Infrared spectroscopy

Question 1

Vibrational spectroscopy

Answer 1

IR (and Raman) spectroscopy are known as vibrational spectroscopy techniques

Question 2

what do IR and Raman spec provide

Answer 2

information regarding the presence or absence of functional groups (i.e. the structure of the molecule)

Question 3

where does IR radiation lie

Answer 3

between the visible and microwave regions of the electromagnetic spectrum

Question 4

IR radiation wavelengths and frequencies

Answer 4

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.

Question 5

what 3 things is the IR region divided into

Answer 5

• Near IR
• Mid IR
• Far IR

Question 6

most forensic science work is done in what IR region

Answer 6

Mid-IR

Question 7

obtaining data in Far-IR region

Answer 7

more difficult (Raman spectroscopy is better in this respect)

Question 8

see powerpoint for

Answer 8

wavenumber and frequency of IR regions

Question 9

what is wavenumber the reciprocal of

Answer 9

wavelength

Question 10

see powerpoint for

Answer 10

wavenumber/wavelength the equation

Question 11

what is the unit for wavenumber

Answer 11

cm-1

Question 12

what 4 regions can Mid-IR be divided into

Answer 12

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.

Question 13

what do the position, size and shape of the peaks gives information on what

Answer 13

the type of chemical bond present (i.e. chemical group) in the sample.

Question 14

basis of IR spec

Answer 14

lies in the vibration of the atoms within a molecule, as all chemical bonds naturally vibrate (elastic)

Question 15

in order for the chemical bonds to absorb IR radiation

Answer 15

the frequency of the IR radiation must match the frequency of the bonds natural vibration

Question 16

what does absorption of IR radiation stimulate

Answer 16

molecular vibrations

Question 17

what does radiation in the M-IR region cause

Answer 17

stretching and bending vibrations of the bonds in most covalent bonds.

Question 18

see powerpoint for

Answer 18

energy, planck’s constant, speed of light equation

Question 19

what are the 6 modes of vibration

Answer 19

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)

Question 20

what are vibrations known as

Answer 20

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.

Question 21

how can a transition to a higher state occur

Answer 21

only if the incident photon is directly comparable in frequency (energy) to the energy difference between the two states

Question 22

the loss of radiation corresponds to what

Answer 22

transitions from ground to higher excited states

Question 23

what must there be to absorb IR radiation

Answer 23

change in a bonds dipole moment.

Question 24

when does a dipole moment occur

Answer 24

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

Question 25

see powerpoint for

Answer 25

strong and weak spectrums

Question 26

see powerpoint for

Answer 26

examples of spectra

Question 27

what two types of infra-red instrument are commonly used

Answer 27

fourier-transform (FTIR) spectrometers

filter photometers (dispersive), most modern instrument

Question 28

fourier-transform instruments

Answer 28

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

Question 29

see powerpoint for

Answer 29

FT IR Spectrophotometer set up

Question 30

FT IR Spectrophotometer source

Answer 30

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

Question 31

FTIR advantages

Answer 31

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.

Question 32

what do Filter Photometer instruments transmit

Answer 32

very narrow bands of radiation at a selected wavelength.

do not produce whole spectrum but can be used for quantitative analysis

Question 33

what is a filer photometer instrument

Answer 33

• Scanning instrument 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

Question 34

how is the IR spectrum obtained in filter photometers

Answer 34

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

Question 35

running an IR spectrum

Answer 35

The sample should be pure (i.e. single substance)

Choose the appropriate method of sample preparation.

Ensure the sample is dry, solvents are transparent, solids uniformly ground etc.

The IR spectrophotometer has been calibrated with a standard (polystyrene film).

The spectrum obtained is well resolved and of adequate intensity.

Question 36

IR and UV/VIS

Answer 36

• 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.

Question 37

standard operating procedures

Answer 37

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.

Question 38

gases

Answer 38

• or low boiling liquids may be obtained by expansion of the sample into an evacuated cell.

Question 39

typical gas cell length

Answer 39

10cm

Modern technology now allows gas cell lengths to be longer by using coiled cells

Question 40

liquids may be examined

Answer 40

neat or in solution

Question 41

liquids

Answer 41

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.

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

Question 42

non-aqueous solutions

Answer 42

can also be run as thin films.

Aqueous and very weak solutions 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.

Question 43

how can the spectrum of a solid be determined

Answer 43

by producing a Mull or potassium bromide disc.

Question 44

how are mulls prepared

Answer 44

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 2um to avoid excessive scattering of radiation.

Question 45

how is the mull examined

Answer 45

The mull is examined as a thin film between two NaCl/KBr plates

Question 46

common mulling agents

Answer 46

Common Mulling agents include Nujul (high boiling petroleum oil) and Fluorolube , which is a polymer containing fluorine instead of hydrogen

Question 47

a potassium bromide disc is prepared by

Answer 47

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

Question 48

KBr disc holders

Answer 48

KBr discs should be very thin, transparent and fragile.

They must be handled with great care and the minimum of contact 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.

Question 49

what two transmission methods can polymer samples (plastics) be examined by

Answer 49

Films of less than 10um can be examined in film form using special holders.

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.

Question 50

what is Attenuated Total Reflectance (ATR-FTIR)

Answer 50

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).

Question 51

what is ATR-FTIR used to analyse

Answer 51

numerous forensic samples such as: drugs of abuse, explosives, fibres, inks, paint, body fluids and fingermarks

Question 52

modern IR spectrophotometers

Answer 52

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.

Question 53

how does ATR work

Answer 53

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.

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.

Question 54

ATR crystals

Answer 54

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.

Question 55

coupling of IR microscopy with FTIR spectrophotometer

Answer 55

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

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.

Question 56

give 3 applications of Infra-red spectroscopy

Answer 56

qualitative analysis

counterfeit drugs

quantitative infra-red spectroscopy

Question 57

qualitative analysis - application

Answer 57

• 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).

Question 58

counterfeit drugs - application

Answer 58

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. It is estimated that global sales from counterfeit drugs will reach $75million in 2010. 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

Question 59

quantitative infra-red spec - application

Answer 59

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.