Sample preparation is the most important part in IR spectral analysis, Sample should be transparent to IR radiations so it allows the radiations to pass through them, Hence the salts like KBr, NaCl, AgCl are used for mixing of sample in order to obtain the accurate IR spectrum of a sample with good intensity, sharp peaks and high resolution.
Solid, Liquid and gaseous samples can be examined in IR spectroscopy.
Various Technique for the sample preparation are as follows:
a) For Solid Sample:
1. Solid run in solution Technique :
In this method the concentrated solution of sample is prepared with appropriate solvents by dissolving the sample in solvent or either by placing the sample about 2-5 mg of sample directly on the plates and then add a drop of solvent to dissolve the sample and then sacn the sample to get the IR spectrum of sample and then obtain spectrum of pure solvents as a blank.
Solvents used must be non-aqueous because it may dissolves the KBr plates and the water content can mask the peaks of sample in the spectrum. Solvent should not absorb the IR radiation.
Various solvents can be used for this method are: Carbon disulphide (CS2), Carbon tetrachloride (CCl4), Dichloromethane (CH2Cl2)
2. Solid Film Technique :
This technique is used for the examination of amorphous solids. Sample is melted and then cooled and deposited on the KBr cell as a thin film.
3. Mull Technique (Nujol-Mull Technique) :
Generally in this method the powdered sample is mixed with nujol agent (mineral oil) and mulled to forma a paste and then transferred to the KBr, NaCl or AgCl plates.The Nujol (mineral oil) itself shows the characteristics spectrum it shows bands in the region at about 2857, 1449, 1389 cm-1.
Only the average particle size of the solids gives the accurate IR spectrum by this technique therefore the solid sample is finely crushed in the mortar to reduce its particle size about 1 to 2 micron, about 5 to 10 mg of finely powdered soild sample is placed on the KBr plate and add a drop of Nujol (mineral oil) and then place the second plate on the top of sample mixture and gently rubbed to distribute the sample mixture uniformly between plates and then plce the plates in sample compartment of IR spectrophotometer and scan the sample to obtain the IR spectrum of a sample.
Nujol agent (mineral oil) used in this techniques are : High boiling petroleum, Halocarbon oil, perfluorokerosene.
4. Pressed pellets Technique :
As its name in this technique the pellets of KBr and sample is prepared. Crush or grind the dry powder of KBr in mortar do not crush the potassium bromide to finely powder as reduced particle size or fine powder of potassium bromide gain the moisture from the environment due to its hygroscopic nature and forms the bands in certain IR region. Scan the KBr alone as background. Prepare a homogeneous mixture of KBr and sample. The concentration of sample in KBr should be in between 0.2 to 1%. Concentration of sample must be lower (Beer’s law) because higher concentration of a sample may results in the complete absorption of IR radiation or scattering of IR radiation from sample which gives the noisy spectra. Transfer the mixture of KBr and sample about 80 mg into the die set and pressed by using a hydrullic press to form compressed thin pellets then place dye set in the sample compartment of spectrophotometer and then scan the sample to obtain the IR spectra.
b) For Liquid Sample:
Liquid samples are usually handled pure without mixing them with any solvent because all solvents have their own characteristics absorption spectra. For liquid samples highly polished salt (NaCl, KBr or AgCl) plates are used. A drop of liquid sample is placed on the face first plate and the second plate is placed on the top to form a uniform film of a sample wipe off the excees liquid spilled from the edge of the plate then place the plates in the sample compartment of spectrophotometer and run the spectrum.
Volatile liquids can’t be examined with thixs method as they evaporate very fast.
Aqueous liquid can’t be examined with this method as the surface of plates containing NaCl and KBr dissolves into water.
c) For Gaseous sample:
A special sample cell is used for the gaseous sample made up of NaCl, KBr with long path length i.e 5 to 10 cm and the vapors of gas are placed into the cell and directly place in the path of Infrared radiation.
d) Attenuated Total Reflection Technique (ATR) :
This is the recent technique developed by Fahrenfort. Samples which strongly absorbs the Infrared radiations such as rubber, polymer and some aqueous solution can be analysed with this technique.
In this technique There is no need of sample preparation. Sample can be directly placed in the ATR assembly and examined.
Two types of ATR crystals are available i.e 45º‐ZnSe and 45º‐G. For solid samples flat crystal is used and for liquid trough crystal is used.
Sample Cells :
Sample cells are meant for placing the sample under investigation. Sample cells are available with fixed path lengths from0.025 to 1.0mm and with variable path length 6.0mm. For gaseous samples the path length for sample cell is 5 to 10cm. Sample cells have transparent window made up of plates of KBr or NaCl. If water is to be used as solvents then plates should be made up of CaF2 or BaF2 as water dissolves the surface of KBr and NaCl.
Always use the clean KBr plates to avoid contamination and after completion of analysis the plates must be cleaned thoroughly with acetone or ethanol and dry them with tissue paper and store them in the respective case.
Mortar and pestle used for sample crushing should be washed with acetone and then with purified water and then dry.
Do not prepare the sample near the IR instrument as spillage of sample can harm the instrument and cause contamination always prepare sample in the work bench of laboratory.
In pressed pellets technique the sample and KBr ratio should be accurate and do not grind the KBr to very fine powder.
Keep the KBr and NaCl plates in dessicator and prevent them from moisture.