Nearly all organic or inorganic compounds having covalent bonds absorb various frequencies of infrared radiation. Depending on the type of bonds present, a number of selected frequencies of IR radiation will be absorbed by a molecule.

With modern FT-IR instruments, it is relatively easy to record spectra from solids, liquids, gases and polymer films. In fact, a choice of sampling techniques exists for all states of matter. Some of the most common sample preparation methods are discussed in this section.

One of the easiest and the fastest methods to analyze a solid sample is by the mull technique. The solid sample is ground into a fine powder, suspended in a mineral oil such as Nujol, and the resulting mixture is ground to a smooth paste. Then, a small amount of paste is placed in between a pair of NaCl plates and the spectrum is recorded. The disadvantages to this technique are the lack of controlling the thickness of the paste in between the NaCl plates, and scattering of light by the solid particles. The latter can be minimized by using a liquid of the same refractive index as the solid sample.

The traditional technique of analyzing solid samples is by the Pellet Method. In this method, the sample is first ground into a fine powder. The powder is then mixed with dry KBr, and ground further. An agate mortar and pestle is normally used to grind the materials (agate is preferred since it is an inert material; KBr corrodes stainless steel). However, using an electrical mill for this purpose is much more convenient. A capsule made of polystyrene, stainless steel, or agate is used to contain the sample during the milling process. Stainless steel is popular, however, it should be noted that KBr will corrode this material. Small stainless steel balls are placed in the capsule with the sample and KBr and a stopper is used to close the capsule. The mill, depending on design, either vibrates the capsule, or rocks it back and forth at very high speed.

The finely ground material is then removed from the mill, placed in a KBr die and several tons of pressure is applied with a hydraulic press to coalesce the sample into a transparent or semi-transparent disk. Die sizes range from 1mm, for micro samples, up to the traditional 13mm diameter disks. When necessary, a vacuum line is connected to the die to remove entrained air, and to a limited extent, entrained moisture. As entrained moisture can cause the finished disk to be opaque the KBr powder should be kept dry by means of a desiccator or a heated oven. Moist samples are not suitable to this technique as they will cloud the disk. Such samples should be dried before analysis.

Assembly and Loading of Sample Step 1 • Place the base on a flat surface and insert the ‘0-ring in the groove • Place the die assembly over the post in the base. • Insert the anvil POLISHED SIDE UP into the bushing of the die. Step 2 • Load the prepared sample through the bushing hole and onto the anvil. • Level the sample matrix material using a microspatula or a clean, dry, glass rod. Step 3 • Place the second "0-ring" fl the groove and place the barrel on top. • Insert the plunger and place the "0-ring" and washer around the plunger. Pressing the Pellet • Place the complete die in a hydraulic press and connect the vacuum line. • After two minutes under vacuum, press the pellet. • After an additional two minutes, release the pressure and the vacuum • Wait one minute before removing the die from the press. Conversion for 13mm Die: lbs. total load = 0.205 x psi Examples: 25,000 psi - 5125 lbs. total load (gauge reading) Caution: DO NOT exceed 12,000 pounds total load on the gauge (60,000 psi at sample)

Usually for liquid samples as a solution, a demountable sodium chloride cell is used. The cell consists of two NaCl plates situated in between two supporting metal plates. At the two ends of the cell, there are two openings which are used to fill up the cell. Spectra of neat liquids can be recoded as a thin film pressed between two salt plates.

In the HATR system, a crystal of an IR transmitting material having a high refractive index, of 2.2 or more, is held in a horizontal plane. The IR beam from the spectrometer is directed into the crystal at an angle which exceeds the critical angle. Internal reflection takes place in the crystal. The sample is placed in optical contact with the surface at which this internal reflection occurs. At this reflection point, some energy is lost to the sample, corresponding to the absorption bands of the sample. The beam finally exits the crystal and mirrors of the accessory guide the IR the remaining energy to the instrument's detector.

For samples of films and polymers, a flat plate system is used. The sample is placed on the crystal and a sample clamp used to provide optical contact. The sample clamp normally has a facility which enables it to accommodate samples of varying thickness and also to provide a range of contact pressures to optimize the experiment, by controlling band intensity.

* Polystyrene, polyethylene, nylon, saran, Teflon, polyester, mylar, and any other available thin films.

* Bring samples of wool, synthetic fabric such as polyester, soda bottles and cans.

			Recycling Symbols
CODE	TYPE	NAME	FORMULA	DESCRIPTION	SOME EXAMPLES
PETE		polyethylene terephthalate		usually clear or green, sinks in water, rigid, glossy	soda bottles, peanut butter jars, vegetable oil bottles
HDPE		high density polyethylene		semi-rigid, sinks in water	milk and water jugs, juice and bleach bottles
PVC		polyvinyl chloride		semi-rigid, glossy, sinks in water	detergent / cleanser bottles, pipes
LDPE		low density polyethylene		flexible, not crinkly	6-pack rings, bread bags, sandwich bags
PP		polypropylene		semi-rigid, low gloss	margarine tubs, straws, screw-on lids
PS		polystyrene		often brittle, glossy	styrofoam, packing peanuts, egg-cartons, foam cups

The objective of this experiment is to record spectra from solutions, and then identify functional groups present in an unknown chemical provide by your TA.

* Perkin Elmer Paragon 1000PC Fourier-transform infrared spectrometer.

* Demountable sodium chloride cell

* Unknown (labeled UNK? supplied by the laboratory instructor)

1) Record infrared spectra of cyclohexane, ethyl acetate, and methyl ethyl ketone using a demountable NaCl cell. Use air (cardboard cutout) as background. If spectra of neat liquids are out of range, adjust their concentrations by diluting them with CCl₄. (about 5 to 10 times dilution).

2) Obtain the infrared spectrum of the unknown.

3) Correlate functional groups to absorption peaks observed in all the spectra.

Present your spectra of cyclohexane, ethyl acetate, and methyl ethyl ketone in an appropriate Figure. Correlate major abortion bands and to functional groups of the compounds tested.

The objective of this experiment is to determine the concentration of methyl ethyl ketone (MEK) in an unknown sample.

Theory: In the absence of chemical interactions, infrared absorbances should obey Beer’s Law, A = Elc, where A is the absorbance, E is the molar absorptivity, l is the path length, and c is the concentration. In this experiment you will prepare a standard curve by measuring the absorbance due to MEK in several standard solutions and plotting the data as absorbance vs. concentration. According to Beer’s Law, the plot should be linear. You will then record the spectrum of an unknown sample of MEK and utilize the standard curve to determine the unknown concentration.

* Perkin-Elmer Paragon 1000PC Fourier-transform infrared spectrometer.

* Demountable sodium chloride cell

* 10 mL volumetric flasks (6)

* Syringe

* Perkin Elmer Paragon 1000PC Fourier-transform infrared spectrometer.

* KBr pellet apparatus (die, hydraulic press, grinder, etc.)

* Potassium bromide

* Benzoic acid

* Nujol

(1) The method of making a KBr pellet will be demonstrated by the laboratory instructor.

(3) Use a portion of this mixture to make the pellet, and obtain its spectrum.

(4) Grind approximately 0.1 g of benzoic acid into a fine powder.

(5) Add approximately 1 g of Nujol, and grind the mixture into a smooth paste.