PART 3 1. 2 Absorption Spectrometer Dr. S. M. Condren SourceWavelength SelectorDetector Signal...
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Spectroscopic Methods PART 3 1
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Transcript of PART 3 1. 2 Absorption Spectrometer Dr. S. M. Condren SourceWavelength SelectorDetector Signal...
Spectroscopic Methods
PART 3
1
IR Instrumentation
2
IR InstrumentationAbsorption Spectrometer
Dr. S. M. Condren
Source Wavelength Selector DetectorSignal Processor ReadoutSample
Components of Optical Instruments
(a) Construction materials
Dr. S. M. Condren
Components of Optical Instruments(b) wavelength selectors for spectroscopic instruments.
Components of Optical Instruments(c) Sources.
Dr. S. M. Condren
Components of Optical Instruments(d) Detectors for spectroscopic instruments.
Dr. S. M. Condren
Sources
IR RegionNernst glower - rare earth
oxidesglobar - silicon carbide rodincandescent wire - nichrome
wire
Dr. S. M. Condren
Wavelength Selection
Filtersinterference filtersinterference wedgesabsorption filters
Dr. S. M. Condren
Wavelength Selection
MonochromatorsComponents
entrance slitcollimating element (lens or
mirror)prism or grating as dispersing
elementfocusing element (lens or
mirror)exit slit
Dr. S. M. Condren
Wavelength Selection
“Two types of monochromators: (a) Czerney-Turner grating monochromator(b) Bunsen prism monochromator."
Dr. S. M. Condren
Prism Monochromators
UV-Visible-Near IR QuartzIR NaCl
Cornu type
Littrow type
Angular dispersion of prisms
dq dq dn--- = ----- · -----dl dn dl
where q => anglel => wavelength
n => refractive index
Dr. S. M. Condren
Resolving Power ofPrism Monochromators
R => resolving power
l dnR = ------ = b · ----- dl dl
where b=> length of prism base
Dr. S. M. Condren
Interference and DiffractionDiffraction Monochromators
Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
Diffraction increases as aperture size
If l is large compared to the aperture, the waves will spread out at largeangles into the region beyond theobstruction.
DiffractionVideo
1
Video 2
Diffraction Pattern From a Single Slit
Ingle and Crouch, Spectrochemical Analysis
Diffraction Pattern From a Single Slit
sin2
W x
Ingle and Crouch, Spectrochemical Analysis
For Destructive Interference:
x = /2
W sin =
Diffraction Pattern From a Single Slit
Ingle and Crouch, Spectrochemical Analysis
For Destructive Interference:
x = /2
W sin = 2
sin4
W x
Diffraction Pattern From a Single Slit
Ingle and Crouch, Spectrochemical Analysis
For Destructive Interference:
W sin = m
m = ±1, ±2, ±3, …
Eugene Hecht, Optics, 1998.
Diffraction Gratings
Plane or convex plate ruled with closely spaced grooves (300-2400 grooves/mm).
http://www.olympusmicro.com/primer/java/imageformation/gratingdiffraction/index.html
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:
m = (AC + AD)
m = 0, 1, 2, 3, …
Ingle and Crouch, Spectrochemical Analysis
Grating Equation
m = (AC + AD)
AC = d sin a
AD = d sin b
Combine to give Grating Equation:
d(sin a + sin b) = m
Ingle and Crouch, Spectrochemical Analysis
Grating Equation
Grating Equation only applies if:d > l/2
Are you getting the concept?
At what angle would you collect the 1st order diffracted light withl = 500 nm if a broad spectrum beam is incident on a 600groove/mm grating at qi = 10°? For l = 225 nm? For l = 750 nm?
Fourier Transform IR (FTIR)
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.
Fourier Transform IR (FTIR)
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.
Fourier Transform IR (FTIR)
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.
Fourier Transform IR (FTIR)
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.
Fourier Transform IR (FTIR)
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 FT-IR spectrometer.
Fourier Transform IR (FTIR)
The beam splitter is just a piece of semi-reflective material, usually mylar film sandwiched between two pieces of IR-transparent 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.
Fourier Transform IR (FTIR)
Michelson Interferometer"Schematic of a Michelson interferometer illuminated by a monochromatic source."
Dr. S. M. Condren
Fourier Transform IR (FTIR)
"Illustrations of time doamin plots (a) and (b); frequency domain plots (c), (d), and (e)."
Dr. S. M. Condren
Fourier Transform IR (FTIR)
“Comparison of interferograms and optical spectra.”
Dr. S. M. Condren
FT-IR
Dr. S. M. Condren
