Fourier-Transform Infrared Spectroscopy
Unknown Analysis Experiment
Dr. Gergens - Mesa College
Identification of an Unknown
In this experiment, an unknown liquid or solid will be issued for FTIR identification. Since no two molecules of different s tructure have
exactly the same infrared absorption spectrum, the infrared spectrum can be used for molecules much as a fingerprint can be u sed
for humans. By comparing the infrared spectrum of your unknown to an infrared spectrum of a substance thought to be i dentical, one
can establish whether or not they are in fact identical. Word processing the data correctly and carefully will allow you to make quick
comparisons between spectra. The unknown will be a substance on a list of possibilities provided in laboratory.
Procedure
0.
Review the documentation provided for working the instrument before coming to class.
1.
RUN your sample. Please ask for help if you need assistance in working the instrument.
2.
SCALE the spectrum and do a PEAK PICK.
3.
LABEL your spectrum. Use the peak picks function to select an area in the upper left hand corner and input the following: your
name, unknown number, date, and how the sample was prepared (i.e., thin film, neat, KBr, Nujol mull).
4.
SAVE your spectrum to USB as a <WindowMetaFile> file. A file name should be less than eight characters long.
5.
PRINT a hard copy of the spectrum.
6.
Have your instructor inspect and initialize your spectrum with their approval.
7.
PRESS and HOLD the eject button so your USB ejects safely.
8.
KEEP TRACK of your LOGIC in our laboratory notebooks. Identify broad visual patterns, stretching frequencies of vibration (c m–
), intensity, and shape. Concentrate on trying to establish the presence or absence of conspicuous peaks. For example, the
region at 1710 cm– is the first region to consider whether there is a carbonyl, C=O. The absence of peaks may also be noted in
the correlation table. Have your instructor help you if you are not clear as to how to interpret an infrared spectrum.
9.
Record stretching frequencies of vibration (cm –), intensity, and shape into a correlation table in your notebook. This does not
mean you need to record all absorbencies, only pertinent ones.
10.
Predict the functional group classification for your compound. Your unknown will be a substance on a list of possibilities provided
in the laboratory. By process of elimination, narrow your search to a few possibilities. Be sure to report all possibilitie s for your
unknown and discuss how you eliminated each one in your conclusion.
11.
Research reference spectra for your unknown. Reference data are available from a variety of sources:
a.
Organic chemistry textbooks, laboratory manuals, and spectroscopy handbooks. Our library has several on reserve.
b.
The World Wide Web. The SDBS (Japanese) site is most useful. Just Google “SDBS.” Additional spectroscopy
links are located on the organic laboratory home page for this course.
c.
Data from colleagues. Work together, share your data.
d.
Spectral libraries. The Aldrich Spectral Library of FTIR Spectra, or Satler Reference Spectra are available in science
library on the campus of USCD or SDSU.
e.
Employer reference library. You may already work in a laboratory that has a reference library.
f.
Simulated spectra. Some spectral data can be generated by spectral simulation programs (i.e., Beaker).
12.
Correctly identify your unknown.
13.
Complete the post laboratory exercise and input that data into the appropriate course management software.
FTIR Formal Report
1.
Correctly identify the unknown. A tutorial review of the fundamentals infrared spectroscopy will be provide to you.
2.
Apply the word processing skills learned in previous experiments to complete a data page like the one on the next page. In
Microsoft Word, use the <INSERT>, <PICTURE>, <FROM FILE> commands to insert your FTIR data file. Scale all images in an
appropriate manner. You may have to use the command <PASTE SPECIAL> under <INSERT> to paste in images.
3.
Write a formal conclusion identifying your unknown and your logic you used to determine its identity. Include the points (a-g) in
the conclusion.
a.
Begin your conclusion exactly as shown using similar language and paste an image of your unknown into the report.
QuickTime™ and a
decompressor
are needed to see this picture.
b.
Write a spectral interpretation for the unknown compound, BUT don't over do it. In the discussion, interpret the most important
spectral features used to correctly identify the unknown. Validate you LOGIC.
c.
Describe why some functional groups were eliminated while others were retained in the identification.
d.
Describe how certain unknowns from the list of possibilities in the laboratory were eliminated.
e.
Describe and discuss any anomalies in the appearance of the infrared spectrum for the unknown relative to the appearance for
the reference spectrum. The unknown and reference spectra may match, but not exactly. This could be due to differences in
sample preparation, which may cause slight variances between the appearance of the unknown and reference spectra. Perhaps
the reference spectrum was run as a KBr pellet, but your sample was prepared as a thin film. Or, during sample preparation t he
sample became wet due to moisture in the air. In this case, an O—H stretch at 3500 cm– region appears in the spectrum but is
absent from the reference spectrum. Discuss these types of notable differences in the conclusion.
f.
YOUR DATA must be word process EXACTLY as the sample report and of that shown on the next page.
g.
Give a bibliography and reference all sources for spectral data, and other literature used in the analysis and identification.
Grading - This assignment will be graded with the following criteria in mind:
1.
The appearance of your FTIR spectrum of the unknown sample. Did your place your UNKNOWN number and your name on
your spectrum when you worked up the data?
2.
A correlation table with all pertinent absorbencies needed to unambiguously identify the unknown.
3.
Correctly aligned and word processed data like that shown in the sample report and as shown on the next pages.
4.
The correct functional group classification for the unknown.
5.
The exact identification of the unknown.
6.
A typed conclusion and explanation and logic used to correctly identify your unknown.
7.
The overall appearance of word-processed report.
8.
A bibliography of references used in the analysis.
Correlation Table
Specific Functional Group
Observed Frequency Vibration (cm–)
Intensity
Shape
acid O–H
3500-2800
moderate
broad
ester C=O stretch
1761.7
strong
sharp
acid C=O stretch
1693.6
strong
sharp
C=C stretch
1617.0
weak
sharp
C–O stretch
1200.0
strong
sharp
C–H bend
1472.3
weak
sharp
Experimental FT-IR Spectrum - Aspirin
Actual FT-IR Spectrum - Aspirin1
O
C-O-H
O C-CH3
O
mp 135°C
1.
http://aist.go.jp/RIODB/SDBS/sdbs