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An Illustrated Guide to USP Standards Using the Acetaminophen and Acetaminophen Capsules Monographs


Acetaminophen Monograph, Illustrated
1. Acetaminophen

C8H9NO2 151.17

Acetamide, N-(4-hydroxyphenyl)-.
4’-Hydroxyacetanilide [103-90-2].

 1. The monograph begins with an official title, using the United States Adopted Name (USAN), as outlined under Nomenclature <1121>, followed by descriptive information, including a graphic formula, chemical formula, molecular weight, chemical names, and Chemical Abstracts (CAS) registry number.
2. >>Acetaminophen contains not less than 98.0 percent and not more than 101.0 percent of C8H9NO2, calculated on the anhydrous basis. 2. The first item, introduced by a bold-face double-chevron symbol (>>), is the Definition. In the Definition, the content of the article is specified. It is usually given as a percentage of the chemical formula, based on the Assay, calculated on the anhydrous or dried basis. The assayed content of a synthetic drug substance normally should not be less than 98.0 percent and not more than 102.0 percent. The tightness of the tolerance depends on the precision of the assay used as well as on the ability to produce a drug substance of high purity without incurring unreasonable costs. For articles of lesser purity, which are derived from natural sources or fermentations, as well as for biologics (see Biologics <1041>), the content might be expressed in micrograms per milligram or in units per milligram.
3. Packaging and storage—Preserve in tight, light-resistant containers. 3. A discussion on packaging and storage is found in the General Notices under Preservation, Packaging, Storage, and Labeling. The proper packaging and storage conditions should be derived and documented from stability studies on the bulk drug. These standards are also important and applicable to storage and repackaging within the community pharmacy.
4. USP Reference standards <11>—USP Acetaminophen RS. 4. The Reference Standards section notifies the analyst of the official USP Reference Standard(s) used in the monograph and refers to the general test chapter USP Reference Standards <11> for additional information and instructions. Reference Standards are supplied by USP. (See also the section on Reference Standards in the General Notices).
5. Change to read: 5. Although technically not part of the monograph, this is a key revision phrase, which denotes that an official revision has occurred (see No. 6). The superscript black box is the beginning of the change and the subscript black box with a numeral signals the end of the change. The number at the end denotes the Supplement that the revision becomes official. (The official date of each Supplement is listed on the front cover of each Supplement.) Other revision phrases include "Add the following;" and "Delete the following:"
6. Identification

A: Infrared Absorption <197K>.

B: Ultraviolet Absorption <197U>—

Solution: 5 µg per mL.3
Medium: 0.1 N hydrochloric acid in methanol (1 in 100).

C: It responds to the Thin-layer Chromatographic Identification Test <201>, a test solution in methanol containing about 1 mg per mL and a solvent system consisting of a mixture of methylene chloride and methanol (4:1) being used.

 6. Identification tests are discussed in Procedures under Tests and Assays in the General Notices and Requirements. They are "…provided as an aid in verifying the identity of articles. Such tests, however specific, are not necessarily sufficient to establish proof of identity…Other tests and specifications in the monograph often contribute to establishing or confirming the identity of the article under examination."

The most conclusive test for identity is the infrared absorption spectrum (see Spectrophotometry and Light-scattering <851>). When taken together, absorption bands characteristic of individual functional groups are unique for a given chemical compound with few exceptions. Conformance with both infrared absorption and ultraviolet absorption test specifications "leaves little doubt…regarding the identity of the specimen under examination" (see Spectrophotometric Identification Tests <197>). If no suitable infrared spectrum can be obtained, the Thin-layer Chromatographic Identification Test <201> is a good substitute. Care has to be taken to ensure that the chromatographic system separates the article from other closely related drug substances. When a drug substance is a salt, identification of the base or acid used is also provided. This is particularly important if an active principle is available in several salt forms. Identification tests for the most frequently used acids or bases can be found in Identification Tests—General <191>.
7. Melting range <741>: between 168° and 172°. 7. For many organic compounds the melting range or temperature is a convenient criterion of identity and purity. Generally, for a melting range to be useful it should not exceed 3° or 4°. This test should not be specified when the substance melts with decomposition; rather, such characteristics are given in USP in the Reference Tables under Description and Solubility.
8. Water, Method I <921>: not more than 0.5%. 8. If water is the only residual solvent, or if it is present as a hydrate, it must be determined for the reasons given for Procedures under Tests and Assays in the General Notices. Water Determination <921> describes the various methods that might be applicable for a given article.
9. Residue on ignition <281>: not more than 0.1%. 9. Residue on ignition can be regarded as a purity test because it limits contamination with inorganic matter (salts) in an organic compound. Such contamination would not be readily detectable by the assay, particularly a chromatographic one. It also serves as an identity test for compounds with heavier inorganic counterions or inorganic functional groups.
10. Chloride <221>—Shake 1.0 g with 25 mL of water, filter, and add 1 mL of 2 N nitric acid and 1 mL of silver nitrate TS: the filtrate shows no more chloride than corresponds to 0.20 mL of 0.020 N hydrochloric acid (0.014%).
Sulfate <221>—Shake 1.0 g with 25 mL of water, filter, add 2 mL of 1 N acetic acid, then add 2 mL of barium chloride TS: the mixture shows no more sulfate than corresponds to 0.20 mL of 0.020 N sulfuric acid (0.02%).
Sulfide—Place about 2.5 g in a 50-mL beaker. Add 5 mL of alcohol and 1 mL of 3 N hydrochloric acid. Moisten a piece of lead acetate test paper with water, and fix to the underside of a watch glass. Cover the beaker with the watch glass so that part of the lead acetate paper hangs down near the pouring spout of the beaker. Heat the contents of the beaker on a hot plate just to boiling: no coloration or spotting of the test paper occurs.

 10. These tests are provided as general procedures where limits of chloride or sulfate salts are specified.
11. Heavy metals, Method II <231>: 0.001%. 11. This limit test, which actually determines heavy metals relative to a lead standard, should be used whenever contamination with toxic metals introduced during the manufacturing process is suspected. With modern methods of synthesis and modern supplies of acids, the need for such a test requirement seems to have lessened somewhat, at least in developed countries, and it has been possible to lower the limits for heavy metals in a number of articles.
12. Free p-aminophenol—Transfer 5.0 g to a 100-mL volumetric flask, and dissolve in about 75 mL of a mixture of equal volumes of methanol and water. Add 5.0 mL of alkaline nitroferricyanide solution (prepared by dissolving 1 g of sodium nitroferricyanide and 1 g of anhydrous sodium carbonate in 100 mL of water), dilute with a mixture of equal volumes of methanol and water to volume, mix, and allow to stand for 30 minutes. Concomitantly determine the absorbances of this solution and of a freshly prepared solution of p-aminophenol, similarly prepared at a concentration of 2.5 µg per mL, using the same quantities of the same reagents, in 1-cm cells, at the maximum at about 710 nm, with a suitable spectrophotometer, using 5.0 mL of alkaline nitroferricyanide solution diluted with a mixture of equal volumes of methanol and water to 100 mL as the blank: the absorbance of the test solution does not exceed that of the standard solution, corresponding to not more than 0.005% of p-aminophenol.

Limit of p-chloroacetanilide— Transfer 1.0 g to a glass-stoppered, 15-mL centrifuge tube, add 5.0 mL of ether, shake by mechanical means for 30 minutes, and centrifuge at 1000 rpm for 15 minutes or until a clean separation is obtained. Apply 200 µL of the supernatant liquid, in 40-µL portions, to obtain a single spot not more than 10 mm in diameter to a suitable thin-layer chromatographic plate (see Chromatography <621>) coated with a 0.25-mm layer of chromatographic silica gel mixture. Similarly apply 40 µL of a Standard solution in ether containing 10 µg of p-chloroacetanilide per mL, and allow the spots to dry. Develop the chromatogram in an unsaturated chamber, with a solvent system consisting of a mixture of solvent hexane and acetone (75:25), until the solvent front has moved three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots in the chromatogram by examination under short-wavelength ultraviolet light: any spot obtained from the solution under test, at an Rf value corresponding to the principal spot from the Standard solution, is not greater in size or intensity than the principal spot obtained form the Standard solution, corresponding to not more than 0.001% of p-chloroacetanilide.

 12. Toxic impurities, arising out of the synthesis or degradation of an article, are those possessing undesirable biological properties. They must be controlled by suitable tests to a level not considered harmful. The manufacturer must notify USP concerning the presence of such impurities and should provide methods and validation data for a limit test. Suitable limit tests employ either chromatographic methods or specific and sensitive spectrophotometric and chemical methods.
13. Readily carbonizable substances <271>—Dissolve 0.50 g in 5 mL of sulfuric acid TS: the solution has no more color than Matching Fluid A. 13. This nonspecific test is applied to substances that are not readily carbonized by sulfuric acid in order to limit the presence of concomitant impurities that are readily carbonized.
Change to read:

14. Organic volatile impurities, Method V <467>: meets the requirements.1

Solvent—Use dimethyl sulfoxide as the solvent.

 14. This limit test determines organic volatile impurities relative to a standard preparation containing chloroform, benzene, 1,2-dioxane, methylene chloride, and trichloroethylene. The limits are 50, 100, 100, 500, and 1,000 ppm, respectively. The test is required for bulk substances and excipients that are used in chronic-systemic administered dosage forms. The three main analytical methods used are based on the use of gas chromatography.
15. Assay—Dissolve about 120 mg of Acetaminophen, accurately weighed, in 10 mL of methanol in a 500-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Concomitantly determine the absorbances of this solution and of a Standard solution of USP Acetaminophen RS, in the same medium, at a concentration of about 12 µg per mL in 1-cm cells, at the wavelength of maximum absorbance at about 244 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of C8H9NO2 in the Acetaminophen taken by the formula:

10C(AU/AS), in which C is the concentration in m g per mL, of USP Acetaminophen RS in the Standard solution, and AU and AS are the absorbances of the solution of Acetaminophen and the Standard solution, respectively.

 15. Tolerances in the Definition are based on the Assay. They, therefore, should be as precise as possible. The Assay does not have to be stability-indicating, but the monograph, taken as a whole, should assure that any degradation would be detected and can be limited by a chromatographic or other specific test. An ideal combination is a chromatographic test for ordinary impurities with a precise titrimetric assay. Microbial assays for antibiotics (see Antibiotics—Microbial Assays <81>) are currently replaced by HPLC (see Chromatography <621>) assays, wherever possible. However, for antibiotics that are mixtures of several active components, the microbial assay is still the preferred one and is sometimes coupled with a chromatographic test to quantitate the individual components. Biologics, proteins, and peptides may require very specialized biological assays.
* The explanations of each of the monograph sections are based on an article that appeared in the Pharmacopeial Forum Vol. 15 No. 5, A Guide to USP Standards by Klaus G. Florey, Ph.D. Dr. Florey was a member of the USP Committee of Revision from 1970 to 1995.