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Recent developments in photonics are finally making Raman instrumentation accessible to larger basic laboratories.
Raman spectroscopy has been considered an exotic analytical technique and its rare use in science education curricula has been relegated to advanced analytical, inorganic, or physical chemistry classes in which Raman spectroscopy is mentioned as part of quantum mechanics, group theory, and vibrational spectroscopy lectures. Unfortunately, until recently, hands-on experiments of this vibrational spectroscopy technique were available in very few institutions, not only due to the high cost of the spectrometers, but also to the accessibility of the units, which are often relegated to research laboratories in universities with minimum access to undergraduate students and even to graduate students who do not use the instrument for their specific research. Today, a low-resolution Raman spectrometer with fairly good sensitivity and ~10-cm-1 resolution can be obtained for as little as $3500 (US), while a complete Raman system with laser and optical fiber probe (1) goes for around $13,000, which is similar to the cost of educationally designed near-infrared (NIR) or atomic absorption (AA) spectrometers or a liquid chromatography (LC) or gas chromatography (GC) system.
Thanks to recent advances in the miniaturization of spectrometers and lasers, which prompted the proliferation of low-resolution Raman spectroscopy (LRRS) (2) for chemical detection and simple Raman molecular structure analysis, more universities and even technical colleges are including Raman instrumentation as part of their lecture and laboratories' curriculum. Furthermore, the technique is finding its way not only into the chemical departments of these institutions but also into fields such as pharmacy, forensic sciences, geology, materials engineering, biomedical sciences, and nanotechnology, among others. Continuous developments in photonics might make this technique available for larger basic science laboratories and it can become as common as regular UV–vis spectroscopy, pH measurements, and wet chemistry analysis.
LRRS has the advantage of the heavy use of personal computers, software packages, handheld capabilities, and instant results generation, which are in tune with current science education philosophies and trends.
Once the cost and availability hurdles are dissipated, Raman spectroscopy has all the potential to be one of the first choices to pedagogically introduce the concept of molecular spectroscopy to students at levels as early as high school (3,4).
The technique's inherent advantages, such as no required sample preparation, its nondestructive nature, and the flexibility to analyze liquids or solids directly or through clear containers, make LRRS highly attractive to students in all kinds of scientific fields.
Experiments in the analytical chemistry laboratory can be as simple as determining qualitatively what kind of alcohol is present in a hand sanitizer (by first creating a small spectra library of several known alcohols) to use for fingerprinting comparison and subsequent identification of the unknown substance. The Raman spectra generally are well defined with sharp distinctive peaks, as can be seen in Figure 1.
Figure 1: Simple Raman spectra of ethanol and cyclohexane showing clear, well-defined peaks.
More elaborate experiments could encompass the introduction of chemometric analysis that would be required to actually determine the amount of alcohol present in a household product.
In physics education, miniaturized low-cost spectrometers are commonly used in optics curricula to explain the Raman effect, either using laser pointers focused through a microscope's optics or more elaborate optical trains commonly encountered in optic laboratories that use the Raman scattering to shift a laser's original wavelength.
Materials and metallurgical engineers as well as geologists are now more acquainted with the use of Raman in their search for new materials, including nonlinear optic and nanometric materials, polymers, graphemes, ceramics and composites, catalysts, corrosion products, and the use of secondary Raman techniques such as surface enhanced Raman spectroscopy (SERS) for plasmon effect studies or new SERS materials synthesis and characterization studies (5–9).
So far, the two main scientific fields that are exploiting the innovations on LRRS are the forensics and pharmaceutical sciences. Forensic laboratories and crime field investigators are becoming more cost efficient and productive using portable, low-cost Raman spectrometers in their investigations, either by using them as prescreening tools for further analysis of samples or as chemical identification tools for samples that can range from automotive paints to explosive residues. Until recently, students considering entering this field had intrinsic knowledge of crime scene processing and its relationship to the scientific and legal communities. Today, major universities offer forensic scientist and forensic chemist degrees and graduate degrees, creating a new generation of experts who are commonly used to defend the forensic evidence in a court of law. Instrumentation such as handheld LRRS systems can now be used by nonscientific personnel, including detectives or investigators, to collect evidence that can then be analyzed by the same technique or even complementary techniques in the laboratory.
Furthermore, the same evidential sample can be taken to court and analyzed again in front of the witnesses and jury after a short explanation of Raman spectroscopy by the forensic experts as well as using simple demonstrations of chemical fingerprinting identification. These kinds of demonstrations can be accomplished only by a technique that can examine unaltered, and even untouched samples, preserving the evidence in its original condition for further analysis.
LRRS is gaining a predominant role in almost all scientific forensic areas including toxicology, pharmacology, archaeology, biochemistry, and chemistry. Thousands of papers have been published on the use of Raman in forensic science in applications as diverse as the determination of blood of different species (for example, it can be determined if a blood stain is human, canine, or feline) to the brand of automobile in a hit-and-run accident (by differentiating the different automobile manufacturers' paints and coatings) (10–12).
Pharmaceutical sciences are following the forensics trend closely with an exponential increase of the use of Raman spectroscopy. Besides the research and development market, the pharmaceutical industry was one of the major users of this analytical technique, thanks to their understanding of the complementary features of Raman and NIR and FT-IR spectroscopy, and their use regarding the determination of molecular structures. Again, pharmaceutical science college curricula included the technique as part of advanced vibrational spectroscopy courses, which focused more on the use of NIR spectroscopy due to cost and availability. The use of fiber-optic probes and technology for Raman analysis has two major fields in this industry and the incremental use of the technology has created full semester courses of vibrational spectroscopy in several universities' pharmaceutical science departments.
The first application of LRRS relates to the pharmaceutical industry's constant demands to become more efficient by increasing production capacity, reducing operational costs, and improving manufacturing practices, which resulted in the impetus to create the well-known process analytical technology (PAT) initiative. In-line Raman spectroscopy has become a major resource for this initiative, but the applications do not stop in the laboratory or in process control. Tighter restraints in quality and the ongoing increase of pharmaceutical counterfeiting has created a market for portable Raman devices that can be used either on the receiving docks of the pharmaceutical company for raw material analysis and verification, or at the retail consumers' level by providing brand and authentication of medications in drug stores around the world (13,14).
Although Raman spectroscopy is used most commonly for chemical analysis and substance identification, a newer generation of out-of-the-laboratory applications using fiber-optic probes currently is booming with novel applications. For example, handheld devices are making their way into biomedical and medical applications for in situ and in vivo disease and cancer detection in animals and humans, not only in research centers, but also in dentists' offices for cavity determination and in the operating room to help with fast detection of malignant tumors of oncology patients. These life science fields had previous experience with some fluorescence spectroscopy, but it was never practical to use vibrational spectroscopy in them because low-cost mobile techniques were based upon NIR spectroscopy, which in turn was almost useless, due to the inevitable presence of water in living tissue (15–17).
The importance of Raman spectroscopy theory and applications also can be highlighted in fields in which the actual science behind the technique is of secondary importance, and would not be understood by the majority of the students in such fields. Some of these fields include art, archaeology, and restoration, where students can use LRRS portable devices for pigment and substrate identification and analysis in museums, excavations, or art exhibits. It is definitely worth mentioning the hundreds of published papers and the flourishing amount of scientific conferences related to the use of Raman spectroscopy in these fields (18,19).
Also worth highlighting is that the previously mentioned fields (biomedical and arts) require analytical techniques that can acquire as much information as possible from samples or objects while at the same time minimizing the amount of damage created by the analytical technique.
The Raman spectroscopy "boom" has reached not only Hollywood, when featured in different TV series from CSI and NCIS to more educational and realistic places such as the Discovery Channel and National Geographic, but it is moving into our homes and even into outer space. As mentioned earlier, it will not take too long to see small Raman instruments that will be better known as product identification devices or scanners in drug stores and supermarkets to authenticate drugs, or inspect fresh meats and produce for freshness and cleanliness (20,21).
Furthermore, several research agencies around the world, including NASA, are working with Raman spectrometer designs that will be used to manufacture small systems for missions going to the moon, Mars, and even to Europa (one of Jupiter's moons) to analyze soil and rock samples in situ and send the information back to earth. Commercial handheld devices have been used by NASA scientists to analyze sediments and rocks in Antarctica, which is considered to have a terrain and climate conditions very similar to those they expect to find on the surface of Mars.
The same instrument was also used to look for signs of life (organic residues) in giant gypsum crystals found in the Naica cave of Northern Mexico. These experiments were aimed at proving the feasibility of using Raman spectroscopy in the aforementioned space exploration missions in two completely different climate conditions such as the frigid -58 °F in Antarctica and then in the 138 °F geothermic temperature of the 1000-ft underground formation (22–26).
Jorge Macho is with Ocean Optics, Dunedin, Florida.
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