Sample preparation for X-ray fluorescence analysis IV. Fusion bead

Rigaku Journal, 31(2), 2015 12

Sample preparation for X-ray ﬂuorescence analysis

IV.Fusion bead methodpart 1 basic principals

Mitsuru Watanabe

1.Introduction

For the analysis of powders by XRF, sample

inhomogeneity due to segregation, grain size and

mineralogical effects inﬂuence X-ray intensity and can

cause analysis errors. It is therefore recommended

to analyze powder samples after ﬁne pulverizing

as described in “Sample Preparation for X-ray

Fluorescence Analysis II. Pulverizing methods of

powder samples

(1)

.” However, when inhomogeneity

can not be sufﬁciently removed by pulverization and

more accurate analysis is required, fusion bead (Fig. 1)

method is advisable.

The fusion bead method ﬁrst established in the

1950s

(2)

, has since progressed such that it is not only

applicable to powders as oxides but also to non-oxides

such as metals, carbides, sulﬁdes which had previously

been considered to be difﬁcult

(3),(4)

. Characteristics of

the fusion bead method are that (a) analysis error

due to grain size and mineralogical effects can be

removed, (b) matrix effect is reduced due to dilution, (c)

standard samples can be prepared by mixing of reagents.

Characteristics of fusion bead and pressed powder

methods are compared in Table 1.

In this article, general preparation methods, equipment,

reagents and other important considerations for powders

with typical grain size and drying conditions are

described. If analysis must be performed in accordance

to a speciﬁc standard test method, adhere to its

prescribed conditions. In a following issue on fusion

beads, various applications such as for ferroalloy, silicon

carbide and copper concentrate samples will be reported.

2.Preparation method

Figure 2 shows the general procedure for preparing a

fusion bead.

2.1.Weighing

Dry powder sample, ﬂux, oxidizing agent are weighed

to 0.1 mg precision.

2.2.Mixing

Transfer weighed sample and ﬂux into a crucible.

Be sure that the specimen is thoroughly mixed prior to

fusion and add releasing agent if fusion bead is difﬁcult

to remove from the mold. Add oxidizing agent if

oxidation of sample is required.

2.3.Oxidization/Calcination

Samples containing metals, carbides, sulﬁdes are

mixed with oxidizing agent and oxidized at around

600

–

800°C. In cement raw meal and limestone, CaO

in sample can actually be present as CaCO

. For such

samples, sudden increase in temperature can cause

signiﬁcant foaming due to release of CO

caused by

the chemical reaction CaCO

→CaO+CO

. Calcination

at around 800°C before fusion reduces risk of sample

overﬂow and air being trapped in the ﬁnished fusion

bead.

Fig. 1. Fusion beads.

SBU WDX, X-ray Instrument Division, Rigaku Corporation.

Technical articles

Rigaku Journal, 31(2), 2015 13

Sample preparation for X-ray uorescence analysis IV. Fusion bead method―part 1 basic principals

2.4.Fusion

Samples are fused at 1000

–

1200°C for a certain

period of time. Keep the crucible stationary until the

sample and the ﬂux melt, and then begin to swing to

homogenize and remove air bubbles. The higher the

temperature applied, the higher the ﬂuidity during

fusion, but can increase volatilization of ﬂux and

analytes, and decrease crucible lifetime. By keeping

fusing time ﬁxed, ﬂux volatilization is consistent

and therefore its effect can be ignored. However, to

minimize variation in sample volatilization due to

differences in sample composition, it is advisable to fuse

at lower temperatures.

When the fusion is completed, stop swinging and

keep the crucible horizontal for fusion/casting combined

vessels, or pour the fused sample into a mold.

2.5.Cooling

It is important to cool down the fused sample while

maintaining its property as a glass. Rapid cooling

can cause the sample to crack, while cooling too

slowly can cause crystallization. Because of this, initial

passive slow cooling followed by active cooling is

recommended. Cooling time will vary according to

sample, ﬂux type, dilution ratio. Figure 3 illustrates

sample cool down timeline.

3.Instruments

3.1.Fusion types

There are three types of fusion machines for preparing

fusion beads, namely high frequency (HF) induction,

Table 1. Comparison between fusion bead method and pressed powder method.

Fusion bead method Pressed powder method

Grain size effect Not affected Affected

Mineralogical effect Not affected Affected

Matrix effect Reduced due to dilution Affected by elemental composition

Standard sample Possible to prepare by mixing reagents Need to be similar to test sample

Grain size of sample 106 μm (140 mesh) or less 46 μm (330 mesh) or less

X-ray intensity Reduced due to dilution No change

Storage In desiccator to avoid deliquescence In desiccator

Handling Easy since in glass form Care required not to break

Preparation time 15

–

30 min. including weighing Several minutes

Fig. 2. General fusion bead preparation method.

Fig. 3. Sample cool down timeline.

Rigaku Journal, 31(2), 2015 14

Sample preparation for X-ray uorescence analysis IV. Fusion bead method―part 1 basic principals

electric furnace and a gas burner types. Fusion

temperature range slightly varies depending on type

but is typically around 1000

–

1250°C and have a swing/

cooling mechanism. Room ventilation is recommended

especially when many fusion bead samples are prepared

with oxidizers or releasing agent.

(a) HF induction type

Due to direct heating by high frequency coil, heating

is efﬁcient and rapid

(5)

(b) Electric furnace type

There are mufﬂe and non-mufﬂe type electrical

furnaces. It is necessary to turn power on before use and

time is required to reach high enough temperatures.

Temperature is controlled by adjusting gas ﬂow

rate. Since combustible gases such as propane is used,

environment needs to be carefully monitored.

3.2.Vessels (Crucible, Mold)

Regarding use of vessels, there are two different

approaches for forming the fusion bead namely crucible/

mold combined and separate types. The fusion instrument

mentioned above uses combined type vessels (Fig. 4).

(a) Crucible/mold combined

Sample is fused and cooled down in the crucible,

where the crucible is also used as a mold.

(b) Crucible/mold separate type

Sample is fused in the crucible and then transferred

into a mold to form the fusion bead. Preheating the mold

to around 800°C before pouring into the mold prevents

cracking of the fusion bead.

Most vessels are made of platinum and gold (95 and

5 mass%) to assure release of the formed fusion bead. To

improve hardness and prevent deformation, some have

added rhodium and consist of reinforced platinum or

platinum alloy. Whatever the material, the ﬂatness of the

mold’s surface in contact with the sample is critical for

sample detachment and should therefore also be mirror

polished.

4.Analysis sample and reagents

4.1.Analysis sample

Analysis sample is pulverized to smaller than 106 μm

particle size (140 mesh), dried for over two hours in

1105° C air atmosphere, then cooled down and stored

in a desiccator.

4.2.Flux

Flux is available as ﬁne powders or in granular and

globular forms, and some already contain releasing

agents. In any case, it should not contain any water

and depending on analysis requirement, the appropriate

purity grade should be chosen.

Impurity levels can vary from lot to lot, so it is

desirable to have a substantial amount of the same lot to

minimize analysis error. Flux is a hygroscopic substance

and therefore it should be dried before use. Place the

ﬂux in a platinum tray and heat in an electric furnace

at 200

–

250°C below its melting point for 4 hours, and

then cool down and store in a desiccator (for example,

lithium tetraborate is dried at 650

–

700°C for 4 hours).

Fusing temperature is typically 200

–

250°C higher than

its melting point to improve ﬂuidity. Table 2 shows the

physical properties of various types of ﬂux

(6),(7)

(a) Lithium tetraborate Li

=Li

O2B

Lithium tetraborate is the most commonly used ﬂux

Fig. 4. Crucible/mold in one type.

Table 2. Physical properties of various ﬂuxes.

Lithium tetraborate Lithium metaborate Mixed ﬂux Sodium tetraborate

Formula Li

LiBO

, LiBO

Melting

point

930°C 845°C 875°C (66 : 34)

870°C (50 : 50)

825°C (12 : 22)

840°C (20 : 80)

741°C

Acid/Base Acidic Basic Weak acid

–

Weak base Acidic

Suitable

sample

Lime stone

Cement

Silicate, Rock

Refractory

Silicate, Rock

Refractory

Metal, Mineral ore

Remarks High melting point

High solubility for

basic oxides

Crystalizes when cooled Low melting point

High solubility for

acidic oxides

Lowest melting point

High deliquescence

Na analysis not possible

Rigaku Journal, 31(2), 2015 15

Sample preparation for X-ray uorescence analysis IV. Fusion bead method―part 1 basic principals

in XRF analysis. Since it is a relatively more acidic

compound compared to lithium metaborate (LiBO

it is suitable for fusing samples such as lime stone and

cement which contain basic oxides (CaO, MgO, Na

O, etc.). However, its 930°C melting point is the

highest among the various ﬂux types and consequently

fusion temperature must be relatively high. Therefore,

this ﬂux requires special attention to sample and ﬂux

volatilization as well as damage to the vessel.

(b) Lithium metaborate LiBO

=Li

Lithium metaborate is a relatively more basic

compound compared with lithium tetraborate, and is

therefore suitable for fusing samples such as silicates,

rocks and refractories which contain acidic oxides (SiO

ZrO

, TiO

, etc.). It has a low melting point of 845°C

and tends to cause crystallization instead of vitriﬁcation

when cooled. It is therefore not used by itself but rather

in combination with lithium tetraborate as a mixed ﬂux.

, Lithium

metaborate LiBO

)

As this is a mixture of lithium tetraborate and lithium

metaborate, its acidity/basicity can be adjusted by the

mixing ratio. It also has the advantage of lowering the

melting point for easier fusion compared to pure lithium

tetraborate.

Lithium tetraborate to metaborate ratios are typically

66% to 34%, 50% to 50%, 35.3% to 64.7% (12 : 22

ﬂux), 20% to 80%, respectively. The melting point of

12 : 22 ﬂux is 825°C which is the lowest among the

mixed types.

(d) Sodium tetraborate Na

This is an acid compound and has the lowest melting

point of 741°C and samples can therefore easy be fused.

However, its high deliquescence makes it unsuitable for

long term use, and the Na in the ﬂux restricts analysis of

Na in sample.

4.3.Releasing agent

Halides such as iodides or bromides are used as

releasing agents to facilitate removal of the cooled

fusion bead from the mold. Addition of releasing agent

increases surface tension of sample during fusion since

halogen remains on the surface and therefore surface

area of sample in contact with the vessel is reduced. As a

result, the cooled fusion bead becomes easier to remove

from the mold. In addition, it improves removal of air

bubbles from the sample due to lower viscosity. Typical

iodides are LiI, NaI, KI and NH

I, and bromides are

LiBr, NaBr and KBr.

Bromides compared to iodides act as stronger

releasing agents and therefore can be effective in

smaller quantities, but tend to remain in the fusion bead.

Halogens remaining in the sample can interference

analysis lines. Table 3 shows analysis and interference

lines due to the releasing agent.

Releasing agent can be added before heating or during

fusion. Consistent addition of releasing agent quantity

can be difﬁcult since typical amounts are less than 1 mg.

By preparing 5

–

50% (w/v) solutions in advance and

addition by a micropipette can improve consistency. The

iodide solution should be kept in a light-resistant bottle

as many iodide materials tend to liberate iodine due to

air oxidation or light. Moreover, as described below,

lithium ﬂuoride in addition to acting as a releasing agent

also lowers the viscosity during fusion.

When releasing agent quantity is insufﬁcient, the

sample’s surface area in contact with mold’s internal

wall is relatively large and become difﬁcult to remove.

On the other hand, when too much is added, surface

tension becomes too high such that sample does not

fully cover the bottom of the vessel resulting in a

crescent or ball shaped fusion bead. Effect of releasing

agent quantity on sample surface tension is shown in

Fig. 5 above. Adequate amount of the releasing agent

depends on type of sample, dilution ratio, and surface

condition of the mold.

Fig. 5. Effect of releasing agent quantity on sample surface

tension.

Table 3. Example of interference line from releasing agent.

Releasing agent Analysis line Interference line

Bromide (Br) Al-Kα (λ=0.8340 nm) Br-Lα (λ=0.8375 nm)

Iodide (I) Ti-Kα (λ=0.2750 nm) I-Lβ

(λ=0.2752 nm)

Table 4. Physical properties of various oxidizing agents.

Oxidizing agent

Lithium

nitrate

Sodium

nitrate

Potassium

nitrate

Strontium

nitrate

Ammonium

nitrate

Lithium

carbonate

Sodium

carbonate

Vanadium

oxide

Formula LiNO

NaNO

KNO

Sr(NO

)

Melting point 264°C 306.8°C 339°C 570°C 169.6°C 618°C 851°C 690°C

Decomposition temperature 600°C 380°C 400°C 570°C 210°C 618°C 851°C 1750°C

Deliquescence Yes Yes Yes Yes Yes No No No

Remarks

originated

, NO

originated

Oxidation

catalyst

Rigaku Journal, 31(2), 2015 16

Sample preparation for X-ray uorescence analysis IV. Fusion bead method―part 1 basic principals

4.4.Oxidizing agent

Samples which include metals, carbon and sulfur

are fused with oxidizing agent since they can react

and form an alloy with the platinum in the vessel and

cause irreversible damage. It is possible to oxidize the

sample using a strong oxidant such as nitric acid and

then dried prior to fusion. However, for some samples

it may be easier to perform oxidization during the

pre-heating stage prior to fusion. Typical oxidizing

agents are nitrates such as LiNO

, NaNO

, KNO

and

Sr(NO

)

which cause oxidation at high temperatures.

Since decomposition temperature varies depending on

oxidizer, often times a mixture of several different

nitrates are used.

Other than ammonium nitrate, elements in the

oxidizer remain in the fusion bead and therefore a

consistent amount should be added. Generally, since

nitrates easily dissolve in water, preparation of oxides as

water solution in advance and addition by a micropipette

improves consistent addition of a homogeneous mixture.

Carbonate is occasionally used as another type of

oxidizing agent. Since carbonate has higher melting

point and decomposition temperature compared with

nitride, it is mostly used for oxidization of ferroalloy or

metal sample. Vanadium oxide works as an oxidation

catalyst for the oxidization of sample by oxygen in the

air. Table 4 is a compilation of the physical properties of

various oxidizing agents

(8)

4.5.Other reagents

(a) Lithium ﬂuoride

Lithium ﬂuoride works not only as a releasing

agent but also acts to lower the viscosity and melting

temperature. For mixed ﬂux of lithium tetraborate 90%

and lithium ﬂuoride 10%, the melting point is as low as

780°C, and therefore volatilization of sample and ﬂux

can be reduced drastically. Adhesion to the vessel is low

as its ﬂuidity is very high.

(b) Oxide of heavy element

In case of wide elemental concentration range such

for geological samples, addition of heavy element based

oxides (La

, CeO

, etc.) further reduces absorption/

excitation effects due to matrix, and the linearity of the

calibration line is improved (Heavy element dilution

effect)

(9)

5.Other sample preparation considerations

5.1.Trapped air

As mentioned above, samples having high

concentration of Ca such as limestone, cement raw meal

etc. cause signiﬁcant foaming when fused, and they air

may be trapped in the fusion bead. Calcination at lower

temperature prior to fusion can reduce the air being

trapped in the fusion bead.

5.2.Residual sample unfused

Sample with high quartz concentration such as high

silica materials are difﬁcult to fuse completely, and

the unfused residual causes analysis error. Thoroughly

mixing sample and ﬂux before fusion is advisable in

this case. When unfused residue is observed, change

the ﬂux type considering the acid-base, or fuse it again.

Pulverization of samples can also reduce residuals since

coarse powders require longer fusion time.

6.Vessel maintenance

Fragments of fusion bead attached to the vessel

and difﬁcult to remove are soluble in 30% (w/v) citric

acid solutions. Heated citric acid solutions or diluted

hydrochloric acid can speed up removal.

Thermal stress from repeated use of the vessel can

cause mosaic-like patterns to appear on the surface due

to sample, oxidizer or releasing agent residue in the

micro-cracks. This not only can lead to the fusion bead

to crack during cooling and make releasing sample

difﬁcult, but also can be a cause of analysis error. In

such cases, polish the vessel surface with a cloth or

ﬁbrous buff with alumina paste or diamond paste with

particle size less than 1 μm.

Polishing allows repeat use of the vessel, but

eventually the bottom surface begins to deform making

preparation of ﬂat samples difﬁcult. When the damage to

the vessel surface increases, problems such as cracking,

air being trapped and releasing become more serious. In

such cases, recasting of vessel may be required.

7. Standard

Following standards regarding XRF analysis by

fusion bead method are published, and detailed sample

preparation methods are also described

(10)

JIS M 8205:2000 Iron ores

—

X-ray ﬂuorescence

spectrometric analysis

JIS R 2216:2005 Methods for X-ray ﬂuorescence

spectrometric analysis of refractory products

JIS R 5204:2002 Chemical analysis method of cement

by x-ray ﬂuorescence

ISO 4503:1978 Hardmetals

—

Determination of

contents of metallic elements by X-ray ﬂuorescence

—

Fusion method

ISO 9516-1:2003 Iron ores

—

Determination of various

elements by X-ray ﬂuorescence spectrometry

—

Part

1: Comprehensive procedure

ISO 12677:2011 Chemical analysis of refractory

products by X-ray ﬂuorescence (XRF)

—

Fused cast-

bead method

ISO 29581-2:2010 Cement

—

Test methods

—

Part 2:

Chemical analysis by X-ray ﬂuorescence

8.Summary

This article describes general principles of fusion

bead preparation regarding operation, instruments,

reagents and precaution. In a following issue regarding

fusion beads, various fusion bead preparation methods

with practical application examples will be discussed.

References

( 1 ) A. Morikawa: Rigaku Journal (English version), 30 (2014), No.

Rigaku Journal, 31(2), 2015 17

Sample preparation for X-ray uorescence analysis IV. Fusion bead method―part 1 basic principals

2, 23

–

27.

( 2 ) F. Claisse: Norelco Reporter January

–

February, 3 (1957), No.

1, 3

–

( 3 ) M. Watanabe, H. Inoue and Y. Yamada, M. Feeney, L Oelofse

and Y. Kataoka: Adv. in X-ray Anal., 56 (2013), 177

–

184.

( 4 ) M. Watanabe, Y. Yamada, H. Inoue and Y. Kataoka: Adv. in

X-ray Chem. Anal. Japan, 44 (2013), 253

–

259.

( 5 ) Rigaku Corporation: Guide for X-ray ﬂuorescence spectroscopy,

(2008), 101

–

103 (in Japanese).

( 6 ) H. Bennett and G. Oliver: XRF Analysis of Ceramics, Minerals

and Allied Materials, John Wiley and Sons Ltd., (1992).

( 7 ) F. Claisse and J. S. Blanchette: Physics and Chemistry of Borate

Fusion

—

For X-ray Fluorescence Spectroscopists

—

, Fernand

Claisse Inc., (2004).

(8) Kagaku binran kisohen I, Ed. The Chemical Society of Japan,

Maruzen, Tokyo, (2004), 113

–

363 (in Japanese).

( 9 ) K. Norrish and J. T. Hutton: Geochim. Cosmochim. Acta, 33

(1969), 431

–

453.

(10) H. Homma: Keiko Xsen bunseki no jissai, Ed. I. Nakai, Asakura

Shoten, Tokyo, (2005), 70

–

71 (in Japanese).