9 Jun 2017

Checklist for making XRF pellets | Spectroscopy Guides

For the most consistently pressed XRF pellet, follow these basic points and ensure your X-Ray Fluorescence analysis gets off to a smooth, homogeneous start.

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The range of sample types for X-Ray Fluorescence analysis is potentially endless. Therefore, although these guidelines offer advice for producing well-pressed samples, they are subject to change, according to the particular sample being analyzed and its individual XRF sample preparation needs. But whatever happens, the key to all analysis sample preparation is consistency in the preparation procedures.

Checklist: making XRF pellets

1) Confirm spectrometer's XRF sample preparation needs

The sample requirements of the spectrometer should be determined. To produce a sample for XRF analysis, you will need to know what size sample the XRF spectrometer will accept. Common sizes for round XRF pellets are 32 mm or 40 mm diameter.

Learn about XRF Pellet Dies

2) Are you milling your sample to a powder for XRF?

Powders to be pressed should be as fine as possible but also with consistent particle size. This provides an optimal distribution and evenness of any x-ray fluorescence sample and therefore offers better accuracy, repeatability and consistency of results.

specamill-FTIR-and-XRF-sample-grinder.jpeg

Learn about the Specamill

3) Standard or ring XRF pellet die?

a) Standard XRF pellet die

It is possible to directly press a sample in a die and depending on the sample holder of the spectrometer, it may be possible to use the pressed sample as it is. However, the most common method is to use a crushable aluminium support cup. These are low cost cups made from thin aluminium.

The cup should be a good fit in the die. Upon placing the cup in the die, the milled sample is then placed into the cup. The die is assembled and pressed. The aluminium cup is crushed and acts as a support for the sample. The sample can then be placed into the sample holder of the spectrometer.

Learn about XRF consumables, including cups and powders

b) Ring XRF pellet die

Some spectrometers offer an automated loading system or a tool that can be used to load the sample. In these cases, samples are pressed into a metal ring. There are different rings available and this again depends on the requirements of the spectrometer.

The most common is the 51.5 mm diameter ring which has an inside diameter of 35 mm into where the sample is pressed. In this case, a special ‘ring die’ is needed which can hold the ring as the sample is pressed. Also, in situations where many samples are to be prepared for analysis at a later time, the rings offer good protection for the sample. 

4) Do you have a high-quality XRF pellet die?

All dies should be constructed from a high-quality stainless steel. The pressing face used to contact the sample as it is pressed (referred to as die pellets) should be perfectly flat and smooth, commonly polished to a mirror finish.

Pellet-Die-Spares-FTIR-or-XRF.jpeg

Learn about XRF Pellet Die spare parts

If iron, for example, is to be analyzed in the sample, there is the option to use Tungsten Carbide die pellets to avoid contamination of the sample. 

5) Ensure you have the correct load for XRF pellet pressing

The pressing procedure can be simply described as a load applied to the sample in order to compress it so that the powder will bind and form a solid pellet.

Learn about the 2 ton Mini-Pellet XRF Press
 
If a foodstuff is to be analyzed, the load needed may be very low, perhaps as low as 2 tons. Pharmaceutical products may require a higher load to make the sample bind, perhaps 20 tons. Slag waste, mineral ores and fillers used in industrial manufacturing, depending on their makeup, may require a higher load of 25 tons upward.

However, some materials in all these categories may not bind at all or break up very easily even when supported in a cup. It is with samples like these that a wax binder could be considered at the milling stage. Wax binders are available in powder or tablet form.

There are no set rules as to how much to add, but this should be the minimum amount to safely bind the sample when pressed.

6) Manual or automatic XRF pellet press?

The lowest cost hydraulic XRF presses are the manual type. These are available offering a load up to 25 tons, but the effort needed by the operative to achieve 25 tons can be difficult.

If you want more information on using the Atlas Manual Hydraulic Press, Autotouch Automatic Hydraulic Press or Evacuable Pellet Dies for XRF pellet preparation, get in touch for a price and demonstration.

Learn about the Manual Hydraulic XRF Pellet Press

Alternatively, power operated presses offer consistency and repeatability. Basic power operated presses offer a load range that can be adjusted by the user.

Autotouch-Hydraulic-Press-for-FTIR-or-XRF-pellets.jpeg

Learn about the Autotouch Hydraulic XRF Pellet Press

However, perhaps the best option is a press that offers a programable pressing cycle. With these presses options are:

  • timer capability to offer maximum consistency
  • auto top-up; some samples will compress while under load. The Auto-Top up feature will automatically top the press load up if compression while under load is lost
  • Step function – rather than immediately ramp the load on a sample up to the load that is needed, programmability offers the option to gradually step the load up. This is a significant advantage for lighter samples or to help air or gasses escape from a sample, as air/gas pockets will give errors in analysis
  • other considerations can be power supply. A 3-phase power requirement will require special installation; a standard mains plug/voltage may be preferable
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