Infrared Spectrocopy

Infrared (IR) light or radiation has the correct energy to enhance stretching and bending of bonds, which are vibrational transitions.  Because molecules have many possible vibrational states, each with a slightly different energy and thus a different frequency of radiation required to enhance the vibration, many absorption bands appear in the infrared spectra of even the simplest molecules.  Today, infrared spectroscopy is typically used to identify the presence of functional groups.

IR spectra can be obtained on solids, liquids, and gases.  Our organic students typically analyze only liquid samples.  The department owns two Fourier transform infrared (FT-IR) spectrometers that are located in Salem 104.  Both spectrometers are controlled through a windows based program called OMNIC.

Galaxy FT-IR

Genesis FT-IR

To use the IR, follow these simple steps:   Preparing a sample: 1.      Obtain two salt plates from a desiccator.  To open the desiccator, slide off the lid.  Do not attempt to remove the lid by pulling straight up on it.  (Please keep the desiccator closed as much as possible.) 2.      Clean the plates with an organic solvent such as acetone.  Since the plates are made of salt (usually KBr), never wash the salt plates with water.  Place the plates on a Kimwipe, cover the plate with acetone, and then gently wipe the plate with a Kimwipe.  Clean both sides.  (Handle plates only on the edges to avoid obtaining an IR spectrum of oils from your fingers.)

3.      Once the acetone has evaporated from the plates, add one drop of your sample to one plate.  Place the second plate on top of the first to sandwich your sample between the two salt plates. 4.      If a salt plate sample holder is not installed inside the IR spectrometer, slide one into place.  (The holder will only fit in one of the two available tracks.)  Place your sample into the holder and close the IR compartment.

Obtaining a spectrum using the OMNIC Software on the computer attached to the Genesis:

5.      If the software is not already open, open it by double-clicking the “OMNIC" icon on the desktop. If the software is already open, close any existing windows within the program.

6.      Use the dropdown menus to carry out the following tasks:

a.    Obtain the data.

For the Galaxy:
- Collect: Collect Background. Make sure the light path is clear (no sample or salt plates in IR) while obtaining the background. Click OK.
- When asked if you want to add the spectrum to a window, select NO.
- Collect: Collect Sample Place your sample in the sample holder and select OK.
- When collection of data is complete, you will be prompted for a spectrum title. Enter your name and some type of sample identification and hit enter.
- You will be asked if you want to add the spectrum to a window. Click YES. Otherwise, your data will be discarded.

For the Genesis II:
- Collect: Collect Sample The default is currently set to prompt the user to prepare for a background scan. Make sure the light path is clear while obtaining the background. Click OK when the pathway is clear.
- A "Prepare to Collect Sample" window will pop up after the background scans are complete. Place the sample in the sample holder and select OK.
- When collection of data is complete, you will be prompted for a spectrum title. Enter your name and some type of sample identification and hit enter.
- When asked if you want to add the spectrum to a window, click YES.
- To change the y-axis from absorbance to percent transmittance, Process: % Transmittance.

Please Note: If someone used the instrument before you and you forgot to close old windows before collecting your data, your spectrum will simply add to the one(s) already on the screen. To delete any old spectrum, click on it. The selected spectrum and its title will appear in red. Once you have selected the spectrum you wish to delete, use Edit: Clear to delete it permanently.

b.      (Optional) Analyze: Find Peaks If too many peaks have been labeled, you may move the threshold line down by sliding the bar running the left side of the screen. The line may not appear to move, but you will notice fewer and fewer peaks labeled as you slide the threshold lower and lower. Click the Replace button to put this new window of labeled peaks into your active window.

c.      File: Print

Please Note: If you have trouble printing 1) make sure you are printing to the Salem 104 printer. 2) Make sure the Salem 104 printer is installed. Each user who logs into the computer must install the printer. 3) make sure you are logged into the network.

Though it is typically unnecessary for the organic labs, you may save your spectrum if you wish.

Cleaning up:

7.  Clean your plates with acetone or ethanol (NEVER water!) and place them back in the desiccator.  Throw away any used Kimwipes and place disposable pipets in a broken glass container.