1. For 3 rd year students- Biophysics Department – Faculty of Science – Cairo University Prepared by: Dr. Heba Mohamed Fahmy and Dr. Alaa Mamdouh

2.   FTIR is an Acronym for Fourier-Transform Infrared Spectroscopy.  It may also be called infrared or IR spectroscopy.  FTIR is a chemically-specific analysis technique. It can be used to identify chemical compounds, and substituent groups. What is FTIR

3.  What is infrared? Infrared light occurs between 0.7 and 1000 µm. in the electromagnetic spectrum, between the visible and microwave regions.

4.  Why does absorption occur? FTIR spectroscopy is an absorption technique. It is a kind of vibrational spectroscopy. A diatomic molecule vibrates like this:

5.  Why does absorption occur? When a quanta of infrared light interacts with the molecule, it may absorb the energy and vibrate faster This is the basis of FTIR spectroscopy

6.  Why does absorption occur? The absorption leads to an increase in vibrational energy level: This leads us to the selection rules for infrared absorption: NB: Quantised vibrational energy levels!

7.   1. Infrared absorption only occurs when infrared radiation interacts with a molecule undergoing a change in dipole.  2. Infrared absorption only occurs when the incoming infrared photon has sufficient energy for the transition to the next allowed vibrational energy state.  If these two rules are not met, no absorption can occur. When can absorption occur?

8.   Rule 1 has important implications: N2 (nitrogen) has no infrared spectrum, as there is no dipole change. However, CO (carbon monoxide ) does have one When can absorption occur?

9.  How does a spectrometer work? As infrared is an absorption technique, infrared passes through the sample, and is then detected. A dispersive ( traditional ) spectrometer works as follows:

10.   IR source: Usually a glowing ceramic element i.e. it is very hot - do not touch!  Sample: often there are two sample compartments  Dispersive element: usually a prism(as shown) or a grating.  IR detector: these are usually semiconductor detectors. Some are room - temperature, some are liquid nitrogen cooled.  If you need to cool the detector, fill with liquid N2, then leave it to reach thermal equilibrium for 20 minutes. Otherwise signal sizes can vary quite a lot. How does a spectrometer work?

11.  How does an FTIR spectrometer work? FTIR spectrometers are very different to dispersive spectrometers - and more complicated! All FTIR spectrometers are based on the Michelson Interferometer: How does the Michelson interferometer work? As the name implies, it relies on interference of the infrared waves. At the interferometer, the light strikes a beam splitter which passes 50% of the light to mirror 1 and 50% of the light to mirror 2. Mirror 1 is a moving mirror, which oscillates back and forth, whereas mirror 2 is stationary. On reflection from these mirrors, the beam splitter recombines the light which is then guided on towards the sample. This light is absorbed by the sample, and passes to the detector as usual.

12.   Unlike the dispersive spectrometer, all frequencies from the source are measured at one time.  Because of the interference between the light reflecting from the stationary mirror, and that from the moving mirror, the energy observed by the detector shows an unusual pattern over time. This is called an interferogram How does the Michelson interferometer work?

13.   Because of the complicated nature of the signal, it must be converted back to a spectrum by a computer. This is the FOURIER TRANSFORM.  On most instruments, you may observe the raw data, i.e the interferogram. This can be useful for troubleshooting. For example, the peak - to peak distance ( in volts ) is a measure of the quality of spectrum you will get.  The FTIR is a single beam spectrometer. That is, it does not automatically measure a background for you. You should measure a background spectrum before your sample. How does the FTIR work?

14.   It can identify unknown materials.  It can determine the quality or consistency of a sample.  It can determine the amount of components in a mixture. What information can FT-IR provides?

15.   1- Speed  2-Sensitivity  3- Mechanical Simplicity  4- Internally Calibrated Some of the major advantages of FT-IR over the dispersive technique include: