How can fusion bead preparation be used for XRF analysis of ferroalloys?

Ferroalloys must be oxidized before fusion because an insufficiently oxidized metallic sample can alloy with the platinum crucible and cause irreversible damage. For ferrosilicon, lithium tetraborate flux is first spread over the bottom of the crucible to prevent direct contact between the sample and platinum. The sample is mixed with lithium carbonate, sodium carbonate, and potassium nitrate as oxidizing agents and progressively heated from 500 to 600°C before fusion at 1200°C. The same general procedure can be applied to ferromanganese, silicomanganese, and magnesium ferrosilicon.

Why must ferroalloys be oxidized before preparing XRF fusion beads?

Ferroalloys contain metallic components that can react with platinum at fusion temperatures. If the material is insufficiently oxidized and comes into direct contact with the platinum crucible, an alloy can form and permanently damage the crucible. Proper oxidation converts the material into forms that can be safely dissolved in the flux. Fine sample particle size, complete oxidation, and a protective layer of flux between the sample mixture and crucible are therefore critical.

How can sulfide ores such as copper ore be prepared as fusion beads without strong-acid pretreatment?

Sulfide ores can be oxidized during a preheating step instead of first being treated with strong acids such as nitric acid. For copper ore, the dried sample is mixed with lithium tetraborate/lithium metaborate flux and immersed in lithium nitrate solution and distilled water. Heating at 800°C for 10 minutes oxidizes sulfur in the sample. After cooling, lithium bromide is added as a releasing agent, and the material is fused at 1050°C. This approach avoids the corrosive fumes and equipment degradation associated with heated strong acids.

Why is silicon carbide difficult to prepare for XRF analysis by conventional methods?

Silicon carbide combines extreme hardness, wear resistance, chemical stability, heat resistance, and oxidation resistance. Its hardness makes fine pulverization difficult and can increase contamination from grinding vessels. Coarse particles can also increase errors from particle-size and mineralogical effects. During heating, silicon carbide forms a protective silicon dioxide layer that inhibits further oxidation, making conventional fusion preparation unusually slow.

How can the preparation time for silicon carbide fusion beads be reduced?

A procedure without added oxidizing agents requires oxidation at approximately 840°C for 30 hours before final heating and fusion. A faster procedure uses a mixture of lithium carbonate, vanadium oxide, and strontium nitrate as oxidizing agents. After oxidation at 800°C for 20 minutes, the sample can be fused with lithium tetraborate at 1200°C. Including weighing and mixing, a fusion bead can be prepared in approximately 40 minutes rather than more than 30 hours.

How does the flux-to-sample dilution ratio affect XRF analysis of fusion beads?

Increasing the dilution ratio lowers analyte concentration in the bead and therefore reduces X-ray intensity. The effect becomes particularly significant for longer-wavelength analytical lines because absorption by the flux is greater. When a standard method specifies a dilution ratio, that ratio should be followed. Otherwise, the ratio can be selected according to sample solubility in the flux. Using a lower dilution ratio, when practical, increases the sample fraction and can improve analytical precision.

Why does fusion bead thickness affect XRF intensity and analytical-line selection?

Some analytical lines can penetrate deeply into fusion beads because the beads consist largely of light elements such as lithium, boron, and oxygen. If a bead is thinner than the saturation thickness for the selected analytical line, changes in bead thickness can produce changes in measured intensity. For example, saturation thickness is approximately 0.65 mm for Fe-Kα, 1.15 mm for Zn-Kα, and 2.8 mm for Sr-Kα. Consistent bead thickness is therefore especially important for shorter-wavelength lines that require greater thickness to reach saturation.

How should an analytical line be selected when fusion bead thickness cannot be kept constant?

A longer-wavelength analytical line may be preferable when bead thickness is below the saturation thickness of a shorter-wavelength line and consistent thickness cannot be achieved. For zirconium, for example, Zr-Kα requires a bead thickness of about 3.8 mm to reach saturation, whereas Zr-Lα saturates at only about 30 μm. Using the L line can therefore reduce sensitivity to bead-thickness variation. The tradeoff is lower X-ray intensity, potentially requiring longer measurement times, along with greater sensitivity to surface contamination and deliquescence.

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Sample Preparation for X-ray Fluorescence Analysis V.

Fusion bead method—part 2: practical applications

Mitsuru Watanabe

Winter 2016 Vol.32 No.1 , 17-21

The general preparation method of fusion bead, equipment, reagents and other important considerations were described in the previous article “Sample preparation for X-ray fluorescence analysis IV Fusion bead method—part 1 basic principles.” In this article, the preparation methods of various applications such as ferroalloy, sulfide and carbide are described.

Conventionally, these samples had been prepared as fusion beads after they were oxidized completely by mixing with a strong acid followed by drying. However, raising the temperature of the strong acids such as nitric acid degrades the working environment and the surrounding equipment corrodes due to its oxidizing power. Therefore, it has been necessary to work in a well-ventilated environment.

In the preparation method described here, oxidation reaction progresses slowly in the platinum crucible by oxidizing agent or oxidation catalyst without the use of strong acids. Thus it is possible to prepare fusion beads in a short time in a conventional environment. It should be noted that the described procedure assumes that grain size of the powder samples and the drying temperature of the samples and reagents are those for common fusion bead preparation methods.

If the grain size of the sample is coarse and the fusion bead is prepared without spreading the flux on the bottom of the platinum crucible, the sample will be oxidized insufficiently and form an alloy with the platinum in the crucible and cause irreversible damage. In this case, recasting of crucible may be required. For this reason, it is necessary to perform sample preparation carefully. Other additional analytical considerations regarding measurement of fusion bead are described at the end.

Highlights

Fusion bead preparation can be extended to difficult non-oxide samples—including ferroalloys, sulfides, and silicon carbide—by using controlled oxidation before fusion.
Oxidizing agents or catalysts can replace conventional strong-acid pretreatment, simplifying preparation, shortening processing time, and reducing problems associated with corrosive acid fumes.
Careful control of oxidation, dilution ratio, bead thickness, and analytical-line selection is essential for protecting platinum crucibles and obtaining accurate XRF results.

Summary

Fusion bead preparation is widely used for XRF analysis because dilution reduces matrix effects while fusion eliminates errors associated with particle size, mineralogical effects, and sample inhomogeneity. Although traditionally applied primarily to oxide materials such as refractories and cement raw materials, appropriately designed oxidation procedures make the method suitable for non-oxide materials including ferroalloys, sulfides, and carbides.

These materials present special preparation challenges because they must be sufficiently oxidized before fusion. Metallic samples such as ferrosilicon can alloy with a platinum crucible and permanently damage it if they contact the crucible before complete oxidation. Sulfide ores require oxidation of sulfur, while silicon carbide is particularly resistant to oxidation because of its chemical stability and formation of a protective silicon dioxide layer. Oxidizing agents and controlled heating can overcome these problems without the strong-acid pretreatment traditionally used for such materials.

Ferrosilicon can be oxidized gradually using a mixture of lithium carbonate, sodium carbonate, and potassium nitrate before fusion. Copper sulfide ores can be oxidized during preheating with lithium nitrate. For silicon carbide, adding oxidizing agents reduces preparation time dramatically: a procedure requiring more than 30 hours without an oxidizing agent can be reduced to approximately 40 minutes.

Analytical conditions must also be considered when designing a fusion bead method. Increasing the dilution ratio reduces X-ray intensity, particularly for longer-wavelength analytical lines, so the lowest practical dilution ratio can improve analytical precision. Bead thickness can affect shorter-wavelength analytical lines because X-rays may penetrate the entire bead before reaching saturation intensity. When consistent bead thickness cannot be maintained, selecting a longer-wavelength analytical line can reduce thickness-related variation, although lower intensity and greater sensitivity to surface condition may require longer measurement times and careful bead handling.

Frequently asked questions

: Ferroalloys must be oxidized before fusion because an insufficiently oxidized metallic sample can alloy with the platinum crucible and cause irreversible damage. For ferrosilicon, lithium tetraborate flux is first spread over the bottom of the crucible to prevent direct contact between the sample and platinum. The sample is mixed with lithium carbonate, sodium carbonate, and potassium nitrate as oxidizing agents and progressively heated from 500 to 600°C before fusion at 1200°C. The same general procedure can be applied to ferromanganese, silicomanganese, and magnesium ferrosilicon.
: Ferroalloys contain metallic components that can react with platinum at fusion temperatures. If the material is insufficiently oxidized and comes into direct contact with the platinum crucible, an alloy can form and permanently damage the crucible. Proper oxidation converts the material into forms that can be safely dissolved in the flux. Fine sample particle size, complete oxidation, and a protective layer of flux between the sample mixture and crucible are therefore critical.
: Sulfide ores can be oxidized during a preheating step instead of first being treated with strong acids such as nitric acid. For copper ore, the dried sample is mixed with lithium tetraborate/lithium metaborate flux and immersed in lithium nitrate solution and distilled water. Heating at 800°C for 10 minutes oxidizes sulfur in the sample. After cooling, lithium bromide is added as a releasing agent, and the material is fused at 1050°C. This approach avoids the corrosive fumes and equipment degradation associated with heated strong acids.
: Silicon carbide combines extreme hardness, wear resistance, chemical stability, heat resistance, and oxidation resistance. Its hardness makes fine pulverization difficult and can increase contamination from grinding vessels. Coarse particles can also increase errors from particle-size and mineralogical effects. During heating, silicon carbide forms a protective silicon dioxide layer that inhibits further oxidation, making conventional fusion preparation unusually slow.
: A procedure without added oxidizing agents requires oxidation at approximately 840°C for 30 hours before final heating and fusion. A faster procedure uses a mixture of lithium carbonate, vanadium oxide, and strontium nitrate as oxidizing agents. After oxidation at 800°C for 20 minutes, the sample can be fused with lithium tetraborate at 1200°C. Including weighing and mixing, a fusion bead can be prepared in approximately 40 minutes rather than more than 30 hours.
: Increasing the dilution ratio lowers analyte concentration in the bead and therefore reduces X-ray intensity. The effect becomes particularly significant for longer-wavelength analytical lines because absorption by the flux is greater. When a standard method specifies a dilution ratio, that ratio should be followed. Otherwise, the ratio can be selected according to sample solubility in the flux. Using a lower dilution ratio, when practical, increases the sample fraction and can improve analytical precision.
: Some analytical lines can penetrate deeply into fusion beads because the beads consist largely of light elements such as lithium, boron, and oxygen. If a bead is thinner than the saturation thickness for the selected analytical line, changes in bead thickness can produce changes in measured intensity. For example, saturation thickness is approximately 0.65 mm for Fe-Kα, 1.15 mm for Zn-Kα, and 2.8 mm for Sr-Kα. Consistent bead thickness is therefore especially important for shorter-wavelength lines that require greater thickness to reach saturation.
: A longer-wavelength analytical line may be preferable when bead thickness is below the saturation thickness of a shorter-wavelength line and consistent thickness cannot be achieved. For zirconium, for example, Zr-Kα requires a bead thickness of about 3.8 mm to reach saturation, whereas Zr-Lα saturates at only about 30 μm. Using the L line can therefore reduce sensitivity to bead-thickness variation. The tradeoff is lower X-ray intensity, potentially requiring longer measurement times, along with greater sensitivity to surface contamination and deliquescence.

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