Question

In ftir why does collecting more data point in the time domain improves the resolution in the frequency? I know it is because of the longer mirror movement distance but why

Answer 1

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.

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, 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.

Hope this helps