Fourier Transform Infrared Spectroscopy System (FTIR)

 

Infrared spectroscopy reveals information about molecular vibrations that cause a change in the dipole of moment of molecules. It offers a fingerprint of the chemical bonds present within materials.  FTIR is a very powerful analytical tool for examining both inorganic and organic materials. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Fig.1 Schematic diagram of a generic Michelson Interferometer used in FTIR.

 

FTIR spectrometry uses the technique of Michelson interferometry, as illustrated in Fig. 1.  A beam of radiation from the source, is focused on a beam splitter, where half the beam is reflected to a fixed mirror and the other half of the beam is transmitted to a moving mirror which reflects the beam back to the beam splitter from where it travels, recombined with the original half beam, to the detector.  The IR intensity variation with optical path difference (interferogram) is the Fourier transform of the (broadband) incident radiation. The IR absorption spectrum can be obtained by measuring an interferogram with and without a sample in the beam and transforming the interferograms into spectra.

Fig. 2 The MIR 8000TM FTIR Spectrometer System at the Optical Characterisation Facility.

 

Fig.2 illustrates how the MIR 8000TM works.  The scanner modulates the radiation from the source or from sample; the A/D board digitise  the analog signals from the detection system and sends them to a computer.  The data acquisition and scanner control system control the scanner as well as acquire and manipulate data.

 

The scanner is the heart of MIR 8000TM.  Two corner cubes and a retro-reflector  make the system immune to tilt and shift with a minimum of realignment required, as shown in Fig. 3.  Moreover, the Scanning Mirror Control System of MIR 8000TM is based on two in-quadrature HeNe laser interferograms that provide position and direction of information.  Fine tuning is available to position the ZPD, zero path difference, exactly in the centre of the scan.

 

Fig. 3 Oriel’s optical layout includes corner cubes and a retroreflector mounted to the beam splitter.

 

 

Fig.4 Modular IR Fourier Spectrometer 8000.

 

Modular IR Fourier Spectrometer 8000 in our Facility provides FTIR transmission and reflection spectra in the range of 1.7-28 mm.  Normal FTIR suits liquid, and solid bulk materials and requires some sample preparation. We also have the Attenuated Total Reflection  (ATR) accessory. The attenuated total reflection (ATR) is a versatile and powerful technique for infrared sampling.  Materials are either too thick or too strongly absorbing to be analysed by transmission spectroscopy can be routinely analysed using ATR spectroscopy.  ATR is also useful for layered sample when only the surface of the material needs to be analysed.  The 2 mm spot size can produce high quality spectra from a small mount of samples. 

 

 

 

 

 

 

 

 

 

 


Fig. 5 The diagram for multiple bounce attenuated total reflection.

 

As show in Fig. 5, in attenuated total reflection, light is reflected internally at the critical angle, when the light is travelling from a high refractive index material to a lower refractive index material.  However, at each internal reflection, the infrared beam (IR) actually penetrates a short distance (~ 1 mm) from the surface of the crystal into sample placed on top of it.  This unique physical phenomenon enables us to obtain infrared spectra of samples placed in contact with the crystal.  Fig. 6 shows the attenuated total reflection accessory used in our laboratory.

 

Fig. 6 Attenuated total internal reflection setup.

 

 

Specifications  for MIR 8000TM in the Optical Characterisation Facility: