13 FTIR Spectroscopy

Once upon a time, IR absorption spectra were measured analogous to UV-visible absorption spectra. That is, using a dispersive instrument with an IR light source, monochromator, and an IR sensitive photodetector. Such technology is now obsolete, having been replaced by Fourier transform (FT) IR spectrometers.

The Michelson Interferometer and Fourier Transform

FTIR absorption spectrometers have, in sequence, an IR light source (e.g. globar), an interferometer, the sample, and an IR-sensitive detector (e.g. DTGS or MCT).

The key feature of an FTIR spectrometer is a Michelson interferometer, which uses a beam-splitter to divide the light from the IR source into two paths: one path has a fixed mirror and the other path has a mirror that is moved over a distance of several millimeters to centimeters. The light reflected off both mirrors is then recombined. The different wavelengths in the recombined beam of light interfere with one another to produce an interferogram when the moving mirror is scanned over a distance. An interferogram is a plot of light intensity versus the distance the mirror has moved and looks like a beat pattern for interfering waves. A mathematical Fourier transform (FT) operation on the interferogram yields a spectrum of light intensity as a function of wavenumber. Transmittance can then be calculated from measurements of a blank and a sample.

The FTIR design has two critical advantages versus a dispersive instrument. The multiplex or Fellgett advantage arises for the high speed at which spectra may be acquired (all wavenumbers acquired concurrently). Spectra can thus be measured many times and averaged to improve the signal-to-noise ratio. The throughput or Jacquinot Advantage arises from the absence of slits in the instrument, which results in higher light intensities and signal-to-noise ratios that are ten-fold better for FT instruments than for dispersive instruments. Another advantage of FTIR is that spectral resolution is determined by how far the mirror moves, such that resolution < 1 cm^–1 is readily achieved.

Sample Cells

As glass and quartz have strong IR absorption, sample cells for IR spectroscopy tend to made from (or have windows made from) inorganic crystals (e.g. KBr, NaCl, CaF₂, AgBr, ZnSe). Conventional transmission measurements, with light passing through the sample, are common for IR absorption spectroscopy, as are reflectance measurements. These formats may also be coupled to a microscope for imaging, where an FTIR absorption spectrum is obtained for each image pixel.

Another mode of FTIR spectroscopy is attenuated total reflection (ATR). In ATR, light is coupled into a dovetail-prism-shaped crystal (e.g. ZnSe, Ge, diamond) by total internal reflection and the sample is applied to the top face of the crystal. At each reflection point, the light field leaks into the sample as an evanescent wave and can be absorbed. The multiple reflections within the crystal are analogous to a long path length.

Not Quite Quantitative

In principle, the IR absorbance follows the Beer-Lambert Law; however, in practice, it is often challenging to match measurement conditions between sample and blank and obtain a linear relationship between absorbance and concentration. FTIR spectroscopy thus tends to be used either qualitatively or semi-quantitatively. When peak intensities are quantified, they are usually quantified relative to the baseline of the spectrum rather than 100% transmission.

Connections

• FTIR spectrometers, AAS spectrometers (Ch. 11), and UV-visible spectrometers (Ch. 7) all have the general design of a light source at their front end, a photodetector at their back end, and an intermediate sample cell.
• A major difference in the design of a modern IR spectrometer versus AAS spectrometers (Ch. 11) and UV-visible spectrometers (Ch. 7) is the absence of wavelength selection. AAS and UV-visible spectrometers have grating-based dispersive designs, whereas FTIR spectrometers do not!
• Another difference in instrument design for FTIR spectrometers versus AAS and UV-visible spectrometers is the types of light sources (Ch. 3) and photodetectors (Ch. 5), which are matched to the IR region of the spectrum instead of the UV-visible region.
• The Michelson interferometer operates based on interference between light waves (Ch. 2).
• As with UV-visible spectrophotometry (Ch. 7), the measurement of IR absorption requires sample cells made from materials that are transparent to the relevant wavelengths.
• The Beer-Lambert law (Ch. 6) applies to all absorption measurements, but, as with AAS and AES (Ch. 10), it is often less straightforward to apply for IR absorption.

Post-Reading Questions

1. How many gratings and slits are in a FTIR absorption spectrometer?
2. Name at least three differences between an FTIR absorption spectrometer and a UV-visible spectrophotometer.
3. What are three advantages of FTIR spectrometers versus dispersive IR spectrometers?
4. Why are sample cells for IR absorption not made of glass?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Explain the design and operational principles of a Michelson interferometer, including how it downshifts frequencies.
• Distinguish between time-domain and frequency-domain measurements.
• Understand the Nyquist theorem and how it is important to FTIR absorption measurements.
• Draw a labeled block diagram for a FTIR spectrometer.
• Produce a flow chart that shows how interferograms are converted to transmission spectra.
• Sketch diagrams for different FTIR sampling modes.