FTIR: Fourier Transform Infrared Spectroscopy Jack Nathaniel

Applications FTIR is used to identify unknown molecules (usually organic). It is now used where a dispersive spectrometer was formerly used. The advantage of an FTIR over dispersive spectroscopy is that an FTIR is much faster.

Sample Properties Samples are usually organic compounds, which are usually colorless and don’t absorb light in the visible spectrum, and the bonds are usually too weak for UV spectroscopy. The best way to get a spectrum to identify these compounds is IR spectroscopy.

Instrument Components

Data After collecting the interferogram and transforming it, the results are absorption spectra like that below. We look at the location, size, and shape of the valleys in the infrared spectrum. Using this, we figure out the identity of the molecule. The wavenumber (cm^-1) is the unit usually used here, is equal to

Sample Preparation There are several ways to use an FTIR. You can sample liquids, solids in solution, and pellets. The samples are typically held between KBr or NaCl plates. These specific salts are used because they don’t absorb IR light in the frequencies used. I found all this info on a website should i credit them yes dad

Sampling solids in solution Add several mg of your compound on the plates and add a drop of solvent. Run a spectrum of this. Alternatively, make a solution in a test tube and dispense it onto the plates with a pipette. Run a spectrum. Remember to run a spectrum of your solvent too to prevent problems. Also, make sure your solvent doesn’t have water in it unless you want to dissolve your KBr plates.

Solids as Nujol mulls Grind up the solid and place 5 - 10 mg on the face of a KBr plate. A small drop of mineral oil is added and a second plate is placed on top. Gently rubbing the 2 plates will even distribute the powder and remove and air bubbles from within.

Sampling pellets Put some KBr in the oven. Heat it to around 150 deg C. Working quickly, transfer it out, put it in a mortar, and then grind it up. Add around 1:50 mass ratio of Sample to KBr. Grind the sample +KBr mixture. Use a hydraulic press to squish them into pellets. Remove the pellets, tape them into place in the holder, and run the spectrum.

Sampling Liquids A small drop of the liquid is placed between one of the KBr plates. The other one is put on top and turned to smear the liquid out in between the plates. They are then placed in the holder and a spectrum is run. If it is bad, clean the plates with a solvent that will clean the sample off and then ethanol. Then wipe off the plates and redo it.

Theory The frequency of the wave absorbed by the molecule matches the frequency of the vibrations of the bonds of the atoms. The vibrational frequency is related to the strength of the bonds as well as the mass of the atoms at either end of the bond. Through changes in frequency with different structural arrangements, the identity of the molecule can be figured out. asymmetric symmetric scissoring twisting wagging rocking stretching stretching Do the molecule dance

Works Sighted "FT‐IR Sample Preparation - NIU - Chemistry Analytical Lab." FT‐IR Sample Preparation - NIU - Chemistry Analytical Lab. N.p., n.d. Web. 11 May 2016. Pavia, Donald L., Gary M. Lampman, George S. Kriz, and James R. Vyvyan.Introduction to Spectroscopy. 4th ed. Belmont: Brooks/Cole, 2009. Print.