Figure 1. Representative Raman spectra (stacked) collected from cellulose (blue), nylon (red), cellulose triacetate (black), polyethylene terephthalate (PET) (green) and polytetrafluoroethylene (PTFE) (purple) using 532nm laser excitation.
Raman spectroscopy is a vibrational spectroscopy method which is based upon inelastic scattering of a monochromatic excitation source. This type of scattering is often referred to as “Raman scattering.”
As such, Raman spectroscopy is undoubtedly a spectroscopic technique which can stand on its own; however, it is routinely considered a complimentary technique when compared to Fourier transform infrared spectroscopy (FTIR). By featuring Raman spectroscopy as its own entity, it is our purpose to showcase and provide insight on applications where Raman spectroscopy can provide a unique advantage. In this featured application, we would like to discuss how Raman spectroscopy can aid in identification and discrimination of various types of fiber materials.
As previously discussed in a preceding blog, Gateway Analytical utilizes Bruker Optics SENTERRA Raman Microscopes for our purpose of analytical and R&D-based investigations. The SENTERRA Raman Microscope is built on an optical microscope platform. This allows for confocal Raman spectroscopy to be performed utilizing various magnifications by means of choosing different microscope objectives. The ability to change between microscope objectives quickly is one unique advantage, because one can focus on minute regions that would otherwise be inaccessible when using FTIR, even with a FTIR microscope attachment. This is a large advantage of Raman spectroscopy over FTIR, especially in regard to fiber analysis.
Commonly, FTIR is a routine method for fiber identification, as it does exceptionally well in identifying organic compounds. However, FTIR analysis would ordinarily be preceded by some means of sample preparation. When utilizing Raman for this purpose, samples may be placed directly underneath the microscope and analysis can commence. In addition, fiber samples may not be individually separated, but sewn tightly, or even lightly stretched, in a woven pattern. In this case, preparation time for FTIR would include teasing individual fibers apart. When utilizing Raman spectroscopy, spectral data can be obtained solely by focusing on individual fibers within the weaving or material substrate. In turn, this allows for fast and efficient discrimination of various fibers which may be present in a material.
One might argue in favor of FTIR over Raman spectroscopy, as the laser excitation sources of Raman do have the potential to cause fluoresce in certain compound. However, the Raman microscopes do provide flexibility by utilizing several different laser excitation sources which are commonly 532nm, 633nm, 785nm and 1064nm. The ability to change between multiple laser excitation sources can be critical depending on the chemical nature of the fibers being analyzed. Combining the ease of spectral data collection with the discrimination power of Raman spectroscopy, much time can be saved throughout the course of analysis and data reporting.
Figure 1 depicts a spectral overlay (stacked) of various fibers collected using Raman spectroscopy at 532nm laser excitation.
In conclusion, Raman spectroscopy has a long history of being a complementary technique to FTIR, but is an excellent spectroscopic method on its own. One featured application of the unique and important role of Raman spectroscopy is seen in the analysis and discrimination of various fiber types. Using a confocal Raman spectroscopy system, one can reduce preparation time while increasing analytical output. The highly discriminative power of Raman spectroscopy among various fibers allows for fast and efficient spectral data collection as well as discrimination, especially when comparing to known standards.