploited for their separation from other materials , such methods might not fractionate components of the same class , it is in this later area that new techniques are constantly being developed .

sublimation   may sometimes be possible on the whole drug as in the isolation of ( caffeine ) from tea or for the purification of materials present in a crude extract . modern equipment employs low pressures with a strict control of temperature .

fractional distillation has been widely used for the isolation of the components of volatile oils . gas chromatography is now used to separate minor components of a mixture in a pure state .

steam distillation is much used to isolate volatile oils and hydrocyanic acid from plant material .

some groups of compounds lend themselves to fractional liberation from a mixture , ex : a mixture of alkaloid salts in aqueous solution when treated with aliquots of alkali , will give first the weakest base in the free state followed by base liberation in ascending order of basicity . if the mixture is shaken with an organic solvent after each addition , then a fractionated series of bases will be obtained . a similar scheme can be used for organic acids soluble in water – immiscible solvents in this case , starting with a mixture of the acid salts , it is possible to fractionally liberate the acids by addition of mineral acids .

a method much used in traditional isolations and still valuable for the resolution of often otherwise intractable mixtures . the method exploits the differences in solubility of the components of a mixture in a particular solvent . frequently derivatives of the particular components are employed ( picrates of alkaloids , osazones of sugars ) .

1- partition chromatography

2-  partition chromatography on paper

3- hplc

4- tlc

5- counter – current  chromatography

6- glc

7- gel – filtration chromatography

8- electro chromatography

9- affinity chromatography

all above kinds of chromatography cited in the chromatography lecture .

chemists are coming to rely more & more on the use of physical techniques to establish structures of new compounds & to identify known compounds in plant sources . ultraviolet ( u.v. ) , infrared , mass & nuclear magnetic resonance ( nmr ) spectroscopy together with x-    crystallographic & optical rotary dispersion methods have all played a significant role in these developments , various modifications of mass spectrometry ( ms ) have become of increasing importance for the structural characterization & determination of the active constituents of plants .

          dr. saad a. ihsan

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