As evident from the discussion on the occurrence and type of adulteration in crude drugs, qualities of drugs offered for sale in the market cannot always be guaranteed. Thus, it is necessary to evaluate all commercial varieties of crude drugs before or after being purchased. I any case, all samples of drugs must be evaluated before use or consumption.
Evaluation is an elaborate process of establishing the correct identity of a drug and of determining its quality ad purity. Evaluation of a drug can be done on the spot by using some Organoleptic methods or in the laboratory by the use of microscopical, chemical, biological and physical methods. The Organoloptic evaluation of drugs involves the use of the organs of sense and depends on the macroscopic appearance and sensory characters of the drug, such as its gross morphology, shape and size, colour and external markings and odour and taste.
The Microscopical evaluation of drugs is done the aid of microscopes and utilizes various microscopic characters of the drugs, such as trichomes, calcium oxalate crystals, starch grains, pollen grains, etc. and their histological features, such as types and arrangements of various cells and tissues. This method of evaluation is indispensable in the evaluation of powdered drugs,
as they possess very few macroscopicl characters other than colour, odour and taste. Microscope is also essential for determining some important physical constants like stomata! number, stomata} index, palisade ratio, vein-islet number, vein termination number, etc. of leaf drugs. This type of microscopical determinations is otherwise known as Quantitative microscopy.
The Chemical evaluation of drugs involves both qualitative and quantitative determinations of their active principles. In this method characteristic qualitative chemical tests are employed to identify crude drugs and their constituents. Quantitative chemical assays are used to determine their quality and purity. This method of evaluation is now widely used in the examination of crude drugs for its accuracy and reliability. Various specific chemical tests have been established for identification and quality determination of different drugs.
The Biological evaluation of crude drugs is very useful in determining the potency of drug sample. In this type of evaluation the extent of pharmacological activity of the drug or its constituents is taken as the basis of quality. Since living organisms or their isolated living tissues are used, this method is also called the biological method or bioassay. Many drugs, particularly the antibiotics, toxins and toxoids and also vitamins are assayed by this method.
The Physical evaluation of crude drugs is accomplished by the determination of various physical constants using various physico-chemical techniques. The common physical constants used to evaluate crude drugs and their extracted chemical principles include specific gravity (particularly of the fats and volatile oils and some crude drugs as Nutgalls), optical rotation (of some alkaloids in solution and of volatile oils), refractive index (particularly of the volatile and fixed oils), melting points (of isolated alkaloids and their derivatives), ash values (of most crude drugs), extractive values (of most crude drugs) and stomata! number, stomata] index, palisade ratio, vein-islet number and vein termination number (of entire and powdered leaf drugs).
All the above methods and types of evaluation are appropriately used to achieve the various objectives of evaluation of crude drugs, that is, to establish their identity (correct source), purity (absence of adulterants) and quality (presence of required amount of active constituents).