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 WinFirst.

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.

Placing samples inside IR:


The holes in the sample holder and instrument side panels align to allow light to pass from the IR source through the sample, and then to the detector.

Genesis

Galaxy

Obtaining a spectrum:

5.      Open the IR software by double-clicking the “Start IR” icon if it is not already open.  (Be careful not to open the software using the WinFIRST icon.  You cannot scan new samples unless you open the software from the Start IR icon.)

Use this icon to scan a new sample or recall one saved previously. This icon should only be used to recall a spectrum you saved previously.

6.      Click the  “Simple Menus” button at the lower right corner of the screen.

7.      IF you are instructed to use a particular method, use the “Scan” dropdown menu and select “Scan Method.”  Select the desired method.

8.      Use the dropdown menus to carry out the following tasks (in order):

a.       Scan: Background Scan Make sure the light path is clear (no sample or salt plates in IR) while obtaining the background. (“100% Complete” appears in the bottom right corner when background scan is complete.)

b.      Scan: Sample Scan (Spectrum appears on screen when complete.)

c.       Math: Autobaseline (Sometimes performing an autobaseline calculation multiple times is necessary to make the spectrum baseline look reasonable.)

d.      Math: Peaks (select singles and “OK”)  Click at the bottom of an absorbance and the wavenumber of that absorbance will appear on your spectrum.  However, if the peak is full scale, then it will not be possible to read the wavenumber when it prints. (If you can't read the label on the screen, you won't be able to read the label in the printout.)

e.       To label full-scale peaks: If the “Notate” window is not open, then open it using the dropdown menu “Tools-Annotator.”  Click the second icon, top row in the Notate window.  A box should pop up instructing you to “Pick Two Points.”  First click at the bottom of the peak you want to label and then click in an open space where you want the text box to appear.  Repeat as necessary.

f.        To edit text: Double-click on the title and alter the text to include at least your name. The top right icon in the Notate box allows you to type text on the spectrum. The pencil icon allows you to edit text lines in the spectrum. The pencil eraser icon in the Notate window clears all peak labels.

g.       File: Plot  Select the Salem 104 printer, click “Plot” and then “Done.”

Posted on the wall near each IR is a “frequently encountered problems” sheet.  If you have trouble, first see if the solution is posted on that sheet.

Saving a spectrum:

9.      There are several formats in which you can save the data from the “File – Save As” menu.  If you save to the hard drive, please use your WFU username as the filename to avoid confusion, and save in a course-titled folder (such as Chm223L.)

  • If you want to be able to open the spectrum again in WinFirst, choose “Mattson Data File.”  You may save one copy of your spectrum in a course folder.  The file path on the Galaxy, for example, is C:/First/Data/Galaxy/Chm223L/.
  • You may save to a floppy disk and open the spectrum in WinFirst on any networked Wake Forest computer by following the instructions listed on the linked web page.
  • Data may be exported to Microsoft Excel and many other graphing programs as an ASCII Table.  When opening the file in Excel, check “Space” as a delimiter to get wavenumbers and absorbencies in separate columns.
  • To use Dr. Bell’s IR program, save the file to the A drive as “SpectraCalc & GRAMS.”

Cleaning up:

10.  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.

Return to course index