Experimental IR Spectroscopic Methods (4)

Question 1

6 main IR sampling techniques

Answer 1

transmission, internal reflectance/ATR, external reflectance, diffuse reflectance, photo acoustic, emission

Question 2

IR Spectroscopy can analyze…

Answer 2

liquids, solids, gases

Question 3

In ATR, what happens if sample absorbs evanescent wave?

Answer 3

attenuation of incoming radiation occurs at wavelength of IR absorption bands

Question 4

What are the properties of optical materials needed in the IR compared to UV-Vis?

Answer 4

transparent window; quartz glass does not transmit IR

Question 5

What is the refractive index value for common IR transmitting materials?

Answer 5

1.38 - 4

Question 6

What are the differences in path lengths needed in the IR compared to UV vis?

Answer 6

10-100 µ versus 1 cm

Question 7

What re the differences between IR and UV vis transmission experiments?

Answer 7

IR transmission demountable with replaceable spacers to vary pathlength

Question 8

Why can fringes be present in IR transmission measurements?

Answer 8

they occur in empty cell spectrum from internal focus of windows

Question 9

How can path length be determined from the fringes in an IR cell?

Answer 9

number of fringes between two wavenumbers

Question 10

How does ATR differ from transmission IR

Answer 10

In transmission the incident radiation does not pass through a crystal to interact with sample. IR light goes directly into the sample to be absorbed and any other light will be transmitted into detector

Question 11

What conditions must be present in order for radiation to undergo internal reflection?

Answer 11

radiation must approach interface between 2 phases from higher refractive index medium; angle of incidence must be greater than critical angle

Question 12

What is the evanescent wave?

Answer 12

standing electric field in phase 2

Question 13

What is the depth of penetration in ATR?

Answer 13

distance from surface of ATR crystal penetrates into phase 2

Question 14

How are the depth of penetration and the evanescent wave intensity related?

Answer 14

Distance where intensity wave falls 1/ e or. 37% original value

Question 15

What are common values for the depth of penetration in Mid IR

Answer 15

1-6 µm

Question 16

What type of optical materials are commonly used in ATR

Answer 16

Zinc selenide, diamond, germanium (refractive index must be above 2.4)

Question 17

What type of samples are useful to study by ATR?

Answer 17

very thick, very absorbing, very thin

Question 18

What is specular reflectance?

Answer 18

mirror like reflection f incident IR radiation from surface of sample

Question 19

describe external reflectance IR

Answer 19

nondestructive analysis of surfaces

Question 20

How does specular reflectance differ from reflection-absorbance?

Answer 20

highly directional, measure optical properties; in Ra, IR beam passes through sample and back

Question 21

What type of samples are useful to study by external reflectance

Answer 21

thin films on reflective substrates or from monomolecular layers

Question 22

Advantages of diffuse reflectance

Answer 22

effective; powdered samples with minimal sample prep

Question 23

How does diffuse reflectance compare to internal and external?

Answer 23

Multiple beam reflection

Question 24

What are the physical principles behind diffuse reflectance?

Answer 24

radiation strikes powder, slightly penetrates, then reemerges

Question 25

Does beer’s law apply to diffuse reflectance spectra?

Answer 25

yes

Question 26

Physical principles behind photo acoustic IR

Answer 26

light absorbance; sample in closed cell with gas; chopper; absorbs by vibrations and heats up, changing pressure which is then picked up by a microphone

Question 27

Advantages of photoacoustic

Answer 27

can be used on difficult samples with minimal prep as long as it fits in sample cup; can detect components of mixtures and monitor gas in atmosphere

Question 28

disadvantages of photo acoustic

Answer 28

not widely used

Question 29

physical principle behind IR emission spectroscopy?

Answer 29

emits IR radiation upon relaxing at high temperatures

Question 30

What is the source in ir emission

Answer 30

sample itself

Question 31

what is emission ir used for

Answer 31

not widely used in lab, but for environment monitoring