Winter 2008
CH241L
PCC Sylvania
Jenkins
Experiment #3: Infrared Spectroscopy of an Unknown Liquid and Solid
Pre-Lab
Complete items numbered 1-8 (omit number 3) as described in your lab syllabus. Make sure your data
table includes the appropriate physical properties (chemical structure, molecular weight, melting points
for solids, boiling points for liquids, hazards, and purpose) for all the possible unknowns. Remember to
write in pen in your lab notebook, and to turn in your carbon copies before you start the lab. START
YOUR PROCEDURE AND OBSERVATIONS ON A SEPARATE PAGE. Your procedure will need to
include the necessary steps to prepare a solid sample for FTIR analysis. You will need this to complete
your lab.
Additional Pre-lab questions to be written in your pre-lab and turned in at the start of lab.
1. What will happen to the spectrum if the solid sample is not ground fine enough?
2. What will happen to the spectrum of a liquid sample if too much sample is put on the salt plate?
3. What will happen to the spectrum of a liquid sample if too little sample is put on the salt plate?
Questions of the Day:
How is the identity of an organic compound confirmed with Fourier Transform Infrared Spectroscopy
(FTIR)? How does the sample preparation differ between a solid and a liquid? What do the peaks in a
spectrum represent? What are the most common errors in the spectra?
Procedure
You will work in pairs, assigned by the instructor, and will divide the work accordingly. The instructor will
provide each pair with a liquid unknown and a solid unknown. Note physical characteristics; color,
crystal structure, etc. Obtain melting point, boiling point, and index of refraction. Using the salt plate,
obtain an IR spectrum of your liquid unknown. Print enough copies to attach to each of your notebooks
(You can push Plot for a large spectrum or Print for a small spectrum). Mark all important bands. Make
a KBr pellet with your solid unknown, obtain an IR, and mark all important peaks. In addition, obtain an
IR of the empty salt plates and a KBr pellet free of any compounds. Analyze your IR spectra: Identify
functional groups. From the list of possible unknowns try to identify your unknown. Once you think you
have identified your unknown, obtain and print a reference spectrum in the lab from the following
website
http://www.aist.go.jp/RIODB/SDBS/cgi-bin/cre_index.cgi.
The following is the list of possible unknowns:
Liquids
Solids
Heptaldehyde
Oxalic acid
Toluene
Maleic anhydride
Cyclohexanol
Benzoic acid
Methyl benzoate
1-octadecanol
Methyl ethyl ketone
Phenol
Cyclopentanone
Urea
2-Butanol
Cyclohexene
p-Xylene
Post Lab Questions: Answer each Question of the Day. Your Discussion section should clearly explain
how you identified your unknown. Your spectra (both lab and reference) should be numbered in an
appendix and you should refer to these in your discussion.
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Winter 2008
CH241L
PCC Sylvania
Jenkins
Background
Paragon 1000 FT-IR Spectrometer
The Perkin-Elmer Paragon 1000 FT-IR spectrometer is a bench-top instrument that provides
interferometer alignment ports, a screen, a sample compartment, a disk drive, and a power switch in
one self-contained unit.
Warning!
DO keep the Paragon 1000 dry. Avoid spilling liquid into the instrument, especially into the top of the
display-screen housing, which contains a high-voltage supply. Clean all external spills immediately. If
anything that is spilled enters the main body of the Paragon 1000, switch off the power and call a
Perkin-Elmer Service Engineering.
DO NOT stare into the laser beam. The Paragon 1000 contains a low power, visible (red) laser;
momentary exposure to the beam is not dangerous, but deliberate, direct viewing of the beam along its
axis could damage your eye.
Read the sections pertaining to infrared spectroscopy in Chapter 12 of the textbook. The following web
sites will provide you with additional information (and some have movies):
http://www.forumsci.co.il/HPLC/FTIR_page.html
http://www.chem.uic.edu/web1/OCOL-II/WIN/SPEC/IR/IRF.HTM
http://orgchem.colorado.edu/hndbksupport/irtutor/tutorial.html
http://www.chem.ualberta.ca/~orglabs/spectroscopy/specmaster.html
http://orgchem.colorado.edu/hndbksupport/movies.html
For preparation of samples – KBr
http://www.ijvs.com/volume1/edition1/section1.html
http://orgchem.colorado.edu/hndbksupport/movies.html
http://www.dq.fct.unl.pt/qoa/jas/ir.html
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Winter 2008
CH241L
PCC Sylvania
Jenkins
Infrared radiation, which is electromagnetic radiation of longer wavelength than visible light, is detected
not with the eyes but by a feeling of warmth on the skin. When absorbed by molecules, radiation of this
wavelength (typically 2.5 to 25 um) increases the amplitude of vibrations of the chemical bonds joining
atoms. The presence and also the environment of functional groups in organic molecules can be
identified by infrared spectroscopy. The small quantity of sample needed, the speed with which a
spectrum can be obtained, and the wide application of the method combine to make infrared
spectroscopy one of the most useful tools available to the organic chemist.
Scanning and Viewing:
1. Before the Paragon 1000 can collect a sample spectrum it needs a valid background
spectrum against which it can ratio the sample scan. Make sure that the sample slide is
empty and press the green long SCAN button, then BACKG, and finally the button below
EXECUTE on the screen. When the scan is complete, the background spectrum is
displayed briefly. It is immediately replaced with the default region. Default region: The
Paragon 1000 contains four memory regions for the temporary storage of spectral data.
These regions are called spectrum X, Y, Z, and Backg. The Paragon 1000 always
designates one to be the default region. We use region X. To display the background
spectrum, press BACKG.
2. Open the sample compartment by lifting the cover using the recess at the front. Place the
sample on the sample holder and close the sample compartment.
3. Press the green scan button, choose a region W, Y, or Z, indicate 4 scans, and press the
button below execute. Wait until the four scans are done and the computer has finished
calculating and has returned to “ready.”
Viewing More Than One Spectrum
The Paragon 1000 can display up to three spectra at one time. This helps you to compare spectra, for
example, when you are comparing the product to the starting material.
Press the Z button and hold it down while you press X. Release both keys. Both X and Z spectrum
regions are displayed, overlaid. The two spectra are displayed in different colors.
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Winter 2008
CH241L
PCC Sylvania
Jenkins
Using the Vertical Cursor and Mark Buttons
You can use the vertical cursor to determine the wavenumber and ordinate value of a peak and the
mark button to display the values on the printed spectrum.
1. Press shift then vcursor (vertical cursor). A vertical line appears.
2. Move the line with the arrow buttons to the peak you want to mark.
3. Press shift then mark and go to the next peak.
4. Before plotting the spectrum, remove the vertical line by pressing shift then vcursor again.
Automatic Expansions
The autex button (automatic expansion) button will expand the spectrum vertically to fill the display
area. This only changes how the spectrum appears on the screen, the data is not changed. To use the
automatic expansion: press shift then autex.
Plotting the Spectrum
To plot the spectrum, press plot. The print button will print the entire screen.
Spectra of Neat Liquids:
To run a spectrum of a neat (free of water!) liquid, remove the NaCl plates from the dessicator. Be
sure to handle the NaCl plates very carefully. They are very expensive, very breakable, and not easy
to replace. Pick up the plates by the edge only. You do not want an IR spectrum of your oily
fingerprint. Place a drop of the liquid on one plate, and carefully place the other NaCl plate on top.
Press the plates together gently to remove any air bubbles. Place the plates in a vertical position on
the sample holder and obtain a spectrum. Mark and expand your spectrum as necessary and then plot.
While your spectrum is plotting, carefully remove the NaCl plates and rinse them with acetone. Replace
the dry plates in the envelopes and dessicator for the next student.
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Winter 2008
CH241L
PCC Sylvania
Jenkins
Potassium Bromide (KBr) Disk
The spectrum of a solid sample can be run by incorporating the sample in a potassium bromide (KBr)
disk. This procedure needs only one disk to cover the entire spectral range, since KBr is completely
transparent to infrared radiation. Although very little sample is required, making the disk calls for
special equipment and time to prepare it. Since KBr is hygroscopic, water is a problem.
Recommended Method
1. Transfer weighed amounts of sample, approx 2 mg, and KBr powder, approx. 200 mg into an
agate mortar. The KBr powder must be of spectroscopic grade purity, and be spectroscopically
dry.
2. Grind the powders together, with an agate pestle, until the sample is well dispersed and the
mixture has the consistency of fine flour.
3. Gather all the equipment for making the KBr pellet.
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Winter 2008
CH241L
PCC Sylvania
Jenkins
4. Place one of the screws into the bolt and add one small scope of the KBr mixture.
5. Place the second screw into the bolt and hold by the vise clamp.
6. Apply pressure to the KBr sample by tightening the second screw.
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Winter 2008
CH241L
PCC Sylvania
Jenkins
7. Remove both of the screws and place the bolt on the sample holder in the sample compartment
of the FT-IR. The KBr should transformed into a translucent window.
The spectrum quality is affected by the quality of the disk. The flatness of the baseline is dependent on
the particle size and dispersion of the sample in the KBr powder. Check the disk and spectrum for the
following faults:
A. If the disk breaks on removal from the die, this indicates that the disk is too thin caused by too little
powder, or too much pressure for too long. Remedy this effect by increasing the sample load.
B. If the disk is not translucent, this can have numerous causes.
1.
2.
3.
4.
5.
6.
Uneven distribution of powder in die
Too much sample
Too much KBr powder
Poorly dispersed sample
Water in disk
Pressed at too low pressure or for too short a time
All except the water in the disk can be remedied by re-grinding and pressing with adjusted amounts.
Water in the disk can be present due to a wet sample or wet KBr powder. Heating the disk at approx
100°C for a few minutes and repressing the disk will sometimes remove residual water.
C. Truncated Bands. If the spectrum contains bands which have a flattened turning point and do not
reach 0 % T, this is caused by a poorly dispersed sample or holes in the disk. Check the disk visually
and if necessary repeat the preparation.
D. Sloping baseline. This is usually due to a poorly dispersed sample.
The faults listed above have been commonly found, sometimes in publications, but is not necessarily
exhaustive. Use of other halide salts can overcome some effects, or extend the range of the spectrum
examined, e.g. CsI extends the lower wavenumber range from 400 to 200 cm -1. Poor sample
preparation can lead to avoidable errors in interpretation of the resultant spectrum. A little care can
avoid the need for repetitious sample preparation or embarrassing errors in results.
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