Limit Tests in Pharmaceuticals
Limit Test: Limit tests are quantitative or semi-quantitative tests designed to identify and control small quantities of impurities, which are likely to be present in the pharmaceutical substance.
Impurity: The impurities in pharmaceuticals are the unwanted chemicals that remain with the active pharmaceutical ingredients (APIs), or develop during formulation, or upon aging of both API and formulated APIs to medicines. The presence of these unwanted chemicals even in small amounts may influence the efficacy and safety of the pharmaceutical products.
TYPES OF INORGANIC IMPURITIES
- Toxic impurities: This kind of impurities are very harmful and can even cause death when taken once or short period of time. Example: Arsenic
- Cumulative impurities: This kind of impurities shows their toxicity when it taken over a period of time. Example: Heavy Metals
- Harmless impurities: Some impurities may not cause harm to body but reduce the therapeutic activity of active ingredient when it is present in large quantities. Example: Chlorides, Sulphates
On considerations of above classification, the pharmacopoeia has fixed the permissible limits of each impurity. For toxic impurities the permissible limit is as less as 5-10 PPM, whereas for cumulative impurities the permissible limit is 20 PPM. For harmless impurities the limits are still high.
Impurity profiling (i.e., the identity as well as the quantity of impurity in the pharmaceuticals), is now getting receiving important critical attention from regulatory authorities because practically it is impossible to remove all the impurities from any pharmaceuticals. Some remain in trace even after purification, so it is only desirable that the substance should be sufficiently pure and can be used safely. The different pharmacopoeias (IP, BP, USP etc.) specify the limits up to which various impurities can be tolerated in pharmaceuticals. The limit tests help to check and indicate the presence of various inorganic impurities in pharmaceuticals.
Obviously the quantity of any impurity present in official pharmaceuticals is often small, and therefore, the normal visible-reaction-response to any test for that impurity is also quite small. Hence, it is essential to design the individual test in such a manner so as to avoid possible errors in the hands of various analysts. This is accomplished by taking into consideration the following three cardinal factors.
- Specificity of the tests: Any test used as a limit test must, of necessity, give some form of selective reaction with the trace impurity. Many tests used for the detection of inorganic impurities in official inorganic chemicals are based upon the separations involved in inorganic qualitative analysis. A test may be demanded which will exclude one specific impurity, but highly specific tests are not always the best; a less specific test, which limits several likely impurities, at once, is obviously advantageous, and in fact can often be accomplished. An example of such a test is the heavy metals test applied to alum, which not only limits contamination by lead, but also other heavy metal contaminants precipitated by thioacetamide as sulphide at pH 3.5.
- Sensitivity: The degree of sensitivity required in a limit test varies enormously according to the standard of purity demanded by the monograph. The sensitivity of most tests is dependent upon a number of variable factors all capable of strict definition, and all favorable towards the production of reproducible results. Thus the precipitation of an insoluble substance from solution is governed by such factors as concentration of the solute and of the precipitating reagent, duration of the reaction, reaction temperature, and the nature and concentration of other substances unavoidably present in solution. As a general rule, cold dilute solutions give light precipitates, whereas more granular ones are obtained from hot concentrated solutions. Many of the limit tests, however, are concerned with very dilute solutions, which are often slow to react, and here sensitivity of the reaction can often be increased by extending the duration of the reaction or by raising the reaction temperature. Similar considerations apply in the design of colour and other tests employed as limit tests.
- Control of personal errors: It is essential to exclude all possible sources of ambiguity in the description of a test. Vague terms such as 'slight precipitate,' should be avoided as far as possible. The extent of the visible reaction to be expected under the specified test conditions should be clearly and precisely defined. This is usually accomplished in one of three ways.
(a) Tests in which there is no visible reaction: A definite statement is incorporated in the wording of the test, which states that there shall be no colour, opalescence or precipitate, whichever is appropriate to the particular test. One example of this type of requirement is the test for barium, and calcium in dilute hypophosphorus acid, where the additions of dilute sulphuric acid under precisely controlled condition shall produce 'no turbidity, or precipitate' within one hour. The time factor is used here as a means of increasing the sensitivity of the test. Tests such as these which give negative results do not necessarily imply the complete absence of the impurity, the test as laid down merely indicating the absence of an undesirably large amount of the impurity
(b) Comparison methods: Tests of this type require a standard containing a definite amount of impurity, to be set up at the same time and under the same conditions as the test experiment. In this way the extent of the reaction is readily determined by direct comparison of the test solution with a standard of known concentration. The official limit tests for chlorides, sulphate iron and heavy metals are based on this principle. The limit tests for lead and arsenic are, in practice, also comparison methods. They are, however, so designed that they can be readily applied as quantitative determinations.
(c) Quantitative determinations: Quantitative determination of impurities is only applied in special circumstances, usually in those cases where the limit is not readily susceptible to simple and more direct chemical determination.
The method is used in the following different types of tests:
- Limits of insoluble matter
- Limits of soluble matter
- Limits of moisture, volatile matter, and residual solvents
- Limits of non-volatile matter
- Limits of residue on ignition
- Loss on ignition
- Ash values
- Precipitation methods.
Differences between assay and limit test
- The assay is quantitative whereas the limit test is semi-quantitative or qualitative test.
- The assay result provides the exact amount of substances whereas the limit tests for range of impurities.
- The assay will be done for substances as well as impurities whereas the limit tests particularly for impurities.
"We will Keep you Updated as we receive More Information"
Last Updated : 23-10-2019 06:23:24 pm
Madhusudhana Reddy Induri
Dr. Reddy working as Associate Professor, Department of Pharmaceutical Analysis, KVSR Siddhartha College Pharmaceutical Sciences, Vijayawada