Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy are classified as non-destructive analytical techniques involving an interaction between electromagnetic radiation and molecular vibrations to supply information regarding the chemical composition and structure of a sample.
While both techniques share some similarities, they are somewhat distinct as they display key differences.
In essence, FTIR is based on the absorption of infrared radiation by molecules and measures fluctuations in dipole moment during molecular vibrations. Conversely, Raman spectroscopy is based on the inelastic scattering of monochromatic light and detects the changes by measuring polarizability during molecular vibrations.
When used in applications requiring bulk chemical analysis, both FTIR and Raman spectroscopy are able to analyze solids, liquids, and gases, but by nature, Raman spectroscopy offers a series of unique sampling advantages. Furthermore, Raman typically requires little to no sample preparation.
This article outlines a few application examples that shed some light on some of the sampling advantages Raman spectroscopy can offer. All the spectral data used to demonstrate these advantages were collected using a Thermo Scientific™ DXR3 Flex Raman Spectrometer armed with a fiber optic probe.
(Fiber optics are easily obtainable for Raman spectroscopy because of the use of visible and NIR light and do not necessitate the use of exotic or rare materials associated with FTIR fiber optics.)
Preliminary tests for all samples were conducted using both 532 nm and 785 nm excitations, and it was concluded that the 785 nm laser set offered a better balance between fluorescence and Raman scattering efficiency overall.
Sampling Through Containers
Raman leverages the power of laser light for excitation, which facilitates precise and localized analysis at the light beam’s focal point. This makes Raman spectroscopy well-suited for the analysis of materials inside containers, through container walls, as long as laser light is able to permeate the container.
While the container walls may contribute to the total spectra, subtracting these contributions allows for the extraction of the Raman spectra of the contents inside the container, eliminating the need to open it.
This offers serious advantages in applications such as illicit drug detection, pharmaceutical counterfeit analysis, and toxic chemical identification; risks of contamination are eliminated by keeping the analyte contained, which also preserves evidence fidelity and prevents accidental operator exposure to toxic chemicals or high-potency drugs such as fentanyl.
Figure 1. Raman spectra of chemicals stored in glass bottles. (a) Liquid in a large brown bottle; (b) solid in a small brown vial; and (c) solid in a small clear vial. Contributions from glass fluorescence have been subtracted. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
Figure 1 displays spectra obtained from Raman analyses of chemicals in both liquid and solid forms contained in various types and sizes of glass containers. The transparency of glass enables the visible lasers used for Raman excitation to pass through unhindered without generating significant Raman scattering signals that would interfere with the analysis.
The relatively small contributions from the glass fluorescence, as shown in Figure 1, were deducted from all spectra, which presented Raman spectra with flat baselines and sharp features. Materials were quickly identified by cross-referencing against libraries.
Figure 2. Raman spectra of two pharmaceutical products analyzed through blister packaging. The contributions from the packing were minor but have been subtracted from these spectra. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
Another application similar in scope is the analysis of samples within packaging, such as the pharmaceutical products contained in blister packaging. Figure 2 exhibits the direct analyses of two over-the-counter (OTC) pharmaceutical products housed in bister packaging in different forms: a solid tablet (Cinnarizine) and a soft gel.
Blister packs are preformed plastic materials, but they do not contribute much to the observed Raman spectra due to the fact the packaging is usually relatively thin. On the other hand, thick plastic containers are more challenging as they interrupt the sample’s Raman spectra by contributing to varying degrees, depending on material type, thickness, and compositional complexity.
Figure 3. Raman spectra from the analysis of acetaminophen inside a white container. (a) Spectrum obtained through the container, (b) Spectrum of the container, (c) Spectrum of acetaminophen inside the container (subtraction result). Components were identified by searching against libraries. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
An example can be seen in Figure 3, which shows a container comprised of polyethylene (PE) with titanium dioxide (TiO₂) as a pigment.
As shown in Figure 3b, the container itself has significant Raman features. By deducting the spectrum of the container (Figure 3b) from the total spectrum (Figure 3a), a high-quality Raman spectrum of the content inside (Figure 3c) was acquired nevertheless.
Even though the container was fairly opaque, the contents were confirmed to be acetaminophen by cross-referencing with the available library.
Analysis of Aqueous Solutions
While water’s strong dipole moment leads to significant absorption in the mid-infrared region, often causing interference with sample spectra in FTIR analysis, it is a relatively weak Raman scatterer. Consequently, Raman spectroscopy can be more effective in aqueous environments.
Figure 4. Raman spectra from the analyses of an energy drink and water. (a) The spectrum of an energy drink with multiple components. (b) The spectrum of water. (c) The result of the subtraction of the water spectrum from the energy drink spectrum. Peaks associated with caffeine, potassium sorbate and L-phenylalanine are marked in the spectrum. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
Figure 4 exhibits spectra from the Raman analysis of an energy drink poured into in a vial.
The energy drink is a composition of several ingredients, including 0.4 % (0.004 g/ml) caffeine. As demonstrated in Figure 4b, water exhibits relatively weak and broad Raman peaks.
After subtracting water contributions from the total spectrum (Figure 4a) to obtain the corrected spectrum (Figure 4b), the final spectrum (Figure 4c) reveals the presence of anticipated ingredients, including caffeine, L-phenylalanine, and potassium sorbate.
Analysis of Inorganic Samples
Both FTIR and Raman have the capacity to analyze inorganic samples such as minerals, but there are hurdles to overcome when using the common sampling techniques in FTIR.
It can be a laborious and time-intensive process preparing samples for transmission FTIR measurements. FTIR reflection measurements can be further complicated by a combination of diffuse and specular reflection.
Attenuated Total Reflection (ATR) relies on the variations in refractive index (RI) between the ATR crystal and the sample, but an inorganic material may possess a higher RI. However, sampling with Raman spectroscopy is as easy as simply focusing the excitation laser on the surface.
Raman spectroscopy offers easy access to low-frequency vibrations typically associated with inorganic materials and generally exhibits narrow and well-defined peaks that are particularly sensitive to molecular symmetry. Thus, Raman is well-suited for identifying any changes in crystal structures, including the differentiation of polymorphs.
Figure 5. Analysis of three large geological samples: (a) calcite; (b) hematite deposits; and (c) cerussite deposits. Spectra were baseline corrected. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
The Raman analysis of three geological samples is displayed in Figure 5.
The spectra were cross-referenced against libraries for identification. Figure 5a is representative of the bulk mineral analysis performed using Raman. Figures 5b and 5c show the Raman analysis of multi-component mineral samples with small deposits on the surface.
It can be seen in Figure 5b that the presence of hematite (Fe₂O₃) in an agglomeration of different minerals was detected, whereas Figure 5c reveals the presence of cerussite (PbCO₃) on the surface of galena (PbS). This is a likely consequence of the weathering of PbS.
Figure 6 displays the Raman analysis of two different paints (white and yellow) on a painting. The white pigment was determined to be the rutile form of titanium dioxide (TiO₂). Rutile and anatase account for two of the most stable crystalline forms of TiO₂.
Raman spectroscopy is highly sensitive to crystal symmetry, making it an appropriate technique for the identification of crystal phases and polymorphs. The yellow pigment was determined to be chrome yellow (lead chromate, PbCrO₃). The analyzed section appears to contain a hint of titanium dioxide.
Flexibility of Fiber Optic Probe for Raman Sampling
Fiber optic probes offer greater flexibility and ease of movement during analysis. This is beneficial when analyzing samples that are not easy to access or manipulate, such as the 2-L glass bottle in Figures 1a, the large plastic container in Figure 3, the geological samples in Figure 5, and the painting example in Figure 6.
All these samples are too large to be placed into the sample compartments of most commercial Raman instruments. A fiber optic probe enables direct and tactile analysis of these samples without having to transfer the material into smaller containers or destroy the samples.
Figure 6. (a) Raman spectrum of the white paint on the painting; and (b) Raman spectrum of the yellow paint on the painting. Both spectra are baseline corrected. Components were identified by searching against libraries. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
Summary
This article has covered the analyses of a variety of samples using Raman spectroscopy. All samples were analyzed without having to perform any sample preparation.
As a result, precise and localized analysis at the focal point of the Raman-exciting laser allows sampling through containers such as glass bottles, blister packaging, and plastic containers, which limits potential cross-contamination and analyst exposure to hazardous materials.
The packaging and containers may make some contribution to the Raman spectra, but this depends on the type of container material, the container thickness, and the materials’ chemical compositional complexity. However, such contributions can be easily deducted when finalizing the results.
Water is considered a weak Raman scatterer, meaning Raman has a reduced water sensitivity, making it a suitable technique for aqueous solution analysis and preferential over FTIR.
As exhibited in the analysis of an energy drink, the Raman bands from water can be deducted from the spectrum of the aqueous solution, which reveals the ingredients present in the solution.
Raman spectroscopy is perfectly suited for the analysis of inorganic materials, particularly when identifying crystal phases and polymorphs, as shown by the analysis of minerals and pigments in a painting.
Fiber optic probes offer tactile flexibility and ease of maneuverability. This is of great benefit when analyzing samples that may be hard to access or manipulate, such as large containers, geological samples, and paintings.
Thermo Scientific DXR3 Flex Raman Spectrometer. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis
The DXR3 Flex has a range of sampling options available. Fitting a fiber-optic probe and ensuring it is properly configured is arguably the most suitable and versatile accessory for the analysis of bulk solids and liquids.
The benefits, both from those characteristic of Raman spectroscopy and those resulting from the use of fiber optic probes, offer several opportunities across a wide range of applications when using the DXR3 Flex Raman Spectrometer.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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