Sample preparation is a crucial step in Infrared (IR) spectroscopy.
Different states of matter require specific techniques to ensure accurate spectra are obtained.
Let's delve into the various sample handling techniques for IR spectroscopy based on the physical state:
1. Solid Sampling:
a) KBr Pellet Technique:
A common method for solid samples involves grinding the sample with potassium bromide (KBr) and then pressing this mixture into a thin pellet.
KBr is transparent to IR radiation, allowing the sample's spectrum to be obtained without interference.
b) Mull Technique:
The solid sample can be ground with a small amount of mineral oil to produce a mull (a thick, paste-like substance).
This mull is then spread between two salt plates (like NaCl), which are transparent to IR radiation.
c) Diffuse Reflectance (DRIFT) Spectroscopy:
For solid samples that can't be prepared using the above methods, a technique known as Diffuse Reflectance Infrared Fourier Transform spectroscopy can be used.
The solid sample is mixed with a reflective medium and the reflected IR radiation is measured.
2. Liquid Sampling:
a) Thin Film:
A thin film of the liquid sample can be placed between two salt plates, typically made of NaCl, and then analyzed.
This method is straightforward but only suitable for liquids that do not attack the salt plates.
b) Solution Technique:
The liquid sample can be dissolved in a solvent that is transparent in the desired IR region, and a drop of this solution can be placed between salt plates or in a liquid cell.
3. Gas Sampling:
a) Gas Cells:
These are specially designed cells that can hold gaseous samples.
The cells usually have salt windows and a path length that can be varied. They are filled with the gas sample to a desired pressure and then sealed before analysis.
4. Solution Sampling:
a) Salt Plates:
A drop of the solution can be placed between two salt plates.
After the solvent evaporates, a thin film of the solute remains, which can then be analyzed.
b) Liquid Cells:
These are designed for solution samples and typically consist of two salt plates separated by a thin spacer, creating a cell with a known path length.
The solution is added to this cell, and its IR spectrum is measured.
When dealing with IR spectroscopy, it's vital to remember that the sample handling materials (like KBr or salt plates) must not absorb IR radiation in the regions of interest.
Also, special care must be taken to avoid contamination, ensure uniformity of the sample, and to avoid any conditions that might change the sample's properties between preparation and analysis.