Infra-Red Spectroscopy of Solids and Solutions
Experiment
This experiment consists of many parts and is intended to familiarize the common sample
preparation techniques practiced to obtain IR spectra (in the transmission mode).
Obtaining the Spectrum - FTIR.
Run the background* (air). Obtain the IR spectrum of the polystyrene film, by placing the
polystyrene film in the IR beam. This spectrum can be used to calibrate the instrument. For all
other samples, run the appropriate background, set each sample in the IR beam, close the cover
and obtain the spectrum after 2 minutes (to ensure the expulsion of atmospheric carbon dioxide).
*
Run the background of either a) air b) salt plates with no sample or c) the fixed path length cell
with the solvent; as needed before obtaining the IR spectrum of the sample. Always allow CO2
free - moisture free air to pass through the cell compartment of the instrument for about 2
minutes with the sample compartment cover closed, before running any data collection for the
spectrum generation.
IUPAC: Tables of wavenumbers for the calibration of IR spectrometers
Wavenumber /cm-1
3027.1 *
2924
2850.7 *
1944.0 *
1871.0 *
1801.6 *
1601.4 *
1583.1 *
1495
1454
1353
1332
1282
1181.4 *
1154.3 *
1069.1 *
1028.0 *
906.7 *
842
752
698.9 *
Intensity
9
10
7
3
3
3
9
5
10
10
5
5
3
4
4
6
8
3
3
10
10
* Wavenumbers accurate to better than 1 cm-1
[Butterworths - London (1961)]
Sample Preparation: Solids
Mulls.
In this method the solid material is suspended in oil (e.g. Nujol – a hydrocarbon liquid, or
Flourolube 1370-4000 cm-1 – a fluorinated hydrocarbon liquid, depending on the region of
interest in the IR region.. The suspension (paste) will then be held between two polished (see
page 7) salt plates of KBr or NaCl, as a thin layer.
Grind for about 2-3 minutes a dry sample (~0.01g) of the solid (glycine) using an agate mortar
and a pestle to a fine powder (would cake at the edge of the pestle and attains a glossy look, 1-2µ
particle size < IR radiation wave length). Make a fine, smooth, consistent paste of the powder in
the oil by adding a drop of the hydrocarbon oil and further grinding to a paste. Continue
grinding for five more minutes until the desired consistency is achieved.
Transfer a small amount of this paste on to one salt plate, these plates should be handled at their
edges at all times. Cover the ‘paste’ with another plate and press them to remove any air pockets
and to spread the sample evenly. Place the plates in a de-mountable cell holder and set the
screws to slightly less than finger-tight. Then, tighten the screws to finger tight in a diagonal
fashion and in a cyclic fashion.
Obtain the IR spectrum using the FTIR spectrometer as described below. If the Nujol peaks
overwhelm the set of observed peaks the sample preparation is unsatisfactory or if the peaks are
broad the solid sample had not been well grounded and in either case requires a new sample. If
the peaks are too strong reduce the sample quantity within the plates; which may be achieved by
gently squeezing the plates and/or scraping off material with a clean blade. Reset the cell but do
not use screws to squeeze the sandwiched sample.
De-mountable cell holder
Clean the salt plates with CCl4 moistened paper towel and dry them with lint-free paper towels
after use.
KBr Pellet:
A thin wafer of an intimate mixture of the sample in spectra-grade KBr is employed in this
technique.
Take a dry glycine sample (100-mg) in a clean and dry agate mortar and grind it vigorously to
pulverize the sample to smaller particle size. Transfer 1-5mg of the glycine sample into a plastic
vial, add 500mg of dry spectra-grade KBr solid to it and mix well. Transfer this mixture to a die
assembly and spread evenly; this sample will eventually be pressed to ~10,000-15,000 pounds
under vacuum. Hold the sample at this pressure for about two minutes. The finished KBr pellet
must be transparent to visible light. Mount the KBr pellet containing the sample on to a holder to
obtain its IR spectrum.
Thin Film Preparation:
Dissolve ~0.01g of the solid (Vanillin) in about 2mL of ethanol. Evaporate a few drops of the
solution on a polished NaCl crystal. Let the volatile ethanol evaporate. Obtain the IR spectrum
of the thin film formed.
Sample Preparation of Solutions
Prepare a ~10% solution of (v/v) benzene or toluene in tetrachloro-ethylene as solvent (by
mixing ~100µL of benzene in ~900µL of solvent. Fill a fixed path IR cell with the solvent close
the cell with Teflon stoppers; obtain the spectrum of the cell with tetrachloro-ethylene as the
background in a fixed path length IR cell. Remove the solvent completely. Introduce the
solution of benzene into the fixed path length cell with a clean hypodermic syringe. Close the
Teflon stoppers. Run the spectrum as before. Clean the cell immediately with acetone followed
by a stream of nitrogen. Store in the proper container and cover it tightly.
Obtain the IR of the neat liquid as a capillary film between two NaCl plates.
Calibration of the fixed path length IR-Cell – Interference Fringe Method:
The interference fringes arising from the constructive and destructive interference of
electromagnetic waves is measured in this experiment.
Set the output in %T mode. Collect the background file for air. Place the empty cell in the sample
compartment and run the ‘IR’ of a fixed path length cell (ZnSe cell, for our purpose). This will
generate a fringe pattern. The relationship between the path length t, the number of peaks, n,
between two wave numbers say,ν 0 , (the reference peak) and the nth peak (fringe) left of reference
peak, say ν n , is (for linear frequency instruments, as the one we have);
t=
10n
2(ν n −ν O )
The thickness of the cell spacing is t (in millimeters), and wavenumber is in cm-1. The above
equation can be modified to generate a linear plot of wave number vs. fringe number (n).
A typical output from an empty cell is shown below. The wave numbers at the minimum
transmission positions are shown.
Upon rearranging the equation shown above for a series of wave numbers, we get;
5
t
ν n = n +ν 0
Pick any peak near the middle of the plot as a ‘reference peak’ on the low frequency side, say it is
ν 0 ; and find the 1st peak on the high wavenumber side of the ‘reference peak’, ν 1 . The index for
the latter peak is set to 1, i.e. n =1. Continue to find the frequencies for a series of consecutive
peaks on the high wavenumber side (n = 2, 3... 8) and record their corresponding, n and ν n values.
Plot ν n vs. n; and calculate the path length of the cell and associated uncertainty of the path
length from the slope of the best fit line data.
FTIR:
Instrument Setup
The Nicolet FTIR spectrometer is left running continuously to stabilize the system. Open the
OMNIC software. On the right and bottom of the OMNIC window there will be a Bench
Status indicator. It shows a green check mark indicating the system is ready to be used. If a
red cross or a yellow circle shows up it would require the attention of a service person.
Open the Experiment Setup window (under the Collect menu). Choose the Collect tab and
check that the following settings have been entered before clicking on OK.
Number of Scans: 32
Resolution:
4
Final Format:
Transmittance
Correction:
None
File Handling:
Auto save
Background Handling: Collect background after 300 min.
Background and Sample Spectrum
This spectrometer expects you to collect a background spectrum that it will use for the next five
hours. Open the spectrometer sample area by turning the handle in the center of the front face of
the spectrometer to insert the background "cell"; if there is no background "cell" employed (i.e.
air is the background) leave the door closed.
Keep the spectrometer compartment door closed at all times other than that at “cell” exchanges
into the sample compartment.
Select Collect Background either from the Collect menu, or using the toolbar icon, click OK. As
the instrument collects data, the (background) spectrum will appear on the computer screen. At
the bottom left of the spectrum a box will show you the progress of scans as the 32 scans are
accumulated and averaged. When the experiment is finished, you will be asked whether or not to
add the spectrum to window 1; click Yes.
Select Collect Sample either from the Collect menu, or using the toolbar icon, click OK. Provide
a Title. When the experiment is finished, you will be asked whether or not to add the spectrum to
window 1; click Yes.
To delete the background spectrum from appearing on the screen, click on the background
spectrum, click Edit on the menu bar and select Clear. Click File, select Print.
Polystyrene film IR spectrum.
Spectrum of Nujol (paraffin, long straight chain alkane, mineral oil)
Spectrum of Fluorolube (florinated alkane, mineral oil)
Polishing crystals (NaCl or KBr)
Precautions: Wear gloves or finger cots. Avoid conditions of excessive humidity. Work in a
dust free atmosphere. Keep polishing kit and components clean and free from dust, grit or oil.
Alcohol — Felt Method
Stretch the felt over one of the ground glass pads and hold it in place with the spring holder.
• Pour a small amount of the polishing compound (600) onto the rear of the pad. Moisten the rear
third of the pad with ethyl alcohol.
Rub the salt surface briskly, figure 8 pattern, over the portion of the felt containing the alcohol
and polishing compound, gradually working down into the dry portion of the felt. About thirty
strokes should be enough to polish the surface. A dozen strokes or so at the dry end will buff the
surface clean of any residual rouge and alcohol. Examine the surface and repeat the procedure if
needed.
After polishing and buffing, rub the sides of the block on the felt to clean off any residual
polishing compound. As polishing proficiency increases, 50:50 water/alcohol may be used. This
increases the quality of the polish but requires greater speed.
Aqueous Solution — Glass Method
Prepare a solution of 25% water in alcohol.
Put a few drops on the clean ground surface of one of the glass pads.
Rub the salt surface against the glass until the water/alcohol solution has nearly evaporated.
Examine the surface and repeat the process, if necessary; usually three times will be sufficient.
Buff on dry felt, if necessary.
Clean-Up
Wipe the agate mortar and pestle with tissue paper. Rinse with acetone or water and wipe again.
Wipe the NaCl crystals with tissue paper moistened with alcohol. Return to the container. Store
the container in a dessicator.
Store the IR grade KBr bottle in a dessicator.
Draw any liquid sample remaining from the IR cell, use a syringe. Do not wipe or touch the
cell windows. Rinse the interior of the cell with acetone by passing acetone using a syringe. Dry
the cell with a stream of nitrogen. Store the cell in a dessicator.