1 Aug 2018

XRF Sample Preparation Techniques: Pellets versus Loose Powder | Spectroscopy Guides

We recently compared the effectiveness of loose powder against prepared pellets for the x-ray fluorescent spectroscopic analysis of cement.

Some users choose to carry out XRF without pressing the sample into a pellet. Despite the extra time involved in sample pellet preparation, which involves the use of a hydraulic laboratory press, our results show the importance of preparing high quality XRF pellets for the analysis of samples like cement or similar. Read more and download our free application note.

What are the advantages of using pellets for XRF?

X-Ray Fluorescence spectroscopy (XRF) is highly sensitive to the preparation of the sample material. Pressed cement samples show a higher signal-to-noise ratio and this allows the detection of the lightest elements compared to its loose powder form.

When quantifying the elemental composition, significant discrepancies are observed between the expected values and the loose powder samples. When the same batch of cement is used to prepare a pressed pellet these discrepancies disappear.

A PDF of this study can be found here: Advantages of Pellet Preparation for XRF.

What factors affect XRF analysis procedure?

When a sample is analyzed using an XRF spectrometer, there are certain characteristics that can impact the signal received.

 
Sample
 
 
 
 
 
(Filter)
 
Detector
 
X-ray source
 
Sample
 
(Filter)
 
Detector
 
X-ray source
Two components make up the analytical signal for XRF spectroscopy:
  1. X-ray emission peaks at characterstic wavelengths that correspond to electron transitions taking place within the atoms of the analyzed sample
  2. a continuous background of X-rays which are scattered by the loosely bound outer electrons, over which the emission peaks are super imposed

How XRF affects electrons 1How XRF affects electrons 2How XRF affects electrons 3

These characteristic X-rays are generally due to surface atoms at depths between 1-1000 µm below the sample's surface. The exact depth depends on its atomic weight; if an element is lighter it is harder to detect than a heavier element. 

The background scattering and intensity of the emission peaks are all affected by particle size, mineral composition and particle density. However the grinding of samples to a very fine particle size and then compressing into a smooth and flat XRF pellet reduces the background scattering and improves the detection of emissions.

What techniques were used to analyze cement using XRF?

Type 1 Portland Cement was analyzed. The spectra were all recorded on a high-throughput wavelength dispersive XRF device with vacuum capability.

1) Loose powder XRF preparation method

10 grams of cement was placed into an open-ended XRF sample cup and then covered using 6 µm Mylar® film.

XRF sample used to house powders for pellet pressing

CLICK HERE FOR PRODUCT PRICING

2) XRF sample pellet preparation method

The cement was finely milled and homogenized using 20 wt% of cellulose binder (SpectroBlend®) in a P6 Planetary Ball Mill. Then pellets were compressed in aluminium sample cups at 20 tonnes using a 25 T Atlas® Power Press and a 40 mm ApexTM Quick Release Die.

An Apex Quick Release XRF Pellet Die

CLICK HERE FOR PRODUCT PRICING

Quality comparisons between pellets and powder, for XRF

There are clear differences in quality between the XRF spectra taken from loose sample powder and taken using sample pellets. Read more detail on the comparison of XRF sample preparation techniques below.

How heavy and light elements compared in pellets and loose powder

The spectra taken using the loose powders show clear differences from the pressed pellets. The pellets show a higher signal-to-noise ratio and this allows the lightest elements to be detected easily above the background. This is further improved in the pellets because thin films were not necessary, allowing the measurement to take place under a vacuum.

The quantification of the composition

It is critical for accurate quantification that the user can clearly detect all elements in the sample. In the loose powder samples, underestimation of the lighter Al, Mg, and Na elements leads to overestimation of Fe and Ca in the cement. The sample pellets, on the other hand, provide a quantification in line with the range etablished by standard lab experiments.

XRF spectra of cement, comparing loose powder to sample pellet.XRF spectra of cement, comparing loose powder to sample pellet.

Figure 1: Spectral comparison for (A) heavy elements and (B) light elements in cement samples

Which is better for XRF sample preparation: pellets or powder?

Pellet preparation is essential for the accurate quantification of sample composition using XRF spectroscopy. It allows the user to detect the sample's lightest elements while preventing the underestimation of other light elements.

Table 1: Results of Portland Cement fundamental parameter quantification
Compound Powder (%) Pellet (%) Expected
SiO2 7.75 18.90 19.0-21.8
Al2O3 1.16 4.35 3.9-6.1
Fe2O3 5.70 2.32 2.0-3.6
MgO 0.12 1.06 0.8-4.5
CaO 78.57 65.60 61.5-65.2
Na2O Not detected 0.25 0.2-1.2

Loose powder is quick and convenient for detecting heavy elements using XRF spectroscopy, however quantification is unreliable as it is unable to detect lighter elements.

Download our free application note to print off this study.

XRF analysis is more effective when carried out using compressed sample pellets, instead of loose powder. Read this application note for more information.

Get in touch: Collaborate with Specac

Through working with Specac, Dr Tom Knott of the University of Leicester and his team's work has been championed via application note and online editorial. The research will be shared internationally via various promotional channels. Are you working in a field that uses, or could use, our FTIR, XRF, polarizer, heated platen or hydraulic pressing equipment? Do you want your work reported to a wider audience?

To promote your research with us, please contact collab@specac.co.uk with a simple brief, detailing:

  • what your work involves
  • what equipment you currently/can use
  • why your work is important

To learn more about what spectroscopy can do, check out #SpectroscopySolutions for more insights into the applications XRF and FTIR can fit.

Look up #SpectroscopyGuides for tips and tricks to help your analysis.

Also, check out our #SpectroscopySolutions presence on YouTube!