Infrared Spectroscopy

1
Infrared Spectroscopy

• Department of Natural Sciences
• UM-Dearborn

Written by Bette Kreuz Produced by Ruth Dusenbery
2
Info Slide 1

• Infrared spectroscopy is an instrumental method
  of analysis that can be used to identify and
  quantify samples ranging from pharmaceuticals to
  diesel emissions.

3
Slide 1 Introduction
4
Info Slide 2

• All modern instruments are Fourier Transform
  instruments.
• In all transmission experiments radiation from a
  source is directed through the sample to a
  detector.

5
Slide 2
Sample Compartment
IR Source
Detector
6
Info Slide 3

• The measurement of the type and amount of light
  transmitted by the sample gives information about
  the structure of the molecules comprising the
  sample.

7
Slide 3
Infrared Source
Detector
Sample Compartment
8
Info Slide 4

• In the IR region of the electromagnetic spectrum,
  the absorption of radiation by a sample is due to
  changes in the vibrational energy states of a
  molecule.

9
Slide 4
10
Info Slide 5

• The pattern of absorption as a function of
  wavelength is called an IR spectrum.

11
Slide 5
12
Info Slide 6

• To obtain an IR spectrum, the sample must be
  placed in a container or cell that is
  transparent in the IR region of the spectrum.
• Sodium chloride or salt plates are a common means
  of placing the sample in the light beam of the
  instrument.

13
Slide 6
IR transparent Salt Plates
14
Info Slide 7

• These plates are made of salt and must be stored
  in a water free environment such as the
  dessicator shown here.

15
Slide 7
Dessicator Water-free Environment for
Water-sensitive Salt Plates.
16
Info Slide 8

• The plates must also be handled with gloves to
  avoid contact of the plate with moisture from
  ones hands.

17
Slide 8
18
Info Slide 9

• To run an IR spectrum of a liquid sample, a drop
  or two of the liquid sample is applied to a salt
  plate.

19
Slide 9
20
Info Slide 10

• A second salt plate is placed on top of the first
  one such that the liquid forms a thin film
  sandwiched between the two plates.

21
Slide 10
22
Info Slide 11

• The two plates are then secured in a sample
  holder that is compatible with the particular
  instrument being used.

23
Slide 11
24
Info Slide 12

• The cell holder is then placed in the beam of the
  instrument.

25
Slide 12
26
Info Slide 13

• The light beam traverses the sample compartment,
  as illustrated by the red line.

27
Slide 13
Light Path (shown by red line)
28
Info Side 14

• The sample is then scanned by the instrument
  utilizing predesignated parameters.
• A relevant background scan should already have
  been taken.

29
Slide 14
Click Here to Start Scan
30
Info Slide 15

• A satisfactory spectrum has well defined
  peaks-but not so intense as to cause flattening
  on the bottom of the peaks.
• Major peaks can be labeled
• using the peak function of the software

31
Slide 15
Well-defined peaks are labeled with the
Wavenumbers of the Absorption Maxima.
32
Info Slide 16

• The spectrum can then be printed using the print
  function of the software.

33
Slide 16
34
Info Slide 17

• A sample of a printout of an IR spectrum.

35
Slide 17
Sample of a printout of an IR spectrum.
36
Info Slide 18

• The salt plates are cleaned by rinsing into a
  waste container with a suitable organic
  solvent-commonly cyclohexane.
• NEVER WATER!

37
Slide 18
CYCLOHEXANE Solvent
38
Info Slide 19

• Cloudy plates must be polished to return them to
  a transparent condition.
• To polish cloudy windows, rotate salt plate on
  polishing cloth.

39
Slide 19
40
Info Slide 20

• The clean plates and cell holder are stored in
  the moisture free atmosphere of a dessicator.

41
Slide 20