In this article, I discuss the sample preparation for IR
The native form of a sample is unsuitable for analysis. This could be due
to
(i) The complex nature of the object, which could provide false measurements
due to interferences or masking agents.
(ii) The size of the object is too large
to analyze in its entirety (e.g., laboratory sample of contaminated soil).
(iii) The awkward shape of the object, preventing it from fitting within the instrument in
which the measurement is to be made.
To overcome these problems, some sort
of sample preparation must be performed. In many cases, sample preparation is
required before any quantitative analysis, and both can have dramatic impacts on
the measured results and their accuracy.
IR Spectroscopy can be done on any pure sample regardless of the physical state (solid, liquid, or gas). Cells must be constructed of ionic substances— typically sodium chloride or potassium bromide. Potassium bromide plates are more expensive than sodium chloride plates but have the advantage of usefulness in the range of 4000 to 400 cm−1. Sodium chloride plates are used widely because of their relatively low cost. There are many methods for the sample preparation for IR
Solids
- A solution of the solid sample can sometimes be used (but also must consider the properties of the solvent). The three common methods of Sample Preparation of pure solid is:
- The KBr Pellet method
A solid sample mixed with solid KBr (transparent in the mid-IR region), then ground and pressed in a special die to yield a transparent “window”
2. The IR Polyethylene Card method.
3. Nujol mull
Dispersion of the solid in a heavy hydrocarbon liquid (Nujol); particle size of the suspended solid must be very small to prevent light scattering •
SOFTWARE
You can find premium software in very cheap rates.
Procedure
KBE PELLET METHOD
- The first method involves mixing the finely ground solid sample with powdered potassium bromide and pressing the mixture under high pressure. Excessive grinding of the potassium bromide is not required. The finely powdered potassium bromide will absorb more humidity (it is hygroscopic) from the air and therefore lead to an increased background in certain ranges. Under pressure, the potassium bromide melts and seals the compound into a matrix. The result is a KBr pellet that can be inserted into a holder in the spectrometer.
- The main disadvantage of this method is that potassium bromide absorbs water, which may interfere with the spectrum that is obtained.
- If a good pellet is prepared, the spectrum obtained will have no interfering bands since potassium bromide is transparent down to 400 cm−1.
- After use, the mortar and pistil should be cleaned with acetone and double distilled water, then be put back on top of the oven for drying.
Sample/KBr ratio
The concentration of the sample in KBr should be in the range of 0.2% to 1%. The pellet is much thicker than a liquid film, hence a lower concentration in the sample is required (Beer's Law). Too high a concentration usually causes difficulties obtaining clear pellets. The IR beam is absorbed completely, or scattered from the sample which results in very noisy spectra.
 |
| A large benchtop hydraulic press |
 |
| A simple pellet press with cell holder |
NUJOL MULL
The second method, a Nujol mull, involves grinding the compound with mineral oil (Nujol) to create a suspension of the finely ground sample dispersed in the mineral oil. The thick suspension is placed between salt plates. The main disadvantage of this method is that the mineral oil obscures bands that may be present in the analyzed compound. Nujol bands appear at 2924, 1462, and 1377 cm−1. Good results are obtained by this method only if the average particle size of the solid is somewhat less than the wavelength of light the particles are to transmit. Samples should therefore be ground in a mortar to reduce the average particle size to 1 to 2 microns. About 5 to 10 mg of the finely ground sample are then placed onto the face of a KBr plate, a small drop of mineral oil is added and the second window is placed on top.
The KBr plates must be thoroughly cleaned after this procedure to prevent contamination of future samples. Wipe the windows with a tissue, then wash several times with methylene chloride, then ethanol. Use the polishing kit in the lab to polish the window surface. Wear gloves to prevent fogging. The cleaned surface should be clean and free from scratches.
 |
| In this photograph, the sample, ferrocene, two clean and polished KBr plates, an agate mortar and pestle, a mounting card, and a spatula are displayed as the base minimum requirements for preparing a sample through a Nujol mull. Of course, a small bottle of mineral oil is also necessary. |
.
 |
| Mulling ferrocene into mineral oil with a mortar and pestle. |
 |
| The prepared mull from an agate mortar and pestle being applied to a polished KBr plate. |
 |
| Sandwiched KBr plates with a Nujol mull of ferrocene. |
 |
| A series of plates indicating various forms of physical damage with a comparison to a good plate |
Poly Ethylene Card
Method
1) Dissolve/Mix ~ 100 mg (0.1 g)
of solid sample in ~ 2 mL of
methylene chloride.
2) Add a drop or two of the mixture to one of the
polyethylene surface of the
card.
3) Allow the methylene chloride
to evaporate (~ 30 seconds)
before putting into the IR
machine
Another common method used with solids is to dissolve the organic compound in a solvent, most commonly carbon tetrachloride (CCl4). Again, as was the case with mineral oil, some regions of the spectrum are obscured by bands in the solvent
LIQUID
- The one common preparation using a pure liquid is:
- The Solution Cell method.
- “Neat”: pure liquid used •
- Solid NaCl plates are most commonly employed as “windows” (NaCl does not absorb infrared light in the mid-IR region)
- A solution of the liquid sample can sometimes be used, but solvent may also absorb in the infrared region •
- Water and alcohols (e.g., methanol, ethanol, etc.) are seldom used as solvents because they absorb strongly in the infrared region and dissolve NaCl
Procedure
A drop of a liquid organic compound is placed between a pair of polished sodium chloride or potassium bromide plates, referred to as salt plates. When the plates are squeezed gently, a thin liquid film forms between them. A spectrum determined by this method is referred to as a neat spectrum since no solvent is used. Salt plates break easily and are water-soluble. Organic compounds analyzed by this technique must be free of water. The pair of plates are inserted into a holder that fits into the spectrometer.
 |
| A sealed solution cell with two injection ports and a schematic of its construction. |
 |
A sealed solution cell with two injection ports and a schematic of its construction |
Gases
Gases, or liquids with low boiling points, allowed to expand into an evacuated
sample cell
0 Comments
Thanks for visiting blog. if you have any query please let me know.