1 Scope
This standard specifies the flame atomic absorption spectrometry, the inductively coupled plasma emission spectrometry and the inductively coupled plasma mass spectrometry for the determination of manganese in foods.
This standard is applicable to the determination of manganese in foods.
Method I Flame atomic absorption spectrometry
2 Principles
After the digestion treatment, the test sample is introduced into atomic absorption spectrometer; after flame atomization, the manganese absorbs the resonance line at 279.5 nm. The absorbance value of manganese is in direct proportion to the content of manganese within a certain concentration range, and the content of manganese is quantified by comparing with standard series.
3 Reagents and Materials
Unless otherwise stated, the reagents used in this method all refer to guaranteed reagents and the water refers to Grade 2 water specified in GB/T 6682.
3.1 Reagents
3.1.1 Nitric acid (HNO3).
3.1.2 Perchloric acid (HClO4).
3.2 Preparation of reagents
3.2.1 Mixed acid [perchloric acid+nitric acid (1+9)]: take 100 mL of perchloric acid, slowly add into 900 mL of nitric acid, and then mix well.
3.2.2 Nitrite acid solution (1+99): take 10 mL of nitric acid, slowly add into 990 mL of water slowly, and then mix well.
3.3 Standard substance
Manganese metal standard substance (Mn): with purity of greater than 99.99%.
3.4 Preparation of standard solutions
3.4.1 Manganese standard stock solution (1000 mg/L): accurately weigh 1 g (accurate to 0.0001 g) of manganese metal, add nitric acid to dissolve and transfer into a 1000-mL volumetric flask, add nitric acid solution to dilute to the scale, then mix well and keep in a polyethylene bottle for storage under 4 ℃. Or the standard solution certified by the State and awarded to the Reference Material Certificate may be used.
3.4.2 Manganese standard working solution (10.0 mg/L): accurately pipette 1.0 mL of manganese standard stock solution into a 100-mL volumetric flask, dilute to the scale with nitric acid solution, and keep in a polyethylene bottle for storage at 4 ℃.
3.4.3 Manganese standard series working solutions: accurately pipette 0 mL, 0.1 mL, 1.0 mL, 2.0 mL, 4.0 mL and 8.0 mL of manganese standard working solutions respectively into six 100-mL volumetric flasks and then dilute to the scale with nitric acid solution, and mix well. The mass concentration of manganese in these standard series working solutions is 0 mg/L, 0.010 mg/L, 0.100 mg/L, 0.200 mg/L, 0.400 mg/L and 0.800 mg/L, respectively. The concentration range of the standard solution can also be adjusted according to the concentration of manganese in the actual sample solution.
4 Apparatus and Equipment
4.1 Atomic absorption spectrometer: equipped with flame atomizer and manganese hollow cathode lamp.
4.2 Analytical balances: with the sensitivities of 0.1 mg and 1.0 mg.
4.3 Acetylene gas in steel cylinder and air compressor used for analysis.
4.4 Sample smashing devices: homogenizer, high-speed pulverizer.
4.5 Muffle furnace.
4.6 Adjustable temperature control electric hot plate.
4.7 Adjustable temperature control electric heating furnace.
4.8 Microwave digestion apparatus: equipped with a PTFE digestion tank.
4.9 Constant temperature drying oven.
4.10 Pressure digestion tank: equipped with a PTFE digestion tank.
5 Analysis Procedures
5.1 Preparation of test sample
5.1.1 Solid samples
5.1.1.1 Dry samples
For samples of low water content such as beans, grains, mushrooms, tea, dried fruits and bakery products, etc., take the edible parts and smash uniformly by high-speed pulverizer when necessary; for powdery samples such as solid dairy products, protein powder and flour, etc., shake well.
5.1.1.2 Fresh samples
For samples of high water content such as vegetables, fruits and aquatic products, etc., wash and clean, air-dry, take the edible parts to homogenize uniformly; for samples such as meat and egg, etc., take the edible parts to homogenize uniformly.
5.1.1.3 Quick-freezing and canned food
After the quick-freezing food and canned food is thawed, take the edible parts to homogenize uniformly.
5.1.2 Liquid samples
For samples such as soft drinks and condiments, etc., shake well.
5.1.3 Semi-solid sample
Stir well.
5.2 Digestion of test sample
5.2.1 Microwave digestion
Weigh 0.2 g~0.5 g (accurate to 0.001 g) of test sample, place into the microwave digestion tank; for samples containing ethanol or carbon dioxide, heat on the electric hot plate at low temperature to remove ethanol or carbon dioxide. Add 5 mL~10 mL of nitric acid, cover the cap to stand for 1 h or overnight, tighten the outer tank and then place in the microwave digestion apparatus for digestion (refer to Table A.1 for the digestion conditions). Take out the inner tank after cooling, remove acid to nearly dryness on the adjustable temperature control electric hot plate at 120 ℃~140 ℃, dilute to 25 mL or 50 mL with water, and then mix well for use later; perform the blank test at the same time.
5.2.2 Pressure tank digestion
Weigh 0.3 g~1 g (accurate to 0.001 g) of test sample into the PTFE pressure digestion tank; for samples containing ethanol or carbon dioxide, heat on the electric hot plate at low temperature to remove ethanol or carbon dioxide. Add 5 mL of nitric acid, cover the cap to stand for 1 h or overnight, tighten the outer tank and then place in the constant temperature drying oven for digestion (refer to Table A.1 for the digestion conditions). Take out the inner tank after cooling, remove acid to nearly dryness on the adjustable temperature control electric hot plate at 120 ℃~140 ℃, dilute to 25 mL or 50 mL with water, and then mix well for use later; perform the blank test at the same time.
5.2.3 Wet digestion
Weigh 0.5 g~5 g (accurate to 0.001 g) of test sample into the glass or PTFE digestion vessel; for samples containing ethanol or carbon dioxide, heat on the electric hot plate at low temperature to remove ethanol or carbon dioxide. Add 10 mL of mixed acid, cover the lid to stand for 1 h or overnight and then place on the adjustable temperature control electric hot plate or electric heating furnace for digestion. If the solution becomes brownish black, add mixed acid after cooling until white smoke appears and the digestion solution shows colorless and transparent or slightly yellow, cool down, dilute to 25 mL or 50 mL with water, and then mix well for use later; perform the blank test at the same time.
5.2.4 Dry digestion
Weigh 0.5 g~5 g (accurate to 0.001 g) of test sample into the crucible, perform carbonization over low heat on the electric furnace until no smoke appears, place in the muffle furnace for ashing for 5 h~8 h under 525 ℃±25 ℃ and then cool down. In case of incomplete ashing with black carbon granules, dropwise add a little nitric acid to wet them after cooling, transfer into the muffle furnace to continue ashing until it becomes white ash after drying on the hot plate. Cool to room temperature and take out, dissolve with nitric acid solution, dilute to 25 mL or 50 mL with water, and then mix well for use later; perform a blank test at the same time.
Note: The dry digestion is recommended for formula foods for infants and young children.
5.3 Instrument conditions for reference
Adjust the instrument to optimal conditions, main reference conditions are as follows: absorption wavelength of 279.5 nm, slit width of 0.2 nm, lamp current of 9 mA and gas flow of 1.0 L/min.
5.4 Plotting of standard curve
Respectively introduce the standard series working solutions into the atomic absorption spectrometer to determine their absorbance values, and plot the standard curve by taking the concentrations of the standard working solutions as horizontal axis and the absorbance values as vertical axis.
5.5 Determination of test sample solution
Under the same experiment conditions as those for the determination of standard curve working solutions, respectively introduce the blank solution and the test sample solution into the atomic absorption spectrometer to determine the absorbance value of manganese and then obtain the concentration of manganese in the solution to be tested from the standard curve.
6 expression of Analysis Result
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