How are powder samples prepared for XRF analysis using the pressed powder method?

The powder is placed in a ring or cup and compressed with a press using flat or cylinder-type dies. Fine pulverization generally improves pellet formation. If the powder does not form a mechanically stable pellet, a binder can be mixed with it before pressing. The die, ring or cup, sample quantity, and pressing conditions should be kept consistent to obtain reproducible XRF results.

When should a binder be used to prepare pressed powder pellets for XRF?

A binder is useful when a powder does not readily form a stable pellet or when particles may detach from the pellet and contaminate the XRF sample chamber. Materials with spherical particles, such as SiO₂ and burned ash, can be particularly difficult to pelletize. Typical sample-to-binder ratios are 10:1 or 10:2. The binder must be sufficiently pure and free of elements that could interfere with the analysis, and accurate weighing and thorough mixing are required.

How does pelletization pressure affect XRF measurements of pressed powder samples?

Increasing pelletization pressure increases sample density and therefore can increase measured X-ray intensity. Above a certain pressure, the intensity reaches a saturation region. Preparation reproducibility can be improved by using the same sample quantity and pressure for every pellet and, when practical, selecting a pressure high enough to reach the intensity saturation region.

How can cracking and breakage of pressed powder pellets be prevented?

The behavior of both the powder and its supporting ring after pressure release should be considered. PVC rings tend to expand after pressure release and are better suited to samples that also expand, whereas aluminum rings are appropriate for samples that do not expand. Pressurizing too quickly can also trap air within the sample. Releasing pressure several times before reaching the final pressure allows trapped air to escape and reduces cracking or flaking.

How can contamination be minimized when preparing pressed powder samples for XRF?

Die surfaces should be cleaned before each pellet is prepared because residue from previously pressed samples can contaminate subsequent samples. Preparing samples in order from lower to higher elemental concentrations can further reduce carryover problems. A polypropylene or polyester film can also be placed between the sample and die to prevent powder from sticking to the die surface. Repelletizing broken samples can introduce additional contamination from ring or cup materials.

When is loose powder preparation preferable to pressed pellets for XRF analysis?

Loose powder preparation is useful when the material is difficult to pelletize even with a binder, as can occur with graphite and mica, or when the sample needs to be recovered after XRF measurement for another analytical technique. The tradeoff is poorer preparation reproducibility compared with pressed pellets, so reproducibility should be evaluated before routine use.

How do sample films affect XRF analysis of loose powders?

The film required to contain a loose powder absorbs some of the emitted X-rays and therefore decreases analytical sensitivity. The effect is especially severe for light elements from boron through fluorine, which may not be measurable through the film. Film impurities can also contribute to the measured spectrum, so blank measurements of the film are recommended. Film strength is important because rupture under vacuum can release powder into the sample chamber.

How can very small quantities of powder be prepared for XRF analysis?

Double-pellet and embedded-pellet techniques can be used when there is insufficient material to produce a conventional pellet. A small quantity of sample is pressed onto or into a previously prepared cellulose or boric acid pellet. Dedicated sample cells can also accommodate approximately 100 mg of powder for measurement under vacuum without pelletization. Sample quantity should be carefully controlled when calibration accuracy depends on maintaining consistent sample thickness.

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Sample Preparation for X-ray Fluorescence Analysis III.

Pressed and loose powder methods

Gakuto Takahashi

Winter 2015 Volume 31, No. 1 , 26-30

There are two main sample preparation techniques for measurement of powders with XRF—pressed and loose powder methods—neither requiring any chemical processes. In either case the proper sample preparation and accessories need to be selected to prevent breakage of the pressed powder during measurement. When a thin film for analysis surface (hereafter “sample film”) or a binder is used, it is recommended to select the proper sample preparation method to minimize analysis errors of target elements. This note describes key points and considerations for sample preparation by pressed and loose powder methods. In addition, sample preparation technique for the analysis of small quantities of powder sample is introduced.

Highlights

Pressed powder preparation generally provides better reproducibility than loose powder preparation, but pellet density, pressure, binder selection, and contamination control can significantly affect XRF results.
Loose powder methods allow difficult-to-pellet samples to be measured and recovered, but sample films reduce X-ray intensity and can make light-element analysis impractical.
Specialized preparation methods extend XRF to very small powder quantities, including double-pellet, embedded-pellet, and dedicated sample-cell approaches for samples down to about 100 mg or less.

Summary

Powder samples for X-ray fluorescence analysis can be prepared using pressed or loose powder methods without chemical treatment. The appropriate method depends on sample characteristics, available quantity, target elements, required precision and accuracy, and whether the sample must be recovered after measurement.

For pressed powder preparation, the powder is loaded into a ring or cup and compressed using flat or cylinder-type dies. Sufficient pulverization improves pellet formation, while binders can be added when powders are difficult to pelletize. Binder purity and mixing are important because contamination or variations in the sample-to-binder ratio can introduce analytical errors. Typical sample-to-binder ratios are 10:1 or 10:2.

Pelletization pressure also affects XRF intensity because increasing pressure increases sample density. X-ray intensity eventually reaches a saturation region, so maintaining constant sample quantity and pressure—and preferably operating where intensity has saturated—improves preparation reproducibility. Ring material should also be matched to the expansion behavior of the sample after pressure release. Gradual pressurization with intermediate pressure releases can help remove trapped air and prevent cracking or flaking.

Contamination can arise from material remaining on die surfaces or from rings and cups during repelletization. Cleaning dies between samples, preparing lower-concentration samples first, and using polypropylene or polyester films between the powder and die can reduce these problems.

Loose powder preparation is useful for materials such as graphite and mica that are difficult to pelletize or when the sample must be recovered for subsequent analysis. Its preparation reproducibility is generally poorer than that of pressed pellets. The film covering the powder also attenuates X-rays, with particularly severe effects for light elements from boron through fluorine. Film integrity must be considered during vacuum measurements because breakage can contaminate the instrument.

Small quantities can be handled through specialized techniques. Double and embedded pellets use cellulose or boric acid support pellets, while dedicated cells permit vacuum analysis of quantities around 100 mg without pelletization.

Frequently asked questions

: The powder is placed in a ring or cup and compressed with a press using flat or cylinder-type dies. Fine pulverization generally improves pellet formation. If the powder does not form a mechanically stable pellet, a binder can be mixed with it before pressing. The die, ring or cup, sample quantity, and pressing conditions should be kept consistent to obtain reproducible XRF results.
: A binder is useful when a powder does not readily form a stable pellet or when particles may detach from the pellet and contaminate the XRF sample chamber. Materials with spherical particles, such as SiO₂ and burned ash, can be particularly difficult to pelletize. Typical sample-to-binder ratios are 10:1 or 10:2. The binder must be sufficiently pure and free of elements that could interfere with the analysis, and accurate weighing and thorough mixing are required.
: Increasing pelletization pressure increases sample density and therefore can increase measured X-ray intensity. Above a certain pressure, the intensity reaches a saturation region. Preparation reproducibility can be improved by using the same sample quantity and pressure for every pellet and, when practical, selecting a pressure high enough to reach the intensity saturation region.
: The behavior of both the powder and its supporting ring after pressure release should be considered. PVC rings tend to expand after pressure release and are better suited to samples that also expand, whereas aluminum rings are appropriate for samples that do not expand. Pressurizing too quickly can also trap air within the sample. Releasing pressure several times before reaching the final pressure allows trapped air to escape and reduces cracking or flaking.
: Die surfaces should be cleaned before each pellet is prepared because residue from previously pressed samples can contaminate subsequent samples. Preparing samples in order from lower to higher elemental concentrations can further reduce carryover problems. A polypropylene or polyester film can also be placed between the sample and die to prevent powder from sticking to the die surface. Repelletizing broken samples can introduce additional contamination from ring or cup materials.
: Loose powder preparation is useful when the material is difficult to pelletize even with a binder, as can occur with graphite and mica, or when the sample needs to be recovered after XRF measurement for another analytical technique. The tradeoff is poorer preparation reproducibility compared with pressed pellets, so reproducibility should be evaluated before routine use.
: The film required to contain a loose powder absorbs some of the emitted X-rays and therefore decreases analytical sensitivity. The effect is especially severe for light elements from boron through fluorine, which may not be measurable through the film. Film impurities can also contribute to the measured spectrum, so blank measurements of the film are recommended. Film strength is important because rupture under vacuum can release powder into the sample chamber.
: Double-pellet and embedded-pellet techniques can be used when there is insufficient material to produce a conventional pellet. A small quantity of sample is pressed onto or into a previously prepared cellulose or boric acid pellet. Dedicated sample cells can also accommodate approximately 100 mg of powder for measurement under vacuum without pelletization. Sample quantity should be carefully controlled when calibration accuracy depends on maintaining consistent sample thickness.

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