5 Sep 2017

What is X-ray powder? | Spectroscopy Guides

What is XRF powder?

Spectroscopy is an analytical technique used in diverse industries for sample characterisation, quality control, and contaminant detection. X-ray fluorescence spectroscopy (XRF) involves shining bright X-rays at a sample and measuring how much light is emitted, 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. Table 1 shows a list of common binders and their elemental composition of three key binding agents.

The reason for a variety in choice is because some binders may contain trace elements that can affect measurement accuracy. If you're looking at Na, say, then you don't want there to be any Na in your binder.  If your application involves accurate measurements of oxygen (O), Boric acid is perfect.

Table 1. The elemental make-up of some commercially available binding agents; other trace elements may also be present. ® X-Ray Mix, SpectroBlend and Chemplex are registered trademarks of Chemplex Industries, Inc.

Additive Chemical Composition
X-Ray Mix® C: 48.7%; H: 8.1%; B: 0.6%; O: 42.6%
SpectroBlend® C: 81.0%; H: 13.5%; N: 2.6%; O: 2.9%
Boric Acid O: 77.6% H: 4.9%; B: 17.5%

Three key questions remain.

Binder powder needed for XRF pellet

Pre-mixed binders are available to be added to samples during the milling stage of preparation to control concentration. Specac offers pre-weighted ¼ and ½ gram tablets. Typically, a thick wax binder might be better for hard gritty powders while a thin, fluid wax is preferred for finer powders.

a smooth, well-prepared XRF pellet

An XRF pellet of milk powder

A 1:5 ratio of binder to sample is enough to produce a strong and stable pellet.

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.

an automatic hydraulic press, perfect for pressing XRF pellets

A good starting condition is 25 T for 2 minutes and depends on the sample type. Automatic presses like the Atlas® Autotuch™ 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. 

For more information on sample preparation: