![<?echo $_SERVER['SERVER_NAME'];?>](/template/twentyseventeen/skin/images/header.jpg)
Professor Jin Qinhan, Department of Chemistry, Jilin University
Foreword: The international advanced analytical laboratories attach great importance to sample preparation, and sampling and sample pre-processing inputs account for about 67% of all analysis and data processing inputs. The sample preparation equipment should be a separate working system, not an accessory to the analytical instrument. Because the performance of analytical instruments such as ICP spectrometers is good, it mainly affects the analysis results. The pre-treatment of the sample will not only affect the analysis results, but also affect the work efficiency and the safety of the operation. Therefore, you should be very careful when choosing. In view of the wide variety of microwave sample processing instruments on the market, there are many kinds of microwave sample processing instruments. In order to facilitate the general users to purchase the corresponding instruments, this paper only makes some necessary research on the imported microwave sample processing instruments.
1. High efficiency, element recovery and safety are the three main principles for evaluating microwave sample processing instruments.
Microwave sample processing equipment is now available for digestion/extraction/drying/concentration/Kjeldahl/proteolysis/synthesis. However, it is generally used for digestion and extraction of organic/inorganic analytical samples. The basic working methods are closed, open and continuous. The emergence and popularization of microwave chemical instruments is due to their ability to greatly speed up the chemical reaction rate, which can greatly improve the efficiency of sample preparation. Microwave sample pretreatment equipment is a very important experimental equipment, which can improve the efficiency of sample preparation by tens or hundreds of times. Not only that, it also affects the accuracy of the final analysis and critical laboratory safety management. From a safety point of view, the use of high-pressure digestion in a closed type can greatly improve efficiency, but high pressure also poses a potential danger. It must be clear that efficiency gains cannot be at the expense of safety and precision. Laboratory safety and sample element recovery are prerequisites for increased efficiency. Therefore, our goal should be to ensure safety and ensure element recovery, on the basis of which to improve operational efficiency. Some manufacturers use the closed tank automatic process to eliminate the side deflation mechanism for safety purposes. Although this can avoid accidents such as explosions, the high temperature and high pressure will cause serious element loss, which will affect the consistency of sample digestion. The recovery rate of the elements. In terms of safety, users should strictly check whether the instrument meets the safety radiation and explosion-proof standards of the United States, Canada, the European Community and China's relevant industrial electrical appliances. In short, the overall performance of the instrument should be comprehensively evaluated based on the three factors of high efficiency, elemental recovery rate and laboratory safety.
1) High efficiency: refers to the overall efficiency of the microwave processing method compared to the conventional sample preparation method in terms of operation and reaction time.
2) Element recovery rate: refers to the ability of the digestion process mechanism to ensure that the element to be tested is completely retained without loss.
3) Safety: Refers to the radiation hazard and the ability to prevent accidents caused by high voltage during the operation and reaction of microwave processing equipment.
The market for microwave sample processing equipment is currently confusing. It is difficult for users to distinguish the difference in technical content between various brand instruments. It is often found that the instruments purchased by the heavy money may be very simple. Therefore, users should not only focus on analytical instruments such as ICP/AAS when purchasing instruments, but also increase the importance of sample pretreatment instruments. Analytical instrument manufacturers should encourage users to freely choose the safest sample preparation instrument for their application needs. For example, sometimes the automated release digestion system is more suitable for large sample volume laboratory applications than the high pressure closed digestion system, and is safer. Help the market to establish a responsible and responsible scientific attitude, allowing users to conduct an objective and prudent comprehensive evaluation based on the three principles of high efficiency, element recovery and safety of microwave chemical sample pretreatment instruments. The actual analytical chemistry level is generally improved by increasing the sample preparation level.
2. Basic functions and methods of microwave sample pretreatment instruments
There are twenty or thirty kinds of professional microwave chemical products, which are mainly divided into continuous flow sample processing series, high temperature sample processing series, open sample processing series and closed sample processing series. For example, the large Spectroprep continuous flow sample processing system used by the US Environmental Protection Agency can process 180 samples per hour and can process thousands of samples per day. The robot automatically operates from the label/injection/stirring/reagent addition/digestion/extraction/filtration/ Cooling / constant volume and other processes, but the unit price is up to 100,000 US dollars. The following are just some of the features of open and closed:
1) Open microwave sample pretreatment instrument
At present, atmospheric pressure open digestion technology is in the ascendant. Open type refers to the treatment of samples under normal pressure conditions [1], which has the advantage of being able to process larger quantities of samples, such as: digestion of organic samples up to 10 grams / can, sample extraction up to 50 grams / can, but it is also possible Use high boiling acid to speed up. Compared with the closed type, the safety is also higher. The disadvantage is that the ability to handle difficult samples is slightly worse than that of the closed type. Unconventional temporary open operation in a closed chamber can contaminate and corrode the entire operating system, and the use of the instrument in a short period of time will be scrapped, so it is not advisable. The state-of-the-art, open-ended microwave sample processing system features automation and flexibility with a multi-channel system that automatically adds a variety of reagents, eliminating the need for manual reagent addition and transfer of samples, allowing digestion, extraction, evaporation, and volume reduction in one go. Waiting for the whole process. The instrument uses a multi-channel focusing non-pulsating microwave radiation host and can automatically control its temperature change. Its condensing and recirculating system can achieve an element recovery rate of 99.5%. These instruments are currently only produced by French PROLAB and American CEM ( PROLAB has been acquired by CEM). Due to the flexible and diverse processing capabilities of open sample pretreatment technology, as a routine tool for laboratories, the overall market trend is growing, with market share in some advanced countries approaching and exceeding closed digestion instruments.
2) Closed microwave sample pretreatment instrument
The purpose of closed digestion is to digest insoluble samples and prevent any loss of elements when performing trace or trace element analysis. The advantage of closed digestion is that there is no element loss and the reaction is very rapid under high pressure. The professional open digestion device solves the problem of element recovery through the recirculation system, and the closed digestion as the name suggests is to solve this problem with a high pressure sealing method. Therefore, the sample of the entire reaction process is required to be in a strictly sealed state. Because of this, the disadvantage is that the sample processing volume of the general instrument is small, the processing amount of organic matter can only be about 0.5 g, and there is a risk of high pressure. Therefore, very strict and complex control capabilities and explosion-proof design are required. Due to the high temperature caused by the high pressure digestion reaction, many elements and compounds are in a gaseous state, and if the seal is poor, the element loss is easily caused. In order to prevent explosion, some manufacturers have the function of venting at high pressure in the digestion cylinder. This can not be called closed digestion, and it is not suitable for the instrument with venting function when processing trace element analysis samples. Therefore, the so-called automatic exhaust function should be avoided when designing the closed tank to ensure the recovery and integrity of the elements to be tested, otherwise it is difficult to ensure the reliability of the analysis results (especially the volatile trace elements).
3. The pressure level of the closed microwave sample processing instrument and the latest safety design ideas
The closed reaction vessels can be divided into three types: low pressure type (15-30 bar), medium pressure type (40-60 bar) and high pressure type (70-100 bar). Generally, the higher the pressure, the stronger the digestion ability. When choosing a closed reaction vessel, you must first carefully consider your application requirements. Remember that the preference for high pressure cannot be misguided. Then you should know the reputation of each manufacturer and the safety performance of the instrument in detail, especially pay attention to whether it meets safety standards. . Some non-professional manufacturers define medium voltage as high voltage, please be sure to take the data as the standard. Since the household microwave oven is designed according to the kitchen cooking, its structure is weak and cannot withstand the acid gas corrosion and mechanical strength that may be leaked during high pressure digestion, and can only be operated at medium and low pressure. Of course, common organic and inorganic samples can generally be solved with medium pressure. Pressurization and safety are two important concepts that are both contradictory and complementary. When using, you should try not to use excessive pressure. For high-voltage equipment that is often used in laboratories, operations and management personnel should pay sufficient attention to safety. The general accident is due to the lack of understanding of the performance of the purchased instruments by the experimenters, the lack of sufficient understanding of the chemical reactions, and the inadvertent operation.
It is necessary to pay attention to the improper use of high-voltage instruments, which may cause serious consequences. For example, in June 1998, Dr. Brandt, a 32-year-old German chemical doctor, was killed by the operation of the Milestone high-pressure instrument and caused a sensation [2]. This has led to a re-examination of the need for high-pressure reactions and the superiority of open microwave instruments, thereby strengthening the development and research of open microwave series. The latest safety design theory of closed instruments has changed from how to prevent explosions to how to reduce the losses caused by explosions, that is, assuming that explosions will definitely happen, which is also called passive safety. According to reports, in 1998, CEM introduced the MARS-5 model with intelligent power output. The passive safety design instrument can withstand various explosion tests. The design purpose is to ensure that even if the operation is not correct, almost the entire instrument will not be damaged. The possibility, and will not hurt the operator, it should be said that passive safety design reflects the new direction of the development of high-pressure microwave chemical instruments [3]. Its security mechanism is roughly as follows:
1) Active safety system: 1 Through the automatic control of pressure and temperature in the main control tank, the initiative to prevent abnormal conditions is improved. 2 The IR infrared scanning monitoring at the bottom of the chamber is performed to confirm the temperature difference and pressure difference between each tank and the main control tank. , prompt prompts and shutdown to prevent accidents. 3 cavity pressure vibration monitoring system, can test the accidental overpressure in the tank approaching the critical point and stop the power transmission operation of the instrument. The response frequency of the closed loop system is controlled to 120 times per second. The power output of the magnetron is controlled based on monitoring the temperature/pressure feedback signal within the reaction tank. Thereby reducing the chance of an explosion.
2) Passive safety system: Based on the assumption of control failure, the protection response of the system in the event of high pressure out of control or explosion is: 1 pressure tank double safety relief; 2 automatic reagent and acid mist sensing system; 3 based on 3D directional explosion protection Theoretical vertical blasting high pressure tank design. After a large number of simulation experiments, in the event of an explosion, the composite fiber material can be used to limit the shock wave to be released only in the vertical direction, thereby ensuring lateral safety; 4 the explosion-proof door system is the last passive safety protection, which can be used when the danger occurs. The automatic parallel ejection advances the lateral impact pressure.
3) Operational safety: During the process and at the end of the reaction, the operator is instructed and warned of the temperature and pressure in the container and the automatic cooling value to prevent accidental operation.
4. Microwave power control mode, the concept of pulsed microwave and non-pulsed microwave
Once the chemical reaction process exceeds a certain critical point, a large amount of gas may be quickly released to exceed the upper pressure limit (110 bar) of each tank and it is difficult to control. Therefore, it is necessary to carefully monitor the reaction process at any time, and change the microwave power output to adjust it in time. Generally, according to the control ability, it can be divided into three levels: low, medium and high. The control ability is different and the program input is different.
1) Switching pulse control: The traditional method is to use a fixed power output, but intermittently turn off the microwave to change the total output power, which is characterized by switching pulse microwave. For example, the microwave is turned off 5 times in 1 second, and the power is 50%. Switched control is the first generation of control technology. Researchers have found that this type of control is not only difficult to control, but may also directly affect the results of the reaction, and accidents occur in the switching mode. The microwave is defined as a pulse and a non-pulse according to the power transmission mode, that is, the intermittent emission is a pulsed microwave, and the discontinuous emission is a non-pulsed microwave. Studies have shown that pulsed microwaves generate high-threshold electromagnetic pulses at the moment of switching. For the digestion of samples containing organic lipids and alcohols, the reaction product with nitric acid may stimulate a critical explosion, and the reaction mechanism is similar to explosive detonation. Non-pulse technology should also be used in the extraction reaction, because the high threshold pulse microwave is also very easy to destroy the extracted organic molecular morphology, and can not guarantee the integrity of the molecular organic morphology, thus affecting the consistency and reliability of the results.
2) Automatic power frequency conversion control and non-pulse technology: This is the second generation control technology, characterized by automatic power change, and the output is non-pulsed microwave. The feature is that it is not necessary to turn off the microwave emission, and under the condition of continuous microwave emission, the microwave power output is automatically changed linearly according to the temperature and pressure feedback signal, and the reaction state is adjusted. Not only does it increase the reaction rate, but it is also very safe. Since the closed-loop response is based on an accurate and reliable in-canister temperature and pressure sensing device, the overall machine technology is improved, and the cost is also increased accordingly. Non-pulsed microwaves automatically adjust the power output of the microwave under continuous microwave emission. The characteristic is that the microwave can continue to output regardless of the power, without pulse stimulation. The experimental results show that this method is easier to control the microwave assisted reaction and improve the stability and safety of the digestion reaction. The recovery and stability of the organic extraction reaction are also improved. High-power microwave instruments are preferably non-pulsed because their thresholds are too high and are potentially dangerous. Therefore, the development of non-pulsed microwave chemical instruments is very beneficial to the study of reaction kinetics, which actually represents a new direction in the development of microwave technology. [4]
5. The difference between temperature and pressure detection and temperature control of microwave instruments
Temperature and pressure control technology = temperature and pressure data monitoring + power feedback mechanism is the basic formula for evaluating the control system. The characteristics of temperature and pressure control performance are digital display and automatic power feedback mechanism. Due to the immature monitoring technology, non-professional manufacturers generally refrain from introducing sensing methods. Affected by the reliability of their detected data, they do not want users to purchase their temperature and pressure detection devices, only listed as non-standard options. All manufacturers, including non-professional manufacturers, claim to have temperature-controlled voltage control functions in publicity. In fact, there are some unrealities in this. Because they deliberately confused the conceptual differences between monitoring and control. Detecting does not mean having control, and control is based on the power feedback mechanism of the detected data. In addition, if a machine needs manual power/time setting, it proves that its machine can't automatically change the power, and there is no closed-loop control system. How to control the process? Therefore, users should pay special attention to the purchase of such instruments. The actual situation.
Some manufacturers describe the intracavity vibration safety system as a pressure control system. There is no real-time voltage control numerical display for the voltage control system. It is difficult for the user to verify the reliability of the pressure control system. Temperature control also has a temperature setting function but no temperature display function. In addition, you should also pay attention to the sensing method. Strictly speaking, all sensors that generate induction arcs, such as thermocouples, are not suitable for use in microwave fields, because arcing and heat release may cause potential hazards (such as detonation redox reactions). The hydrogen released, but still used by some non-professional manufacturers, is dangerous.
6. How to choose microwave sample digestion instrument
What grades the user purchases depends on the actual work needs of the laboratory, such as sample processing batch requirements, ease of digestion, and other factors. The performance and price levels of such products vary greatly. In the international market, the current open and closed quotas are roughly the same, while the domestic market is mainly closed. In terms of production technology, currently available microwave chemical equipment can be roughly divided into two categories: professional and household microwave oven modified.
General household microwave oven modification manufacturers have a relatively simple product model, mainly focusing on closed digestion. Convenient conditions for retrofitting based on domestic microwave ovens due to closed digestion. I hope that users should be vigilant even for foreign manufacturers. Because non-professional manufacturers are difficult to sell in Europe and the United States, they tend to focus on the third world instrument market. Some manufacturers are accustomed to a large number of false propaganda, which makes it difficult for users to distinguish the level of their control technology from the promotional materials. Here are four simple basic methods you can use to measure their product levels:
1) Look at the pictures of the instruments, non-professional products have obvious traces of household microwave oven transformation [5]. The control is connected to the microwave oven in a separate way, often with a laptop next to it, emphasizing how big the screen is to attract users. A certain manufacturer actually divided the host, program input, detection, and exhaust parts into four. Professional manufacturers use an integrated model, because the split-type instrument is easily corroded and difficult to operate on a platform filled with chemical reagents.
2) See if the instrument series is complete. Non-professional manufacturers often only produce one type of closed machine. In the same kind of rack, it is called a so-called multi-function. Professional manufacturers tend to design and produce high-reliability special equipment, such as fully automatic open focus microwave equipment, closed high-pressure equipment and large-scale continuous flow systems. The special machine is the development trend of laboratory instruments, so the large manufacturers and More models. [6][7]
3) See if the instrument passes the current US, Canada and EC related industrial electrical radiation and explosion safety standards, professional manufacturers' products pay attention to safety, with intelligent BITS fault self-test system, various active and passive safety protection devices, and Low-end instruments are generally not available.
4) See if the instruments meet the current international production quality and management standards such as: ISO9001, ISO9002, etc., professional manufacturers generally have quality standards. However, some foreign manufacturers do not have basic quality standards. In order to deceive Chinese users, they write the so-called software methods and the false claims of ISO9002, TQM and GLP on the Internet, deliberately giving users the illusion of quality standards. Users should carefully check Prevent from being fooled.
It can be seen from the situation of some professional manufacturers that the characteristics of advanced microwave sample pretreatment instruments should be: complete professional varieties, strong automation control and high safety. As the country strengthens safety management and attaches importance to laboratory safety, it is hoped that users will strive to verify the safety and reliability of products, purchase instruments that meet international safety standards, and do not take risks to buy instruments without safety standards.
The relevant North American and European Community safety standards are as follows:
1). Emission and safety Approvals US. And Canada Emissions & Safety US: ETL* approved to UL standard3101 (Laboratory Equipment).3 Canada: TEL** approved to standard CAN/CSA C22.2 No.1010.1 (Laboratory Equipment)
2). European Community Emissions & Safety- Conforms to 1EC standard EN55011 (Emissions for industrial, Scientific, and Medical Equipment). 2 Conforms to EC standard EN50082-2 (Electro –magnetic Compatibility – Part 1) 3 Conforms to EC standard IEC 1010 -1 (Safety requirements for electrical equipment of measurement, control, laboratory use-part 1). HHS standards under 21 CFR, Part 1030. 10, subparts(C) (1), (C) (2) and (C) (3).
main reference
1) Zou Mingqiang, Wang Danian, Xie Shujuan, Liu Li, Zhang Hanqi, Jin Qinhan, Trace Analysis, 1990, (1-2): 82
2) SONNABEND, 25JULY 1998 NR. 172
1). HM kingston, Stephen J. Heswell, Microwave enhanced chemistry. Printed in USA. American Chemical Society, 1997. 502
2). R&D 100 Magazine, 1997, 1998
3). Quevauvller, P., Imbem, J. Olle M. Evaluation of the use of microwave over systems for the digestion of envitonmental sample s Mikrochim. Acta 1993, 112, 147~154
4). Collins, MJ, Cruse, BW Goechius RJUS Patent 3 909 598, 1975
5). Collins, MJ, Cruse, BW Goechius RJUS Patent 4 457 632, 1984
You may want a canvas backpack for school, a canvas and leather backpack for work and a rucksack backpack for casual daily activities – but how about having a modern, trendy, gorgeous canvas backpack to meet all these needs? Many of our canvas backpacks, travel backpacks, outdoor backpacks, heavy duty backpacks and student backpacks are convertible.
We are committed to providing a comprehensive collection of highest quality, stylish, tasteful and highly functional canvas backpack for all ages. Canvas backpack will always be a staple of quality and attention to detail. Our products feature premium canvas, resilient to wear and tear, cotton lining textures, genuine leather reinforcements and design elements, and so on.
Canvas Backpacks,Black Canvas Backpack,Canvas Leather Backpack,Casual Black Canvas Backpacks
KAIPING XIN SUI YING LEATHER WARE CO.,LTD , https://www.paliobag.com