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5.17 ORDINARY IMPURITIES Limit

5.17 ORDINARY IMPURITIES Limit

 ​           This test, where called for in the individual monograph, is provided to evaluate the presence of ordinary impurities in official articles. Ordinary impurities are defined as those species in drug substances and/or drug products that have no significant, undesirable biological activity in the amounts present. These impurities may arise out of the synthesis, preparation, or degradation of compendial articles. In certain instances, impurities that pose a potential health risk may be detected. Because these impurities would not be individually identified by the strict use of this Appendix, a separate evaluation may be necessary to ensure that the detected impurities fit the requirements set forth in the definition of Ordinary Impurities. Selections of tests and assays allow for anticipated amounts of impurities that are unobjectionable for the customary use of the article.

 ​           Reporting and specifications The value of 2.0 per cent, unless otherwise specified in the individual monograph, was selected as the general limit for the total amount of ordinary impurities in monographs where documentation did not support adoption of other values.

 ​           Where a monograph sets limits on concomitant components and/or specified impurities/degradation products, these species are not to be included in the estimation of ordinary impurities unless so stated in the individual monograph. Concomitant components are defined as species characteristic of many drug substances that are not considered to be impurities in the Pharmacopoeial sense. Examples of concomitant components are geometric and optical isomers (or racemates) and antibiotics that are mixtures. Any component that can be considered a toxic impurity because of significant undesirable biological effect is not considered to be a concomitant component.

Method

 ​           Unless otherwise specified in an individual monograph, estimation of the amount and number of ordinary impurities is made by relative methods rather than by strict comparison to individual Reference Substances. Nonspecific detection of ordinary impurities is also consistent with this classification.

 ​           Typical evaluation methods used for ordinary impurities are thin-layer chromatographic (TLC) techniques (Appendix 3.1). Tests for related substances or chromatographic purity may also be used to evaluate the presence of ordinary impurities. Other methods (e.g., HPLC, HPTLC, etc.) may also be used with adequate justification as an alternate method. Unless otherwise specified in the individual monograph, use the following method.

 ​           Test solution Prepare, in the solvent specified in the monograph, a solution of the substance under test having a known concentration of 10.0 mg per ml.

(Note Heat or sonication may be used to dissolve the drug substance where use of such does not adversely affect the compound.)

 ​           Standard solutions Prepare, in the solvent specified in the monograph, solutions of the Reference Substance or designated substance having known concentrations of 10, 50, 100, and 200 μg per ml. (Note Heat or sonication may be used to dissolve the drug substance where use of such does not adversely affect the compound.)

Procedure

 ​           Use a thin-layer chromatographic plate coated with a 0.25-mm layer of silica gel GF254, and mobile phase specified in the monograph. Apply equal volumes (20 μl) of Test solution and Standard solutions to the plate, using a stream of nitrogen to dry the spots.

 ​           Allow the chromatogram to develop in a preequilibrated chamber until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the chamber, and air-dry. View the plate using the visualization technique(s) specified. Locate any spots other than the principal spot in the chromatogram of Test solution, and determine their relative intensities by comparison with the chromatograms of the appropriate Standard solutions. See discussion above with regard to reporting and specifying total ordinary impurities.

Key for Visualization Techniques

 ​           (1) Use UV light at 254 nm and at about 366 nm.

 ​           (2) Use iodoplatinate TS.

 ​           (3) Mix 10 ml of acetic potassium iodobismuthate TS with 20 ml of glacial acetic acid, and dilute with water to make 100 ml.

 ​           (4) Ninhydrin spray Dissolve 200 mg of ninhydrin in 100 ml of ethanol. Heat the plate after spraying.

 ​           (5) Acid spray In an ice-bath, add slowly and cautiously, with stirring, 10 ml of sulfuric acid to 90 ml of ethanol. Spray the plate, and heat until charred.

 ​           (6) Acid-dichromate spray Add sufficient potassium dichromate to 100 ml of sulfuric acid to make a saturated solution. Spray the plate, and heat until charred.

 ​           (7) Vanillin Dissolve 1 g of vanillin in 100 ml of sulfuric acid.

 ​           (8) Chloramine T-trichloroacetic acid Mix 10 ml of a 3 per cent w/v solution of chloramine T with 40 ml of a 25 per cent w/v solution of trichloro acetic acid in ethanol. 

Prepare immediately before use.

 ​           (9) Folin-C Add 10 g of sodium tungstate and 2.5 g of sodium molybdate to 70 ml of water, add 5 ml of phosphoric acid and 10 ml of hydrochloric acid, and reflux this solution for 10 hours.

 ​           (10) KMnO4 Dissolve 100 mg of potassium permanganate in 100 ml of water.

 ​           (11) DAB Mix 1 g of 4-dimethylaminobenzaldehyde in 100 ml of 0.6 M hydrochloric acid.

 ​           (12) DAC Mix  100 mg of 4-dimethylaminocinnamaldehyde in 100 ml of 1 M hydrochloric acid.

 ​           (13) Ferricyanide Mix equal volumes of a 1 per cent w/v solution of iron(III) chloride and a 1 per cent w/v solution of potassium hexacyanoferate (III). Use immediately.

 ​            (14) Fast blue B: Reagent A Dissolve 500 mg of Fast Blue B Salt in 100 ml of water.

 ​           Reagent B 0.1 M sodium hydroxide.

 ​           Spray first with A, then with B.

 ​           (15) Alkaline iron(III) cyanide Dilute 1.5 ml of a 1 per cent w/v solution of potassium hexacyanoferrate(III) with water to 20 ml, and add 10 ml of 15 per cent w/v solution of sodium hydroxide.

 ​            (16) Iodine spray Prepare a 0.5 per cent w/v solution of iodine in chloroform.

 ​           (17) Expose the plate for 10 minutes to iodine vapours in a pre-equilibrated closed chamber, on the bottom of which there are iodine crystals.

 ​           (18) Solution A Dissolve 0.5 g of potassium iodide in 50 ml of water.

 ​           Solution B Prepare a solution of 0.5 g of soluble starch in 50 ml of hot water.

 ​           Just prior to use, mix equal volumes of Solution A and Solution B.

 ​           (19) PTSS Dissolve 20 g of 4-toluenesulfonic acid in 100 ml of ethanol, spray the plate, dry for 15 minutes at 110º, and view under UV light at 366 nm.

 ​           (20) o-Tolidine Spray Dissolve 160 mg of o-tolidine in 30 ml of glacial acetic acid, dilute with water to make 500 ml, add 1 g of potassium iodide, and mix until the potassium iodide has dissolved.

 ​           (21) Mix 3 ml of a 10 per cent w/v solution of chloroplatinic(IV) acid with 97 ml of water, followed by the addition of 100 ml of a 6 per cent w/v solution of potassium iodide to prepare the spray reagent.

 ​           (22) Iodine-Methanol Spray Prepare a mixture of equal volume of iodine TS and methanol.

APPENDICES • 5.17 ORDINARY IMPURITIES Limit
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หมายเหตุ / Note : TP II 2011 PAGE 486-487