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.
Overview
In order to prevent the interference of civil microwave power to radio communication, radio, television and radar, the frequency of civil microwaves for industrial, scientific research, medical and household use is 2450 MHz. Therefore, the frequency used by the internationally accepted microwave digestion apparatus is basically 2450 MHz, and the industrial microwave digestion apparatus and the household microwave oven are basically within this Hertz range.
First, let's take a look at the characteristics of microwaves so that we can understand what substances absorb microwaves. What substances reflect microwaves? What substances have no effect on microwaves?
(1) Substances that absorb microwaves?
A substance of a polar molecule absorbs microwaves (substances that are large in loss factor), such as water, acid, and the like. Their molecules have a permanent dipole moment (ie, the centers of the positive and negative charges of the molecules do not coincide). Polar molecules rapidly change orientation in the microwave field with the frequency of the microwave, and rotate back and forth, causing the molecules to collide with each other and absorb the energy of the microwave to increase the temperature. The food we eat contains water, which is a strong polar molecule and can therefore be heated in a microwave oven.
(2) Substances that reflect microwaves
Metal materials do not absorb microwaves and can only reflect microwaves. Such as copper, iron, aluminum and so on. A metal (stainless steel plate) is used as a furnace for the microwave oven to reflect back and forth on the heating material. Metal containers cannot be placed in the microwave oven, and the reflected microwaves are detrimental to the magnetron.
(3) Substances that penetrate microwaves
The insulator can pass through the microwave, which absorbs almost no energy from the microwave. Such as glass, ceramics, plastic (polyethylene, polystyrene), polytetrafluoroethylene, quartz, paper, etc., they are transparent to microwaves, and microwaves can penetrate them forward. None of these substances absorb the energy of the microwaves or absorb very little microwaves. The strength of the microwave absorbed by the substance is essentially related to the complex permittivity of the substance, that is, the larger the loss factor, the stronger the ability to absorb microwaves [2]. Household microwave oven containers are mostly plastic products. The material used for the microwave sealed digestion solution tank is polytetrafluoroethylene, engineering plastics and the like.
working principle
A sample of 0.2 g to 1.0 g was weighed and placed in a digestion tank, and about 2 mL of water was added thereto, and an appropriate amount of acid was added. Usually, nitric acid, hydrochloric acid, hydrofluoric acid, hydrogen peroxide, etc. are used, and the can is covered and placed in a 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.
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.
progress
At present, microwave digestion instrument manufacturers have been extended to dozens of companies by APL Opler, Shanghai Hao, CEM, Milestone, and the comprehensive strength of each enterprise is quite different. How to choose a suitable microwave digestion instrument?
First of all, according to the type of test sample, which microwave digestion instrument is selected? According to the tolerance range of the microwave digestion instrument, it is generally divided into ultra-high pressure, high pressure, medium pressure microwave digestion instrument. Generally difficult to digest samples, such as plastics, paints, etc., must choose ultra-high pressure microwave digestion instrument, otherwise there will be incomplete digestion or damage to the instrument.
The second is selected according to the number of samples per batch, batch processing 1-6, 1-8, 1-12, 1-20, 1-40. Prices also range from 50,000 to 250,000. At present, high-throughput microwave digestion devices with more than 20 bits have been widely recognized.
The third is according to the temperature monitoring control mode, which is divided into the main control tank mode and the full tank temperature monitoring mode. At present, the high-throughput microwave digestion instrument adopts the full tank monitoring mode, and only the microwave digestion apparatus with partial batch processing below 10 uses the main Control tank mode.
Application field
Microwave digestion equipment is mainly used for the detection of heavy metals and harmful elements such as chromium, cadmium, mercury, arsenic and lead.
The main application areas are:
(1) Environment, including the atmosphere, soil, water, etc.
(2) Food, including dairy products, grain and oil, beverages, tea, etc.
(3) Other fields, such as paints, plastics, toys, etc.
Sample digestion
1. Food sample digestion:
(1) Move the digestion device into the kitchen and heat it to 120 °C.
(2) Liquid sample (such as white wine, purified water, natural water, etc.) draw 1-10.0ml (sample into the digestive tract, add three glass beads), put the digestive tube into the digestion apparatus and heat it to near dry at 120 °C, add 1.0 Ml (digestion solution 240 ° C constant digestion to near dry, take off and let cool, determine the volume to determine the volume).
(3) General foods (such as color wine, beverages, fruit sugar, granules, noodles, porridge, etc.) Weigh 0.500-1.000g samples into the digestive tract, add three glass beads, add 10.0ml digestive juice, digest The tube is placed in a digestion apparatus at 160 ° C for about 1-3 h) until the digestive juice is clarified, and the temperature is digested to completion at 240 ° C, and it is taken to a cold, and the volume is measured to a certain volume.
(4) Foods containing a large amount of oil and sediment (such as meat, sausage, soybean oil, milk powder, toffee, chocolate, etc.) Weigh 0.300-0.500g sample into the digestive tract, add three glass beads, add 10ml of digestive juice, The digestive tube is placed in a digestion apparatus and digested at 160 ° C for 3 h. If the solution has been clarified at 240 ° C, the temperature is digested to near dryness, and the cooling is taken off, and the volume is adjusted to a certain volume for measurement.
2. Water sample digestion:
1.00-10.0ml sample into the digestive tract, add three glass beads, put the digestive tube into the digestion apparatus and heat it to near dry at 120 °C, add digestive agent (clean water added 0.50ml digestive agent, sewage added 2ml digestive agent) 240 °C Constant temperature digestion to near dry take off and cool, constant volume for a certain volume.
3, sample digestion:
Weigh 0.200g sample into the digestive tract and add three glass beads to 5.0ml of digestive juice on the digestion instrument for digestion at 160 ° C for 30min. 240 ° C constant digestion to near dry, take off and let cool, constant volume to a certain volume for determination.
4, cosmetics digestion:
Take 0.100-0.200g of cosmetics into the digestive tract, add 3 glass beads to add 5-10ml digestive juice, digest at 1200h for 1-2h until the solution is clarified at 240 °C and digest to near dryness, remove and let cool, and make a certain volume for determination. [2]
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