Infrared Absorption Spectroscopy Flashcards
a region of the electromagnetic
spectrum covering the range from just above the
visible (7.8 π₯ 10β7) to approximately 10β4 m.
Infrared (IR)
Frequencies are given in wavenumbers rather than in
hertz
spectroscopic method used for
identifying pure organic and inorganic compounds. With the
exception of some homonuclear molecules, all molecular
species absorb infrared radiation.
Infrared Spectroscopy
What are the advantages of IR spectroscopy?
- Qualitative and quantitative analysis
- Sample preparation
- Sensitive and time saving
- Itβs versatility
- Easy for interpretation
Why does a molecule absorb some wavelengths of IR radiation?
A pair of atoms joined by a covalent bond
can be thought of as being like balls on the
end of a vibrating spring. The bond can
vibrate with different amounts of energy at a
frequency that depends on the masses of the
atoms and the strength of the bond.
Why does a molecule absorb some wavelengths of IR radiation?
At room temperature, most bonds will vibrate
with the lowest possible amount of energy. But
if radiation of the right frequency is supplied,
the bond can absorb energy and vibrate with
greater amplitude
is an instrument that uses infrared radiation to obtain a complete spectrum of the analyte for qualitative
identification.
Fourier Transform Infrared Spectrometer
identifies chemical bonds in a molecule by
producing an infrared absorption spectrum.
FTIR
Advantages of FTIR
- High sensitivity, resolution and speed of data
acquisition - All wavelengths are detected and measured
simultaneously using Michelson interferometer that can
be recorded as an interferogram and subsequently
decoded by Fourier transform which is a mathematical
operation that is conveniently carried out by the
computer. - The measured spectrum is complete with different
display options (%T, A, zoom, peak, height, and peak
area)
Parts of an Infrared Spectrum:
- Wavenumber
- Transmittance
- Peaks
- Functional Group Region
- Fingerprint Region
tells us about the very particular energy of infrared light.
Wavenumber
percentage of a [articular wavenumber that reaches the
detector.
Transmittance
means that IR light that corresponds to that
wavenumber is not being absorbed by the sample.
100% transmittance
shows that the IR light of the particular wavenumber is not making
through to the detector and is being absorbed by the sample.
Peaks
region where most of the information is used to interpret the IR spectrum (4000 β 1500 ππβ1)
Functional Group Region
serves as a unique fingerprint of a compound that we can
use to compare with a fingerprint of a known substance in a database. This region
important in finding an exact match of the sample. (1500 - 400 ππβ1)
Fingerprint Region
Two major instruments used to measure IR absorption
- Dispersive spectrometer
- FTIR
- It was introduced in the mid-1940s.
- It provided the robust instrumentation
required for the extensive application of
this technique
Dispersive spectrometer
Dispersive spectrometer components
- source of radiation
- monochromator
- detector
The higher the temperature, the faster
will be the movement of molecules. These moving
molecules emit energy in the form of infrared
radiation.
As temp increases, intensity of every emitted wavelength increases
Limiting facts that require consideration when choosing an IR source.
- The material should be thermodynamically stable; otherwise it would
quickly break down and need replacing. - The material should not an IR absorber.
Source of Radiation of a Dispersive Spectrometer:
- Nernst glower
- Globar
- Nichrome Coil
- capable of hotter
temperatures than a Globar - fabricated from a mixture of
refractory oxides - uses a small ceramic rod that
was heated to incandescence - can reach temperatures of
2200 K
Nernst Glower
- Silicon Carbide Rod
- most ubiquitous IR source (present, appearing, or found everywhere.)
- a resistively heated silicon
carbide rod - An electric current is passed through the bar which become very hot, producing large amounts of IR radiation
Globar
- Nichrome and Kanthanl wire coils
where also once popular IR
sources. - They too did not require water
cooling, ran at lower
temperatures than a Globar, and
possessed lower emissivity
Nichrome Coil
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.
Monochromator
are the dispersive components used in conjunction with slit
mechanisms.
Prisms or gratings
Narrower slits enable the
instrument to better distinguish more closely spaced frequencies
of radiation, resulting in better resolution.
Wider slits allow
more light to reach the detector and provide better system
sensitivity.
Thus, certain compromise is exercised in setting the
desired slit width
amount of spectral
detail in a band based on the number
and width of spectral bands
Spectral Resolution
Most detectors used in dispersive IR spectrometers can be categorized into two classes:
- thermal detectors and
- photon detectors
- They sense the change of temperature of a detector material due to
absorption of the IR radiation. - It includes Golay detectors, thermistors, and thermocouple.
- The thermal detectors also have a broader IR spectral response
which allows various wavelengths in the IR region to be detected.
However, thermal detectors are less sensitive and slower
(milliseconds domain) than quantum detectors (microseconds
domain)
Thermal Detector
expansion of a non-absorbing gas
It a small hollow cell filled with a non-absorbing gas such as xenon.
Golay detector (pneumatic detector)
electrical resistance of semiconductor
The resistance variations of the thermistor will
appear as variations in the voltage drop across, or current
through the thermistor is an indication of the resistance
variations experienced by a thermistor in response to
incident radiation.
Thermistor
voltage of junction of dissimilar metals
consists of two dissimilar metals (often bismuth
and antimony). connected in series. To detect radiation, one junction is
blackened to absorb the radiation and when the metals are heated by IR
radiation, a small voltage, proportional to the temperature at the junction
between the 2 metals, is sent out
Thermocouple
- Rely on the ineteraction of IR radiation and a semiconductor
- More expensive as these materials are not widely available
- Photon detectors count photons of light. A photon detector has
some surface that absorbs photons and produces some effect
(current, voltage) proportional to the number of photons
absorbed. - 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.
Photon detector
FTIR instruments are relatively inexpensive, sturdy, stable, flexible, and
fast.
- Instead of viewing each component frequency sequentialy, all
frequences are examined simultaneously in FTIR spectrometer. - It replaced the dispersive spectrometer for most applications due to
their superior speed and sensitivity
FTIR
FTIR spectrometer components
- Source of Radiation,
- Interferometer, and
- Detector
The same types of radiation source are used for both despersive and FTIR spectrometer.
However, the source is more often water-cooled in FTIR instruments to provide better power
and stability
Radiation source
The monochromator in the dispersive spectrometer is being replaced by interferometer.
The most commonly used a Michaelson Interforemeter.
Interferometer
made of a special material that transmits half of the radiation striking it and
reflects the other half
Beam splitter
is nothing more than a flat highly reflective
surface.
Stationary mirror
There is present only one
moving part in an FTIR spectrometer, its oscillating mirror. The air bearings eliminate friction that would
inevitable cause the moving parts of the mirror to break down, as is the case for the mechanical
bearings.
Moving mirror
Most detectors used in FTIR can be also categorized into two classes, as in Dispersive
Spectrometer:
- thermal detectors and
- photon detectors
The two most popular detectors for FTIR are
deuterated triglycine sulfate (DTGS) and mercury
cadmium telluride (MCT) detectors.
A very sensitive room-temperature detector for mid-infrared range measurements that
employs temperature-sensitive ferroelectric crystals of deuterated triglycine sulfate.
Deuterated triglycine sulfate (DTGS) Detectors
temperature at
which certain magnetic materials undergo a sharp change in
their magnetic properties
Curie point (Curie Temperature)
is a photon/ quantum detector that depends on the quantum nature of radiation
and also exhibits very fast responses.
a nearly ideal material for infrared sensor applications
because of its strong IR absorption, its adjustable wavelength sensitivity, and its favorable
semiconductor properties.
Mercury Cadmium Telluride (MCT) Detector
It is an analytical chemistry technique used
in quality control and research for determining the
content and purity of a sample as well as its
molecular structure.
Molecular structure and molecular conformation
Nuclear magnetic resonance spectroscopy (NMR)
