Vibrational Spectroscopy for Pharmaceutical Analysis . Part IV. Fourier Transform Infrared (FT-IR) Spectroscopy Rodolfo J. Romañach, Ph.D. LAB INSTRUMENTS. Spectrum One – Perkin Elmer. Scimitar-Varian . Thermo Nicolet 6700. Tensor 27 – Bruker Optics. ABB - 100. PROCESS INSTRUMENTS.
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Part IV. Fourier Transform Infrared (FT-IR) Spectroscopy
Rodolfo J. Romañach, Ph.D.
Spectrum One – Perkin Elmer
Thermo Nicolet 6700
Tensor 27 – Bruker Optics
ABB - 100
Hamilton Sundstrand RefinIR
Mettler Toledo ReactIR™4000
Fellgett or Multiplex Advantage
Schematic of a Michelson Interferometer. Figure 8, page 87 Chalmers and Dent.
Schematic showing summation of two cosine function interferogram. Chalmers and Dent page 88.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH, 2002, pages 60 – 61, 70
For a polychromatic source, the interferogram represents the summation of all the individual cosine functions corresponding to each of the wavelengths (wavenumbers in the source).
They are only in phase at the centerburst (position of zero path difference.
Problem with CO2 Band.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH, 2002, pages 67-69
Strong CO2 and water bands but the baseline is flat, instrument is working well.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH, 2002, page 40.
The following parameters must be taken into consideration when taking spectra:
1. Apodization & Number of Points
The moving mirror does not travel to plus or minus infinity, so the interferograms is truncated (cut) by a boxcar function.
Schematic of Boxcar Truncation of the interferogram for a single frequency source. Chalmers and Dent figure 11, page 90.
Software will have a command requesting how many points you want to keep away from centerburst. Most of the information is in first 100 points, but most users keep 300 – 500 points. System has ADC with dynamic range of 1016 or better.
Effect of Boxcar Truncation
You might choose double sided interferogram and 300 points, you are specifying the truncation of the spectrum.
There is a tradeoff between eliminating the lobes and having band widening with reduction in intensity.
Effect of Triangular Apodization.