Introduction
In physical evaluation method of herbal drugs, herbal dugs are evaluated on the basis of some important physical properties. It is often used to determine solubility, specific gravity, viscosity, melting point, total ash, heavy metal determination, and determination of foreign matters, refractive index and other physical characteristics.
1. Ash value
Ash value is helpful in determining the quality and purity of crude drugs, especially in powder form. The objective of ashing vegetable drugs is to remove all traces of organic matter, which may otherwise interfere in an analytical determination. On incineration, crude drugs normally leave an ash usually consisting of carbonates, phosphates and silicates of sodium, potassium, calcium and magnesium.
Hence an ash determination furnishes a basis for judging the identity and cleanliness of a drug and gives information relative to its adulteration with inorganic matter. Ash standards have been established for a number of official drugs. Usually these standards get a maximum limit on the total ash or on the acid insoluble ash permitted. There are different types of ash value: they are total ash, acid insoluble ash, water soluble ash, sulphated ash.
- Total ash value: The ash content of a crude drug is the residue remaining after incineration of the crude drug. This represents the amount of inorganic salts adhering to or occurring naturally in the drug. At times, this may be extended to include inorganic matter added for the purpose of adulteration. Ash determination gives basis for the evaluation of the identity and purity of a crude drug. It also gives an idea of the crude drug’s extent of adulteration with inorganic matter.
- Water soluble ash values: The water soluble ash is subjected to greater reduction than in the total ash. It is thus used in the detection of material exhausted by the water and as such is an important indication for the presence of materials substituted for the genuine article.
- Acid-insoluble ash values: Ash, which is insoluble in dilute hydrochloric acid, is referred to as acid-insoluble ash. Acid-insoluble ash value is often preferred to total ash value. This preference is based on the fact that majority of the crude drugs often contain calcium oxalates in large but variable amounts, and as such, total ash values may vary within limits for genuine drug specimens.
- Sulphate ash value: This is a process which entails the conversion of all oxides and carbonates to sulphates at higher temperatures. Sulphated ash produces a more consistent ash value. It is used to identify low grade products.
2. Determination of extractive yields
Various methods have been employed in the identification and evaluation of crude vegetable drugs. It is pertinent to highlight the fact that, in most cases, the amount of drug soluble in a given solvent is an index of its polarity. Thus, on this basis, the determination of ethanol-soluble extractive and water soluble extractive are used as a means of evaluating drugs whose constituents are not readily estimated by other means.
3. Determination of moisture content
The most widely accepted method of moisture content determination of plant and food materials is the gravimetric method, as modified by the Association of Official Analytical Chemists. It is the most important and widely used measurement in samples that absorb and retain water. Moisture content determination looks very simple in concept, but in practice the accurate determination is complicated by a number of factors which may vary considerably from one sample to another. Among the factors are the relative amounts of water available and the ease with which the moisture can be removed. Air or vacuum oven drying at 70oC – 80oC are considered to be reliable methods provided that there is no chemical decomposition of the sample and water is the only volatile constituent removed. Sample should be dried to a constant weigh.
4. Foreign organic matter:
It is not possible to collect a herbal ingredient without small amounts of related parts of plant or other plants. Standards should be set in order to limit the percentage of such unwanted plant contaminants. Herbal drugs should be made from the stated part of the plant and be devoid of other parts of the same plant or other plants. They should be entirely free from molds or insect, including excreta and visible contaminant such as sand and stones, poisonous and harmful foreign matter and chemical residues. Animal matters such as insects and “invisible” microbial contaminants, which can produce toxins, are also among the potential contaminants of herbal medicines.
Macroscopic examination can easily be employed to determine the presence of foreign matter, although, microscopy is indispensable in certain special cases (for example, starch deliberately added to “dilute” the plant material). Furthermore, when foreign matter consists, for example, of a chemical residue, TLC is often needed to detect the contaminants.