<title>A Basic Experiment in Horizontal Attenuated Total Reflectance Infrared Spectroscopy (HATR-IR)</title>

<author><firstname>Andy</firstname><surname>Surface</surname></author>

The HATR accessory for the FTIR Spectrometer tests the IR absorbance of various solids, semi-liquids, and even volatile liquids, by <quote>bouncing</quote> IR radiation off of a substance through a crystal with a high refractive index and a large spectral range. Hampden-Sydney owns a FTIR insturment with the HATR accessory, and the following describes a two basic HATR-IR experiments, one which obtains the spectra of a liquid (Olive Oil) and the other which obtains the spectra of a solid (parafilm) so that future HATR-IR experiments can be accomplished by following the same (or a similar) procedure.

Before running any test or blank on the HATR accessory, a proper installation of it into the FTIR instrument must be done. Everything is preset on the HATR accessory, so all that is necessary is to check to make sure that the hardware on it is in good shape, and that it attaches firmly and securely to the mount in the FTIR instrument. Check the transmission value (similar to running a blank) on the accessory before actually running a blank just to make sure that it has been installed correctly. The transmission value should be at least 18% at 1000cm^-1 for both ZnSe crystal plates.

When a blank is run on the crystal, it is important to note the spectra produced, especially along the ~2400cm^-1 line (CO₂'s IR absorbance peak). Several blanks should be run to observe any CO₂ changes and any other contaminant peak changes. If the blank runs can be averaged together in the computer, do so, before running the actual IR experiment.

Additionally, if the crystals are dirty or have not been cleaned correctly, they should be cleaned very carefully. The best way to do this is with the mildest solvent possible for the compound that is suspected as the contaminant on the crystal. For most cases the best solvent to use is isopropyl alcohol, but if absolutely necessary acetone is an okay solvent to use. In extremely difficult cases dimethylformamide can be used as well. The solvent must be applied with something like a Q-Tip and it must be gently removed with a Q-tip as well. No <quote>Kleenex</quote> or other paper products may be used on the crystals because they can be abrasive and scratch the crystal (which is extremely soft ZnSe—and most scratches on it may not even be able to be seen).

When placing the sample in the crystal or on it (depending on which crystal is being used), the success of the experiment depends critically upon the substance having an intimate contact with the crystal. If it is a solid being tested, it must be flat and placed on the crystal plate. If it is a powder, it must be a soft powder and it must be placed in the trough. If it is a liquid, then it must be placed inside the trough crystal as well. The press must be screwed firmly on top of the crystal once the sample has been put in it, and then it must be locked into position. The sample must not be abrasive or have any grit in it that may damage the crystal. One other item to be noted: it is not recommended that the sample be anymore than 30C different than the temperature of the crystal as it may damage the indium seal that seals the crystal, or even damage the crystal itself.

One final note on running an HATR FTIR test on any sample, ATR spectra are slightly different than normal transmission spectra as the IR waves detected are not so much based on what passes through a sample but how deeply the IR waves penetrate the sample. Depending upon the accuracy being sought in the IR spectra from the IR instrument there are other optional ATR corrective filters that can be added to the computer program that compensates for crystal absorbance and other factors. This corrective filter adapts the ATR spectra to look more like transmission spectra.

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