topic 4 ir spectroscopy

Question 1

4 components of infrared spectrometer

Answer 1

1. radiation source (Nernst Glower or Globar)
2. monochromator (except FT-IR which uses interferometer)
3. sample cell (metal halide)
4. detector (bolometer or thermocouple)

Question 2

3 conditions for radiation source of infrared spectrometer

Answer 2

1. must be able to emit radiation over infrared region i.e. 400-4000 cm^-1
2. must have sufficient intensity such that transmitted energy can be detected at the end of the optical path
3. must be stable (I0 = 100%)

Question 3

3 problems with using metal halides as sample cell/prism in infrared spectrometer

Answer 3

1. mechanical instability
2. thermal instability
3. water solubility

Question 4

how to prepare samples for infrared spectrometry for solid (3), liquid (2) and gas (1)

Answer 4

solid sample preparation

1. KBr pellet technique
2. mull technique
3. cast film technique

liquid sample preparation

1. KBr disc technique
2. liquid cell technique (CaF2 is used if water is present in sample)

gas sample preparation
KBr or NaCl sample cell technique
*difficult to analyse as concentration is low. we can use a cell with longer path length, which will increase absorbance.

Question 5

theory of infrared absorption (2)

Answer 5

each molecule has a complete absorption spectrum unique to its functional groups.

in molecules, atoms are held by chemical bonds and are in continuous synchronised movement.

Question 6

3 conditions for molecule to absorb infrared radiation

Answer 6

1. change in bond length or angle due to vibrational or rotational motion causing net change in dipole moment
2. molecule must be vibrating at a natural frequency equal to frequency of incoming radiation
3. radiation energy must correspond to energy difference between ground state and excited state of molecule

Question 7

compare the two radiation sources for IR (3)

Answer 7

• Nernst Glower is more intense
• NG made of mixture of rare earth oxides (zirconium, cerium, thorium); G made of sintered silicon carbide
• both heated to temp betw 1000-1800 deg C

Question 8

compare the two detectors used in IR (3)

Answer 8

• bolometer consists thin metal conductor; thermocouple consists two dissimilar metal wires connected at two ends
• bolometer measures radiation incident on it by taking change in resistance; thermocouple does so by taking potential difference
• both are heat detectors