Microwave digestion usually refers to the wet digestion that uses microwaves to heat the digestion solution (various acids, some lyes, and salts) and samples in a closed container, and dissolves various samples quickly under high temperature and pressurized conditions.
At present, microwave digestion technology is widely used in the digestion of various samples in various inspection institutions such as food, medicine, environmental protection, feed, fertilizer, health inspection, geology, chemical industry, etc., Especially suitable for accurate determination of trace, trace and ultra-trace elements in various samples by atomic absorption, ICP-emission spectrometer, atomic fluorescence, ICP-MS, etc.
Microwave Digestion Features
Microwave digestion technology has become increasingly mature. Microwave digestion has achieved the characteristics of convenience, speed, completeness, low blank space and simple operation. However, for novices, they are often worried about the potential safety hazards of microwave digestion. The high pressure in the microwave digestion process is the highest. It can reach 100-150bar and the temperature usually reaches 180-240℃. At the same time, accompanied by strong acid vapor, under the triple pressure of “high temperature, high pressure, and strong acid”, it is inevitable for experiment operators to have safety concerns. In order to standardize the operation process, ensure the normal, safe and effective use of the instrument, and ensure that the microwave-treated samples meet the laboratory management requirements, the user must be fully proficient in the microwave digestion operation process and safety precautions.
Operation process and precautions
1. Preparation before the test:
Check that the instrument is operating normally. Check that the rotor is clean and the container has been cleaned.
2. Weighing:
Accurately weigh the prepared sample with a ten-thousandth balance, the general sample weight is 0.05-0.5g, (determine the sample weight according to the test situation). Principles and precautions for weighing samples:
① Ensure that no sample adheres to the inner tube and seal or O-ring;
② The weight of the unknown sample in the initial test is controlled within 0.1g, and the safety of microwave digestion must be guaranteed for the digested sample;
③For the samples with violent reaction, control the sample weight within 0.1g.
3. Add digestion reagents:
Place the digestion tank in a fume hood, add digestion reagents, and mix well. It is recommended that the total reagent volume be at least 6mL, and at the same time, ensure that the solvent volume and solvent (acid) type in each digestion tank are consistent. For organic samples with an amount greater than 0.2g, it is best to preheat them in a heating device (such as LABTECH’s VB24) at low temperature for more than half an hour to avoid excessive reactions. Pay attention to rinse the sample stuck to the tube wall to the bottom of the tube as much as possible.
4. Inspection of the sealed upper cover:
Check the seal for damage and replace it if damaged. Use a flarer to expand the opening and seal it. Then, put the reaction tube cap on the reaction tube, put the reaction tube into the ceramic outer sleeve, and tighten the top cover and the venting screw by hand. Place the top cap on the reaction tube with the vent screw seated in the recess of the protective sheath. Put the protective cover on the top cover, make sure the distance between the protective case and the protective cover is less than 2mm. Be careful not to apply excessive force to prevent damage to the threads. Press the sealing cap fully onto the sealer for at least 3 seconds. The new seal should be pressed for more than 10 seconds. In order to ensure effective sealing, please start the experiment within 15 minutes after flaring, otherwise reflaring is required.
5. Installation of reaction tank:
Make sure all parts are dry and free from acid residue. Put the inner tube into the outer tube and screw the screw cap clockwise to tighten. Place the rotor on the turntable in the oven chamber and gently move it to a fixed position. After the main control bottle is inserted into the temperature sensor, it is placed in the middle of the stand and connected. Verify that the temperature sensor is not twisted. In order to ensure that the rotor can rotate stably, it is recommended that the sample vials be placed symmetrically when digesting a small number of vial samples. For 41 rotors, it is recommended to put more than 4 samples. When the number of tanks is less than the above number, blanks can be used to fill the position, clean the digestion tubes and seal them, so as to ensure the corresponding relationship between the number of reaction tubes and the power.
6. Cover the protective cover:
Put the protective cover on the rotor, align with the positioning hole, screw the protective cover clockwise, and the bayonet is locked.
7. Selection method:
Turn on the power and enter the main program interface. According to the sample, start writing the program. For the sample digestion procedure, refer to the method provided, as in the following example: only useful procedures are written. The 41-position rotor is suitable for environmental samples, such as soil digestion, organic samples (such as animal and plant tissues, etc.), such as biological samples, 0.5g (maximum) plus 5mL nitric acid 1mL, H2O2 (maximum use 2mL), for the initial time and For power setting, we recommend setting according to the number of samples, for example, less than 8:
| Step | Time | Power | Tempreture1 | Tempreture2 |
| 1 | 00:10:00 | 1000 | 120℃ | 120℃ |
| 2 | 00:10:00 | 1000 | 170℃ | 120℃ |
| 3 | 00:20:00 | 1000 | 170℃ | 120℃ |
| Vent cooling | 30 min |
After editing, press START to start the program. Regarding the setting of power and time for a large number of samples: under normal circumstances, the number of containers is 20, and the temperature can be raised to 180 degrees in 20 minutes (5mL nitric acid 1500W maximum power). Under the same conditions, it may take 30 minutes to fill up 41 containers Climb up to 180 degrees.
8. Pressure relief at the end of digestion:
After digestion, open the door, transfer the rotor to the fume hood, and slowly loosen the air release screw. Note: align the air outlet with the pressure relief baffle. Wait for a while until the acid gas is exhausted, remove the top cover and put the inner tube and ceramic outer tube into the corresponding tube rack. Generally, the lid is opened when the temperature is below 45°C, and acid distillation or constant volume is carried out in order to be suitable for analysis.
9. Clean the top cover, reaction inner tube and constant volume:
①Open the digestion tank, press the side of the screw cap, and remove the screw cap. Put the inner tube and pressure sleeve into the corresponding tube holder.
②Rinse the inside of the sealing cap with a small amount of distilled water for 2 to 3 times, and collect the solution into the digestion tube.
③ Rinse the sealing cap with a small amount of distilled water for 2 to 3 times and pour it into the digestion solution.
④ Place the rinsed sealed cap in a clean place to dry.
⑤ Make constant volume to the corresponding digestion products.
10. Cleaning of sealing cap and digestion inner tube:
①After rinsing the inner tube of digestion with distilled water, you can soak it in acid overnight to remove the residue, or add 6mL HNO3 to run the cleaning procedure.
② The “O” ring on the inner tube should be removed during acid removal or cleaning.
Summary
To sum up, the use of microwave digestion must be strictly in accordance with the regulations. It is strictly forbidden to study the digestion process under different reagents or different types. Any non-standard operation will have potential safety hazards.
How to choose the correct microwave digestion instrument
At present, there are many brands of microwave digestion instruments on the market, how to correctly choose a suitable microwave digestion instrument is an important problem faced by laboratory staff and purchasers. According to the author’s experience, the correct choice of microwave digestion instrument mainly considers safety, accuracy, failure rate and cost of consumables.
Safety
The safety protection measures of microwave digestion instruments are generally divided into active and passive methods.
Active safety protection – take measures to eliminate hidden dangers before danger comes;
Passive safety protection – measures that can reduce losses after danger arrives.
Accuracy
At present, most of the microwave digestion instruments on the market adopt the dual control mode of temperature and pressure, and the mode of independent control of temperature or pressure is gradually eliminated by the market. The higher the temperature, the higher the sample reaction speed and the degree of digestion; the higher the pressure, the higher the pressure resistance requirements of the digestion tank, and the higher the relative risk. Therefore, the authenticity and accuracy of the data read by the temperature sensor and the pressure sensor are very important.
Failure rate
1. Word of mouth
Understand the word-of-mouth effect of manufacturers from the same industry, including machine quality and maintenance service capabilities.
2. Inspection
If conditions permit, you can visit the production workshop of the instrument manufacturer or the existing users of the manufacturer; you can also visit the industry exhibition to experience and try the instrument in the exhibition.