Microwave digestion usually refers to the acid dissolution of various samples by using microwave rapid heating in a closed container (there is also microwave digestion in an open container, which will not be discussed here). The two characteristics of closed container reaction and microwave heating determine the advantages of complete/fast/low blank, but inevitably bring about high pressure (possible hidden danger of overpressure) and small amount of digested sample.
High pressure (up to 100-150bar)/high temperature (usually 170-220°C) or strong acid vapor brings psychological pressure to the laboratory in terms of safety. The current commodity microwave digestion system generally has temperature measurement or pressure measurement or even temperature control or pressure control technology.
Therefore, the safety has been guaranteed, but as a potential user (or user), you should understand some of its characteristics, not only for choosing a microwave digestion system, but also for safer use.
Ⅰ. What are the application fields of microwave digestion instrument?
Microwave digestion has been widely used in food, textile, plastic, geology, metallurgy, coal, biomedicine, petrochemical, environmental monitoring, sewage treatment, battery manufacturing, cosmetics and other fields.
Ⅱ. What are the advantages of microwave digestion?
Microwave digestion, as an efficient sample pretreatment method, can well meet the requirements of modern instrument analysis for sample pretreatment process. It has the advantages of fast heating speed, uniform heating, less reagent consumption, low blank, energy saving and high efficiency. Especially in the analysis and detection of volatile elements, the integrity of the sample can be well maintained, and the sample recovery rate is very high.
Ⅲ. What are the temperature control methods of the microwave digestion instrument? What are the characteristics of each?
At present, the microwave digestion temperature control methods in the market include infrared temperature control, thermocouple temperature control, platinum resistance temperature control, optical fiber temperature control and other temperature control methods. The working method of infrared temperature control is to scan and monitor temperature infrared data at a certain distance. It is a non-contact temperature control, so its accuracy is poor and the temperature control accuracy is not high.
The thermocouple controls the temperature by testing the relative temperature through the potential difference between the hot and cold ends. Since it is easy to cause the antenna effect to interfere with the uniformity of the microwave field, it is easy to generate electric sparks and cause safety accidents. And there is a self-heating effect under the microwave field, that is, the actual temperature in the tank cannot be measured.
Platinum resistance temperature control utilizes changes to affect the absolute conductivity of the free electron beam in the platinum conductor. The thermodynamic temperature is tested through impedance changes, and the output signal responds with high precision. But there will also be an antenna effect, which is easy to generate electric sparks and cause safety accidents.
Optical fiber temperature control adopts direct optical fiber temperature measurement method, which is not affected by the microwave field, can provide high-precision measurement, has timely information feedback, accurate temperature control, and no potential safety hazards. It is currently the most ideal microwave digestion temperature control method.
Ⅳ. What active safety measures should microwave digestion have?
The high-precision temperature and pressure control system is adopted, and the operator can understand the machine’s travel situation by observing the data and curves of temperature and pressure changes. Its software module can actively stop the operation when the slope is out of control, which greatly reduces the possibility of explosion.
Equipped with a real-time temperature and pressure abnormal monitoring system, when the high-precision temperature and pressure control system fails, the system will sense and stop operation in time as a backup measure to ensure safety.
Select high-strength and high-temperature-resistant container materials.
Ⅴ. How to ensure personal safety when active safety measures fail?
Pressure tank safety relief:
Assuming that the control fails, when the pressure is close to the design pressure limit, the pressure tank can release the overpressure to ensure safety.
Vertical Orientation Explosion Protection:
The latest high-pressure vessel structure design is based on the three-dimensional directional explosion-proof theory. Even if the pressure relief measures fail and the tank explodes, the super-strong aerospace shell material can limit the vertical release of shock waves, absolutely ensuring the safety of personnel in the lateral direction.
High-strength explosion-proof safety door:
Another passive safety protection method provided by the three-dimensional directional explosion-proof mechanism, its super-sensing shock wave automatic explosion-proof door can automatically pop up in parallel to release the lateral impact pressure in advance when danger arises, and the multi-layer hard steel door structure provides sufficient strength to ensure personal safety.
Ⅵ. How does the microwave digestion instrument prevent microwave leakage?
The main body should adopt a rectangular industrial resonant cavity enclosed by a metal wall;
The furnace door is equipped with triple independent interlocking sensing equipment. When the furnace door is opened, the power supply is cut off. The microwave device cannot work if the furnace door is not closed.
Ⅶ. What are the advantages of non-pulse frequency conversion microwave control technology?
According to the power transmission mode, microwaves are divided into pulsed microwaves and non-pulsed microwaves. The traditional fixed power output is characterized by switch-type pulsed microwaves. This control method is not only difficult to control, but may also directly affect the digestion effect. The current development direction of microwave is automatic power frequency conversion control and non-pulse technology. Its feature is that the power changes automatically, and the output is non-pulse microwave. Its advantage is that there is no need to turn off the microwave emission. Under the condition of continuous microwave emission, the microwave power output is automatically linearly changed according to the temperature and pressure feedback signal, the reaction state is adjusted, the temperature control is more accurate, and the experiment is ensured to be carried out safely and smoothly.
Ⅷ. What material is generally used for microwave digestion vessels?
The materials commonly used in high-end microwave digestion instruments on the market are:
The inner liner is made of TFM, the outer sleeve is made of aerospace composite material, the vessel cover is made of PFA, and the vessel frame is made of PEEK.
Ⅸ. What substances cannot be digested with microwaves? What are the precautions?
Explosives (TNT, nitroglycerin, etc.), propellants, pyrotechnics, perchlorates, glycols (ethylene glycol, propylene glycol, etc.), aviation fuels, acetylene compounds, various ethers, ketones, short carbon chains alkanes etc.
Note: Do not digest phenol, triethylamine and animal fat with nitric acid.
Ⅹ. What are the environmental requirements for microwave digestion?
The microwave digestion instrument should be placed on a firm and stable table. The top of the furnace body and the left and right sides must not be covered and there must be a gap of more than 5 cm. There should be more than 10 cm of space on the rear wall to keep it well ventilated.
Keep away from heating sources to avoid hot air and water vapor entering the microwave oven and cause malfunctions, and keep away from tap water sources to avoid the risk of electric leakage due to splashing water.
Do not get close to strong magnetic materials or electrical appliances with magnetism, because the external magnetic field will interfere with the uniform distribution of the magnetic field in the furnace and reduce the heating efficiency.
Other normal laboratory temperature and water and electricity conditions.
