We get some FAQs here in the analytical lab…one recurring question here at Gateway is “Why did/didn’t you do Raman?” During foreign particulate matter investigations, the use of spectroscopy is pretty common. Spectroscopy allows us to study the way a material interacts with energy, in order to identify a material. Sometimes, a scientist may use FTIR (Fourier Transform Infrared Spectroscopy)(1) …sometimes, Raman(2) may be chosen as the choice method; however, there are times where both methods are employed.
FTIR and Raman spectroscopy are complimentary techniques. Both are based on molecular vibrations; however Raman focuses on scattering of light, along with the change in polarizability of a molecule… FTIR uses absorption of light, and depends on a change in dipole moment. However, there several key differences about the two methods that speaks directly to their functionality in the identification of unknown materials. It is the understanding of materials and having the ability to ascertain which technique(s) would be valuable that is key in choosing your method.
Generally speaking, FTIR can give us a pretty good identification on “organic” materials. So, when we are looking at particles that consist of plastics, rubbers, fibers, etc., FTIR usually can give us the identification, without certain limitations sometimes encountered with Raman. Sometimes we do run into situations where a good FTIR spectrum can’t be attained. This happens often with thermally degraded materials. Using Raman, we can detect the presence of amorphous carbon, which can be indicative of thermal degradation or the presence of carbon black…when we look at all of the overall data (including the texture and appearance of the particulate) we can usually say if something was likely burnt. Additionally, some materials do not lend themselves well to FTIR (such as various minerals, carbides, graphitic materials, etc.), and are better suited for Raman analysis. Other materials that often give great Raman results include dyes, various oxides, drug products, etc. When we are dealing with inclusions embedded in other materials, such as glass/rubber/etc., sometimes Raman is more beneficial to use than FTIR, because with a confocal system, you can actually just focus in on the region of interest without having to isolate and prepare the material from the matrix. Raman spectroscopy can analyze solids and liquids…even gases. Raman does not have issues with interference from water, like FTIR does. Using Raman, many times we can even gain an understanding of the molecular structure of a material (3) and even identify polymorphs.
After this little rant, you are probably thinking… “Well…why do FTIR at all? Raman is the way to go!” Raman, while very useful, does have its own set of limitations. As mentioned, some materials tend to fluoresce heavily with Raman spectroscopy…this is an interference to the signal which can mask vibrational information. Some materials are just not Raman active and will not produce a spectrum. Also, we are limited by our databases. There are a lot more extensive databases for FTIR spectroscopy than for Raman. Also, Raman instruments tend to be a bit on the pricier side than are their FTIR counterparts.
So when we get that “Why did/didn’t you do Raman?” our analysts often have variable answers, as the answers are totally sample dependent. Usually it is during the actual analytical process that a trained analyst starts “getting a hunch” as to what methods would be beneficial, that the actual Raman vs. FTIR decision is made.