Raman Spectroscopy

February 15, 2011

Raman spectroscopy has become a workhorse technique for molecular analysis with a wide range of applications both in the laboratory and in the field. Participants in this Technology Forum are David Tuschel of Horiba Scientific, Rob Morris of Ocean Optics, and Scot Ellis of Thermo Fisher Scientific.

Raman spectroscopy has become a workhorse technique for molecular analysis with a wide range of applications both in the laboratory and in the field. Participants in this Technology Forum are David Tuschel of Horiba Scientific, Rob Morris of Ocean Optics, and Scot Ellis of Thermo Fisher Scientific.

FT-IR has completed a major transition from research technique to analytical laboratory workhorse to staple technique in quality control laboratories. Will Raman follow this path?

Tuschel: The principal driving force for the transition had been improvements in instrumentation and controlling software to make data acquisition much easier. That occurred first with FT-IR and now Raman spectroscopy. The hardware and software developments over the last decade have contributed to the growth of Raman spectroscopy among users who are not primarily Raman spectroscopists. Having said that, the key to the effective and correct use of any analytical method is the proper analysis and interpretation of the data. Consequently, the continued development of data analysis software tools to support users who are not principally Raman spectroscopists is and will be the key to an expansion (I wouldn’t say transition) of Raman spectroscopy as exclusively a research technique to include analytical laboratory measurement.

Morris: Absolutely. In fact, it’s already happening. For example, Raman is a standard technique in markets such as pharmaceuticals and forensics. If you run a quick search of “Raman spectroscopy” in Google Scholar, Google’s search mechanism for scholarly papers and citations, you’ll find nearly a million references. Raman has become another easily accessible tool in the researcher’s toolkit.

Ellis: I think it will, though I do believe Raman also has a long research track ahead. FT-IR gained most of its technical advances when spectroscopic research was its own end. Its utility improved over the years as designs advanced to meet the needs of users in analytical labs in industry and instruments became easier and more trustworthy, without giving up sensitivity or other performance aspects. Most Raman instrumentation today is still very academic research oriented, and experts are most successful with them. Recently, however, a few Raman microscopes with research level performance have been designed with that same philosophy that made FT-IR a standard tool in most labs. As instruments and software design incorporate more of the technique expertise under the hood, more analytical problems in industry, crime labs, and government labs will be quickly solved with Raman. Even today, there are those turning to Raman as a quality control tool.

What are the most significant ways in which Raman instrumentation has changed in the past 10 years and how have these changes impacted Raman users?

Tuschel: The migration from large frame gas lasers to compact lasers that don’t require heat exchangers for water cooling or 208-V high amperage electrical service has made it easier for people to use Raman spectroscopy without having to become laser jocks. Solid state lasers can be obtained to provide a variety of excitation wavelengths at sufficient power, which was the principal advantage of the gas laser. Secondly, the continued improvement in laser light cutoff filters has allowed Raman spectroscopists to obtain spectra at very low energy shift using only a single grating spectrograph.

Morris: Three areas immediately come to mind. First, the use of CCD detectors in Raman systems replaced more light-sensitive (and often more expensive) single-element detectors such as the photomultiplier tube and avalanche photodiode. So, while the user may have sacrificed some optical resolution in the process, for simple applications such as identifying spectral features, CCD spectrometer-based systems have proved more than sufficient.

Second, the emergence of laser diodes as excitation sources helped reduce overall system cost and complexity. Unlike single-mode lasers that demand careful control of the laser line, diode lasers often have multiple modes with the instrument utilizing one of the available modes for exciting the sample. There are some drawbacks associated with diode laser use, but those tradeoffs have been mitigated through a variety of techniques.

Last, as Raman has matured, developers of Raman spectral libraries have expanded their collections. Libraries are available with most organic and inorganic substances encountered in the field across a range of markets. This allows users to more easily qualify samples and in some cases, to add data typical of a quality control lab setting. Building data libraries and applying chemometric techniques to Raman data are also more easily accomplished.

Ellis: Raman is moving in multiple directions and new powerful capabilities continue to become available for spectroscopic research. Many of these are still for the Raman specialist to configure and make successful, and so much of their benefit remains in the academic research laboratory. Still, the evolution is rapid and exciting as breakthrough Raman analyses are performed everyday. Perhaps the most significant changes to the technique over the years, however, are those recent products that are moving Raman outside of the academic laboratory into manufacturing industries, applied academic fields and government, military and law enforcement use. Additionally, Raman is becoming easier to set up, configure and maintain. For example, there are now some systems that can be serviced, upgraded or configured with different options in minutes by everyday users. These are the areas that are giving Raman the credit that it deserves as a robust, powerful analytical tool and that help move the technique out of the domain of experts alone. With these changes, Raman will grow even more explosively. For that reason, I feel the instrumentation changes enabling those broader uses are the most significant — miniaturization driving uses outside of the lab and solution-oriented spectroscopic intelligence mixed with robust, foolproof designs allowing general laboratories to turn increasingly to the technique.