Establishment of identity of powdered drugs depends on the microscopic recognition of characteristic cell types and cell contents. Microscopic analysis is indispensable for the identification of the correct species and/or the right plant part that is to be present. For example, pollen morphology may be used in the case of flowers to identify the species and presence of leaf stomata to identify the plant part used. Stinging nettle (Urtica urens) is a classic example where the aerial parts are used to treat rheumatism, while the roots are applied for benign prostate hyperplasia. The basic aim of microscopic examination is to determine the size, shape and relative positions of different cells and tissues, chemical nature of cell walls and form and chemical nature of cell contents.

Powdered drugs or adulterants which contain a constant number, area or length of characteristic particle/mg such as starch grains, epidermis, trichome ribs, fibres etc. can even be determined quantitatively using lycopodium spores as indicator diluent. Other microscopic determinations are percent foreign organic matter in powdered drugs, calculation of vein-islet number, palisade ratio, stomatal numbers and stomatal indices for leafy drugs. Average fibre length (Ceylon and cassia cinnamon), length of epidermal cells (Indian and European squill) number of characteristic sclereids/mg (clove stalks and coconut shells) may be estimated for comparison with known standard for genuine drugs to identify the adulterant or the variety present in the powdered drug.

Prior to microscopic examination the sample has to be treated with chemical reagents. Dried material requires softening before preparation for microscopy preferably by being placed in a moist atmosphere (for leaves) or by soaking in water. Bark, wood and other dense and hard materials need to be soaked in water or equal parts of water, ethanol and glycerol overnight. Botanical sections of the plant material may need to be made and sections of the drug material are necessary for the examination of mucilage or water-soluble cell components. Disintegration serves isolation of specific tissues and bleaching and defatting techniques for observing deeply coloured materials and fatty seeds respectively. Clearing agents and suitable stains are required to highlight cell walls and cell contents. Powdered material is to be mounted in a few drops of water, glycerol/ethanol TS or chloral hydrate before microscopic examination.

Examination of microscopic morphology of both powdered and unground drugs is valuable to establish the identity of many adulterants. For instance, varieties of senna may be identified by vein-islet numbers and palisade ratios, adulterants of belladonna herb by stomatal index and stomatal number or by trichomes.

Knowledge of the microscopic features of genuine materials and of frequently encountered adulterants is essential for their detection in powdered drugs. For example, ginger is characterized by non-lignified vessels, varieties of aloes by the presence or absence of aloin crystals and cinchona bark by the absence of sclereids.