Introduction to Spectroscopy - Vibrational Spectroscopy for Tissue Analysis - page 12

Table 1.2
Comparison between FTIR and Raman Spectroscopy

Infrared Raman

Physical
effect

Absorption. Analysing the change in
the dipole moment of the
molecules (strong: ionic bonding
such as; O−H, N−H, C=O).

Scattering. (Monitoring the emission
of scattered light). Observes change
in the polarisation of molecules
(strong: covalent bonding such as;
C=C, C−S, S−S, aromatics).

Sample
preparation

Sample preparation required in
majority of cases (part from using
photo-acoustic sampling technique,
which eliminates the need of
sample preparation).

Little or no sample preparation

Problems Water (has as strong signal) Fluorescence can be an issue and

some glasses do possess extremely
high fluorescence. In certain cases
quartz glass is required to do analysis.

Materials Mainly organic compounds. Certain

inorganic molecules can be
analysed. Synthetic and natural
materials can also be analysed.

Nearly a complete range of samples,
organic, inorganic, polymers, and
biological (both in wet and dry
conditions) can be analysed.

Resolution: 1-20 µm (varies with beam splitters) 0.05-8 µm (varies with lasers)

Frequency
range

4000-400 cm −1 (Mid-IR)
Far and Near-IR also possible,
making analysable range to 30,000
to 50 cm −1

4000-50 cm −1

Samples Non-aqueous Aqueous

is due to changes in dipole moment during molecular vibration, whereas
Raman spectroscopy involves a change in polarisability [14].
Another difference existing between the techniques is their resolution.
While Raman has lateral resolution of 1-2 µm and confocal resolution
of 2.5 µm, FTIR does not provide the possibility of confocal resolution, and
its lateral resolution is 10-20 µm [29]. It also must be mentioned that FTIR is
mainly for organic compounds, but Raman can deal with nearly unlimited
types of samples.
Mahadevan-Jansen and Richards-Kortum [11] describe the advan-
tages of FTIR as producing spectra with a superior signal-to-noise ratio
and an improved resolution as compared to some of the conventional
IR spectrometers, but note that it may increase spectra collection times.
Chowdary et al. [42] report Raman spectroscopy as being superior to FTIR
techniques based on having higher spatial resolution, less sample prepa-
ration, reduced water absorption bands, and less harmful near-infrared
radiation.