In the first part of this article, we explored the principles of Raman scattering and spectroscopy. Now, in this second and final part, we will delve into implementation issues and instrumentation based on these principles.
Q: What are some of the appealing aspects of Raman scattering and spectroscopy?
A: Raman spectroscopy offers several advantages. It is non-invasive, non-contact, and does not require special sample preparation before analysis. Analysis can be conducted on raw samples or through transparent or opaque containers (known as SORS). Additionally, Raman spectroscopy can be used for both quantitative and qualitative analysis.
Q: How does Raman scattering create a chemical fingerprint?
A: Each peak on the Raman spectrum corresponds to a different frequency of light absorbed by the sample, which excites a vibration. These unique frequencies, specific to the molecule and its bonds, create a distinctive fingerprint that enables the identification of various substances.
Q: What are some challenges in this process?
A: Let’s consider oxygen and nitrogen, two fundamental gaseous elements. When illuminated by an appropriate laser wavelength, oxygen exhibits a peak Raman shift around 1550 cm-1, while nitrogen shows one around 2330 cm-1, as illustrated in Figure 1, making them easily distinguishable.
Raman scattering and spectroscopy challenges
Q: It sounds straightforward, so what’s the issue?
A: Most molecules of interest are complex, with numerous chemical bonds and therefore multiple molecular vibrations. Consequently, Raman spectra for most molecules will exhibit numerous peaks. This is where libraries come into play to address matching requirements.
Q: How about optical “access”?
A: Traditional Raman spectroscopy instruments illuminate a sample with a laser, and the sensor for the return scatter is positioned alongside the laser source (co-axial illumination). To enhance optical “reception,” a technique called spatially offset Raman spectroscopy (SORS) is employed; akin to using multiple antennas in RF reception.
SORS utilizes multiple measurements to explore the subsurface of a sample, often enabling identification of materials within sealed, thick, colored, or seemingly opaque containers that possess optical “windows” at certain wavelengths allowing light transmission. It is particularly valuable in enhancing the speed, efficiency, and safety of hazardous material identification, airport security liquids screening, and pharmaceutical raw material verification.
Q: What is another challenge in successful Raman spectroscopy?
A: Sample fluorescence poses a challenge. Fluorescence occurs when a substance absorbs light and emits lower-energy light. This differs from the Raman effect, where light matching the frequency of molecular vibrations is absorbed, and the remaining light is scattered.
Light absorbed and emitted due to fluorescence does not correspond to vibrations, rendering it unhelpful in Raman spectroscopy. However, the detector in a Raman spectrometer cannot differentiate between light emitted due to fluorescence and that from Raman scattering. Thus, if the sample fluoresces during a Raman experiment, the emitted light will appear on the Raman spectrum. Fluorescence disrupts the Raman spectrum as fluorescence peaks are broader and stronger than Raman peaks, making Raman peaks challenging to differentiate.
Q: What can be done about fluorescence?
A: Occasionally, using a visible light laser of a different color (wavelength) suffices to avoid fluorescence. However, the optimal approach to circumvent fluorescence in Raman experiments is to utilize a laser in the near-infrared (NIR) region, such as at 830 nanometers (nm) and 1,064 nm, as these generally induce less fluorescence in the sample. For this purpose, a specialized type of Raman spectrometer employing infrared light is employed for analysis, known as a Fourier transform (FT) Raman spectrometer.
Physical implementation
Q: What does a typical Raman spectrometer entail?
A: Raman spectrometers are either dispersive or employ a Fourier-transform technique (FT-Raman) akin to Fourier-transform infrared (FTIR). In a dispersive Raman spectrometer, the key components, as depicted in Figure 2, include:
The spectrometer comprises these elements:
- The excitation source: A single-wavelength line source (a laser) is utilized to excite or initiate the analyte to an excited state. The laser wavelength choice is crucial as the Raman signal intensity is inversely proportional to the fourth power of the laser wavelength (∝ λ4); moreover, the wavelength should not induce interfering fluorescence.
- Detector: The photodetector records the signal intensity at each scattered wavelength.
- Narrow-bandpass optical filter: This filters out Rayleigh and anti-Stokes scattering in most conventional Raman spectrometers.
- Diffraction grating: This disperses and separates Raman bands based on wavelength.
- Entrance slit where the sample light is focused: Typically, the narrower the slit, the better the spectral resolution.
- A combination of notch filter and grating filters: These further isolate the single-wavelength laser beam to ensure narrow bands.
A benchtop version is depicted in Figure 3:
Figure 3. This top-down view of a Raman spectrometer constructed as an optical benchtop. (Image: Edmund Optics)
Raman spectroscopy units you can purchase
Post navigation
