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Although there are many different methods of introducing analytical instruments for samples of different states. Many methods are effective for the specific requirements of a particular sample. However, the most extensive and priority is to introduce liquid into analytical instruments such as atomic absorption spectrometers, atomic fluorescence spectrometers, and plasma emission spectrometers.
Advantages of liquid introduction to analytical instruments:
1) After the solid sample is processed and decomposed and converted into a liquid, the elements are present in the solution in an ionic state, eliminating the measurement error caused by the state of occurrence of the element and physical properties. -
2) When performing the analysis, according to different types of samples, 0.1-1 g of solid sample is generally weighed for chemical treatment, which has a good sampling representative.
3) Analysis of the liquid pattern basically eliminates the fractionation phenomenon of evaporation of each element from the solid sample, so that the evaporation behavior of each element tends to be uniform, improving the accuracy and precision of the analysis.
4) The evaporation behavior of each element tends to be consistent, which creates favorable conditions for simultaneous determination of multiple elements.
5) A standard solution and a matrix element matching solution of each element can be easily prepared by using a compound of each element (high purity).
6) The solution atomization method can measure 70 elements; for the plasma emission spectrometer, the simultaneous or sequential determination of multi-element of primary, secondary and trace concentrations can be carried out without changing the analysis conditions. . -
The introduction of liquid into analytical instruments requires first the problem or disadvantage of converting a solid sample into a solution:
1) Chemical pretreatment is required, which increases manpower, material resources and costs. A chemical pretreatment chamber is required.
2) Some chemical treatments require a certain amount of expertise.
3) After the sample is decomposed, the dilution is more than 50 times, which reduces the absolute sensitivity of the element in the sample. -
4) Contaminants or salts that are not conducive to the measurement are sometimes introduced during the chemical treatment process.
B
Sample preparation and decomposition requirements
Although the conversion of solid samples into liquid samples poses problems, solution injection still has many outstanding advantages, so it is still used in most laboratories.
The following should be noted when the solid sample is chemically processed into a liquid sample:
1) The solid sample weighed should be processed according to the specified requirements (such as crushing, sampling, etc.), which is uniform and representative;
2) The measured elements to be measured in the sample should be completely dissolved in the solution (whether the sample is "fully soluble" can be determined as needed). Set a chemical treatment method that is reasonable, has few links, is easy to grasp, and is suitable for processing a large number of samples.
3) When applying chemical treatment, the measured elements can be arbitrarily separated according to requirements. The purpose of the separation is to separate the matrix and other elements that interfere with the measured elements to improve the measurement accuracy. The premise that must be considered is that “the measured elements must be fully enriched, there must be no loss, and the separated components. They do not have to be separated very cleanly.
4) Sample contamination should be avoided during the entire process, including preparation of solid samples (fragmentation, sieving, sampling), laboratory environment, reagent (water) quality, vessels, etc.
5) Sometimes it is necessary to consider the total solid dissolved amount (total salt) in the test solution. Excessive total solid dissolved amount will cause matrix effect interference, spectral line interference and background interference. In general, for a test solution having a total dissolved solid content of about 10 mg/ml (1%) in a sample, the sample introduction system is not blocked when the analysis is not performed for a long time.
In routine analytical work, the total dissolved solids content of the test solution is desirably as low as possible, generally controlled at about 1 mg/ml (0.1%), sometimes controlled at 0.5 mg/ml if the sensitivity of the measured elements is satisfied. (0.05%) or so. Therefore, sample pretreatment of inorganic element analysis uses acid decomposition instead of alkali fusion as much as possible, and the dilution factor is about 1000 times.
6) Many samples cannot be dissolved by acid (wet method) under normal pressure conditions, such as corundum, chromite, zircon, etc. At this time, alkali fusion (dry method) is required, but the sample should be taken into consideration during alkali fusion. The relationship between the total dissolved solids content and the content of the measured elements in the sample. - The use of a closed tank pressurized (wet) solution to solve the decomposition of a large number of samples requiring alkali fusion. The method of pressurizing the dissolved sample of the sealed can is more widely applied to the decomposition of the sample as the sealed container device is improved in safety, reliability, convenience and the like, especially the application of the microwave technology.
The sample preparation method can be roughly divided into the following basic ways:
Acid decomposition - open container, acid decomposition - closed container, alkali metal solvent melting method, microwave digestion
1
Acid decomposition - open container
Open cell acid decomposition is the most common method of sample decomposition in chemical analysis laboratories. Its advantage is that it facilitates large-scale sample analysis operations. There are two main methods for the decomposition and dissolution of biological and plant samples:
a) dry processing
Put the sample into platinum or porcelain crucible, slowly heat it up in a muffle furnace, ash for a few hours at 450-550 ° C, then dissolve the residue with a small amount of aqua regia, and dilute to volume with water. In the test. -
This method is simple, economical and fast, but the main disadvantage is that it will cause volatilization loss of some elements, such as As, Hg, Se, Cd, Pd, Zn and so on. Some of these elements are not volatile per se, but during the ashing process, they may be lost by the formation of some volatile compounds (such as chlorides) (most biological samples contain chlorine). For some non-volatile elements, they are easily adsorbed on the wall of the vessel after ashing and converted into a poorly soluble phase, which is difficult to dissolve.
The addition of nitrates of zirconium, magnesium and aluminum as an aid to the ashing process to enhance the oxidation process reduces and eliminates this loss, preventing the analyte from volatilizing and converting into readily soluble nitrates. However, an increase in salt will increase the total salt content of the test solution, which will increase the background and deteriorate the detection limit.
For plant or animal tissue samples, a small amount of nitric acid or sulfuric acid is added to the ashing agent instead of the above-mentioned aiding agent. Add 1-1.5g sample to the crucible and small beaker, then add 5ml concentrated nitric acid or 0.5ml concentrated sulfuric acid, cover the surface dish, heat it in a fume hood and evaporate it to carbonization, then move it into the muffle furnace and gradually heat up to 450. -500 ° C for about 4 hours. After taking out, add 2ml of aqua regia to dissolve the residue. After dissolving, the solution is about 1ml, and the volume is to be tested.
b) wet processing
Generally, nitric acid can be used to decompose organic substances. For example, beer or beverage can be added with nitric acid, and the organic substance is digested at a low temperature for a long time, and then measured. Or the sample is evaporated to dryness, and then concentrated nitric acid is added to digest the organic matter. - Decomposition of milk and decomposition of plants using nitric acid and perchloric acid, plus hydrogen peroxide can completely decompose organic samples.
summary:
The advantages of 1 decomposition with acid are:
The operation is simple; a large amount of silicon is removed, and the salinity in the test solution is greatly reduced compared with the alkali fusion. However, some minerals such as corundum, zircon, zircon, cassiterite, chromite, rutile, monazite, etc. cannot be dissolved by the above-mentioned acids, and it is necessary to dissolve the sample by alkali fusion.
2 There is still a small amount of residue after acid decomposition of the sample.
If it is still necessary to measure the components, the residue can be filtered, and the filter paper (quantitative filter paper) is placed in a small crucible together with the residue, dried, ashed (500-600 ° C), and cooled. Then put a small amount of sodium peroxide and sodium hydroxide and use as little as possible. The mixture was melted in a muffle furnace at 480 ° C, neutralized with an acid to form a lumpy extract, and this solution was combined with the ortho acid solution to carry out measurement. Note that if you have this step, the above method should combine the two solutions to make up the volume. At this time, the standard solution for the measurement should be added with alkali-saturated sodium chloride to match the matrix.
The method of decomposing the sample three times cannot be used to analyze elements such as Hg, Se, As, Ge, Sn, and Te because their chlorides will volatilize.
4 When the sample is decomposed by this method, Cr will lose about 10% in the form of CrOCl3.
5 When the organic matter in the sample is slightly higher, a few drops of perchloric acid may be added dropwise at the operation method (III), and the white smoke is exhausted by heating.
6 Decompose the sample with HF/HClO4, dip it with hydrochloric acid, transfer it to a volumetric flask, and immediately transfer it to a plastic volumetric flask for storage to prevent the zinc in the volumetric flask glass from being brought into the glass.
2
Acid decomposition - closed container
The sealed container digestion sample has the following advantages over the open container digestion sample method:
1 The pressure generated inside the sealed container raises the boiling point of the reagent, and thus the digestion temperature is high. This increased temperature and pressure can significantly shorten the decomposition time of the sample and make some insoluble materials easy to dissolve.
2 Volatile element compounds such as As, B, Cr, Hg, Sb, Se, Sn will remain in the container and thus these elements will be stored in solution.
3 The amount of reagents is greatly reduced, which saves costs and reduces the emission of toxic gases.
4 The amount of reagent is reduced, and it is a closed container, which reduces the possibility of contamination.
3
Microwave digestion
Microwave digestion is a major advance in the decomposition of samples in closed containers. Under microwave irradiation, energy is transmitted through a container (PFA or TFM material) to rapidly heat the digestion medium (liquid phase, usually a mixture of inorganic acids), and can also be sampled. The absorption of molecules increases the kinetic energy and produces internal heating. This action causes the surface layer of the solid material to rupture by expansion and vibration, so that the exposed new surface layer is etched by the acid.
"Microwave digestion" "strong" finishing articles
This effect produces much higher dissolution efficiencies than those that rely solely on acid heating and avoids the use of alkali-thawed samples. In recent years, microwave digestion technology has developed rapidly, especially the high-pressure closed microwave digestion technology has developed rapidly and has been widely used.
4
Alkali metal melt decomposition
Alkali metal melt decomposition method is mainly used in geological silicates, ceramics, refractories, metals, alloys and other fields. The main fluxes are lithium metaborate (LiBO2), lithium tetraborate (Li4B4O7), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), sodium peroxide (Na2O2) and the like. After the sample is melted, the frit is extracted with water and acidified with acid, the advantages and disadvantages of which have been discussed above.
5
Supplement: separation and pre-enrichment
Separation and enrichment are two nouns of different concepts, but in reality they are complementary systems. That is to say, there is enrichment when there is separation, and vice versa. The basic substance of a sample is an essential part of its composition, accounting for a large percentage.
Separation and enrichment methods can be roughly divided into four categories:
1 to generate volatile compounds for separation and enrichment; 2 solution extraction method; 3 ion exchange chromatography; 4 coprecipitation method.
These basic substances are often not the elements that require measurement, and the content of the matrix elements will be relatively high, which will generate excitation interference and spectral interference, affecting the determination of trace elements (accuracy and detection limits).
For this purpose, the matrix element and the element to be tested are separated. Separation not only removes the matrix effect produced by the matrix elements, but at the same time allows the analytical solution to achieve pre-enrichment. Because a large amount of matrix elements are separated, the total salt of the analytical solution is reduced, which reduces the dilution factor of the analytical solution or can be concentrated by evaporation, typically up to several orders of magnitude.
Several issues to be aware of separation and enrichment—:
(1) There are few separation and enrichment steps, and the operation is simple and easy to grasp. It is best to combine separation and enrichment with sample decomposition and play a role in the decomposition process.
(2) During the separation and enrichment process, all the elements to be tested are enriched and must not be lost, and the basic elements do not have to be completely cleanly separated without affecting the measurement.
(3) The reagents and utensils used in the separation and enrichment process must not contain the elements to be tested to avoid contamination.
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