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Determination of Iodine in Libyan Food Samples Using Epithermal Instrumental Neutron Activation Analysis U. M. EL-GHAWI*

AND

A. A. AL-SADEQ

Tajura Research Center, P. O. Box 84462, Tripoli, Libya Received April 8, 2005; Revised August 20, 2005; Accepted September 5, 2005

ABSTRACT Epithermal instrumental neutron activation analysis (EINAA) has been used to determine the iodine content of many individual food materials that constitute the typical Libyan diet. The selected samples include different varieties of local and imported foods such as wheat and barley products, rice, bread, legumes such as chick peas and lentil, table salt, and commonly used spices, including thyme and fenugreek. Both conventional and anticoincidence γ-ray spectrometry techniques have been employed. Epithermal INAA in conjunction with anticoincidence counting has been found to provide the most reliable results. For quality control purposes, a number of NIST biological reference materials were analyzed. The range of daily dietary intake has been calculated as 100–180 µg of iodine per day, which is within the recommended range. Bread was identified as a significant source of iodine in the Libyan diet, as it contributed 99 µg/d. Index Entries: Iodine; food; cereals; iodized table salt; average daily dietary intakes (ADDI) of iodine; recommended dietary allowance (RDA) of iodine; epithermal instrumental neutron activation analysis (INAA).

INTRODUCTION Iodine has been known to be an essential trace element in humans since the 19th century (1). The only known role of iodine is reported to be *Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research

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a component of thyroid hormones thyroxin (T4) and triiodothyronine (T3). These hormones accelerate cellular reaction, increase oxygen consumption and basal metabolic rate, and influence reproduction, growth and development, energy metabolism, differentiation, neuromuscular function, and protein synthesis. Each step of the synthesis of these hormones is stimulated by thyroid-stimulating hormone (TSH). The term “goiter” was used for many years to refer to the effects of iodine deficiency. However, other effects have become known and the term “iodine deficiency disorders” (IDDs) is now commonly applied. Iodine deficiency results in lowered production of T3 and T4, which leads to release of TSH by the pituitary gland (2,3). This results in an increased activity of the thyroid gland and enlargement, so that the available iodine can be used more efficiently. The condition is common when intake drops below 20 µg/d. The World Health Organization (WHO) has estimated that about 1.6 billion people worldwide are at risk of iodine-deficiency diseases (1,2). Unfortunately, there are not any real recorded data for goiter in Libya; however, the south part of Libya, which is far from the seashore, includes most of the Libyan cases of goiter. Most iodine enters living organism via the food chain. The richest sources of iodine in the diets are generally marine fish and seaweed. The importance of iodine in medicinal, nutritional, environmental, and epidemiological scientific studies requires the use of analytical tools that give an accurate measurement of traces of iodine in biotic materials. A review of the most commonly employed techniques for measuring iodine, including colorimetry based on catalytic reactions, gas chromatography spectrometry, ion-selective electrodes, X-ray fluorescence spectrometry; isotopic exchange, and neutron activation has been reported by Rao et al. (4). Other methods used for determination of iodine such as intracavity laser spectrometry, inductively coupled plasma–mass spectrometry (ICP-MS), fluorimetry, potentiometry, high-performance liquid chromatography (HPLC), and isotope dilution analysis (IDA) have been reviewed by Unak et al. (5). Most radioactive nuclides originated from activation with thermal or resonance neutrons are produced by (n,γ) reactions. In the thermal neutron region, the (n,γ) activation cross-section of most nuclides follows the 1/v law (inversely proportional to the neutron velocity). Some nuclides mainly follow the 1/v law also in the epithermal region, whereas others show strong resonances in their cross-section in that region. Therefore, the ratio of thermal to epithermal activation shows a large variation between different target nuclides, as conveniently illustrated by the ratio of resonance activation integral/thermal neutron cross-section (I0/σ0) of the nuclides concerned. Whereas this ratio is of the order of 0.5 for nuclides following the 1/v law in the resonance region, it might be as high as 100 in other cases. This means that the radionuclide distribution originating from epithermal activation might deviate strongly from that apparent when the whole reactor spectrum is employed, and that forms the basis of epithermal instrumental neuron activation analysis (EINAA) (6). Neutron activation Biological Trace Element Research

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analysis (NAA) has a very good sensitivity for iodine determination. However, iodine is difficult to measure in biological materials by NAA because of its generally low content in biological materials as well as the high background resulting from the thermal neutron activation products of other elements in the material. This is particularly true of dietary samples, which have high activities because of 38Cl, 42K, 56Mn, and 24Na. Therefore, preconcentration NAA (7) or radiochemical NAA methods (8–11) have been used. Because iodine can be activated efficiently by epithermal neutrons (a resonance integral cross-section of 147 ± 6 barns) and Cl, K, Mn, and Na are not, epithermal (more accurately, epi-cadmium in this case) irradiation can be used to lower the background activities so that iodine can be detected with a superior detection limit (4,11,12). Anticoincidence γ-ray spectrometry can also be used to lower the Compton background arising from other activation products. The combination of EINAA and anti-coincidence (AC) counting has the potential to achieve iodine analysis with high sensitivity and low detection limits without the need for chemical separation (13). The aim of the present study is to estimate the levels of iodine in Libyan food by EINAA together with the Compton suppression counting system.

EXPERIMENT Comparator Standards Standards were prepared from various stock solutions prepared by dissolution of KIO3 (reagent grade) in deionized distilled water. Comparator standards were prepared by pipetting 100-µL aliquots onto finely ground sucrose into 1.2-mL polyethylene vials, and then dried under a ventilated box hood before heat-sealing the cap. Eppendorf pipets were carefully calibrated prior to use for dilution and transfers. The comparator standards were of identical geometry and contained approximately similar amounts of iodine as the samples. The actual iodine masses in the standards amounted to 0.5 µg and 2.0 µg.

Standard Reference Materials and Food Samples Standard reference materials (SRMs) obtained from the US National Institute of Standards and Technology (NIST) were used to evaluate the precision and accuracy of the method. The materials were dried according to the procedures prescribed by the issuing agency. The wheat and barley flours, spices, and iodized salt samples were heated at 45°C for 48 h to remove moisture. The dry chick beans, lentil, thyme, and fenugreek were powdered prior to heating. All samples were rehomogenized to minimize subsampling uncertainties. Samples were taken from the above materials, weighed directly to fill the full volume of the 1.2-mL polyethylene vials, and heat-sealed. Five vials were prepared for each sample. The mass of these materials used varied between 500 and 1000 mg. Biological Trace Element Research

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El-Ghawi and Al-Sadaq Table 1 Iodine Content of Some Biological Reference Materials by EINAA and Anticoincidence Counting System (in ng/g)

Precision and accuracy of measurements were evaluated by the analysis of SRMs. The biological reference materials used in this work include NIST SRMs 1549 (Non-fat Milk Powder), 1566a (Oyster Tissue), 1515 (Apple Leaves), 1547 (Peach Leaves), and 1577b (Bovine Liver). The agreement between the concentrations of iodine in standard reference materials obtained in the present work with the certified concentration was quite good, as shown in Table 1.

Irradiation All samples and comparator standards were placed in a 27-mL polyethylene vial containing a polyethylene spacer to ensure the exact positioning of the vials for reproducing identical irradiation geometries. These vials were irradiated in the outer epi-cadmium irradiation position of the Dalhousie University Slowpoke-2 reactor facility. At this position, the epicadmium flux was 2.5×109 n/cm2/s when the reactor is operated at a power of 4 kW (4). The food samples and standards were irradiated, cooled, and counted respectively for ti = 20, td = 3, and tc = 15 min. These timing parameters are mainly dependent on the sample matrix and the flux of the reactor. The cooling time was 3 min to let 23Ne at 439.9 keV to decay, because the half-life of this isotope is 37.24 s. The samples were counted only when dead times of multichannel analyzer (MCA) systems were below 6%. For the iodized salt and spice samples, the time of irradiation (ti) was reduced to 2 and 10 min, respectively. The actual detection limit depends on levels of the elements whose neutron-induced radionuclides produce the Compton continuum background below the analytical peak of 128I at 442.9 keV (i.e., 80Br, 38Cl, 56Mn, and 24Na). Table 2 shows favorable ratios of resonance integral to thermal neutrons cross-section for the production of 128I compared to the interfering radionuclides.

Counting The principal detector used in this work, in both conventional and anticoincidence γ-ray spectroscopy, consisted of an EG&G ORTEC (HP-Ge) detector with a resolution of 2.0 keV at 1332.5 keV and a relative efficiency of 25%. The guard detector used in anticoincidence γ-ray spectrometry Biological Trace Element Research

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Table 2 Nuclear Reactions and Parameters of Iodine Radioisotope and Those of Interfering Elements

consisted of a 10-in. × 10-in. NaI(Tl) annulus with five photomultiplier tubes (PMTs) supplied by Harshaw and a 3-in. × 3-in. Na(Tl) plug. The peak-to-Compton plateau ratio of this system is 368:1 at the 662-keV photopeak of 137Cs.

RESULTS AND DISCUSSION The richest sources of iodine in human diets are generally marine fish, seaweed, and iodized table salt. Concentrations in other foods tend to be highly variable, subject to differences in soils, fertilization, and food processing and preparation. Milk can be a significant contributor to the daily diet, although its content can be quite variable depending on the content of animal feeds and use of iodophor. Regulation in many countries requires the control of the level of iodine intake through dietary materials. For example, iodization of table salt at 76 µg/g has been mandatory in Canada since 1949 (4) and the Libyan national program of supplying iodine in table salt was started in 1995. The focus of this study was the amount of iodine included in a typical Libyan diet. Wheat is by far the commonest staple food in Libya. Estimates show that about 450 g of wheat or wheat products such as bread, pasta, and couscous are used daily by the average adult person, with much of it in the form of bread. Other cereals, such as barley and rice, are used occasionally. Libya is a Mediterranean country and several fruits, vegetables, and legumes are available throughout the year depending on the season (e.g., the main fruit product in Libya in summer is watermelons, and in winter, it is oranges). The main vegetables are potato and onion. The main legumes are beans, green peas, chick peas, and lentil. The commonly used spices are red and black paper, curry, cumin, thyme, coriander, and fenugreek. For this study, various dietary materials were purchased from the market to represent the typical daily intake for a Libyan. The specified weights of individual components (Table 3) were taken as if to cook the meal in traditional Libyan style. The average daily dietary intakes of iodine from the mentioned products have been calculated for the Libyan population, and parameters such as socioeconomic, cultural, and regional Biological Trace Element Research

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El-Ghawi and Al-Sadaq Table 3 Daily Dietary Intake (in g) of Various Food Materials by Adult Libyan Population (Tripoli Region)

Table 4 Iodine Content of Individual Food Items

variations have been considered. The average iodine concentration in the different individual food items consumed in Libya is given in Table 4; it can be seen that the concentration of iodine ranges from 39 to 40000 µg/kg. Bread was identified as a significant source of iodine in the Libyan diet, as it contributed 99 µg/d. The high iodine content of bread comes principally from the addition of iodized salt to the dough. The measured contents of iodine in wheat flour ranged from 39 to 48 µg. Because Libyan consumers are highly dependent on imported cereals, the amount of this trace element in bread, pasta, couscous, and rice will be Biological Trace Element Research

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affected by the place of origin: its soil and fertilization and irrigation practices. The other main source of iodine is spices; the average daily consumption of spices by the Libyan population is 6 g/d, which is about 7.4 µg/d of iodine, whereas the Indian population consumes approx 25 g spices per day per person (14). The results in Table 4 implicate spices and bread baked with iodized salt as significant contributors to iodine intake. The measured daily intake of iodine in legumes such as lentils and chick peas ranged from 4.1 to 5.4 µg/d. The average daily consumption of table salt by the Libyan population is 5–6 g/d, whereas in China, table salt consumption is 10–30 g/d (15). Most of the Libyan table salt products are iodized with potassium iodate (KIO3), which contains 30 ± 4 µg/kg of iodine. Tunisian iodized salt, which is available in the Libyan market, has a higher concentration of iodine, containing 40 ± 5 µg/kg of iodine. Overall, the results shows that the daily intake of iodine by the Libyan population is in the range of 100–180 µg/d, whereas iodine intakes in most European countries are 150–200 µg/d (16). The US Recommended Dietary Allowance (RDA) for iodine is 150 µg/d (17). It is worth noting that the normal cooking practice in Libya for preparing the traditional food is heating of the ingredients of the diet for 30–90 min at and above 100°C. This results in the loss of appreciable amounts of iodine because iodine is volatile at temperatures of 58°C and above. During various cooking processes (e.g., frying, grilling, and boiling), iodine losses are reported to be 20%, 23%, and 58%, respectively (1). On the other hand, Chavasit et al. (18) mentioned that the loss of iodine in iodated salt was not significantly affected by cooking method, kind of cooking utensil, or pH. The losses become higher with the addition of sugars, food additives, fortificants, and spices. In addition, iodine is unstable under the storage conditions found during manufacturing, distribution, and sale of salt in most developing countries. The effects of packaging materials and environmental conditions were investigated by Diosady et al. (19). They confirmed that iodized salt is susceptible to major losses of iodine when stored at high temperature and humidity. Considering this, the estimated range of 100–180 µg/d would be further reduced if these conventional cooking losses were taken and the storage conditions. Three methods are available for evaluating human dietary nutrition status: selective study of individual foodstuffs, duplicate portion study and total diet study (20). In this study, the market basket method is applied. The average iodine intake per capita might vary significantly among different regions of a country as well as among different countries. Table 5 illustrates the various estimates of the mean daily iodine intakes for adults in various geographical locations including Libya, as found in this research. Table 5 indicates that the figure of 100–180 µg/d discovered in Libya would reach the international standards and be just sufficient to Biological Trace Element Research

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El-Ghawi and Al-Sadaq Table 5 Daily Dietary Iodine Intake for Adults in Selected Countries

meet the nutritional requirements. However, of this 100–180 µg/d of iodine, 80% is acquired through the consumption of iodized salt. Many people in Libya, and other countries, need to restrict salt intake in order to reduce the risk of high blood pressure, heart disease, renal disease, and cirrhosis. Salt intakes for these people are usually limited somewhere between 0.5 and 2 g/d and some severe cases require intakes of less than 0.5 g/d. For this group, alternative iodine sources should be investigated. Biological Trace Element Research

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CONCLUSIONS Epithermal INAA in conjunction with anticoincidence counting has proven to be a reliable, accurate method to measure iodine concentration, and the samples can be counted directly after irradiation without further pretreatment (preconcentration or radiochemical NAA). The conventional EINAA without an anticoincidence counting system can be used to assess the iodine levels in diet samples down to approx 500 µg/kg. This study suggests that iodized table salt contributes around 80% of the iodine in the average Libyan diet. Thus, people with low salt intakes are at risk of low iodine intakes; hence, fortification of alternative food components (as well as salt) could be considered. The study has highlighted bread as the major dietary source of iodine, which is a function of the addition of iodized salt. There is a need to carry out human diet studies, which involve the measurement of urinary iodine.

ACKNOWLEDGMENT One of the authors (El-Ghawi) thanks the IAEA for providing a fellowship, and Dr. Jiri Holzbecher for his assistance in irradiation of samples at Dalhousie University, Halifax, Canada. The assistance of Tajura Research Center in providing the diet samples is also acknowledged.

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