Studies of Total, Organic and Inorganic Iodine Species by Instrumental and Preconcentration Neutron Activation with Compton Suppression Spectrometry
A. Chatt Trace Analysis Research Centre Department of Chemistry Dalhousie University Halifax, Nova Scotia, B3H 4J3 Canada Safe LOW Power Kritical Experiment (SLOWPOKE) Univ_Milano_Fisica_Chatt_Seminar_Iodine_2012_10_11
Iodine Geochemical Cycle • 70% of iodine in earth’s crust is found Atmosphere in oceanic sediments. • It is transformed into CH 3 I by marine organisms, and transported into the atmosphere. Land Soils • From the atmosphere it could return to the ocean or to the land. • In land, one part is released as CH 3 I by plants, another remains in soils or goes back to oceans through rivers Oceanic • It returns to oceans through sediments brines and also by organic activity from subducting sediments Muramatsu et al, Earth Planet. Sci. Lett, 192 (2001) 583-593
Introduction It is an essential micronutrient WHO estimates that about 1.6 billion for humans. Lack of it produces people are afflicted by IDD; 1% of iodine deficiency disorders (IDD). whom suffers from cretinism. Iodine It was accidentally iod (violet) and ine for discovered in 1811 its chemical resemblance by Bernard Curtois. to bromine and chlorine. WHO has set the Recommended Dietary Allowance (RDA) of iodine as: The daily average intake of iodine in 50 g d -1 from 0 to 6 month 90 g d -1 from 6 month to 6 years Canada has been estimated as 6 times 120 g d -1 from 7 to 10 years 150 g d -1 > 10 years 200-300 g d -1 during pregnancy and lactation the Recommended value. RDA and RNI values for some countries U.K. (140 g d -1 ) USA, EU, Australia (150 g d -1 ) Canada (160 g d -1 )
Introduction Q: What are the reasons for this high iodine intake in Canada since late 80’s? A: Sources of iodine for humans Milk Seafoods Iodized table salt (76 ppm) Iodine levels in milk increased significantly since 1970’s Milk is one of the major contributors of iodine (~30%)
Introduction Annual sales (L) divided by Canadian 60 Whole, 3.25% 50 Partially 2% population 40 Partially 1% 30 Skimmed 20 Chocolate 10 Buttermilk 0 Years http://www.dairyinfo.agr.ca , Average: 7 – 8 L per month per person A glass of milk (250 mL) contains about 125 µg of iodine which is approx. 78% of the Canadian RNI. Health Canada recommends that adults take 4 glasses of milk per day. This will lead to an intake of 500 µg iodine per day which is half of the Provisional Maximum Tolerable Daily Intake (PMTDI) recommended by FAO and WHO.
Iodine Isotopes: 115 I - 141 I The most used and/or important Nuclide Half-life Production/Usage 123 I 13.1 h Diagnosis in nuclear medicine 125 I 60.14 d Diagnosis, RIA 127 I STABLE 128 I 25 min Iodine determination by NAA 129 I 1.59 x 10 7 a Fission product of high yield 130 I 12.36 h Fission product of high yield 131 I 8.041 d Diagnosis and therapy in nuclear medicine 132 I 2.285 h Fission product of high yield 133 I 20.8 h Fission product of high yield
Methods for Iodine Determination Year Method Detection Limit, ppb 1920’s Colorimetric 50 1940’s Electrochemical 50 1970’s GC 10 1970’s INAA 30 1970’s PNAA 12 1970’s RNAA 2 1980’s LC 100 1990’s ICP-MS / ID-MS 4
Principles of NAA Nuclear reaction: 127 I(n, g ) 128 I Target nuclide: 127 I Product nuclide: 128 I Isotopic Half-life 25.0 min 100% abundance Thermal n (6.2 0.2) b g Energy 443 keV cross-section Resonance g Branching (147 6) b 0.16 integral ratio
Classification of NAA Techniques Neutron Activation Analysis (NAA) Non-destructive or Destructive Instrumental Analysis (INAA) Radiochemical Pre-irradiation Separation Separation (RNAA) Advantages: Preconcentration Derivative Speciation (PNAA) (DNAA) (SSNAA) Sensitive Sample is retained, Very Low levels Simultaneous Interference can be measured, minimum reagent measurement speciation eliminated no reagent blanks blanks of non- before High precision and sensitive irradiation accuracy elements
Advantages of SSNAA Simultaneous multielement specificity Simultaneous speciation of elements which are not chemically similar Simultaneous speciation of elements which are rather difficult to determine by other techniques Virtually free from matrix interferences Applicable to solids and liquids
Advantages of SSNAA Almost all the elements can be determined Qualitative as well as quantitative analysis Excellent sensitivity and detection limits High precision (low overall uncertainty) Excellent accuracy Extensive linear range (ppb to percent) Small sample size Can be combined with preconcentration steps No effect on species change
Advantages of SSNAA Enhanced quality assurance capabilities 3-D spectroscopy alternative nuclides alternative gamma-rays various decay times multiple counting
Comparison of experimental conditions, sensitivities and detection limits for iodine in different matrices Detection limits (mg kg -1 ) Neutron Irradi- t i :t d :t c Sensi- NIST NIST NIST IAEA CRM SRM-1549 SRM-1566 SRM-1572 H-9 shield ation (min) tivity (Non-fat (Oyster (Citrus (Human site ( counts Milk Tissue) Leaves) Mixed / g) Powder) Diet) None Cd-site 20:3:20 2177 0.24 0.31 0.17 0.18 None Cd-site 10:1:10 640 0.30 0.35 0.24 0.26 Cd-bottom Cd-site 10:1:10 636 0.28 0.34 0.23 0.24 (1 mm thick) Flex/boron Thermal 20:6:20 1600 0.24 0.17 0.11 0.15 (3.2 mm thick) Flex/boron Cd-site 20:3:20 1372 0.12 0.16 0.10 0.13 (3.2 mm thick) Thick boron Thermal 20:3:20 1408 0.11 0.14 0.09 0.10 (5 mm thick)
Iodine content of selected biological reference materials by EINAA (in ng g -1 , dry weight basis) Material No. of This work Agency Value Literature Values measurements (Mean SD) NIST SRM-1549 8 3115 170 3380 20 3710 140, 3150 75 (Non-fat Milk 3200 300, 3500 84, 3200 Powder) NIST SRM-1566 6 2735 215 2800 b 3209 134, 2500 200, (Oyster tissue) 2720 200, 2800 300 NIST SRM-1572 8 1760 165 1840 30 1290 50, 1870 60, (Citrus Leaves) 1890 45, 1460 NIST RM-8415 8 1900 180 1970 460 2040 200, 1875 94 (Whole Egg Powder) NIST RM-8435 8 2400 260 2300 400 2377 70 (Whole Milk Powder) NIST SRM-1570 6 1180 100 c 1325 55, 1080 160, (Spinach) 1160 40, 1200 120 NIST RM-8431 6 690 40 c 813 55 (Mixed Diet) IAEA CRM H-9 6 370 45 c 407 17, 372 30, 382 23 (Mixed Human Diet) NIST RM-8418 6 65 11 c 60 13 59 3, 62 4 (Wheat Gluten) a irradiated in Cd-site only; b information value; c irradiated with Flex/boron shields in Cd-site
Results • The lowest detection limits for all materials were obtained with the 5.0mm thick boron shields followed by the Flex/boron shield and Cd-site combination, as expected. • The 5.0-mm B 4 C loaded polymer container gave the best results. However, the mechanical integrity of these containers was not that great and they started to chip off on repeated use. • Nevertheless, these containers were useful to determine iodine levels down to 200 ppb. • The 3.2-mm B 4 C loaded Flex/boron shields in combination with the Cd-site are the next best to analyze samples down below 500 ppb.
Interaction of Gamma Radiation with Matter The background observed in a γ -ray spectrum has two components: specific background peaks and the Compton continuum. The specific background peaks are due to radioactive isotopes present in the environment ( i.e. members of the natural decay series, very long-lived nuclides, products of interactions with cosmic radiation), and artificially produced nuclides. All background nuclides and, often of greater importance, all nuclides in the sample contribute to the Compton continuum which results from the partial absorption of photons
Anti-coincidence The purpose of anti-coincidence counting is the reduction of the Compton continuum ( i.e. the background under peaks of interest). Anti- coincidence γ -ray spectrometers consist in part of a "principal" detector crystal surrounded as completely as possible by a second detector (known as the "guard" detector) which is used to detect scattered radiation. Anti-coincidence refers to the electronic treatment of the signals coming from these two detectors. The signal from the principal detector, in general terms, produces the spectrum while the signal from the guard detector is used to gate the principal signal either "on" or "off" prior to its reaching the MCA.
Performance of the DUSR Anti-coincidence System System P/Cp P/Ce P/Ta Conventional 93.4 89.5 0.07 Anti-coincidence 582 410 0.17 Improvement Ratio 6.26 4.58 2.43 P/Cp: Peak-to-Compton plateau (Cp 358-382 keV) P/Ce: Peak-to-Compton edge (Ce 475-481 keV) P/Ta: Peak-to-total area Nuclide: 137-Cs Worst case scenario
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