KBr Pellet Press and Making Pellets for FTIR
This procedure is specifically relevant to those who are using Fourier Transform Infrared Spectroscopy (FTIR) material characterization technique in the lab, it does not need to be followed for general pellet making.
Pellets for FTIR are typically required to be within 3 mm up to 13 mm, the exact diameter will depend on the manufacturer of your FTIR equipment.
The goal is to create a thin, transparent pellet which contains the sample you are interested to characterise held in an IR transparent medium (typically KBr), thus these are also known as KBr Pellets.
FTIR is a very sensitive technique, so you only need a few mg of your sample and the vast majority of your pellet is KBr (or another IR-transparent material). The typical ratio of KBr to sample is 100:1
Equipment you’ll need to make your KBr FTIR pellet:
- A Pellet Press Die Set of the diameter you need – these can be selected from our standard die sets or our low profile die sets.
- A Pestle and mortar – we like using agate pestles and mortar due smooth surfaces (so you don’t lose your precious sample powder) and due to lower contamination from the very hard material.
Please note: KBr is hydroscopic meaning it will start to absorb water from the air as soon as you open the bottle. If the KBr absorbs a lot of moisture, then this can show up in the FTIR measurement. If you are in a very humid environment or your samples will have a long exposure time in air, then you may wish to perform grinding and pressing in a glovebox and/or press with a vacuum die. It should also be noted that if you are grinding the KBr in open air anyway, using a vacuum die for a few minutes will not make a huge difference, so preparation in a glovebox is preferred over simply using a vacuum die. If you wish for us to make you a vacuum die, then we certainly can!
- Clean your die set and pestle and mortar thoroughly with solvent and DI and dry in a warm oven.
- Take a small amount of your sample powder and grind it in your pestle and mortar. For a ½ inch (12.7 mm) diameter sample, take around (1-2 mg). If you are making a smaller diameter pellet, use less powder remembering it scales with the area of the disk (square of the radius/diameter), or consult your FTIR manufacturer’s instructions.
- Add KBr powder, briefly grind together to mix. This can be done in a glove box if preferred to reduce moisture absorption. Over grinding will increase surface area and therefore moisture absorption. Around 200 - 250 mg of is sufficient for a ½ inch (12.7 mm) diameter sample to produce a disc approximately 2 mm thick. Use less if you are making a smaller diameter pellet, remembering it scales with the area of the disk (square of the radius/diameter)
- Put the pellet press die sleeve onto the base plate, add the sample + KBr mixture and put plunger on top.
- Press with force per unit area of around 8,000 to 10,000 psi (55-69 MPa). The force to be applied will depend on the diameter of your pellet. A ½ inch (12.7 mm) diameter pellet, has an area of 0.196 inch2, requiring 1960 lb (around 1 ton) to reach 10,000 psi. If you are making a ¼ inch (6 mm) pellet, then you will need 0.25 Ton to reach the same force per unit area. This is important so that you don’t over / under press.
- Remove the base plate, turn the die sleeve over, add the release ring to the top and press the pellet through to release it. If you are using a low-profile die set then you can just press against the inbuild prongs (not release ring required).
- The resultant pellet you are aiming for is around 2mm thick and clear / transparent. If you pellet is too thick or not transparent then you make need adjust the amount of powder being used.
Note – depending on your FTIR equipment, you make be able to examine your pellet while still in the sleeve. This may be more applicable to those using our low profile die sets.
Any questions about making your pellets or how we can make something to make your life in the lab easier, please email us at email@example.com