Chapter 16 Quality control

STANDARDS APPLICABLE TO CRUDE DRUGS 121

STANDARDS APPLICABLE TO VOLATILE AND FIXED OILS 127

The quality control of crude plant drugs is of paramount importance. In the past, the monographs of national pharmacopoeias adequately covered this aspect for drugs used in the allopathic system of medicine and the British Herbal Pharmacopoeias (1983, 1990, 1996) contained descriptions, tests and quantitative standards for those species commonly employed by medical herbalists. However, there was no control on the plant materials used in the many herbal products manufactured for general retail sale. Under current EU regulations, herbal products can only be manufactured under licence in conformity with the ‘Rules and Guidance for Pharmaceutical Manufacturers and Distributors 2007’, as set out by the Medicines and Healthcare products Regulatory Agency (MHRA) and published by the Pharmaceutical Press, London. Also, the BP/EP 2007 includes a monograph ‘Herbal Drugs’, which gives requirements relating to definition, production, identification, various tests, pesticide residues, heavy metal content, microbiological quality and, where necessary, limits for aflatoxins and radioactive contamination. Quality control personnel are required to have particular expertise in herbal medicinal products in relation to the above.

One possible problem in devising standards for crude drugs concerns the requirement for an assay of the active constituents when the latter may not have been precisely ascertained. Furthermore, one of the tenets of herbal medicine is that the maximum effectiveness of the drug derives from the whole drug or its crude extract rather than from isolated components. In cases where an assay is lacking it is therefore important that the crude drug is properly authenticated, its general quality verified and all formulations of it prepared in accordance with good manufacturing practice. Attention should also be paid to the shelf-life of the crude drug and its preparations.

Although official standards are necessary to control the quality of drugs their use does raise certain problems. Of necessity, to accommodate the considerable variation that occurs between different batches of a natural product it is necessary to set reasonable standards which allow the use of commercial material available in any season. This has resulted in a tendency for producers or manufacturers to reduce all of their material to the lowest requirement; for example, in a good year the majority of the alkaloid-rich leaves of belladonna herb may be removed and used for the economical manufacture of galenicals and the residue of the herb, containing much stem, used to give the powdered drug. Similarly, high-quality volatile oils may be mixed with lower grades and still remain within official limits.

STANDARDS APPLICABLE TO CRUDE DRUGS

There are a number of standards, numerical in nature, which can be applied to the evaluation of crude drugs either in the whole or the powdered condition.

Sampling

Before a consignment of a drug can be evaluated, a sample must be drawn for analysis; considerable care must be exercised to ensure that this sample is truly representative. With large quantities of bulky drugs a different method of sampling is required from that involving broken or powdered drugs. The BP gives no specific instructions for this but the methods of sampling used in the USA are fully described in older editions of the USP. EU guidelines specify that sampling should be carried out by those with particular expertise.

Preliminary examination

In the case of whole drugs the macroscopical and sensory characters are usually sufficient to enable the drug to be identified. The general appearance of the sample will often indicate whether it is likely to comply with such standards as percentage of seed in colocynth, of ash in valerian or of matter insoluble in alcohol in asafoetida. However, drugs may comply with the descriptions given in the pharmacopoeias and yet be unsatisfactory, as it is often difficult specifically to describe deterioration of drugs owing to faulty harvesting, shipment or storage or deterioration due to age. In such cases the trained worker will be able to infer much of the history of the sample from its appearance. The following examples will serve to indicate the type of evidence to look for.

If leaves and similar structures are baled before being properly dried, much discoloured material may be found in the middle of the bale. Overdrying, on the other hand, makes leaves very brittle and causes them to break in transit. If starch-containing drugs break with a horny fracture, it may usually be inferred that the temperature of drying has been too high and that the starch has been gelatinized. A pale colour in the case of chamomiles indicates that the drug has been collected in dry weather and carefully dried, while the colour of the fractured surface of gentian is a good indication as to whether it has been correctly fermented. Some drugs are particularly liable to deterioration if, during shipment or storage, they become damp (e.g. cascara). Under moist condition moulds readily establish themselves on drugs having a high mucilage content (e.g. psyllium, linseed, squill and cydonia). Evidence of insect attack must also be looked for.

The price of certain drugs depends largely on such factors as size and colour, which are not necessarily related to therapeutic value. This applies to such important drugs as senna leaflets, senna pods, chamomile flowers, ginger, nutmegs and rhubarb.

Foreign matter

The difficulty of obtaining vegetable drugs in an entirely pure condition is fully recognized, and pharmacopoeias contain statements as to the percentage of other parts of the plant or of other organic matter which may be permitted. Table 16.1 gives examples of various official types of limit applicable to specific drugs. Drugs containing appreciable quantities of potent foreign matter, animal excreta, insects or mould should, however, be rejected even though the percentage of such substances be insufficient to cause the rejection of the drug on the percentage of foreign matter.

Table 16.1 Examples of BP limits for foreign matter.

Drugs	Foreign matter limits
Leaves and herbs
Bearberry leaf	8% Foreign matter of which 5% stems and 3% other foreign matter. Leaves of different colour to official description 10%
Birch leaf	3% Fragments of female catkins, 3% other foreign matter
Lemon balm	10% Stems having a diameter 1 mm, 2% other foreign matter
Wild thyme	3% Foreign matter (involves recognition of Thymus vulgaris and T. zygis)
Wormwood	5% Stems with diameter greater than 4 mm, 2% other foreign matter
Fruits and seeds
Hawthorn berries	2% Foreign matter, 5% deteriorated false fruits
Juniper berries	5% Unripe or discoloured cone berries, 2% other foreign matter
Psyllium seeds	1% Foreign matter including greenish unripe fruits. No seeds of other Plantago spp.
Inflorescences
Calendula flowers	5% Bracts, 2% other foreign matter
Elder flowers	8% Fragments of coarse pedicels and other foreign matter, 15% discoloured brown flowers
Lime flowers	2% Foreign matter, absence of other Tilia spp.
Rhizomes and roots
Couch grass rhizome	15% Greyish-black pieces of rhizome in cut drug
Marshmallow root	2% Brown deteriorated root, 2% cork in peeled root
Valerian root	5% Stem bases, 2% other foreign matter
Barks
Quillaia bark	2% Foreign matter
Cascara bark	1% Foreign matter

In the case of whole drugs a weighed quantity (100–500 g, according to the type of drug), of a carefully taken sample is spread in a thin layer on paper. It is examined at ×6 magnification and the foreign matter is picked out and weighed and the percentage recorded. Details will be found in the appropriate BP Appendix. For foreign organic matter in powdered drugs, see ‘Quantitative Microscopy’.

Methods based on distillation have been widely used for moisture determination. The sample to be analysed is placed in a flask together with a suitable water-saturated immiscible solvent (toluene, xylene, carbon tetrachloride) and pieces of porous pot and is distilled. The water in the sample has a considerable partial pressure and co-distils with the solvent, condensing in the distillate as an immiscible layer. A simple apparatus (Fig. 16.1A) originally devised by Dean and Stark permits the direct measurement of the water obtained and the less dense solvent (toluene, xylene) is continuously returned to the distillation flask. The method is employed in the USP and in the BP/EP for some volatile oil-containing drugs (Roman chamomile flowers, lovage root, eucalyptus, peppermint and sage leaves) and aniseed and star-anise fruits. To accommodate the loss of water due to solubility in the solvent the BP specifies a preliminary distillation of the solvent with added water (about 2 ml); the exact volume of water separating as a layer is read off and then the drug (sufficient to give a further 2–3 ml water) added to the flask and distillation resumed. Water separated from the drug is calculated from the combined final volume. Heavier-than-water solvents require the receiver shown in Fig. 16.1B. The method is readily applicable to crude drugs and food materials but has the disadvantage that relatively large quantities of the sample (5–10 g) may be required.

Fig. 16.1 A, Apparatus for the determination of moisture in crude drugs by distillation and for volatile oils heavier than water; B, receiver of apparatus for the determination of water in crude drugs (heavy entrainment) and for volatile oils in drugs; C, receiver for determination of volatile oil in drugs as used by the BP 1980; D, receiver for determination of volatile oil in drugs as used by both the EP and the BP.

Gas-chromatographic methods have become increasingly important for moisture determination because of their specificity and efficiency. The water in the weighed, powdered sample can be extracted with dry methanol and an aliquot submitted to chromatography on a column on either 10% carbowax on Fluoropak 80 or Porapak, a commercial polymer suitable for gas–liquid chromatography (GLC). The water separated by this means is readily determined from the resulting chromatogram. Teflon-6 coated with 10% polyethylene glycol 1500, with n-propanol as an internal standard has also been employed for the determination of moisture in crude drugs.

Chemical methods

The most extensively employed chemical method for water determination is probably the Karl Fischer procedure, which finds use not only in the pharmaceutical, but also in the food, chemical and petrochemical industries. It is used in the BP and is particularly applicable for expensive drugs and chemicals containing small quantities of moisture [very small quantities of water (10 μg to 10 mg) are determined quantitatively by coulometric titration, see below]. Dry extracts of alkaloid-containing drugs, alginic acid, alginates and fixed oils (e.g. arachis, castor, olive and sesame oils for BP parenteral use) may usefully be evaluated. For crude drugs such as digitalis and ipecacuanha the powdered material can first be exhausted of water with a suitable anhydrous solvent (dioxan) and an aliquot taken for titration.

The Karl Fischer reagent consists of a solution of iodine, sulphur dioxide and pyridine in dry methanol. This is titrated against a sample containing water, which causes a loss of the dark brown colour. At the end-point when no water is available, the colour of the reagent persists. The basic reaction is a reduction of iodine by sulphur dioxide in the presence of water. The reaction goes to completion by the removal of sulphur trioxide as pyridine sulphur trioxide, which in turns reacts with the methanol to form the pyridine salt of methyl sulphate, see formulae below.

In the absence of methanol, the pyridine sulphur trioxide reacts with another molecule of water. The reagent requires standardization immediately before use and this can be done by employment of a standard solution of water in methanol or by use of a hydrated salt—for example, sodium tartrate (Na₂C₄H₄O₆.2H₂O). To eliminate interference from atmospheric moisture, the titration is carried out under an atmosphere of dry nitrogen, the end-point being recorded amperometrically. Equipment is now available for a completely automated determination, thus eliminating the manual aspects of sample handling and weighing, introduction to the Karl Fischer cell, titration and data completion. Although the BP Karl Fischer reagent contains pyridine, as above, the latter has been replaced by other bases (e.g. imidazoles) in some commercial reagents. Alternatives to methanol, such as diethylene glycol monoethyl ether, have been introduced to improve reagent stability.

The principal drawbacks of the Karl Fischer method are the instability of the reagent and the possibility of substances in the sample, other than water, which may react with the reagent.

The coulometric method for the quantitative determination of water relies on the same basic reactions as indicated above for the Karl Fischer procedure. However, the iodine is produced electrochemically at the anode by oxidation of iodide and reacts immediately with the sulphur dioxide and water from the sample. When all the water is used up, iodine is produced in excess and this is the electrochemical end-point. If necessary, moisture in a solid sample can be evaporated and passed into the reaction vessel in a stream of dry inert gas. The method is employed for the measurement of the very small amounts of water permissible in fixed oils used for the preparation of parenteral dosage forms; examples include a maximum of 0.1% for soya, olive and evening primrose oils.

Other chemical methods for water determination include treating the sample with various carbides, nitrides and hydrides and measuring the gas evolved; gas chromatography has been employed for the analysis of the liberated gas.

Spectroscopic methods

Water will absorb energy at various wavelengths throughout the electromagnetic spectrum and this fact can be made a basis for its quantitative determination (see later in this chapter for a general discussion on spectroscopy). Measurements can be made in both the infrared and ultraviolet regions; interfering substances must be absent. The method is particularly suitable for very small quantities of water (e.g. trace quantities in gases). Nuclear magnetic resonance (NMR) spectroscopy has been employed for the determination of moisture in starch, cotton and other plant products.

Electrometric methods

Conductivity and dielectric methods have both been utilized for moisture determination but have not, as yet, found extensive application to pharmaceutical products.

Extractive values

The determination of water-soluble or ethanol-soluble extractive is used as a means of evaluating drugs the constituents of which are not readily estimated by other means. But as suitable assays become available (e.g. with the anthraquinone-containing drugs), some of the previously used extractive tests are no longer required as pharmacopoeial standards. In certain cases extraction of the drug is by maceration, in others by a continuous extraction process. For the latter the Soxhlet extractor is particularly useful and has been in use for many years, not only for the determination of extractives (e.g. fixed oil in seeds) but also for small-scale isolations (Fig. 16.2). A development of the Soxhlet technique is also shown in Fig. 16.2; in this apparatus extraction is by boiling solvent followed by percolation; finally, evaporation yields the extract and the recovered solvent ready for the next sample. Some examples of the types of extractive used are given in Table 16.2.

Fig. 16.2 I, Soxhlet continuous extraction apparatus. A, powdered drug for extraction in thimble and plugged with suitable fibre e.g. defatted tow or cotton wool; solvent refluxes into thimble and syphons into flask B, containing boiling solvent, when receiver is full. II, Three-stage continuous extraction and solvent recovery: left, extraction by boiling with solvent; centre, percolation stage; right, removal of solvent. A, sample for extraction; A₁, exhausted drug; B, solvent; B₁, recovered solvent; C, solvent containing soluble plant constituents; D, final extract. (Soxtec System, Tecator Ltd.)

Table 16.2 Extractives employed for drug evaluation.

Drug	Method of evaluation
Gentian, liquorice, many drugs of BHP	Percentage of water-soluble extractive
Quillaia	Percentage of ethanol (45%) extractive
Valerian, cocillana, asafoetida	Percentage of ethanol (60%) extractive
Ginseng, hop strobiole	Percentage of ethanol (70%) extractive
Ginger, ipomoea and jalap	Percentage of ethanol (90%) extractive
Benzoin, catechu and Tolu balsam, myrrh	Limits of ethanol-insoluble matter
Colocynth	Limit of light petroleum extractive
Crushed linseed	Percentage of ether-soluble extractive