Lecture 3 - Spectroscopic Techniques

Question 1

What type of spectroscopy is raman?

Answer 1

vibrational spectroscopy

Question 2

what is raman based on?

Answer 2

photons interacting with the molecular vibrations in materials and are scattered

Question 3

what are the two types of photon scattering?

Answer 3

elastic and inelastic scattering

Question 4

what is elastic scattering?

Answer 4

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

Question 5

what is inelastic scattering?

Answer 5

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

Question 6

what are the two types of raman scattering?

Answer 6

stokes and anti stokes

Question 7

what is meant by stokes and anti-stokes?

Answer 7

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

Question 8

what do scattered photons emit?

Answer 8

certain wavelenghts of light which correspond to their energy

Question 9

in raman spectroscopy how is the majority of light scattered?

Answer 9

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

Question 10

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

Answer 10

the molecule’s energy increases

Question 11

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

Answer 11

virtual states

Question 12

name 6 types of molecular vibrations

Answer 12

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

Question 13

to be raman active what must a molecule have?

Answer 13

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

Question 14

if something changes polarisability what does this mean?

Answer 14

the shape of the electron cloud around the molecule shifts

Question 15

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

Answer 15

the dipole moment

Question 16

what corresponds to a change in polarisability?

Answer 16

the stretching of a molecule

Question 17

what is the mutual exclusion rule?

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

what is a centrosymmetric molecule

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

summarise raman spectroscopy (5 points)

Answer 23

• 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

Question 24

summarise IR spectroscopy (5 points)

Answer 24

• 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

Question 25

what is requires for a bond to experience a dipole

Answer 25

a difference in electronegativity between the atoms

Question 26

what aspect of raman spectroscopy is good for use in forensics

Answer 26

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

Question 27

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

Answer 27

red, blue, green or near IR

Question 28

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

Answer 28

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

Question 29

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

Answer 29

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

Question 30

what is a CCD in a raman spectrometer

Answer 30

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

Question 31

what are the three types of raman instruments

Answer 31

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

Question 32

as a raman device gets smaller what is compromised?

Answer 32

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

Question 33

what can the peaks of raman spectra provide

Answer 33

structural information of the molecule

Question 34

why does the raman spectrum of diamond have only one peak

Answer 34

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

Question 35

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

Answer 35

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

Question 36

how are raman spectra useful in identifying organic species

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

- 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

Question 39

why is raman considered a non destructive technqiue

Answer 39

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

Question 40

name 7 uses of raman spectroscopy in forensic science

Answer 40

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)

Question 41

what is chemometric useful for

Answer 41

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

Question 42

what does a lumpy humpy raman spectrum mean

Answer 42

that fluorescence is present