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Analytical Chemistry > Forensic IR Spectroscopy > Flashcards

Forensic IR Spectroscopy Flashcards

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1
Q

example of IR spectroscopy

A

look on ppt

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2
Q

Vibrational spectroscopy

A
  • IR (and Raman) spectroscopy are known as vibrational spectroscopy techniques
  • Both IR and Raman spectroscopy provide information regarding the presence or absence of functional groups (i.e. the structure of the molecule)
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3
Q

IR radiation

A
  • IR radiation lies between the visible and microwave regions of the electromagnetic spectrum
  • IR radiation has wavelengths longer than visible radiation and shorter than microwave radiation, and has frequencies that are lower than visible radiation and higher than microwave radiation
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4
Q

what is the IR region divided into

A

Near IR
Mid IR
Far IR

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5
Q

what region is most work done in forensic science

A
  • Most work in forensic science is done in the M-IR region, Obtaining data in the F-IR region is technically more difficult (Raman spectroscopy is better in this respect)
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6
Q

wave number of the 3 regions

A

look on ppt

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7
Q

wavenumber

A
  • Wavenumber is the reciprocal of wavelength

look on ppt for equation

  • where lamda is measured in centimetres
  • The unit for wavenumber is therefore cm-1 (‘reciprocal centimetres’
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8
Q

The MID-IR spectrum can be divided into four regions-

A

4000-2500cm-1
high energy vibrations of single bonds (C-H , O-H , N-H)

2500-2000cm-1
vibrations of triple bonds (CC , CN)

2000-1500cm-1
medium energy vibrations of double bonds (C=C , C=O)

1500-400cm-1
low energy vibrations of bonds gives complex specific bands, called the fingerprint region. The molecular structure given here is unique for each molecule.

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9
Q

mid infra red

A

look on ppt

the position size and shape of the peaks (or no peaks) gives information on the type of chemical bond present (i.e chemical group) in the sample

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10
Q

IR theory

A
  • The basis of IR spectroscopy lies in the vibration of the atoms within a molecule, as all chemical bonds naturally vibrate (elastic)
  • In order for chemical bonds to absorb IR radiation, the frequency of the IR radiation must match the frequency of the bonds natural vibration
  • Absorption of IR radiation stimulates molecular vibrations
  • Radiation in the M-IR region will cause stretching and bending vibrations of the bonds in most covalent bonds.
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11
Q

energy equation

A

look at ppt

E = energy; h = Planck’s constant (6.626 x 10-34 J/s) and c = speed of light ( 3 x 108 m/s)

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12
Q

Modes of vibration

A
  • Symmetric stretching vibration (both atoms move away or towards the centre)
  • Symmetric bending vibration in the same plane (scissoring)
  • Symmetric bending vibration out of plane (twisting)
  • Asymmetric stretching vibration (as one atom moves towards the centre the other moves away)
  • Asymmetric bending vibration in plane (rocking)
  • Asymmetric bending vibration out of plane (wagging)
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13
Q

fundamental absorptions (vibrations)

A

Theses vibrations are known as fundamental absorptions (result from excitation from the ground state to the first excited state)

i.e. electronic transitions between 2 energy states. Energy is required for a transition from a lower to a higher level and released in a transition from a higher to a lower level.

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14
Q

absorption

A
  • Transition to a higher state can occur only if the incident photon is directly comparable in frequency (energy) to the energy difference between the two states
  • The loss of radiation corresponds to transitions from ground to higher excited states
  • The material is subjected to a range of frequencies of radiation.
  • Pattern of absorptions – spectrum
  • To absorb IR radiation, there must be a change in a bonds dipole moment
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15
Q

When does a dipole moment occur

A
  • A dipole moment occurs when there is a separation of a charge due to differences in electronegativity i.e. when a atoms in a molecule share electrons unequally, as one atom is more electronegative than another. Good example is water hence why it is a polar solvent as the oxygen atom as lone electrons so giving it a partial negative charge and both hydrogens with a partial positive charge.
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16
Q

example of good , weak and strong spectrums

A

look on ppt

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17
Q

alcohol

A

Alcohol (R-OH): O-H produce broad band with rounded tip (plus C-O and C-H regions)

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18
Q

ketones

A
  • Ketones, aldehydes, esters, amides, carboxylic acids and other carboxylic acid derivatives contain the c=o functional group
  • C=o band typically appears between 1760-1630 cm-1 depending on the type of molecule and carbonyl bands always have a strong intensity that is relatively broad
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19
Q

amine

A

Primary (RNH2) and Secondary (R2NH) both produce broad bands)

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20
Q

nitrile group

A

(CN) absorb between 2300- 2200 cm-1 and band is intense

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21
Q

nitro group

A

N-O stretching at 1537 and 1358 cm-1

22
Q

IR instrumentation what 2 types commonly used

A

fourier-transform (FTIR) spectrometers

and filter photometers (dispersive), most modern instrument

23
Q

Fourier-transform instruments

A

only one beam of radiation is used and this means that all the required frequencies pass through the instrument simultaneously (non-dispersive).

24
Q

FT IR Spectrophotometer

A

look on ppt

Source: including tungsten-halogen lamps, or an electrically heated filament or high pressure mercury lamp

25
Q

FTIR has a number of advantages over dispersion instruments :

A
  • The whole spectrum can be run in seconds rather than minutes.
  • The sensitivity of the technique is greater because of lower ‘background noise’.
  • The spectrum of a solvent or a known impurity can be subtracted from the spectrum by the computer.
  • A small sample can give a spectrum by adding the information from several scans to give a single spectrum.
26
Q

Filter photometer instruments

A

transmit very narrow bands of radiation at a selected wavelength.
- They do not produce a whole spectrum but can be used for quantitative analysis

27
Q

Scanning instrument

A
  • which uses diffraction gratings (frequency separation devices) to resolve the IR radiation into individual frequencies (wavelengths). A specific frequency is isolated via an exit slit and passed to the detector
  • The IR spectrum is obtained by scanning (moving) the grating over a given wavenumber region after the beam of IR radiation has passed through the sample. Each wavelength is therefore analysed separately

look at ppt

28
Q

running an IR spectrum

A
  1. The sample should be pure (i.e. single substance)
  2. Choose the appropriate method of sample preparation.
  3. Ensure the sample is dry, solvents are transparent, solids uniformly ground etc.
  4. The IR spectrophotometer has been calibrated with a standard (polystyrene film).
  5. The spectrum obtained is well resolved and of adequate intensity.
29
Q

sample preparation and handling

A
  • IR is not as sensitive as UV/Vis because the energy is less, but good, reproducible spectra can be obtained from very small amounts of sample.
  • It is important that standard operating procedures (SOPs) are used when producing an IR spectrum. This is to ensure the resultant spectrum can be validated when compared to the reference library data.
30
Q

Gases-

A

or low boiling liquids may be obtained by expansion of the sample into an evacuated cell.
Typical gas cell lengths are 10cm.
Modern technology now allows gas cell lengths to be longer
by using coiled cells.

31
Q

Liquids :-

A

may be examined neat or in solution. A ‘thin film’ (0.01nm thickness )of a
neat liquid is produced using sodium chloride (NaCl) or potassium
bromide (KBr) plates, where a drop of the liquid is placed on one plate and another plate placed on top. The plates are polished discs of 2cm diameter and 0.5cm thickness.

32
Q

the nacl/kbr plates

A
  • NaCl/KBr are expensive and must be handled with care. They must not come into contact with water or they dissolve! . They are stored in a warm oven to protect them from atmospheric moisture.
  • The plates hold together by capillary action and placed in a cell holder in the IR instrument
33
Q

non aqueous solutions

A

can also be run as thin films.

34
Q

aqueous and very weak solutions

A

are examined using special cells
made of zinc sulphide, potassium bromide or silver chloride. Cells of 0.1-1.0mm thickness take volumes of 0.1-1.0cm of 0.05-10% solutions.

35
Q

Solids :-

A

the spectrum of a solid can be determined by producing a Mull or potassium bromide disc.

36
Q

how are mulls prepared

A

by grinding 2-5 mg of sample in a smooth agate mortar. Grinding is continued after the addition of 1-2 drops of mulling oil. The suspended particles must be less than 2m to avoid excessive scattering of radiation. The mull is examined as a thin film between two NaCl/KBr plates. Common Mulling agents include Nujul (high boiling petroleum oil) and Fluorolube , which is a polymer containing fluorine instead of hydrogen

37
Q

A potassium bromide (KBr) disc is prepared by

A
  • grinding about 1mg of sample with 100mg of dry powdered potassium bromide using a pestle and mortar.
  • The very fine powder is then put into a circular die and placed under mechanical pressure of 10,000-15,000psi under vacuum for about five minutes
38
Q

KBr disc holder:

A
  • KBr discs should be very thin, transparent and fragile.
  • They must be handled with great care and the minimum ofcontact with fingers. If the disc is too thick or too much sample has been added, the disc can be broken and half of it reground with more KBr to make a new disc.
39
Q

Polymer samples (plastics) can be examined by two transmission methods.

A
  1. Films of less than 10m can be examined in film form using special holders.
  2. Thick film samples are examined by the deposited film method. The sample is dissolved in a solvent, drops of the solution are placed on a NaCl plate and the solvent is removed leaving a thin film.
40
Q

Attenuated Total Reflectance (ATR-FTIR)

A
  • Attenuated total reflectance (ATR) is a new method used to obtain spectra of solids regardless of thickness and revolutionised M-IR spectroscopy.
  • Little or no sample preparation is required and enables small quantities of liquid and solid samples to analysed directly (i.e. non-destructive).
41
Q

Attenuated Total Reflectance (ATR-FTIR) uses

A
  • Used to analyse numerous forensic samples such as: drugs of abuse, explosives, fibres, inks, paint, body fluids and fingermarks
  • Most modern IR spectrophotometers are able to take an ATR attachment which can be fixed into the sample compartment of the machine. ATR-FTIR has further increased the use of this technique
42
Q

How does ATR work

A
  • works by total internal reflection.
  • The IR beam is passed through a special ATR crystal with a high refractive index, which is in contact with the sample.
  • The IR beam ‘bounces’ through the crystal forming an evanescent wave penetrating the sample as it crosses the crystal.
  • The reflected beam is collected by the detector as it exits the crystal.
  • The IR beam is passed down the crystal and reflects back off the sample to give a reading. Spectra can also be obtained from pastes and semi-solids by spreading them over the crystal.

look at ppt

  • The evanescent wave extends a few microns from the crystal surface into the sample, where It interacts with the electronic dipoles of the molecules. It is attenuated in regions of the IR spectrum where the sample absorbs the IR beam
  • The attenuated wave then returns back to the IR beam and is detected once it exits the crystal, generating an IR spectrum.
43
Q

ATR crystals

A
  • must have a high refractive index.
  • The best ATR crystal is diamond but this is very expensive.
  • Other common crystals are germanium, zinc selenide, thallium halides and silicon (for FAR-IR).
  • Powdered solids can be placed over the crystal in a very thin layer (1-2mm only) to obtain a spectrum.
  • The crystals must be cleaned before use, usually by being washed with a solvent such as water, methanol or isopropanol. ATR can be used on liquid samples – the liquid is poured into a cell surrounding the crystal.
44
Q

IR Microscopy

A
  • The coupling of an IR microscope with an FTIR spectrophotometer allows the infra-red analysis of samples as small as 10m in diameter. Get a magnified image of the sample and a IR spectrum
45
Q

IR microscopy process

A
  • The sample is placed between two slides and put on the
  • microscope stage platform.
  • Visible light is used to focus the microscope on to the sample.
  • The light source is then changed to IR and the detector measures the amount of IR absorbed by the sample.
  • Fine fibres such as silk can be viewed by this method.
  • Fibres are usually flattened using a roller knife before viewing.
46
Q

Applications of IR spectroscopy

A
  • The major use IR is for qualitative analysis (identification) of a sample. Some quantitative analysis (amount) is possible. Some forensic applications have been included in the above slides.
47
Q

Qualitative analysis (IR)

A
  • An IR spectrum can be used to identify a sample only if it can be matched to a reference spectrum. If no reference is available, the spectrum will enable identification of a type of compound (e.g. an alcohol or carboxylic acid etc) but full identification may not be possible unless other techniques are employed as well (such as NMR, MS).
48
Q

Counterfeit drugs

A
  • The World Health Organisation (WHO) estimates that counterfeit drugs account for 50% of those sold from internet sites around the world. In Africa, Latin America and Asia approximately 10-30% of drugs sold are counterfeit. The type of counterfeit activity is given below in order of prevalence:
    • Products without active ingredients
    • Products with wrong ingredients
    • Products with incorrect quantities of active ingredients
  • Products with correct quantities of active ingredients but with fake packaging
    • Products with high levels of impurities and contaminants
    • Copies of an original product
49
Q

Quantitative infra-red spectroscopy

A
  • The recent increase in government regulations on atmospheric contamination has demanded the development of sensitive, rapid and highly specific methods for a variety of chemical compounds.
  • IR absorption methods meet this requirement.
  • A variety of atmospheric pollutants can be determined with a simple, portable filter photometer equipped with a separate interference filter for each analyte species.

The Occupational Safety and Health Administration (OSHA) have set maximum tolerable limits for approximately 400 chemicals. IR spectrometry can be used to determine more than half of these chemicals.

50
Q

compound exposure and detection table

A

look at ppt

Analytical Chemistry (20 decks)

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