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This standard replaces GB 5009.93-2010 National Food Safety Standard - Determination of Selenium in Foods, GB/T 21729-2008 Determination of Selenium Content in Tea, SN/T 0860-2000 Method for the Determination of Selenium in Canned Mushroom for Export - Fluorometry and SN/T 0926-2000 Method for the Determination of Selenium in Tea for Import and Export - Fluorimetry.
The following main changes have been made with respect to GB 5009.93-2010 (the previous edition):
——The hydride-atomic fluorescence spectrometry is retained as Method I, and the fluorescence spectrophotometry as Method II;
——Inductively coupled plasma mass spectrometry is added as Method III.
National Food Safety Standard
Determination of Selenium in Foods
1 Scope
This standard specifies hydride-atomic fluorescence spectrometry, fluorescence spectrophotometry and inductively coupled plasma mass spectrometry for determination of selenium content in foods.
This standard is applicable to determination of selenium in various foods.
Method I Hydride-atomic Fluorescence Spectrometry
2 Principle
After hot digestion of the specimen with acid, in 6mo1/L hydrochloric acid medium, hexavalent selenium in specimen is reduced to quadrivalence selenium, which is reduced to hydrogen selenide in hydrochloric acid medium with sodium borohydride or potassium borohydride as the reductant; hydrogen selenide is brought into an atomizer by carrier gas (argon gas) for atomization; under the irradiation of selenium hollow cathode lamp, ground state selenium atom is excited to upper state and emits fluorescence with characteristic wavelength after being deactivated to ground state, whose fluorescence intensity is in direct proportion to the selenium content and compared with standard series for quantitation.
3 Reagents and Materials
Unless otherwise specified, analytically-pure reagents and Class-II water (defined in GB/T 6682) are adopted for the purpose of this method.
3.1 Reagents
3.1.1 Nitric acid (HNO3): guaranteed reagent.
3.1.2 Perchloric acid (HClO4): guaranteed reagent.
3.1.3 Hydrochloric acid (HCl): guaranteed reagent.
3.1.4 Sodium hydroxide (NaOH): guaranteed reagent.
3.1.5 Hydogen peroxide (H2O2).
3.1.6 Sodium borohydride (NaBH4): guaranteed reagent.
3.1.7 Potassium ferricyanide [K3Fe(CN)6].
3.2 Preparation of reagents
3.2.1 Mixed acid of nitric acid and perchloric acid (9+1): mix 900mL nitric acid and 100mL perchloric acid uniformly.
3.2.2 Sodium hydroxide solution (5g/L): weigh 5g sodium hydroxide, dissolve in 1000mL water and mix uniformly.
3.2.3 Sodium borohydride alkali solution (8g/L): weigh 8g sodium borohydride, dissolve in sodium hydroxide solution (5g/L) and mix uniformly. Prepare immediately prior to use.
3.2.4 Hydrochloric acid solution (6mol/L): measure 50mL hydrochloric acid, slowly add into 40mL water, scale the volume with water to 100mL after cooling, and mix uniformly.
3.2.5 Potassium ferricyanide solution (100g/L): weigh 10g potassium ferricyanide, dissolve in 100mL water and mix uniformly.
3.2.6 Hydrochloric acid solution (5+95): measure 25mL hydrochloric acid, slowly add into 475mL water and mix uniformly.
3.3 Standard product
Selenium standard solution: 1000mg/L, or selenium standard solution of certain concentration approved and awarded with reference material certificate by the State.
3.4 Preparation of standard solutions
3.4.1 Selenium standard intermediate solution (100mg/L): accurately pipet 1.00mL selenium standard solution (1000mg/L) into a 10mL volumetric flask, add hydrochloric acid solution (5+95), dilute the solution to the scale and mix uniformly.
3.4.2 Selenium standard working solution (1.00mg/L): accurately pipet 1.00mL selenium standard intermediate solution (100mg/L) into a 100mL volumetric flask, dilute with hydrochloric acid solution (5+95) to the scale and mix uniformly.
3.4.3 Selenium standard series solutions: accurately pipet 0mL, 0.500mL, 1.00mL, 2.00mL and 3.00mL selenium standard working solutions (1.00mg/L) into 100mL volumetric flasks respectively, add 10mL potassium ferricyanide solution (100g/L), dilute with hydrochloric acid solution (5+95) to the scale and mix uniformly for test. Mass concentrations of such selenium standard series solutions are 0μg/L, 5.00μg/L, 10.0μg/L, 20.0μg/L and 30.0μg/L respectively.
Note: mass concentration of selenium in standard series solutions may be determined according to the sensitivity of instruments and the actual selenium content in the sample.
4 Instruments and Apparatus
Note: all glassware and polytetrafluoroethylene digestion inner tanks shall be soaked in nitric acid solution (1+5) overnight, flushed with tap water repeatedly and finally washed clean with water.
4.1 Atomic fluorescence spectrometer: equipped with selenium hollow cathode lamp.
4.2 Balance: with sensitivity of 1mg.
4.3 Electric hot plate.
4.4 Microwave digestion system: equipped with polytetrafluoroethylene digestion inner tank.
5 Analysis Steps
5.1 Preparation of specimen
Note: specimen shall be free from contamination in the process of sampling and preparation.
5.1.1 Grain and bean samples
Crush and store the samples in plastic bottles after foreign substances are removed from them.
5.1.2 Such samples as vegetables, fruits, fish and meat
Clean the samples with water, dry them in the air, take the edible parts to make into homogenate and store them in plastic bottles.
5.1.3 Such liquid samples as beverage, wine, vinegar, soy sauce, edible vegetable oil and liquid milk
Shake the samples well.
5.2 Digestion of specimen
5.2.1 Wet digestion
Weigh 0.5~3g (accurate to 0.001g) solid specimen or accurately transfer 1.00~5.00mL liquid specimen, put into a conical flask, add 10mL mixed acid of nitric acid and perchloric acid (9+1) and several glass beads, and cover the watch glass for cold digestion overnight. Heat it on electric hot plate the next day and timely supplement nitric acid. Continue to heat it until the residual volume is about 2mL where the solution turns to be clear and colorless with white smoke, but don't evaporate to dryness. Cool, add another 5mL hydrochloric acid (6mol/L), continue to heat it until the solution turns to be clear and colorless with white smoke. After cooling, transfer to 10mL volumetric flask, add 2.5mL potassium ferricyanide solution (100g/L). Scale the volume with water and mix uniformly for test. Meanwhile, carry out reagent blank test.
5.2.2 Microwave digestion
Weigh 0.2~0.8g (accurate to 0.001g) solid specimen or accurately transfer 1.00~3.00mL liquid specimen, put into a digestion tube, add 10mL nitric acid and 2mL hydrogen peroxide, shake and mix uniformly, digest in microwave digestion system, see Annex A for the recommended microwave digestion conditions (digestion conditions may be set by oneself according to different instruments). At end of the digestion, after cooling, transfer the digestion solution into a conical flask, add several glass beads, continue to heat on electric hot plate to nearly dryness, but don't evaporate to dryness. Add another 5mL hydrochloric acid solution (6mol/L), continue to heat until the solution turns to be clear and colorless with white smoke, cool, transfer into a 10mL volumetric flask, add 2.5mL potassium ferricyanide solution (100g/L), scale the volume with water and mix uniformly for test. Meanwhile, carry out reagent blank test.
Contents
Foreword i
1 Scope
2 Principle
3 Reagents and Materials
4 Instruments and Apparatus
5 Analysis Steps
6 Expression of Analysis Results
7 Accuracy
8 Other
9 Principle
10 Reagents and Materials
11 Instruments and Apparatus
12 Analysis Steps
13 Expression of Analysis Results
14 Accuracy
15 Other
Annex A Temperature Rise Procedures of Microwave Digestion
