How to prepare a XRF pellet sample for Spectral Analysis

Preparation of Pressed Pellets for XRF Analysis

1. Objective

This procedure describes the method for preparing solid samples for X-ray Fluorescence (XRF) analysis by pressing fine powders into homogeneous, stable pellets. Proper preparation eliminates errors caused by particle size and ensures high accuracy and reproducibility.

2. Scope

This method is applicable to inorganic powders, such as geological materials (rocks, minerals, soils), cements, slags, and certain industrial by-products. It is designed for samples that can be ground to a fine particle size.

3. Principle

The sample is ground to a specific particle size (typically <75 µm) to minimize mineralogical effects. It is then mixed homogeneously with a grinding aid or binder (optional) and placed in PMH-A XRF pellet die. Under high pressure (20-40 tons ), the particles are forced together, forming a solid, coherent disc. The resulting pellet presents a flat, smooth surface to the X-ray beam, ensuring consistent excitation and measurement.

4. Equipment and Materials

Spectroscopy Press: A hydraulic press capable of applying at least 30 tons of force (e.g., a 40-ton automatic or manual press).

Grinding Mill: A ring and puck mill, vibratory disc mill, or planetary ball mill.

Pellet Die Set: Typically made of chrome-plated steel or tungsten carbide.

Common accessories: Steel rings (for "unbounded" pressing) or aluminum cups (for "bounded" pressing).

Analytical Balance: Precision to 0.0001 g (0.1 mg).

Drying Oven: Capable of maintaining 105°C ± 5°C.

Binders (Optional but recommended):

Cellulose (e.g., Herzog H-Binder, C-Wax)

Boric acid (H₃BO₃)

Starch

Mortar and Pestle: For manual mixing or breaking up agglomerates.

Sieves: 200 mesh (75 µm) or finer for particle size verification.

Sample Containers: Plastic vials or bags with labels.

Personal Protective Equipment (PPE): Safety glasses, dust mask (N95 or equivalent), and lab gloves.

5. Safety Precautions

Dust Inhalation: Fine powders can be hazardous. Always perform weighing and grinding inside a fume hood or using a downdraft table. Wear a dust mask.

Noise: Grinding mills can be very loud; use hearing protection if necessary.

Manual Handling: The press dies and pellets are heavy. Lift with proper technique to avoid back strain.

High Pressure: Never place fingers or tools between the press pistons during operation.

6. Procedure

Step 1: Sample Collection and Pre-treatment

Ensure the sample received in the laboratory is representative of the bulk material.

If the sample is moist, dry it in an oven at 105°C for 2-4 hours (or until constant weight) to remove free moisture. Note: Overheating may cause loss of volatiles.

Step 2: Grinding (Communition)

Weigh the required amount of sample (e.g., 20-50g depending on the mill type).

Grind the sample in a disc mill until at least 95% passes through a 200 mesh (75 µm) sieve.

Note: Consistency in grinding time and pressure is critical for reproducibility. If the sample is very hard, pre-crush it in a jaw crusher before fine grinding.

Check particle size occasionally with a sieve.

Step 3: Mixing (with Binder)

Note: This step is optional but highly recommended for non-self-binding samples (e.g., most rocks and minerals).

Weigh the ground sample and the binder into a mixing vessel.

Typical Ratio: 5g to 10g of sample mixed with 1g to 2g of binder (e.g., 20% cellulose wax).

Mix thoroughly using a mortar and pestle or a laboratory mixer/mill to ensure the binder is evenly distributed throughout the sample. This coating helps the particles slide together under pressure and adds mechanical strength to the pellet.

Step 4: Pressing the Pellet

Option A: Unbounded Pressing (Using a Steel Ring)

Assemble the die base and place the polished steel ring inside.

Pour the prepared sample mixture into the cavity of the ring, filling it completely.

Carefully spread the powder evenly with a spatula to create a flat surface.

Insert the plunger/piston into the die.

Place the die assembly centrally on the press table.

Activate the press and apply the pre-set pressure.

Typical Pressure: 30 tons (or specific pressure of ~200 MPa).

Dwell Time: Hold the pressure for 15-30 seconds. Automatic presses handle this with a "hold" function; manual presses require the operator to maintain pressure by watching the gauge.

Release the pressure slowly.

Disassemble the die and carefully eject the pellet. The pellet should be solid, with a shiny, smooth surface free of cracks or lamination.

Option B: Bounded Pressing (Using Aluminum Cups)

Place an aluminum sample cup upside down on the die base.

Fill the cup with the prepared powder, ensuring it is level.

Insert the plunger.

Press as described in Option A (Step 6). The powder will be compressed into the cup, forming a pellet mechanically locked into the aluminum ring for extra stability.

Step 5: Labeling and Storage

Immediately label the pellet with a permanent marker on the back or side (not the analysis surface).

Place the pellet in a desiccator (silica gel) to keep it dry and free from contamination until analysis.

The analysis surface must not be touched by fingers to avoid contamination from skin oils and salts.

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