To study and perform various IR Sampling Techniques in IR Spectroscopy

Aim: To study and perform various IR Sampling Techniques in IR Spectroscopy.

Requirements: 

IR Sampling Apparatus, Sample (Solid, liquid or gas), Nujol, NaCl or KBr, and other Chemicals.

Theory:

IR spectroscopy is used for the characterization of solid, liquid or gas samples. Material containing sample must be transparent to the IR radiation. So, the salts like NaCl, KBr are only used.

Sampling of solids

Various techniques used for preparing solid samples are as follows

Mull Technique: In this technique, the finely crushed sample is mixed with Nujol (mulling agent) in a marble or agate mortar, with a pestle to make a thick paste. A thin film is applied onto the salt plates. The Plates are squeezed together to adjust the thickness of the Sample between IR transmitting windows. This is then mounted in a path of IR beam and the spectrum is recorded.

Demerit:

• Although Nujol in transparent throughout IR region, yet it has a disadvantage that it has absorption maxima at 2915, 1462, 1376 & 719 cm.
• So when IR spectrum of solid sample is taken in Nujol mull, absorption bands of solid sample that happen to coincide with the absorption bands of the Nujol mull will be hidden (but others will be clearly seen in IR spectrum) and then interferes with the absorption of the sample.
• This method is good for qualitative analysis but not for quantitative analysis

Solid run in Solution: In this technique, solid sample may be dissolved in a non-aqueous solvent provided that there is no chemical interaction with the solvent and the solvent is not absorbed in the range to be studied. A drop of solution is placed on the surface of alkali metal disc and solvent is evaporated to dryness leaving a thin film of the solute (or the entire solution is placed in a liquid sample cell) which is then mounted in spectrometer.
If the solution of solid can be prepared in a suitable solvent then the solution is run in concentration of cells for liquids.
Some solvents used are chloroform, carbon tetrachloride, acetone, Cyclohexane etc

Demerit:

• This method can’t be used for all solids because suitable solvents are limited in number & there is no single solvent which is transparent throughout IR region.

Precautions:

• Solute chemical interaction with the solvent must be taken into consideration especially for compounds having property of H-bonding.
• The solvent should not absorb in the studied range.

Case film or solid film technique: If a solid in polymer resins & amorphous solids, the sample in dissolved in any reasonable volatile solvent & this solution is poured on a rock salt plate (NaCl or KBr) & solvent is evaporated by gentle heating.
If the solid is amorphous or non-crystalline nature then a thin homogenous film of the sample is deposited on the surface of a KBr or NaCl cell by evaporation of a solution of the solid and ensured that the film is not too thick to pass the radiation which can be mounted and scanned directly.
Sometimes polymers can be "hot pressed" onto plates.

Merit and Demerit:

• This method is useful for rapid qualitative analysis but becomes useless for carrying out quantitative analysis.

Pressed pellet technique: In this technique a small amount of finely ground solid sample is intimately mixed with about 100 times its weight of powdered Potassium bromide, in a vibrating ball mill. This finely ground mixture is then pressed under very high pressure (25000 p sig) in evacuable die or minipress to form a small pellet (about 1-2 mm thick and 1cm in diameter).

The resulting pellet is transparent to IR radiation and is run as such.
The powder (KBr + sample) is introduced in between the 2 bolts and the upper screw A is tightened until the powder in compressed to a thin disc.

After compressing the sample bolts A & A' are removed and a steel cylinder with pellet inside it is placed in path of the beam of IR spectrometer and a blank KBr pellet of identical thickness is kept in the path of reference beam.

Advantages of this technique over mull technique

• The use of KBr eliminates the problem of bands which appear in IR spectrum due to the mulling agent as in this case no such bands appear.
• KBr pellets can be stored for longer periods of time.
• As concentration of the sample can be suitably adjusted in pellets, it can be used for quantitative analysis.
• The resolution of spectrum in KBr is superior to that obtained with mulls.

Demerits:

• It always has a band at 3450cm^-1, from -OH group of moisture present in the sample.
• The high pressure involved during the formation of pellets may bring about polymorphic changes in crystallinity in the samples, (Especially inorganic complexes) which may cause complications in IR spectrum. In some cases, even substitution of the ligand by bromide may be possible in inorganic complexes.
• This method is not successful for some polymers which are difficult to grind with KBr.

Sampling of liquids

Liquid sample cells can be sandwiched using liquid sample cells of highly purified alkali halides, normally NaCl. Other salts such as KBr and CaF₂ can also be used. Aqueous solvents cannot be used because they cannot dissolve alkali halides. Organic solvents like chloroform can be used. The sample thickness should be selected so that the transmittance lies between 15-20%. For most liquids, the sample cell thickness is 0.01-0.05 mm. Some salt plates are highly soluble in water, so the sample and washing reagents must be anhydrous.

Sampling of gases

The sample cell is made up of NaCl, KBr etc. and it is similar to the liquid sample cell. A sample cell with a long path length (5 – 10 cm) is needed because the gases show relatively weak absorbance.