(1) The sample can be completely decomposed by the flux. After high temperature melting, the sample and flux can form a uniform single-phase glass bead.

(2) The melting temperature is suitable with little volatility.

(3) The solvent can not contain the component elements to be measured or interference elements, and it is also necessary to pay attention to the flux content.

(4) The surface of the glass bad must be smooth and flat.

(5) The glass bead must be easily demolded.

Commonly used fluxes are sodium tetraborate(Na₂B₄O₇), lithium tetraborate(Li₂B₄O₇), and lithium perborate(LiBO₂). Compared with sodium salt, lithium salt of boric acid has a lower mass absorption coefficient, which is conducive to the light elements analysis. The melt flow provided excellent performance and the melting temperature is high. Among them, Li₂B₄O₇ has the best solubility with poor mechanical performance of the bead, but the mechanical strength by LiBO₂ is better. Therefore, the two fluxes are often mixed in a certain proportion in melting process to give full play to the advantages of both.

The flux can be determined from the following aspects: flux properties and characteristics, sample composition and experimental equipment for sample preparation.

(1) Flux properties and characteristics

1) Lithium tetraborate (Li₂B₄O₇)

Lithium tetraborate with the melting point: 920℃ has the highest melting point of all lithium borate. Its crystal is slightly hygroscopic and it is currently the most commonly used flux at a low price. Li₂B₄O₇ is easily reacted with alkaline oxides such as CaO, MgO, K₂O, and Na₂O, so it is also called an "acidic" flux. It can be widely used in areas such as steel, ceramics and geology to produce excellent and stable glass beads.

2) Lithium borate (LiBO₂)

The melting point of lithium perborate is 850℃. In independent melting, it crystallizes when cooling down. And when melting the oxides, the bead often crystallizes except at low rate of dilution and with some acidic oxides. LiBO₂, known as "alkaline" flux, is prone to react with Al₂O₃, SiO₂, P₂O₅, sulfate, etc.. Because LiBO₂ has a lower melting point and is more hygroscopic than Li₂B₄O₇, it is not recommended to use LiBO₂ independently, but to mix with Li₂B₄O₇ for use. In this way, it can expand the range of fusible oxides and increase the melting degree. LiBO₂ is often used as a flux in preparing samples for AA and ICP analysis.

3) Sodium borate（Na₂B₄O₇）

Sodium borate has a melting point of 741℃. If it did not have high hygroscopicity, it would be an excellent flux for glass beads. The glass bead made with it is almost never crystallized or ruptured, but it must be kept in the dryer to avoid hydration layer on the surface. The mixture of water and sodium tetraborate(Na₂B₄O₇•10H₂O) is not used for melting samples, because it contains almost 50 % of water. When heated, it produces a large amount of gas. In some cases, part of the mixture will eject from the crucible. Na₂B₄O₇, rarely used, has weaker acidity than Li₂B₄O₇, because it contains alkaline elements such as Ca, Mg, K. However, when mixed with Li₂B₄O₇, it is an excellent flux for melting metal oxides.

4) The mixed flux of 66 % Li₂B₄O₇ and 34 % LiBO₂

The uniform mixture flux of 66 % Li₂B₄O₇ and 34 % LiBO₂ is a universal flux with a melting point of 875℃, obtaining advantages in their independent uses. This mixed solvent bring about excellent performance on ceramics, geological samples and rocks containing bauxite.

5) The mixed solvent of 35.34 % Li₂B₄O₇ and 64.7 % LiBO₂

The mixed solvent of 35.34 % Li₂B₄O₇ and 64.7 % LiBO₂ has a melting point of 825℃. It is recommended for melting aluminum silicate, bauxite, aluminum earth salt, iron ore and so on.

(2) Selection of flux based on sample composition

Depending on the sample composition, the type of flux required can be qualitatively selected. Li₂B₄O₇ is a better choice for non-metallic oxides, but LiBO₂ is better if the sample contains bauxite or bauxite. The common problem with earthworms is that at a low melting temperature(1000-1100℃), it is difficult to melt in Li₂B₄O₇. The certain amount of LiBO₂ not only increases the alkalinity of the system, but also reduces the melting temperature, enabling  oxides to better evenly permeate into the melting system.

(3) Selection of flux based on experimental equipment for sample preparation

In order to obtain accurate results of XRF analysis, the sample must completely penetrate into the flux and be completely evenly distributed in the melting material. Besides, the proper melting temperature and the effective rotating and stirring during the melting process are also important factors to obtain good results. Generally the melting temperature should be set higher than the flux general melting point of 100-150℃.

(4) General suggestion

In some cases, in order to avoid the loss of volatile oxides such as Na₂O, K₂O, SO₃, etc., the temperature of the melting sample should not exceed 1100℃. While 1250℃ is the highest temperature which must be observed to avoid the loss of evaporation of the solvent. In conditions that the heating device is not hot enough or it is melting volatile substances(such as S), Li₂B₄O₇ can be mixed with LiBO₂ flux.

In order to determine whether the sample is completely melted, it is suggestive to adapt the method of observing glass beads. The melting difference often leads to crystallization, glass bead fragmentation, opacity, and other phenomena. In cases that some samples are difficult to dissolve, such as chromium iron ore, a mixture of Li2B4O7, LiBO2 and LiPO3 flux is a good choice.

(5) Sample and flux particle size range

In general, the sample particle size requirements between 80 μm-200 μm. The finer the grinding sample, the more the effective contact area with the flux, and the better the melting effect.