Raman Spectroscopy Flashcards
Summarize the theory of IR absorption.
- IR spectra are results of low energy absorption derived from vibrations and to some extent rotations of molecules
- Absorption is possible when there is a net change in the dipole moment (caused by vibrations or rotations of the molecule)
In an IR spectrum, lower transmittance = […]
more absorbance
What are the two types of instruments in IR instrumentation?
- Dispersive and Fourier Transform
Describe dispersive IR instrumentation.
- Components are similar to UV-VIS spectrometers
- i.e., radiation source
- monochromator or filter
- sample holder
- detector connected to amplifier system
Largely now replaced by FT-IR
Describe FT IR spectroscopy instrumentation.
- Uses an interferometer rather than monochromator, and polychromatic IR light to generate an interferogram.
Describe the interferogram before and after the Fourier Transformation.
- Before: The interferogram is intensity of signal vs. time (or mirror position)
- Mirror position changes at a fixed velocity
- After: Intensity (absorbance/transmittance) vs. frequency
- An IR spectrum
The FT takes the signal, and determines the amount of each wavelength/wavenumber that went into the signal.
What does Fourier Transformation accomplish?
- Takes the signal and determines the amount of each wavelength/wavenumber that went into the signal.
What is Raman spectroscopy?
How does it work?
- Used to study the molecular vibrations and rotations of a sample
Uses monochromatic laser light, usually at VIS or near-IR wavelengths.
Works by shining a light source, usually a laser, onto a sample and measuring the light that is scattered by the sample.
What information does Raman spectroscopy provide? [4]
- Chemical structure
- Phase and polymorphy
- Crystallinity
- Molecular interactions
Non-destructive chemical analysis
Describe Raman spectra. [3]
- Molecule specific
- Contain information about the vibrational modes of the molecule
- Have sharp features, allowing identification of the molecule by its spectrum
Describe the Raman Effect.
- When a Raman event is observed, energy from incident laser photons is transferred to the analyte, resulting in emitted photons that are shifted to different frequencies.
- This inelastic form of scattering only affects approximately 1 in 10 million photons an is known as Raman scattering, or the Raman effect.
Inelastic scattering
What is the difference between elastic and inelastic scattering?
i.e., Raleigh vs. Raman scattering
- Elastic: most of the light is scattered without a change in energy - this is termed Rayleigh scattering
- Inelastic: a small proportion of photons is scattered with a loss or gain of energy to molecular vibrations - this phenomenon is termed the Raman effect, or Raman scattering
What is the difference between Stokes and Anti-stokes shift?
- Both are forms of Raman scattering
- Stokes: when emitted photons are shifted to a lower energy
- Anti-stokes: when emitted photons are shifted to a higher energy
Are stokes or anti-stokes shift more commonly used in Raman applications?
- Stokes (emitted photons are shifted to a lower energy)
Stokes shift is when emitted photons are shifted to a lower energy.
True or False?
True.
Stokes shift is when emitted photons are shifted to a higher energy.
True or False?
False.
Stokes shift is when emitted photons are shifted to a lower energy.
Anti-stokes shift is when emitted photons are shifted to a higher energy.
True or False?
True.
Anti-stokes shift is when emitted photons are shifted to a lower energy.
True or False?
False.
Anti-stokes shift is when emitted photons are shifted to a higher energy.
Describe the frequency shift of the scattered light in Raman scattering.
Compare Stokes to Anti-stokes.
- Proportional to the vibrational energy of the molecules in the sample
- By analyzing the frequency shifts of the scattered light, we can determine the molecular structure of the sample and identify its chemical composition.
What are the ‘virtual levels’ of photon molecular interactions?
Compare the energy shifts between UV-VIS, IR, and Raman spectroscopy.
- Virtual levels refers to Rayleigh and Raman scattering effects.
- Not as high as the first excited state
What are the characteristics of Raman scattering? [3]
- Very weak effect: only 1 in 10^7 photons is Raman scattered
- True scattering process: virtual state is a short-lived distortion of the electron cloud which creates molecular vibrations
- Strong Raman scatterers: C=C and pi-bonds
What works well with Raman spectroscopy?
- Organic materials including active pharmaceutical ingredients, organic solvents, polymers, harmful narcotics, and explosives
- Polyatomic inorganics such as magnesium sulfate, sodium bicarbonate, titanium dioxide, and calcium phosphate
- Molecules containing only single bonds: C-C; C-H; or C-O (e.g., aliphatic; sugar; starch; cellulose)
- Highly polar small molecules such as ethanol
What does not work well with Raman spectroscopy?
- Materials with no covalent bonds: purely ionic species (e.g., NaCl)
- Highly fluorescent samples including plant-based materials
- Black or dark-coloured samples as the materials may completely absorb the laser light
- Any substance with weak Raman signal within the region being examined (e.g., water in the region 200 to 2000 cm^-1)
- Most metals and elemental substances
Compare between Raman and IR spectroscopy.
Raman
* Observes the shift in vibration from an incident source
* Change in polarizability of a bond is required for a vibrational mode to be Raman active
* Occurs at all wavelengths
* Weak signal
* Sharp spectral features for molecular fingerprinting
IR
* Examines the wavenumber at which a functional group has a vibrational mode
* A change in dipole moment is required for a vibrational mode to be IR active
* Only observed in IR spectral regions
* High water absorption
* Broad spectral features
Complementary techniques
Why are IR spectroscopy and Ramn spectroscopy complementary?
- Many molecules that are inactive or weak in the IR will have intense Raman signals.
What are the limitations of Raman spectroscopy?
- Materials with no covalent bonds and intensely coloured materials cannot be characterised
- A Raman event is observed on average every 10^6 excitation photons, limiting the sensitivity of the technique and its applicability for contamination detection or trace analysis
- Fluorescence is a much more efficient optical process and can mask Raman bands in the spectrum.
What is SERS?
- Surface enhanced Raman spectroscopy
- Increases Raman signal to allow analysis of single molecules.
What are the advantages of SERS?
- Fingerprint information
- Non-destructive data acquisition
- Ultrahigh sensitivity
- Consistent for quantification
What are the two key components of SERS?
- Nanosubstrate and instrument
Describe dispersive Raman instrumentation.
Describe FT-Raman instrumentation.
Describe laser sources for Raman spectroscopy.
Describe confocal Raman microscopes.
- Confocal pinole: the Raman scattered light is collected from a small, well-defined area of the sample using a tightly focused laser beam and a confocal pinhole
- Beam splitter: efficient separation of the Raman scattered light from the excitation laser, and the acquisition of high-resolution Raman spectra.
Compare dispersive and confocal Raman.
Dispersive Raman instrument
- Faster and more sensitive
- Lower spatial resolution and can suffer from unwanted fluorescence or background signals
Confocal Raman instrument
- Acquisition of high-resolution, 3D images of the sample
- Slower and less sensitive
- Higher spatial resolution and are less susceptible to background signals
Are portal Raman spectrometers feasible?
Yes - useful for online and in-field detection of food chemical and microbiological hazards.
What are the general applications of Raman and SERS spectroscopy? [8]
- Structural chemistry
- Solid state
- Analytical chemistry
- Applied materials analysis
- Process control
- Microspectroscopy/imaging
- Environmental monitoring
- Biomedical
Describe magnetic/AuNPs nanocomposites (Au-MNN) for SERS detection of pesticides on vegetables.
Describe an innovate SERS mapping method for monitoring pesticide penetration.
