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

Sample handling and interpretation of IR spectra


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


       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


       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


       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)



Mid IR (Vibration - rotation region)



Far IR(Rotation region)



Most used




       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



Stretching Vibration

(change in bond length & bond angle remains the same)



Bending Vibration

(bond length remains the same & bond angle may or may not change)







       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


       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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