Lecture 3 - Spectroscopic Techniques Flashcards
What type of spectroscopy is raman?
vibrational spectroscopy
what is raman based on?
photons interacting with the molecular vibrations in materials and are scattered
what are the two types of photon scattering?
elastic and inelastic scattering
what is elastic scattering?
when the photon leaves the molecule in the same quantum state = Rayleigh scattering
what is inelastic scattering?
when the photon leaves the molecule in a different quantum state = Raman scattering
what are the two types of raman scattering?
stokes and anti stokes
what is meant by stokes and anti-stokes?
stokes = the photon loses energy to the molecule
anti-stokes = the photon gains energy from the molecule
what do scattered photons emit?
certain wavelenghts of light which correspond to their energy
in raman spectroscopy how is the majority of light scattered?
elastically (Rayleigh), for every 1 in 10 million photons we get raman scattering
if a photon loses energy to the molecule what has happened to the energy of the molecule?
the molecule’s energy increases
raman scattering does not consider electronic transitions so what does it consider?
virtual states
name 6 types of molecular vibrations
symmetric stretch
asymmetric stretch
in plane scissoring
in plane rocking
out of plane wagging
out of plane twisting
to be raman active what must a molecule have?
anisotropic polarizability (refers to the ability of a molecule to be polarised by an external electric field)
if something changes polarisability what does this mean?
the shape of the electron cloud around the molecule shifts
what needs to change for a molecule to be IR active?
the dipole moment
what corresponds to a change in polarisability?
the stretching of a molecule
what is the mutual exclusion rule?
in centrosymmetric molecules (molecules with a plane of symmetry in the centre) a vibrational mode can either be raman or IR active not both
give an example of a molecule that is raman active but IR inactive
H2 - with a symmetrical stretch there is a change in polarisation but no change in dipole
give an example of a molecule that is both raman active and IR active
HCl - with a symmetrical stretch there is a change in polarisation and a change in dipole
what is a centrosymmetric molecule
a molecule with a plane of symmetry - normally in the centre
when a vibrational mode can be both IR and raman active what do we see
one interaction to be stronger than the other e.g water there is a weak raman interaction but strong IR interaction
name a solvent that is good to use in raman and why
water - because the weak raman interaction but strong IR interaction means the water wont interfere with the raman signal of the sample much
summarise raman spectroscopy (5 points)
- involves light scattering
- requires a change in polarisability (C=C, C-C, C-H)
- little to no sample prep
- can measure through transparent packaging
- can be done on aqueous samples
summarise IR spectroscopy (5 points)
- IR light absorption
- requires a change in dipole moment (O-H N-H C=O)
- sample prep normally needed
- short optical path length needed
-done on non aqueous samples
what is requires for a bond to experience a dipole
a difference in electronegativity between the atoms
what aspect of raman spectroscopy is good for use in forensics
the little to no need for sample prep - maintains integrity of the sample
can be done through transparent packaging e.g evidence bags
in a raman spectrometer, what 4 types of light could the laser be
red, blue, green or near IR
what is the purpose of a Rayleigh filter in a raman spectrometer
to filter out the rayleigh scattered light from the sample so only the raman scattered will be detected
what is the purpose of a diffraction grating in a raman spectrometer
the raman scattered light that passes the rayleigh filter is split up due to holes in the diffraction grating
what is a CCD in a raman spectrometer
a charge couples device - the type of detector which takes in the light and converts it to an electrical charge which is the signal then turned into a spectra
what are the three types of raman instruments
benchtop - good for lab analysis (biggest)
handheld = good for on scene analysis (smallest)
portable = mid size - can also be taken to scenes
as a raman device gets smaller what is compromised?
the resolution of the spectrum (the ability to distinguish between spectral features and bands in the raman spectrum)
what can the peaks of raman spectra provide
structural information of the molecule
why does the raman spectrum of diamond have only one peak
becasue diamond is made of one type of atom and one type of bond multiple times over
for what type of material is raman better than IR and why is this
inorganic materials (raman can also be used for organic)
this is because FTIR normally begins at 500cm-1 and the vibrations of a of metal ligand bonds are generally in the range of 100-700 cm-1 - these can be seen in Raman spectra but not IR
how are raman spectra useful in identifying organic species
they can be used complimentary to IR spectra, helping with assigning functional groups
why is signal intensity in raman spectroscopy sometimes an issue and how can raman signal intensities be improved
due to only 1 in 10million photons scattered being raman scattered signal intensity can be a problem
but these can be improved using advanced techniques such as SRS (stimulated raman spectroscopy) for example
give some reasons as to why raman spectroscopy is used in forensics (10)
- rapid (90secs)
- non destructive
- high spatial resolution
- can do organic and inorganic profiling
- sensitive
- minimal sample prep
- use of an automated stage allows sample mapping (take measurements at different points)
- can take a reading at specific points we choose
- green technique = not using single plastic swabs
- allows for chemometric analysis
why is raman considered a non destructive technqiue
because laser light is used to but too much power cna burn the sample
name 7 uses of raman spectroscopy in forensic science
sample material identification
sample matching
sample heterogeneity
structural determination (which polymorph is present)
forgery detection (glass and diamonds)
foreign substances present in fingerprints (drugs)
chemometrics (the ability to group similar samples for predictions and sample classification)
what is chemometric useful for
identifying unknown samples by assigning them to a potential classification group - an advanced statistical method aiming to reduce subjectivity and bias
what does a lumpy humpy raman spectrum mean
that fluorescence is present
