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In assaying chemical substances, two immediate concerns are posed: identity and purity. To this end, the chemical and pharmaceutical industries have developed reference standards—agreed upon formulations of given chemicals which are used as a standard of comparison for analytical laboratories and production areas.
The identity of a chemical substance is determined by comparison with a known standard through any of a number of analytical means. Most commonly used today are various modes of gas and liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. Less commonly used in modern practice, though still seen, are reagent reactions and X-ray crystallography.
Chromatography methods rely upon differences in mass of the components of a mixture. The mixture is decomposed or adsorbed into a neutral carrier medium and the degree of adsorption into a test vessel (usually referred to as a column) is indicative of the mass of the various components of the mixture. Color differences may also be observed in liquid chromatography methods.
Spectroscopy relies on the resonance of the chemical bonds of the substance upon exposure to various wavelengths of radiation. Different chemical bonds (or, in the case of NMR, different energy levels of atoms) absorb radiation at different wavelengths; the absorption of the radiation creates resonance. The actual mechanics of this process are dependent upon principles of quantum mechanics outside the domain of this class.
In a reagent reaction, the substance to be tested is caused to react with another chemical (the reagent) via titration (the slow addition of one substance to another via a buret). The amount of the substance needed to trigger a reaction (usually denoted by a change in color, temperature, or heat exchange) is then compared to the reference standard. In X-ray crystallography, crystalline substances are identified via exposure to a high-intensity X-ray beam. The diffraction pattern induced by the crystal lattice structure of the sample is then compared to that of a reference standard. Both of these methods are extremely prone to human error and are not commonly seen in industrial environments today.
Once the substance has been identified, the question of purity may then be addressed. In reality, using more modern methods of testing, both questions may be answered at once: chromatography and spectroscopy methods will also identify any contaminants present. Most modern lab apparatus of this sort is computerized and will also contain a database which may be customized for the individual user’s needs. Thus, the sample and any possible contaminants may be identified in one sweep. Using older methods of analysis, the process is much more labor-intensive and, therefore, more prone to human error.
The organization in the United States responsible
for the development and dissemination of chemical standards is the Chemical
Science and Technology Laboratory of the National Institute of Standards
and Technology. In addition, numerous private companies and organizations
(notably the U.S. Pharmacopoeia, or USP) also produce standards which hold
equal weight with those of the NIST. These standards are, in turn,
reconciled with international standards; as yet, however, there is no world
governing body for chemical standards. In addition to defining the
standards of identity and purity themselves, these organizations also develop
protocols which testing laboratories use to identify chemical substances;
these protocols can, in some circumstances, carry the force of law.
For example, pharmaceutical manufacturers are required by the FDA to follow
USP protocols in testing and identification of their products. These
protocols are also useful in other quality activities, such as ISO-9000,
third party audits, and the formulation of the corporate quality plan in
general as they establish accountability to higher standards and define
a document trail for future reference.
