USP 751 Microscope for Ophthalmic Ointments

USP 751 Test Microscope: Metal Particles in Ophthalmic Ointments

The USP 751 test was designed to limit to a level considered to be unobjectionable, the number and size of discrete metal particles that may occur in ophthalmic ointments.

USP 751 Test Procedure

The USP 751 test procedure requires extruding the contents of 10 tubes individually into separate, clear, flat-bottom 60mm petri dishes that are free from scratches. Cover the dishes, and heat at 85ºF for two hours, increasing the temperature slightly if necessary to ensure that a fully fluid state is obtained. Take precautions against disturbing the melted sample, and allow each to cool to room temperature and to solidify.

Remove the covers, and invert each petri dish on the stage of a USP 751 microscope adjusted to furnish 30x magnification and equipped with a calibrated eyepiece micrometer for 30x. In addition to the near vertical illumination, direct an illuminator from above the ointment at a 45º 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 50µm or larger in any dimension. (The grid reticle installed in the USP 751 microscope is calibrated for 30x and has each square equal to 50µm to make this measurement easy to perform). The requirements for the test 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 that 8 such particles. If these results are obtained, repeat the test on 20 additional tubes. The requirements are met if the total number of metal particles that are 50µm 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.

USP Staff Liason: Catherine Sheehan, B.Sc. Scientist
Expert Committee: EGC05 Excipient General Chapters
USP29-NF24 Page 2709
Phone: 301-816-8262

USP 751 Microscope

USP 751 MicroscopeThe USP 751 microscope is calibrated using a NIST certified stage micrometer and meets all the above mentioned requirements.

The microscope comes with the eyepiece micrometer pre-installed. While performing all tests, the focus knob will be set to 3x, for a total of 30x magnification.


If you have any questions about the USP 751 microscope, or would like a digital version of this microscope, please contact Microscope World.