Lecture 3 - Spectroscopic Techniques Flashcards

1
Q

What type of spectroscopy is raman?

A

vibrational spectroscopy

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2
Q

what is raman based on?

A

photons interacting with the molecular vibrations in materials and are scattered

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3
Q

what are the two types of photon scattering?

A

elastic and inelastic scattering

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4
Q

what is elastic scattering?

A

when the photon leaves the molecule in the same quantum state = Rayleigh scattering

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5
Q

what is inelastic scattering?

A

when the photon leaves the molecule in a different quantum state = Raman scattering

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6
Q

what are the two types of raman scattering?

A

stokes and anti stokes

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7
Q

what is meant by stokes and anti-stokes?

A

stokes = the photon loses energy to the molecule
anti-stokes = the photon gains energy from the molecule

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8
Q

what do scattered photons emit?

A

certain wavelenghts of light which correspond to their energy

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9
Q

in raman spectroscopy how is the majority of light scattered?

A

elastically (Rayleigh), for every 1 in 10 million photons we get raman scattering

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10
Q

if a photon loses energy to the molecule what has happened to the energy of the molecule?

A

the molecule’s energy increases

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11
Q

raman scattering does not consider electronic transitions so what does it consider?

A

virtual states

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12
Q

name 6 types of molecular vibrations

A

symmetric stretch
asymmetric stretch
in plane scissoring
in plane rocking
out of plane wagging
out of plane twisting

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13
Q

to be raman active what must a molecule have?

A

anisotropic polarizability (refers to the ability of a molecule to be polarised by an external electric field)

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14
Q

if something changes polarisability what does this mean?

A

the shape of the electron cloud around the molecule shifts

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15
Q

what needs to change for a molecule to be IR active?

A

the dipole moment

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16
Q

what corresponds to a change in polarisability?

A

the stretching of a molecule

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17
Q

what is the mutual exclusion rule?

A

in centrosymmetric molecules (molecules with a plane of symmetry in the centre) a vibrational mode can either be raman or IR active not both

18
Q

give an example of a molecule that is raman active but IR inactive

A

H2 - with a symmetrical stretch there is a change in polarisation but no change in dipole

19
Q

give an example of a molecule that is both raman active and IR active

A

HCl - with a symmetrical stretch there is a change in polarisation and a change in dipole

20
Q

what is a centrosymmetric molecule

A

a molecule with a plane of symmetry - normally in the centre

21
Q

when a vibrational mode can be both IR and raman active what do we see

A

one interaction to be stronger than the other e.g water there is a weak raman interaction but strong IR interaction

22
Q

name a solvent that is good to use in raman and why

A

water - because the weak raman interaction but strong IR interaction means the water wont interfere with the raman signal of the sample much

23
Q

summarise raman spectroscopy (5 points)

A
  • 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
24
Q

summarise IR spectroscopy (5 points)

A
  • 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
25
Q

what is requires for a bond to experience a dipole

A

a difference in electronegativity between the atoms

26
Q

what aspect of raman spectroscopy is good for use in forensics

A

the little to no need for sample prep - maintains integrity of the sample
can be done through transparent packaging e.g evidence bags

27
Q

in a raman spectrometer, what 4 types of light could the laser be

A

red, blue, green or near IR

28
Q

what is the purpose of a Rayleigh filter in a raman spectrometer

A

to filter out the rayleigh scattered light from the sample so only the raman scattered will be detected

29
Q

what is the purpose of a diffraction grating in a raman spectrometer

A

the raman scattered light that passes the rayleigh filter is split up due to holes in the diffraction grating

30
Q

what is a CCD in a raman spectrometer

A

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

31
Q

what are the three types of raman instruments

A

benchtop - good for lab analysis (biggest)
handheld = good for on scene analysis (smallest)
portable = mid size - can also be taken to scenes

32
Q

as a raman device gets smaller what is compromised?

A

the resolution of the spectrum (the ability to distinguish between spectral features and bands in the raman spectrum)

33
Q

what can the peaks of raman spectra provide

A

structural information of the molecule

34
Q

why does the raman spectrum of diamond have only one peak

A

becasue diamond is made of one type of atom and one type of bond multiple times over

35
Q

for what type of material is raman better than IR and why is this

A

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

36
Q

how are raman spectra useful in identifying organic species

A

they can be used complimentary to IR spectra, helping with assigning functional groups

37
Q

why is signal intensity in raman spectroscopy sometimes an issue and how can raman signal intensities be improved

A

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

38
Q

give some reasons as to why raman spectroscopy is used in forensics (10)

A
  • 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
39
Q

why is raman considered a non destructive technqiue

A

because laser light is used to but too much power cna burn the sample

40
Q

name 7 uses of raman spectroscopy in forensic science

A

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)

41
Q

what is chemometric useful for

A

identifying unknown samples by assigning them to a potential classification group - an advanced statistical method aiming to reduce subjectivity and bias

42
Q

what does a lumpy humpy raman spectrum mean

A

that fluorescence is present