IR Spectroscopy

Question 1

IR Instrumentation: dispersive verses non-dispersive?

Answer 1

Dispersive spreads the light out using a monochromator.

Non-dispersive measures light as a function of time. (Time Domain Spectroscopy)

Question 2

Why is non-dispersive used more in IR?

Answer 2

Because the IR has a low energy, making it hard to measure. (This is why we mostly see FTIR.)

Question 3

IR Sources:

Answer 3

1. Nernst glower: rare earth oxides (most intense in Near IR).
2. Nichrome wire: cheap and not as intense.
3. Globar: silicon carbide bar (long wavelengths) most ubiquitous IR source.

Question 4

Optical Components (cells used for samples):

Answer 4

Note that these are mostly inorganic salts; therefore, no water may be used (no aqueous solutions).

1. NaCl
2. AgCl
3. KBr
4. KRS-5; TlBr-TlI

Question 5

IR Detectors / Transducers:

Answer 5

1. Thermal Detectors
2. Pyroelectric
3. Photoconducting (PCT)

Question 6

Thermocouple (Thermal Detectors IR):

Answer 6

temperature difference between two metals will produce a potential difference.

Question 7

Bolometer (Thermal Detectors IR):

Answer 7

resistance change as a function of temperature.

Question 8

Pyroelectric (IR Detectors):

Answer 8

temperature dependent capacitance, fast response time to track changes in the time domain, commonly used in nondispersive instruments.

Question 9

Photoconducting (IR Detectors):

Answer 9

semiconducting material (PbS) where IR absorbance promotes nonconducting valence electrons into the conducting state thus lowering resistance.

Question 10

Describe Fourier Transform:

Answer 10

Collecting all of the signal at the same time, and transforming the data from a time domain to a frequency domain.

Question 11

Limitations of FT:

Answer 11

Frequency is too high to monitor at a time varying signal (10e14 Hz) sources.

Question 12

Calculating modulated frequency:

Answer 12

blank