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GB 5009.11-2024: PDF in English

GB 5009.11-2024
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
GB 5009.11-2024
National food safety standard - Determination of total
arsenic and inorganic arsenic in food
ISSUED ON: FEBRUARY 08, 2024
IMPLEMENTED ON: AUGUST 08, 2024
Issued by: National Health Commission of the People’s Republic of China;
State Administration for Market Regulation.
Table of Contents
Foreword ... 4
1 Scope ... 5
Chapter 1: Determination of total arsenic in food ... 5
Method I: Hydride generation atomic fluorescence spectrometry ... 5
2 Principle ... 5
3 Reagents and materials ... 6
4 Instruments and equipment ... 8
5 Analysis procedure ... 8
6 Expression of analysis results ... 11
7 Precision ... 11
8 Others ... 12
Method II: Inductively coupled plasma mass spectrometry ... 12
Method III: Graphite furnace atomic absorption spectrometry ... 12
9 Principle ... 12
10 Reagents and materials ... 12
11 Instruments and equipment ... 13
12 Analysis procedure ... 14
13 Expression of analysis results ... 15
14 Precision ... 15
15 Others ... 16
Chapter 2: Determination of inorganic arsenic in food ... 16
Method I: Liquid chromatography-atomic fluorescence spectrometry ... 16
16 Principle ... 16
17 Reagents and materials ... 16
18 Instruments and equipment ... 19
19 Analysis procedure ... 20
20 Expression of analysis results ... 25
21 Precision ... 25
22 Others ... 26
Method II: Liquid chromatography-inductively coupled plasma mass spectrometry . 26
23 Principle ... 26
24 Reagents and materials ... 26
25 Instruments and equipment ... 29
26 Analysis procedure ... 29
27 Expression of analysis results ... 32
28 Precision ... 33
29 Others ... 33
Annex A Reference conditions for microwave digestion ... 34
Annex B Reference conditions for instruments ... 35
Annex C Reference conditions for instruments ... 36
Annex D Chromatograms of LC-AFS method ... 38
Annex E Chromatograms of LC-ICP/MS method ... 42
National food safety standard - Determination of total
arsenic and inorganic arsenic in food
1 Scope
Chapter 1 of this Standard specifies the methods for the determination of total arsenic
in food.
Methods I and II of Chapter 1 of this Standard apply to the determination of total arsenic
in food. Method III applies to the determination of total arsenic in food (except milk
powder and prepared milk powder, oils and fats and their products, condiments, and
special dietary foods).
Chapter 2 of this Standard specifies the methods for the determination of inorganic
arsenic in food.
Chapter 2 of this Standard applies to the determination of inorganic arsenic in cereals
and their products, aquatic animals and their products, edible fungi and their products,
oils and fats and their products, condiments, supplementary foods for infants and young
children, algae and their products.
Chapter 1: Determination of total arsenic in food
Method I: Hydride generation atomic fluorescence
spectrometry
2 Principle
After the sample is digested, thiourea is added to pre-reduce the pentavalent arsenic to
trivalent arsenic, and then sodium borohydride or potassium borohydride is added to
reduce the trivalent arsenic to generate arsine, which is loaded into the quartz atomizer
by argon gas and decomposed into atomic arsenic. The atomic arsenic produces atomic
fluorescence under the excitation of the emitted light of the arsenic hollow cathode lamp.
Under fixed conditions, its fluorescence intensity is proportional to the arsenic
concentration in the solution being tested. It is quantitatively determined by external
standard method.
3.2.5 Magnesium nitrate solution (150 g/L): Weigh 15.0 g of magnesium nitrate,
dissolve with water and dilute to 100 mL, and mix well.
3.2.6 Hydrochloric acid solution (1 + 1): Measure 100 mL of hydrochloric acid, slowly
pour it into 100 mL of water, and mix well.
3.2.7 Sulfuric acid solution (1 + 9): Measure 100 mL of sulfuric acid, slowly pour it
into 900 mL of water, and mix well.
3.2.8 Nitric acid solution (2 + 98): Measure 20 mL of nitric acid, slowly pour it into
980 mL of water, and mix well.
NOTE: This method can also use sodium borohydride (20 g/L) as the reducing agent: weigh 20 g of
sodium borohydride, dissolve it in 1000 mL of 5 g/L sodium hydroxide solution, and mix well. The
concentration of potassium borohydride or sodium borohydride solution can be adjusted according
to the sensitivity of the instrument. Freshly prepare each time before use.
3.3 Reference material
Arsenic trioxide (As2O3, CAS number: 1327-53-3) reference material: purity ≥ 99.5 %.
3.4 Preparation of standard solutions
3.4.1 Arsenic standard stock solution (100 mg/L, calculated as As): Accurately weigh
0.0132 g of arsenic trioxide that has been dried at 100 °C for 2 h, add 1 mL of sodium
hydroxide solution (100 g/L) and a small amount of water to dissolve, transfer to a 100
mL volumetric flask, add an appropriate amount of hydrochloric acid to adjust its
acidity to near neutral, and dilute to the mark with water. Store in a refrigerator at 2 ℃
~ 8 ℃ away from light, the validity period is 1 year. Or arsenic standard solution
certified by the country and awarded a reference material certificate.
3.4.2 Arsenic standard use solution (1.00 mg/L, calculated as As): Accurately pipette
1.00 mL of arsenic standard stock solution (100 mg/L) into a 100 mL volumetric flask,
and dilute to the mark with nitric acid solution (2 + 98). Store in a refrigerator at 2 ℃
~ 8 ℃ away from light, the validity period is 3 months.
3.4.3 Arsenic standard series solutions: Take 7 25 mL volumetric flasks or colorimetric
tubes, accurately add 0.00 mL, 0.05 mL, 0.10 mL, 0.25 mL, 0.50 mL, 1.50 mL and 3.00
mL of arsenic standard use solution (1.00 mg/L) in sequence (equivalent to arsenic
concentrations of 0.0 μg/L, 2.0 μg/L, 4.0 μg/L, 10.0 μg/L, 20.0 μg/L, 60.0 μg/L and
120.0 μg/L respectively), add 12.5 mL of sulfuric acid solution (1 + 9) and 2 mL of
thiourea + ascorbic acid solution, add water to the mark, mix well, and let stand for 30
min before determination. Freshly prepare each time before use.
NOTE: The mass concentration range of arsenic in the standard series solutions can be fine-tuned
according to the sensitivity of the instrument and the actual arsenic content in the sample.
For thawed quick-frozen food, it shall take the edible parts and crush evenly; for canned
food, it shall crush or homogenize evenly.
5.1.2 Liquid samples
For samples such as beverages, condiments, milk and their products, oils and fats and
their products, it shall homogenize.
5.1.3 Semi-solid samples
Stir evenly.
5.2 Digestion of samples
5.2.1 Wet digestion
Weigh 0.5 g ~ 2.5 g (accurate to 0.001 g) of solid samples or 5.0 g ~ 10.0 g (accurate to
0.001 g) of liquid samples into a digestion bottle or digestion tube, add 20 mL of nitric
acid, 4 mL of perchloric acid and 1.25 mL of sulfuric acid and leave overnight. The next
day, heat and digest at 120 ℃ ~ 200 ℃ step by step. If there are still undecomposed
substances or the color becomes darker when the digestion solution reaches about 5 mL,
add 5 mL ~ 10 mL of nitric acid, and then digest to about 2 mL. Repeat this 2 to 3 times,
taking care to avoid carbonization, and continue heating and digestion until the
digestion solution is about 1 mL, which is colorless and clear, and the digestion bottle
or digestion tube is filled with white smoke (for samples of aquatic animals and their
products, edible fungi and their products, fish oil and their products, krill oil and their
products, algae and their products which have high organic arsenic, raise the digestion
temperature to 280 ℃ ~ 300 ℃ and continue to heat and digest until the digestion
solution is about 0.5 mL, which is colorless and clear, and the digestion bottle or
digestion tube is filled with white smoke). After cooling, slowly add about 10 mL of
water along the wall of the digestion container, and then evaporate until the digestion
bottle or digestion tube is filled with white smoke. Cool down, transfer the digestion
solution with water into a 25 mL volumetric flask or colorimetric tube, add 2 mL of
thiourea + ascorbic acid solution, dilute to the mark with water, mix well, and leave it
for 30 min before determination. Do a blank test at the same time.
5.2.2 Dry ashing method
Weigh 1.0 g ~ 2.5 g (accurate to 0.001 g) of solid samples or weigh 4.0 g (accurate to
0.001 g) of liquid samples (excluding samples of oils and fats), and place it in a 50 mL
~ 100 mL crucible. Add 10 mL of magnesium nitrate solution (150 g/L) and mix well,
evaporate to dryness over low heat, and cover the dry residue with 1 g of magnesium
oxide (for samples of oils and fats, weigh 1.00 g in a 50 mL ~ 100 mL crucible, and
directly add 0.2 g of magnesium oxide to cover on oils and fats), carbonize on an electric
furnace until there is no black smoke, then transfer it to a 550 ℃ muffle furnace for
ashing for 4 h. Take it out and let it cool, carefully add 5 mL ~ 10 mL of hydrochloric
acid solution (1 + 1) to neutralize the magnesium oxide and dissolve the ash, transfer it
to a 25 mL volumetric flask or colorimetric tube, add 2 mL of thiourea + ascorbic acid
solution to the volumetric flask or colorimetric tube, wash the crucible with sulfuric
acid solution (1 + 9) in batches, combine the washing liquids and dilute to the mark,
mix well, and leave it for 30 min before determination. Do a blank test at the same time.
5.2.3 Microwave digestion method
Weigh 0.2 g ~ 0.8 g (accurate to 0.001 g) of solid samples and samples of oils and fats
and their products, or weigh 1.0 g ~ 3.0 g (accurate to 0.001 g) of samples containing
more water or liquid samples into a digestion tank, add 5 mL ~ 8 mL of nitric acid, and
leave it for more than 30 min. For samples that are difficult to digest such as meat, oils,
and fats, add 0.5 mL ~ 1 mL of hydrogen peroxide, cover the safety valve, and put the
digestion tank into the microwave digestion system. According to different types of
samples, set up appropriate microwave digestion procedures (see Annex A, Table A.1),
and perform digestion according to relevant steps. After digestion, reduce acid to 1 mL
~ 2 mL at 135 °C ~ 145 °C. Transfer the digestion solution to a 25 mL volumetric flask
or colorimetric tube, wash the digestion tank 3 times with a small amount of sulfuric
acid solution (1 + 9), combine the washing liquid into the volumetric flask or
colorimetric tube and add 2 mL of thiourea + ascorbic acid solution, dilute to mark with
sulfuric acid solution (1 + 9), mix well, and leave it for 30 min before determination.
Do a blank test at the same time.
NOTE: The microwave digestion method does not apply to samples with complex matrices such as
aquatic animals and their products, edible fungi and their products, fish oil, krill oil and their
products, aquatic condiments, algae and their products, which have a high organic arsenic content.
5.2.4 Pressure tank digestion method
Weigh 0.2 g ~ 1.0 g (accurate to 0.001 g) of solid samples and samples of oils and fats
and their products, or weigh 1.0 g ~ 5.0 g (accurate to 0.001 g) of fresh samples or
liquid samples into a digestion inner tank, and add 5 mL of nitric acid and soak
overnight. Cover the inner cover, tighten the stainless steel jacket, put it into a constant
temperature drying oven, keep it at 140 ℃ ~ 160 ℃ for 3 h ~ 4 h, naturally cool to
room temperature, then slowly loosen the stainless steel jacket, take out the digestion
inner tank, and place it on a temperature-controlled electric heating plate at 135 ℃ ~
145 ℃ to reduce acid to 1 mL ~ 2 mL. Transfer the digestion solution to a 25 mL
volumetric flask or colorimetric tube, wash the digestion tank 3 times with a small
amount of sulfuric acid solution (1 + 9), combine the washing liquid into the volumetric
flask or colorimetric tube and add 2 mL of thiourea + ascorbic acid solution, dilute to
mark with sulfuric acid solution (1 + 9), mix well, and leave it for 30 min before
determination. Do a blank test at the same time.
NOTE: The pressure tank digestion method does not apply to samples with complex matrices such
as aquatic animals and their products, edible fungi and their products, fish oil, krill oil and their
products, aquatic condiments, algae and their products, which have a high organic arsenic content.
10.1.4 Sodium hydroxide (NaOH).
10.2 Preparation of reagents
10.2.1 Nitric acid solution (2 + 98): Measure 20 mL of nitric acid, slowly pour it into
980 mL of water, and mix well.
10.2.2 Nitric acid solution (3 + 97): Measure 30 mL of nitric acid, slowly add it to 970
mL of water, and mix well.
10.2.3 Sodium hydroxide solution (100 g/L): Weigh 10.0 g of sodium hydroxide,
dissolve with water and dilute to 100 mL, and mix well.
10.2.4 Palladium nitrate solution (1 g/L): Weigh 0.1 g of palladium nitrate, dilute to 100
mL with nitric acid solution (3 + 97), and mix well.
10.3 Reference material
Arsenic trioxide (As2O3, CAS number: 1327-53-3) reference material: purity ≥ 99.5 %.
10.4 Preparation of standard solutions
10.4.1 Arsenic standard stock solution (100 mg/L, calculated as As): Same as 3.4.1.
10.4.2 Arsenic standard use solution (1.00 mg/L, calculated as As): Same as 3.4.2.
10.4.3 Arsenic standard series solutions: Take an appropriate amount of arsenic
standard use solution (1.00 mg/L), use nitric acid solution (3 + 97) to prepare standard
series solutions with arsenic mass concentrations of 0.0 μg/L, 2.0 μg/L, 5.0 μg/L, 10.0
μg/L, 20.0 μg/L and 30.0 μg/L, respectively.
NOTE: The mass concentration range of arsenic in the standard series solutions can be fine-tuned
according to the sensitivity of the instrument and the actual arsenic content in the sample.
11 Instruments and equipment
11.1 Atomic absorption spectrometer: it is equipped with a graphite furnace atomizer.
11.2 Electronic balance: the minimum division is 0.01 mg, 0.1 mg and 1 mg.
11.3 Temperature control electric heating plate: the temperature control accuracy is
±5 ℃.
11.4 Microwave digestion system.
11.5 Constant temperature drying oven: the temperature control accuracy is ±2 °C.
11.6 Pressure digestion tank.
11.7 Homogenizer.
11.8 High-speed crusher.
NOTE: Glassware and polytetrafluoroethylene digestion inner tanks need to be soaked in (1 + 4)
nitric acid solution for 24 h, rinsed repeatedly with tap water, and finally rinsed with water.
12 Analysis procedure
12.1 Preparation of samples
Same as 5.1.
12.2 Digestion of samples
12.2.1 Microwave digestion method
Weigh 0.2 g ~ 0.8 g (accurate to 0.001 g) of solid samples and samples of oils and fats
and their products, or weigh 1.0 g ~ 3.0 g (accurate to 0.001 g) of samples containing
more water or liquid samples into a digestion tank; add 5 mL ~ 8 mL of nitric acid and
leave it for more than 30 min. For samples that are difficult to digest such as meat and
aquatic animals and their products, add 0.5 mL ~ 1 mL of hydrogen peroxide, cover the
safety valve, and put the digestion tank into the microwave digestion system. According
to different types of samples, set up appropriate microwave digestion procedures (see
Table A.2) and perform digestion according to relevant steps. After digestion is
completed, reduce acid to 0.5 mL ~ 1 mL at 140 ℃ ~ 145 ℃, dilute with water to 25
mL, and mix well for later use. Do a blank test at the same time.
12.2.2 Pressure tank digestion method
Weigh 0.2 g ~ 1.0 g (accurate to 0.001 g) of solid samples and samples of oils and fats
and their products, or weigh 1.0 g ~ 5.0 g (accurate to 0.001 g) of fresh samples or
liquid samples into a digestion inner tank, and add 5 mL of nitric acid and soak
overnight. Cover the inner cover, tighten the stainless steel jacket, put it into a constant
temperature drying oven, keep it at 140 ℃ ~ 160 ℃ for 3 h ~ 4 h, naturally cool to
room temperature, then slowly loosen the stainless steel jacket, take out the digestion
inner tank, place it on a temperature-controlled electric heating plate to reduce acid to
0.5 mL ~ 1 mL at 140 ℃ ~ 145 ℃, dilute with water to 25 mL, and mix well for later
use. Do a blank test at the same time.
12.3 Reference conditions for instruments
See Table B.1 in Annex B for reference conditions for instruments. It shall subtract
background when using deuterium lamp, self-absorption or Zeeman effect.
12.4 Plotting of standard curve
17.1.8 Hydrogen peroxide (H2O2): 30 %.
17.1.9 Hydrochloric acid (HCl).
17.1.10 Ammonia (NH3 · H2O).
17.1.11 n-hexane [CH3(CH2)4CH3]: chromatographically pure.
17.2 Preparation of reagents
17.2.1 Hydrochloric acid solution (7 + 93): Measure 70 mL of hydrochloric acid,
dissolve with water and dilute to 1000 mL, and mix well.
17.2.2 Nitric acid solution (0.15 mol/L): Measure 10 mL of nitric acid, dissolve with
water and dilute to 1000 mL, and mix well.
17.2.3 Nitric acid + hydrogen peroxide solution (0.15 mol/L nitric acid + 0.45 %
hydrogen peroxide): Measure 10 mL of nitric acid and 15 mL of 30 % hydrogen
peroxide, add water to dilute to 1000 mL, and mix well. Freshly prepare each time
before use.
17.2.4 Potassium hydroxide solution (100 g/L): Weigh 10 g of potassium hydroxide,
dissolve with water and dilute to 100 mL, and mix well.
17.2.5 Potassium hydroxide solution (5 g/L): Weigh 5 g of potassium hydroxide,
dissolve with water and dilute to 1000 mL, and mix well.
17.2.6 Potassium borohydride solution (20 g/L): Weigh 20 g of potassium borohydride,
dissolve with 5 g/L potassium hydroxide solution and dilute to 1000 mL, and mix well.
Freshly prepare each time before use.
17.2.7 Mobile phase A (15 mmol/L ammonium dihydrogen phosphate, pH = 6.0):
Weigh 1.73 g of ammonium dihydrogen phosphate, dissolve in 1000 mL of water, adjust
the pH to 6.0 with ammonia water, and mix well. Ultrasonically degas in an ultrasonic
water bath for 30 min for later use.
17.2.8 Mobile phase B (1 mmol/L diammonium hydrogen phosphate, pH = 9.0): Weigh
0.132 g of diammonium hydrogen phosphate, dissolve in 1000 mL of water, adjust the
pH to 9.0 with ammonia water, and mix well. Ultrasonically degas in an ultrasonic water
bath for 30 min for later use.
17.2.9 Mobile phase C (30 mmol/L diammonium hydrogen phosphate, pH = 8.5):
Weigh 3.96 g of diammonium hydrogen phosphate, dissolve in 1000 mL of water, adjust
the pH to 8.5 with ammonia water, and mix well. Ultrasonically degas in an ultrasonic
water bath for 30 min for later use.
17.2.10 Mobile phase D (5 mmol/L dipotassium hydrogen phosphate + 1 mmol/L
ammonium nitrate, pH = 10.9): Weigh 1.14 g of dipotassium hydrogen phosphate and
0.08 g of ammonium nitrate, dissolve in 1000 mL of water, adjust the pH to 10.9 with
ammonia water, and mix well. Ultrasonically degas in an ultrasonic water bath for 30
min for later use.
17.2.11 Mobile phase E (25 mmol/L dipotassium hydrogen phosphate + 40 mmol/L
ammonium nitrate, pH = 9.2): Weigh 5.7 g of dipotassium hydrogen phosphate and 3.2
g of ammonium nitrate, dissolve in 1000 mL of water, adjust the pH to 9.2 with
ammonia water, and mix well. Ultrasonically degas in an ultrasonic water bath for 30
min for later use.
NOTE: This method can also use sodium borohydride (20 g/L) as the reducing agent. Weigh 20 g
of sodium borohydride, dissolve it in 1000 mL of 5 g/L sodium hydroxide solution, and mix well.
The concentration of potassium borohydride or sodium borohydride solution can be adjusted
according to the sensitivity of the instrument. Freshly prepare each time before use.
17.3 Reference materials
17.3.1 Arsenic trioxide (As2O3, CAS number: 1327-53-3) reference material: purity ≥
99.5 %.
17.3.2 Potassium dihydrogen arsenate (KH2AsO4, CAS number: 7784-41-0) reference
material: purity ≥ 99.5 %.
17.4 Preparation of standard solutions
17.4.1 Arsenite [As(Ⅲ)] standard stock solution (100 mg/L, calculated as As):
Accurately weigh 0.0132 g of arsenic trioxide that has been dried at 100 °C for 2 h, add
1 mL of 100 g/L potassium hydroxide solution and a small amount of water to dissolve,
transfer to a 100 mL volumetric flask, add an appropriate amount of hydrochloric acid
to adjust its acidity to near neutral, and add water to dilute to the mark. Store in a
refrigerator at 2 ℃ ~ 8 ℃ away from light, the validity period is 1 year. Or arsenite
standard solution certified by the country and awarded a reference material certificate.
17.4.2 Arsenate [As(Ⅴ)] standard stock solution (100 mg/L, calculated as As):
Accurately weigh 0.0240 g of potassium dihydrogen arsenate that has been dried at
100 °C for 2 h, dissolve with water, transfer to a 100 mL volumetric flask and dilute to
the mark with water. Store in a refrigerator at 2 ℃ ~ 8 ℃ away from light, the validity
period is 1 year. Or arsenate standard solution certified by the country and awarded a
reference material certificate.
17.4.3 Organic arsenic standard solutions: Monomethylarsenic (MMA),
dimethylarsenic (DMA), arsenic betaine (AsB), purchase standard solutions certified
by the country and awarded a reference material certificate.
17.4.4 Organic arsenic standard intermediate solutions (10.0 mg/L, calculated as As):
Accurately pipette a certain amount of monomethylarsenic, dimethylarsenic and arsenic
betaine standard solutions respectively, and dilute with an appropriate amount of water
Weigh about 0.5 g ~ 1.0 g of the sample (accurate to 0.001 g) into a 50 mL
polypropylene centrifuge tube, and add 20 mL of 0.15 mol/L nitric acid solution. Make
hot extraction in a thermostat at 90 ℃ for 2.5 h, and shake for 1 min every 0.5 h. After
the extraction is completed, take it out and cool it to room temperature, centrifuge at
8000 r/min for 15 min, take the supernatant, filter through a 0.45 μm filter membrane,
and wait for determination. Do a blank test at the same time.
19.2.1.2 Microwave-assisted extraction method
Weigh about 0.5 g ~ 0.8 g of the sample (accurate to 0.001 g) into a microwave
extraction tank, add 15 mL of 0.15 mol/L nitric acid solution, cover the inner cover,
tighten the outer cover, place in a microwave digestion instrument, extract using
gradient heating method, and the microwave-assisted extraction procedure is shown in
Table C.1 in Annex C. After the extraction is completed, take it out after cooling,
centrifuge at 8000 r/min for 15 min, take the supernatant, filter through a 0.45 μm filter
membrane, and wait for determination. Do a blank test at the same time.
19.2.2 Aquatic animals and their products
Weigh about 0.5 g ~ 1.0 g of the sample (accurate to 0.001 g) into a 50 mL
polypropylene centrifuge tube, and add 20 mL of 0.15 mol/L nitric acid + 0.45 %
hydrogen peroxide solution. Make hot extraction in a thermostat at 90 ℃ for 2.5 h, and
shake for 1 min every 0.5 h. After the extraction is completed, take it out and cool it to
room temperature, and centrifuge at 8000 r/min for 15 min. Take 5 mL of the
supernatant and place it in a centrifuge tube, add 5 mL of n-hexane, shake for 1 min,
centrifuge at 8000 r/min for 15 min, and discard the upper layer of n-hexane. Repeat
this process once more. Pipette the lower layer of supernatant, filter it through a 0.45
μm filter membrane, and wait for determination. Do a blank test at the same time.
19.2.3 Supplementary foods for infants and young children
19.2.3.1 Cereal supplementary foods for infants and young children (except
products with added algae)
Weigh about 1.0 g of the sample (accurate to 0.001 g) into a 50 mL polypropylene
centrifuge tube, add 20 mL of 0.15 mol/L nitric acid solution, make hot extraction in a
thermostat at 90 ℃ for 2.5 h, and shake for 1 min every 0.5 h. After the extraction is
completed, take it out and cool it to room temperature. Centrifuge at 8000 r/min for 15
min. Pipette the supernatant, filter through a 0.45 μm filter membrane, and wait for
determination. Do a blank test at the same time.
19.2.3.2 Cereal supplementary foods for infants and young children (products with
added algae)
Weigh about 0.5 g ~ 1.0 g of the sample (accurate to 0.001 g) into a 50 mL
polypropylene centrifuge tube, and add 20 mL of 0.15 mol/L nitric acid + 0.45 %
hydrogen peroxide solution. Make hot extraction in a thermostat at 90 ℃ for 2.5 h, and
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.