Spectroscopic Methods

Spectroscopic Methods PART 3

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IR Instrumentation

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IR InstrumentationAbsorption Spectrometer

Dr. S. M. Condren

Source Wavelength Selector DetectorSignal Processor ReadoutSample

IR Instrumentation

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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

SourcesIR Region

Nernst glower - rare earth oxides

globar - silicon carbide rodincandescent wire - nichrome

wire

Dr. S. M. Condren

Wavelength SelectionFilters

interference filtersinterference wedgesabsorption filters

Dr. S. M. Condren

Wavelength SelectionMonochromators

Componentsentrance 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 MonochromatorsUV-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 l

If l is large compared to the aperture, the waves will spread out at largeangles into the region beyond theobstruction.

DiffractionVideo

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Video 2

Diffraction Pattern From a Single Slit

Ingle and Crouch, Spectrochemical Analysis

Diffraction Pattern From a Single Slit

q sin2W x

Ingle and Crouch, Spectrochemical Analysis

For Destructive Interference:

x = l/2

W sin q = l

Diffraction Pattern From a Single Slit

Ingle and Crouch, Spectrochemical Analysis

For Destructive Interference:

x = l/2

W sin q= 2 l

q sin4W x

Diffraction Pattern From a Single Slit

Ingle and Crouch, Spectrochemical Analysis

For Destructive Interference:

W sin q = m l

m = ±1, ±2, ±3, …

Eugene Hecht, Optics, 1998.

Diffraction GratingsPlane 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:

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

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.

Michelson Interferometer

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

Video

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