ACP Home | Organic Chemistry IOrganic Chemistry II  | Lecture | Laboratory | Laboratory SyllabusGeneral Chemistry

IR Spectroscopy
Experimental Procedure

Liquid unknowns
Obtain a set of salt plates and set one of the plates on a Kimwipe.  Hold the plates on the sides to avoid getting fingerprints on the plates which can interfere with the analysis.  Using a disposable, glass pipet, place 2-3 drops of the liquid onto the center of the plate (Figure 1).  Set the second salt plate on top of the first, distributing the liquid evenly over the surface of the plate, creating a film.

Figure 1:  Salt plates:  Liquid Sample Preparation for IR Spectroscopic Analysis


Figure 2:  KBr Pellet:  Solid Sample Preparation for IR Spectroscopic analysis

Figure 3:  ATR-IR

Solid Unknowns
Solids can be analyzed using the ATR-IR spectrophotometer or using a KBr pellet.  For analysis of a sample with the ATR instrument, follow the the instructions (click here).  For preparation of the KBr pellet, follow the step-by-step tutorial for preparing a KBr pellet for IR analysis on the General Chemistry I website or follow the procedure provided below.  Obtain a mortar and pestle, dry potassium bromide (KBr) and a minipress.  Keep the pieces of the mini-press together and be sure to return them intact.  The mini-presses are manufactured to fit together precisely; the screw of one mini-press will not necessarily fit properly into another mini-press.  In the mortar, combine the solid sample (1 mg) and KBr (100 mg) in a 1:100 ratio.  The concentration of the sample in the pellet should be in the range of 0.2-1.0%.  Gently grind the two solids together to make a homogenous mixture.  Insert one of the screws into the minipress.  Using a spatula, transfer approximately 200 mg of the sample/KBr mixture to the mini-press.  Gently tap the mini-press on the bench to distribute the solid over the flat surface of the screw inside the mini-press so that the screw is not visible.  Screw in the second screw and tighten the screw using a socket wrench.  Wait one minute and carefully remove the screws, leaving behind a "KBr window".  Preparation of the KBr pellet for IR analysis is often difficult and causes frustration for many students.  Frequently the window cracks as the screws are being removed.  Sometimes this can be avoided by adjusting the ratio of sample to KBr; ie., from 1:100 to 1:50 or 1:150. Cracks will also occur if the KBr is not very dry. Place the KBr in the oven to dry it out if you have persistent problems.  Overtightening the screws in the mini-press will also cause the window to crack.

Running the Spectrum
Proceed to the IR spectrophotometer to run the IR spectrum.  Run a background scan if one has not yet been run.  Place the sample in the sample holder of the instrument.  Click on the "run spectrum" icon.  The spectrum will appear in approximately 2 minutes or less. The scale of the y-axis or % transmittance of the spectrum should be between  ~0 and 80-90%.  The scale will automatically adjust, but it should not be too small a range, otherwise the integrity of the spectrum is compromised and may be difficult to interpret.

   Figure 6.3:  Setting the Salt Plates into the IR Spectophotometer 


Figure 6.4:  Setting the Mini-Press in the Instrument Sample Holder

To clean the salt plates, carefully place them in a 50 ml beaker containing ~15-20 ml of CH2Cl2.  Swirl the plates around in the beaker and dry them off with a Kimwipe. Return the plates to the dessicator when you have completed your analysis. To clean the mini-press, rinse with water, then place the mini-press in a 50ml beaker containing 15-20 ml of CH2Cl2.  Swirl the mini-press around in the beaker and dry it off using a Kimwipe.  Be sure that all of the pellet is removed by the cleaning process.  Wash both the mini-press and the screws.  Return the mini-press and screws to the container unassembled.

Interpreting the Spectrum
Interpretation of the IR spectrum can be accomplished by following the steps outlined below:
1. Identify and label (1, 2, 3,etc) all the major peaks in the IR spectrum between 1500-4000 cm-1
2. List each peak in a table and record the frequency (or frequency range) for each peak.
3. Assign each peak to a specific bond type (-C=O, -C-H, etc..) using the information provided in Table 6.2 in the background section.
4. Prepare a table listing each peak by its frequency, shape, and bond type.
5. Review the list of possible compounds (Table 2, background section) corresponding to your IR spectrum.
6. Collect the structures for all of the compounds on the list using the Merck Index, Chemfinder, or other resources.
7. Narrow the list of possible structures by eliminating those compounds that do not have bond types consistent with the unknown.
8. Sketch a hypothetical IR spectrum for each of the remaining compounds on the list.
9. Eliminate any compounds whose theoretical IR spectrum is not an exact match with your spectrum, i.e, no extra peaks and no missing peaks.
10.  The unknown can then be identified as the remaining compound.

benzoic acid
ethyl acetate
methyl benzoate
benzyl alcohol

Table 2:  List of Possible Unknowns

ACP Home | Organic Chemistry IOrganic Chemistry II  | Lecture | Laboratory | Laboratory SyllabusGeneral Chemistry