Technology Forum: Raman

Raman spectroscopy is a non-destructive technique that requires little or no sample preparation. Participants in this forum are Tom Tague, of Bruker Optics, and Robert Chimenti, of B&W Tek.

What are typical applications for Raman spectroscopy, and why is it sometimes chosen over NIR or conventional IR methods?

Tague: Raman analysis can be successfully employed for many types of samples. In many ways, it is possible to characterize samples as well or better than Fourier-transform infrared (FT-IR). Raman has additional benefits to FT-IR analysis in that the intrinsic spectral linewidth is somewhat more narrow than IR, visible–near infrared (NIR) excitation allows for data collection through glass and quartz holders, and the Raman spectrum of water is very weak. Raman allows data collection in not only the fingerprint region of the spectrum (4000–400 cm^-1), but the far infrared (FIR) region as well (400–40 cm^-1). The increase in spectral range facilitates the analysis of inorganic molecules to include geological and mineral samples. Solid-state modes are also readily analyzed by Raman in the FIR region. These solid-state modes can provide vital differentiating information in the analysis of polymorphic species.

Raman allows data collection through vials and water, while still providing access to fundamental vibrational information in the mid-infrared (MIR) region. Fundamental vibration band analysis in the MIR is highly specific allowing for the identification of unknowns. The NIR region contains overtone and combination bands that are less intense and overlap significantly thereby complicating the analysis of unknowns. Because the intrinsic bandwidth of Raman spectral is narrow, univariate quantification can frequently be employed in lieu of more complex multivariate quantitative analysis typically employed in the NIR.

Chimenti: Raman spectroscopy allows for the nondestructive and noncontact interrogation of molecular bond structure, and as a result makes it the ideal choice for measuring material composition, stress/strain, and crystallinity. Raman spectroscopy has long been used in research communities for applications such as mineral analysis, art conservation and restoration, degree of saturation (C=C), and forensic analysis. In recent years, there has been a strong emphasis on using Raman spectroscopy for biomedical applications including cancer research. Additionally, with the miniaturization of lasers, spectrometers, and integrated computers, portable handheld Raman spectrometers have gained rapid acceptance in the fields of pharmaceuticals, security, and safety.

While Raman is complementary to NIR and FT-IR in many ways, it offers several unique advantages especially when looking at materials in aqueous solutions. Raman and FT-IR are both advantageous for their high specificity, which arises from their sharp, well-defined spectral features. On the other hand, Raman and NIR offer a much higher degree of flexibility in sampling due to the noncontact nature of the technique. As a result, Raman provides the user with “the best of both worlds” affording them a much higher degree of utility than either of the other two methods, which is particularly important for field measurements. Due to its versatility, handheld Raman spectrometers are becoming the method of choice for on-the-spot material identification and verification, counterfeit detection, and at-line quality control.

How does the instrumentation for Raman, NIR, and IR methods compare in terms of cost, dependability, safety, automation, support, and availability?

Tague: State-of-the-art Raman, NIR, and MIR instrumentation comes with validation packages ensuring good instrument performance. Raman analyzers are typically considerably more expensive than their FT-IR and NIR counterpoints, due to the relatively high cost of the excitation laser and detector. MIR and NIR systems are very robust and dependable. Raman systems can be more costly to maintain over time due to laser lifetimes and associated replacement costs. Virtually all IR and analytical Raman systems are Class I laser-safe products. Automation can be employed equally on IR and Raman systems and in fact typically use the same automation tools and software. The support and availability of Raman and IR systems are very similar. There are several vendors that provide quality IR and Raman systems with reasonable support. Raman and IR microscopes have very similar associated costs.

Chimenti: When comparing these instruments in terms of cost it can get a bit tricky since Raman, NIR, and FT-IR instrumentation vary according to the application requirements. Depending on the application, the cost of these types of instrumentation can vary from less than $10,000 to well over $250,000; therefore it is more important to look at the overall cost of ownership. As was stated in response to the previous question, Raman is both nondestructive and noncontact and as a result does not require the use of consumables or sample preparation, which lowers the overall cost. When comparing Raman to NIR instrumentation, it’s important to consider that NIR requires detailed multivariant analysis performed by a scientist or highly skilled technician, due to low spectral information content. The ease of which qualitative analysis can be performed with Raman completely eliminates the need for a technical user. Additionally, in comparison to FT-IR there are no moving parts inside of a Raman spectrometer, which increases the dependability of the unit and reduces the overall cost. Finally, Raman is the only one of these techniques to operate in a spectral region, which allows for the use of low-cost charge-coupled-device (CCD) cameras, which are significantly less expensive than InGaAs or other IR detectors.

All three techniques can be fairly well automated depending on the application of interest. For example, precision stages in conjunction with a microscope can allow for hyperspectral and topographical information to be automatically mapped, while intuitive onboard software packages are used in handheld units to automate the collection and analysis of data at the click of a single button. Raman and NIR differ from FT-IR in that they are both commonly integrated into process systems for applications such as reaction monitoring due to the high degree of sampling flexibility. As far as safety is concerned, there is no doubt that the use of a laser inside of a Raman spectrometer has the potential to be hazardous, but this is easily mitigated through the use of Class 1 enclosures and other laser shields for bench-top systems and the use of highly divergent beams with short nominal ocular hazard distances (NOHDs) with handheld devices. Finally, for the question about support and availability, all three of these technologies have matured to the point where they are all readily available and offer a high level of customer support. This holds especially true for handheld systems.