The microwave digestion technique destroys the initial form of the target component in the sample under microwave heating and releases it in the highest or higher valence state of the inorganic ion. Microwave heating differs from conventional heating methods in that it does not “heat” through internal and external heat conduction, ie “internal heating”, ie the molecular polarization of the sample and reagent generating medium is under the action of an alternating magnetic field generated by the microwave. . As the magnetic field changes, the molecules alternately align, causing the molecules to oscillate at high speed, causing the molecules to vibrate and collide with each other, causing the temperature to rise rapidly. Under strong collision and agitation, the acid is in better contact with the sample, causing the sample to decompose rapidly. The high pressure generated in the closed container increases the boiling point of the digested acid, allowing the sample to be digested for a certain period of time. High boiling point, greatly reducing digestion time.
Microwave digestion instrument commonly used reagents
(1) Nitric acid: Nitric acid is a strong oxidizing agent that oxidizes and corrodes metals and organic substances, making it a soluble nitrate that dissolves most of the sulfides. It is usually used in conjunction with hydrogen peroxide to completely eliminate digestion. It is mainly used in organic samples such as fats, beverages, proteins, pigments and polymers, as well as in metal oxides and soils.
(2) Sulfuric acid: Sulfuric acid is an effective solvent for many substances. It completely destroys almost all organic compounds and undergoes rapid dehydration and carbonization. Hot sulfuric acid can be used in organic tissues. Hydroxides, metals and ores, but their temperature must be strictly monitored because they have boiling points that exceed the extreme temperatures of many microwave digestion tanks, in most cases as auxiliary acids.
(3) Hydrofluoric acid: suitable for digestion of silicon-containing samples, which can be converted from a sample to silicon tetrafluoride together with nitric acid, titanium oxide, tungsten, cerium, zirconium and the like.
(4) Aqueous solution: It is a mixture of 1 volume of nitric acid and 3 volumes of hydrochloric acid, and its effectiveness is mainly derived from nitrosyl chloride (NOCI) produced by the reaction of the two. It is suitable for the digestion of inorganic substances such as gold, platinum, plant tissue and wastewater. Aqueous water can filter metals in wollastonite, but it does not dissolve completely.
Application in environmental monitoring
Environmental samples involving microwave digestion include soil, solid waste, coal, coal fly ash, marine sediments, sludge, wastewater, and the like. Many environmental samples are complex, deposited products, complex matrix components, heavy metals and pesticide residues. Due to the diversity of environmental samples and the complexity of the matrix, it is necessary to test and test the components. Consult a large body of literature to determine the nature of the sample and the required digestion reagents. Environmental samples usually contain some organic matter, which is difficult to digest completely with acid under normal pressure. Closed microwave digestion can solve this problem well. In addition, some volatile elements do not cause damage.
Microwave digestion has been widely used for the analysis of metal elements in environmental samples. A method for determining elements in solid waste by microwave digestion (USEPA 3051) has been tested by a large number of experiments, and the reliability of microwave digestion has been verified.
Chemical oxygen demand (COD) is one of the main indicators of water quality monitoring. The classic method is potassium dichromate reflux, but it consumes more sample and reagents and uses microwave digestion technology for longer reflow times (greater than 2 h). When is the speed.
Research on non-metallic elements has focused on sulfur, nitrogen and phosphorus. By microwave heating digestion, more than 10 samples can be digested at the same time, and the analysis speed is greatly improved compared with the traditional analysis method.
Microwave digestion is an advanced and effective sample processing method that can meet the requirements of sample processing in modern instrumental analysis, especially in the analysis and detection of volatile elements.