FTIR Gas Analysis: An On-Line Primer

What is FTIR?

Infrared (IR) spectroscopy is a chemical analytical technique, which measures the infrared intensity versus wavelength of light. In infrared spectroscopy, infrared radiation is passed through a sample resulting in a spectrum, which represents its molecular absorption and transmission.

FT-IR stands for Fourier Transform InfraRed, the preferred method of infrared spectroscopy. It was developed in order to measure all of the infrared frequencies simultaneously.

FT-IR can be used to identify unknown materials, amount and quality of a sample and the amount of components in a mixture. An FT-IR spectrometer produces an output signal from an optical device called an interferometer. The interferometer splits and recombines a beam of light such that the recombined beam produces a wavelength-dependent interference pattern.

Michelson Interferometer
Michelson Interferometer

The Michelson Interferometer

The essential component of the FTIR, the Michelson Interferometer, consists of a perpendicular mirrors with a beam splitter between them. When one of the two mirrors is translated, all optical frequencies are converted into cosine waves of intensity; the result is the complex time variation of intensity called the interferogram.

An interferogram is the sum of all cosine waves for all optical frequencies. The spectrum is calculated from the interferogram by computing its cosine Fourier transform. This in effect, decodes the individual frequencies in the spectral analysis.

Once the interferogram is Fourier transformed, a single beam infrared absorption spectrum is collected. In order to produce a sample spectrum, a single beam sample is ratioed over a reference background (like an empty cell). The resultant trace, or sample spectrum, is a plot of absorbance versus wavenumber. It is in the spectral analysis of the absorbance spectrum where qualitative and quantitative analysis is derived.

Spectral Signal Processing