Transcript lecture 4 FT-IR Instrument

FT-IR Instrument
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Most commercial instruments separate and measure
IR radiation using
i) Dispersive spectrometers or
ii) Fourier transform spectrometers.
Dispersive Spectrometers
Dispersive spectrometers, introduced in the mid-1940s and widely used since,
provided the robust instrumentation required for the extensive application of this
technique.
This consists of three basic components: radiation source, monochromator, and
detector
Radiation source
The common radiation source for the IR
spectrometer is an inert solid, heated electrically
to 1000 to 1800 °C. Three popular types of
sources are Nernst glower (constructed of rareearth oxides), Globar (constructed of silicon
carbide), and Nichrome coil. They all produce
continuous radiations, but with different radiation
energy profiles.
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Monochromator
•The monochromator is a device used to disperse a broad
spectrum of radiation and provide a continuous calibrated series of
electromagnetic energy bands of determinable wavelength or
frequency range.
•Prisms or gratings are the dispersive components used in
conjunction with variable-slit mechanisms, mirrors, and filters.
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Detectors
Most detectors used in dispersive IR spectrometers can be categorized into two
classes:
•Thermal detectors and
•Photon detectors.
Thermal detectors include thermocouples, thermistors, and pneumatic
devices (Golay detectors).
They measure the heating effect produced by infrared radiation. A variety of
physical property changes are quantitatively determined: expansion of a
nonabsorbing gas (Golay detector), electrical resistance (thermistor), and voltage
at junction of dissimilar metals (thermocouple).
Photon detectors rely on the interaction of IR radiation and a semiconductor
material. Nonconducting electrons are excited to a conducting state. Thus, a
small current or voltage can be generated. Thermal detectors provide a linear
response over a wide range of frequencies but exhibit slower response times
and lower sensitivities than photon detectors.
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General Concepts Interferometry
• Optical Interfrometry is an optical measurement technique that provides extreme
precise measurements of distance, displacement or shape and surface of objects.
• It exploits the phenomenon of light waves interference .
Where under certain conditions a pattern of dark and light bars called interference
fringes can be produced. Fringes can be analyzed to present accurate
measurements in the range of nanometer.
• The recent developments in laser, fiber optics and digital processing techniques
have supported optical interferometry .
• Applications ranging from the measurement of a molecule size to the diameters
of stars.
Interference
• Interference
is
a
light
phenomenon .It can be seen in
everyday life. e.g.. colures of oil
film floating on water.
• In electromagnetic waves ,
interference between two or
more waves is just an addition or
superposition process. It results
in a new wave pattern .
Superposition of two waves
•
When two waves with an equal amplitudes are superposed the
output wave depends on the phase between the input waves.
Y = y 1 + y2
y1=A1 sin (wt + θ1 )
y2=A2 sin (wt + θ2)
Where:
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Since the energy in the light wave is intensity I ,which is proportional to
the sum of square amplitudes A^2
where: A=A1^2+A2^2+2A1A2 cos (θ1 – θ2)
If
A1=A2=A then:
A=2A^2+2A^2 cos (θ1 – θ2)
If
y1&y2 in phase ,cos(0)=1 hence,
Y = 4A^2 ,it gives a bright fringe.
If
y1&y2 out of phase by (π) ,cos (π)=-1 hence,
Y = 0 ,it gives a dark fringe
Optical Path Length [OPL]
•
When light beam travels in space from one point
to another, the path length is the geometric length
d multiplied by n (the air refractive index) which
is one:
OPL = d
•
Light beam travels in different mediums will
have different optical path, depending on the
refractive index (n)of the medium or mediums.
OPL = n d
Fourier transform spectrometers
• Superior speed and sensitivity
• Instead of viewing each component frequency sequentially, as in a
Dispersive IR spectrometer, all frequencies are examined simultaneously
in Fourier transform infrared (FTIR) spectroscopy.
Components
•
•
•
•
Michelson
Interferometer
Source
Michelson Interferometer
Sample
Detector
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Sources
• Black body radiators
• Inert solids resistively heated to 1500-2200 K
• Max radiation between 5000-5900 cm-1 (2-1.7
mm), falls off to about 1 % max at 670 cm-1 (15
mm)
• Nernst Glower – cylinder made of rear earth
elements
• Globar- SiC rod
• CO2 laser
• Hg arc (Far IR), Tungsten filament (Near IR)
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Michelson interferometer
•
Configuration:
Michelson interferometer consists of a coherent
light source, a beam splitter (BS), a reference mirror
,a movable mirror and a screen .
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Applications:
There are many measurements that Michelson
interferometer can be used for, absolute distance
measurements, optical testing and measure gases
refractive index.
•
Work method:
The BS divides the incident beam into two parts one
travel to the reference mirror and the other to the
movable mirror .both parts are reflected back to BS
recombined to form the interference fringes on the
screen.
Sample
• Sample holder must be transparent to IR- salts
• Liquids
– Salt Plates
– Neat, 1 drop
– Samples dissolved in volatile solvents- 0.1-10%
• Solids
– KBr pellets
– Mulling (warm)(dispersions)
• Quantitative analysis-sealed cell with NaCl/NaBr/KBr windows
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Detector
The two most popular detectors for a FTIR spectrometer are
• deuterated triglycine sulfate (DTGS) and
•mercury cadmium telluride (MCT).
The response times of many detectors (for example, thermocouple and
thermistor) used in dispersive IR instruments are too slow for the rapid scan
times (1 sec or less) of the interferometer.
The DTGS detector is a pyroelectric detector that delivers rapid responses
Because it measures the changes in temperature rather than the value of
temperature.
The MCT detector Is a photon (or quantum) detector that depends on the
quantum nature of radiation and also exhibits very Fast responses. Whereas
DTGS detectors operate at room temperature, MCT detectors must be
maintained at liquid nitrogen temperature (77 °K) to be effective. In general, the
MCT detector is faster and more sensitive than the DTGS detector.
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FTIR Advantages
• Better speed and sensitivity (Felgett advantage). A complete spectrum can be
obtained during a single scan of the moving mirror, while the detector observes all
frequencies simultaneously.
• Increased optical throughput (Jaquinot advantage). Energy-wasting slits are not
required in the interferometer because dispersion or filtering is not needed.
• Internal laser reference (Connes advantage). The use of a helium neon laser as
the internal reference in many FTIR systems provides an automatic calibration in an
accuracy of better than 0.01 Cm –1 . This eliminates the need for external
calibrations.
• Simpler mechanical design. There is only one moving part, the moving mirror,
resulting in less wear and better reliability.
•Elimination of stray light and emission contributions. The interferometer in FTIR
modulates all the frequencies. The unmodulated stray light and sample emissions (if
any) are not detected.
•Powerful data station. Modern FTIR spectrometers are usually equipped with a
powerful, com- puterized data system. It can perform a wide variety of data
processing tasks such as Fourier transformation, interactive spectral subtraction,
baseline correction, smoothing, integration, and library searching.
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