X-ray fluorescence spectroscopy (XRF) is an analytical technique used in several industries for sample characterisation, quality control, and contaminant detection. It involves illuminating the sample with X-rays and measuring the intensity of light emitted through fluorescence of the excited elements in the material, revealing its elemental makeup.
This fundamental technique can quickly show manufacturers how pure their steel is or reveal to chemists how pure their pharmaceuticals are. The types of samples that can be tested span hard solids, soft powders, liquids and gels.
This note focuses on the analysis of solid and powder samples. First, samples are typically ground to fine powders and then compressed into a solid pellet. The grinding helps improve homogeneity while the binding agent stabilises the pellet.
One of the limits of XRF analysis is sample heterogeneity. Large particle size effects and poor sample mixing affects the accuracy and precision of XRF analysis. Practical challenges such as powder stability to radiation and aging are also a concern. These are influenced by grinding speed and time, the grinding materials and the choice of sample binder.
Tonnage needed for XRF pellet
A fully and evenly compressed pellet is essential for XRF analysis. When a load is applied to some samples, they can readjust due to escaping gasses or contain void spaces that undermine structural integrity of the pellet.
A good starting condition is 25 T for 2 minutes and depends on the sample type. Automatic presses like the Autotouch™ series have an inbuilt top-up function to reapply pressure to a pellet containing really compressible samples. For greater tonnage loads, 40T presses are also available.
XRF pellet thickness required
The thickness of the pellet will depend on your die diameter and how much sample you add to the die. Samples with light elements don’t need to be as thick as those containing heavier elements. This is due to the inherent properties of heavier atoms. For a standard 32 mm die, 10g is usually sufficient, while a 40-mm die may require 15 g of sample.
XRF powder and spectroscopy
It is possible to directly press a sample in a die using just the sample or a crushable aluminium support cup. 32 and 40 mm dies are the industry standard for XRF sample sizes but smaller pellets can be made for one-off tests and laboratory investigations.
For automated analyses, samples are pressed into a metal ring, which offers good protection for the sample as the sample is contained within a metallic ring. Find Specac’s range of dies here.
Did you know, FTIR spectroscopy is a complementary technique that uses infra-red light to find out about molecular structure?
For highly quantitative analysis, compressed pellets can also be formed but different binding agents are needed. Typically, KBr powder is milled with the sample as it creates a transparent and stable matrix that houses the sample.
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