4.2: IR Spectroscopy - Chemistry LibreTexts

Infrared Optical Materials and Handling

While it is all well and wonderful that substances can be characterized in this fashion one still has to be able to hold the substances inside of the instrument and properly prepare the samples. In an infrared spectrometer (Figure \(\PageIndex{1}\))

the sample to be analyzed is held in front of an infrared laser beam, in order to do this, the sample must be contained in something, consequently this means that the very container the sample is in will absorb some of the infrared beam.

This is made somewhat complicated by the fact that all materials have some sort of vibration associated with them. Thus, if the sample holder has an optical window made of something that absorbs near where your sample does, the sample might not be distinguishable from the optical window of the sample holder. The range that is not blocked by a strong absorbance is known as a window (not to be confused with the optical materials of the cell.)

Windows are an important factor to consider when choosing the method to perform an analysis, as seen in Table \(\PageIndex{1}\) there are a number of different materials each with their own characteristic absorption spectra and chemical properties. Keep these factors in mind when performing analyses and precious sample will be saved. For most organic compounds NaCl works well though it is susceptible to attack from moisture. For metal coordination complexes KBr, or CsI typically work well due to their large windows. If money is not a problem then diamond or sapphire can be used for plates.

Table \(\PageIndex{1}\) Various IR-transparent materials and their solubilities and other notes. M. R. Derrick, D. Stulik, and J. M. Landry, in Scientific Tools in Conservation: Infrared Spectroscopy in Conservation Science. Getty Conservation Institute (1999).

Material	Transparent Ranges (cm -1)	Solubility	Notes
NaCl	40,000 - 625	H2O	Easy to polish, hygroscopic
Silica glass	55,000-3,000	HF	Attacked by HF
Quartz	40,000-2,500	HF	Attacked by HF
Sapphire	20,000-1,780	-	Strong
Diamond	40,000-2,500 and 1,800-200	-	Very strong, expensive, hard, useless for pellets
CaF2	70,000-1,110	Acids	Attacked by acids, avoid ammonium salts
BaF2	65,000-700	-	Avoid ammonium salts
ZnSe	10,000 - 550	Acids	Brittle, attacked by acids
AgCl	25,000-400	-	Soft, sensitive to light.
KCl	40,000-500	H2O, Et2O, acetone	Hygroscopic, soft, easily polished, commonly used in making pellets.
KBr	40,000-400	H2O, EtOH	Hygroscopic, soft, easily polished, commonly used in making pellets.
CsBr	10,000-250	H2O, EtOH, acetone	Hygroscopic soft
CsI	10,000-200	H2O, EtOH, MeOH, acetone	Hygroscopic, soft.
Teflon	5,000-1,200; 1,200-900	-	Inert, disposable
Polyethylene	4,000-3,000; 2,800-1,460; 1,380 - 730; 720- 30	-	Inert, disposable

Proper handling of these plates will ensure they have a long, useful life. Here follows a few simple pointers on how to handle plates:

• Avoid contact with solvents that the plates are soluble in.
• Keep the plates in a dessicator, the less water the better, even if the plates are insoluble to water.
• Handle with gloves, clean gloves.
• Avoid wiping the plates to prevent scratching.

That said, these simple guidelines will likely reduce most damage that can occur to a plate by simply holding it other faults such as dropping the plate from a sufficient height can result in more serious damage.