Why is sample preparation important in XRF analysis?

XRF measures X-rays emitted from the material presented to the instrument, so the physical condition and uniformity of that material directly influence the measurement. Surface roughness, particle size, mineral composition, segregation, contamination, and other forms of heterogeneity can change measured X-ray intensities even when the overall elemental concentrations are unchanged. Good preparation minimizes or controls these effects and improves accuracy and reproducibility.

How are metal samples prepared for XRF analysis?

Metal surfaces are typically ground or machined to remove surface contamination and near-surface elemental segregation while producing a flat analytical surface. Hard metals such as iron and steel can be prepared with a belt sander or grinder, while softer metals such as copper and aluminum alloys can be prepared with a lathe or milling machine. Surface roughness should be comparable between unknown samples and calibration standards because roughness affects X-ray intensity, particularly for lighter elements. For high-precision measurements of major elements, a very flat surface is desirable.

What are the main sources of error when analyzing powder samples by XRF?

Three major heterogeneity effects are mineralogical effects, segregation, and grain-size effects. Mineralogical effects occur because an element can produce different X-ray intensities depending on the minerals or compounds in which it occurs. Segregation results from nonuniform mixing or nonuniform elemental distributions within particles. Grain-size effects occur because particle dimensions influence the generation and detection of fluorescence X-rays. Pulverization can reduce these effects but does not necessarily eliminate them.

Why does particle size affect XRF measurements of powders?

Coarse or uneven particle sizes can cause differences in the portions of the sample that are irradiated and from which fluorescence X-rays reach the detector. The grain-size effect is especially important for low-energy X-rays from light elements because their effective analysis depth is shallow. Pulverizing a sample to finer, more consistent particle sizes reduces this effect. Unknowns and standards should also be prepared under comparable conditions so that any remaining heterogeneity affects them similarly. The diagrams on page 3 illustrate how particle geometry can create irradiated and shadowed regions.

What is the difference between a pressed powder pellet and a fusion bead for XRF analysis?

A pressed pellet is produced by pulverizing a powder and pressing it into a compact specimen. This is relatively straightforward, but mineralogical, segregation, and grain-size effects can remain. A fusion bead is produced by heating the powder with a flux, such as lithium tetraborate, at approximately 1000–1200°C to create a homogeneous glass disk. Fusion eliminates heterogeneity effects and also allows calibration standards to be prepared from pure chemical reagents. However, the typical sample-to-flux ratio of about 1:10 dilutes the sample and reduces sensitivity for trace components.

How are liquid samples prepared for XRF analysis?

Liquids can be poured directly into a sample cell fitted with a thin film. Polypropylene-type films can be used for acidic and alkaline solutions, while polyester-type films can be used for materials such as lubricating and fuel oils. The film itself should be checked for impurities because contaminants in the film can interfere with elements being measured. Other preparation approaches include depositing and drying a controlled quantity of liquid on a carrier or chemically concentrating trace metals before analysis.

How can light elements in liquids be measured with greater sensitivity by XRF?

A micro-droplet method can be used in which a fixed quantity of liquid is pipetted onto filter paper or another specialized carrier and dried. The resulting specimen can be measured under vacuum without a conventional liquid-sample film. Removing the film improves sensitivity for low-energy X-rays from light elements such as Mg, Na, and F. Specialized carriers can also accommodate larger liquid volumes to further improve sensitivity.

How can XRF be used to measure trace metals in aqueous solutions?

Trace metal ions can first be reacted with a chelating reagent to form a precipitate. The precipitate is collected by filtration, the filter is dried, and the concentrated material on the filter is measured by XRF. Concentrating the metals before measurement improves the ability to analyze trace-level components that would be difficult to measure directly in the original aqueous solution.

Read Article

Sample Preparation for X-ray Fluorescence Analysis I.

Outline of sample preparation

Yasujiro Yamada

Winter 2014 Volume 30, No. 1 , 26-29

XRF (X-ray fluorescence) analysis as a technique is widely used in academia, research and development and industry as an analysis tool for the determination of elemental composition of materials. Unlike wet chemical and other instrumental techniques which require the use of hazardous chemicals and difficult preparation methods to dissolve samples for analysis purposes, the quick, accurate, sensitive non-destructive analysis technique of XRF makes it attractive as an analytical technique, this coupled with the fact that it can be used by non-technical users and does not require expert knowledge and high skill levels to produce good, reliable, reproducible analytical data. XRF is considered to be less labor intensive and more environmentally friendly that the aforementioned methods.

XRF has many advantageous features as mentioned above but all of these features rely on a good sample to present to the XRF system.

It would not be an over statement to say that XRF analysis data quality is directly linked to the quality of the sample preparation technique.

A series of articles designed to cover all aspects of sample preparation for XRF will appear in the journal over the course of the next several months.

Highlights

XRF data quality depends strongly on sample preparation; even a capable instrument cannot compensate for a poorly prepared or nonrepresentative sample.
Metal samples generally require a flat, ground surface with surface roughness comparable to the calibration standards, especially when measuring light elements or performing high-precision analysis.
Powder samples are particularly susceptible to mineralogical, segregation, and grain-size effects. Pulverization reduces these sources of error, while fusion can eliminate heterogeneity by converting the powder into a uniform glass bead.
Liquid samples can be measured directly through a sample film or prepared using micro-droplet and concentration methods when greater sensitivity is required.

Summary

X-ray fluorescence (XRF) is a technique for determining the elemental composition of materials. It offers several practical advantages over many wet-chemical methods because analysis can be fast, sensitive, nondestructive, and relatively easy to perform without hazardous sample-dissolution procedures. However, obtaining reliable XRF results depends heavily on how the sample is prepared.

Sample preparation differs for metals, powders, and liquids. Metal samples are normally ground or machined to remove surface contamination and elemental segregation and to provide a flat surface. Hard metals such as iron and steel can be prepared using a belt sander or grinder, while softer materials such as copper and aluminum alloys can be machined with a lathe or milling machine. Surface roughness affects measured X-ray intensity, particularly for lighter elements, so samples and calibration standards should have comparable surface finishes.

Powders introduce additional complications because the material may not be uniform at the microscopic level. Three important sources of error are mineralogical effects, segregation, and grain-size effects. Pulverizing the material and pressing it into a pellet reduces some of these problems. When greater uniformity is required, the powder can instead be mixed with a flux and heated to approximately 1000–1200°C to form a homogeneous glass disk called a fusion bead. Fusion eliminates heterogeneity effects, although dilution by the flux reduces sensitivity for trace elements.

Liquids can be poured directly into a sample cell fitted with a thin film. Alternatively, a controlled amount can be deposited on a filter or specialized carrier and dried. This micro-droplet approach allows measurement without a sample film and can improve sensitivity for light elements such as Mg, Na, and F. Trace metals in aqueous solutions can also be chemically concentrated, collected on a filter, dried, and measured.

XRF sample preparation is not simply a preliminary handling step. The preparation method determines how representative and uniform the material presented to the X-ray beam will be and therefore has a direct effect on the accuracy and reproducibility of the measurement.

Frequently asked questions

: XRF measures X-rays emitted from the material presented to the instrument, so the physical condition and uniformity of that material directly influence the measurement. Surface roughness, particle size, mineral composition, segregation, contamination, and other forms of heterogeneity can change measured X-ray intensities even when the overall elemental concentrations are unchanged. Good preparation minimizes or controls these effects and improves accuracy and reproducibility.
: Metal surfaces are typically ground or machined to remove surface contamination and near-surface elemental segregation while producing a flat analytical surface. Hard metals such as iron and steel can be prepared with a belt sander or grinder, while softer metals such as copper and aluminum alloys can be prepared with a lathe or milling machine. Surface roughness should be comparable between unknown samples and calibration standards because roughness affects X-ray intensity, particularly for lighter elements. For high-precision measurements of major elements, a very flat surface is desirable.
: Three major heterogeneity effects are mineralogical effects, segregation, and grain-size effects. Mineralogical effects occur because an element can produce different X-ray intensities depending on the minerals or compounds in which it occurs. Segregation results from nonuniform mixing or nonuniform elemental distributions within particles. Grain-size effects occur because particle dimensions influence the generation and detection of fluorescence X-rays. Pulverization can reduce these effects but does not necessarily eliminate them.
: Coarse or uneven particle sizes can cause differences in the portions of the sample that are irradiated and from which fluorescence X-rays reach the detector. The grain-size effect is especially important for low-energy X-rays from light elements because their effective analysis depth is shallow. Pulverizing a sample to finer, more consistent particle sizes reduces this effect. Unknowns and standards should also be prepared under comparable conditions so that any remaining heterogeneity affects them similarly. The diagrams on page 3 illustrate how particle geometry can create irradiated and shadowed regions.
: A pressed pellet is produced by pulverizing a powder and pressing it into a compact specimen. This is relatively straightforward, but mineralogical, segregation, and grain-size effects can remain. A fusion bead is produced by heating the powder with a flux, such as lithium tetraborate, at approximately 1000–1200°C to create a homogeneous glass disk. Fusion eliminates heterogeneity effects and also allows calibration standards to be prepared from pure chemical reagents. However, the typical sample-to-flux ratio of about 1:10 dilutes the sample and reduces sensitivity for trace components.
: Liquids can be poured directly into a sample cell fitted with a thin film. Polypropylene-type films can be used for acidic and alkaline solutions, while polyester-type films can be used for materials such as lubricating and fuel oils. The film itself should be checked for impurities because contaminants in the film can interfere with elements being measured. Other preparation approaches include depositing and drying a controlled quantity of liquid on a carrier or chemically concentrating trace metals before analysis.
: A micro-droplet method can be used in which a fixed quantity of liquid is pipetted onto filter paper or another specialized carrier and dried. The resulting specimen can be measured under vacuum without a conventional liquid-sample film. Removing the film improves sensitivity for low-energy X-rays from light elements such as Mg, Na, and F. Specialized carriers can also accommodate larger liquid volumes to further improve sensitivity.
: Trace metal ions can first be reacted with a chelating reagent to form a precipitate. The precipitate is collected by filtration, the filter is dried, and the concentrated material on the filter is measured by XRF. Concentrating the metals before measurement improves the ability to analyze trace-level components that would be difficult to measure directly in the original aqueous solution.

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