Infrared Spectrometry - EIU



Infrared Spectrometry

Section 7I, Chapters 16 & 17

Remember the energy diagram of a molecule as previously discussed.

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The energy of infrared radiation corresponds with the vibrational (as opposed to electronic for UV-visible radiation) energy levels of molecules.

Molecular vibrations are basically dependent on two parameters:

1. The masses of the atoms involved in the vibration

2. The strength of the bond holding the atoms together.

Thus infrared spectrometry provides a lot of structural/functional group information (as opposed to electronic UV-visible spectrometry) in molecules.

Even though there is totally different information content between the two techniques the spectrum, in a fundamental sense, is generated the same way.

Historically the instrumentation of UV-Vis and IR spectrometers were very similar. They were often both single channel, usually double beam instruments.

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Now single channel infrared spectrometers are extinct, and multichannel infrared spectrometers have not yet been perfected.

Fourier transform infrared spectrometers rule this region of the electromagnetic spectrum – single beam, single channel, multiplexing.

The UV/Vis instruments discussed thus far work in the frequency domain. That is, the measurements are made as a function of frequency (or wavelength). Fourier transform (FT) spectrometers work in the time domain. That is, measurements are made as a function of time, then a Fourier transform (an advanced mathematical manipulation to the data) is done to convert that to a frequency domain spectrum.

The basics of optical FT spectroscopy first introduced in Section 7I.

(Note that FT spectroscopy also dominates NMR instrumentation, and is also a major player in MS instrumentation (FT-MS, ICR). The principles are the same, the details of how the time domain information is generated differs.)

Time domain and frequency domain spectra have the same information content.

As you know, for some historical reasons the x-axis in infrared spectrometry is in units of cm-1. The mid-IR region where fundamental molecular vibrations occur is from 4000 – 200 cm-1. This corresponds to frequencies of 1.2 x 1014 – 6.0 x 1012 s-1.

You can conceive of acquiring a time domain spectrum with the following instrumental configuration.

Detectors do not respond to power variations at these high frequencies, they have a finite “rise time”.

The rise time for a PMT or vacuum phototube is

10-8 – 10-9 s.

The rise time for a typical IR detector is 10-3 – 10-4 s.

Since the detector rise time is ................
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