At present, most of the tests focus on accuracy, efficiency and speed; this is a development goal of the current analytical laboratory! How to achieve this goal, in addition to improving the analytical means of analytical instruments, but also the means of sample pretreatment equipment. Briefly understand the related equipment and methods of wet digestion Wet digestion is a method of destroying organic or reducing substances in samples with acid or alkali and heating.
Commonly used acid hydrolysis systems are: nitric acid-sulfuric acid, nitric acid-perchloric acid, nitric acid-hydrochloric acid, hydrofluoric acid, hydrogen peroxide, etc., which can completely destroy the organic and reducing substances in the test object; Caustic soda solution is often used for alkaline digestion. Digestion can be carried out in crucibles (nickel, PTFE) or high-pressure digestion tanks.
There are two main types of wet digestion equipment:
Conventional heating and digestion equipment mainly include: electric furnace, water bath, electric heating plate, orifice digestion device such as our graphite digestion instrument, heavy metal digestion instrument, etc. The electric furnace is a traditional digestion equipment in the laboratory, which is simple and practical. However, due to the innovation of laboratory technology and the requirements of laboratory management, the electric furnace is an open flame equipment. At the same time, it is difficult to control the temperature and easy to be damaged, now most laboratories abandon this earliest digestion equipment.
The water bath is mainly used for some low-temperature digestion, such as: sewage, cosmetics, etc. Compared with the electric furnace, the stability and safety of the water bath are higher, and it is also more convenient to operate, but it is only suitable for some low-temperature digestion treatment, and the application field is limited.
The electric heating plate is a conventional digestion equipment that has been recognized and accepted by domestic laboratories in the past 10 years! No matter large or small analytical laboratories, they are all recognized by its advantages of good temperature control, high stability and strong safety. To a large extent, it helped the experimenters solve some problems faced by the electric furnace digestion. According to statistics, my country’s annual demand for electric heating plates is more than 12,000, and it is still growing at a rate of 20% every year, but electric heating plates also have some disadvantages: large energy consumption, low thermal energy utilization rate, and small heating effective area , The amount of processed samples is limited, and the uniformity of the experimental results is not strong.
At present, after innovation on the basis of the electric heating plate, a device called a hole-type digester has appeared. It heats the sample in a surrounding heating method, which effectively increases the utilization rate of heat energy. At the same time, it uses With the advantages of higher temperature control accuracy, small difference between holes, high efficiency of the whole machine, energy saving, and environmental protection, it occupies the front end of conventional digestion equipment in Europe and America. It can save energy by more than 75% than electric heating plates, and the speed of digesting samples is faster than electric heating plates. -6 times, the amount of samples processed is 2-3 times that of the electric heating plate, it can be said that it is a technological innovation of heating equipment.
At present, domestic equipment manufacturers have also begun to develop and produce such products, and our laboratory technicians have also begun to recognize the advantages and value of such products. The current domestic demand for hole-type digesters is growing at a rate of 300% per year.
Due to the purpose of energy saving and environmental protection advocated by the country, this will also be a direction for the future development of our laboratory. We can predict that this kind of product will completely replace the electric heating plate in the laboratory as a new trend of conventional heating and digestion equipment.
The second is microwave digestion, a new digestion technology developed and applied in the 1980s! Conventional heating and digestion can only process some simple samples, and some insoluble samples can only be completely digested under high pressure and airtight devices. This is the advantage of microwave digestion, it can process almost all samples quickly and accurately.
There are many wet digestion methods at present, and different digestion equipment can be selected according to different samples, and different digestion methods can be drawn up to achieve an accurate, efficient and fast test result. All digestion should be carried out in the following aspects:
1. Avoid loss of components to be tested;
2. Do not introduce interfering substances;
3. It must be safe and fast without causing difficulties to the subsequent operation steps;
4. The solution obtained after digestion must be easy to detect. In this way, the above-mentioned different digestion equipment and methods can be selected according to the experimental requirements!
Wet digestion is currently the most direct, effective and economical sample pretreatment method for elemental analysis. With the innovation and development of laboratory equipment technology. Hole digester and microwave technology will be the two mainstream experimental equipment products for laboratory wet digestion!
There are many methods of wet digestion, and different methods will be selected for different samples and different instruments and equipment. At present, the samples of wet digestion can be divided into three categories: samples with high organic content, samples with low organic content, and samples that are simple and easy to digest.
The choice of acid system is different for different samples! Hydrochloric acid is suitable for the digestion system below 80°C, nitric acid is suitable for the digestion system at 80-120°C, sulfuric acid is suitable for the digestion system at about 340°C, and the mixed acid of hydrochloric acid-nitric acid is suitable for the digestion system at 95- For the digestion system at 110°C, the mixed acid of nitric acid-perchloric acid is suitable for the digestion system at 140-200°C, the mixed acid of nitric acid-sulfuric acid is suitable for the digestion system at 120-200°C, and the nitric acid-hydrogen peroxide is suitable for the digestion system at 95-130°C. It is very important to choose a suitable acid system to accelerate the destruction of organic matter. At the same time, accurate temperature control is required to achieve the ideal digestion effect!
With the development and updating of instruments, equipment and experimental methods, the future laboratory will develop into a green and environment-friendly laboratory. The recovery, separation, purification and reuse of reagents, the minimization of energy consumption, and fully automated experimental schemes will all be the future development direction of the laboratory.
Let’s take a look at the comparative experiment of digesting and clarifying 1g of meat:
| Instrument | Sample | Single acid added | Digestion time | Remark |
| Electric heating plate | 5-10 | 20-50ml | 4-6 hours | The amount of sample to be processed is small, the energy consumption is large, the amount of acid added is large, and the processing time is long |
| Graphite digester | 20 | 15ml | 2 hours | Large amount of processing samples, moderate energy consumption, moderate amount of acid added, moderate processing time |
| Microwave digester | 12 | 8ml | 30min | Moderate sample volume, moderate energy consumption, less acid addition, and short processing time |
During the digestion process, there are obvious differences in the amount of acid added and energy consumption for different instruments and equipment selected. The more advanced the equipment, the less energy consumed during the experiment. Experimental consumables, electrical energy, and operating time are all energy consumption. How to avoid energy waste in experiments is also a problem that all laboratories have to solve at present. Reasonable use of reagents, recycling and reuse of reagents, improving the utilization rate of electric energy, and improving the work efficiency of laboratory personnel are not only a selection of experimental methods, but more importantly, the screening of instruments and equipment!
During the digestion process, the content of organic matter in the samples is often different, and the amount of acid it requires is also inconsistent; the temperature setting is different, and the amount of acid volatilization loss is also inconsistent. How to add an appropriate amount of acid to achieve complete digestion? The automatic acid adding and replenishing system will realize this step for the experiment. Its working principle is to detect the volume of acid liquid left in the current container through the liquid level sensor. When it is less than the set value, it will automatically add a certain amount of acid liquid. acid. The advantages brought are: the sample will not be burned dry, the acid will not be lost in a large amount, the human operation is eliminated, and the time for acid removal is reduced. At the same time, this is also a manifestation of the automated laboratory.
When digesting volatile elements, such as arsenic, mercury, etc., the digestion temperature must be strictly controlled, especially for non-closed digestion devices. Excessive temperature will cause the loss of elements, so this requires a real-time temperature monitoring and control system. At present, the real-time feedback of the temperature of the digestion liquid by the external temperature sensor is a very good application prospect. It can help the experimenter know how to set a reasonable temperature when measuring different elements! Avoid element loss and achieve the purpose of rapid sample processing.
After the digestion is complete, the amount of acid in the container should be driven to 1-2ml. From digestion to acid removal, a large amount of acid gas (carbon dioxide, nitrogen dioxide, sulfur dioxide, etc.) will be produced. At present, most domestic laboratories conduct digestion experiments in fume hoods, and a large amount of acid gas is discharged outside through the fume hood. In the atmosphere, it causes serious pollution. Although my country’s laboratory management standards do not explicitly prohibit the discharge of such pollution. However, in laboratories in some developed countries in Europe and the United States, an exhaust gas treatment device will be installed at the exhaust terminal of the fume hood to treat the exhaust gas in the fume hood through acid-base neutralization and activated carbon adsorption. In this way, the pollution caused by the experiment to the atmosphere is reduced, and a green and environmentally friendly laboratory is realized. Moreover, the digestion devices in European and American laboratories generally have separate waste gas recovery devices that are matched with instruments and equipment.
In the elemental analysis laboratory, how to establish an accurate, efficient and fast testing laboratory first involves the problem of sample pretreatment. The fast or slow processing, good or bad all directly affect the detection of samples.
To sum up, there are many kinds of equipment and methods for wet digestion, but some details still need to be optimized. Only by choosing reasonable equipment supporting schemes, accurate and convenient experimental methods, and a good laboratory management system can the requirements of the new laboratory be met—accurate, efficient, fast, energy-saving, and environmentally friendly!