Whether the analysis and determination of elements is accurate is very important to experimental research, and sample pretreatment before elemental analysis is an important process in analytical chemistry research. There are wet digestion and dry digestion methods in the pretreatment of samples for elemental determination. Dry digestion is to ash the sample to be tested in a high-humidity furnace and then add acid to dissolve it. The whole digestion process takes a long time and consumes a lot of energy, and the elements to be measured are easily polluted and lost. Wet digestion is to add nitric acid, perchloric acid, sulfuric acid, hydrogen peroxide, caustic soda and other solutions to the sample, and then digest at high temperature to make the sample clear and translucent. Traditional heating equipment such as electric heating plates, electric furnaces, etc. have been gradually replaced by other digestion equipment such as microwave digestion due to problems such as low temperature control accuracy, uneven heating of samples, uneven sample digestion, serious environmental pollution, and easy loss of digestion elements. It has been gradually replaced by other digestion equipment such as microwave digestion instrument and automatic graphite digestion instrument.
Automatic graphite digestion instrument, also known as programmed temperature control graphite digestion instrument, is used as an inorganic sample pretreatment equipment and has a complementary effect with microwave digestion instrument. The automatic graphite digestion instrument makes the experiment safer because of its open digestion process; In terms of operation, the fully automated computer operation replaces the manual operation, so that the addition of acid solution, the digestion of the sample, and the acid removal, concentration, and constant volume of the digestion solution are all automatically completed by the instrument, realizing the unattended operation of the digestion process. which realizes the unattended digestion process, and has shown its remarkable advantages in the sample pretreatment process, and has been adopted by more and more laboratories at home and abroad. The remote control makes it safer for the experimenters not to be in direct contact with the acid mist generated during the digestion process, and the built-in exhaust system does not need to purchase additional fume hoods.
The following introduces the automatic graphite digestion instrument, so that everyone can further understand the automatic graphite digestion instrument.
Ⅰ. Principle of automatic digestion instrument
The automatic digestion instrument uses high-purity graphite blocks to destroy the organic matter or reducing substances in the sample to be tested with acid under high temperature conditions, and finally makes the sample clear and transparent. The high-purity graphite block has excellent thermal conductivity, suitable heat capacity and machinability. The drilled graphite block can heat the batch of samples uniformly around the three-dimensional, so that all parts of the sample are heated evenly.
Ⅱ. Features of Automatic Graphite Digester
2.1 Digestion process safety
Different from the microwave digestion method, the automatic digestion instrument belongs to the open digestion method, and the digestion process is safer. It can process samples in large quantities under normal pressure conditions, avoiding safety hazards caused by pressure.
2.2 High batch processing capacity
Automatic graphite digestion instrument, which can digest 60 samples at the same time, with strong sample batch processing ability, and dual temperature and dual modes can digest more samples.
2.3 Wide range of sample weighing
The weighted sample of microwave digestion method is usually only 0.2~0.5g, and the digestion amount of a single sample is relatively small. When plant selenium is digested by the automatic graphite digestion instrument with program temperature control, the digestion amount of a single sample can reach more than 10.0g, which is much higher than the conventional sample weight of the microwave digestion method, and can meet the analysis requirements of samples with a large sample size.
2.4 It is convenient to weigh samples
The digestion tube made of polytetrafluoroethylene is resistant to acid and alkali, and its diameter is larger than that of microwave digestion tube. It is more convenient to weigh samples in the digestion tube. For samples that are easily adsorbed on the tube wall (such as powdery and pasty samples), the reflux of acid vapor can be used to bring the sample back to the bottom of the tube in the early stage of digestion, so that the sample can be digested more completely.
2.5 high automation program
The digestion tube rack can be automatically lifted, oscillated, and shaken evenly. It can automatically cool the digestion tube before adding the digestion solution and shake after adding the liquid. In the later stage of the digestion program, the acid removal and constant volume operations of the digestion solution are automatically completed, which saves the sample transfer process, reduces the possibility of sample contamination, improves the experimental efficiency, and reduces the labor intensity.
2.6 Wide choice of acids
At present, the acid solutions commonly used in automatic digestion systems include: nitric acid-sulfuric acid, nitric acid-perchloric acid, nitric acid-hydrochloric acid, etc. Generally, acids that can be used for microwave digestion can be used in the digestion program of automatic digestion equipment. Sulfuric acid has a boiling point of 337°C, which is 240°C higher than the temperature of the polytetrafluoroethylene digestion tube, and it will react with the vessel at high temperature, destroy the surface finish of the digestion tube, and cause element adsorption, so it is generally less used for microwave digestion. Perchloric acid is a strong oxidizing agent, which will cause a violent oxidation reaction when it meets organic matter, causing the organic matter to decompose rapidly, and at the same time, the pressure will rise sharply, which may cause an explosion. Most of them are used in normal pressure pretreatment, and rarely used in closed digestion tanks. Both can be combined with other acids to enhance the effect of sample digestion in fully automated digestion procedures.
2.7 Sample digestion effect is good
As a constant temperature device for sample digestion, the automatic digestion instrument changes the traditional flat-plate heating method into a hole-type, surrounding three-dimensional heating method. The heating area of the heating body is larger, so that the bottom and side of the heating body can be heated at the same time, and the heating is more uniform. The temperature is controlled by computer, so that the temperature fluctuation range is small, and the heating rate is more stable, which effectively avoids the phenomenon that the sample with high organic content will overflow when the heating rate of the sample is too fast.
2.8 No pollution to the experimental environment
Although the automatic graphite digestion instrument is an open digestion, it has an exhaust system and filtering function inside, the acid mist during the digestion process of the instrument can be discharged to the outside of the laboratory through the exhaust system, no need to purchase fume hood equipment, saving costs.
Ⅲ. Application of automatic digestion instrument in analysis and detection
At present, the automatic digestion instrument is being used in many fields, such as: environment, food, medicine, chemical industry, scientific research, electronic products, etc. like:
- The automatic digestion instrument ICP method was used to measure seven heavy metal elements in the national standard material soil. It was found that compared with the electric heating plate-atomic absorption method stipulated in the national standard, this method has good parallelism, accurate data, and greatly saves experimental time.
- Established an analysis method for the determination of heavy metal content in dust by ICP-MS with a fully automatic digestion instrument. The results show that the automatic digestion instrument is heated evenly, the temperature is controlled accurately, the sample is thoroughly digested, and the acid reagent is automatically added quantitatively, which avoids human contact and eliminates interference. The linear relationship, accuracy, precision, and detection limit of the experiment all meet the analysis requirements, and can be used for the simultaneous determination of multiple elements in a large number of samples.
- The selenium content in rice was determined by automatic sample graphite digestion and dilution system and atomic fluorescence spectrometry, the experimental conditions were optimized, and satisfactory results were obtained.
- The mercury and arsenic in the soil were determined by the automatic digestion instrument-atomic fluorescence method, which improved the sensitivity of the analysis, shortened the analysis time, and the measurement precision and accuracy met the requirements.
- Three different digestion methods were used to measure the content of heavy metals in the soil. The results showed that the automatic graphite digestion was controlled by a program, and its precision and accuracy were better than the other two digestion methods, which was suitable for batch sample analysis.
- The automatic graphite digestion instrument was used instead of the water bath to heat the aqua regia hydrolysis method to treat the soil samples, which proved that the measurement results of the automatic graphite digestion method were closer to the standard value than the aqua regia digestion method.
- The chemical oxygen demand in the waste water was measured by using the temperature-programmed graphite digestion apparatus, and satisfactory analysis results were obtained.
Ⅳ. Discussion of results
As a new sample pretreatment equipment, the automatic digestion instrument, with its high degree of automation, can complete the operations of liquid addition, digestion, acid removal, concentration and cooling, constant volume, and mixing at one time. It improves the experimental efficiency, reduces the labor intensity, and has shown obvious advantages. Compared with the traditional wet digestion technology, it is more suitable for the requirements of modern analysis experiments. The application of automatic digestion instrument in the field of analysis has the characteristics of fast and efficient, simplified operation procedures, unattended operation, good reproducibility of results, and high accuracy. The automatic digestion instrument not only shortens the time for sample pretreatment, but also reduces the contact time between toxic and harmful gases and heat sources and experimenters, reducing the burden on experimenters. It can be predicted that the automatic digestion instrument will have a broader application prospect in more fields.
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