Lab 3: FTIR
1. Purpose
Analyze the organic compositions and create “fingerprints” of polymeric materials using FTIR.
2. Theory
Fourier Transform Infrared (FTIR) spectroscopy is an analytical method that utilizes absorption of infrared (IR) spectrum radiation, l ~ 740nm – 300mm, by polar molecules to undergo vibrational and rotational transitions. Typical IR measurements are collected in the range of 400 - 4000 cm-1.[1],[2]
Molecules in the ground state configuration (lowest possible energy) have atoms in a “fixed position” with respect to each other. However, the distances and bond angles between two or more atoms within the same molecule may change from time to time, corresponding to vibrational or rotational movements within the molecule, Fig. 3.1. During these vibrations and rotations, molecules possess higher kinetic energies, and exist in excited states. These excited states are reached by absorbing discrete energies matching the excitation energy necessary to jump between the ground and excited states. The excited states available are determined by the quantum mechanical properties of each molecule, which are beyond the scope of this course. Suffice it to say that the electron configuration, type of chemical bond, and the masses of the individual atoms that form the molecule dictate the energy states available.
Only polar molecules can be excited by a photon having energy in the IR region. The specific chemical structure of each molecule results in unique vibrations and rotations of the sample. Thus each molecular species experiences a characteristic set of bending and stretching modes observed in the IR spectrum. After excitation, the molecule returns to its ground state and the rotational and vibrational energy is converted into heat.[3] Vibrational transitions occur at higher energies and often occur with rotational transitions.1
Water and carbon dioxide molecules have strong dipole moments and are a common sources of noise and major absorption peaks in an FTIR spectrum. To remove these absorptions from the spectrum, a non-polar gas should be used to purge the system. A typical gas used for this purpose is nitrogen, N2.
FTIR uses a Michelson Interferometer equipped with a monochromator and collimator mirror to select and align a single wavelength of light that is incident on the sample (See Figure 3.2 below).
Figure 3.2: The Michelson Interferometer[4]
IR radiation is split by a half-silvered mirror that is at a 45° angle to the incident beam. One beam, with half the intensity, is incident on a stationary mirror at a fixed distance from the splitter and subsequently reflected back to the splitter. The other beam is incident on a movable mirror which is at a varying distance from the splitter; this beam too is reflected back through the beam splitter. The two reflected beams interfere constructively (if the difference in path lengths is an exact multiple of the wavelength) or destructively (if the difference in path lengths is half the wavelength) when they recombine at the splitters.
The intensity amplitude at the detector alternates between constructive and destructive interference, as the movable mirror travels away from the splitter at a constant rate, resulting in the creation of a simple sine wave interference pattern. Each peak indicates that the mirror has traveled one half the wavelength of the beam wavelength. The moving mirror is controlled by an electromagnetic drive and rides along an air bearing. The position of the mirror is monitored using a laser beam shining on a corner of the mirror.
The system used in the Lab is FTIR-8300 from Shimadzu. The optical system is illustrated in Fig 3.3. The IR beam from Source 1 is reflected by Collimator Mirror 4 into the interferometer at a 30° incident angle, where it is then divided by Beam Splitter 5. The resultant beams are incident upon Moving Mirror 7 and Fixed Mirror 8. Both beams are reflected back to and through the beam splitter and proceed to Collecting Mirror 9. Mirror is equipped with an automatic alignment mechanism to optimize interference efficiency. From the collecting mirror, the parallel infrared beams are convergently focused down onto the center of the sample compartment. Collecting Mirror 11 gathers the diverging beams that passed through the sample and reflects it to Detector 12 to form the interferogram. Beam spot size at the center of the sample compartment is approximately 9.1 × 9.1 mm.
Fig 3.3 FTIR-8300 optical system. 1. Light Source, 2. Spherical Mirror, 3. Aperture, 4, Collimator Mirror, 5. Beam Splitter, 6. Compensator, 7. Moving Mirror, 8. Fixed Mirror, 9. Collecting Mirror, 10. Image Position, 11. Collecting Mirror, 12. Detector, 13. Laser, 14. Laser Detector.
3. Experimental Procedure
CAUTION: YOU MUST WEAR RUBBER GLOVES WHILE HANDLING THE POLYMER POWDER.
(1) Turn on the instrument using switch on lower right front panel. The system is controlled with IRSolution software. If the software is not running already, double click the red “FTIR” icon on the desktop to open the software. The system should be connected to the software automatically and a IRSolution –[Measure] window will appear.
(2) From the Measurement pull down menu, select Easy Scan.
(3) Parameter Settings: Choose transmittance (%T) for measurement mode. Set resolution at 4 cm-1, scan range between 400 and 4600 cm-1, and number of scan at 20. The whole scan process will take approximately 1 minute. At “Data file”, type in your sample name and short description in Text box. NOTE: sample name must be input before carrying out the experiment.
(4) Place blank KBr disk onto the sample holder as illustrated in Fig 3.4, and click “yes” when asked to execute the background measurement (BKG) to start background scan. A background spectrum will appear on the screen. Collect the background measurement at least once prior to collecting the data of each of your first samples.
Fig 3.4 Inner chamber of FTIR-8000 and the sample holder
(5) Your sample powders, Polystyrene (PS; made in Lab 1), Polyethylene glycol (PEG), Polyvinyl chloride (PVC) and Poly(methyl methacrylate) (PMMA) will be dissolved in THF (concentration around 10 to 50 mg/ml) and found in marked vials. Apply one drop of PS solution onto the surface of the KBr glass, allowing the solvent to evaporate completely. Apply another drop if not enough material is deposited. Repeat as necessary until a noticeable film is formed on the KBr disk. Place the KBr sample into sample holder.
(6) Place a sample in holder, one at a time, and collect data. The sample spectrum will appear.
(7) Click 'OK" when asked if you are all right.
(8) Print file to XPS document writer
Ø Save file to: Local Drive/Program Files/Shimadzu/IRsolution/data/MSE 453,553
§ Select a name that indicates polymer sample and your group designation
(9) Export data as ASCII .txt file
Ø Open File drop down menu and select Export
Ø Save in: Local Drive/Program Files/Shimadzu/IRsolution/data/MSE 453,553
(10) Save .txt file onto thumb drive or e-mail file to yourself for
graphing and analysis
(11) Wash out the sample using THF. Use kimwipe to gentlely wipe out the solution and clean the sample area. BE CAREFUL, TRY NOT TO SCRATCH THE KBr GLASS.
(12) Repeat procedures 5-11 for other polymer samples, that is, polyethylene glycol (PEG), polyvinyl chloride (PVC), and poly(methyl methacrylate) (PMMA).
4. Assignments
(1) Look into the spectra in the literature and compare them with those you obtain. Are there any major differences? If so, explain some sources for these differences.
(2) Assign major peaks in the spectra, that is, identify the vibration modes
(3) The FTIR spectra (from 1500 cm-1 to 1900 cm-1) of bleached softwood cellulose paper with 99.5% celluose content were obtained under following conditions. Explain the reasons for the observed spectra difference.
a) Under normal condition (i.e., in air)
b) Vacuumed to 10-3 torr for 15 min
c) Heated in dry air at 100 °C for 30 min
d) Sample c) after being put into a chamber filled with deuterated water vapor for 24 h.
Hint: look into the chemical structure of cellulose in the textbook, and see if there exits specific interaction (secondary force).
1
[1] “Infrared Spectroscopy”, http://chem.unl.edu/powers/421-821/Lectures/421-821-chapter-16-17.ppt
[2] The following reference is available in the library for interpreting FTIR data:
Aldrich Library of FT-IR Spectra, by Pouchert
[3] “FT-IR spectroscopy: FTIR (Fourier Transform InfrarRed) measuring principle and technical details”, http://www.uow.edu.au/~arndt/measprin.htm
[4] “Infrared Absorption Spectroscopy: Instrumentation”, http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec3.htm