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Trace evidence > Lecture 3: Spectroscopic Techniques > Flashcards

Lecture 3: Spectroscopic Techniques Flashcards

1
Q

What are the two types of scattering?

A
  • Elastic (Rayleigh scattering)
  • Inelastic (Raman scattering)
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2
Q

What is elastic scattering?

Rayleigh scattering

A

Elastic scattering is when the molecule leaves in the same state that it entered.

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3
Q

What is inelastic scattering?

Raman scattering

A

Inelastic scattering is when the molecule leaves in a different quantum state, this can be higher or lower.

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4
Q

What are the two different types of inelastic (Raman) scattering?

A

Stokes & Anti-stokes

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5
Q

What is stokes?

A

Stokes is when the photon has lost energy to the molecule

Light comes in and leaves with less energy so the molecule has gained energy.

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6
Q

What is anti-stokes?

A

The photon has gained energy from the molecule

The light comes in and leaves with more energy so the molecule has lost energy.

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7
Q

What is the predominant type of scattering?

A

Elastic / Rayleigh

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8
Q

What are the odds of inelastic / Raman scattering?

A

Every 1 in 10 million you will get Raman scattering

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9
Q

What happens when a photon interacts with a molecule?

A

When photons interact with molecule they only go to a virtue state and then drop back down again.

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10
Q

What are the different types of molecular vibrations?

A
  • Symmetrical stretching
  • Antisymetrical stretching
  • Inplane scissoring
  • In-plane rocking
  • Out-of-plane wagging
  • Out-of-plane twisting
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11
Q

What must a molecule have to be Raman active?

A

For a molecule to be Raman active it must have anisotropic polarisabilty

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12
Q

What is anisotropic polarisabilty?

A
  • Anisotropic = non uniform
  • Polarisability = ease of which you can distort electrons
  • You need a change in polarisability to pick up Raman signals.
  • Anisotropic polarizability refers to the ability of a molecule or an object to polarize in different directions when subjected to an electric field.
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13
Q

Is H2 Raman active?

A
  • H2 has symmetrical stretch.
  • Change in polarisation so it’s raman active.
  • However with hydrogen and hydrogen, because there’s the same charge there is no dipole so it is IR inactive.
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14
Q

If we consider linear molecules doing a symmetric stretch..

A
  • If we consider linear molecules, doing a symmetric stretch, the atoms are moving apart and then together which causes a change in polarisability.
  • The shape of the electron cloud is changing therefore it is raman active.
  • The polarisability occurs as a function of the intermolecular distance.
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15
Q

What is a centrosymmetic molecule?

A

Centrosymmetric molecule is when there is symmetry around the center.

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16
Q

What rule applies in a centrosymmetric molecule?

A
  • In a centrosymmetric molecule a vibrational mode may be either Raman or IR active, but cannot be both.
  • Where a mode is active in both Raman and IR, one tends to be a weak and the other strong e.g water.

The mutual exclusion rule

17
Q

Why aren’t aqueous samples a problem for Raman scattering?

A
  • Water is a weak raman scatterer but a strong IR interaction.
  • This is why aqueous samples aren’t a problem for raman as they don’t obscure anything of interest in the spectrum.
18
Q

Raman

A
  • Light scattering
  • Requires change in polarisability
  • Little or no sample preparation necessary - You use a laser light and shine it on the sample
  • Maintains the integrity of the sample.
  • Measure through transparent packaging - bonus for forensics as you can do it through evidence bags
  • You can have aqueous samples
19
Q

Infrared

A
  • IR absorption
  • Requires change in dipole moment
  • Sample preparation usually necessary
  • Short optical path length required
  • Non-aqueous samples
  • Water is a problem.
20
Q

What is the difference between different Raman spectrometers?

A

The smaller you get the more you have to compromise on spectrum resolution

21
Q

What does more bonds mean on a Raman spectrometr?

A

It will mean a more complex spectrum.

22
Q

What can Raman spectroscopy be used on?

A
  • Organic materials - Allows functional group assignments, complemenatary to IR, can differentiate polymorphs
  • Inorganic materials - Usually superior to IR for inorganics
23
Q

How can you enhance Raman spectra?

A

Raman signal intensity may be improved using advanced techniques:
* Stimulated Raman Spectroscopy (SRS)
* Coherent Anti-stokes Raman (CARS)
* Resonance Raman Spectroscopy (RRS)
* Surface Enhanced Raman Spectroscopy (SERS)
* Tip-enhanced Raman Spectroscopy (TERS)
* Coherent Stokes Raman Spectroscopy (CSRS)
* Surface Enhanced Resonance Raman (SERRS)

24
Q

What are the pros of Raman Microspectroscopy

A
  • Non destructive
  • High spatial resolution (Good for trace evidence)
  • Sensitive
  • Minimal sample preparation
  • Chemometrics
  • Parameter optimisation
  • Automated stage
  • Green technique
  • Maintains sample integrity
25
Q

What can Raman microscopy be used for?

List of 6

A
  • Sample matching
  • Sample heterogeneity
  • Structure determination
  • Forgery detection
  • Foreign substances in fingerprints
  • Material identification
26
Q

What is chemometrics?

A
  • Ability to group similar samples.
  • Can predict properties and characteristics of samples or to identify and unknown.
  • Useful for small spectral differences that are difficult to distinguish visually.
  • Way of grouping samples and determining differences in spectral.
27
Q

What is the benefit of chemometrics?

A

Advanced statistical method where you can
- discriminate between samples effectively,
- remove difference of opinion and subject of biased,
- removes subjectivity

28
Q

What has to be true for a molecule if a photon loses or gains energy?

A

If the photon gains or loses energy then the opposite will be true for the molecule as there has to be an equilibrium.

29
Q

What has to be true for a molecule to be IR active?

A

There has to be a change in dipole.

30
Q

Is HCl raman active or IR active?

A
  • HCl is both raman active and IR active.
  • Cl is a minus and therefore H becomes a plus meaning there is a change in dipole so we can see this on an IR spectrum.
  • However, because of the stretching there is also a change in polarisability it’s also Raman active.
31
Q

What is the general path light takes in a raman microscope?

A
  • Laser source (blue, green, red, near-infrared) produces light which hits focussing mirrors, goes through a beam splitter and hits more mirrors and then comes through a microscope objective.
32
Q

What happens to the laser light after it hits the sample?

A

The laser light hits the sample and interacts with the sample molecules. It scatters the light and then it goes back through the same objective, bouncing off the focussing mirrors and back through the beam splitter. This time, it goes through the focussing lens and then through the Rayleigh filter which is very important.

33
Q

What is the purpose of the filter in a raman microscope?

A

The purpose of the filter is to filter out the rayleigh scattered light which is stronger than the stokes and anti-stokes scattering

34
Q

What happens to the light after it goes through the filter?

A
  • Whatever light is left over goes through a diffraction grating
  • Then it gets captured in a charged couple device detector
35
Q

What does the charged couple device do in a raman microscope?

A

Takes light and converts it into an electrical charge which produces a signal and then the algorithm within the software turns it into a spectrum.

36
Q

What does a diffraction grating do in a raman microscope?

A

It is a metal with tiny grooves that allows light to come through and breaks it up into its component parts.

37
Q

What is raman spectroscopy better at compared to IR?

A

Raman is typically superior to IR for the investigation of inorganics.

38
Q
A

Trace evidence (22 decks)

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