FTIR Spectroscopy

Infrared spectroscopy is an important technique in organic chemistry.  It is an easy way to identify the presence of certain functional groups in a molecule.  Also, one can use the unique collection of absorption bands to confirm the identity of a pure compound or to detect the presence of specific impurities.

This page will step you through the principles of operation of the FTIR Spectrometer, and how it differs from classical continuous-wave (CW) instruments.  To skip directly to spectral interpretation, click here.

Before delving into FT spectrometry, let's review the principles of a classical spectrometer.  If you have used an optical or UV spectrometer, the principles are identical:
 

• A source generates light across the spectrum of interest.
• A monochromater (in IR this can be either a salt prism or a grating with finely spaced etched lines) separates the source radiation into its different wavelengths.
• A slit selects the collection of wavelengths that shine through the sample at any given time.
• In double beam operation, a beam splitter separates the incident beam in two; half goes to the sample, and half to a reference.
• The sample absorbs light according to its chemical properties.
• A detector collects the radiation that passes through the sample, and in double-beam operation, compares its energy to that going through the reference.
• The detector puts out an electrical signal, which is normally sent directly to an analog recorder.  A link between the monochromater and the recorder allows you to record energy as a function of frequency or wavelength, depending on how the recorder is calibrated.

Although very accurate instruments can be designed on these principles, there are several important limitations.  First, the monochromater/slit limits the amount of signal one can get at a particular resolution.  To improve resolution, you must narrow the slit and decrease sensitivity.  Second, there is no easy way to run multiple scans to build up signal-to-noise ratios. Finally, the instrument must be repetitively calibrated, because the analog connection between the monochromater position and the recording device is subject to misalignment and wear.

Now let's look at an FT instrument.

We still have a source, a sample and a detector, but everything else is different.  Now, we send all the source energy through an interferometer and onto the sample.  In every scan, all source radiation gets to the sample!  The interferometer is a fundamentally different piece of equipment than a monochromator.  The light passes through a beamsplitter, which sends the light in two directions at right angles.  One beam goes to a stationary mirror then back to the beamsplitter.  The other goes to a moving mirror.  The motion of the mirror makes the total path length variable versus that taken by the stationary-mirror beam.  When the two meet up again at the beamsplitter, they recombine, but the difference in path lengths creates constructive and destructive interference:  an interferogram:

The recombined beam passes through the sample.  The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram.  The detector now reports variation in energy versus time for all wavelengths simultaneously.  A laser beam is superimposed to provide a reference for the instrument operation.

Energy versus time is an odd way to record a spectrum, until you recognize the relationship between time and frequency:  they are reciprocals!  A mathematical function called a Fourier transform allows us to convert an intensity-vs.-time spectrum into an intensity-vs.-frequency spectrum.

The Fourier transform:
 
A(r) and X(k) are the frequency domain and time domain points, respectively, for a spectrum of N points.
 
 

An interferogram

A spectrum

Now, you have to do the Fourier transform for every point in the interferogram.  You may like to do exponential functions by hand, but I (and most organic chemists) are far too lazy for that.  Fortunately, even a slow computer can efficiently perform this operation.  The output of the detector is digitized, and a small computer program will do the transform in a matter of seconds (or less).  All modern FT instruments are computer-interfaced.

There are several advantages to this design:

• All of the source energy gets to the sample, improving the inherent signal-to-noise ratio.
• Resolution is limited by the design of the interferometer.  The longer the path of the moving mirror, the higher the resolution.  Even the least expensive FT instrument provides better resolution that all but the best CW instruments were capable of.
• The digitization and computer interface allows multiple scans to be collected, also dramatically improving the signal-to-noise ratio.
• Most of the computer programs today allow further mathematical refinement of the data:  you can subtract a reference spectrum, correct the baseline, edit spurious peaks or otherwise correct for sample limitations.

The one minor drawback is that the FT instrument is inherently a single-beam instrument; it cannot use the "channel ratio" trick used in CW operation.  One result is that IR-active atmospheric components (CO₂, H₂O) will appear in the spectrum.  Usually, a "Background" spectrum is run, then automatically subtracted from every spectrum.  The spectrum below is such a background scan.

 

You can see CO₂ as the strong doublet at around 2300 cm^-1, and water as the "spiky" peaks in the 3800 and 1600 cm^-1.  The "bell curve" shape of the spectrum reflects the output spectrum of the source:  strong in the middle, but falling off at the ends.