MRG LabNotes 1999

Using the Perkin-Elmer FT-Infrared Spectrometer

As you probably already know, you can use infrared (IR) spectroscopy to determine the structure of a polymer, or other chemical due to characteristic vibrations of chemical bonds. This is a very useful analytical tool, as you might have guessed, but to get useful information, you must perform the technique properly.

The first thing you need to do is run a background spectrum. This is so that you only get the data from your sample, and not a spectrum containing the other chemicals which are always present. For example, if your sample is on a substrate, such as a salt plate or a KBr pellet, you need to run a background spectrum of the substrate (glass is not an acceptable IR substrate. Do you know why?). Also, the carbon dioxide and other chemicals in the air also vibrate when hit with IR radiation, and they will show up in your spectrum as well if you do not have a background spectrum. These gaseous chemicals can mostly be eliminated by purging the sample chamber with nitrogen.

Now that you know why you need a background spectrum, here's how to aquire one:

Below you will find an illustration of the keyboard of the spectrometer. Familiarize yourself with the keyboard, and refer to the picture when reading the procedures.

1. MAKE SURE THE NITROGEN GAS IS TURNED ON
2. Press Enter to exit screen saver mode (only if screen saver is running)
3. Press Backg
4. Press Scan
5. At this point the computer asks how many scans you would like. A list of numbers should appear at the bottom of the screen. Press the blank button below the number of scans you wish to aquire (4 or 16 should be sufficient)

Now the spectrometer will aquire your background spectrum and display it on the screen. Wait for the scan to finish and the screen to say READY. At this point you should insert the sample you prepared earlier into the spectrometer. Here's how you aquire a spectrum of your sample:

1. Press X
2. Press Scan
3. Choose number of scans from list at bottom of screen by pressing corresponding blank button on the top row. It is advisable to use the same number of scans as you did for background.
4. The computer will scan the sample and return your spectrum.
5. To save the spectrum to disk: press Copy (one of the blank buttons)
6. Press (from) X
7. Press (to) disk
8. Give your file a name by pressing the blank keys
   • Choose a group of letters
   • Choose a letter
   • Repeat until you have typed the filename
9. Press Enter
10. You may enter a comment in the same way as the title if you wish
11. Press Enter

The computer will now save your spectrum to an ancient five-and-a-half inch floopy disk (if the drive is being polite).

That wasn't so hard, was it? Now all you have to do is assign the peaks on your spectrum, and you will know something about your polymer's structure. You can find some of these vibrations and their frequencies in the Macrogalleria on the Infrared Vibrational Modes page. They may also be found in tables in refernce books. You can also learn more about these vibrations and their frequency ranges in the PSRC computer lab. Just try the "Spect" program and go to the IR section.

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