12 Jun 2017

KBr pellet faults | Spectroscopy Guides

Generally, it is easy to produce a good quality KBr pellet if a decent Evacuable Pellet Die is used correctly, in conjunction with an appropriate Hydraulic Press. However, some faults in the produced sample pellet may occur due to a variety of reasons. This guide advises users how to make a KBr pellet for FTIR (or other purposes) with minimum error, by identifying some common faults or mistakes. 


The faults described below are for pure KBr or other halide salts which do not contaminate the sample. When the sample powder is added to the halide salt for pellet making, the clarity of the disc will depend to a large extent on the quantity and type of sample. Usually 0.1 to 2% weight to weight of sample to KBr is perfectly adequate. The overall quality of a pellet is largely dependent upon the quality of the KBr or halide salt powder used, which should always be of a spectroscopic grade of purity.

KBr pellet faults and fixes

A table of KBr pellet making faults, causes and solutions

KBr pellet fault Cause of KBr pellet fault Fix for KBr pellet fault
1) Sample KBr pellet not clear. Lacks optical clarity. Sample damp, contaminated KBr powder or insufficient pressure when pressing. Dry the sample powder or sample and increase the pressure via the hydraulic pellet press.
2) Sample KBr pellet clear, but shows opaque spots. Powder not uniformly flat in pellet die, leaving large particles which do not vitrify when pressed. Sieve pellet powder to extract coarse grains, then re-grind and re-press.
3) Sample KBr pellet cloudy. Insufficient pellet evacuation time or leaky seals. Check seals on the pellet die and lengthen evacuation period.
4) Sample KBr pellet clear at first, but becomes cloudy. Damp powder or damp atmosphere. Dry the pellet powder or sample, check seals on the pellet die and lengthen evacuation period.

To ensure that a sample pellet is produced which will enable accurate spectra of samples to be obtained, it is essential that the sample is thoroughly blended with the halide salt powder. Blending can be achieved either by using a mortar and pestle or by using a grinding mill such as the Specamill.

If a pellet breaks apart, then possibly not enough total mixture is being used in the die. Under compression the disc has become too thin and fragile. Excess moisture in a KBr disc could cause it to fragment and insufficient pressure or load on the sample when in the die can mean that the KBr/sample mixture has not been sufficiently compacted.

Evacuable FTIR pellet dies are essential to KBR pellet-making, pressing sample powder for FTIR analysis.

For a 13mm diameter pellet die with a total KBr/sample mixture mass of 400mgs, a load of 6 to 7 tons on the press gauge will produce a disc of approximately 1 to 1.5mm thickness.

Specac manufacture a wide range of KBr and XRF pellet preparation equipment, including Pellet Dies, various Pellet Presses, Mills and Grinders and Film-Makers.

Our Manual Hydraulic Press, in particular has been used for the preparation of cocoa powder for X-Ray Fluorescence analysis, as well as lasting for over 30 years at Sun Chemical Ltd.

The Autotouch Hydraulic Pellet Press provides an automatic, powered pellet press solution. And the Mini-Press provides a small, portable pellet press solution. All of these hydraulic presses are easy-to-use, consistent and durable.

Download a PDF of this KBr pellet guide here.

If you want more information on using the Atlas Pellet Dies for preparing spectroscopy pellets, or for any other application, get in touch for a price and demonstration.

Please see some reviews of our hydraulic presses and pellet dies

  • Mini-Press "such a simple device to use that gives high quality KBr pellets every time....The pellet holder easily attaches to a variable temperature cell holder for VT experiments. This is one of the best lab products I have ever used for IR spectroscopy" – Martin Lemaire of Brock University
  • Evacuable Pellet Die – "these Dies are the best of their kind. I have tried other brands and they do not work as well or last the same. If properly cleaned and stored they are eternal" – Leopoldo Suescun of the Faculty of Chemistry, University of the Republic
  • Autotouch Hydraulic Press – "the machine is powerful, reliable, robust and only needs minimum maintenance" – Li Chen of SCP Science
  • Manual Hydraulic Press – "the quality of the product is excellent and we never had problem with the results"Margirose Ancheta of Chanelle Pharmaceuticals Mfg. Ltd

Check out #SpectroscopyGuides, our new series of how-tos, tips and more.