Determination of Lead in Soy Sauce by AA-1800H Graphite Furnace Atomic Absorption Spectrometry - Master's thesis - Dissertation

Determination of Lead in Soy Sauce by AA-1800H Graphite Furnace Atomic Absorption Spectrometry

Key words: graphite furnace atomic absorption spectrometry; soy sauce; lead; matrix modifier; aesthetic analyzer ; lead is a storage of harmful elements, widely distributed in nature, there are many sources of lead in food, Pipes, container packaging materials, utensils and coatings, including animal and plant raw materials, food additives and food contact, will transfer lead to food. Long-term consumption of foods containing lead is harmful to the human body, causing chronic poisoning of lead. In severe cases, it may cause diseases such as hemoglobin deficiency anemia, vasospasm and hypertension. Therefore, it is particularly important to test the lead content in foods, especially in the seasoning soy sauce, which is indispensable in people's daily lives. The China National Accreditation Service for Conformity Assessment has used heavy metal detection in soy sauce as an important part of laboratory proficiency testing. At present, the determination method of lead content in laboratory soy sauce is GB/T 5009.12-2003. Determination of lead in food] First method - graphite furnace atomic absorption spectrometry. The author used the wet digestion method to digest the sample and determined by graphite furnace atomic absorption spectrometry. It is considered that the following points should be noted in the test: The reagent blank reagent blank is related to the purity of the reagent used in the sample and the cleanliness of the container. As we all know, lead is widely distributed in nature. Therefore, the detection of lead is a trace analysis with low pollution and low limit. The lower the blank, the higher the accuracy, so the whole experimental blank is required to be low. During the experiment, the pollution should be strictly controlled. The main points of attention are as follows: 1. The experimental water should meet the requirements of GB/T 6682-2008 analytical laboratory water specification and test method secondary water; 2. The experimental reagent should be excellent grade pure, if not met The reagents required for purity can be purified by chemical methods. However, during the purification process, the possibility of secondary contamination of the solvent should be avoided. At the same time, the reagents selected for the experiment should also be based on the non-staining elements to be tested (in the experiment). If the absorption signal of the element to be tested is not detected within the sensitivity range of the instrument, it can be considered that the selected reagent does not contaminate the element to be tested); 3. The glass instrument used in the experiment should be soaked with acid, and other equipment should be exhausted. Probably clean, glass equipment such as emergency, can be boiled with 10-20% nitric acid for 1 hour, then rinsed with tap water, and then rinsed with deionized water. It should be noted here that the nitric acid solution of the soaking equipment cannot be used repeatedly for a long time, and the lead and the like in the solution are increased due to long-term use, which causes pollution. Sample pretreatment The determination of lead in soy sauce by graphite furnace atomic absorption spectrometry requires pretreatment of the sample, ie digestion of the sample. There are many methods for digesting the sample. The author uses the wet digestion method. When the wet method is used to digest the soy sauce sample, it is required. Note: 1. The reagents used, such as nitric acid and perchloric acid, are corrosive and dangerous. During the experiment, a large amount of acid mist and smoke will be generated, so the digestion should be carried out in a fume hood. 2. During the digestion process, Slowly heat at low temperature to prevent the temperature from being too high, and a large amount of foam will instantly cause the liquid to overflow, which will affect the accuracy of the result. Once the digestion liquid turns brownish black, it should be cooled and then added with nitric acid to continue digestion until white smoke is formed, and the digestive juice is clear or transparent. Slightly yellowish; 3. It is important to note that when digesting the sample with perchloric acid, the operating procedures should be strictly observed, and only a small amount of organic components should be present when the temperature reaches 200 degrees Celsius, otherwise the oxidation potential of perchloric acid is here. It will rise rapidly at temperature and cause a violent explosion! Therefore, it is recommended to add a mixture of nitric acid and perchloric acid for a night before digestion. To make the organic component in the sample oxidize first, or to add nitric acid first, to destroy the easily oxidized substance, and then add nitric acid or perchloric acid; 4. The digestion solution cannot be evaporated to prevent the loss of the measured element; Too much influence on the elements determined by the graphite furnace method, especially the damage to the graphite tube is very large. Therefore, the concentration of the acid in the digestion solution should not be too high. After the lysate is clear and transparent, it is generally necessary to add water to dissolve the salt while catching the acid. When the acid is used, the temperature should be controlled to prevent the temperature from being too high, causing the liquid to splash, causing the loss of the element, and the experimental result is low. Reasonable application of matrix modifier during sample determination Digestion solution is determined by atomic absorption spectrophotometer after constant volume measurement. Note: 1. Adjust the instrument to the best condition, especially the appropriate depth and left and right position of the injection, and inject the sample. Must be accurate and stable, it determines the linearity of the standard curve and the reproducibility of the experiment; 2. According to the sensitivity of the instrument and the approximate content of lead in the soy sauce sample, the range of the standard curve is reasonably selected, so that the signal measurement value of the sample falls. Within the curve range; it should be noted that the acidity of the standard curve should be consistent with the sample blank and the acidity of the sample; 3. The composition of the soy sauce sample is complex, especially the content of sodium chloride is high, and the lead is directly determined by graphite furnace atomic absorption spectrometry. The absorption is severe, and the non-atomic absorption signal is extremely strong at the time of atomization, and it is difficult to obtain the absorption signal of lead, thereby affecting the measurement result, and therefore it is necessary to select a suitable matrix modifier. Commonly used matrix modifiers for the determination of lead in foods are ammonium dihydrogen phosphate, magnesium nitrate, ammonium phosphate, palladium nitrate, etc. (note that these reagents must be pure grade). For the sample of soy sauce, the author chose to use as a matrix modifier. The main principles are as follows: NaCl+ NH4NO3→NH4 Cl+ NaNO3 dissolves enough volatile NH4NO3 in high salt sample and converts NaCl (1465°C evaporation) into NH4. Cl (vaporized at 340 ° C) and NaNO3 (evaporated at 500 ° C), because the evaporation temperature of ammonium chloride and sodium nitrate in the graphite furnace are lower than 500 ° C, which overcomes the interference of NaCl on the determination of trace heavy metal elements. During the experiment, the ashing stage can be seen at the beginning of the injection of a large amount of sample smoke from the graphite tube injection hole indicating that ammonium chloride and sodium nitrate are volatilized. In this way, sodium chloride can be eliminated in the ashing stage, thereby avoiding the interference of the sodium chloride on the measurement. Even with the presence of a very small residual matrix, the signal can be easily compensated with a xenon lamp background corrector. Therefore, an excess solution is added to the digestion solution of the soy sauce sample containing a large amount of salt, so that the high non-atomic absorption signal at the time of atomization is lowered to an easily controllable degree, and the recovery rate is greatly improved. The standard addition test is a standard method for adding the standard substance to the sample, and determining the accuracy of the method by measuring the recovery rate. The system error of the method can also be found by multiple recovery tests. Since the measured lead is a trace element, it is easy to lose. In order to ensure the accuracy of the experimental data, it is recommended to carry out the spike recovery test at the same time. The amount of the scalar is preferably half of the lead content in the soy sauce sample. Detailed introduction of the experimental process 1. Method principle The sample is digested by mixed acid solution, and the digestive solution is injected into the graphite furnace. The ground state atom lead produces characteristic absorption of the wavelength emitted by the hollow cathode lamp, and its absorbance is proportional to the number of ground state atoms in the vapor. According to this principle, the concentration of lead is determined from the standard curve by measuring the absorbance value, and then the lead content is determined. 2. Instruments and reagents AA-1800H atomic absorption spectrophotometer, lead element hollow cathode lamp (American company); lead standard solution, number GBW 08619, concentration value 1000 μg / ml ± 2 μg / ml; nitric acid (excellent level Pure), perchloric acid (excellent grade); water for deionized water (18.23 MΩ·cm). Pipettes and volumetric flasks used in the test are Class A. 3. Preparation of standard curve Accurately absorb 1.00ml of lead standard solution and make up to 100ml of 0.5% nitric acid, then accurately absorb 2.00ml lead stock solution and make up to 500.0ml with 0.5% nitric acid to obtain lead standard solution 40μg/L. 4. Sample Pretreatment - Digestion Accurately draw 5.00 ml of the sample, place it in a 150 ml Erlenmeyer flask, add several glass beads, add 20 ml of mixed acid solution, and place overnight. Heat digestion until the solution is clear or slightly yellow. The heating was stopped, and after cooling, 10 ml of water was added to continue heating until white smoke was emitted. The digest was transferred to a 50 ml volumetric flask and made up to volume with 0.5% nitric acid. At the same time, the reagent is blank. 5. Sample determination 5.1 Spectral conditions: Wavelength (nanometer): 283.3 Slit (nanometer): 0.7L Lamp current: 10mA Signal type: AA-BG Signal measurement: Peak area 5.2 Graphite furnace heating procedure: Drying: 100 °C Slope rise time 5s retention time 15s; 140°C ramp time 15s retention time 10s ashing: 700°C ramp time 10s retention time 15s atomization: 1800°C ramp time 0s retention time 4s impurity: 2600°C ramp time 1s retention time 5s