Sample preparation in any analytical chemistry technique is important but in XRF is particularly critical because different sample preparation methods and conditions can yield very different results even if the analysis is only qualitative, therefore the analysts in XRF must be well trained in the chosen sample preparation technique.

Ways to prepare samples for XRF

There are 4 ways samples can be loaded in a spectrometer according to their preparation:

Source: https://www.911metallurgist.com/blog/sample-preparation-methods-for-xrf-analysis

 

    1. No preparation

    2. Loose powder

    3. Pressed pellets

    4. Fused beads

Pressed pellets give results that are between fused beads and loose powders in terms of analytical quality and speed of results. Fusion bead is used instead of pressed pellet when the interferences coming from different crystals (the “mineralogical effect”) need to be eliminated from the analysis. If the same element exists in two or more crystal configurations it will emit slightly different intensities of the same wavelength, making quantitative analysis less reliable.

Pressed pellets, on the other hand, improve results by compressing powder materials into a dense solid form that is free of voids and has a smooth, flat analytical surface to reduce variations in distance to the detector. All this lowers the level of scattered background and improves detection of low atomic weight elements.

Major and minor constituents

It is useful to compare analysis of major elements (which make up the bulk of the sample) and minor elements (which occur in smaller, possibly trace level amounts). For analysis of minor elements, pressed pellets may be superior to fused beads owing to the dilutions required for the latter technique.

Another important distinction is between heavy and light elements. Heavy elements emit characteristic wavelengths of a higher energy, and therefore are easier to detect. Light elements produce longer, lower energy wavelengths and may be difficult to detect in loose powders due to the inhomogeneity and low density of these samples.

Which industries use pressed pellets for their XRF analysis?

Cement

Cement is a primary building material in the modern world; as a major constituent of buildings, concrete structures and roads the consistency of its composition during production is essential.

A major method of quality control (QC) for cement production is XRF spectroscopy as it can provide fast and accurate analysis in cement manufacturing using in-line methods to ensure 24/7 results. XRF analysis is used at several stages of the cement analysis process:

      • Evaluation of raw materials at the quarry
      • To assess intermediate products (clinker, gypsum, limestone)
      • Approval of the finished product (cement) to the required standards such as ASTM C114 [1] and ISO/DIS 29581-2 [2]

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In all of these areas, robust and simple to use XRF instruments that are operated by non-expert production staff provide cost-effective chemical composition analysis.

XRF analyzers typically measure the concentrations of MgO, Al2O3, SiO2, SO3, K2O, CaO and Fe2O3 in cement materials to assess quality and are also used to monitor sulfate addition to gypsum.

Click here to read our Application note “Analyzing cement using XRF spectroscopy”

Slag composition and controlling steel chemistry

The ability to rapidly analyse the chemical composition of blast furnace slag is central to the operation and control of a blast furnace. XRF spectrometers are the most common analysis tools available to analyse powder samples of slag produced in iron and steel making. The role of the chemical composition of blast furnace slag in the control of the iron production process is well established.3,4 The fast, accurate, and cost-effective elemental analysis of blast furnace slag is a key part of quality and process control in iron production.

XRF is cost-effective, user-friendly fast, non-destructive and environmentally friendly analysis method with a very high accuracy and reproducibility. All of the elements of the periodic table from beryllium up to the rare earths can be measured qualitatively and quantitatively in powders, solids and liquids.5 Concentrations of up to 100% maybe analyzed directly, without any dilution, with reproducibility’s of better than ±0.1% and typical limits of detection (LODs) from 0.1 to 10 ppm.

Mining Exploration & Grade Control

Exploration of iron deposits involves determining the type of mineral ore (hematite, magnetite, taconite) the likely yield of iron (Fe), manganese (Mn), and other profitable elements, as well as determining so-called ‘penalty’ elements that might negatively affect the quality of steel produced using the ore. These include phosphorus (P), silicon (Si), aluminum (Al), and sulfur (S).

Once the mine is operational, the produced ores need to be accurately characterized before shipment. Quantification of a few elements, such as Fe and Mn plus the important penalty elements that may be present can be achieved with Wavelength Dispersive XRF, while full major and minor element analysis can be quickly gathered using an Energy Dispersive XRF instrument.

Nutritional analysis in the dairy industry

Milk analysis, dairy analysis, and milk powder testing for iron, phosphorus and calcium, sodium and potassium are some of the essential processes in the regulation of the dairy industry and the QA of infant formula. X-ray fluorescence analysis (XRF) of milk and dairy products is not yet widespread in dairy industry, although the method has a lot of potential, as dried samples may be analyzed directly without any chemical treatment and XRF equipment is easily available.6

 

XRF spectrometry is a comparative technique and it requires a set of calibration standards in order to perform its quantitative measurements and this can be a difficulty in the dairy industry because of the wide range of sample types. In a recent study6 the concentrations of minerals (Na, Mg, P, S, Cl, K, and Ca) and trace elements (Mn, Fe, Ni, Cu, Zn, Rb, Sr, and Br) in different types of milk, dairy products, and infant formulas were determined using wavelength dispersive X-ray fluorescence analysis (WDXRF).

The technique used freeze dried samples pressed as tablets of 4 g and calibrations were established using available plant and milk standard reference materials. The results demonstrated the suitability of WDXRF for the quantitative analysis of Na, Mg, P, S, Cl, K, Ca, Zn, Rb, Sr, and Br in all of the dried samples. Trace elements such as Mn, Fe, Ni, and Cu were at the limit of detection (LOD) in most samples. The main disadvantage of XRF was the high LODs for trace elements (Cd, Hg, Pb, and As).

 

Click here to read our Application note “High throughput, high accuracy sample prep for XRF”

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References

      1. ASTM, Standard C 114-08, “Standard Test Methods for Chemical Analysis of Hydraulic Cement”, Annual Book of ASTM Standards, Volume 04.01, ASTM International, West Conshohocken, PA, 2008, pp. 150–157.

      1. DIN EN ISO 29581-2 (Draft standard, 2007-07), “Methods of testing cement Chemical analysis of cement – Part 2: Analysis by X-ray fluorescence” (ISO/DIS 29581-2:2007), 30 pp

      1. Goro Okuyama, Koji Yamaguchi, et al., Effect of Slag Composition on the Kinetics of Formation of Al2O3–MgO Inclusions in Aluminium Killed Ferritic Stainless Steel, ISIJ International, Vol. 40 (2000), No. 2, pp. 121–128

      1. Ko-ichiro Ohno, Masashi Kaimoto, et al., Effect of Slag Melting Behaviour on Metal- Slag Separation Temperature in Powdery Iron, Slag and Carbon Mixture, ISIJ International, Vol. 51 (2011) No. 8, p1279-1284

      1. Global CCS Institute, IGCC solids disposal and utilisation, 01 May 2012

    1. Galina V. Pashkova, X-ray Fluorescence Determination of Element Contents in Milk and Dairy Products Food Anal. Methods (2009) 2:303–310