Epithermal INAA: I in food samplesPeter Vermaercke
International WorkshopIWIRAD 2005 on
“Applications of the Ionising-Radiations to Industry, Health and Environment”
20 and 21 June 2005, Bucharest, Romania
I in food samples: EINAA is probably the only technique suited
• Why determination of I in food samples?
• Why EINAA?• Validation results of EINAA• Iodine concentrations in some
typical food stuffs
Why determination of I in food samples?
• Iodine is an essential dietary element which is required for synthesis of the thyroid hormones. A shortage or an overdose of iodine creates thyroid disorder.
• The WHO and the Institute of Medicine at the National Academy of Sciences developed dietary reference intakes for iodine. These rise from 100 g/day for children up to 150 g/day for adults. The same institute also developed tolerable upper Intake levels for iodine intake: these range from 600 g/day for children up to 1000 g/day for adults.
150 g/day dietary reference intake
1000 g/dayTolerable intake level
Determination of I is not an easy task
The narrow range between minimum and maximum intake levels sets the requirements for the analysis technique for the determination of iodine in different sorts of foods:
• detection limits at a level of about 0.1 g
• an accuracy preferably better than 10 %
0
0,2
0,4
0,6
0,8
150 g/day
1000 g/day
A lot of techniques have a problem monitoring I
IRMM intercomparison study in milk powder:
values ranged from 0,1 g/g to 0.6 g/g
This is mainly due to the additional complications in chemical analysis when the solid samples are being digested with acids and the formation of
volatile iodine species is not sufficiently suppressed.
Nuclear Techniques have to find a “niche”
• No sample preparation which eliminates the problems of the chemical techniques for I
• Only few samples (1 to 3) where the cost of ICP is not too low in comparison to INAA (for larger numbers we can not compete!)
• Customers for whom accurate results are needed: e.g. food federal agency who don’t want to take any risk (for them accreditation ISO 17025 is a basic requirement)
Using INAA: I hidden by Na, Cl, Mn, K, …
1
10
100
1000
10000
100000
0 500 1000 1500 2000 2500 3000
Energy (keV)
Co
un
ts p
er c
han
nel
128I
56Mn
80Br
24Na 38Cl 38Cl24Na
Let’s have a look at the cross-section data
0,01
0,1
1
10
100
1000
10000
100000
1,E-05 1,E-04 1,E-03 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08
Energy (eV)
Cro
ss
se
cti
on
(b
)
Cl-38
Na-23
I-127
ECd=0,55 keV
1
10
100
1000
10000
100000
0 500 1000 1500 2000 2500 3000
Energy (keV)
Cou
nts
per
ch
ann
el
128I
56Mn
80Br
24Na 38Cl 38Cl24Na
Result using EINAA: suppression of the background leading to much lower detection limits
The set-up
• Rabbit-system with a flux of about 3E11 n/cm2s
• 1 mm of Cd leads to Cd ratio of about 20 to 30 for Al, Na, K, Mn, Cl
• Measurement of 40 % HPGe• 127I (n,)128I reaction resulting in a -peak at
442.9 keV with a half-life of 24,99 m. • Irradiation time: 5 m, counting time: 5 m,
decay time: 3 m• As standard I-solutions were used• Samples: about 1 g• Usually three sub samples taken because I
is not distributed homogeneous (usually homogeneity higher than 10 %)
Radioprotection
• Due to the fact that the Cd-capsules are dismantled and re-used and, if a lot of samples have to be measured, the activity in the Cd increases, the dose to the NAA-operator increases
• Working a whole morning (dismantling is performed behind a lead window) leads to a Hp(10) of about 3 Sv, whereas otherwise it is about 1 Sv
Excellent validation parameters
22labref
lab
scoreuu
xEn
ref
Reference Material
No. of measurements
This work (mg/kg)
Ref. Value (mg/kg)
En-score
Within-Lab reproducibility
BCR-151 Milk Powder
7 5,24 0,29 5.35 0,14 -0,6 2,4 %
BCR 279 Sea Lettuce 8 148 9 153 10 -0.7 1,9 %
• No significant bias
• Good Rw (r even at about 1 %)
• Specificity: the peak has some interference with the 23Na(n,p)23Ne reaction, with a half-life of 37 s (so td > 3 m)
• MDA: for several matrices ranging from 50 ng/g up to 100 ng/g
Measurement uncertainty
• The K0 -software is used to correct for sample filling height, sample density, matrix, measurement position
• The largest contributions to uncertainty are the reproducibility and the counting statistics. As reproducibility is at a level of 2,5 % we can say that for reasonable counting statistics (levels of about 10 ppm) the expanded uncertainty U is at a level of 6 %, at about 0.5 ppm U is about 10 %
What are the Iodine levels in Iodine rich foods?
Some manufactures claim Iodine levels: are they correct?
Values differ in some cases from manufacturer specs
Product This work
(g/g) Serving size
Amount (g)
% Daily Value
Value supplier (g/g)
MDA (ng/g)
Yogurt 0,20 1 portion 25 17% 40 Bambix milk 0,18 1 cup 36 24% 0,11 40 Milk 0,17 1 glass 41 28% 40
Cheese spread 0,48 1 portion 10 6% 100 Cod 3,10 1 portion 620 414% 50 Plaice 0,59 1 portion 118 79% 50
Crackers 1,18 4 crackers 47 32% 1,15 200
Bread 0,6 4 slices 60 40% 200 Ionised salt 19,54 1 teaspoon 39 26% 25 200
Tab Water 0,02 1 liter 5 4% 15-20
Nori 9,14 1 part 22 15% 500
Seaweed Extract 1200,00 1 teaspoon 2400 1600% 3000
Sea Lettuce 150,23 1 teaspoon 300 200% 1000
The daily I intake mostly comes from the consumption of bread and fish, adding salt during cooking and drinking of milk.
Iodine Rich Foods
0% 10% 20% 30% 40% 50%
Seaweedextract
Cod
Sea Lettuce
Plaice
Bread
Crackers
Milk
Ionised salt
Bambix milk
Yogurt
Nori
Tab Water
Cheese spread
% Daily Value
1600 %
414 %
200 %
80 %
Eating less bread might not be the solution (for I anyway)
Conclusion
• As INAA has to find “niche” markets to survive it has to focus on the validation of the chemical methods or to substitute where they fail (mostly sample preparation)
• EINAA is an excellent technique for the determination of I, for which our federal food safety agency has now officially recognised only EINAA
• Manufacturers are not always right when they label their products, is there a market for this?