FTIR and Raman: Better Together
Both FTIR and Raman spectroscopy yield
a unique spectrum which can be used as a chemical fingerprint. The two
techniques are said to be complementary because they have different selection
rules. In order for a bond
to be IR active, it has to be polar. To be Raman active, a bond must
simply be polarizable. FTIR
is sensitive to the functional groups on molecules because they are the
ones that are more often polar. Raman is more sensitive to the carbon backbones
of molecules, especially the multiple bond regions. These regions are electron
dense and therefore have high polarizability.
The combination of these two techniques can give a high degree of confidence
to the identification of an unknown substance. Where one of the techniques
might leave a gap in information, the other technique can fill in the blanks.
Both techniques have advantages over other analytical techniques. (8)
Highlights of Infrared Spectroscopy:
(4,8)
- High range of compounds have known spectra
that can be easily matched
- Low detection limits for trace substances
- Easy sample preparation
Highlights of Raman Spectroscopy: (4,8)
- Can analyze without opening sample bags
or containers
- Completely non-destructive
- No sample preparation
- Spectra contain fewer and narrower peaks
than IR, making it easier to resolve parts of a mixture
Both have been:
- adapted for use in the field
- coupled with a microscope to decrease
necessary sample size
- enhanced with devices to increase their
sensitivity
- Attenuated Total Reflectance (ATR) for FTIR
- Surface Enhanced Raman Spectroscopy (SERS)
for Raman
- Fourier Transform (FT) for both
- a proven method in the lab with libraries
of known spectra
Portable Raman Device
Using either of these techniques alone can
provide good spectral matching for many different compounds in the lab and
in the field. Using the two together simply extends the range of compounds
and mixtures that can be matched and increases the confidence in the matching.
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