Light microscopy (LM)
Light microscopy (LM) is an alternative compendial technique to light obscuration for the analysis of subvisible particle in parenterals.
LM is listed in the pharmacopeias Ph. Eur. 2.9.19, USP <788> and USP <787>. For analysis, particles from at least 25 ml of sample are collected on a filter membrane and subsequently dried. The particles on the membrane are counted and sized under a microscope using 10- and 25-µm graticules. Particle counts are reported for the size classes ≥10 and ≥25 µm. Possibilities to improve the detection of proteinaceous particles captured on a filter are staining procedures, e.g., by Coomassie Blue or commercially available kits.
Light microscopy can be used as a viable option for batch release testing in cases that light obscuration [LO] is not feasible, e.g., due to high viscosity.
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S. Zölls, R. Tantipolphan, M. Wiggenhorn, G. Winter, W. Jiskoot, W. Friess, A. Hawe, Particles in therapeutic protein formulations, Part 1: overview of analytical methods., J. Pharm. Sci. 101  914–35. doi:10.1002/jps.23001.