Infrared Spectroscopy

Infrared Spectroscopy

What is spectroscopy? • The study of the interaction between radiation and matter as a function of wavelength (λ). • Historically, spectroscopy referred to the use of visible light dispersed according to its wavelength. • Later, the concept was expanded greatly to comprise any measurement of a quantity as function of either wavelength or frequency. • A plot of the response as a function of wavelength — or more commonly frequency — is referred to as a spectrum

Spectrometry & Spectrometer • Spectrometry - the spectroscopic technique used to assess the concentration or amount of a given species. • Spectrometer - instrument that performs such measurements. (a.k.a spectrograph) • Spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.

Types of spectroscopy • Electromagnetic spectroscopy involves interactions of matter with electromagnetic radiation, such as light. • Electron spectroscopy involves interactions with electron beams. • Mass spectrometry involves the interaction of charged species with magnetic and/or electric fields • Acoustic spectroscopy involves the frequency of sound.

Measurement process • Most spectroscopic methods are differentiated as either atomic or molecular, based on whether or not they apply to atoms or molecules. • They also can be classified on the nature of their interaction: • Absorption spectroscopy • Emission spectroscopy • Scattering spectroscopy

Measurement process • Absorption spectroscopy uses the range of the electromagnetic spectra in which a substance absorbs. • (E.g. Atomic absorption spectroscopy, Infrared spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy) • Emission spectroscopy uses the range of electromagnetic spectra in which a substance radiates (emits). • Scattering spectroscopy measures the amount of light that a substance scatters at certain wavelengths, incident angles, and polarization angles. One of the most useful applications of light scattering spectroscopy is Raman spectroscopy.

Infrared Spectroscopy • Infrared spectroscopy - a technique used to identify chemical compounds based on how infrared radiation is absorbed by the compounds' chemical bonds. • The most common technique used is absorption spectroscopy. • Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels.

Infrared Absorption For a molecule to show infrared absorpsions it must possess a specific feature: an electric dipole moment of the molecule must change during the vibration. A dipole moment, µ is defined as the charge value multiplied by the separation distance between positive and negative charges. µ = qd (C.m)

Infrared Absorption The infrared spectrum of a sample is collected by passing a beam of infrared light through the sample. Examination of the transmitted light reveals how much energy was absorbed at each wavelength. From this, a transmittance or absorbance spectrum can be produced, showing at which IR wavelengths the sample absorbs. Analysis of these absorption characteristics reveals details about the molecular structure of the sample.

Infrared Absorption Simple spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures.

Modes of Vibration The interactions of infrared radiation with matter may be understood in terms of changes in molecular dipoles associated with vibrations. Vibrations can involve either change in bond length (stretching) or bond angle (bending). Some bonds can stretch in-phase (symmetric stretching) or out-of-phase (asymmetric stretching) Bending vibrations can be in-plane (scissoring, rocking) or out-of-plane (wagging, twisting) bending vibrations.

Vibrations • Atoms in the molecule are subjected to number of vibrations.

Modes of Vibration • The degree of vibrational freedom for polyatomic molecules containing (N) atoms is given by 3N – 5 (linear molecules) and 3N – 6 (non-linear molecules). • Two other concepts are also used to explain the frequency of vibrational modes : • the stiffness of the bond and • the masses of the atoms at each end of the bond

Modes of Vibration The stiffness of the bond can be characterized by proportionality constant termed the force constant, k. The reduced mass, µ provides a useful way of simplifying our calculations by combining two-bodies problem as one-body. (1/µ) = m1m2/(m1+m2) The relation between force constant, the reduced mass and the frequency of absorption is: υ= (1/2π)(k/µ) or υ= (1/2πc)(k/µ)

Theoretical principles • In infrared spectroscopy wavelength is measured in “wave numbers” which have the units cm-1 • IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. • For a molecule to absorb IR, the radiation must interact with the electric field caused by changing dipole moment

Calibration This device is precisely calibrated by using polystyrene calibration film. Size of peaks  amount of material www.internationalcrystal.net www.chemistry.oregonstate.edu/courses/ch361-464/ch362/irinstrs.htm - 9

Background Spectrum • This background spectrum can be used to compare with the sample measurement to determine % transmittance • Peaks in this region are characteristic of specific kinds of bonds, thus can be used to identify whether a specific functional group is present.

Example of C-H(functional group) spectra • Peaks in the region of (3000- 3100) cm-1 indicates that sp2 hybridized C-H bond are present in the sample • And peaks in range of (2800-3000)cm-1 indicates that sp3 hybridized C-H bond are present in a sample

Acids: 1650-1700cm-1Esters: 1740-1750cm-1Aldehydes: 1720-1750cm-1Ketones: 1720-1750 cm-1Amides:1650-1715 cm-1

FTIR Spectrum of Sample (98% N,N-Dimethylamphetamine Hydrochloride)

What information can FT-IR provide • It can determine the amount of components in a mixture • It can determine the quality or how consistent a sample is • It can identify unknown materials

How FTIR works? • Source: Infrared energy is emitted from a glowing black-body source. Ends at the Detector • Interferometer: beam enters the interferometer where the “spectral encoding” takes place • Interferogram signal then exits the interferometer • Beamsplittertakes the incoming beam and divides it into two optical beams • Sample: beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample • Detector: The beam finally passes to the detector for final measurement • Computer: measured signal is digitized and sent to the computer where the Fourier transformation takes place • Movingmirror in the interferometer is the only moving part of the instrument • Fixedmirror

Qualitative -Because each different material is a unique combination of atoms, no two compounds produce the exact same infrared spectrum -the size of the peaks in the spectrum is a direct indication of the amount of material present Quantitative and Comparative -Since it is sensitive, accurate, and has software algorithms, the quantitative methods can be developed and calibrated easily to perform various analysis -FT-IR is comparative due to the background spectrum that is compared to the sample Is FT-IR Qualitative, Quantitative, or Comparative?

Speed Sensitivity Mechanical simplicity Internally calibrated Destructive Too sensitive that it would detect the smallest contaminant Advantages/disadvantages

Forensic Lab use: • A Forensic Scientist would use FT-IR to identify chemicals in different types of samples: • Paints • Polymers • Coatings • Rugs • Contaminants • Explosive residues

Sample Preparation - Gaseous Gaseous samples require little preparation beyond purification, but a sample cell with a long pathlength (typically 5-10 cm) is normally needed. The walls are of glass or brass. Longer pathlengths are necessary to analyse complex mixtures and trace impurities. The cell pathlength can be measured by the method of counting interference fringes. L = n/2(υ1 – υ2)

Sample Preparation - Liquids Liquid samples – use solution cells. Two types of solution cells – permanent and demountable. Permanent cell – The pathlength need to be calibrated regularly if quatitative work is to be undertaken. Diffucult to clean and can be damaged by water. Demountable cell – Easy to maintain as it can be readily dismantled and cleaned. The windows can be repolished, a new spacer supplied and the cell reassembled.

Sample Preparation - Solids Solid samples can be prepared in four major ways. Crush the sample with a mulling agent in a marble or agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured. Grind a quantity of the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass.

Sample Preparation - Solids • The third technique is the Cast Film technique, which is used mainly for polymeric materials. The sample is first dissolved in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. • The final method is to use microtomy to cut a thin (20-100 micron) film from a solid sample. This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.

Typical Method • A beam of infrared light is produced and split into two separate beams. • One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. • The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. • The two signals are then compared and a printout is obtained. • A reference is used for two reasons: • To prevent fluctuations in the output of the source affecting the data. • To allow the effects of the solvent to be cancelled out (the reference is usually a pure form of the solvent the sample is in).

Typical Method

Uses and Applications Infrared spectroscopy is widely used in both research and industry . It is applied for detection and identification of different elements/compounds in solving problems in the fields of forensics, medicine, oil industry, atmospheric chemistry, polymer degradation, pharmacology, etc. Among the more common spectroscopic methods used for analysis is FTIR spectroscopy, where chemical bonds can be detected through their characteristic infra-red absorption frequencies or wavelengths.

Uses and Applications The instruments are now small, and can be transported, even for use in field trials. With increasing technology in computer filtering and manipulation of the results, samples in solution can now be measured accurately (water produces a broad absorbance across the range of interest, and thus renders the spectra unreadable without this computer treatment). Some machines will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage.

Analysis of Polymers Many polymer degradation mechanisms such as UV degradation and oxidation, amongst many other failure modes, can be investigated using infra-red spectroscopy.

UV Radiation Many polymers are attacked by UV radiation at vulnerable points in their chain structures. Polypropylene suffers severe cracking in sunlight unless anti-oxidants are added. The point of attack occurs at the tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage. Polyethylene is also susceptible to UV degradation, especially those variants which are branched polymers such as LDPE. The branch points are tertiary carbon atoms, so polymer degradation starts there and results in embrittlement.

UV Radiation IR spectrum showing carbonyl absorption due to UV degradation of polyethylene

Oxidation Polymers are susceptible to attack by atmospheric oxygen, especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. Oxidation tends to start at tertiary carbon atoms because free radicals here at more stable, so last longer and are attacked by oxygen. The carbonyl group can be further oxidised to break the chain, so weakening the material by lowering the molecular weight, and cracks start to grow in the regions affected.

Oxidation IR spectrum showing carbonyl absorption due to oxidative degradation of polypropylene

Infrared Spectroscopy