Wavelength Tech Forum: Raman Spectroscopy

June 15, 2009

Joining us for this discussion are Julien Bradley, Ahura; Tom Tague, Bruker Optics; Scott Sutherland, GE Homeland Protection, Inc.; and Scot Ellis, Thermo Fisher Scientific, Inc.

The increased popularity of Raman techniques and instrumentation shows no sign of waning any time soon. With new applications being discovered every day and portable Raman instruments now being used by firefighters and first responders, the future growth of Raman technology seems to point only upward.

Joining us for this discussion are Julien Bradley, Ahura; Tom Tague, Bruker Optics; Scott Sutherland, GE Homeland Protection, Inc.; and Scot Ellis, Thermo Fisher Scientific, Inc.

Several handheld and field-portable Raman instruments were introduced at Pittcon this year and they are growing in popularity. Will there come a time when they have the sensitivity of benchtop instruments?

Bradley: Perhaps there will be a time when handhelds have the sensitivity of benchtop instruments, but that would add unnecessary cost and complexity with little value. While field-portable Raman (NIR and FT-IR for that matter) instruments have made tremendous advances in recent years, it is important to note that in general they are designed to balance sensitivity and resolution with such considerations as power consumption, size, ruggedness, and ease of use for their intended application. Handheld instruments must be good enough to provide accurate and trusted answers for the applications that they are designed (this keeps the size, cost, and complexity to a minimum). Other attributes become far more relevant to the operator like usability, field-support, battery life, temperature performance, IP rating, drop-testing, etc. Field-portable instruments are not designed to compete with laboratory instruments. As far as chemical selectivity, particularly with mixtures analysis in real-world scenarios for in-situ testing, this is certainly an area where handheld instruments are making tremendous strides to be as or more effective than their benchtop counterparts.

Tague: Sensitivity depends on what the goal of the measurement is. For low resolution work, handheld Raman units are very adequate for the task. Optical constraints limit the facility of handheld units, where high spectral resolution AND spectral range are required.

Sutherland: Because most applications for which a handheld Raman spectrometer is the best solution also require long battery life and low cost, it is unlikely in the near future that handheld systems will be able to take advantage of many of the advances that have allowed benchtop Raman systems to achieve high sensitivity. However, as the cost and power requirements of back-thinned and deep depletion CCD’s decreases, they may become practical for handheld systems. Coupled with the addition of newer, smaller, higher throughput, and lower cost spectrometer technology (often from the telecommunications industry), such as transmission grating spectrometers, and high throughput coded-aperture spectrometers, the performance of future handheld systems should continue to improve, and even approach that of some current benchtop Raman instruments.

Ellis: There may come a time, but ultimately the technologies will go where the markets want them to go. There is still the possibility for both laboratory and portable Raman spectroscopy to evolve in many directions. In the case of portables, most of the demand is driven by applications where a measurement is not convenient or effective with a lab instrument and in many cases, the user's goal is to get a "good enough" answer at the measurement point to make a decision. These products will be used by those who tend to be more practical than spectroscopic in nature. More than likely, lab instruments will continue to evolve in terms of performance and sampling capability where handhelds will evolve towards being fit for task. These are not absolutes, however, and we do see some elements of instrument design crossing over. Both types of instruments will continue to get better in ways that correspond to their matching applications.

What effect have developments in Raman technology had on the food and beverage industry?

Bradley: At my company, we have not seen Raman spectroscopy have an impact, yet, on the methods and techniques employed by the raw and processed foods industry. Of course, the food and beverage industry is increasingly concerned with the dangers of contaminated or adulterated raw materials and has been actively looking for easy-to-use, reliable techniques to ensure the quality and safety of the ingredients used in production. With the advent of handheld Raman instruments that don’t require trained chemists to operate, the food and beverage industry is evaluating Raman as a promising tool to improve their quality processes.

Tague: This is an emerging market, where the impact has yet to be felt.

How has Raman spectroscopy changed medical diagnostics?

Bradley: At this point, I am not aware of any broadly used diagnostic tool that employs Raman spectroscopy. However, Raman spectroscopy is very well suited to a number of diagnostic applications that currently have inadequate solutions. My company is actively working on several applications that will address these unmet needs.

Tague: To my knowledge, clinical trials have not yet been completed to validate the use of Raman spectroscopy for diagnostic work. It is my opinion that Raman can be utilized in this important area. Only when 510k approved products become available, can we say a change has been made.

Sutherland:Raman spectroscopy offers a number of key features that are useful in medical diagnostics. Since the method is often nondestructive, the laser light and generated Raman signals can be carried by optical fibers. There are numerous examples of Raman spectroscopy being used in the field of medical diagnostics. Raman spectroscopy has been used for characterization of gallstones and in the diagnosis of breast cancer. The recent development of spatially offset Raman spectroscopy (SORS) and its analogs, which allows Raman spectra to be obtained in turbid samples and several millimeters below the surface of opaque and translucent samples, such as skin, indicate the possibility that some of these analyses may be used for in vivo screening.

Raman spectroscopy has also been demonstrated as a powerful tool for non-invasive glucose monitoring using a different Raman spectroscopy method to correct for potential interferences from biological tissues. Finally, research at the University of Utah School of Medicine has shown that resonance Raman detection of Lutein and Zeaxanthin, carotenoids, which help protect against age-related macular degeneration, can be detected nondestructively at diagnostically relevant levels in the retina.

The use of Raman spectroscopy for the study of objects of art has been increasing in recent years. What developments in the technology have led to this growth? Do you expect the growth of this particular application to continue? Why?

Bradley:Miniaturization has clearly helped. Portable Raman and XRF are two technologies that can provide researchers a tremendous amount of analytical information without moving or disturbing the artifact of interest. For this fundamental reason, Raman and XRF will surely continue to grow in popularity and become standards for in-situ analysis.

Tague:The system design for some systems was altered to accommodate larger works of art with high spatial resolution. Handheld Raman units can be used to access works of art that could not be reasonably moved.

Sutherland:When analyzing art and architecture for the purposes of authentication or characterization, the use of methods that require either small amounts of sample or can be used nondestructively are of great utility. Raman spectroscopy fits that need quite well, and can be used to identify inorganic and organic pigments, as well as the minerals (and decay products) from sculptures and buildings.

In my opinion, I do not believe that there are necessarily any major developments in the technology that have fueled the growth in the use of Raman spectroscopy for this application, as much as it is a realization of what kind of information the method can provide to this field. However, one development that has definitely been beneficial is the miniaturization of micro-Raman optics.

Early commercial Raman microscopes were large, bulky add-ons to laboratory Raman systems, and keeping them up and running and aligned was not always a pleasant task. More recent generations of Raman microscopes are much more robust and, in some designs, particularly well suited for in-situ analysis or works of art. Microscope optics and video cameras have been decoupled from the laser and spectrometer using fiber optics, and miniaturized for easy positioning. By using microscope optics and alignment cameras, sample spot sizes can be greatly reduced, allowing even miniscule contaminants to be located and easily interrogated. In addition, in at least one higher end system designed with analysis of objects of art in mind, multiple laser wavelengths operating in the same instrument allow the operator to choose the optimum wavelength for different types of pigments, primarily to reduce fluorescence and absorption (which might cause local heating and damage of the sample). Finally, improvements in the use of confocal apertures provide the precision needed to analyze multiple layers one at a time, collecting Raman data only from the region of interest.

Given that there have been books published just on this specific application, and we are preparing for the 5th International Congress on the Application of Raman Spectroscopy in Art and Archaeology, coupled with the advent of smaller, more powerful Raman instruments, there appears to be no reason to expect that Raman spectroscopy will not continue to provide valuable information in this field for years to come.

Ellis: Historically the nature of the Raman technique and the adaptability of instrumentation have been ideal for the study of art and historical objects and this alone will drive growth as awareness increases. But the area will grow more quickly with instruments that accelerate getting answers and useful information rather than just acquiring spectral data and whose designs make using Raman confidently as easy as pushing a button. Recently, however, we have put our emphasis into pushing spectroscopy know-how and chemical and materials data into the instrument and software so that they solve the specific task. For example, the combination of specific natural material and mineral libraries with software that automatically breaks down and identifies the components in a mixture make a Raman system capable of complete pigment characterization quickly and easily. Add the capability to handle newer, advanced Raman calibrations and corrections automatically to optimize and standardize data and you give a whole new population of researchers and conservators access to use and share Raman as a powerful tool.

What is new and exciting in the Raman field? Where is Raman technology headed?

Bradley:Raman technology is being drawn into all manner of industries and applications. The fact that end-users are not limited to chemists, spectroscopists, or chemometricians allows for innovate uses without being inhibited by what was previously possible only in a laboratory setting. Where the applications demand selective chemical identification, noncontact analysis, and the end-user seeks a technique that allows them to be more effective and more efficient, then a Raman-based handheld instrument may be a good choice.

In the pharmaceutical industry during the 1940s, Raman spectroscopy was the preferred analytical technique, but was soon displaced for decades by near infrared technology. In recent years, commercial innovations in the pricing, packaging, and spectral library development of Raman spectroscopy have led regulatory and pharmacopeial authorities around the world to make significant updates to policy and standards.

Miniaturization has dominated the scene recently and will likely continue to do so for the next several years, but Raman-AFM combinations, imaging systems, and continued laser development into the UV have also certainly been interesting to watch.

Tague:Greater consideration is being given to combined analysis, such as AFM and Raman, XRD and Raman, etc. This is a more solutions-oriented approach that has great importance to the customer.

Sutherland:As for current and future trends, I guess the more appropriate question might be, “Where is Raman technology not headed?” In recent years, the field has seen some significant advances, such as the use of high-throughput coded aperture spectrometers for increased throughput, and the use of SORS for improvements in analysis of samples inside opaque containers (HDPE plastic bottles, envelopes, etc.).

In addition, surface-enhanced Raman spectroscopy (SERS) seems to be getting fresh legs. Commercially available substrates now allow for routine use of SERS for many applications, including the analysis of dipicolinic acid for identifying anthrax spores. In the near future, the use of antibody-labeled SERS tags will allow for the rapid, sensitive, and selective identification of bacteria, viruses, and toxins, at threat-dosage levels, using portable Raman equipment, to help first responders to be better prepared for potential biological attacks.

There has been significant activity toward dispersive-based Raman systems which employ longer wavelength lasers, on the order of 1 micron, as advances in NIR spectrometers, particularly from DWDM (dense wavelength division multiplexing) from telecommunications, begin to filter into the spectroscopy world. By combining these improvements with advances in low noise InGaAs and other NIR sensitive arrays, this area seems ripe for growth, both in laboratory systems to compliment FT-Raman instruments, and even in handheld field-based Raman instruments.

Another area that appears to be poised for significant advances is standoff remote analysis. There have been publications on short-range (1-50 meters) standoff Raman data collection since the early 1980s. However, it appears that more groups are throwing their hats into the ring, and there are now commercially available products, employing either near-infrared or deep ultraviolet excitation, and these devices continue to get smaller and easier to use. Deep UV excitation provides the benefit of significantly higher Raman signals, so that lower powered laser sources can be employed. An additional advantage of using deep UV wavelengths is that the fluorescence interference that is often the bane of Raman experiments is sufficiently red-shifted from the excitation source that Raman data can be collected in a fluorescence-free region. And for portable and transportable Raman applications, working in the deep ultraviolet allows Raman data to be collected in the solar blind region, so that Raman data collection in direct sunlight is no more difficult than collecting the same data in pitch blackness. Such technology is being deployed for the analysis of weapons of mass destruction in real time from moving vehicles.

These possible growth areas for Raman technology and applications are only the tip of the iceberg. There are many more exciting advances and applications on the horizon for Raman spectroscopy.

Ellis:Raman technology is rapidly branching in many different directions at the moment. Many of the applications are very exciting and bring promise for helping solve issues important to health, safety, and the environment. There is a significant and more fundamental development that should not be overlooked and that is the maturation occurring within Raman laboratory instruments. Raman historically has been a specialized technique in which instruments were tailored to the user and the specific application. Setting up the optimal configuration and getting high-quality data took careful set-up and expertise in the technique and the instrument; results would be unique to an instrument and even to a user. This slowed the growth of Raman. Only recently have some Raman designs evolved to the workhorse standards that allowed FT-IR to grow so rapidly. Now we are seeing more emphasis on making instruments suitable to volume applications. Developments in areas of calibration, alignment, laser power regulation, and data cleansing go a long way to making the results of these instruments comparable from one to the next. Because of this progress, Raman is ready for more routine and practical industrial and other applications and should see rapid growth as a general analytical and even quality control tool.

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