Chapter 11: IR

Question 1

Near IR wavelength

Answer 1

14000-4000 cm^-1

Question 2

Mid-Infared wavelength

Answer 2

4000-400 cm^-1

Question 3

Far Infared

Answer 3

400-10 cm^-1

Question 4

Where are aromatics on the IR?

Answer 4

1600-2000 cm^-1

Question 5

What msut occur for a molecule to absorb an IR photon?

Answer 5

There must be a dipole-dipole moment. (asymmetric)

Question 6

What analysis does IR do? Qualitative or Quantitiative?

Answer 6

Qualitiative.

Question 7

Draw out a diagram for a Michaelson Inferometer.

Answer 7

Question 8

Fourneir transform the following graph from a Power v.s. Time graph to a Power v.s. Frequency graph.

Answer 8

Question 9

What are the four advatanges in FT instruments? Explain.

Answer 9

-Faster collection times: A typical FTIR spectrum can be obtained in less than one second. Scanning IR spectrometers are slow and can take up to 15 minutes for a single scan.10

-Multiplex advantage (signal-to-noise): Because Fourier transform data collection is so fast, it is possible to collect many spectra in a relatively short amount of time, store them in the computer, and signal average the results. As the number of averaged spectra approaches infinity, the average value of the “white” noise approaches zero.12 In practical terms, the S/N ratio increases as the square root of the number of spectra (n); for example, averag- ing 64 spectra will provide an 8× increase in S/N.
-Throughput advantage (Jaquinot’s advantage): .Inadispersiveinstrument, only a small portion of the total energy from the source that passes through the sample reaches the detector. In a Fourier transform instrument, all of the radiant power reaches the detector. The result is better detection limits and better S/N ratios.

-Better Resolution:he resolution of a dispersive spectrometer is related to the slit width of the monochromator. Given the fact that the energy of an IR photon is already relatively weak, narrow slit widths results in an unacceptable attenuation of the signal. To increase signal, larger slit widths are required in order to “see” he sample; however, larger slit widths result in lower resolution. In an Fourier transform instrument, the resolution is dependent on the distance traveled by a moving mirror (see Section 11.3) and thus can be adjusted by the user as needed without sacrificing throughput of the source radiation.

Question 10

What are two types of infared detectors?

Answer 10

-Thermal
-Pyroelectric

Question 11

What type of detector is NOT used in FTIR and why?

Answer 11

Thermal detectors
The response time on these are too slow.

Question 12

Name three types of pyroelectric detectors.

Answer 12

-Deuterated triglycine sulfate
-LiTaO3
-Lead zirconium titanate