07.10.2010

Sandrine Kouao Céline Monnard Group B12

Analytical Chemistry

FT-IR/ATR Spectroscopy
Assistant : Peiyu Ge

Introduction
IR spectroscopy is used to determine functional groups in molecules. Infrared radiations of specific frequencies are absorbed depending on the type of bonds vibrations, which are characteristic of the chemical functions. This technique can therefore allow us to identify compounds by comparison of spectra with a data base.

Theoretical part
The energy of a radiation is given by: E = hν = hc/λ = hc , with the wavenumber defined by: =1/ λ [cm-1]. The infrared area goes from 4000 to 400 cm-1 and this corresponds approximately to the energy of vibrations in molecules. Vibrational modes with a change in dipole moment absorb IR radiations, there are several different modes: stretching, bending, scissoring, rocking, wagging and twisting. Therefore each absorption peak on the spectrum is characteristic of the type of the bond. The intensity and frequency of absorbed light depend on the mass of the atoms forming the bond, and on the bond strength. With the spectrum it is then possible to identify specific bonds and thus functional groups.

For our analysis we use a FTIR (Fourier transform) spectrometer.

Infrared light goes first through an interferometer, which modulates the wavelengths of the radiation. The beam is then separated; half of it is directed onto a fixed mirror and the other part on a mobile mirror. Afterward, the two beams are recombined and can give constructive or destructive interference depending on the mirror position. After passing through the sample, the signal is recorded as a function of this position and thanks to a reverse Fourier transform, a spectrum can be built with the wavelengths and intensities of absorbed light.

Experimental part
In this experiment we measured the spectra of five samples. We then assign the peaks to functional groups and compare the obtained structures with the molecule