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PART 3
1
2
Absorption Spectrometer
Source
Sample
Wavelength Selector
Dr. S. M. Condren
Detector
Signal Processor
Readout
4
(a) Construction materials
Dr. S. M. Condren
(b) wavelength selectors for spectroscopic instruments.
(c) Sources.
Dr. S. M. Condren
(d) Detectors for spectroscopic instruments.
Dr. S. M. Condren
IR Region
Nernst glower - rare earth oxides
globar - silicon carbide rod
incandescent wire - nichrome wire
Dr. S. M. Condren
Filters
interference filters
interference wedges
absorption filters
Dr. S. M. Condren
Monochromators
Components
entrance slit
collimating element (lens or
mirror)
prism or grating as dispersing
element
focusing element (lens or mirror)
exit slit
Dr. S. M. Condren
“Two types of
monochromators:
(a) CzerneyTurner grating
monochromator
(b) Bunsen prism
monochromator."
Dr. S. M. Condren
UV-Visible-Near IR
IR
NaCl
Cornu type
Littrow type
Quartz
dq dq
dn
--- = ----- ----dl dn
dl
where
q => angle
l => wavelength
n => refractive index
Dr. S. M. Condren
R => resolving power
l
dn
R = ------ = b  ----dl
dl
where b=> length of prism base
Dr. S. M. Condren
Diffraction Monochromators
Diffraction
If l is large compared to the
aperture,
the waves will spread out at
large
angles into the region beyond
the
obstruction.
Video
1
Video
2
Diffraction increases as aperture
size  l
Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
Diffraction Pattern From a Single Slit
Ingle and Crouch, Spectrochemical Analysis
Diffraction Pattern From a Single Slit
For Destructive
Interference:
x = l/2
W sin q = l
Ingle and Crouch, Spectrochemical Analysis
W
x
sinq
2
Diffraction Pattern From a Single Slit
For Destructive
Interference:
x = l/2
W sin q= 2 l
Ingle and Crouch, Spectrochemical Analysis
x
W
sinq
4
Diffraction Pattern From a Single Slit
For Destructive
Interference:
W sin q = m l
m = ±1, ±2, ±3, …
Ingle and Crouch, Spectrochemical Analysis
Diffraction Gratings
Plane or convex plate ruled with
closely spaced grooves (3002400 grooves/mm).
Eugene Hecht, Optics, 1998.
http://www.olympusmicro.com/primer/java/imageformation/gratingdiffraction/index.html
Grating Equation
Two parallel monochromatic
rays strike adjacent grooves
and are diffracted at the
same angle (b).
Difference in optical
pathlength is AC + AD.
For constructive
interference:
ml = (AC + AD)
m = 0, 1, 2, 3, …
Ingle and Crouch, Spectrochemical Analysis
Grating Equation
ml = (AC + AD)
AC = d sin a
AD = d sin b
Combine to give Grating Equation:
d(sin a + sin b) = ml
Grating Equation only applies if:
d > l/2
Ingle and Crouch, Spectrochemical Analysis
Are you getting the concept?
At what angle would you collect the 1st order diffracted light with
l = 500 nm if a broad spectrum beam is incident on a 600
groove/mm grating at qi = 10°? For l = 225 nm? For l = 750 nm?
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Modern infrared spectrometers are very
different from the early instruments that were
introduced in the 1940s. Most instruments
today use a Fourier Transform infrared (FT-IR)
system.
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In early experiments infrared light was passed through
the sample to be studied and the absorption measured.
This approach has been superseded by Fourier
transform methods.
A beam of light is split in two with only half of the
light going through the sample.
The difference in phase of the two waves creates
constructive and/or destructive interference and is a
measure of the sample absorbance.
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The waves are rapidly scanned over a specific
wavelength of the spectra and multiple scans
are averaged to create the final spectrum.
This method is much more sensitive than the
earlier dispersion approach.
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A Fourier transform is a mathematical
operation used to translate a complex curve
into its component curves. In a Fourier
transform infrared instrument, the complex
curve is an interferogram, or the sum of the
constructive and destructive interferences
generated by overlapping light waves, and the
component curves are the infrared spectrum.
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An interferogram is generated because of the
unique optics of an FT-IR instrument. The key
components are a moveable mirror and beam
splitter. The moveable mirror is responsible for
the quality of the interferogram, and it is very
important to move the mirror at constant
speed. For this reason, the moveable mirror is
often the most expensive component of an FTIR spectrometer.
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The beam splitter is just a piece of semireflective material, usually mylar film
sandwiched between two pieces of IRtransparent material. The beam splitter splits
the IR beam 50/50 to the fixed and moveable
mirrors, and then recombines the beams after
being reflected at each mirror.
Michelson
Interferometer
"Schematic of a
Michelson
interferometer
illuminated by
a
monochromatic
source."
Dr. S. M. Condren
"Illustration
s of time
doamin
plots (a)
and (b);
frequency
domain
plots (c),
(d), and (e)."
Dr. S. M. Condren
“Comparison of
interferograms
and optical
spectra.”
Dr. S. M. Condren
Dr. S. M. Condren
37