Experimental IR Spectroscopic Methods (4) Flashcards
6 main IR sampling techniques
transmission, internal reflectance/ATR, external reflectance, diffuse reflectance, photo acoustic, emission
IR Spectroscopy can analyze…
liquids, solids, gases
In ATR, what happens if sample absorbs evanescent wave?
attenuation of incoming radiation occurs at wavelength of IR absorption bands
What are the properties of optical materials needed in the IR compared to UV-Vis?
transparent window; quartz glass does not transmit IR
What is the refractive index value for common IR transmitting materials?
1.38 - 4
What are the differences in path lengths needed in the IR compared to UV vis?
10-100 µ versus 1 cm
What re the differences between IR and UV vis transmission experiments?
IR transmission demountable with replaceable spacers to vary pathlength
Why can fringes be present in IR transmission measurements?
they occur in empty cell spectrum from internal focus of windows
How can path length be determined from the fringes in an IR cell?
number of fringes between two wavenumbers
How does ATR differ from transmission IR
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
What conditions must be present in order for radiation to undergo internal reflection?
radiation must approach interface between 2 phases from higher refractive index medium; angle of incidence must be greater than critical angle
What is the evanescent wave?
standing electric field in phase 2
What is the depth of penetration in ATR?
distance from surface of ATR crystal penetrates into phase 2
How are the depth of penetration and the evanescent wave intensity related?
Distance where intensity wave falls 1/ e or. 37% original value
What are common values for the depth of penetration in Mid IR
1-6 µm
What type of optical materials are commonly used in ATR
Zinc selenide, diamond, germanium (refractive index must be above 2.4)
What type of samples are useful to study by ATR?
very thick, very absorbing, very thin
What is specular reflectance?
mirror like reflection f incident IR radiation from surface of sample
describe external reflectance IR
nondestructive analysis of surfaces
How does specular reflectance differ from reflection-absorbance?
highly directional, measure optical properties; in Ra, IR beam passes through sample and back
What type of samples are useful to study by external reflectance
thin films on reflective substrates or from monomolecular layers
Advantages of diffuse reflectance
effective; powdered samples with minimal sample prep
How does diffuse reflectance compare to internal and external?
Multiple beam reflection
What are the physical principles behind diffuse reflectance?
radiation strikes powder, slightly penetrates, then reemerges
Does beer’s law apply to diffuse reflectance spectra?
yes
Physical principles behind photo acoustic IR
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
Advantages of photoacoustic
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
disadvantages of photo acoustic
not widely used
physical principle behind IR emission spectroscopy?
emits IR radiation upon relaxing at high temperatures
What is the source in ir emission
sample itself
what is emission ir used for
not widely used in lab, but for environment monitoring
