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Instrumental Test 3

Terms in this set (28)

- probes quantum state transitions associated with molecular vibrations in the physical forms of gas, liquid, solutions, and crystalline or amorphous solids

-based on scattering of radiation

- changes the frequency because of interaction between the vibrational energy level and incident radiation of the molecules

plot- intensity of the scattered radiation produces Stokes

Rayleigh - electron falls down to original ground state and there is no energy change, light from same wavelength is re-emitted

Stokes Scattering - electron is excited and then falls to vibrational level - molecule absorbed a certain amount of light

Anti-Stokes - electron excited from vibrational level reaches a virtual level and then falls to ground level

-Antistokes and Rayleigh scattering has the same wavelength as the radiation source

-IR and Raman spectroscopy yields complementary formations.

-Raman active vibration may be inactive in the IR range and vice versa.

-Raman yields more useful information than IR in certain regions
-Raman is insensitive to water and so high-quality spectra for hydrated samples can be determined

Advantages of FT-Raman over Dispersie Raman
- use infrared which eliminates fluorescence and photo decomposition
- good frequency precision for high-resolution

Disadvantages of FT-Raman over Dispersive
-detectors have low sensitivity
- difficulties with aqueous solutions
- Rayleigh scattering requires a lot of filtering

Instrumentation
1. laser source of visible or near-infrared monochromatic radiation
2. Common sources:
Argon laser
Neodymium YAG source
Krypton laser
Diode laser
3. sample illumination device
4. Spectrometer.

Operation
- similar to fluorescence but with high power illumination.
- FT Raman instrument use interferometry and Fourier transform methods

Advantages
-super spectral resolution
- frequency accuracy
- high throughput advantages over dispersive based instrument.
1. Electron Impact (EI) Ionization - uses a beam of energetic electrons generated by a filament to cause fragmentation, which provides structural information, of molecules.
methane reacts with high energy electrons to produce several reactive ions such as CH4 + , CH3 + and CH2 + .
-The fragment ions provide structural information.

2.Chemical Ionization (CI) - uses a large excess of reagent gas such as methane or ammonia and energetic electrons to ionize the analyte.
-produces less fragmentation of the analyte.
-Molecular weight information is obtained
from ions in the mass spectrum.
-methane reacts with high energy electrons
to produce several reactive ions such as
CH4 + , CH3 + and CH2 + .
-The predominant ions (CH4 +● and CH3 + )
react rapidly with additional reagent gas
-species collide w/analyte and ionize analyte
by proton or hydride transfer
-Proton transfer produces (M+1)+ ions and
hydride transfer produces (M-1)+ ions. These
ions undergo less fragmentation due to their
low energy.

3. Field Ionization (FI) - uses a powerful electric field concentrated in the tip of an emitter wire to vaporize and ionize a sample.
-Field Desorption (FD) - utilizes a powerful electric field concentrated in the tip of an emitter wire to ionize a sample coated on the emitter.
the majority of the solvent must be allowed to evaporate before the emitter is introduced into the mass spectrometer source.
application of a high potential between the emitter and another electrode not to far away from the emitter causes desorption of ions and ionization of the analyte