The following test is designed to limit to a level considered to be unobjectionable the number and size of discrete metal particles that may occur in ophthalmic ointments.

Extrude, as completely as practicable, the contents of 10 tubes individually into separate, clear, flat-bottom, 60-mm Petri dishes that are free from scratches. Cover the dishes, and heat at 85 for 2 hours, increasing the temperature slightly if necessary to ensure that a fully fluid state is obtained. Taking precautions against disturbing the melted sample, allow each to cool to room temperature and to solidify.
Remove the covers, and invert each Petri dish on the stage of a suitable microscope adjusted to furnish 30 times magnification and equipped with an eye-piece micrometer disk that has been calibrated at the magnification being used. In addition to the usual source of light, direct an illuminator from above the ointment at a 45 angle. Examine the entire bottom of the Petri dish for metal particles. Varying the intensity of the illuminator from above allows such metal particles to be recognized by their characteristic reflection of light.
Count the number of metal particles that are 50 µm or larger in any dimension: the requirements are met if the total number of such particles in all 10 tubes does not exceed 50, and if not more than 1 tube is found to contain more than 8 such particles. If these results are not obtained, repeat the test on 20 additional tubes: the requirements are met if the total number of metal particles that are 50 µm or larger in any dimension does not exceed 150 in all 30 tubes tested, and if not more than 3 of the tubes are found to contain more than 8 such particles each.

Auxiliary Information—
Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
General Chapter Robert H. Lafaver, B.A.
(EGC05) Excipient General Chapters
USP32–NF27 Page 296