XRF spectrometer, sample preparation technology
Solid samples
1. The main disadvantage is that of a solid sample usually cannot be used in a variety of adding method: such as standard adding method (or dilution), low (or high) absorption dilution method, such as internal standard method. If all the samples have already contains the appropriate, a certain concentration of internal standard elements, the above at the end of the two methods are available. In addition, also can’t for chemical enrichment and separation. Surface structure and sometimes difficult to agree. May can’t get the ready-made sample, and it’s difficult and synthetic.
2. Sample preparation methodSolid samples available raw or processed large pieces of material or raw materials (such as pig iron, steel ingot, etc.). In addition, also can put the furnace melt casting directly to the small mold. In order to prevent the slow cooling occurs when composition segregation, it is best to chill. Approved by polishing materials, or the grinding wheel surface, generally is satisfactory, but the latter still needs further polishing, in order to reduce the surface roughness, and removal of machining damage and no typical surface layer. Polishing method has many kinds, including:
(1) the belt grinding first and then use polishing machine polishing, the sand paper in turn from coarse to fine particle size, and (2) use lathe, milling machine or planer for processing.For sheet and foil, must be careful operation, to ensure that the surface not warp, wrinkles, and creases. Special attention should be paid to not irradiation time is too long, so as to avoid heat deformation. Liner plate and foil must be on a piece of rigid supports, or put them together. The preparation of solid samples should pay attention to:
XRF spectrometer, sample preparation technology
(1) the analysis of the sample surface can not have porosity, deposition and porous phenomenon.
(2) to prevent segregation. Cause segregation factors: alloy composition and density; Mold material, shape and thickness; Alloy melting temperature, pouring temperature and the cooling speed of casting sample, etc.
(3) the cooling rate of the sample. When the same sample chemical composition due to thermal process different measured X-ray intensity, containing a high percentage of C steel sample this kind of phenomenon is especially outstanding. Cooling rate is inconsistent, the light elements C, Mg, Si, P., S etc. There is a big impact; And V, Cr, W, often due to the formation of carbides and the impact analysis. Therefore, required to make the sample of the calibration curve and analysis of the heat treatment process to maintain consistent of the sample. In addition, solubility in the metal matrix and element, element of the low solids fusibility can affect the uniformity of the metal, rapid cooling can form fine grained, metallographic structure and the big grain boundary segregation and inhomogeneity.
The microstructure of fine grains can be formed by rapid cooling, while the boundary of large grains is prone to segregation and non-uniformity.
XRF spectrometer, sample preparation technology
For the metal samples that are not suitable for direct analysis, such as cutting samples, wire rods and metal powders, the sample can also be prepared by induction remelting centrifugal casting method.
The principle is to put the appropriate size of the sample into the crucible, in the hydrogen atmosphere through high-frequency induction heating to remelt, under the action of centrifugal force into the special mold, and then quickly cool the metal sample. Centrifugal casting method can eliminate the matrix effect of the sample, and can be used to add method: dilution method (common diluent of pure iron), internal standard method. The standard sample can also be synthesized artificially. But the equipment is expensive and the sample preparation cost is high.
High frequency induction centrifugal casting molten furnace
Note:
(1) by molten metal to maintain a certain size. If metal particles is too small, each tiny particles on the electric potential is small, cannot produce enough large eddy current heating melting sample.
(2) to prevent the crucible of the contamination of the sample. Shorten the melting time also can reduce the contamination.
(3) the loss of components. Such as Mn, Y easy damage elements, such as the higher levels, melting the longer, the more serious the loss.Under the protection of atmosphere pressure protection, to reduce or overcome loss phenomenon. And the volatilization of Mn can also add a certain amount of aluminium as deoxidizer.
(4) in the case of ensure accuracy of analysis, can add pure iron as diluent, can make easier for sample preparation, and reduce the analysis error.
3. The surface treatment:Solid surface sometimes cannot represent the whole sample, at this time, must find out is to analysis the surface or the whole material, or both. In addition, the surface composition and surface structure of samples is difficult to guarantee are the same. In the surface treatment, may take inclusions, causes the composition analysis results on the low side. Surface may be contamination or wear abrasive, must remove all traces of abrasive, lubricant and cutting coolant. Using aluminum oxide, silicon carbide and cerium oxide abrasive, the surface may be with these elements. If the abrasive is sprinkled in the package lead or tin-coat polishing wheel, the surface may be with lead, tin, these elements. Must be removed from the surface oxide and other corrosive, and need to have a certain degree of finish. The finish of the sample directly affect measured X-ray intensity, the higher the average roughness intensity. Lighter elements are especially sensitive. Such as AIKa, MgKa finish best in 20 ~ 50 um, and 100 um shortwave heavy elements also can satisfy the requirement of analysis. Analysis of line strength and direction of grinding surface, the incident ray, and the rays constitute a plane and ground surface grinding direction is parallel to absorb the minimum, vertical when the absorption is the largest. Use sample spinning can make this averaging them out. In essence, the surface roughness is different when the incidence of an X-ray and fluorescence X-ray optical path change along with the thickness of surface grinding grain.
The change of the surface roughness before and after grinding
The analysis of the powder sample can be directly put loose powder in a certain measure containers, are attached to the film can be measured, more is made into tablet or fused measurements. The main source of powder sample error
(1) the effect of powder sample particle size effect refers to the analysis of the measured sample element of fluorescence intensity changes and the size change of the sample. In general, by analyzing the sample size is smaller, the higher fluorescence intensity, light elements. The smaller the atomic number, the granularity of the sensitive; The same elements, the smaller the particle size, the better stability of the sample preparation. General requirements size less than 200 mesh.
(2) segregation segregation yuan table refers to the components in the differences in the distribution of the sample
Segregation has two kinds:
Intergranular segregation: dust particles mixed unevenly between A and B;
Element segregation, element distribution of particle size distribution of non uniformity.
If in the multi-step fully mix or fine crushing cases still cannot solve, can use other sample preparation methods, such as melting, dissolving, etc.
(3) Mineral effect Due to the different chemical structure or microscopic crystal shape of minerals, the same content of the same element 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 X-ray fluorescence analysis of powder samples. The common methods for preparing powder samples are tablet pressing and melting. Below we will introduce these two methods in detail.
1. The tablet method
The tablet method is the preparation of samples that have been crushed or ground under pressure to form.
The sample preparation process of the tablet method
Powder sample pressure molding
Advantages:
(1) sample preparation is simple, fast and suitable for mass production and rapid analysis
(2) sample preparation equipment is simple, the main is flour mill, tablet machine, mold and so on.
(3) can be used for standard addition method and the high and low dilution in order to reduce the matrix effect.
(4) compared with loose samples, the powder samples of tablet can reduce the surface effect and improve the analysis accuracy.
(2) : not completely overcome effectively eliminate mineral effect and size effect. Generally used to control the production, and shall not be used for the fixed value of sample composition.
(3) should be paid attention to in the process of sample preparation matters:
(1) sample to dry.
(2) samples after grinding to achieve a certain particle size and uniformity
(3) samples and standard samples the sample preparation of pressure and the holding time.
(4) unloading speed not too fast, at a constant speed to drop.
5. Keep the crush container and pelleting mould clean, prevent the sample room with each other.
6. Loading density.
Can use the following method to reduce the particle size effect
:1) porphyrization to the point of no size effect;
(2) to all sample and prototype using standardized grinding methods, so they basically have the same size or size distribution;
3 dry dilution. Thinner powder with particle, which contains the analysis element for primary and analysis the quality of the wiring harness is similar to best absorption coefficient;
(4) made in high pressure;
(5) calibration mathematical method;
(4) and grinding aid
The effect of grinding aid is mainly is to improve the grinding efficiency and overcome the fine grinding of the agglomeration phenomenon, improve the uniformity and prevent the sample adhesion on the crushing container when crushed.The commonly used grinding aid are:(1) of the liquid such as ethanol, ethylene glycol, triethanolamine and n-hexane, possesses the advantages of can be dried volatile;
(2) of the solid such as all kinds of stearic acid, etc. In addition, the grinding aid can also reduce and delay the sample particles in the process of crushing and grinding to the reunion phenomenon.
(5) binderBinder’s main function is to make some cohesion more bad of the powder samples increase bonding performance in sample preparation.
Join the binder has the following advantages:
(1) cohesion, low powder can be made into strong pressure;
(2) the granularity and density uneven powder adding binder, sample loading and pressure for a little while can get better uniformity;
(3) can get higher bulk density and a smooth surface.
(4) due to dilution, reduce the absorption enhancement effect.
But to join the binder also has some disadvantages, because join binder are mostly light matrix, low absorption of diluent, can reduce the matrix effect. Increased, but causes scattering background analysis of other elements of the measuring strength will be decreased, adverse to the trace elements, decreased the sensitivity of light elements. At the same time, the sample preparation time increased. Commonly used solid binder with methyl cellulose, microcrystalline cellulose, boric acid, low pressure polyethylene, paraffin wax, dry starch, pulp powder, etc.; Commonly used liquid binder with ethanol, its advantage is liquid can be volatile, the residues in the samples can be ignored. Using the binder should pay attention to its purity, not contain obvious interference elements; Nature and stable not easy absorption of moisture, air drying, the X-ray irradiation not easy and broken; Must be quantitative to join, join the amount is commonly 2% ~ 10% of the total weight.
(6) additivesTo absorb – enhancement effect can be added internal standard calibration. Within the target particle size must be the same as the sample size, or mixed together and then to grind them. Best to join the internal standard, in the form of solution to the internal standard solution and sample powder evenly mixed. To reduce the absorption enhancement effect, can add low absorption of diluent, such as lithium carbonate, boric acid, carbon, starch, etc., for light matrix analysis element powder samples, in order to make the calibration curve is more close to the line, you can add high absorption of buffer, such as lanthanum oxide or tungsten acid. For convenience of grinding, can add powdered inert abrasive, such as alumina, silicon carbide. When using a mortar powder, often use this method.
If the mixing of powder particle size are small, or their size, shape, density are basically the same, can be directly in the form of powder mixed; Thicker, or if the powder particle size and shape is different, it must be in front of the mixed respectively to grinding, or mixed together to grinding. If the density difference is very big, you can add a certain volume of heavy component standard solution to powder after weighing of light components.
(7) substrate
In order to avoid the addition of binder lower intensity, or only a small amount of powder sample, can use boric acid edge substrate tablet.
(8) grinding tool
Available agate, silicon carbide, boron carbide mortars manually grinding. Can dry grinding, can also add ethanol or ether, grinding to dry, so repeated several times. Prototype’d better use grinding of grinding. Line voltage, the smaller the particle size, analysis the higher the strength; Granularity is constant, the higher the pressure, the higher analysis line strength. Using the binder or thinner, can make the strength decreases with the increase of particle size effects become apparent, and the intensity of the effects of the increases with pressure and weak.
2. Melting method
After melting method is to put the pieces of the oxide powder sample with flux mixed in certain proportion, eutectic melting at high temperatures, make samples and flux form even vitreous body.
XRF spectrometer, sample preparation technology
The molten legal process
(1) melting method has many advantages:
1) can eliminate composition, density and particle size inhomogeneity, completely eliminate the mineral effect and size effect.
(2) through the fluxing agent, can reduce or even eliminate absorption enhancement effect, melting process is also the course of the dilution, greatly reducing the matrix effect, the enhanced effect of absorption – also decreases.
(3) easy to use standard adding method, standard dilution method and internal standard method, can join internal standard or heavy absorption agent to reduce or compensate the matrix effect, or add some interference effects of secondary matrix elements, with a fixed the concentration of this element.
(4) the prototype of synthesis is also easier, can according to need to use pure oxides pure reagent preparation of appropriate synthetic standard samples to meet the needs of all kinds of samples, and can get a wider range of calibration curve.
(5) of glass for long time preservation. Glass surface smooth and even, the prototype is easy to save, radiation resistant performance is good.
The main drawback:
Cannot be directly molten metal samples, must pass a pre oxidation treatment.
XRF spectrometer, sample preparation technology
The main drawback:
(1) cannot be directly molten metal samples, must pass a pre oxidation treatment.
2 because of the addition of flux and the flux, the sample was diluted, the strength of the analysis element is reduced, the analysis of the lighter elements line intensity is greatly reduced, the trace of the concentration of components and the secondary has been greatly reduced, adverse to the light and the determination of trace elements.Fused to spend a lot of time.
To the preparation of glass wafer, still need certain skills.
(4) in addition, in the process of storage, transparency will be lost, or will happen due to the stress rupture. Glass wafer to melt and remakes.
(2) the selection of flux
(1) the sample can be completely decomposed by flux. After high temperature melting, samples and single-phase vitreous flux can form uniform.
(2) the molten temperature appropriate, volatile small.
(3) cannot contain a test element or interference elements in flux, pay attention to the content of impurities in flux.
(5) of glass surface to smooth level off
Those of the fuse link to easy mold release.
Commonly used flux has sodium tetraborate, four boric acid lithium, lithium metaborate. Relative to the sodium salt of boric acid lithium salt, mass absorption coefficient is a little lower, is conducive to the analysis of the lighter elements, made of melt flow performance is good, high melting temperature. Lithium tetraborate melting ability is good, but almost on the mechanical properties of the fuse link, and lithium metaborate has good mechanical strength, is often used by mixing these two kinds of flux, mixed flux can balance the advantages of various aspects.
XRF spectrometer, sample preparation technology
(3 flux
The effect of flux
(1) adjusting the flux of acid, alkaline, be helpful for flux can better melting sample.
(2) reduce the decomposition temperature of the flux.
(3) increase the liquidity of the melt, to make a sample made more uniform and improve the accuracy of sample preparation.
(4) oxidation, for some samples with a reducing, join oxidizing fluxing agent, can be pre oxidation so as to save the crucible, still can make some volatile components into salt in the melt. According to different purposes, can add different other material before melting. In order to reduce the melting point, can be added in the lithium tetraborate lithium carbonate. In order to make the acidic and alkaline sample is more easy to dissolve, can add respectively in lithium tetraborate lithium carbonate and lithium fluoride. In the corresponding sodium tetraborate can add sodium carbonate and sodium fluoride respectively. Melting, carbonate decomposition emits carbon dioxide, but on the melt mixing effect, but may also be formed in vitreous bubble. Commonly used flux has Li, Na, NH4 nitrate and carbonate.
(4) the release agent of choice
Molten glass adhesion or infiltrating the tendency of platinum crucible, and mould, which makes the melting glue on the mould, sometimes leading to fracture. For some, release agent should be chosen to help the fuse link smoothly from the crucible, and mould. Commonly used release agent for halon, such as the LiBr, NH4Br, K1, NH41, etc. Release agent itself does not dissolve in flux, it in the form a layer on the surface of molten glass wrap, enables the fuse link to from the crucible or die. The dosage of the release agent need not many, generally 20 to 50 mg can, can join solid, can also use solution to join. Solution in relatively good, so can more accurately for the amount of control. Release agent can be added to the mixture in the crucible in front of the melt, can also note to join before casting crucible, the latter can only join the solid, but such a release agent usage is less. Used as demoulding agent of bromide and iodide, at high temperatures, Br and 1 is quite volatile, but there are still some melt film residues, residues of release agent helps fuse link out, but need to take into account the Br and 1 to analysis element of interference.
XRF spectrometer, sample preparation technology
(5) the selection and use of the crucible
The most widely used is Pt95% + Au5% of platinum alloy crucible.
Usually, the bottom of the crucible thicker, can increase the strength of the crucible, and under high temperature change in a short time. The greater the slope Angle of the bottom of the pot and the pot body, a fuse link also made easier to peel. See below (a). In addition, the crucible bottom and pot body must have certain radian, under the condition of the slope Angle is larger, the radian in 2.5 ~ 3 mm effect is good, see below (b).
Crucible shape
The crucible can be soaked in HCI (1+1) solution near boiling for a few minutes after use, and cleaned and dried. After demoulding, the melting sheet is often used as the bottom of the analysis surface. If the bottom of the crucible appears deformed, crystalline grains, rough, and even black spots, it will affect the quality of the analysis or make the melting sheet brokenCrack or difficult to demoulding. The crucible must be polished at this time.
The following points should be noted when using the platinum crucible:
A) all the heating or burning of platinum vessels shall be conducted on oxidizing flame furnace or a Bunsen burner, not with the flame of smoke and flame inner cone part contact, in order to avoid the formation of brittle carbide platinum.
B) within the platinum vessel heating or molten alkali metal oxide, and barium hydroxide, sulfur, cyanide, cyanide, nitrate, nitrite, a mixture of Na2CO3 and S, Na2S203 and element silicon; Na202 mixture with NaOH in platinum vessel decomposition of sample, the temperature shall not exceed 510 ℃, so as to avoid serious erosion platinum vessels.
C) cannot be handled within the platinum vessel halogen and can precipitate halogen material, such as aqua regia, bromine water with HCI and oxidant (KCIO3 – KNO3, MnO2, KMnO4, NaNO2, K2Cr207) mixture; A significant erosion of platinum FeCI solution.
D) not within the platinum vessel burns and melt containing a large number of metal (such As Pb, zinc, Bi, Sn, Sb, As, Ag, Hg, Cu, etc.) of the sample and compounds. These elements easily back into the metal, and the generation of platinum alloy, platinum vessel damage. l
E) with reducing agents (such as filter paper, organic matter, etc.), shall not be within the platinum vessel heating or burning material contains a lot of phosphorus, sulfur, lest produce brittleness of phosphorus (sulfur) or platinum.
F) contains a lot of iron samples, can not directly within the platinum crucible melting, must go with most HCI dissolved iron and filter out the sediment with a platinum crucible melting; Polymetallic ore sample, should be treated with HCl – HNO3, the residue in a porcelain crucible ashing, again into platinum crucible melting inside).
G) platinum generate most of the metal in the high temperature alloy, so the metal sample in platinum crucible burn or melt.
XRF spectrometer, sample preparation technology
H) are taken from the crucible of condensation of clinker, do not forcibly knead twist, and do not blow or metal bar with a glass rod dao, lest platinum crucible deformation or damage.
I) platinum crucible heating can not contact with any other metal. Best with asbestos board or brick, below to move hot platinum crucible, must be used with platinum crucible tongs.
J) of unknown sample, shall not use heating or melting platinum vessels.
K) used by platinum vessel, can use the following method to clean:
Using (1 + 1 or 1 + 2) HCI boiling cleaning; When washed with HCI, usable Na2CO3, K2S207 or Na2B407 melting.
(6) melting equipmentSample:
1) high frequency induction furnace high degree of automation, fast speed, of uniform melting, good reproducibility, low energy consumption, safe in operation and melting time is short, the service life of crucible is long. The disadvantage is that the price is relatively high.The muffle furnace
2: completely manual melting equipment, prices of the economy. If melt under the condition of small sample can use, sample preparation must have the skilled operation skills to uniformity good reproducibility was melting, and when use should pay attention to safe operation.
(3) gas furnace, melting fast, must use high calorific value gas, sometimes problems uniformity of samples. Should be paid attention to when used to avoid contact with the reduction of the gas flame crucible, at the same time should pay attention to the safety of use.
(7) some matters needing attention
(1) keep a high temperature harm the platinum crucible components in the samples. Such as metal elemental or reducing substances exist in the sample, must be pre oxidation treatment. Otherwise it will damage the crucible.
(2) some alkaline earth metal can reduce vitreous stability, make sample crisp fragile, especially when found in large Numbers in the sample. At this moment can increase into glass elements such as SiO2, AI203 etc, made of fuse link more stable.
3 pay attention to in the high temperature when volatile elements. Such as sulfur dioxide sulfur in high temperature is easy to generate volatile results caused by the loss. In the melting process must keep high oxidizing atmosphere, appropriate to reduce the melting temperature, to make the sulfur retention quantitatively in the melt film.
(4) glass is broken, and if the fuse link too fast cooling is easy broken, and the cooling speed too slow, cause crystallization precipitation, melting and easy to break. In addition, the dust will also cause the collapse of the glass. Moved to fully in the process of melting sample, making uniform inside the glass, and natural cooling.
XRF spectrometer, sample preparation technology
The cooling speed of the fuse is affected
5 Sometimes the produced fuse has bubbles, which depends on the composition of the sample, the flux used, and the dilution ratio. The bubbles can be removed by shaking them during the melting process to move them towards the edge of the melt. For the small bubbles that can not be removed, the viscosity of the melt can be reduced, such as adding a flux to reduce the decomposition temperature.On the measurement of burning loss, you can burn the sample before melting sample, measure the burning loss, must pay attention to the loss of some components to be measured
.⑦ The participation of sulfide should be avoided in the melting process. When they come into contact with the platinum crucible, they will permanently corrode the surface of the crucible, resulting in glass residues that are more likely to stick to the corroded part and remain in the crucible after each subsequent melting. In addition, the low-priced sulfur is easily converted to sulfur dioxide and lost at high temperatures, and if you want to analyze the total sulfur in the sample, you need to use a lower melting temperature, while using an oxidizer, to convert the low-priced sulfur into sulfate.
The purpose of melting is to produce a bubble-free transparent melt. The opacity of the melt indicates that the melting, decomposition and chemical reaction are not complete. The presence of bubbles in the melt indicates that improper measures to remove the bubbles, or the melting temperature is too low and the melt is too viscous. The melting temperature may need to be raised to achieve complete melting. The heat should be continued until no bubbles are formed, which indicates that the chemical reaction is complete. To dislodge the bubbles, the crucible needs to be turned diagonally