New Capabilities in Raman Spectroscopy

Raman spectroscopy is rapidly growing as an analytical technique. It is used for problem solving in many fields such as pharmaceutical manufacturing, research and development, forensics, and in process monitoring. McCrone Associates has utilized Raman microscopy as part of its analytical arsenal for over twenty years.

Raman is a vibrational spectroscopy method that is complementary to infrared spectroscopy, but the mode of analysis is different. Raman detects chemical function groups based on a change in polarizability of a chemical bond. Thus, it favors symmetric (non-polar) functional groups. Infrared spectroscopy requires a change in permanent dipole moment of a molecule and thus favors polar functional groups.

Raman can be used for identification of unknown materials such as polymers, pharmaceutical ingredients, minerals, and pigments to name a few. It can also be utilized to determine polymorphs, hydration, and crystal structure. For example, two different crystal structure forms of titanium dioxide, rutile and anatase, have unique and differentiable Raman spectra. Similarly, it is also very efficient in determining carbon phases such as disordered carbon, graphite, and diamond. Raman is less useful for amorphous materials such as oils, glass particles, and other amorphous silicates and metals. The choice of analytical technique for any particle problems depends on many factors, but the selection of an analytical technique is normally made after a discussion with the client about the problem and a microscopic examination of the sample. Raman is often utilized after an initial evaluation using IR spectroscopy or SEM/EDS analysis.

Perhaps the most exciting application of Raman is chemical imaging (mapping). In January 2021, McCrone Associates upgraded their Renishaw InVia Raman to the Qontor system. The Qontor upgrade includes LiveTrack™ real time focus tracking and an upgraded detector with Synchroscan™ capability to enable measurement of high resolution spectra with wider spectral range. These improvements will result in higher quality Raman chemical imaging. Image domain analysis with particle statistics will be available. This type of imaging is very useful for addressing questions such as the uniformity of ingredient mixing, and any changes to a particular ingredient after it becomes part of a mixture. Chemical imaging also allows for relatively rapid screening of particle dispersions for specific materials.