Infrared Absorption Spectroscopy

Question 1

a region of the electromagnetic
spectrum covering the range from just above the
visible (7.8 𝑥 10−7) to approximately 10−4 m.

Answer 1

Infrared (IR)

Question 2

Frequencies are given in wavenumbers rather than in
hertz

Question 3

spectroscopic method used for
identifying pure organic and inorganic compounds. With the
exception of some homonuclear molecules, all molecular
species absorb infrared radiation.

Answer 3

Infrared Spectroscopy

Question 4

What are the advantages of IR spectroscopy?

Answer 4

1. Qualitative and quantitative analysis
2. Sample preparation
3. Sensitive and time saving
4. It’s versatility
5. Easy for interpretation

Question 5

Why does a molecule absorb some wavelengths of IR radiation?

Answer 5

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.

Question 6

Why does a molecule absorb some wavelengths of IR radiation?

Answer 6

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

Question 7

is an instrument that uses infrared radiation to obtain a complete spectrum of the analyte for qualitative
identification.

Answer 7

Fourier Transform Infrared Spectrometer

Question 8

identifies chemical bonds in a molecule by
producing an infrared absorption spectrum.

Answer 8

FTIR

Question 9

Advantages of FTIR

Answer 9

• 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)

Question 10

Parts of an Infrared Spectrum:

Answer 10

1. Wavenumber
2. Transmittance
3. Peaks
4. Functional Group Region
5. Fingerprint Region

Question 11

tells us about the very particular energy of infrared light.

Answer 11

Wavenumber

Question 12

percentage of a [articular wavenumber that reaches the
detector.

Answer 12

Transmittance

Question 13

means that IR light that corresponds to that
wavenumber is not being absorbed by the sample.

Answer 13

100% transmittance

Question 14

shows that the IR light of the particular wavenumber is not making
through to the detector and is being absorbed by the sample.

Answer 14

Peaks

Question 15

region where most of the information is used to interpret the IR spectrum (4000 – 1500 𝑐𝑚−1)

Answer 15

Functional Group Region

Question 16

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 𝑐𝑚−1)

Answer 16

Fingerprint Region

Question 17

Two major instruments used to measure IR absorption

Answer 17

1. Dispersive spectrometer
2. FTIR

Question 18

• It was introduced in the mid-1940s.
• It provided the robust instrumentation
  required for the extensive application of
  this technique

Answer 18

Dispersive spectrometer

Question 19

Dispersive spectrometer components

Answer 19

1. source of radiation
2. monochromator
3. detector

Question 20

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

Question 21

Limiting facts that require consideration when choosing an IR source.

Answer 21

• The material should be thermodynamically stable; otherwise it would
  quickly break down and need replacing.
• The material should not an IR absorber.

Question 22

Source of Radiation of a Dispersive Spectrometer:

Answer 22

1. Nernst glower
2. Globar
3. Nichrome Coil

Question 23

• 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

Answer 23

Nernst Glower

Question 24

• 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

Answer 24

Globar

Question 25

* 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

Answer 25

Nichrome Coil

Question 26

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.

Answer 26

Monochromator

Question 27

are the dispersive components used in conjunction with slit mechanisms.

Answer 27

Prisms or gratings

Question 28

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

Question 29

amount of spectral detail in a band based on the number and width of spectral bands

Answer 29

Spectral Resolution

Question 30

Most detectors used in dispersive IR spectrometers can be categorized into two classes:

Answer 30

* thermal detectors and * photon detectors

Question 31

* 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)

Answer 31

Thermal Detector

Question 32

expansion of a non-absorbing gas It a small hollow cell filled with a non-absorbing gas such as xenon.

Answer 32

Golay detector (pneumatic detector)

Question 33

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.

Answer 33

Thermistor

Question 34

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

Answer 34

Thermocouple

Question 35

* 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.

Answer 35

Photon detector

Question 36

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

Answer 36

FTIR

Question 37

FTIR spectrometer components

Answer 37

* Source of Radiation, * Interferometer, and * Detector

Question 38

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

Answer 38

Radiation source

Question 39

The monochromator in the dispersive spectrometer is being replaced by interferometer. The most commonly used a Michaelson Interforemeter.

Answer 39

Interferometer

Question 40

made of a special material that transmits half of the radiation striking it and reflects the other half

Answer 40

Beam splitter

Question 41

is nothing more than a flat highly reflective surface.

Answer 41

Stationary mirror

Question 42

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.

Answer 42

Moving mirror

Question 43

Most detectors used in FTIR can be also categorized into two classes, as in Dispersive Spectrometer:

Answer 43

* thermal detectors and * photon detectors

Question 44

The two most popular detectors for FTIR are

Answer 44

deuterated triglycine sulfate (DTGS) and mercury cadmium telluride (MCT) detectors.

Question 45

A very sensitive room-temperature detector for mid-infrared range measurements that employs temperature-sensitive ferroelectric crystals of deuterated triglycine sulfate.

Answer 45

Deuterated triglycine sulfate (DTGS) Detectors

Question 46

temperature at which certain magnetic materials undergo a sharp change in their magnetic properties

Answer 46

Curie point (Curie Temperature)

Question 47

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.

Answer 47

Mercury Cadmium Telluride (MCT) Detector

Question 48

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

Answer 48

Nuclear magnetic resonance spectroscopy (NMR)