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As a relatively mature component analysis method, X-ray fluorescence spectrometer analysis is widely used in metallurgy, geology, environment, chemical industry, materials and other fields. The objects analyzed by X-ray fluorescence spectrometer are mainly three kinds of solids, powders and liquids, of which solid powder is the most analyzed one. Because many samples such as cement, coal, dust, etc. are powders themselves, for irregular shaped block solids, such as various ores, they are often pulverized into powders due to the fact that direct analytical techniques are still immature. The liquid sample can be placed in the liquid sample cup for analysis, but sometimes the liquid is converted into a solid due to the inability to evacuate, etc., and some pre-separation and enrichment results are often powders. Therefore, the sample preparation technique of the powder sample is An important part of the analysis of X-ray fluorescence spectrometers.
There are two main methods for analyzing powder samples by X-ray fluorescence spectrometer:
1. Melting method. The melting method is a kind of sample preparation method which is applied more, which better eliminates the effects of the particle effect and the mineral effect. However, the melting method also has disadvantages: the sample is diluted by the flux and absorbed, so that the measurement intensity of the light element is reduced; the sample preparation is complicated, and it takes a lot of time; the cost is also high.
2, powder tableting method
The advantages of the powder tableting method are simple, fast, and economical. It is widely used when the analysis workload is large and the analysis accuracy is not too high, and is also commonly used for the analysis of trace elements. In practical applications such as cement, rock, and geochemical samples, powder tableting is a widely used X-ray fluorescence spectrometer.
1, sample preparation
The powder tableting method is mainly divided into three steps: dry roasting, mixed grinding, and tableting. There are methods such as direct compression of the powder, powder dilution tableting, use of a binder substrate, and edging.
1 Additives, grinding aids and other additives
When the adhesion of the sample itself is small, it is important to choose a suitable binder. The binder has two kinds of solid and liquid. The commonly used solid binders are boric acid, methyl cellulose, polyethylene, paraffin, starch, filter paper or chromatographic pulp, lithium carbonate and the like. A mixture of paraffin and styrene is used as a binder. The binder is added in an amount of 10% to 50% of the sample, and too much influences the detection limit of the light element. The addition of the binder causes the strength of the analytical line to decrease, and if the particle size of the binder is large, a graininess effect is also introduced. Comparing several commonly used binders in terms of water absorption, sample robustness, vacuuming time, instrument contamination, sample preparation success rate, cost, etc., it is concluded that low pressure polyethylene is an ideal bond. Agent.
The liquid binder is an organic solvent such as ethanol or polyvinyl alcohol (PVA). The use of a liquid binder facilitates the formation of a uniform, reproducible tablet, and the resulting sample is more durable.
In the process of preparing samples and standards, in addition to the binder, grinding aids, internal standard elements, thinners, etc. may be added. The biggest advantage of liquid binders or grinding aids is that they are not weighed, but are compressed. After drying, the amount of addition should not be too much. Generally, a few milliliters to a dozen milliliters are added to the 100 g sample. The solid binder, the grinding aid, and the like need to be accurately weighed, and are uniformly mixed. Therefore, the preparation of the sample is troublesome, and if the time for cleaning the pulverization container is added, it is sometimes even longer than the fusion method. In high-volume analysis, direct compression or substrate compression is often used.
2 crushing technology
It can be manually ground by agate or tungsten carbide mortar. Now more mechanical vibration mill or ball mill is used, which is very efficient. Generally, the sample can be pulverized to below 74 μm (through a 200 mesh sieve), preferably up to about 20 μm. As the pulverization time is prolonged, the particle size is reduced to a certain extent and no longer becomes fine. If the pulverization continues, a "agglomeration" phenomenon will occur. To increase the pulverization efficiency, a solid or liquid grinding aid can be added. The longer the pulverization time, the more serious the contamination caused by the pulverizing container, so it is important to select a suitable pulverizing container. To compare this pollution, you can analyze the contamination of a very hard substance (such as quartz) after pulverization, or compare the analysis results of two different pulverization methods. This is necessary to improve the sensitivity and accuracy of the analysis when analyzing trace elements. There is also a kind of pollution, which is mutual pollution between different pulverized samples. The container should be cleaned after each pulverization. When the sample volume is large, a small amount of sample can be pre-cleaned twice before pulverization.
3 tablet
Manual or electro-hydraulic machines are commonly used in pressure equipment, and powder samples are placed in aluminum cups or aluminum rings (or plastic rings) and pressed into the corresponding molds. In a vacuum spectrometer, a powder tablet may contain air or other gas to cause sputtering, which both destroys the surface of the sample and contaminates the sample chamber. It can be pressed into a block in a vacuum or measured in a xenon light path. In order to reduce the amount of air that is pressed into the sheet, the sample can be tapped during the loading, and the pressure should be gradually increased while pressing, while keeping pressure for a certain period of time. X-ray fluorescence spectrometer analysis is a kind of surface analysis, especially for light elements. When the effective layer thickness is only a few to a few μm, the surface contamination is a fatal problem, and the surface is required to be smooth. Therefore, the surface of the mold should be washed after each tableting, and the surface of the plug (corresponding to the surface to be tested) should be properly polished at intervals. The sample should also be protected from surface contamination, surface damage, moisture absorption, oxidation, and air adsorption during storage. It is best to measure as soon as possible after tableting. For samples that need to be stored for a long time, such as standard samples and management samples, it is better to seal in a powder state and temporarily compress it when needed.
4 standard sample preparation
X-ray fluorescence spectrometer analysis is a relative analysis, and the preparation of standard samples directly affects the accuracy of the analysis. There are three main sources of standard methods for powder tableting: analysis of samples by other methods; addition of certain components to standards with known composition; and artificial synthesis.
2, method application
1 powder tableting method for the analysis of trace elements
The powder tableting method is mostly used to analyze trace elements and analyze the main elements by melting method.
2 powder compression method for analysis of primary, secondary and trace elements
The powder tableting method is also commonly used for the full analysis of samples such as geology, geochemical exploration, and metallurgy.
3 melt and then pulverize the tablet
Melting a uniform, smooth surface of the melt is a very skillful work, some samples are not easy to demould or easily crack, some have a corrosive effect on Pt-Auå©åŸš, and the method of crushing and compressing after melting (available graphiteå©åŸš instead of Pt-Auå©åŸš) can eliminate the effect of the graininess effect and solve the problem of not easy to form. After melting, the tablet is pulverized to determine the total sulfur in the geological sample, which can reduce the influence of the sulfur valence state on the analysis accuracy during the preservation of the powder sample.
In general, for the routine analysis of coal, cement, rock, soil and other samples, the powder compression method can achieve the analysis accuracy and accuracy of about 5%. The detection limit of most trace elements can reach about 100μg/g. Because of the good stability of X-ray fluorescence spectrometer, the detection limit can be further reduced by extending the counting time. Sample preparation by powder tableting method, combined with automatic sample introduction device and automatic analyzer, can accurately analyze 20-30 elements at a time, which can fully meet the analysis needs of geological, mineral, commodity inspection and other departments.
As mentioned above, the powder tablet is tested according to a certain analysis object to select the sample preparation conditions, including the use of various additives, the pulverization time, the pressure, and the selection of the standard sample. The result of this is that the method proposed for one type of analysis object cannot be applied to other samples. The key is that a practical and effective pulverization technique has not been found, and the powder sample can be crushed to 1-2 μm. This pulverization technique is simple and easy, otherwise the X-ray fluorescence spectrometer is lost and analyzed quickly and conveniently.
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