Digestion instrument is a commonly used sample pretreatment equipment. It can be divided into semi-automatic digestion instrument and automatic digestion instrument according to the degree of automation. According to the principle, it can be divided into electric heating digestion instrument and microwave digestion instrument. Currently, there are relatively mature products, and the application is very widely. In terms of digestion, microwave-enhanced chemical technology has a fast digestion rate, and the sample of a furnace is 10 to 100 times faster than the conventional electrothermal plate method. The digestion effect is good, and the high-pressure sealed can is used for microwave heating, and the sample is thoroughly dissolved. In particular, the sample is digested in a closed digestion tank, greatly reducing the loss of volatile elements. Therefore, making the analysis results more accurate
Application field
It can be applied to sample preparation work such as digestion, extraction, protein hydrolysis and other analytical chemistry. In addition, microwave organic synthesis will replace traditional synthesis methods with its absolute application advantages. Such as atomic absorption spectrometry atomic fluorescence spectrometer, inductively coupled plasma emission spectrometer inductively coupled plasma mass spectrometer high performance liquid chromatography, gas chromatograph and other analytical instrument sample preparation, more and more laboratories using microwave sample processing The system replaces time-consuming, laborious, and polluting methods.
Principle of electrothermal digestion instrument
The electric heating digestion device comprises a heating body, and the heating body is connected with the alternating current power source to form a heating circuit by heating the main circuit, and the heating main circuit comprises an air circuit breaker, a single-phase full-bridge rectifying and filtering unit and a Buck main circuit which are sequentially connected in series; The temperature sensor on the heating body, the temperature sensor is respectively connected with the single chip microcomputer and the PWM control driving unit through the filter amplifying unit, the single chip microcomputer and the PWM control driving unit, and the PWM control driving unit is connected with the Buck main circuit through the isolated driving unit; the PWM control driving unit The current in the heating circuit is sampled by the sampling resistor; the output of the single-chip microcomputer is connected with an output device for displaying the temperature, and the input terminal is connected with the input device with the parameter setting [1].
The electrothermal digestion device adopting the above structure adopts a power switch tube to realize precise adjustment of output power, and then can accurately control the heating process.
Microwave digestion principle
A sample of 0.2 g to 1.0 g was weighed and placed in a digestion tank, and about 2 mI of water was added thereto, and an appropriate amount of acid was added. Usually, HNO3, HCI, HF, H2O2, etc. are used, and the can is covered and placed in the furnace. When the microwave passes through the sample, the polar molecules rapidly change orientation with the microwave frequency. The microwave of 2450MHz changes the direction of the molecule 2.45×109 times per second. The molecules rotate back and forth, colliding with the surrounding molecules, and the total energy of the molecules increases. The sample temperature rose sharply. At the same time, the charged particles (ion, hydrated ions, etc.) in the test solution migrate back and forth in the alternating electromagnetic field by the action of the electric field force, and also collide with the adjacent molecules, so that the temperature of the sample rises. This heating method is absolutely different from the traditional electric furnace heating method.
Digestion instrument
Digestion instrument
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(1) Body heating. When the electric furnace is heated, energy is transmitted by heat radiation, convection and heat conduction, and heat is transmitted from the outside to the inside through the wall of the vessel, and the heat is heated to conduct the test. Microwave heating is a direct way of body heating, and microwaves can penetrate into the interior of the test solution.
At different depths of the sample, the microwaves simultaneously produce a thermal effect, which not only makes the heating faster, but also more uniform. The heating time is greatly shortened, which is faster and more efficient than the conventional heating method. For example, oxides or sulfides can be heated to several hundred degrees Celsius in 1 minute under the action of microwave (2450MHz, 800W). Another example is Mn02 1.5 g in 650W microwave heating for 1min can be heated to 920K, showing that the rate of temperature rise is very fast. The traditional heating method (heat radiation, conduction and convection) has a low utilization of heat energy, and many of the heat is radiated to the surrounding environment, and microwave heating directly acts inside the material, thereby improving energy utilization.
(2) Overheating. Microwave heating can also cause overheating (ie, more than the boiling temperature)
high). When the electric furnace is heated, heat is transmitted from the outer to the inner wall to the sample, and bubbles are easily formed on the surface of the wall, so that the superheat phenomenon is not easily generated, and the temperature is maintained at the boiling point because the gasification absorbs a large amount of heat. In the microwave field, the "heating" method is completely different, and the energy is directly converted inside the system. Due to the lack of a "core" in the interior of the system, it is easy for some low-boiling reagents to be overheated in a closed container. It can be seen that the reagents in the closed sample tank can provide higher temperatures. Conducive to the digestion of the sample.
(3) Stirring. Because the reagent and the polar molecules of the sample are rapidly oriented in the 2450MHz electromagnetic field with the changing electromagnetic field, the molecules collide with each other, which is equivalent to the surface of the reagent and the sample are constantly updated, and the surface of the sample is constantly in contact with the new reagent. Promote the chemical reaction between the reagent and the sample to accelerate. The alternating electromagnetic field is equivalent to a high-speed agitator, which is stirred 2.45×109 times per second, which increases the rate of chemical reaction and accelerates the digestion rate. As a result, microwave heating is fast, uniform, and overheated, constantly creating new contact surfaces. Sometimes the reaction activation energy can be reduced, the reaction kinetics can be changed, the microwave digestion ability can be enhanced, and many samples which are difficult to be digested by conventional methods can be eliminated.
As can be seen from the above discussion, the speed of heating and the speed of digestion are related not only to the power of the microwave, but also to the composition and concentration of the sample and the type and amount of the reagent used. To digest a sample in a short period of time, you should choose the right acid, the right microwave power and time.
characteristic
Compared with ordinary electric furnace heating, the microwave digestion instrument not only has a fast heating speed, but also has the following two unique functions.
(1) Body heating function
When the electric furnace is heated, energy is transmitted by heat radiation, convection and heat conduction, and heat is transmitted from the outside to the inside through the wall of the vessel, and the heat is heated to conduct the test. Microwave heating is a direct way of body heating. Microwaves can penetrate into the interior of the test solution. At different depths of the sample, the microwaves simultaneously produce thermal effects, which not only makes the heating faster, but also more uniform. The heating time is greatly shortened, which is faster and more efficient than the conventional heating method. For example, oxides or sulfides can be heated to a temperature of several hundred degrees Celsius in 1 minute under the action of microwave (2450MHz, 800W). Another example is 1.5 g of manganese dioxide heated in 650 W microwave for 1 min, which can be heated to 920 K. It can be seen that the rate of temperature rise is very fast. The traditional heating method (heat radiation, conduction and convection) has a low utilization of heat energy, and many of the heat is radiated to the surrounding environment, and microwave heating directly acts inside the material, thereby improving energy utilization.
(2) There is overheating
Microwave heating can also cause overheating (ie, higher than the boiling temperature). When the electric furnace is heated, heat is transmitted from the outer to the inner wall to the sample, and bubbles are easily formed on the surface of the wall, so that the superheat phenomenon is not easily generated, and the temperature is maintained at the boiling point because the gasification absorbs a large amount of heat. In the microwave field, the "heating" method is completely different, and the energy is directly converted inside the system. Due to the lack of a "core" in the interior of the system, some low-boiling reagents are prone to overheating in a closed container. It can be seen that the reagents in the closed sample tank can provide higher temperatures. Conducive to the digestion of the sample.
As can be seen from the above discussion, the speed of heating and the speed of digestion are related not only to the power of the microwave, but also to the composition and concentration of the sample and the type and amount of the reagent used. To digest a sample in a short period of time, you should choose the right acid, the right microwave power and time.
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