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    Home > Chemicals Industry > Chemical Technology > Classification of sample pretreatment for heavy metal determination

    Classification of sample pretreatment for heavy metal determination

    • Last Update: 2021-09-06
    • Source: Internet
    • Author: User
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    Common digestion methods include dry ashing, wet digestion, high pressure digestion and microwave digestion


    1.


    (1) Advantages


    (2) Disadvantages


    2.


    Advantages


    Disadvantages


    (1) Nitric acid digestion method


    (2) Nitric acid - hydrochloric acid (aqua regia) digestion method


    (3) Nitric acid- perchloric acid digestion method


    (4) Nitric acid- sulfuric acid -perchloric acid digestion method


    (5) Nitric acid- hydrofluoric acid -perchloric acid digestion method


    (6) Nitric acid-hydrogen peroxide digestion method, and other polyacid digestion methods


    The wet digestion process mainly uses nitric acid, hydrofluoric acid, perchloric acid, hydrochloric acid, hydrogen peroxide and other reagents


    Nitric acid (HNO 3 ), 65% concentration, boiling point 120°C, easy to decompose on heating, is a typical acid that oxidizes organic matter, reacts with organic matter to generate nitric oxide, and is often mixed with perchloric acid, hydrogen peroxide, hydrochloric acid and sulfuric acid


    Hydrogen peroxide (H2O2), boiling point 150℃, slowly decomposes when heated, hydrogen peroxide is an oxidant (2H 2 O→2H 2 O+O 2 ); mixing with nitric acid can reduce nitrogen vapor and accelerate the digestion of organic samples by increasing the temperature Process


    Hydrofluoric acid (HF), 40% concentration of hydrofluoric acid has a boiling point of 108°C; it is used to digest minerals, ores, soil, rocks and silicon-containing vegetables.
    Hydrofluoric acid is the only acid that can decompose silicon dioxide and silicate.
    Class; often mixed with nitric acid or perchloric acid
    .
    In order to avoid damaging the instrument and requiring the removal of hydrofluoric acid, the hydrofluoric acid in the solution can be removed by adding boric acid
    .

    Sulfuric acid (H 2 SO 4 ), 98% sulfuric acid has a boiling point of 340°C, which is higher than the maximum working temperature of Teflon (TFM) tanks; to avoid tank damage, pay attention to the reaction and destroy organic matter through dehydration reaction; 300°C is The critical temperature of the TFM tank is too high for the tetrafluoroethylene (PFA) tank (it will melt at this temperature)
    .
    Therefore, strict temperature control should be performed when using sulfuric acid
    .

    Perchloric acid (HCIO4), with a boiling point of 130°C, is a strong oxidant that can completely decompose organic matter
    .
    However, perchloric acid is prone to explosion when it comes into direct contact with organic matter.
    Therefore, perchloric acid is usually used in combination with nitric acid, or it is added to nitric acid for a period of time before adding perchloric acid
    .
    Perchloric acid is mostly used in the pretreatment under normal pressure, and it is rarely used in closed digestion.
    It should be used with caution and pay attention to safety
    .

    Hydrochloric acid (HCL), boiling point 110°C
    .
    Hydrochloric acid is not an oxidizing agent and is not usually used to digest organic matter
    .
    Under high pressure and high temperature, hydrochloric acid can react with many silicates and some insoluble oxides, sulfates and fluorides to form soluble salts
    .
    Many carbonates, hydroxides, phosphates , borates and various sulfides can be dissolved by hydrochloric acid
    .

    3.
    High-pressure digestion The commonly used sealed container for high-pressure digestion is composed of a polytetrafluoroethylene (PTFE) cup and lid, and a stainless steel jacket that fits closely with it
    .
    The outer sleeve has a screw top or screw cap.
    When tightened, the PTFE cup and the cap are tightly sealed to form a high-pressure air seal
    .

    Care must be taken when using this kind of digestion tank, because the pressure generated by the evaporation of the mixed reactants is 7-12 MPa
    .
    The capacity of samples and reagents must not exceed 10% to 20% of the capacity of the lining.
    The pressure generated by too much solution will exceed the safe rated pressure of the container.
    Similarly, organic substances must not be mixed with strong oxidants in the digestion tank.
    The decomposition temperature must be Strictly control, do not over-temperature
    .
    After the decomposition is completed, the digestion tank must be completely cooled before it can be opened.
    When opening, it should be placed in a suitable fume hood and operated with care
    .

    (1) Advantages
    .
    The advantages of high-pressure digestion include: ①Low cost and less pollution
    .
    ②The closed system can avoid the loss of certain elements (volatile elements, such as As, Hg, Se, Cd, etc.
    )
    .
    ③The pressure generated inside the sealed container increases the boiling point of the reagent, so the digestion temperature is higher
    .
    This increased temperature and pressure can significantly shorten the decomposition time of the sample, and make some difficult-to-dissolve substances easier to dissolve
    .
    ④Reagent consumption is reduced, cost is saved, and pollution is reduced
    .

    (2) Disadvantages
    .
    The disadvantages of high-pressure digestion include: ①There is a possibility of explosion, therefore, you need to pay attention to safety
    .
    ②The maximum temperature that PTFE can withstand is 190~230℃.
    The filling compound in this material may cause system deviation of some elements, which is difficult to control
    .

    4.
    Microwave digestion Microwave digestion is a more effective pretreatment method for the determination of inorganic elements.
    It is carried out completely in a closed environment and can avoid the pollution and loss of target elements as much as possible.
    However, this method has a small sample size and digested samples.
    There are a lot of nitrogen and oxygen compounds in it, and there is a certain matrix interference, which is more suitable for ICP-MS detection (the detection limit is easier to reach)
    .
    If the solution after microwave digestion is used for the determination of arsenic by atomic fluorescence spectrometry (AFS), it is necessary to expel the acid
    .

    (1) Advantages
    .
    The advantages of microwave digestion include: ① Very short digestion time, usually in minutes instead of hours
    .
    ② without loss of volatile elements, including mercury (Hg), arsenic (As) and the like
    .
    ③No acid mist, improve the working environment of the laboratory
    .
    ④ No sample contamination
    .
    ⑤Use the least amount of acid solution, the blank value is low
    .

    (2) Disadvantages
    .
    The disadvantages of microwave digestion include: ①The cost of the instrument is relatively high and the sample volume is limited
    .
    ②The digestion of high-oil samples is often incomplete
    .
    ③It is easy to explode and damage high grease samples
    .
    ④Consumables for microwave tanks are expensive
    .

    Related links: Principles of sample pretreatment for heavy metal determination

     

     

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