Preparing a sample for infrared spectroscopy

Summary

This video explains two methods for preparing samples for IR spectroscopy: the Nujol mull method and the solution method. In the Nujol mull method, a compound is finely powdered and mixed with Nujol to form a paste, which is then transferred to moisture-sensitive potassium bromide discs and pressed into a film for spectrometer analysis. In the solution method, a compound is dissolved in a solvent, and the solution is placed in a special cell with potassium bromide plates. Both methods emphasize careful handling to avoid contamination and ensure accurate results in the IR spectrometer.

Takeaways

• 😀 A nujol mull is a technique used to prepare a sample for IR spectroscopy, involving grinding the compound into a fine powder and mixing it with nujol to create a paste.
• 😀 Potassium bromide discs are used to transfer the sample for IR spectroscopy, and these discs are moisture-sensitive and need careful handling to avoid dissolving.
• 😀 The flexible end of a small spatula is used to transfer the sample to the potassium bromide discs, and care is taken to avoid scratching the discs' surfaces.
• 😀 The two potassium bromide discs are pressed together to create a film that holds the sample in place for analysis in the IR spectrometer.
• 😀 Plate holders are used to support the discs in the spectrometer, with different designs available. The screws on one design help apply even pressure across the plates.
• 😀 Some plate holders have a spring design to hold the sample tightly, but the core function remains the same: to keep the sample securely in place in the spectrometer.
• 😀 An alternative method to prepare a sample for IR spectroscopy is to create a solution by adding the compound to a solvent at a concentration of about 5%.
• 😀 The solution is placed in a solution cell made of two potassium bromide plates clamped together, which must also be stored in a dry environment to prevent damage.
• 😀 After performing a spectrum of the solvent, the solution is transferred to the solution cell, with care taken to expel all air from the cell before sealing it with small plastic plugs.
• 😀 The solution cell, with the sample inside, fits into grooves in the plate holders to ensure it is placed properly in the spectrometer for the infrared radiation path.

Q & A

What is the purpose of making a nujol mull in IR spectroscopy?

A nujol mull is used to prepare a sample for infrared (IR) spectroscopy by creating a paste of the compound mixed with nujol, which allows the sample to be easily placed on potassium bromide discs for analysis.

How is the compound prepared for making a nujol mull?

The compound is ground into a fine powder using a mortar and pestle. A few drops of nujol are then added to the powder, and they are mixed to create a paste.

Why is it important to handle potassium bromide discs with care?

Potassium bromide discs are very moisture sensitive. If they come into contact with moisture or water, they will dissolve, which could interfere with the analysis, so they must be stored with a drying agent and handled carefully.

How are the potassium bromide discs prepared for use in the IR spectrometer?

The sample is transferred to one of the discs using a small spatula. Great care is taken not to scratch the surface of the disc. The two discs are then pressed together to create a thin film that can be used in the IR spectrometer.

What is the role of the plate holders in the IR spectrometer?

Plate holders are used to securely hold the potassium bromide discs with the sample in place during the IR spectroscopy process. They ensure that the sample is positioned correctly in the path of infrared radiation.

What are the different designs of plate holders, and how do they work?

There are various designs for plate holders. One example features two separate plates held together by screws that apply even pressure. Another design includes a spring inside a central cylinder. Despite differences, all plate holders function to keep the sample tightly in place during the analysis.

What is an alternative method for preparing a sample for IR spectroscopy?

An alternative method is to prepare a solution by adding a small amount of the compound to a sample vial, followed by a solvent to create a solution with a concentration of approximately five percent.

How is the solution prepared for use in the IR spectrometer?

The solution is prepared by adding the compound to a vial and then adding the solvent. The solution is transferred into a solution cell that consists of two potassium bromide plates clamped together.

Why is it important to first carry out a spectrum of the solvent before using a solution in IR spectroscopy?

Carrying out a spectrum of the solvent first is important to account for any absorption that the solvent itself may cause in the IR spectrum, ensuring that the final spectrum reflects only the compound of interest.

What precautions must be taken when using the solution cell in the IR spectrometer?

The solution cell, which contains potassium bromide plates, must be kept dry as any exposure to moisture could damage the plates. After filling the solution cell, small plastic plugs are used to prevent any solution from escaping.