Merits and De-merits of different Liquid Sampling accessories used in FT-IR Spectroscopy

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Liquid sampling accessories for FT-IR spectroscopy, including sealed window cells, open window plates, demountable cells, and the horizontal ATR accessory, are the devices used to hold liquid samples for infrared analysis, each offering a different trade-off between ease of cleaning, control of film thickness, and suitability for viscous or highly absorbing liquids.

Liquid samples constitute a large majority of samples requiring identification and characterization using FT-IR Spectroscopy. Liquid samples can be analyzed neat or by diluting in a suitable non-absorbing solvent. For a good spectrum the film thickness can range from 0.02 mm to 0.1 mm.

Unlike sampling in UV-VIS spectral range where glass or quartz is used as cell material in IR region such materials cannot be used due to strong absorption of IR energy. The samples are contained in sample handling devices made from a range of suitable window materials.

Sealed Window Cells

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(Image Courtesy : http://www.internationalcrystal.net/)

Sealed window cells are made from suitable salts into which liquids can be poured and capped. The fixed path length windows are permanently sealed.

Disadvantages

  • Path length cannot be adjusted depending on solution concentration
  • It is difficult to fill the sealed cell with viscous samples
  • After analysis cleaning the cell is time-consuming and not easy.

Open Window plates

Open window plates are available in suitable sizes.These are convenient to use as a drop of the liquid can be pressed between two plates made of NaCl or K Br as a film and placed in the spectrometer window compartment. Low volatility liquids can also be analyzed by pressing a drop between disposable polyethelene plates

Advantage

  • Convenient to handle and clean with solvents after analysis

Disadvantages

  • Film thickness cannot be controlled

Demountable cells

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(Image Courtesy : http://www.perkinelmer.com/)

Demountable cells can be used like sealed cells and offer the following advantages :

  • Can be dismantled for cleaning purpose
  • Spacers of different thickness can be used to prepare films of differing thicknesses
  • Filling the cell is convenient with the help of a syringe

Horizontal ATR accessory

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(Image Courtesy : http://www.internationalcrystal.net/)

The horizontal ATR accessory is a versatile accessory offering spectral analysis of a large type of samples ranging from highly absorbing samples such as aqueous solutions, viscous liquids and slurries. This accessory is based on the internal reflectance of the IR beam within the sampling crystal which forms the bottom surface of the accessory

Advantages

  • Simple to operate. The sample needs only to be poured into the accessory
  • Suitable for all types of liquids as described above
  • Accessory can be cleaned with a suitable solvent after analysis conveniently