6/25/2023 0 Comments Spectrometer grating dispersio![]() Raman spectra of a compound’s polymorphs can be reliably used to confirm crystal forms, or even identify new ones in a research setting or in polymorph screening. Raman spectroscopy is often found to be experimentally more convenient to use than X-ray diffraction for crystal polymorph characterization or screening (2). If one is studying the strain induced in a material by the application of a stress, then the required spectral resolution will be dictated by the resolution of the strain one wishes to determine, and the degree to which a Raman peak position changes with strain. The magnitude of the Raman peak shift will be commensurate with that of the stress and corresponding strain induced in the material. Compressive and tensile stresses will induce corresponding strains in the crystal that can be observed as shifts of a Raman peak position to higher or lower frequencies respectively. ![]() Specifically, strain is the change in the positions of the atoms or the lengths of the chemical bonds within the object induced by the application of stress. Stress is a force per unit area applied to an object, and strain is the effect on the object resulting from the stress. For example, Raman spectroscopy has been used to characterize the stress on a material and the strain induced in it (1). In addition to the phase (solid, liquid, or gas) of the material being studied, other considerations related to spectral resolution are the material characteristic to be analyzed and the question that needs to be answered. Generally, the narrower the Raman bands of the material that one is studying, the greater the spectral resolution required to perform the analysis. For gases, they generally have weak molecular interactions, and therefore tend to have sharp and narrow Raman bands. In liquids, molecular interactions such as hydrogen bonding can induce significant broadening of Raman bands as a result of a greater distribution of the vibrational energy states. Are the materials solids, liquids, or gases? If they are solids, the natural bandwidths of the amorphous materials are significantly greater than those of crystalline compounds or elements. The answer depends on the materials the spectroscopist is working with and the questions that need to be resolved. It is quite common to hear a colleague ask what spectral resolution is required of a spectrometer to perform Raman spectroscopy. Here, we discuss how each of these items affects the spectral dispersion and ultimately the spectral resolution. The spectral dispersion depends on the following four items: the spectrometer’s focal length, the grating groove density, the individual pixel width in the array detector, and the absolute wavelength of the Raman scattered light striking the detector. A Raman spectrometer’s spectral resolution is determined by its spectral dispersion in conjunction with the entrance slit width. The spectral resolution required will depend on the materials you are working with, their natural bandwidths, and the degree to which you need to measure a peak shift or resolve two closely spaced peaks. Questions often arise regarding the spectral resolution required to perform Raman spectroscopy and those components of the spectrometer that contribute toward it.
0 Comments
Leave a Reply. |