Step-by-Step Pelletising Workflow for XRF Analysis

X-ray Fluorescence (XRF) analysis depends as much on sample preparation as it does on the performance of the spectrometer itself. Before any measurement takes place, the physical condition of the sample determines how X-rays interact with the material and how consistent the resulting data will be. Pelletising is widely used because it converts loose powder into a stable, uniform form that can be analysed repeatedly under controlled conditions. When pellet preparation follows a clear and logical sequence, many common sources of variability can be reduced, such as particle size differences, uneven density, and surface irregularities, leading to more dependable XRF measurements.

Why Pelletising is Crucial for XRF Analysis

In XRF analysis, primary X-rays are directed onto the surface of a sample, where they excite atoms within the materials. When the atoms return to a lower energy state, they emit secondary fluorescent X-rays at energies characteristic of the elements present. A spectrometer then measures the emissions to determine elemental composition.

The reliability of XRF analysis depends strongly on the physical condition of the sample. Coarse particles, uneven packing, and mixed mineral phases can alter how X-rays interact with the material, introducing absorption and scattering effects that distort the measured signal. Pelletising reduces these influences by producing a flat, dense, and homogeneous surface, allowing the detected signal to reflect chemistry rather than physical variability.

Pelletising Workflow for XRF Analysis

Step 1: Comminution to a Fine and Uniform Powder

Size reduction is the first step in the pelletising workflow. The goal is to mill the sample to a fine and uniform powder that minimises mineralogical effects in XRF analysis while remaining representative of the bulk material. For most XRF applications, this means decreasing the particle size to below 75 micrometres using consistent milling conditions. Grinding media selection also matters, as materials such as hardened steel or tungsten carbide can introduce contamination if they contain elements being analysed. Choosing chemically appropriate media at this stage helps maintain data quality through the remaining stages of preparation.

Step 2: Weighing and Binder Addition

Once the powder has been milled to the required fineness, it needs to be stabilised before pressing. This is undertaken through adding a binder in a controlled proportion, allowing the material to form a durable pellet that can be handled and analysed reliably. A commonly used ratio is around 80% sample to 20% binder by mass, although the ideal proportion varies depending on the hardness and cohesiveness of the material. High-purity, cellulose-based binders are widely used because they provide mechanical strength without introducing elements that interfere with XRF measurement. Careful weighing and consistent proportioning during binder addition help produce pellets with uniform density and support reproducible analytical performance.

Step 3: Homogenising the Mixture

Binder addition must be followed by thorough homogenisation so that the sample and binder form a single, uniform matrix. Mechanical mixing, typically using a laboratory mill or blender, distributes the binder evenly throughout the powder and prevents localised differences in density or composition. When mixing is insufficient, the X-ray beam applied to XRF analysis can encounter areas of uneven packing within the pellet, which leads to poor repeatability between measurements. Effective homogenisation reduces this variability and ensures the pellet responds consistently across its entire surface during analysis.

Step 4: Preparing the Die and Charging the Sample

With the powder ready for pressing, attention turns to the condition and setup of the pressing tools. Die sets should be clean and free of residual material to prevent cross-contamination and surface defects in the finished pellet. The polished base must be seated correctly within the die body, and an aluminium pellet cup may be used where additional lateral support is needed. When charging the die, the powder should be added in a controlled manner and levelled before pressing to encourage even pressure distribution. Proper die preparation and charging help minimise density gradients and surface irregularities that can affect XRF analysis.

Step 5: Pressing and Dwell Control

Pressing brings the prepared powder together into a single, solid pellet by applying pressure in a controlled and gradual manner. As the load increases to the target level, typically between 15 and 30 tons, the particles are forced into closer contact and the binder begins to distribute between them. Holding such pressure for 30-60 seconds allows trapped air to escape and the internal structure of the pellet to stabilise. Releasing the pressure slowly helps preserve the structure and prevents cracking or surface separation. As long as the compression cycle is applied consistently, the result is a flat, stable pellet that can be handled and analysed repeatedly with confidence.

Step 6: Inspecting and Storing the Pellet

After pressing, the pellet should be inspected to confirm it is ready for analysis. The analytical face must be smooth, flat, and free from cracks, pits, or visible delamination, as any surface defect can scatter X-rays and reduce measurement accuracy. If the pellet is not analysed immediately, it should be stored in a desiccator to protect hygroscopic raw materials from moisture uptake. Maintaining surface integrity during storage is essential for preserving analytical reliability over time.

Bringing the Workflow Together

A well-controlled pelletising workflow directly supports accurate and repeatable XRF analysis by reducing physical variability in every part of sample preparation. When milling, mixing, pressing, and storage are treated as a connected process, laboratories can achieve better consistency and greater assurance in their results. For teams looking to refine or standardise their XRF analysis workflow, XRF Scientific can offer purpose-built preparation solutions, including Orbis Mining pulverisers for controlled size reduction, xrTab cellulose-based binder tablets for consistent pellet formation, and xrPress manual and automatic hydraulic presses for repeatable compression cycles. Contact XRF Scientific to discuss your current sample preparation approach and identify opportunities to improve consistency, throughput, and long-term analytical reliability.