Microwave digestion
Microwave digestion
brief introduction
Microwave digestion usually refers to the use of microwave heating of the digestion vessel (different acids, partial lye, and salts) in the closed vessel and the sample to rapidly dissolve the various samples under high temperature pressurization conditions. (There are also open vessels. Microwave digestion, will not be discussed). The two characteristics of closed vessel reaction and microwave heating determine its advantages of completeness, rapidity, and low blankness, but inevitably brings the disadvantages of high pressure (potential for overpressure) and small amount of digested sample. High pressure (up to 100-150 bar), high temperature (usually 180-240°C), and strong acid vapors give the experimenter psychological pressure. Current commercial microwave digestion systems generally have temperature measurement/manometer or even temperature control/pressure control technologies, and therefore have a greater guarantee of safety. However, as potential users (or users), they should understand some of their characteristics, not only for the selection of microwave digestion systems, but also for safer use.

Smart integration
One-button smart integration: Automatically identifies the type, number, and location of the reaction tanks in the reaction chamber, and automatically detects the installation of the warm-pressure control system. Then, according to the characteristics of the sample and the equivalent, automatically retrieve the application method database, automatic energy optimization data matching calculation, the entire process of intelligent control without setting, but at the same time achieve the entire process of temperature, pressure, power adjustment curve display, 0-40 cans Target tracking real-time temperature status display.
Relief pressure
Whether or not safety can be released in a timely manner is related to safety. It is the most important. The general microwave digestion system controls the microwave heating when it detects that the temperature/pressure reaches the target temperature/pressure, but the actual digestion of the chemical reaction in the tank does not necessarily stop immediately, so it may cause overpressure and must be able to be safe in time. Relieve pressure. If there is no pressure relief method, the pressure resistance of the digester must be much larger than the maximum working pressure in order to ensure a sufficient safety factor. Currently, several microwave digestion tanks manufactured by major import microwave digestion manufacturers have adopted corresponding measures for passive pressure relief protection. Anton Parr (Austria) and Berghof (Germany) use metal explosion-proof membranes, which are used by PreeKem . The plastic explosion-proof membrane, Milestone (Italy) reaction tank for a single spring sheet pressure relief type, CEM (United States) uses the explosion-proof membrane and shrapnel pressure relief, Sineo (China) uses explosion-proof membrane and burst pressure relief.
Temperature measurement method
Temperature measurement methods can be divided into contact with the inside of the digester: contact (fiber temperature measurement), non-contact (infrared temperature measurement) and platinum resistance temperature measurement. Each of the three methods has advantages and disadvantages. The user can inspect the factors such as the accuracy of detection, the uniformity of sample detection, and the speed of detection. In the pressure measurement mode, it is divided into contact pressure measurement and non-contact pressure measurement.
Container material
General fluorine plastic TFM, PTFE, PFA and quartz. TFM belongs to modified PTFE. Melting point 320-340°C. In addition to all the advantages of PTFE, there are some noteworthy property improvements: smaller deformation at high temperature and high pressure, less permeability, better recovery of heavy pressure at high temperatures, and high surface finish. Although PFA is translucent, its maximum use temperature is not as good as that of PTFE. Therefore, TFM is the preferred material for digestion vessels. The PTFE digestion tank can only do medium and high pressure, while the TFM digestion tank can do ultra high pressure microwave digestion.
Container selection
The casing (sheath) and the intercalation tank can be separated. The one-piece can may be made of a single material, but it may be composed of one, two, or three kinds of materials, but these have been combined into one and cannot be disassembled. Combination tanks generally have shells, frames (intermediate layers), intercalated digesters, and some combined tanks do not use intermediate layers. Shell material is composite fiber (CEM patent), PEEK, high temperature ceramics, Ultem-100 (Ultem material has been basically eliminated). The framework mainly uses PFA and high-strength engineering plastics. The intercalation tank is directly in contact with the sample and the acid, and acid resistance, temperature resistance and high pressure performance are the primary considerations.
Comparing the advantages and disadvantages of the two: The combination tank can withstand higher pressure than the single digestion tank with a single material, and the interpolation tank can be removed and replaced (with a small cost). However, the combination tank needs to be disassembled and assembled, and the acid resistance of the housing is always inferior to fluorine plastic, and some housings are brittle at high temperatures. One-piece cans do not need to be disassembled, but the cost is large, heavy, and slow cooling. The single material integrated can only by TFM or PTFE, acid resistance, long service life. Fluoroplastics are tough materials that are not explosive.
There are four types of material used to intercalate digester tanks for intercalation digesters (or one digestion tank): PTFE, TFMTM PTFE, PFA, and quartz. The first three are fluorine plastic types. PTFE: Polytetrafluoroethylene, melting point 320-340°C; TFMTM PTFE (TFM for short): made of less than 1% of additives added by PTFE, belonging to modified PTFE. Melting point 320-340°C. In addition to all the advantages of PTFE, there are some noteworthy property improvements: smaller deformation at high temperature and high pressure, less permeability, better recovery of heavy pressure at high temperatures, and high surface finish; PFA: Perfluorinated Alkoxyethylene, translucent, melting point approx. 308°C, high surface finish; Quartz: very good thermal stability, high melting point, but brittle under high pressure and cannot be sampled using HF. Quartz digestion tank can not be used alone, it must be jacketed.
For the highest practical operating temperature, TFM is 260°C like PTFE. PFA is 10-20°C lower than the previous two. Although quartz can be used at high temperatures, it is limited by materials such as seals and brackets. The maximum temperature for quartz digestion vessels is typically 300°C. The maximum use temperature is usually achieved when using sulfuric acid, the usual digestion temperature is 170-220 °C.
Compared with the three fluorine plastics, TFM is undoubtedly the best material for digestion. Therefore, the currently imported microwave digestion system is a TFM material for the intercalated digestion tanks of high-pressure digestion tanks. PFA and PTFE are generally only used as medium and low pressure digestion vessels. Of course, if transparency is definitely required, PFA can only be used.
Sheath material
General composite fiber materials, PEEK, ceramics, Ulte

Highest temperature

The maximum working temperature cannot exceed the maximum use temperature of the digestion vessel material. Therefore, 260°C is generally the highest operating temperature of all fluoroplastic digesters. Although some company's products can reach 300 °C, but can not reach the maximum pressure at the same time, 300 °C will certainly have an impact on the service life of fluoroplastic digesters. The maximum working temperature of a quartz digestion vessel can be very high, but it is generally limited to 300°C because of its sheath or bracket. In general, most of the microwave digestion can be done at 160-220°C.

greatest pressure

The pressure in the digester is mainly determined by the organic content of the sample, the vapor pressure of the acid, and the temperature. The higher the organic matter content of the sample, the greater the pressure in the digester.

Digestion with acid or lye

Microwave digestion usually uses nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, and hydrogen peroxide. Sulfuric acid has the lowest volatility, and the maximum temperature is generally reached when sulfuric acid is used, but it is extremely easy to damage the digester after exceeding 210°C. Sometimes, for some samples (silicon-aluminum alloys), potassium hydroxide or sodium hydroxide solutions are often used for digestion.

Dissolved product quantity

Different kinds of digesters can digest the largest amount of different samples. The maximum amount of digestible sample is determined by the organic content of the sample, the type of acid, the set temperature, the volume of the digester, and the maximum working pressure of the digester. For microwave sealed digestion, the maximum amount of sample that can be digested is always the relative weakness of all microwave digestion systems.

Microwave oven

The microwave oven cavity is generally divided into the home kitchen remodeling type and the industrial grade professional type, all need the anti-corrosion coating, preferably is the PTFE or the PFA coating. Industrial-grade professional furnace cavities are relatively safer and prices are relatively expensive; retrofit microwave ovens are relatively inexpensive and basic performance can also be satisfied.

Industry application

The cost of elemental analysis method conversion in the pharmaceutical industry is enormous, including instrument procurement, microwave digestion methods and ICP method development, test item determination, training, and documentation. It is easy to digest some samples, but it is not as easy as some raw material samples containing biomolecules. CEM's goal is to allow technicians to quickly and safely assemble digestion tanks, add appropriate amounts of sample and acid reagents, and place them into microwave digestion chambers to complete the digestion process. Jason Keith, Product Manager for Analytical Chemistry Product Lines at CEM, said: "CEM's extensive in-built training courses and pre-programmed methods help achieve this goal."[1]
Can be used for industrial, agricultural, environmental protection, health inspection, metallurgy, geology, medicine, chemical and other departments of various samples of the digestion; especially suitable for use atomic spectroscopy such as atomic absorption spectrometry, ICP-emission spectrometry, atomic fluorescence spectrometry, ICP-MS The accurate determination of trace, trace, and ultra-trace elements in various samples by instruments, as well as anodic dissolution apparatus. APL's microwave digestion instruments are widely used in these fields!
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