Phytochemical Investigation of Herbal Products : Extraction of the plant material ; Separation and Isolation of Constituents ; Characterization of the Isolated Compounds

lecture : ( 10 ) dr. saad ali ihsan

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



1 ) - extraction of plant material :
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 . ( dropingping 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 .

2 ) - separation & isolation of constituents :
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 : -
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 .

distillation : -
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 .

fractional liberation : -
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 .

fractional crystallization :-
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 ) .

adsorption chromatography : -

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 .


3 ) - characterization of isolated compounds : -
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 .



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