ftir & uv vis theory

8/3/2019 Ftir & Uv Vis THEORY

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THEORY

Fourier Transform Infrared (FTIR)

Fourier Transform Infrared (FTIR) spectrometry was developed in order to overcome the

limitations encountered with dispersive instruments. The main difficulty was the slow scanning

process. A method for measuring all of the infrared frequencies simultaneously, rather than

individually, was needed. A solution was developed which employed a very simple optical

device called an interferometer. The interferometer produces a unique type of signal which has

all of the infrared frequencies fixed into it. The signal can be measured very quickly, usually on

the order of one second or so. Thus, the time element per sample is reduced to a matter of a few

seconds rather than several minutes. Most interferometers use a beam splitter which takes the

incoming infrared beam and divides it into two optical beams. One beam reflects off of a flat

mirror which is fixed in place. The other beam reflects off of a flat mirror which is on a

mechanism which allows this mirror to move a very short distance (typically a few millimeters)

away from the beam-splitter. The two beams reflect off of their respective mirrors and are

recombined when they meet back at the beam-splitter.

Because the path that one beam travels is a fixed length and the other is constantly

changing as its mirror moves, the signal which exits the interferometer is the result of these two

beams interfering with each other. The resulting signal is called an interferogram which has

the unique property that every data point (a function of the moving mirror position) which makes

up the signal has information about every infrared frequency which comes from the source. This

means that as the interferogram is measured; all frequencies are being measured simultaneously.

Thus, the use of the interferometer results in extremely fast measurements. Because the analyst

requires a frequency spectrum (a plot of the intensity at each individual frequency) in order to

make identification, the measured interferogram signal cannot be interpreted directly. A means

of decoding the individual frequencies is required. This can be accomplished via a well -known

mathematical technique called the Fourier transformation. This transformation is performed by

the computer which then presents the user with the desired spectral information for analysis.

Infrared spectrophotometer is a powerful tool for identifying pure organic and inorganic

compounds because, with the exception of a few same mononuclear molecules such as O 2, N2

and Cl2, all molecular species absorb infrared radiation. Each molecular species has a unique


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infrared absorption spectrum. Therefore, exact match between the spectrum and of a compound

of known structure and the spectrum of analyte unambiguously identifies the analyte. Infrared

spans a section of the electromagnetic spectrum by wavenumbers, , in the range of 12,800 to 10

cm-1

and the wavelength, , ranging from 0.78 - 1000m. Both wavenumber and wavelength

represent IR absorption. Wavenumber unit is more commonly used in modern IR instruments

that are linear in the cm-1

scale. In the contrast, wavelengths are inversely proportional to

frequencies. Wavenumber and wavelength can be interconvert using following equation:

( )

( ) (eq. 1)

Transmittance, T, is the ratio of radiant power transmitted by the sample (I) to the radiant

power incident on the sample (I0). Absorbance, A, is the logarithm to the base 10 of the

reciprocal of the transmittance.

( ) (eq. 2)

The transmittance spectra provide better contrast between intensities of strong and weak

bands because transmittance ranges from 0 to 100% T whereas absorbance ranges from infinity

to zero. The 0% T and 100% T adjustments should be made immediately before each

transmittance or absorbance measurement. To obtain reproducible transmittance measurements,

it is essential that the radiant power of the source remain constant while the 100% T adjustment

is made and the % T is read from the meter.

Like as stated above before, the infrared region of the spectrum encompasses radiation

with wave numbers ranging from about 12,800 to 10 cm -1 or wavelengths from 0.78 to 1000 m.

From the perspective of both application and instrumentation, the infrared spectrum is

conveniently divided into near-, mid- and far- infrared radiation; rough limits of each are shown

in table below.


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Table 1: Infrared Spectral Regions

RegionWavelength ()

Range, m

Wavenumber ()

Range, cm-1

Frequency () Range,

Hz

Near 0.78 to 2.5 12,800 to 40003.8 x 10

14to 1.2 x

1014

Mid 2.5 to 50 4000 to 2001.2 x 1014 to 6.0 x

1012

Far 50 to 100 200 to 106.0 x 10

12to 3.0 x

1011

Most used 2.5 to 15 4000 to 6701.2 x 10

14to 2.0 x

1013

Figure 1: Instrument Diagram of Basic FTIR

Adapted from http://binoybnair.blogspot.com/2007_10_01_archive.html


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Figure above shows the instrument diagram for a basic FTIR spectrometer. Radiation of

all frequencies from the IR source is reflected into the interferometer where it is modulated by

the moving mirror on the left. The modulated radiation is then reflected from the two mirrors on

the right through the sample in the compartment at the bottom. After passing through the sample,

the radiation falls on the detector. A data acquisition system attached to the detector records the

signal and stores it in the memory of a computer as an interferogram. (Courtesy of Thermo

Electron Corp., Madison, WI).

Figure 2: IR Spectroscopy Apparatus

Adapted from

http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_205:_Larsen/ChemWiki_Module_Topics/H

ow_an_FTIR_Spectrometer_Operates

In most UV-VIS, the cell is located between the monochromator and the detectors in

order to avoid photodecomposition of the sample, which may occur of samples, are exposed to

the full power of the source. Photodiode array instruments avoid this problem because of the

short exposure time of the sample to the beam. In IR radiation, in contrast, is not sufficiently

energetic to bring about photodecomposition. Most samples are good emitters of IR radiation.

Because of this, the cell compartment is usually located between the source and the

monochromator in IR instrument.
http://upload.wikimedia.org/wikipedia/en/f/fc/IR_spectroscopy_apparatus.jpeg


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As stated before, the components of IR instruments differ significantly from UV-VIS

instruments. Thus, IR sources are heated solids and IR detectors respond to heat rather than

photons. FTIR which is widely used nowadays, detect all the wavelengths all the time. They

have greater light-gathering power than dispersive instruments and consequently better precision.

It contains no dispersing element and all wavelengths are detected and measured simultaneously.

Instead of monochromator, an interferometer is used to produce interference patterns that contain

the infrared spectral information. To obtain radiant power as a function of wavelength, the

interferometer modulates the source signal in such a way that it can be decoded by the

mathematical technique of Fourier transformation.

The energy of infrared radiation can excite vibrational and rotational transitions, but it is

insufficient to excite electronic transitions. Variations in rotational levels may give rise to a

series of peaks for each vibrational state. The number of molecule can vibrate is related to the

number of atoms and the number of bonds, it contains. Infrared absorption occurs not only with

organic molecules but also with covalently bonded metal complexes, which are generally active

in the longer wavelength infrared region.

The major type of molecular vibrations are stretching and bending. Infrared radiation is absorbed

and the associated energy is converted into these types of motion. The absorption involves

discrete, quantized and energy levels. However, the individual vibrational motion is usually

accompanied by other rotational motions. Figure below shows major vibrational modes.

Vibrations as stated above fall into stretching and bending. Stretching involves

continuous change in the interatomic distance along the axis of the bond between two atoms.

While bending are characterized by a change in the angle between two bonds and are of four

types; scissoring, rocking, wagging and twisting. Infrared radiation is not energetic enough to

bring about the kinds of electronic transitions that have encountered in UV-VIS. Absorption of

infrared radiation is thus confined largely to molecular that have small energy differences

between various vibrational and rotational states. In order to absorb infrared radiation, a

molecule must undergo a net change in dipole moment due to vibrational and rotational motion.

However, no net change in dipole moment occurs during the vibration or rotation of

homonuclear species such as O2, N2 and Cl2. Consequently, such compounds cannot absorb in

infrared. Absorption causes increase in vibration amplitude/rotation frequency.


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Figure 3: Types of Molecular Vibrations of CH2

As noted that the approximate frequency (or wave number) at which organic functional

group, such as absorbs infrared radiation can be calculated

from the masses of the atoms and the force constant of the bond between them. These

frequencies, called group frequencies, are seldom totally invariant because of interactions with

other vibrations associated with one or both of the atoms composing the group. Table below lists

group frequencies for several common functional groups.


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Functional Group NamesAbsorption Ranges

Frequency (cm--1)Type of Vibration

Alkanes

3000-2800

1500-1450

Alkenes

3100-3000

1675-1600

Alkynes3300-3200

2200-2100

Aromatic Rings

3100-3000

1600-1580

1500-1450

Alcohols, Phenols3600-3100

(Note: Phenols MUST have

Aromatic Ring Absorptions

too)

1300-1000

Nitriles

2300-2200


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Ketones

1750-1625

Aldehydes 1750-1625

2850-2800

2750-2700

Carboxylic Acids

3400-2400

(Note: This peak always

covers the entire region

with a VERY BROAD

peak.)

1730-1660

Ethers

1300-1000

Esters1735

1300-1000

Amines: Primary3500-3200

(TWO PEAKS!)

1640-1560

Amines: Secondary3500-3200

(ONE PEAK!)

1550-1450

Nitro Groups1600-1500

1400-1300


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Amides3500-3100

1670-1600

1640-1550

UV- VIS

The principle of UV described that molecules have ability to absorb ultraviolet or visible light.

UV-vis spectroscopy is useful as an analytical technique for two reasons which are to identify

some functional groups in molecules and determining the content and strength of a substance.

Spectrophotometers are made up of stable source of radiant energy, a transparent sample

container, a device for isolating specific wavelength, a radiation detector which converts

transmitted radiation to a usable signal and a signal processor and readout. A wave is usually

described in terms of its wavelength, , and frequency, , which is the number of peaks passing a

given point per second. Table 1 showed the different regions of wavelength. All the

electromagnetic radiation travels through a vacuum at the same velocity. This velocity (c) is

called the velocity of light which is 2.99792458 X 108 ms-2. It can be connected to wavelength

and frequency as c= .

Region Wavelength (nm)

Far ultraviolet 10 - 200

Near ultraviolet 200 - 380

Visible 780 - 3000

Near infrared 3000 - 30,000

Middle infrared 30,000 - 300,000

Microwave 300,000 - 1,000,000,000

Table 2: The regions of wavelength.

UV-vis spectrum, primarily provide structural information about the kind and extent of the

conjugation of multiple bonds in the compound being analyzed. The absorption of UV-vis


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spectroscopy is caused by transfer of energy from the radiation beam to electrons in orbitals of

lower energy, causing them to be excited into orbitals of higher energy. The excitation process

proceeds by promoting an electron from an occupied to an unoccupied molecular orbital. Every

molecule has their own ground electronic states with all electron spins in paired. Most excited

electronic states also have all paired electron spins. Excited electronic states may be have two

orbital that each of it posses only one electron. The energy of quantum of electromagnetic energyis directly related to its frequency:

E = h (eq. 3)

Where, h= Planks constant = 6.63 x 10-34

Js

=frequency (Hz)

This means, the higher energy the frequency of radiation, the greater is its energy. Since = c/,

the energy of electromagnetic radiation is inversely proportional to its wavelength.

(eq. 4)

Most spectra ofthe solution obey Beers Law. This states that the light absorbed is proportional

to the number of absorbing. The second law is Lamberts law. It tells us that the fraction of

radiation absorbed is independent of the intensity of the radiation. Combining these two laws

gives the Beer-Lambert law:

Log10I0/I= lc (eq. 5)

Where =

I0

= the intensity of the incident radiationI= the intensity of the transmitted radiation

= the molar absorption coefficient

l = the path length of the absorbing solution (cm)

c= the concentration of the absorbing species in mol dm-3


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(Log10Io/Ialso is called absorbance of optical density or A)

The absorbing groups in a molecule are called chromophores. Chromophore (literally color-

bearing) group is a functional group, not conjugated with another group, which exhibits a

characteristic absorption spectrum in the ultraviolet or visible region. Some of the more

important chromophoric groups are list in the table 2.

Name Chromophore Wavelength, (nm) Molar extinction, e

Acetylide -C C 175-180 6000

Aldehyde -CHO 210 1500

Alkynyl -C C- 160-170 8000

Azo -N=N- 285-400 3-25

Carboxyl -COOH 200-210 50-70

Ester -COOR 205 50

Ether -O- 185 1000

Ketone -C O 195 1000

Nitrile -C N 160 -

Table 3 : UV absorption maxima.

There are few types of UV-Vis those are famously and widely used namely single-beam

instrument, double-beam instrument and multichannel instruments. Figures below shows the

block diagram of each instrument:

Figure 4 : Single beam


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Figure 2 : Double beam (in space)

Figure 5 : Multichannel

For single beam instrument, 0% transmittance is set with shutter in the beam path while 100%

transmittance is set with a reference in the beam path. Measurement is made with the sample in

the beam path. While for double beam instrument, sample and reference are measured

simultaneously and the signal from reference is substracted from sample signal. Double beam

instrument is more widely used because: i) its compensate for variations in the source intensity,

ii) compensate for drift in detector and amplifier, iii) compensate for variation in intensity as a

function of wavelength.

ftir & uv vis theory

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