Epithermal INAA: I in food samples Peter Vermaercke International Workshop IWIRAD 2005 on “Applications of the Ionising-Radiations to Industry, Health.

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?