Raman microscopy is an investigational technique for the identification of individual (sub)visible particles dispersed in a liquid formulation.
During Raman microscopy, the liquid sample is placed in a sample holder where microscopic images of the suspended particles can be taken for morphological analysis. A particle of interest is subsequently illuminated with a laser beam and the electromagnetic radiation from the illuminated particle is collected with a lens. The wavelength corresponding to that of the laser line (Rayleigh scattering) is filtered out, while the rest of the collected light is dispersed onto a detector. The recorded Raman scattering information serves as a chemical fingerprint, which is compared to a library for determination of the identity or origin of the particle.
Raman microscopy has a high spatial resolution and can be applied to particles as small as 5 µm. Depending on the particle properties, long acquisition times might be required, typically allowing for only a few particles per sample to be analyzed. Data from Raman microscopy can be combined with results obtained by flow imaging microscopy [FIM]. Hereby, the particle morphology information from both instruments is used to link particle identity to quantitative information.
Raman microscopy is typically used as a research tool during trouble-shooting and root-cause analysis.
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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.
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