Sample handling and interpretation of IR spectra - Instrumental Methods of Analysis B. Pharma 7th Semester

Sample handling and interpretation of IR spectra

Objectives

After this session students will be able to

•       Recognise that all three states of matter can be used for obtaining IR spectrum

•       Discuss the sample handling techniques

•       Identify the conditions for IR absorption  and importance of wave numbers in IR spectroscopy

Sampling technique

•       Solid Samples:

–                      Neat sample

–                      Cast films

–                      Pressed films

–                      KBr pellets

–                      Mull

•       Gas Samples:

–                      Short path cell

–                      Long path cell

•       Liquid Samples:

–                      Neat sample

–                      Diluted solution

–                      Liquid cell

•       Pressed pellet techniques

•       Potassium bromide (KBr) is probably the most widely used matrix material

–      Before use, it is dried for 2 hours at 105˚C

–      Sample + 100 Times KBr (1:100)

–      Mixture is transferred to a die that has a barrel diameter of 13 mm                       

–      Press at least 25000 psi

–      Clear glassy disk of about 1 mm thick obtained

–      Ready for transmission

Sample analysis procedure

1. Blank KBr  prepared

    Sample KBr  prepared

2. Blank KBr  placed in sample holder and the background  scanned

3. Sample pellets were placed in sample holder and  scanned

4. Smoothening

5. Label  the peaks

6. Printing

Sample holders

Liquid Cell        

Fixed Cell

Application

•       Identification of inorganic compounds and organic compounds

•       Identification of components of an unknown mixture

•       Analysis of solids, liquids, and gasses

•       In remote sensing

•       In measurement and analysis of Atmospheric Spectra

–      Solar irradiance at any point on earth

–      Longwave/terrestrial radiation spectra

•       Used on satellites to probe the space

Interpretation

•       Infrared Spectroscopy or Vibrational spectroscopy is concerned with the study of absorption of IR radiation, which results in Vibrational transition

•       IR spectra is mainly used in structure elucidation to determine functional group

•       Energy of molecule = Electronic energy + Vibrational energy + Rotational energy

•       IR spectroscopy - changes in vibration of molecule or absorption of energy due to vibration

•       Wavelengths usually used is 2.5-25 µm

•       IR spectra, we use wave numbers(cm^-1),the reciprocal of the wavelength in centimeters (4000-400cm^-1)

•       Because wave numbers have larger values  & easy to handle than wavelengths which will show only small differences between functional groups

•       Wave numbers -the number of waves present per cm, which can be calculated from the wavelength.                                                                   ϑ = 1 / λ

•       Wave numbers are proportional to frequency and energy

•       Every bond or portion of a molecule requires different frequency for absorption

•       Hence characteristic peak is observed for every functional group or part of molecule

•       IR spectrum is  a finger print of a molecule

•       Criteria for compound to absorb IR radiation:

•       Absorption of IR should cause change in dipole moment

•       Applied IR frequency should be equal to the natural frequency of radiation-Otherwise compound do not give IR peak

•       No net change in dipole moment occurs during the vibration or rotation of homo nuclear species  such as O₂ ,N₂ ,Cl₂; such compounds cannot absorb IR radiation

•       Infrared regions may be categorized into three distinct zones based on their wave numbers and wavelengths as under:

Region of IR	Wave length(µm)	Wave number(cm-1)
Near IR (Overtone region)	0.8-2.5	12,500-4000
Mid IR (Vibration - rotation region)	2.5-50	4000-200
Far IR(Rotation region)	50-1000	200-10
Most used	2.5-25	4000-400

 

•       Principle of IR spectroscopy is based upon the molecular vibrations - composed of stretching and bending vibrations of a molecule

•       In any molecule, atoms or groups of atoms are connected by bonds

•       These bonds are in  a continuous motion in a molecule- as a result, they maintain some vibrations with some frequency, which is characteristic to every portion of molecule

•       This is called natural frequency of vibration

Stretching Vibrations of a CH₂ Group

Bending Vibrations of a CH₂ Group

Bending Vibrations of a CH₂ Group

Molecular vibrations

Vibrations	Type
Stretching Vibration (change in bond length & bond angle remains the same)	Symmetrical
	Unsymmetrical
Bending Vibration (bond length remains the same & bond angle may or may not change)	Inplane •       Scissoring •       Rocking
	Outplane •       Wagging •       Twisting

•       Vibrations seen before are called fundamental absorption

•       They arise from excitation from the ground state to the lowest energy excited state

•       Usually spectra are complicated because of the presence of weak overtone, combination and difference bands

•       Overtones (multiples of given frequency) results from excitation from ground state to higher energy states

Summary

•       IR spectrum is obtained by plotting transmittance against wave numbers

•       IR spectrum can be obtained for solids, liquids and gases

•       The path lengths for solids and liquids are as small as fraction of a millimeter while for gases the normal path length is 10 cm

 

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