There are several factors that can affect the spectral resolution of a Raman confocal microscope. It is important that these factors are known, as changes in spectral resolution can impact how well the system can identify various compounds in a sample.
What is spectral resolution?
Spectral resolution is the most important feature of a spectrometer. This parameter determines the maximum number of spectral peaks that can be achieved using the system. For example, in the case of a system that has a wavelength range of 200 nanometers and a spectral resolution of 1 nanometer, it can resolve up to 200 wavelengths across a spectrum.
The level of resolution required depends on the application and the sample that is under investigation. For instance, in routine analysis, a low/medium resolution may be sufficient, whereas the analysis of polymorphs and crystal will likely require a high resolution. The reason for this is that subtle shifts of such materials that may not be visible in a low-resolution Raman experiment.
Low versus high spatial resolution
A low to medium resolution is required for the basic identification of chemicals, but also to distinguish different materials. When detecting subtle spectral signatures, higher resolution is necessary to characterize small changes in the shape or position of the spectral peak. The different phenomenon can cause changes in the Raman spectra and its resolution.
Particle shape and the state of aggregation
The shape of a particle and status of aggregation can have a strong effect on the spectra, as emphasized by a previous study on titanium oxide (TiO2). The surface effects manifested in the Raman spectra as new bands due to an increase in the volume ratio.
Crystallinity and polymorphism
As the sample becomes crystalline from an initially amorphous compound, the Raman peaks become sharper and more intense. A higher resolution is one way of resolving this.
Materials that have the same chemical formula but different states of the solid form are known as polymorphs. Due to the similarity in their chemical formula, their spectral profiles are also similar. Nonetheless, the effect of solid form on the vibrations in the chemical bonds can lead to changes in the spectral signatures, which often leads to changes in peak shapes and positions.
Intrinsic strain and stress
When a sample is subjected to stress and strain, the material may show changes in Raman spectra. For example, in semiconductors, strain and stress are introduced into the materials to produce the properties of semiconductors or luminescence. In such cases, higher resolution Raman spectroscopy is used as a tool to characterize this effect on the spectra.
Hydrogen bonding and protein folding
The various inter and intramolecular interactions in hydrogen bonding can cause changes in the Raman spectrum. In such cases, high resolution is required to determine the changes caused to these iterations in the spectra.
Although the primary structure of a protein has a maximum effect on Raman spectra, the secondary and tertiary structures also can have local or widespread effects on the vibrational modes of a molecule. This, in turn, is translated to changes in the spectra. As this effect is subtle, it may require high-resolution methods to detect the presence of such changes in the Raman spectra.
- Horiba. What is spectral resolution, and when is it needed? horiba.com
- B&W Tek. Spectral Resolution - Part 5. bwtek.com
- Ocana, M., et al. (1998). Factors affecting the infrared and Raman spectra of rutile powders. Journal of Solid State Chemistry. doi.org/10.1016/0022-4596(88)90176-4.