GB 14883.9-2016 PDF English
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National Food Safety Standard -- Determination of iodine - 131, a radioactive substance in foods
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| GB 14883.9-1994 | English | 359 |
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Examination of radioactive materials for foods. Determination of iodine-131
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GB 14883.9-2016: PDF in English GB 14883.9-2016
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
National food safety standard - Determination of
iodine-131, a radioactive substance in foods
ISSUED ON: AUGUST 31, 2016
IMPLEMENTED ON: MARCH 01, 2017
Issued by: National Health and Family Planning Commission
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Principles ... 4
3 Reagents and materials ... 4
4 Instruments and equipment ... 4
5 Analytical procedures ... 5
6 Expression of analytical results ... 7
7 Others ... 8
8 Principles ... 8
9 Reagents and materials ... 8
10 Instruments and equipment ... 10
11 Analytical procedures ... 10
12 Expression of analytical results ... 12
13 Others ... 13
Appendix A Diagram of sample press ... 14
Appendix B Total correction factor for 131I measurement efficiency at different
heights and apparent densities ... 15
National food safety standard - Determination of
iodine-131, a radioactive substance in foods
1 Scope
This standard applies to the determination of iodine-131 (131I) in various foods.
Method 1 -- γ-spectroscopy method
2 Principles
Contain the fresh food samples directly or pre-treated into a sample box of a
certain shape and volume. On a γ-spectrometer, measure the total-energy
peak’s net area of the γ-ray characteristic peak of the 131I at 364.5 keV in the
sample. Compare it with the standard radioactive source of known activity.
Calculate the radioactivity concentration of 131I. It is best to use γ-spectroscopy
method for the determination of 131I in fresh-fission products within 6 days after
fission; otherwise it shall carry out decay measurements, to exclude the
interference from the short-lived iodine radioisotope.
3 Reagents and materials
137Cs radioactive standard solution: The specific activity is about 1000 Bq/mL.
It has been calibrated by the national legal metrology department and has the
inspection certificate as signed by the statutory recognized organization (it may
also directly use the 131I standard solution).
4 Instruments and equipment
4.1 Low-background γ-spectrometer system
4.1.1 Detector: Coaxial high-purity helium or germanium (lithium) detector. The
energy resolution of the 60Co 1332.5 keV γ-ray total-energy peak is less than 3
keV, the relative efficiency is higher than 15%.
4.1.2 Shield: The main shield is a multi-layer material heavy metal shield whose
equivalent lead equivalent is not less than 10 cm and atomic number of the
Respectively take 5 ~ 10 sample boxes of Φ75 mm × h75 mm. Respectively,
add different volumes of distilled water and 2 mL of 137Cs standard solution, to
make the height within the range of 1 cm ~ 5 cm.
5.1.3 Eh' calibration of benchmark peak’s efficiency
Measure the prepared standard source of 137Cs aqueous solution of different
heights. Use the formula (1) to calculate the detection efficiency Eh' of each at
the 661.6 keV γ-ray total-energy peaks:
Where:
Eh' - The detection efficiency of the standard source of 137Cs aqueous
solution of different heights at the total-energy peak of the 661.6 keV γ-ray;
N - The net area of the total-energy peak of the 661.6 keV γ-ray, n the unit
of count;
A' - The activity of 137Cs standard source, in the unit of Becquerel (Bq);
T' - The measurement time, in seconds (s);
B - The branch ratio of 137Cs 661.6 keV γ-ray, which is 84.62%.
Based on the above measured data, use the least squares method to fit out the
relationship curve of Eh'-H.
5.2 Sampling
Sampling is carried out in accordance with the provisions of GB 14883.1.
5.3 Preparation of sample
5.3.1 Grain samples: Take 500 g of sample. Evenly spread it in an enamel pan
or stainless-steel pan. Bake it in an oven at about 70 °C for about 5 h. Weight
it. Calculate the dry-to-fresh ratio. After the granular grain is dried, it is directly
placed in the sample box whose inside and outside had been cleaned; and
compacted. The fine-powdered grain is compacted by a sample press, so that
the sample’s height is about 4 cm. Record the dry weight and height of the
sample to be tested. Calculate the apparent density.
5.3.2 Vegetable samples: Take about 3 kg of sample. Remove the inedible part.
Wash it clean. Wipe off or dry the surface water drops. Chop it. Weigh the fresh
weight. Spread it in an enamel pan or a stainless-steel pan. Bake it in an oven
at about 70 °C until it is almost dry and soft. Weigh it. Calculate the dry-to-fresh
W - Amount of fresh sample corresponding to the measured sample, in
kilograms (kg) or liters (L);
Eh - Benchmark peak’s efficiency, which uses 137Cs 661.6 keV energy peak
as the reference peak, the value is found from the Eh'-H relationship curve
which is drawn in accordance with 5.1.3;
R - Relative peak efficiency, which may use 137Cs as the reference peak; it
may also be obtained by test method (see 5.1.2 ~ 5.1.3); Use the 133Ba
standard solution which simulates 131I to obtain the Eh-H relationship curve
of the 356.0 keV γ-ray, to calculate the ratio of the detection efficiency of the
356.0 keV peak and the 661.6 keV peak, which is the relative peak efficiency
of 131I;
F - Total correction factor of measurement efficiency, %, as shown in
Appendix B. For more accurate calculation methods, please refer to GB/T
16145;
λ - The decay constant of 131I, in units of hourly (h-1), λ = 0.693/T0, where T0
is the half-life of 131I, which takes 193 h;
t - The time interval between sampling and measurement, in hours (h).
7 Others
The calculation of the judgement limit and detection limit of this method may
make reference to GB/T 16145.
Method 2 -- Radiochemical assay
8 Principles
The fresh sample of food is carbonized and ashed after being immersed in
potassium carbonate solution. The water-leached liquid is extracted and
separated by carbon tetrachloride and source-made in the form of silver iodide.
Use the low-background β-meter to measure the β-radioactivity concentration
of 131I.
9 Reagents and materials
9.1 Reagents
Calibration: Accurately pipette 1.00 mL of iodine carrier solution in a beaker
which contains 20 mL of water. Heat it. Add a few drops of 2 mol/L nitric acid.
Immediately add 3 mL of 1% silver nitrate solution. Stir it. Heat it to make the
settlements accumulate. Cool it down. Extract and filter the settlements on a
quantitative filter paper which is placed in detachable funnel and has been
constant-weighted. Use a small amount of anhydrous ethanol to rinse it. Dry it
at 110 °C for 0.5 h. Weigh it to constant amount.
10 Instruments and equipment
10.1 Drying box.
10.2 Muffle furnace.
10.3 Conical separatory funnel: 250 mL.
10.4 Detachable funnel: 2 cm inner diameter.
10.5 Low-background β-meter: The diameter of the tested sample is not less
than 2 cm. The background is less than 1 count/min.
10.6 137Cs monitoring source: Use an electroplated 137Cs planar source which
has an activity zone’s diameter of 2 cm. The activity of which is the order of
magnitude of 102 decay/min.
11 Analytical procedures
11.1 Drawing of counting efficiency - mass curve
Accurately prepare a series of solutions which contain different amounts of
iodine carrier. Add an equal amount of 131I standard solution. Then operate
according to 11.4.7. Use the actual sedimentation mass of silver iodide as the
abscissa; use the measured radioactivity (I) divided by the activity (I0) of the
added 131I standard solution as the ordinate. Use computer processing or
drawing on ordinary coordinate paper, to obtain the curve of effective counting
efficiency - sample quality. It may, based on the quality of the sample source,
find the corresponding effective counting efficiency. Use the 137Cs monitoring
source to determine the counting efficiency of the monitoring source during
calibration.
11.2 Sampling
Sampling is carried out in accordance with the provisions of GB 14883.1.
11.3 Pretreatment of sample
the 2 mol/L sodium hydroxide solution to adjust the solution to alkaline. Heat it.
When it is still hot, filter the solution into a 100 mL beaker. Use a small amount
of weak-alkaline water to rinse the precipitate. Discard the precipitate (if there
is no obvious rare earth element contamination, this step may be omitted).
11.4.7 Heat the boiling solution. Cool it down. Add 5 mL of 2 mol/L nitric acid.
Whilst stirring it, add 3 mL of 1% silver nitrate solution. Heat it to make the
precipitate concentrate. After cooling it down, use a detachable funnel to extract
and filter the precipitate onto the filter paper which had been constant-weighted.
Use the 1% nitric acid solution to rinse the precipitate for several times. Use
absolute ethanol to rinse it. Dry it at 110 °C. Weigh the sample to constant
amount. Use the low-background β-meter to measure the β-radioactivity of the
prepared sample source and 137Cs monitoring source. The decay rate of the
sample source is calculated according to the formula (3):
Where:
D - The decay rate of the sample source, in units of decay per minute (dpm);
N - The net count rate of 131I measured in the sample, the unit is count per
minute (cpm);
E'Cs - The counting efficiency as measured when using the 137Cs monitoring
source to calibrate the effective count rate;
Ee - The effective count rate of 131I (including self-absorption correction),
which may be found from the counting efficiency-quality curve (see 11.1);
ECs - The counting efficiency as measured when using the 137Cs monitoring
source to determine the sample.
12 Expression of analytical results
The activity concentration of 131I in food is calculated according to formula (4):
Where:
A - The activity concentration of 131I in food, in units of becquerel per kilogram
(Bq/kg) or becquerel per liter (Bq/L);
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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