Infrared spectroscopy Flashcards
Vibrational spectroscopy
IR (and Raman) spectroscopy are known as vibrational spectroscopy techniques
what do IR and Raman spec provide
information regarding the presence or absence of functional groups (i.e. the structure of the molecule)
where does IR radiation lie
between the visible and microwave regions of the electromagnetic spectrum
IR radiation wavelengths and frequencies
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.
what 3 things is the IR region divided into
- Near IR
- Mid IR
- Far IR
most forensic science work is done in what IR region
Mid-IR
obtaining data in Far-IR region
more difficult (Raman spectroscopy is better in this respect)
see powerpoint for
wavenumber and frequency of IR regions
what is wavenumber the reciprocal of
wavelength
see powerpoint for
wavenumber/wavelength the equation
what is the unit for wavenumber
cm-1
what 4 regions can Mid-IR be divided into
4000-2500cm-1 –> high energy vibrations of single bonds (C-H , O-H , N-H)
2500-2000cm-1 –> vibrations of triple bonds (CC , CN)
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.
what do the position, size and shape of the peaks gives information on what
the type of chemical bond present (i.e. chemical group) in the sample.
basis of IR spec
lies in the vibration of the atoms within a molecule, as all chemical bonds naturally vibrate (elastic)
in order for the chemical bonds to absorb IR radiation
the frequency of the IR radiation must match the frequency of the bonds natural vibration
what does absorption of IR radiation stimulate
molecular vibrations
what does radiation in the M-IR region cause
stretching and bending vibrations of the bonds in most covalent bonds.
see powerpoint for
energy, planck’s constant, speed of light equation
what are the 6 modes of vibration
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)
what are vibrations 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.
how can a transition to a higher state 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 what
transitions from ground to higher excited states
what must there be to absorb IR radiation
change in a bonds dipole moment.
when does a dipole moment occur
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
see powerpoint for
strong and weak spectrums
see powerpoint for
examples of spectra
what two types of infra-red instrument are commonly used
fourier-transform (FTIR) spectrometers
filter photometers (dispersive), most modern instrument
fourier-transform instruments
• only one beam of radiation is used and this means that all the required frequencies pass through the instrument simultaneously (non-dispersive)
see powerpoint for
FT IR Spectrophotometer set up
FT IR Spectrophotometer source
including tungsten-halogen lamps, or an electrically heated filament or high pressure mercury lamp
FTIR advantages
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.
what do Filter Photometer instruments transmit
very narrow bands of radiation at a selected wavelength.
do not produce whole spectrum but can be used for quantitative analysis
what is a filer photometer instrument
• 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
how is the IR spectrum obtained in filter photometers
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
running an IR spectrum
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.
IR and UV/VIS
• 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.
standard operating procedures
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.
gases
• or low boiling liquids may be obtained by expansion of the sample into an evacuated cell.
typical gas cell length
10cm
Modern technology now allows gas cell lengths to be longer by using coiled cells
liquids may be examined
neat or in solution
liquids
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
non-aqueous solutions
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.
how can the spectrum of a solid be determined
by producing a Mull or potassium bromide disc.
how are mulls prepared
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.
how is the mull examined
The mull is examined as a thin film between two NaCl/KBr plates
common mulling agents
Common Mulling agents include Nujul (high boiling petroleum oil) and Fluorolube , which is a polymer containing fluorine instead of hydrogen
a potassium bromide disc is prepared by
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
KBr disc holders
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.
what two transmission methods can polymer samples (plastics) be examined by
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.
what is Attenuated Total Reflectance (ATR-FTIR)
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).
what is ATR-FTIR used to analyse
numerous forensic samples such as: drugs of abuse, explosives, fibres, inks, paint, body fluids and fingermarks
modern IR spectrophotometers
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.
how does ATR work
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.
ATR crystals
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.
coupling of IR microscopy with FTIR spectrophotometer
allows the infra-red analysis of samples as small as 10m 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.
give 3 applications of Infra-red spectroscopy
qualitative analysis
counterfeit drugs
quantitative infra-red spectroscopy
qualitative analysis - application
- 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).
counterfeit drugs - application
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
quantitative infra-red spec - application
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.
