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Chemistry
IR Spectroscopy
Experimental Procedure
Figure 1: Salt plates: Liquid Sample Preparation for IR Spectroscopic Analysis |
Figure 2: KBr Pellet: Solid Sample Preparation for IR Spectroscopic analysis |
Figure
3: ATR-IR
|
Solid Unknowns
Solids can be analyzed using the ATR-IR spectrophotometer or using a
KBr pellet. For analysis of a sample with the ATR instrument,
follow the the instructions (click here).
For preparation of the KBr pellet, follow the step-by-step tutorial for
preparing
a KBr pellet for IR analysis on the General Chemistry I website or
follow the procedure provided below. Obtain a mortar and pestle,
dry potassium bromide (KBr) and a minipress. Keep the pieces of
the
mini-press together and be sure to return them intact. The
mini-presses
are manufactured to fit together precisely; the screw of one mini-press
will not necessarily fit properly into another mini-press. In the
mortar, combine the solid sample (1 mg) and KBr (100 mg) in a 1:100
ratio.
The concentration of the sample in the pellet should be in the range of
0.2-1.0%. Gently grind the two solids together to make a
homogenous
mixture. Insert one of the screws into the minipress. Using
a spatula, transfer approximately 200 mg of the sample/KBr mixture to
the
mini-press. Gently tap the mini-press on the bench to distribute
the solid over the flat surface of the screw inside the mini-press so
that
the screw is not visible. Screw in the second screw and tighten
the
screw using a socket wrench. Wait one minute and carefully remove
the screws, leaving behind a "KBr window". Preparation of the KBr
pellet for IR analysis is often difficult and causes frustration for
many
students. Frequently the window cracks as the screws are being
removed.
Sometimes this can be avoided by adjusting the ratio of sample to KBr;
ie., from 1:100 to 1:50 or 1:150. Cracks will also occur if the KBr is
not very dry. Place the KBr in the oven to dry it out if you have
persistent
problems. Overtightening the screws in the mini-press will also
cause
the window to crack.
Figure 6.3: Setting the Salt Plates into the IR Spectophotometer |
Figure 6.4: Setting the Mini-Press in the Instrument Sample Holder |
To clean the salt plates, carefully place them in a 50 ml beaker
containing
~15-20 ml of CH2Cl2. Swirl the plates
around
in the beaker and dry them off with a Kimwipe. Return the plates to the
dessicator when you have completed your analysis. To clean the
mini-press,
rinse with water, then place the mini-press in a 50ml beaker containing
15-20 ml of CH2Cl2. Swirl the mini-press
around
in the beaker and dry it off using a Kimwipe. Be sure that all
of the pellet is removed by the cleaning process. Wash both
the
mini-press and the screws. Return the mini-press and screws to
the
container unassembled.
acetaminophen |
cyclohexane |
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Table 2: List of Possible Unknowns