Sampling of solids in IR spectroscopy

Solids sampling in IR spectroscopy with video tutorial.

1. PRESENTATION ON SAMPLING OF SOLIDS IN INFRARED SPECTROSCOPY T.Manoj Kumar

2. INTRODUCTION : IR spectroscopy refers broadly to that region of electromagnetic spectrum which lies between visible and microwave regions. This region is divided into different sections :

3. BASIC PRINCIPLE : A chemical substance shows marked selective absorption in IR region. After absorption of IR radiation the molecules of chemical substance vibrate at many rates of vibration giving rise to closely packed absorption spectrum called as IR absorption spectrum . Various bands present in the IR spectrum corresponds to characteristic functional group and bonds present in the chemical substance. Thus IR spectrum of a chemical substance is fingerprint for it s identification .

4. SAMPLE HANDLING TECHNIQUES IN IR SPECTROSCOPY: S amples of different phases have to be treated differently. The only common point to the sampling of different phases is that the material containing the sample must be transparent to IR radiation. This condition restricts our selection to only certain salts like NaCl or KBr. However, a final choice of salt will depend on the wavelength range to be studied. Miller (1965) developed appropriate methods to handle samples in the gas, liquid and solid phase .

5. SAMPLE CELL : Sample handling presents a number of problems in IR region. There is rugged window material for cuvette that is transparent and also inert over this region. The alkali halides are widely used, particularly NaCl , KBr and ThBr which is transparent at wavelength as long as 625 cm -1 . AgCl cells are used for aqueous and moist samples but it is soft and easily gets deformed and darkens on exposure to visible light .

6. Cells made up of Teflon and polyethylene can be used, but Teflon shows bands due to C-C & C-F. For frequencies less than 600 cm -1 , a polyethylene cell is used. Because of the tendency of solvents to absorb, IR cells are ordinarily much narrower (0.1-1 mm) than those employed in UV and visible regions. Sample concentrations required is from 0.1-10 %.

7. Precautions : Cell windows are easily fogged by exposure to moisture and require frequent polishing with buffer powder which returns them to their original condition. Since alkali metal halides, with which cell is made up of are hygroscopic so must be protected from moisture by working at a suitable temperature.

8. SAMPLING OF SOLIDS : Generally 4 techniques are employed for preparing solid samples : Solids run in solution. Solid Films. Mull technique. Pressed pellet technique .

9. SOLIDS RUN IN SOLUTION Solids may be dissolved in non-aqueous inert solvent and a drop of this solution is placed on an alkali metal disc and solvent is allowed to evaporate, leaving a thin film of 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 .

10. 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 .

11. SOLID FILMS If a solid is polymer resins & amorphous solids, the sample is 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 solid is non-crystalline, a thin homogenous film is deposited on the plate 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.

12. MULL TECHNIQUE : In this technique a small quantity of sample is thoroughly ground in a clean mortar until the powder is very fine. After grinding, the mulling agent (mineral oil or Nujol) is introduced in small quantities just sufficient to take up the powder (mixture approximates the consistency of a toothpaste). The mixture is then transferred to the mull 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 run .

13. Demerit: Although Nujol is transparent throughout IR region, yet it has a disadvantage that it has absorption maxima at 2915, 1462, 1376 & 719 cm -1 . 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 .

14. This interference can be avoided by using Hexachlorobutadiene in combination with nujol which absorbs in regions 1630-1510 cm -1 , 1200-1140 cm -1 , 1010-760 cm -1 and thus permits the recording of IR spectra of only the sample. This method is good for qualitative analysis but not for quantitative analysis .

15. 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 .

16. Preparing a KBr Disk

17. The powder (KBr + sample) is introduced in between the 2 bolts and the upper screw A is tightened until the powder is compressed to a thin disc. After compressing the sample bolts A & A 1 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 .

18. 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 .

19. Demerits: It always has a band at 3450 cm -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, e ven 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 .

20. CONCLUSION : Form the discussion, one may draw the important conclusion that one may employ the Nujol mull technique may be applied for crystalline compounds (including complexes) in solid state & may reserve the pellet technique for remaining solid samples .

21. REFERENCES : Instrumental methods of chemical analysis, 5 th edition, Gurdeep.R. Chatwal & Sham.K.Anand , Pg.No.2.29. Instrumental methods of analysis, 7 th edition, Willard , Merritt, Dean & Settle, Pg. no. 305-310. Instrumental methods of chemical analysis, 26 th edition, B.K.Sharma , Pg.No.262-264. Organic spectroscopy, third edition, William Kemp , pg. no. 51 .

22. Thank you