Some problems that are necessary to understand whe

2022-10-19
  • Detail

Some problems that need to be understood when choosing a microwave digestion system

microwave digestion usually refers to the acid dissolution of various samples by rapid microwave heating in closed containers (there are also open containers for microwave digestion, which will not be discussed)

the two characteristics of closed container reaction and microwave heating determine its advantages of complete, fast and low blank, but inevitably bring the disadvantages of high pressure (the potential danger of overpressure) and small amount of digested samples. High pressure (up to bar), high temperature (usually ℃), and strong acid vapor have brought psychological pressure to the experimenter in terms of safety

nowadays, microwave digestion systems for commodities generally have temperature/pressure measurement or even temperature/pressure control technology, so they have a great guarantee in safety. However, as a potential user (or user), we should understand some of its characteristics, not only for choosing microwave digestion system, but also for safer use. The points listed below are basically sorted by importance. Please supplement and correct the deficiencies

I. overpressure relief:

whether the pressure can be relieved in time and safely is related to safety, which is the most important. Generally, the control mode of microwave digestion system is to stop microwave heating when it detects that the temperature/pressure reaches the target temperature/pressure, but the chemical reaction in the actual digestion tank does not necessarily stop immediately, so it may cause overpressure, so it must be able to safely relieve the pressure in time. If there is no pressure relief method, the pressure resistance of the digestion tank must be much greater than the maximum working pressure to ensure sufficient safety factor

II. Temperature measurement/pressure measurement method:

it can be divided into contact type and non-contact type based on whether it is in contact with the interior of the digestion tank

if only a temperature/pressure sensor is inserted into one digestion tank, the temperature and pressure of other digestion tanks cannot be controlled, and there is an inevitable hidden danger of exceeding the target temperature/pressure

the best way is to measure the temperature/pressure of all digestion tanks non-contact. Once any digestion tank reaches the set temperature/pressure, it will stop heating. Because although rotary heating, and even some manufacturers carry out "magnetic stirring", the non-uniformity of microwave heating still exists to a certain extent. Therefore, even if the samples are the same, and the sample weight and acid dosage are completely the same, the temperature/pressure in the digestion tank after microwave heating cannot be guaranteed to be completely the same

III. material of digestion container:

generally there are fluoroplastic TFM, PTFE, PFA and quartz. The maximum service temperature of TFM is not lower than that of PTFE, and its surface finish, permeability resistance under high temperature and high pressure, and deformation resistance under high temperature and high pressure are better than that of PTFE. Although PFA is translucent, its maximum service temperature is not as good as PTFE. Therefore, TFM is the preferred material for digestion containers. The digestion container of domestic manufacturers' press that rusts slightly will affect the efficiency of operation is generally PTFE, which can only be used as medium and low pressure digestion container

IV. whether the integrated tank:

the integrated tank does not need to be disassembled and disassembled, which is convenient to use. It is better to use pure TFM digestion tank, but the working pressure is also limited. At present, digestion tanks with working pressure up to 100bar have jackets or reinforcing rings

v. sheath material:

generally peek, ceramic, ultem-100. Its strength is much higher than that of TFM, but its acid corrosion resistance cannot be compared with that of TFM, PTFE, PFA and other fluoroplastics

VI. maximum working temperature of digestion tank:

the maximum working temperature cannot exceed the maximum operating temperature of digestion container material. Therefore, 260 ℃ is generally the maximum operating temperature of all fluoroplastic digesters. Some manufacturers claim that they can reach 300 ℃, but they cannot reach the maximum pressure at the same time. At 300 ℃, the service life of fluoroplastic digestion containers must be affected. The maximum working temperature of quartz digestion container can be very high, but it can only reach 300 ℃ generally due to its sheath or bracket

generally, most microwave digestion can be completed at 170-220 ℃

VII. Maximum working pressure:

the pressure in the digestion tank mainly depends on the content of organic matter in the sample, the vapor pressure and temperature of acid. The higher the content of organic matter in the sample, the greater the pressure in the digestion tank. Nitric acid, hydrochloric acid, hydrofluoric acid and sulfuric acid are usually used. Sulfuric acid is the least volatile, and the maximum temperature is generally only possible when sulfuric acid is used

VIII. Maximum digestible sample quantity:

the maximum digestible sample quantity of different types of digesters is different. The maximum digestible sample quantity depends on the organic matter content of the sample, the type of acid, the set temperature, the volume of the digestion tank, and the maximum working pressure of the digestion tank

if the sample, acid and set temperature are the same, the "100bar, 100ml" digestion tank claims that the original product can be significantly improved in structure, material and other aspects, resulting in changes in performance and technical characteristics of the new measuring appliance to digest 1000mg pure organic matter samples (such as rice flour, wheat flour), Then the digestion tank of "40bar, 100ml" can digest up to 400mg pure organic matter samples (such as rice flour and wheat flour). PV = but generally it is difficult to form NRT

for microwave closed digestion, the relatively small amount of samples that can be digested is always the relative weakness of all microwave digestion systems

IX. microwave oven:

it is divided into household kitchen modified type and professional type. All need anti-corrosion coating, preferably PTFE or PFA coating

Copyright © 2011 JIN SHI