John Siller
Laboratory 6
FTIR and Raman Spectroscopy
Introduction:
The absorbance and scattering of monochromatic light can be used to identify analytes in a
solution. Using electromagnetic radiation in the infrared spectrum, Infrared spectroscopy (IR) analyzes
molecules based on their vibrational frequencies. These frequencies are unique for each molecule and
depend on the arrangement of atoms in the molecule. Only multiatomic molecules with dipole
moments produce vibrations that can be analyzed by IR. Fourier Transform Infrared Spectroscopy (FTIR)
uses an interferometer to accurately determine the wavelengths that activate each molecular vibration.
For Raman Spectroscopy the physical technique is similar to that used in FTIR but the molecular
mechanics are much different. Raman is a scattering technique that measures the scattered light
produced by molecules when they are excited by a source. The light source excites the analyte’s
electrons into a virtual state and then they fall back down to a higher or lower vibrational level than the
original. IR and Raman spectroscopy are important tools used in a range of applications. The qualitative
data produced can be used to identify unknown analytes. In this lab we analyzed samples of toluene, 2butanone, and 2-propanone on the FTIR. Unfortunately the laser source of the Raman spectroscope was
not outputting a strong enough signal, and we were not able to analyze any samples with it. Additionally
this lab was supposed to include using the IR microscope, but the germanium crystal used was broken
and the microscope was out of commission. We were able to run a number of samples of the above
molecules including mixtures of different ratios of the samples on the FTIR so all of the data reported in
this lab is from that instrument.
Procedure:
FTIR
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Make sure instrument is on and back lever is on to ensure interferometer is synced with IR
Open computer program and select IR scans
Check voltage reading and make sure it is between 1 V and 5 V
Run background scan
Return to rapid scan and run samples
Shut down FTIR
Raman
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Fill dewar with liquid nitrogen
Make sure instrument is on and power on the laser
Open software after loading sample
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Set the proper parameters for the experiment
Turn on the diode
Open the shutter switch
Adjust the laser controls and auto scale if noise is detected
Adjust the x, y, and z until center burst is at maximum
Take scan of samples
Shut down the diode and laser and turn of the instrument
Results:
FTIR scans were obtained on mixtures of toluene, 2-butanone, and 2-popanol. Each analyte was
first scanned on its own generating three unique spectra. Samples were prepared using mixtures of each
analyte in a 1:1:1 ratio, a 1:2:1 ratio, a 3:2:1 ratio, a 2:1:1 ratio, a 1:2:3 ratio, and a 1:1:2 ratio of toluene:
2-butanone: 2-propanol. Additionally an unknown mixture was analyzed. The ten spectra can be found
in my lab notebook along with a spectrum of all ten runs. All three base samples were at equal
concentrations and were taken directly from a stock solution. From the data it is easy to recognize the
broad O-H peak, the CH3 peak, and the CH2 peak associated with each molecule. The OH peak is
strongest for the pure 2-propanol spectrum and depletes as the ratio of 2-propanol decreases. It is
interesting that the 1:1:2 O-H peak is larger than the 1:2:3 O-H peak which has a greater concentration
of 2-propanol. A number of peaks are observed in the fingerprint region including the monosubsituted
benzene peak found in toluene and its mixtures.
Conclusion:
The FTIR is very useful in determining the molecular structure of an unknown sample based on
its vibrational frequencies. In Organic we used IR to determine functional groups in a variety of
hydrocarbons. IR is a good instrument to use in correlation with other instruments for identifying an
unknown hydrocarbon. Using the FTNMR and FTIR together allows for an accurate identification of
molecular structure based on functional groups and the number of carbon and hydrogen atoms. The
FTMR is fairly simple to use with minimal sample preparation and no need for calibration standards.
Indexes can be found, and are often associated with IR computer software, that match up each
vibrational energy peak to its corresponding functional group or geometry. The broad O-H peak
associated with alcohols can also be used to determine the presence of alcohols or water in a sample.
The Raman spectroscope can be used when the presence of that O-H peak hinders the identification of
the molecule. It would have been nice to run the Raman spectroscope and IR microscope, but
unfortunately the timing of this lab in the semester did not lie in the range when these instruments
were operational. Troubleshooting why the laser was not working properly was a good experience
though, because not everything will work always and it is important to be able to figure out why
something is not working. Perhaps we will get the opportunity to work with the Raman and IR
microscope next year in our independent study or by helping future Juniors in their instrumental lab.