Introduction to Spectroscopy - Vibrational Spectroscopy for Tissue Analysis - page 8

requiring qualitative and quantitative analysis, the potential of IR spectros-
copy to identify chemical components via fingerprinting analysis of their
vibrational spectrum is unsurpassed [23].
Infrared spectroscopy mainly deals with the infrared region of the electro-
magnetic spectrum and most commonly focuses on absorption spectroscopy
(when the frequency of the IR is the same as the vibrational frequency of a
bond, absorption occurs). When a material is exposed to infrared radiation,
absorbed radiation usually excites molecules into a higher vibrational state.
The wavelengths that are absorbed by the sample are characteristic of its
molecular structure [25]. However, similar to other spectroscopic techniques,
this technique can also identify molecular structure and investigate sample
composition, and the spectral bands are indicative of molecular components
and structures [26].
The technique works on the fact that chemical bonds or groups of bonds
vibrate at characteristic frequencies [27]. A molecule that is exposed to infra-
red rays absorbs infrared energy at frequencies that are characteristic of that
molecule. Infrared spectroscopy is based on the fact that molecules have spe-
cific vibrational modes that can be activated provided that they are hit by
photons with specific energy levels. However, in order for a molecule or a
chemical bond to be IR active, it must be in a dipole condition. This technique
works almost exclusively on samples with covalent bonds. This is different
from the Raman effect, which mainly deals with the polarisability of chemi-
cal bonds [28]. Therefore, in simple terms, IR spectroscopy detects changes in
the dipole moment of the molecules, whereas Raman spectroscopy analyses
change in the polarisation of molecules. For a molecule to be infrared active
it must have a dipole moment.
FTIR spectroscopy is a type of vibrational spectroscopy technique and is
the most useful tool available to a scientist when it comes to solving a problem
that involves discovering the molecular structure, molecular behaviour, or the
identification of unknown organic chemical substances and mixtures [29,30].
When the material under investigation is put into an FTIR spectrometer, it
will absorb the radiation emitted (typically infrared radiation) and the suc-
cessful absorption will display the uniqueness or fingerprint of the material
under investigation. Discovering the frequency of a particular material can
be done without the use of an FTIR spectrometer [31].
Electromagnetic radiation comes in two distinct forms—in the form of
particles or in the form of waves. It is well understood that infrared radia-
tion is, in actual fact, electromagnetic in origin [31]. The energy ( E ) of any
electromagnetic radiation can be expressed by this equation:
E = hν
where h is Planck's constant and ν is the frequency of radiation. This equation
is known as Planck's law. An infrared spectrum plots the absorption of a mate-
rial (or to be more accurate, the basal molecules) onto an infrared spectrum [30].