The primary way to identify plant material is through physical examination of the features of the entire plant, especially the flowers. Those features are compared to descriptions in plant taxonomy books and/or to specimens whose identity has already been established by a botanist.

When a specimen is identified by a botanist, it is said to be authenticated. Sensory information, referred to as organoleptic features (color, texture, smell, taste), can also yield information on the identity of whole, chopped, or milled plant material.

Further information on identity of milled or powdered plant material is provided by microscopic examination that allows for viewing of tissue structures, organization, cell types, and cell contents. Chemical constituents are also important in identification.

Plants contain thousands of chemical components, including basic proteins and sugars necessary for metabolism and structure. They also contain secondary compounds that were originally thought not to be essential to the life of the plant, but have important medicinal qualities for mammals.

The best known of these secondary components are the alkaloids, a group which includes nicotine, caffeine, morphine, and cocaine. Other classes of secondary compounds include phenolics, terpenoids, and steroids (Trease and Evans, 1978).

Chemical analysis can be useful in identification even when physical examination of plant structures is not possible, such as with fine powders and extracts. For any material studied scientifically, or sold in retail, a sample should be retained for a period of time.

This sample could be examined in the future, should any questions arise regarding identity or quality. In the case of fresh plant material, this can be a voucher specimen that is pressed and dried.

If the test material is milled or powdered, or even in final product form, a retained sample, in that form, may still be used to answer any possible inquiries that might arise regarding identity or quality.

Voucher Specimens

Voucher specimens include the whole plant, or representative parts of the plant (ideally including flowers and seeds), pressed, dried, and fastened to an 11.5" by 16.5" card. Included along with the plant material is information as to the location and environment from which the plant was collected.

Prepared in this way, the taxonomic identity of the specimen can be determined by a botanist. The specimen also serves as a lasting record. Millions of plant specimens have been collected and stored in herbaria, to serve as reference collections of the world’s flora.

Most universities and many private organizations that work with plants house herbaria. The largest herbarium in the United States is the National Herbarium in Washington, DC, which houses 4.5 million specimens and contains collections from the exploration of North America in the 1800s (www.nmnh.si.edu).

The world’s largest herbarium is the Royal Botanical Gardens in Kew, England, with over seven million specimens from all over the world, including 250,000 type specimens (specimens that define taxonomic species) (www.rbgkew.org.uk).

Voucher specimens are the most appropriate form of identification for primary suppliers of botanicals, i.e., farms, collectors, and those who have access to the entire plant.

Organoleptic Identification

Sensory, or organoleptic, information is very useful to those experienced with plant materials in establishing identity. Sight, touch, smell, taste, and sound can also assist in assessment of the quality of the material. Organoleptic features are included in the following characterization of cinnamon bark (Cinnamomum verum J. Presl.):

The matt pieces of bark, 0.2 to 0.7 mm thick, in the form of single or double compounds quills, light brown on the outside and somewhat darker on the inside; the surface longitudinally striated and the fracture is short and splintery. The odor is characteristic and pleasantly aromatic. The taste is pungently spicy, somewhat sweet and mucilaginous and only slightly sharp. (Wichtl, 1994, 148)

Microscopic Identification

Characteristic tissue structures, tissue organization, cell types, and cell contents can be viewed under magnification. For example, the glandular (rounded, multicellular) hairs common to mint leaves are quite distinct from the stellate (star-shaped) hairs of witch hazel leaves when viewed under a microscope.

The addition of chemical reagents to the microscope slide can verify the presence and location, or absence, of starch grains, calcium carbonate, and/or oxalate crystals, as well as lignin (a plant cell wall component). Starch grains will appear purple following the addition of iodine, and their size and pattern are indicative of certain plants.

Lignin will appear red with the addition of acidic phloroglucinol solution and is present in the cell walls of woody plants. The appearance of uncharacteristic components is an indication of adulteration or substitution.

Chemical Identification

Simple chemical tests can be performed on milled or powdered plant material by adding a few drops of a particular chemical reagent to the plant material. These tests usually indicate the presence or absence of a characteristic class of chemical constituent, for example, steroids or alkaloids.

The presence of alkaloids, for example, can be determined by a purple (reddish-brown) color reaction following addition of Dragendorff’s reagent (a solution of potassium bismuth iodide).

Extracts of plant material can be analyzed in more detail for characteristic compounds using spectroscopic analysis and chromatographic techniques. Spectroscopic analyses employ light absorption techniques to analyze classes of compounds.

They include ultraviolet (UV), infrared (IR), and Fourier-transfom infrared (FTIR) spectroscopy. Chromatographic techniques allow for the isolation and quantification of individual compounds.

Components of the mixture are separated through chemical affinity to either the mobile phase (liquid or gas) or the stationary phase (solid substance such as silica over which the mobile phase runs).

These techniques include thin layer chromatography (TLC), high performance thin layer chromatography (HPTLC), gas chromatography (GC), capillary electrophoresis (CE), and high performance liquid chromatography (HPLC).

Plant extracts examined using either spectroscopic or chromatographic techniques will display a characteristic profile, or “fingerprint,” which is useful in identification. Even without chemical identification of all the individual components, a particular pattern accompanied with the identification of a few components can be an assurance of identity.

Chemical analysis is often offered as proof of identity of plant material, but it is not necessarily enough by itself. To prove this point, Dr. Alvin B. Segelman (1995) demonstrated that belladonna alkaloids added to sterilized cow dung could pass the U.S. Pharmacopeia chemical identification test.

However, when the material was examined microscopically itwas clear that itwas neither belladonna leaves nor roots. Conversely, if plant material has been depleted chemically, through extraction, it may pass microscopic examination due to the remaining cell structure, but not chemical analysis.

Therefore, both physical characterization (microscopic examination) and chemical analyses are needed for optimal identification. When the results of chemical analysis are given as descriptors of a product, some indication of the test method must be provided.

For example, extracts of St. John’s wort (Hypericum perforatum L.) are often described as standardized to 0.3 percent hypericins. Hypericin is one of a group of biologically active dianthrones (phenolic compounds) in the herb that includes hypericin, pseudohypericin, protohypericin, and protopseudohypericin. The dianthrones in the extract can be measured using either UV spectroscopy or HPLC.

UV spectroscopy will provide information as to absorption of light by compounds at a specific wavelength. Thus UV spectroscopy will indicate the total quantity of the dianthrones in the extract (as well as other compounds that absorb light at the tested frequency), and the results can be described as total hypericins.

In contrast, HPLC analysis allows for the separation and quantification of the individual compounds. HPLC allows the quantities of hypericin, pseudohypericin, and other dianthrones to be determined individually. Therefore, the quantity of total hypericins as determined by UV will be different from the amount of the individual hypericin as quantified by HPLC.

As another example, the U.S. Pharmacopeia (2004) method of measuring the alkaloids in belladonna extract via HPLC will give a slightly different number from the European Pharmacopoeial (Ph Eur) method which measures total alkaloid content via titration (Ph Eur, 2002). Thus in describing the amount of any constituent in a botanical, the method of analysis must be indicated.