Beam Chopping corrects for flame emission but not for scattering. Can distinguish the signal of the flame from the atomic line at the same wavelength. The higher the absorbance the higher the transmittance.
Intro to IR
Near IR- (0.78 to 2.8 micrometers)
Mid IR- (2.5- 50 micrometers)
Far IR- (50-1000 micrometers)
Most common region used is Mid IR btwn 2.5- 15 micrometers
Mid IR: based on diffracting gratings but now current instrumentation are of Fourier transform type Far-IR: useful now that Fourier transform instruments are available. Can measure IR absorption, emission, and reflection in energy in order for a species to absorb IR radiation, it needs to undergo a net change in dipole moment (vibrational/rotational motion) Rotational Vibrational transition not much energy needed
Liquids and solids intermolecular collisions and interaction that cause broadening lines into a continuum. IR sources - consist of inert solid and heated electrically to very high temperatures 1500-2000K to get a continuum of radiation. Nernst Glower- composed of rare earth oxides (ZrO2, Y2O3, and Er2O3) current is passed thru the device so that the device can reach the high temp of 1200-2000K, spectral out in the IR region Globar source- silicon carbide rod heated to 1300-1500K
Disadvantage to using this is water cooling is required to prevent electrical arching. Incandescent wire sources- tightly wound spiral michrome wire heated by electrical current to 1100k
IR absorption requirement
1 source of continuous IR radiation
2 IR transducer that’s sensitive
Mercury Arc Source (Far IR) only for Far IR region only has enough radiant power in this region. Quartz jacketed tube containing Hg vapor pressurized greater than 1 atm, electricity passed thru this Hg vapor (pressure emits a continuum of radiation in the far IR region.) Disadvantage and the advantage to this source is that it only is used for the Far IR region only and can’t be used for any other IR region. Tungston Filament lamp (Near IR region)- ordinary filament lamp, convenient source for the near IR of 4000-12800 cm^-1 , inexpensive but restricted to Near IR region) CO2 laser source- laser produces a band of radiation in 900-1100 cm^ -1, can get 100 closely spaced discreet lines, can tune the laser to a specific line. (More expensive) Advantage- radiant power available in each line (gets several orders of magnitude increase)
Thermal transducer-response depends on the heating effect of radiation, radiation is absorbed by a small black body and resultant temp rise is monitored, detects temperature changes that is produced. Heat capacity of the absorbing element must be as small as possible if detectable temp change is to be produced, radiant power of the spectrophotometer beam is 10^-7 to 10^-9 watts ( only observes a few thousandths of K change in temp) Has to be in a vacuum and shielded from thermal radiation emitted by nearby objects Beam from the source is chopped at a specific frequency different from extraneous noise outside of it. Try to minimize size and thickness of the absorbing element to concentrate the entire beam of IR radiation on the surface of the absorbing element. Thermocouples- consist of a pair of junctions for when 2 pieces of metal (Bismuth and Antimony are fused together btwn metals a potential develops is temp dependent at the junction potential is temp dependent and varies with the change in temp. junction is often blackened to improve absorbing capacity, also sealed in a vacuum chamber with a window that’s transparent to the IR radiation. Can respond to temp difference, and enhance sensitivity by connecting several thermal couples together= thermopile.
Pyroelectric transducers- insulators with special thermal and electric properties constructed from single crystalline wafers. Electric field is applied across material and electric polarization takes place. when there is temp change by radiating it w/ IR radiation...
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