Readers ask: Why Are Salt Plates Used In Infrared Spectra?

Care and Handling of IR Plates. Sodium chloride is chosen because it is transparent to infrared radiation. These plates – called “salt plates” – are rather expensive because each plate is cut from a single giant crystal; they are very fragile and sensitive to moisture, including the moisture in your fingers.

Why are salt NaCl or KBr plates used to hold the sample in FT IR applications?

All Answers (21) You can use KBr or NaCl only if your samples are hydrophobic, since these cell can be affected by samples soluble in water because KBr and NaCl are soluble in water and these cells can be broken. If you have hydrophilic samples you can use a ZnSe cell.

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Why is Nujol used in IR?

For very reactive samples, the layer of Nujol can provide a protective coating, preventing sample decomposition during acquisition of the IR spectrum.

Why is NaCl transparent in IR?

Explanation: N aCl absorptions are transparent to most of the IR spectrum; there are absorbtions at 400−600cm−1, which leaves most of the spectrum transparent.

Why can’t glass or quartz be used in IR spectrometer what materials can be used instead?

An inconvenience when recording IR spectra is that glass cells cannot be used since glass absorbs IR radiation. Liquid samples are often run neat between two salt plates. Since solvents absorb IR radiation, IR cells usually have rather narrow path lengths to keep solvent absorption to acceptable levels.

Why NaCl or KBr is used in IR spectroscopy?

Potassium bromide (KBr, spectroscopic grade) is typically used as the window material because it is transparent in the IR, between 4000–400 cm-1. Alternatively, samples can be contained within a KBr matrix and pressed to form a pellet that is then analysed.

What precautions must you take when using salt plates for IR sample preparations?

Rule 1: ONLY clean NaCl plates with dichloromethane (aka, methylene chloride, CH2Cl2) and cyclohexane (C6H12). NEVER use water (H2O) to clean a salt plate as it will ruin the plate, permanently! Rule 2: NEVER hold the shiny side of a plate between your fingers (to avoid fingerprints). ONLY hold it by the edges.

Which solvent is used in IR spectroscopy?

The most common solvents are Carbon Tetrachloride (CCl4) and Carbon Disulfide (CS2). Chloroform, methylene chloride, acetonitrile and acetone are useful solvents for polar materials. Solids reduced to small particles can be examined as a thin paste or mull.

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Why is water not used as solvent in IR?

Water cannot be used as a solvent for ir spectroscopy because: 1- Water has two strong infrared absorption peaks. 2- Water is a strongly polar solvent that dissolves the alkali halide disks commonly used for IR.

What is the Mull technique?

a) Mull technique: In this technique, the finely crushed sample is mixed with Nujol (mulling agent) in n a marble or agate mortar, with a pestle to make a thick paste. A thin film is applied onto the salt plates. This is then mounted in a path of IR beam and the spectrum is recorded.

Why is KBr pellet used in IR spectroscopy?

We employ the KBr pellet technique because (1) it is relatively simple and (2) it allows us to run the full mid-IR region down to 400 wavenumbers, without regions of the spectrum being obscured by Nujol or Fluorlube bands or without having to run split mulls.

Why must neat liquid and not aqueous solution be used on salt plates?

First, water is a bad solvent because it will dissolve the salt plates. Second, water exibits a broad -OH peak that will cover up a lot of other peaks that you are interested in. These two reasons are good enough to NEVER USE WATER WHEN TAKING AN IR.

Which compounds are used as diluent in IR sampling?

In this method, you can select different diluents (matrixes) for various applications. For mid-IR frequency range, KBr, KCl or diamond dust can be used. For far-infrared testing, high-density polyethylene (HDPE) or diamond dust is suitable. For near- infrared analysis, CsI or KBr can be selected.

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Why does glass absorb infrared?

Infrared radiation is absorbed and emitted by the rotations and vibrations of chemically bonded atoms or groups of atoms and thus by many kinds of materials. For instance, window glass that is transparent to visible light absorbs infrared radiation by the vibration of its constituent atoms.

Why Raman spectra is more useful than IR spectra?

A big advantage of using Raman over IR is that the sample preparation is much easier and less time-consuming. A major problem that comes up with IR analysis is interference. Water cannot be used in IR due to its intense absorption of IR, whereas it can be used as a solvent in Raman spectroscopy.

What material absorbs infrared?

Glass, Plexiglas, wood, brick, stone, asphalt and paper all absorb IR radiation. While regular silver-backed mirrors reflect visible light waves, allowing you to see your reflection, they absorb infrared radiation. Gold, manganese and copper also absorb IR radiation well.

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