1. The X-ray fluorescence spectrometer analysis method is a relative analysis method. Any sample preparation process and steps must have very good repetitive operation possibilities, so the standard sample and the analysis sample used to make the standard curve must undergo the same sample preparation process. X-ray fluorescence is actually a surface analysis method. Excitation only occurs on the shallow surface of the sample. It must be noted whether the analysis surface is representative of the entire sample. In addition, whether the average particle size and particle size distribution of the sample change, whether there is a non-uniform porous state in the sample, and the like. The sample preparation process must also prevent sample loss and contamination due to the multi-step operation.
1. Errors caused by sample preparation and sample itself:
(1) Uniformity of the sample.
(2) Surface effect of the sample.
(3) Particle size and treatment of powder samples.
(4) Spectral interference present in the sample.
(5) The coexistence element of the sample itself affects the matrix effect.
(6) The nature of the sample.
(7) Accuracy of chemical values ​​of standard samples.
2, the cause of the sample error:
(1) The physical state of the sample is different, the particle size, density and smoothness of the sample are different; the sample is contaminated, absorbed, and the liquid sample is heated and expanded, volatilized, foamed, crystallized and precipitated.
(2) The composition of the sample is unevenly distributed, the segregation of the sample components, the mineral effect, and the like.
(3) Inconsistent composition of the sample causes errors caused by differences in absorption and enhancement effects
(4) The chemical binding state of the measured element changes the oxidation of the sample, causing the change of the percent composition of the element; when the chemical valence state of the light element is different, the displacement of the peak occurs or the error caused by the change of the peak shape.
(5) The error caused by the weighing of the sample preparation operation during the sample preparation process, the dilution ratio is inconsistent, the sample melting is incomplete, the sample is pulverized and mixed unevenly, and the purity of the calibration or the reference reagent is insufficient.
3, sample type Sample state generally has solid block samples, powder samples and liquid samples.
(1) Solid block samples include ferrous metals, non-ferrous metals, plated plates, silicon wafers, plastic products, and rubber products, among which metal materials account for a large proportion.
(2) Powder samples include various mineral products, cement and its raw materials, raw materials and by-products of metal smelting such as iron ore, coal, slag, etc.;
(3) Liquid sample oil products, water quality samples, and solutions in which solids are converted by chemical methods.
4. Different samples have different sample preparation methods in the X-ray fluorescence spectrometer analysis method. Metal samples, if they are the right size or shape, or simply cut to meet the requirements of X-ray fluorescence spectrometer analysis, only surface polishing, liquid samples can be directly analyzed, atmospheric dust is usually collected on the filter directly for analysis. The method of sample preparation of powder samples is more complicated. Only the common methods for preparing solid and powder samples are discussed here, and liquid samples are not discussed.
Second, the solid sample
1. The main disadvantage of solid samples is that various addition methods cannot be used in general: such as standard addition (or dilution), low (or high) absorption dilution, internal standard method, and the like. The last two methods described above are still available if all samples already contain an appropriate concentration of internal standard elements. In addition, chemical concentration and separation are not possible. Surface structures and compositions are sometimes difficult to achieve. There may be no ready-made standards, and artificial synthesis is difficult.
2. Sample preparation method. Solid samples can be prepared from unprocessed or processed bulk materials or raw materials (such as pig iron, steel ingots, etc.). Alternatively, the melt of the furnace can be cast directly into a small mold. In order to prevent segregation of components occurring during slow cooling, it is preferred to use chilling. Polished raw materials, or surfaces that are ground by a grinding wheel, are generally satisfactory, but the latter still require further polishing to reduce surface roughness and remove process damage and an unrepresentative surface layer. There are many ways to polish, including:
(1) First belt grinding, then polishing with a polisher, the grain size of the sandpaper is changed from coarse to fine
(2) Processing with a lathe, milling machine or planer. For sheets and foils, care must be taken to ensure that the surface is free of warpage, wrinkles and creases. It is important to note that the exposure time is too long to protect from thermal deformation. Sheets and foils must be lined with a rigid support or glued together.
Pay attention to the preparation of solid samples:
(1) The analysis surface of the sample must be free of pores, precipitates and porous phenomena.
(2) Prevent segregation. The factors causing segregation: the composition and density of the alloy; the material, shape and thickness of the mold; the melting temperature of the alloy, the casting temperature, and the cooling rate of the sample being cast.
(3) The cooling rate of the sample. When the chemical composition of the sample is the same, the X-ray intensity measured by the thermal process is different, and the steel sample with a high C content is particularly prominent. When the cooling rate is inconsistent, the light elements C, Mg, Si, P, S, etc. have a great influence; while V, Cr, W, etc. often affect the analysis due to the formation of carbides. Therefore, it is required that the heat treatment process of the sample for the calibration curve and the analysis sample be consistent. In addition, it is also related to the solubility of the element in the base metal. The low solid solution of the element affects the uniformity of the metal. The rapid cooling can form a fine-grained metallographic structure, and the boundary of the large grain is prone to segregation and no Uniformity. For metal samples that are not suitable for direct analysis, such as cutting samples, wires and metal powders, samples can also be prepared by induction remelting centrifugal casting. The principle is to put a sample of an appropriate size into a crucible, remelt it by high frequency in an argon atmosphere, inductively heat it, inject it into a special mold under the action of centrifugal force, and then rapidly cool to obtain a sample of the metal round block. The centrifugal casting method can eliminate the matrix effect of the sample, and can be added by the method of dilution: dilution method (purified iron with common diluent), internal standard method, and the like. Standard samples can also be synthesized. However, the equipment is expensive and the sample preparation cost is high.
Precautions for high frequency induction centrifugal casting melting furnace:
(1) The molten metal should maintain a certain particle size. If the metal particles are too small, the potential on each of the fine particles is small, and a sufficiently large eddy current cannot be generated to cause the sample to heat up and melt.
(2) Prevent the contamination of the sample by cockroaches. Shortening the melting time also reduces contamination.
(3) Burning of components. For example, Mn, Y and other easily burnt elements, the higher the content, the longer the melting time, the more serious the burning loss. Under the pressure protection of the protective atmosphere, the burning phenomenon can be reduced or overcome. The Mn volatilization can also be added with a certain amount of metal aluminum as a deoxidizer.
(4) In the case of ensuring the accuracy of the analysis, the addition of pure iron as a diluent makes the preparation easier and reduces the analysis error.
3. Surface treatment of the sample: The solid surface sometimes cannot represent the entire sample block. At this time, it must be clarified whether the surface or the entire material is to be analyzed, or both. In addition, the surface structure and surface composition of each sample are difficult to ensure the same. When the surface treatment is performed, the inclusions may be carried away, resulting in a low analysis result of the component. The surface may be stained or abrasive, and all traces of abrasives, lubricants, and cutting coolant must be removed. When using alumina, silicon carbide, and yttria abrasives, these elements may be stained on the surface of the sample. If the abrasive is sprinkled on a lead-coated or tin-coated polishing wheel, the surface of the sample may be stained with lead and tin. Oxides and other corrosives must be removed from the surface of the sample and must have a certain degree of finish. The smoothness of the sample directly affects the measured X-ray intensity. The higher the general finish, the greater the intensity. Light elements are especially sensitive to this. The finish such as AlKα and MgKα is preferably 20~50μm, and the short wave heavy element 100μm can also meet the analysis requirements. The intensity of the analysis line is related to the direction of the polished surface. When the plane formed by the incoming and outgoing rays is parallel to the grinding direction of the grinding surface, the absorption is minimal, and the vertical absorption is maximum. This effect can be averaged by sample rotation. In essence, the optical path of the incident primary X-ray and fluorescent X-ray varies with the thickness of the surface finish when the surface finish is different. Variation of surface finish before and after grinding 3. The main disadvantage of the absence of a solid sample in the powder sample powder sample (no additives). Powder samples are readily available using standard addition methods, dilution methods, low absorption dilution and high absorption dilution methods, internal standard methods, and intensity reference internal standards. The absorption-enhancement effect can be treated by the above method, and it is also easy to prepare a powder standard. Powder methods are generally convenient and fast in a variety of applications. The main disadvantage of powder samples is that trace impurities can be introduced during the grinding and pressing operations, especially when the powder sample itself is an abrasive. It is difficult to ensure the repeatability of the loose powder surface structure. The tableting method can basically eliminate this problem. Some powders are hygroscopic or can react with oxygen or carbon dioxide in the air. It is best to put them into use. The sample chamber of the Mylar membrane seal. In addition, some powders have a small cohesive force and can be mixed with a binder and compressed into tablets. However, the most serious problem with powder samples is the particle size effect of the powder. The intensity of the line of an element in a powder depends not only on the concentration of the element, but also on its particle size. Effect of Particle Size of Powder Samples The analysis of powder samples can be carried out by directly measuring the loose powder in a certain container or by attaching it to a film, and more often by measuring it into a tablet or a molten sheet.
The main source of error in powder samples:
(1) Particle size effect The powder sample particle size effect refers to the change in the fluorescence intensity of the analytical element in the sample to be measured and the change in the particle size of the sample. In general, the smaller the particle size of the sample being analyzed, the higher the fluorescence intensity, especially the light elements. The smaller the atomic number, the more sensitive it is to the particle size; the smaller the particle size of the same element, the better the sample preparation stability. Generally, the particle size is less than 200 mesh.
(2) Segregation segregation refers to the difference in the distribution of component elements in a sample. There are two kinds of segregation: intergranular segregation: uneven mixing between powder particles A and B; element segregation: non-homogeneity of element distribution to particle size distribution. If it cannot be solved with sufficient multi-step mixing or fine pulverization, other preparation methods such as melting, dissolving, etc. may be used.
(3) Mineral effect Due to the different chemical structure or microscopic crystal morphology of minerals, the same element of the same content in different minerals, their fluorescence intensity will be very different. The so-called mineral effect is not only for minerals, but also has a broader meaning in the analysis of X-ray fluorescence spectrometers for powder samples.
Common powder sample preparation methods are tableting and melting. Below we will introduce the two methods in detail.
1. Tableting method The tableting method is a sample preparation method in which a pulverized or ground sample is press-formed. Sample preparation process of tableting method
(1) Advantages:
1 easy to sample, fast, suitable for large production and rapid analysis
2 sample preparation equipment is simple, mainly mill, tablet press and mold.
3 can be used for standard addition methods and high and low dilutions to reduce matrix effects.
4 Compared with loose samples, the powder sample can reduce the surface effect and improve the analysis accuracy.
(2) Insufficient: The mineral effect cannot be effectively eliminated and the particle size effect is completely overcome. Generally used to control production, not for the setting of sample components.
(3) Matters needing attention during the sample preparation process:
1 sample to be dried.
2 The sample is pulverized to a certain particle size and uniform.
3 The pressure and dwell time of the standard sample and the analytical sample should be consistent.
4 The pressure relief speed should not be too fast, and it should be lowered at a constant speed.
5 Keep the smashed container and the tablet of the tablet clean to prevent mutual contamination between the samples.
6 The loading density should be the same.
The following methods can be used to reduce the effect of granularity:
1 research to the extent that there is no particle size effect;
2 Standardized grinding methods are applied to all samples and standards so that they have substantially the same particle size or particle size distribution;
3 dry dilution. Preferably, the diluent powder and the particles containing the analytical element have similar mass absorption coefficients to the primary and analytical strands;
4 pressed into a block under high pressure;
5 mathematical method correction;
(4) The aid of the abrasive aiding abrasive mainly improves the grinding efficiency and overcomes the agglomeration phenomenon during fine grinding, improves the uniformity and prevents the sample from adhering to the pulverizing container during pulverization.
Commonly used abrasives are:
2 bodies such as ethanol, ethylene glycol, triethanolamine and n-hexane have the advantages of being easy to dry and volatile;
2 solid such as various stearic acid and the like. In addition, the abrasive aid can reduce and delay the re-agglomeration of sample particles during the comminution and grinding process.
(5) The main function of the binder binder is to increase the cohesive properties of some powder samples with poor cohesion in the sample preparation. There are several advantages to adding a binder:
1 A powder with a low cohesion can also be made into a strong compact;
2 Adding a binder to the powder with uneven particle size and density, and obtaining better uniformity when loading and tableting;
3 can get higher bulk density and smoother surface;
4 Due to dilution, the absorption-enhancing effect is reduced. However, the addition of binders has some disadvantages. Since the binders added are mostly light substrates and low absorption diluents, the matrix effect can be reduced. However, the scattering background is increased, and the measurement intensity of the analysis element is decreased, which is detrimental to trace elements, and the sensitivity of the light element is lowered. At the same time, the sample preparation time has increased. Commonly used solid binders are methyl cellulose, microcrystalline cellulose, boric acid, low pressure polyethylene, paraffin, starch, dry pulp powder, etc.; the commonly used liquid binder is ethanol, which has the advantage that the liquid can be volatilized in the sample. The residual amount is negligible. The use of binder should pay attention to its purity, can not contain obvious interference elements; and its properties are stable and not easy to absorb moisture and air dry. It is not easy to be broken by X-ray irradiation; it must be added quantitatively, and the addition amount is generally 2%~10% of the total weight.
(6) Additives An internal standard can be added in order to correct the absorption-enhancement effect. The internal standard must have the same particle size as the sample, or they can be mixed together for grinding. It is preferred to add the internal standard solution in a solution to uniformly mix the internal standard solution with the sample powder. In order to reduce the absorption-enhancing effect, low-absorption diluents such as lithium carbonate, boric acid, carbon, starch, etc. may be added. For powder samples of light matrix analysis elements, in order to make the calibration curve closer to a straight line, a high absorption buffer may be added, such as Cerium oxide or tungstic acid. To facilitate grinding, a powdery inert abrasive such as alumina or silicon carbide may be added. This method is often used when grinding powders with a mortar. If the particle size of the powder to be mixed is very small, or their particle size, shape and density are substantially the same, they can be directly mixed as a dry powder; if the powder is coarse, or the particle size and shape are different, it must be separately before mixing. Grind or mix and grind together. If the density differs greatly, a volume of the heavy component standard solution can be added to the weighed light component powder.
(7) Substrate In order to avoid the addition of the binder to reduce the strength, or to have only a small amount of the powder sample, a boric acid rim substrate may be used for tableting.
(8) The grinding tool can be manually ground by agate, silicon carbide or boron carbide mortar. It can be dry-grinded, or it can be added to ethanol or ***, ground to dry, and repeated several times. It is best to use a grinding machine for grinding. When tableting, the smaller the particle size, the higher the intensity of the analytical line; the higher the particle size, the higher the pressure and the higher the intensity of the analytical line. The use of a binder or a diluent makes the effect that the strength decreases as the particle size increases, and the effect that the strength increases with an increase in pressure becomes weak.
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