National Food Safety Standard
Determination of Calcium in Foods
食品安全国家标准
食品中钙的测定
1 Scope
This standard specifies flame atomic absorption spectrometry, titrimetric method, inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry for the determination of calcium content in foods.
This standard is applicable to the determination of calcium content in foods.
Method I Flame Atomic Absorption Spectrometry
2 Principle
After the digestion treatment of specimen, add into lanthanum solution as the releasing agent, and through the atomic absorption flame atomization, the absorbance value determined at 422.7nm is in direct proportion to the calcium content within the range of certain concentration and compare with standard series for quantitation.
3 Reagents and Materials
Unless otherwise specified, guaranteed reagents and Grade II water (defined in GB/T 6682) are adopted for the purposes of this method.
3.1 Reagents
3.1.1 Nitric acid (HNO3).
3.1.2 Perchloric acid (HClO4).
3.1.3 Hydrochloric acid (HCl).
3.1.4 Lanthanum oxide (La2O3).
3.2 Reagents preparation
3.2.1 Nitric acid solution (5+95): measure 50mL of nitric acid, add into 950mL of water and mix them well.
3.2.2 Nitric acid solution (1+1): measure 500mL of nitric acid and mix it uniformly with 500mL of water.
3.2.3 Hydrochloric acid solution (1+1): measure 500mL of hydrochloric acid and mix it uniformly with 500mL of water.
3.2.4 Lanthanum solution (20g/L): weigh 23.45g of lanthanum oxide, add into 75mL of hydrochloric acid solution (1+1) to dissolve it after wetting it with a small amount of water, transfer the solution to a 1 000mL volumetric flask, bring the volume to the scale with water and then mix well.
3.3 Standards
Calcium carbonate (CaCO3, CAS No. 471-34-1): purity>99.99% or the standard calcium solution (in certain concentration) that is approved and awarded with reference material certificate by the nation.
3.4 Preparation of standard solution
3.4.1 Calcium standard stock solution (1 000mg/L): accurately weigh 2.496 3g (accurate to 0.000 1g) of calcium carbonate, dissolve it with hydrochloric acid solution (1+1), then transfer it into a 1 000mL volumetric flask, bring the volume to the scale with water and mix well.
3.4.2 Standard intermediate solution of calcium (100mg/L): accurately pipet 10mL of calcium standard stock solution (1 000mg/L) to a 100mL volumetric flask, bring the volume to the scale with nitric acid solution (5+95) and then mix well.
3.4.3 Standard series solution of calcium: respectively pipet 0mL, 0.500mL, 1.00mL, 2.00mL, 4.00mL and 6.00mL of standard intermediate solution of calcium (100mg/L) into 100mL volumetric flasks, add 5mL of lanthanum solution (20g/L) into each volumetric flask and then add into nitric acid solution (5+95) to bring the volume to the scale and mix well. The mass concentration of calcium in the standard series solution is 0mg/L, 0.500mg/L, 1.00mg/L, 2.00mg/L, 4.00mg/L and 6.00mg/L respectively.
Note: The specific concentration of elements in standard solution series may be determined according to the sensitivity of apparatus and actual calcium content in the sample.
4 Apparatuses
Note: All the glassware and polytetrafluoroethylene digestion inner tanks shall be soaked in nitric acid solution (1+5) over night, flushed with tap water repeatedly and finally washed clean with water.
4.1 Atomic absorption spectrometer: equipped with flame atomizer and chromium hollow cathode lamp.
4.2 Analytical balance: with sensitivity of 1mg and 0.1mg.
4.3 Microwave digestion system: equipped with polytetrafluoroethylene digestion inner tank.
4.4 Adjustable electrothermal furnace.
4.5 Adjustable electric hot plate.
4.6 Pressure digestion tank: equipped with polytetrafluoroethylene digestion inner tank.
4.7 Thermostatic drying oven.
4.8 Muffle furnace.
5 Analysis Procedures
5.1 Specimen preparation
Note: Specimen shall not be contaminated during the sampling and preparation processes.
5.1.1 Grain and beans samples
Pulverize the sample and store it in plastic bottle after foreign substances are removed from it.
5.1.2 Vegetables, fruits, fish and meat samples
Clean the sample with water, dry it in the air, take the edible part to make into homogenate and store the homogenate in the plastic bottle.
5.1.3 Such liquid sample as beverage, liquor, vinegar, soy sauce, edible oils and liquid milk
Shake the sample well.
5.2 Specimen digestion
5.2.1 Wet digestion
Accurately weigh 0.2g~0.3g (accurate to 0.001g) of solid specimen or accurately transfer 0.500mL~5.00mL of liquid specimen to the digestion pipe with scale, add into 10mL of nitric acid and 0.5mL of perchloric acid, digest them on adjustable electrothermal furnace (reference conditions: 120℃/0.5h~120℃/1h, rising to 180℃/2h~180℃/4h and rising to 200℃~220℃). If the digestive solution is sepia, add into nitric acid again, digest to emit white smoke and digestive solution acts like colorless and transparent or slightly yellow. Take out the digestion pipe and after cooling, bring the volume to 25mL with water; dilute according to the actual determination requirements and add certain volumes of lanthanum solution (20g/L) into the dilute solution to make its concentration in final dilute solution be 1g/L, mix well for future use and this is the to-be-determined solution of specimen. Carry out the reagent blank test simultaneously. Wet digestion can also be carried out on adjustable electric hot plate with conical flask according to the above-mentioned operating methods..
5.2.2 Microwave digestion
Accurately weigh 0.2g~0.8g (accurate to 0.001g) of solid specimen or accurately transfer 0.500mL~3.00mL of liquid specimen into the microwave digestion tank, add into 5mL of nitric acid, and digest the specimen according to the operation procedures of microwave digestion (see Appendix A for digestion conditions). After cooling, take out the digestion tank, and catch acid to about 1mL by placing it on 140℃~160℃ electric hot plate. After the digestion tank cools down, transfer the digestive solution to a 25mL volumetric flask, wash the digestion tank for 2~3 times with a small amount of water, combine the washing solution to the volumetric flask and bring the volume to the scale with water. Dilute according to the actual determination requirements and add certain volumes of lanthanum solution (20g/L) into the dilute solution to make its concentration in final dilute solution be 1g/L, mix well for future use and this is the to-be-determined solution of specimen. Carry out the reagent blank test simultaneously.
5.2.3 Pressure tank digestion
Accurately weigh 0.2g~1g (accurate to 0.001g) of solid specimen or accurately transfer 0.500mL~5.00mL of liquid specimen into the digestion inner tank and add into 5mL of nitric acid. Cover up the inner cover, tighten the stainless steel enclosure, put the tank in thermostatic drying oven and maintain for 4h~5h at 140℃~160℃. After cooling down, slowly loosen the outer tank, take out the digestion inner tank, catch acid to about 1mL by placing it on the adjustable electric hot plate at 140℃~ 160℃. After cooling down, transfer the digestive solution to a 25mL volumetric flask, wash the inner tank and inner cover for 2~3 times with a small amount of water, combine the washing solution into volumetric flask, bring the volume to the scale with water and mix well for future use. Dilute the solution according to the actual determination requirements and add certain volumes of lanthanum solution (20g/L) into the dilute solution to make its concentration in final dilute solution be 1g/L, mix well for future use and this is the to-be-determined solution of specimen. Carry out the reagent blank test simultaneously.
5.2.4 Dry-ashing method
Accurately weigh 0.5g~5g (accurate to 0.001g) of solid specimen or accurately transfer 0.500mL~10.0mL of liquid specimen into the crucible, heat with soft fire, carbonize it to smokeless and transfer to the muffle furnace for ashing for 3h~4h at 550℃. Take it out after cooling. For the specimen not ashed completely, add into several drops of nitric acid, heat with soft fire, carefully evaporate to dryness, and transfer the solution to a 550℃ muffle furnace; continuously ash for 1h~2h until the specimen acts like lime, cool down, take it out, transfer it to the graduated tube with proper amount of nitric acid solution (1+1) and bring the volume to 25mL with water. Dilute according to the actual determination requirements and add certain volumes of lanthanum solution into the dilute solution to make its concentration in final dilute solution be 1g/L, mix well for future use and this is the to-be-determined solution of specimen. Carry out the reagent blank test simultaneously.
5.3 Reference conditions of apparatus
See Appendix B for reference conditions.
5.4 Plotting of standard curve
Lead the standard series solution of calcium into flame atomizer by the sequence of low concentration to high concentration, determine the absorbance value and plot the standard curve by taking the mass concentration of calcium as horizontal coordinate and corresponding absorbance value as longitudinal coordinate.
5.5 Determination of specimen solution
Under the same experimental condition as that for determining standard solution, lead the blank solution and to-be-determined solution of specimen into the atomizer respectively, determine corresponding absorbance value and compare with standard series for quantitation.
Contents
Foreword i
1 Scope
2 Principle
3 Reagents and Materials
4 Apparatuses
5 Analysis Procedures
6 Expression of Analysis Results
7 Accuracy
8 Others
9 Principle
10 Reagents and Materials
11 Apparatuses
12 Analysis Procedures
13 Expression of Analysis Results
14 Accuracy
15 Others
Appendix A Reference Conditions of Temperature Programming of Microwave Digestion
Appendix B Reference Conditions of Flame Atomic Absorption Spectrometry
