Ok, ok – First things first. Since Raman and FTIR spectroscopy have some basic similarities, it is only natural that their applications overlap a bit. FTIR and Raman are used to collect information about molecular bonds and their vibrations by making the sample interact with light.
As a result, both collect spectroscopic data about the identity and structure of the investigated material. Basically, IR and Raman spectra provide complementary information about the molecular structure. As a result, combining both spectroscopic techniques may be of great interest to many applications, such as DLC analysis.
What about vibrational Microscopy?
In microscopy, these similarities present us with particular challenges. While Raman microscopes are based on standard light microscopes, FTIR microscopes require special optics to achieve the desired performance.
Many people say that Raman and FTIR are universal techniques for microspectroscopic analysis. Although this is generally true, there are some interesting facts that help us decide whether Raman or FTIR is the best approach.
FTIR utilizes absorption of IR radiation.
This means, if your sample is absorbing to much of it, you won’t get a any information. This is why for transmission and transflection measurements you have to prepare your sample either in a KBr pellet or thinnly sliced cut. Of course, this type of sample preparation is not always practicable, making ATR particularly valuable in FTIR microscopy.
Using the ATR technique makes FTIR non-destructive, applicable to all types of samples and even offers further advantages. By using the ATR crystal as an immersion lens with a magnification factor of 4, even smaller structures can be resolved. Furthermore, FTIR offers an overall higher sensitivity when compared to Raman, especially for organic compounds.
For many samples, e.g. non-aromatic organic compounds, FTIR offers an overall higher sensitivity than Raman. Often this is simply caused by the larger amount of sample material investigated by FTIR microscopy.
Raman is based on inelastic scattering
Light interacts with the sample, is scattered, collected and analyzed. However, if a sample fluoresces strongly, you will not easily obtain high quality spectra. Apart from fluorescence, there are hardly any other disadvantages of Raman microscopy.
Raman spectroscopy is usually non-destructive, can measure through optically transparent materials such as glass, water or plastic. This enables confocal depth profiling of transparent samples down to the micrometer range.
In addition, it provides much more detail on inorganic compounds and low-energy modes such as crystallinity and analyzes samples with spatial resolution down to the nanometer range. This is especially useful for carbon related applications e.g. the analysis of graphene sheets.
It should be noted, that the correct choice of laser, grating and other technical details requires some more expertise and is therefore somewhat less straightforward than using FTIR microscopy.
Which one is better?
Sometimes it’s obvious.
You mainly characterize organic compounds? It is not necessary to use a spectral range up to 50 cm−1 and your samples do not require depth profiling? Most likely, FTIR is the right choice for you.
Do you want to analyze samples behind glass, such as LCD screens? Need more information on the morphology of a drug? Investigate inorganic nanostructures? Well, then your answer is probably Raman.
Sometimes it’s not.
Conclusion? Whether Raman or FTIR spectroscopy, both methods have advantages and of course limitations. But in combination these two methods become a powerful tool for material characterization.
In most cases, the respective application decides which technique is preferred. At our headquarters in Ettlingen, we frequently perform Raman and FTIR microscopy analyses for all kinds of applications, because we always try to offer the best possible solution to our customers.