Use of the Nicolet IR Spectrometer with an ATR Cell

This tutorial will help you with sample preparation, instrument setup and use of OMNIC software. Please follow the menu below based on which experiment you are doing. You should have read the background on instrument design and spectral interpretation before using this area.

Attenuated Total Reflectance

ATR has come into use largely because it is a very quick technique for introducing sample into the spectrometer, and because of the exceedingly small amount of sample required. The basic principle is illustrated below:

(Wikipedia)

The light beam is reflected through a crystal (diamond, in our case) on which the sample is placed. At each point where the light beam reflects, it penetrates the sample to a very shallow depth, where absorption of the incident light occurs. While not much of the beam is affected in each interactions, the multiple reflections allow signal to accumulate. The exiting beam has thus had specific frequencies absorbed by the sample, just as it would in a classical transmission experiment.

One strong advantage is that ATR can easily be used for both liquid and solid samples without any special preparation. The two limitations are low-boiling liquids (which evaporate before the experiment is complete) and gasses; though sealed ATR cells can be used in those cases.

Step 1 Collect a background spectrum (no sample on the cell crystal). Select "Collect Background" from the OMNIC menu.
The background spectrum is complicated: its overall shape reflects the output of the IR source (which varies by frequency); the spectrum picks up atmospheric components water and CO₂.
Step 2 Place a small amount of sample on the crystal: the small opening in the middle of the top plate on the spectrometer. You only need a drop of liquid, or a few milligrams of solid. For a solid sample (only!), screw down the cell anvil finger-tight to compress the solid sample onto the ATR crystal. Do NOT!! tighten too tight. Do NOT!! do this with liquid samples.
Step 3 Collect the spectrum using the appropriate menu entry in OMNIC. It should do a pretty good job of canceling out bands from the atmosphere, but you likely will have small residual peaks remaining. This (blank) spectrum shows excess CO₂ because we were talking (and exhaling) over the cell prior to sample spectrum collection. Save the spectrum to the local drive (C:\) using a unique file name: a good system is your initials, your notebook page number, and a sample number. If you are logged in to the ch362 area, copy the file to the T:\ drive for access from other computers (where you will examine the spectrum, annotate it and print, if necessary).
Step 4 Clean up! Use a cotton ball (NEVER USE KIMWIPES ON SPECTROMETERS--PAPER WILL DAMAGE THE OPTICS (even a diamond ATR cell), moisten with isopropyl alcohol, and thoroughly clean the cell crystal (and anvil) so that the next person can get a clean background.