Classical or Transmission Raman, SERS or TERS: Which Raman Spectroscopy Technique Is Right for You?

Jun 01, 2013
By Spectroscopy Editors
Volume 28, Issue 6

Raman spectroscopy has found its place as an important analytical tool after years of exclusion as an "academic" technique. The intense instrumental and software developments made easier — or simply possible — the use of different types of Raman spectroscopy. However, as is usual in a relatively new field, Raman spectroscopy — or more exactly, Raman spectroscopies — suffer from some misunderstandings, common (and sometimes false) beliefs, and "fashion" attitudes that make the choice of an appropriate technique and the required parameters of the system difficult. This article re-explains and demystifies some definitions and opinions concerning Raman spectroscopy from two distinct sides: academic and industrial. Different Raman techniques are compared with respect to their main field of applications, possibilities, advantages, difficulties, and limits.

Some time ago I read on the packaging of facial cream that the product was tested using Raman spectroscopy. My first thought was "Great, finally Raman has found its place in industry." The second thought was that the cosmetic laboratory found it worth mentioning on the box. As a Raman spectroscopist, I'm obviously happy to see the technique rise to fame as it is truly useful and information-rich, and, thus, well-worthy of wider interest. However, between the high-end research, the analytical market, and the marketing, the knowledge about Raman spectroscopy and technique — popularized too quickly sometimes — can be confusing or lost. I see this clearly in my job as a Raman applications scientist, which offers an interesting view on changing Raman applications, technology, and user profiles. Today's Raman spectroscopy market offers systems ranging from easy-to-use handheld systems dedicated to one application, through automated analytical systems to multipurpose high-end research systems. The intense instrumental and software developments have made easier, or simply possible, the use of different types of Raman spectroscopy, like the plasmon enhancement techniques used in surface-enhanced Raman spectroscopy (SERS) or tip-enhanced Raman spectroscopy (TERS), spatially offset Raman spectroscopy (SORS), transmission Raman, and Raman optical activity (ROA), to name just a few, which made their way independently from classical Raman spectroscopy to become new techniques on their own.

In this contribution, addressed to nonexpert Raman users, I will review some Raman-related techniques. I've focused on those I have personal experience with to show their advantages and limitations.

Classical Raman Microspectroscopy

Raman spectroscopy is a universal and versatile method for analysis of the atomic structure and chemical properties of matter by analysis of the interaction between the light and molecular and crystalline vibrations, similarly to near-infrared (NIR) absorption spectroscopy (1).

Figure 1: Comparison of Raman spectra of two isomers (which have the same chemical formula but different molecular structure): glucose and fructose.
Raman spectroscopy has numerous advantages. The spectra allow a differentiation between even chemically similar species or different polytypes and their analysis can be quite direct (Figure 1). The confocal setup of modern Raman spectrometers gives lateral and axial spatial resolution on a submicrometer level. It does not require any sample preparation and is nondestructive — an important advantage over other techniques, particularly when the integrity of the sample is an issue. Obviously, laser power should be taken into consideration because burning may happen at high power, but this is a problem related more to the sample itself than to Raman spectroscopy.

Raman spectroscopy is not considered very efficient, meaning low sensitivity and long acquisition times. Indeed, compared to a fluorescence signal, for example, Raman does not look very impressive, even if some good Raman scatterers can give a signal in a few milliseconds or less. However, the advantage of Raman spectroscopy over other techniques is that the information is intrinsic to the material and, thus, label-free. Fluorescence itself can be a problem because it may appear in a Raman spectrum. Usually much more intense, it can mask the Raman signal. It can be intrinsic to the sample coming from light diffused in porous or powder samples or from color centers; it can also be related to the preparation of the sample, such as fluorescent markers, resins used to fix the sample, or organic solvents. Methods exist to avoid or reduce it, however some precaution should be taken during the preparation of the sample.

Who Is It For?

Extensive scientific or technical experience is not required to use and understand Raman spectroscopy, at least at the basic level. Of course, more advanced analyses require more-detailed knowledge about the investigated sample. Most often used in academic research, or industrial development, classical Raman can be adapted to dedicated quality control applications. Because Raman spectroscopy can investigate almost all materials except metals and all states of matter, the applications are numerous. The most common applications involve the analysis of molecular or crystalline structure and chemical composition of materials (polymers, carbon, and semiconductors, to name a few), pharmaceuticals, inorganics and minerals, and biological samples, thus, there are few limitations from this point of view. Corrosion or catalysis studies can be more challenging mainly because of the small volumes, but Raman spectroscopy remains an interesting method, considering the complexity of the investigated structures.

lorem ipsum