Assay of Iodine in Foodstuffs

2

Assay of Iodine in Foodstuffs: Methods and Applications Jan Kucˇera  Nuclear Physics Institute, Academy of Sciences of the Czech Republic,   CZ-25068 Rˇež, Czech Republic

Abstract Dietary assessment methods, total diet studies, and the duplicate portion method are evaluated with regard to their adequacy for obtaining iodine intake data. The advantages and shortcomings of catalytic spectrophotometric methods, nuclear analytical methods, spectrometric, electrochemical, and other techniques, which are used for the determination of iodine in foodstuffs and related materials, are reviewed.

Abbreviations AAS Atomic absorption spectrometry AOAC The Association of Official Analytical Chemists CSC Compton suppression counting CSV Cathodic stripping voltammetry EINAA Epithermal instrumental neutron activation analysis ENAA Epithermal neutron activation analysis FAO Food and Agriculture Organization FAQ Food amount questionnaire FFQ Food frequency questionnaire GC Gas chromatography HPGe High-purity germanium HPLC High-performance liquid chromatography IAEA The International Atomic Energy Agency ICP-IDMS Inductively coupled plasma-isotope dilution mass spectrometry ICP-MS Inductively coupled plasma-mass spectrometry ICP-OES Inductively coupled plasma-optical emission spectrometry Comprehensive Handbook of Iodine ISBN: 978-0-12-374135-6

IDD Iodine deficiency disorders INAA Instrumental neutron activation analysis INFOODS International food composition database LC Liquid chromatography NAA Neutron activation analysis PS-NAA Pre-separation-neutron activation analysis QCM Quartz crystal microbalance RDI Recommended daily intake RNAA Radiochemical neutron activation analysis TXRF Total reflection X-ray fluorescence UNICEF United Nations Children’s Fund WHO World Health Organization

Introduction A major amount of iodine enters the human organism via the food chain. Hence, knowledge of iodine levels in foodstuffs and diets is essential for the assessment of iodine intake by man to ascertain whether the recommended daily level (RDI) (WHO, 2001) is met. Lack of iodine leads to iodine deficiency disorders (IDD), while excessive iodine dietary intake can result in pathological problems, namely goitrogenic effect (Underwood, 1971; WHO, 1996). Low iodine intake resulting in IDD is recognized as a global concern, while excessive iodine intake is not so frequent. For most people it is unlikely that they will exceed the upper level of iodine intake, which is given in Table 2.1 (FAO/WHO, 2006), from normal foods and supplements. In general, iodine content in foodstuffs is low, usually in the range of 10–200 g · kg1 fresh mass (Koutras et al., 1985), except for in seafood. Therefore, analytical methods with a sufficiently low detection limit are required for the iodine assay of foodstuffs. The choice of an adequate Copyright © 2009 Elsevier Inc. All rights of reproduction in any form reserved

16  Analytical Techniques

Table 2.1  Tolerable daily intake upper levels (UL) for iodine, g

Age (years) Children 1–3 Children 4–6 Children 7–10 Children 11–14 Adolescents   15–17 Adults (above 18)

UL for EU (EC/SCF, 2002) 200 250 300 450 500 600

Age (years)

UL for USA/ Canada (IOM, 2001)

Children 1–3 Children 4–8 Children 9–13

  200   300   600

Adolescents   14–18 Adults (above 19)

  900 1100

methodology for the assessment of iodine intake is also important. In many countries a significant portion of the daily iodine intake is achieved by using various supplements, namely iodized salt. This portion may not necessarily be equally taken into account by all methods of iodine intake assessment. In this chapter, approaches to assessment of iodine intake from foodstuffs and diets are evaluated, analytical methods for iodine determination in the above matrices are reviewed, and iodine content in selected foodstuffs is listed.

Obtaining Iodine Intake Data The main purpose of iodine assay in foodstuffs is to obtain data on intake by an individual or a population. Commonly used approaches to evaluate intake of various diet components, such as nutrients, vitamins, minerals and trace elements, contaminants, and so on, involve dietary assessment methods, total diet studies, and the duplicate portion method. In general, their ease of execution and the validity of the results obtained differs for all diet components, and this particularly applies to iodine. The use of biomarkers to assess iodine intake, specifically iodine excretion in urine, is also mentioned. Dietary assessment methods The dietary assessment methods are based on taking a dietary history, such as diet recall (usually 24-h recall, but sometimes for longer periods), diet history (an interview about “typical” or “usual” food intake), and food frequency and/or amount questionnaires (food frequency questionnaire, FFQ and/or food amount questionnaire, FAQ, respectively). More details about these methods are discussed by Margetts and Nelson (1997). Intake of iodine from the data obtained is calculated using various national or international food composition databases, such as INFOODS (Scrimshaw, 1997; Schlotke and Møller, 2000; Braithwaite et al., 2006). There are currently over 150 food composition tables in use around

the world (Heintze et al., 1988; Rand, 1991). An older bibliography of food composition tables was published by the FAO (1975). An inventory of European composition databases was prepared by West (1990). The regularly updated International Food Composition Tables Directory can be seen on the FAO home page (http://www.fao. org/infoods/directory_en.stm). Tables of food composition include information about the average nutrient content of the most commonly consumed foods for most nutrients. They are less comprehensive with regard to food contaminants and nonnutritive constituents that frequently occur in foods and are suspected or known to have biological activity. This is especially true for iodine. For instance, a review of minerals and trace elements in 7 major reference and 17 user databases revealed that iodine values were not included in food items in 14 databases, while the percentage of food items for which a value for iodine was reported varied from 2% to 100% (Braithwaite et al., 2006). This suggests that more analyses of foods for iodine content are needed. Moreover, a prominent problem with all dietary assessment methods is the high prevalence of under-reporting, estimated to range up to 70% in certain groups (Macdiarmid and Blundell, 1998). Clearly, errors in the measurement of food intake add to errors arising from differences between the composition of the food consumed and the values recorded in the database. The quality of the dietary components intake tables depends on the quality of the database, the accuracy with which foods can be identified, the quality of the food consumption data, and the accuracy with which the food composition database and the programs (or ­ calculations) are prepared (Greenfield and Southgate, 2003). A special problem of iodine intake data is associated with iodized salt, which is nowadays in use in most developed and many developing countries. It follows from a review of the IDD global situation (WHO, 2001) that, of the 130 countries with IDD, 98 (75%) have legislation on salt iodization in place and a further 12 have it in draft form. If food composition tables with values for cooked foods are not available, the iodine intake from iodized salt may not be fully taken into account by dietary assessment methods. Total diet studies A total diet study consists of purchasing foods commonly consumed, processing them as for consumption (tableready), combining the foods into food composites or aggregates, homogenizing them, and analyzing them for analytes of interest. The analytical results are combined with food intake information for different population groups, and the dietary intake of the food components by the groups is estimated. Intake through drinking water and water used in cooking are also included in the total diet study assessment. The total diet study, also known as the Market Basket Survey/Study, is recommended by the

Assay of Iodine in Foodstuffs: Methods and Applications  17

WHO for accurate estimates of dietary intakes of various dietary components. The total diet studies differ from other surveillance programs because: they focus on components in the diet, not individual foods, the foods are processed as for consumption in the home; thus, they take into consideration the impact of home cooking on the decomposition of less stable chemicals, and the formation of new ones, and assessment of background, rather than regulatory, concentrations of the analytes in the foods is sought.

l

l

l

The accuracy of the total diet study depends on two fundamental data components: (i) the quantity of each prepared food consumed by individuals, usually obtained in a separate study by the dietary assessment method and (ii) the background concentration of analytes of interest in the foods as ready for consumption. In order not to overestimate dietary intakes, the analytical methods used to measure analytes of interest should have appropriately low detection limits. The total diet study is considered well-suited for assessment of iodine intake, especially because it takes into account all factors, which may influence iodine levels in foods prepared as ready for consumption, including iodized salt. Duplicate portion method The most accurate way to assess the nutrient intake of a person is to analyze an exact duplicate of the foods eaten over the survey period. This approach is seldom used because of obvious practical problems, in addition to the cost and the time involved in the analyses. Nevertheless, the duplicate portion method is the method to which all other methods should be compared (Margetts and Nelson, 1997). An example of such a comparison has recently been given by Lightowler and Davies (2002). They compared iodine intakes estimated from weighed dietary records with those obtained by direct analysis of duplicate diets in a group of vegans. A mean daily iodine intake in males was significantly lower when estimated from dietary records (42 g) compared with that determined from duplicate diets (137 g). Conversely, in females the mean daily iodine intake from dietary records (1448 g) was significantly higher than that from duplicate diets (216 g). Variation in the iodine intake determined by the two different methods may be attributed to the absence of iodine content of some foods, in particular foods suitable for vegan consumption, in food composition tables and the variability in iodine content of seaweed. This example again demonstrates that more reliable information on iodine content of foods, incorporating the variation within

Table 2.2  Methods for assessment of iodine intake from diet Method Dietary assessment   methods Total diet studies Duplicate portion   method

Ease of performance

Adequacy and data quality

Relatively easy

Frequently not sufficient

More difficult Most difficult

Good Best

foods, is needed, and that the method of duplicate portion is the best choice for obtaining iodine intake data. Table 2.2 gives an evaluation of the above methods concerning applicability and adequacy of the assessment of iodine intake. Biomarkers of iodine intake There are various well-established biomarkers of intake and/or nutritional status of numerous food components (Margetts and Nelson, 1997; Wilett, 1998). In the case of iodine, a good measure of iodine intake is urinary excretion, because most (more than 90%) of iodine ingested is excreted in urine. Thus, the urinary iodine concentration, even in casual urine samples, is a good marker of iodine nutrition. Urinary iodine concentration varies with fluid intake, so these values have limited use for casual samples from an individual, but they are well-suited for assessing a population group, because individual variations tend to average out. Iodine intake and urinary excretion were compared among adults in the Netherlands (Brussaard et al., 1997). Food consumption was measured by 3-day food records, and 24 h urine was sampled twice. On average, iodine intake (mean of 3 days) in men was in the recommended range of 150–300 g · day1, but average intake in women was not. A mean 24 h urinary excretion sample confirmed this observation. In a British study, iodine intake and iodine deficiency in vegans were assessed by the duplicate portion method and urinary iodine excretion (Lightowler and Davies, 1998). The iodine intake was estimated using chemical analysis of 4-day weighed duplicate diet collections, while the probability of IDD was judged from the measurement of urinary excretion in 24 h urine specimens during the 4 days. A wide variation of iodine intake was found. The mean iodine intake in men was lower than the RDI, and the mean intake in females was above the RDI. The probability of IDD in the group investigated was moderate to severe. The findings highlighted that vegans are an “at risk” group for iodine deficiency, and also raised the question of adequate iodine intake in groups where cows’ milk is not consumed. The examples demonstrate the importance of the evaluation of iodine intake by measurement of ­ urinary

18  Analytical Techniques

excretion. Therefore, analytical methods for iodine determination in urine are also mentioned in this chapter.

Analytical Methods for Iodine Determination in Foodstuffs and Related Materials Reliable data on iodine content in foods can only be obtained by the careful performance of appropriate, accurate analytical methods carried out by trained analysts. The choice of the appropriate methods performed in the state of statistical control and under other quality control measures is the second crucial prerequisite to ensuring the quality of the results obtained. Obtaining a sufficiently homogeneous and representative sample is the third basic prerequisite for arriving at valid and meaningful analytical results. The determination of iodine in food has been a difficult analytical problem for many years, and inconsistent results have been obtained in interlaboratory studies (Heckmann, 1979), although a variety of analytical methods capable of iodine determination at various levels in foodstuffs have been developed. The main difficulty is the volatility of iodine when present in the elementary form or in the forms of its volatile compounds. The procedures for iodine determination differ in decomposition methods, analytical principles, detection limits, specificity, accuracy and precision, robustness, and sensitivity to interference. From the practical point of view, they also differ in the ease of performance, equipment needed, and the time and costs involved. Sample decomposition Most analytical techniques require sample decomposition, which is a delicate problem in iodine determination, because elementary iodine and some of its compounds, such as HI and CH3Cl, are highly volatile. This problem can be circumvented in two ways. First is the conversion of all iodine species into elementary iodine (I2) by distillation or combustion, with subsequent trapping of the analyte for further processing. Secondly, on the contrary, is the conversion of all volatile species to nonvolatile ones, such as iodide or iodate. Sample decomposition procedures of the first type involve distillation (AOAC, 1984), Schöniger ­ combustion in an oxygen atmosphere in a closed flask (Schöniger, 1955, 1956), combustion in a stream of ­ oxygen flowing through a heated tube (Muramatsu et al., 1988; Gu et al., 1997; Gelinas et al., 1998b; Norman and Iyengar, 1998), or pyrohydrolysis-combustion in a wet oxygen flow (Schentger and Muramatsu, 1996). The second type of decomposition procedure comprises dry ashing (fusion) with alkaline ashing aids, such as with NaOH, NaOH  NaNO3, Na2CO3, and oxidative fusion with Na2O2 in a Parr bomb (Merz and

Pfab, 1969). Recently, a quick (3 min) alkaline–­oxidative fusion in a mixture of Na2O2  NaOH at 850–900°C has been developed, which prevents volatilization losses of halogens and many other elements (Kucˇera and Krausová, 2007). Wet ashing using oxidizing acids or acid mixtures can also be used, provided that the oxidation potential is high enough to oxidize iodine to the nonvolatile iodate. This can be achieved by applying mixtures of H2SO4–chromate (Spitzy et al., 1958), H2SO4–HNO3–HClO4 (Gochman, 1966), or HClO3–HNO3 (Knapp and Spitzy, 1970). No losses of iodine were found in wet digestion with HNO3 at temperatures up to 280°C using several pressure- and ­temperature-controlled closed devices (Knapp et al., 1998). Low ­ temperature (90°C) extraction with tetramethyl­ ammonium hydroxide (TMHA) also yielded no measurable losses of iodine from biological samples of various origins (Fecher et al., 1998; Knapp et al., 1998; Rädlinger and Heumann, 1998). Catalytic spectrophotometric methods Since the development of a catalytic spectrophoto­metry method for iodine assay by Sandell and Kolthoff (1934, 1937) based on the catalytic effect of iodide on the discoloration of yellow Ce(IV) by reduction with As(III) in H2SO4/HCl medium, this technique was for a long time the “state of the art” for the determination of iodine in biological materials, including foodstuffs and urine. Among the other possible decomposition methods, ­alkaline ashing is the most commonly used prior to the final determination of iodine by the Sandell–Kolthoff reaction (Aumont and Tressol, 1986). This method of organic matter destruction is involved in the official procedure of the AOAC (1980). Other digestion procedures were also employed to guarantee that all iodine species are converted to iodide and that substances, such as nitrates, thiocyanate, or ferrous ions, that might interfere by reducing or oxidizing the ceric or arsenite reactants, are removed (Zack et al., 1952; Aumont and Tressol, 1986; IDD Newsletter, 1993; Pino et al., 1996; May et al., 1997; Gültepe et al., 2003; Gamallo-Lorenzo et al., 2005). For iodine determination in urine, chloric acid digestion was reported to be highly effective, among other techniques (Zack et al., 1952). A mixture of chloric acid and sodium chromate or a mixture of perchloric acid and nitric acid under reflux, and potassium carbonate fusion were also employed, and proved to provide comparable results (IDD Newsletter, 1993; Dunn et al., 1993). In order to eliminate the use of irritating strong acids, a digestion method using ammonium persulfate was developed (Pino et al., 1996). To speed up the initial analytical step, microwave-assisted alkaline digestion with TMHA combined with microwave-assisted distillation was employed for the determination of iodide and total iodine in edible seaweed (Gamallo-Lorenzo et al., 2005). Automated modes of the Sandell–Kolthoff ­reaction

Assay of Iodine in Foodstuffs: Methods and Applications  19

for measurement of urinary iodine using Technicon AutoAnalyzer II (AAII) systems, with either dialysis or acid digestion, were compared with a method employing manual alkaline ashing. The automated modes yielded higher results, due to interfering thiocyanates which participated in the catalytic reaction (May et al., 1990). However, a comparison of five methods, with both manual and improved automatic digestion, as well as with both manual and automated reading of the Sandell–Kolthoff reaction, yielded agreement among the methods tested, and also with an inductively coupled plasma mass spectrometry method (May et al., 1997). Another comparison of an autoanalyzer method (AAII), where the mineralization takes place in a continuous flow manner with a manual mineralization and discoloration in a 96-well microtiter plate read by a PC-controlled photometer, was performed by Wuethrich et al. (2000). Both implementations of the Sandell–Kolthoff method applied for iodine assay in urine showed no obvious discrepancy. A detection limit of 0.1 mol · l1 (12.7 g · l1) was achieved for the latter method. Other modifications of the Sandell–Kolthoff reaction include the use of a brucine solution (Matthes et al., 1973; Aumont and Tressol, 1986) or diphenylamine4-sulfonic acid (Trokhimenko and Zaitsev, 2004) to stop the As–Ce–I reaction after a selected time. It is not intended to list all the modifications and improvements of the procedure originally developed by Sandell and Kolthoff (1934, 1937) here, but to show that this old principle of iodine determination in biological materials, such as foodstuffs and urine, is still viable, largely applied, and continuously undergoing improvements. Two other kinetic-photometric methods for the determination of total iodine in biological materials were also developed. One is a flow injection method based on the catalytic action of iodide on the color-­fading reaction of the Fe(SCN)2 complex. It was used by Arda et al. (1998) for the determination of iodine in milk. When organic matter was destroyed by alkaline dry ­ashing or, alternatively, Schöniger combustion, a detection limit of 0.99 g was obtained. The second method is based on the catalytic effect of iodine on the oxidation of chlorpromazine by hydrogen peroxide. Tomiyasu et al. (2004) demonstrated, by the determination of iodine in urine and foodstuffs, that a detection limit of 1.6 ng can

be achieved. The main advantage of catalytic spectrophotometric methods is the low cost of the equipment needed. On the other hand, these methods may suffer from interference and are therefore rarely used for the determination of the lower range of iodine levels occurring in foodstuffs and related materials. With the development of analytical techniques, new methods for iodine determination in biological materials and foodstuffs became available. These newer techniques mostly involve nuclear analytical methods and various types of spectrometric, chromatographic, and electrochemical techniques. Neutron activation analysis and other nuclear analytical techniques Neutron activation analysis (NAA) is, nowadays, the second most widely-reported analytical technique for the determination of iodine in foodstuffs and other types of biological materials. The main advantage of this technique is that iodine can be determined nondestructively, using so-called instrumental neutron activation analysis (INAA). By irradiation with thermal and epithermal neutrons (energies of 0.025 eV and  0.5 eV–10 keV, respectively) in a nuclear reactor, the stable iodine 127I is transformed to the 128I radioisotope, which has a half life of 24.99 min. Irradiation of stable 127I with fast neutrons (energy  0.1 MeV) yields the radioisotope 126I, which has a half life of 13.03 days. The  decays of both 128I and 126 I are associated with emission of well-measurable -rays provided that a semiconductor high-purity germanium (HPGe) detector is used for counting, which is nowadays the most common practice. The nuclear reactions involved, and parameters of the radionuclides formed on neutron activation of stable 127I, are listed in Table 2.3. There are several approaches to the use of NAA for iodine determination in foodstuffs and other biological materials. Of the two nuclear reactions of stable 127I, the reaction of 127I(n,)128I with thermal and epithermal neutrons is almost exclusively used, because favorable nuclear parameters (high activation cross-section and resonance integral for thermal and epithermal neutrons, respectively, cf. Table 2.3) provide much lower detection limits compared

Table 2.3  Nuclear reactions and parameters of iodine radioisotopes used in neutron activation analysis

Radioisotope

Nuclear reaction

128

127

I

126

I

I(n,) I→128 Xe 127 I(n,p) 126 I→126 Xe 128

Cross-section (1024 cm2)

Resonance integral (1024 cm2)

Half life

6.2  0.2

147  6

24.99 min

0.0009  0.0001

–

13.03 days

Main -ray energy, keV (absolute intensity) 442.9 (16.9), 526.6 (1.6) 388.6 (34.2), 666.4 (33.2)

20  Analytical Techniques

with the reaction of 127I(n,p)126I with fast neutrons. However, irradiation of biological materials with thermal and epithermal neutrons also results in high background activities from 24Na, 42K, 38Cl, and 80,82Br activation products originating from the elements Na, K, Cl, and Br, which are present in large amounts in this type of sample, including foodstuffs. Thus, the detection limit of INAA is usually not sufficiently low to enable iodine determination in most types of foodstuff samples (cf. Table 2.4). The detection limit of the INAA mode can be moderately improved using cyclic INAA (Elson et al., 1983). Significant improvement of the detection limit is achieved by taking advantage of a high resonance integral for the reaction of 127I(n,)128I (cf. Table 2.3) on irradiation with epithermal neutrons, in so-called epithermal neutron activation analysis (ENAA), nowadays more often called epithermal instrumental neutron activation analysis (EINAA). Activation with epithermal neutrons is achieved by shielding off thermal neutrons from the reactor pile neutron spectrum by irradiation of samples behind filters made of cadmium or boron (Kucˇera, 1979). In such conditions, iodine is activated more selectively (together with other nuclides with high resonance integrals), because the high background activities from the elements Na, K, and Cl whose radioisotopes are formed according to the “1/v” law (v – velocity of neutrons) are suppressed. For this reason, EINAA has been used extensively for iodine determination in foodstuffs since the beginning of the

Table 2.4  Comparison of iodine detection limits, mD, using ­various modes of NAA in a nuclear reactora NAA mode

Nuclear reaction

INAA

127

ENAA

127

RNAA

127

RNAA

127

a 

I(n,) I→128 Xe

128

I(n,) I→128 Xe

128

I(n,) I→128 Xe

128

I(n,p) I→126 Xe

126

mDb (ng · g1)

Experimental conditionsc

200–300

ti  1 min, td  10 min, tc  20 min ti  0.5 min, td  10 min, tc  20 min ti  2 min, td  15–20 min, tc  20 min ti  20 h, td  10 d, tc  5 h

10–40

0.5 400d

Thermal, epithermal, and fast neutron fluence rates of 5  1013 cm2 · s1; 1.5  1013 cm2 · s1; and 3  1013 cm2 · s1; respectively. b  3 criterion. c  ti  irradiation time; td  decay time; tc  counting time; counting with a coaxial HPGe detector having relative efficiency 23%; FWHM resolution 1.8 keV; and the peak-to-Compton ratio 51 for the 1332.4 keV photons of 60Co. d  Counting with a well-type HPGe detector; active volume 150 cm3; FWHM resolution 2.3 keV for the 1332.4 keV photons of 60Co.

eighties until now (Fardy and Mcorist, 1984; Stroube and Lutz, 1985; Stroube et al., 1987; Rao et al., 1995; Hou et al., 1997a, b; Nichols et al., 1998; Kucera et al., 2004; Akhter et al., 2004; El-Ghawi and Al-Sadeq, 2006). Further improvement to the detection limit of EINAA was achieved using Compton suppression counting (CSC, sometimes also called anticoincidence counting), which selectively enhances the signal-to-background ratio of the 128 I radioisotope by suppressing the detection of the background activities. Two groups of authors recently applied EINAA with CSC for iodine determination in food samples and/or food reference materials, and achieved detection limits in the range of 11–20 ng · g1 (Serfor-Armah et al., 2003; Yonezawa et al., 2003). Noteworthy, in one laboratory only a relatively low epithermal neutron fluence rate of 2  1011 cm2 · s1 in the SLOWPOKE reactor was used (Serfor-Armah et al., 2003). An ultimately low detection limit can be achieved using radiochemical neutron activation analysis (RNAA), which consists of sample destruction and selective separation of the radioisotope 128I. Several RNAA procedures have been developed and applied for iodine determination in foodstuffs. Samples are usually decomposed using Schöniger combustion (Dermelj et al., 1990) or alkaline–oxidative fusion (Kucˇera and Krausová, 2007) in the presence of an iodine inactive carrier. However, alkaline and acidic dissolution of cereal grains was also used (Shinonaga et al., 2000). Elementary iodine is liberated employing a redox reaction with Na2SO3 and NaNO2 in dilute H2SO4 or HNO3. The liberated iodine is extracted with chloroform or tetrachlormethane (Dermelj et al., 1990; Kucˇera et al., 2004). The chemical yield of separation, mostly in the range of 85–95%, is determined spectrophotometrically or with the aid of the 131I radiotracer added prior to sample decomposition. Separation of iodine using bismuth sulfide coprecipitation followed by radiochemical purification with palladium iodide and radiochemical isolation by bismuth sulfide coprecipitation was also developed (Rao and Chatt, 1993). For iodine determination in urine, a simple separation procedure consisting of the use of iodinated exchange-resin proved to yield results comparable with the procedure based on iodine extraction (Dermelj et al., 1992). The RNAA procedures developed were employed for the determination of iodine in daily diet samples in Slovenia (Pokorn et al., 1999), Poland (Kunachowicz et al., 2000), in cereal grains in Austria (Shinonaga et al., 2000), in fish samples in Libya (Arafa et al., 2000), in Asian diet samples (Kucˇera et al., 2004; Akhter et al., 2004), and in Polish infant ­formulae (Osterc et al., 2006), just to give a few recent examples. An alternative way to eliminate the radionuclides 24Na, 42K, 38Cl, and 80,82Br in iodine determination by NAA is selective iodine separation prior to irradiation in so-called pre-­separation NAA (PS-NAA). Rao and Chatt (1991) developed a PS-NAA procedure based

Assay of Iodine in Foodstuffs: Methods and Applications  21

on microwave acid digestion in closed Teflon bombs and iodine separation by coprecipitation of iodide with bismuth sulfide, while Norman and Iyengar (1998) used sample combustion followed by trapping the liberated iodine on charcoal. Both procedures were successfully used for iodine determination in various diet samples and biological and environmental reference materials. Another PS-NAA method capable of determination of iodide and iodate in seawater, urine, and milk was developed by Hou et al. (2000), whereas Bhagat et al. (2007) elaborated a PS-NAA procedure for the iodine speciation in milk. Advantages of PS-NAA procedures are that the radiation burden of personnel is minimized compared with RNAA, and iodine speciation can be performed. Although iodine is not so ubiquitous an element in the environment, appropriate measures are taken to prevent contamination of samples during their processing prior to irradiation. The low cross-section of the reaction of 127I(n,p)126I with fast neutrons (cf. Table 2.3) and a low abundance of neutrons with energies higher than 9 MeV, which are needed for this reaction, in the neutron spectrum of a nuclear reactor result in a detection limit which is not sufficient for iodine determination in most types of foodstuffs, even if an RNAA procedure is applied. However, this reaction, which is completely independent in relation to the reaction of 127I(n,)128I with thermal and epithermal neutrons may be useful for cross-checking results in analysis of foodstuff samples with higher iodine contents, using the so-called self-verification principle in NAA (Byrne and Kucˇera, 1997). Detection limits of various NAA modes, which were achieved in the author’s laboratory are compared in Table 2.4. Concerning other nuclear analytical techniques, isotope dilution analysis proved to be capable of iodine determination in milk with a detection limit of 5 g · l1 (Ünak et al., 2004), while a detection limit of 0.1 g · g1 (dry mass) was obtained by radioisotope X-ray fluorescence on analysis of spiked milk samples (Crecelius, 1975). A similar detection limit of 0.5 g · g1 was reported for freeze-dried milk using energy-dispersive X-ray fluorescence analysis, the procedure being applicable also for iodine determination in potable water and egg yolks (Holynska et al., 1993). Total reflection X-ray fluorescence (TXRF) analysis was found to be suitable for iodine determination in iodine-enriched mineral water containing 94–100 mg · l1 of iodine, in seaweed samples with iodine levels of 450–4520 mg · kg1, and in dietary supplement tablets containing 26–205 mg · kg1 of iodine (Varga, 2007). Of the nuclear analytical techniques, NAA is the most important for iodine determination in foodstuffs. The main advantage of NAA is the ultimately low detection limit, which can be achieved using RNAA or PS-NAA, nondestructive performance of INAA and EINAA, freedom from interference and matrix effects of all modes of NAA for the determination of total iodine content,

and a low uncertainty of results achievable, especially by RNAA. For these reasons, and because NAA has recently been recognized as primary method of analysis, NAA is frequently used for the certification of iodine content in food-related reference materials, and for checking accuracy of other methods. Disadvantages of NAA are that access to a nuclear reactor – the most intensive source of neutrons – is needed, and this type of analysis is associated with handling radioactive samples, which requires specially equipped laboratories and adherence to the radiation safety regulations. Spectrometric techniques Mass Spectrometry  Various spectrometric techniques can be used for iodine determination in biological materials, including foodstuffs. Until now, mass spectrometry has been applied most extensively for this purpose. One of the first reports concerned the use of isotope dilution laser resonance ionization mass spectrometry to determine iodine in oyster tissue using the long-life 129I radioisotope to spike the samples. A detection limit of 100 ng of iodine was achieved using the procedure developed (Fasset and Murphy, 1990). Since the introduction of inductively coupled plasma-mass spectrometry (ICP-MS) this method became a powerful tool for iodine determination in various foodstuffs and related materials because of the favorable detection limits and method selectivity. Applications of ICP-MS were reported for iodine determination in fresh milk and milk powders (Baumann, 1990; Vanhoe et al., 1993; Sturup and Buchert, 1996), urine (Allain et al., 1990), milk, plants and tissues (Schramel and Hasse, 1994), food-related certified reference materials and/or candidate reference materials (Kerl et al., 1996; Larsen and Ludwigsen, 1997; Knapp et al., 1998; Gelinas et al., 1998a; Haldimann et al., 2000; Andrey et al., 2001; Resano et al., 2005; Santamaria-Fernandez et al., 2006), seafood (Julshamn et al., 2001), and total diet samples (Haldimann et al., 2000). In these procedures, various sample decomposition and/or treatment methods were employed. For iodine determination in urine the sample preparation involved only 10-fold dilution with a diluent containing europium as an internal standard, followed by direct nebulization in the plasma (Allain et al., 1990). For the determination of iodine in milk and milk powder by flow injection ICP-MS, a simple sample preparation was used based on the dilution of the sample by an alkaline solution containing KOH and TMAH (Sturup and Buchert, 1996). However, incomplete extraction of iodine with TMAH was observed for certain sample types (Fecher et al., 1998; Gelinas et al., 1998a). Low recoveries were explained by the presence of insoluble components, e.g., covalent bond forms of iodine (Haldimann et al., 2000). In such cases total sample mineralization is required, such as combustion in an oxygen

22  Analytical Techniques

stream (Gelinas, 1998a), Schöniger combustion (Knapp et al., 1998), high pressure, usually microwave-assisted, wet ashing in closed vessels using HNO3 (Knapp et al., 1998) or HNO3  HClO4 (Larsen and Ludwigsen, 1997; Knapp et al., 1998). However, if iodine was present as iodide and nitric acid was used in the wet ashing system, the observed signal in ICP-MS was not stable (Julshamn et al., 2001; Vanhoe et al., 1993). For the correction of nonspectroscopic matrix effects ICP-IDMS was developed using a spike with the 129I radioisotope (Rädlinger and Heumann, 1998; Haldimann et al., 2000; Santamaria-Fernandez et al., 2006). To avoid problems with sample decomposition, a procedure of solid sampling – electrothermal vaporization ICP-MS – was developed and applied to iodine determination in nutritional, as well as soil, reference materials (Resano et al., 2005). For the determination of iodine species, mostly iodide and iodate, in aqueous solutions ion chromatographic systems were coupled with ICPMS (Heumann et al., 1994; Stärk et al., 1997). Accurate results can be obtained, especially if ion chromatography is coupled with inductively coupled plasma-­isotope dilution mass spectrometry (ICP-IDMS) (Heumann et al., 1994). The reported range of detection limits for total iodine is down to a few ng · ml1 for milk (Baumann, 1990) and 6–10 ng · g1 for other nutritional samples (Gelinas et al., 1998b; Rädlinger and Heumann, 1998; Resano et al., 2005; Schentger and Muramatsu, 1996). It can be concluded that ICP-MS proved to be very useful to determine iodine in nutritional samples; however, the full exploitation of this technique requires careful optimization of the analytical parameters. Optical Emission Spectrometry  Inductively coupled plasma-optical emission spectrometry (ICP-OES) has not been used for iodine determination in foodstuffs very often. The reason is that the most intensive emission line of iodine of 178.218 nm is interfered by the phosphorus spectral line of 178.222 nm. In green algae Chlorella enriched with iodine, the element determination with ICP-OES after sample solubilization with TMAH using an interference-free iodine line of 182.980 nm appeared feasible (Niedobová et al., 2005a). The detection limit obtained was 3 g · g1. The phosphorus interference was eliminated in procedures in which iodine was separated by precipitation of AgI (Naozuka et al., 2003) or liberation of elementary iodine using vapor generation ICP-OES (Niedobová et al., 2005b). Both procedures were applied for iodine determination in milk samples, and the respective detection limits of 40 g · g1 (dry mass) or 20 g · l1 were achieved. Atomic Absorption Spectrometry  This technique also offers the possibility to determine iodine in aqueous solutions, seaweed, and foodstuffs samples, but its utilization has been limited so far. The reasons are that atomic absorption spectrometry (AAS) equipment is not intended

to make measurements in the vacuum UV range, therefore they require modifications, and due to the lack of a commercially available iodine lamp the radiation source has to be produced individually (Bermero-Barrera et al., 1999). Among spectroscopic techniques, ICP-MS has a prominent position in iodine determination in foodstuffs, due to a low detection limit, high selectivity, and the ease of coupling with chromatographic separation procedures. The highest accuracy is obtained in ICP-IDMS, which has been recognized as the primary analytical method. Since iodine is a monoisotopic element, the only possibility to perform isotope dilution is to use a spike with the 129 I radionuclide. Thus, similar regulations are required in ICP-IDMS for work with radioactivity as for NAA procedures. Another disadvantage of ICP-MS is that it requires the most expensive equipment of all techniques developed for iodine determination (if we do not consider the cost of a nuclear reactor, which usually serves many other purposes than irradiation for NAA). Electrochemical techniques Total iodine in fresh milk was measured using an iodideselective electrode after addition of KCl to increase the electrical conductivity by Crecelius (1975). The iodide electrode method is simple, fast, and inexpensive, providing a detection limit of 50 g · l1, but it may give erroneous results due to insufficient specificity if, for instance, formaldehyde is used for milk storage. A cathodic stripping voltammetry (CSV) method was developed for iodine determination in table salt and egg samples by Yang et al. (1991). The method involves sample treatment by Schöniger combustion and the determination of iodide by CSV of the solid phase formed with the quarternary ammonium salt Zephiramine. For the determination of iodide in table salt, another CSV method was also reported. Iodide was preconcentrated on a carbon-paste electrode via an ion-pairing reaction followed by oxidation to iodine (He et al., 2003). Electrochemical detection of iodine was also used in conjunction with chromatographic methods, which are reported below. Electrochemical techniques found many fewer applications for the determination of total iodine in foodstuffs compared with other techniques. They are inexpensive, but their limited specificity requires careful validation of the procedures developed. On the other hand, electrochemical detection of iodine species separated by chromatographic methods seems to be well-established. Chromatographic methods Urinary iodide was measured using paired-ion reversedphase high-performance liquid chromatography (HPLC) with electrochemical detection employing a silver working electrode (Rendl et al., 1994). A detection limit of 5 g · l1 was achieved.

Assay of Iodine in Foodstuffs: Methods and Applications  23

A head-space flow injection method for online determination of iodide in urine, with chemiluminescence detection with a detection limit of 10 g · l1, was also developed (Burguera et al., 1996). Reversed-phase ion-pair liquid chromatography (LC) with electrochemical detection was successfully used in a collaborative study on the determination of iodine in liquid milk and dry milk powder (Serti and Malone, 1993). Speciation analysis of iodine in milk was performed using size-exclusion chromatography with ICP-MS detection (Sanchez and Szpunar, 1999). For the determination of iodine in nutritional reference materials gas chromatographic (GC) measurement of the 2-iodopentan-3-one derivative with a 63Ni electron-capture detector was used following either combustion of samples in a stream of oxygen or oxidation in a basic solution of peroxodisulfate (Gu et al., 1997). The combination of a miniaturized alkaline ashing step and column-switching HPLC has been shown to be a powerful approach for the determination of total iodine in urine and other types of biological materials, with a limit of quantification of 70 ng · g1 for solid biological materials (Andersson and Forsman, 1997). Recently, a method based on anionexchange chromatographic separation coupled with amperometric detection at a modified platinum electrode was developed by Cataldi et al. (2005). The method was successfully applied to determine iodide content in milk, common vegetables, and waste waters. Chromatographic methods are especially useful for iodine speciation when coupled with ICP-MS, electrochemical detection, or chemiluminescence detection. The total iodine can be determined following various ashing procedures with a moderately low detection limit. Other methods A fluorescence-based method was successfully tested for iodine determination in iodine-supplemented food, such as table salt and milk powder. It is based on the fluorescent quenching capability of iodine/triodide using a highly fluorescent compound consisting of a synthetic metal ion receptor coupled with a signaling element. The detection limit for triodide down to a concentration of 108 mol was reported (Zhao et al., 2003). A quartz crystal microbalance (QCM) method for the determination of iodine in foodstuffs was proposed by Yao et al. (1999). The method is based on sensitive response to mass change at electrodes of piezoelectric quartz crystal. After sample decomposition, iodide in the sample solution is transformed to the elementary iodine in an acidic environment. The free iodine is then adsorbed at gold electrodes of QCM, and the iodine content is estimated through a decrease in QCM frequency. A detection limit of 0.5 g · l1 in aqueous solutions was reported. This method is also applicable for the determination of iodine in urine and other biological samples.

Choice of an Analytical Method Versus the Iodine Level in Foodstuffs and Diets Various analytical techniques for the determination of different levels of total iodine or iodine species in foodstuffs and related materials are presently available. They differ in principles, equipment needed, detection limits, reliability, i.e., accuracy and precision of results, the ease of performance, sample throughput, and analysis cost. The choice of the most appropriate method largely depends on the purpose of the analysis, e.g., whether it concerns routine monitoring and/or screening or whether delicate certification of a foodstuff reference material is to be carried out. Obviously, one of the decisive parameters is whether the method’s detection limit is sufficiently low for the given purpose. For this reason, it appears useful to give the typical iodine levels in various foods to facilitate the choice of the appropriate method(s). Table 2.5 lists the average iodine content of foods (fresh and dry basis), which was adapted from the data reported by Koutras et al. (1985). The iodine content of food varies with geographical location, because there is a large variation in the iodine content of various environmental areas. In plants, the iodine levels depend on the iodine content of the soil in which they are grown. This factor influences the iodine content of the local food chain, and consequently locally produced foodstuffs, as demonstrated in Table 2.6, which compares the iodine levels of several foods in readyto-eat form from two regions of Greece. One is the area of Thessalia, with endemic goiter shown to be due to iodine deficiency, while the other is the region of Athens, which is goiter-free, but where food additives are not used regularly (Koutras et al., 1970).

Table 2.5  Average iodine content of foods (g · kg1) and urine (g · l1) Fresh basis

Dry basis

Food

Mean

Range

Mean

Range

Fresh water fish Marine fish Shellfish Meat Milk Eggs Cereal grains Fruits Legumes Vegetables Urineb

30 832 798 50 47 93 47 18 30 29 130

17–40 163–3180 308–1300 27–97 35–56

116 3715 3866 179 450a 395a 65 154 234 385

68–194 471–4591 1292–4987 96–346 335–540a

a

22–72 10–29 23–36 12–201 20–300

34–92 62–277 223–245 204–1636

Recalculated from the data given by Koutras et al. (1985) using a dry/fresh mass ratio determined in the author’s laboratory as 10.4 and 23.4% for medium-fat milk and eggs, respectively. b Estimated from the data given in WHO (2001).

24  Analytical Techniques

Table 2.6  Iodine content of water, milk, and food items in areas with and without endemic iodine-deficiency goiter (Koutras et al., 1970) Athens Food item Drinking water, g · 100 ml1 Cows’ milk, μg · 100 ml1 Sheep’s milk, g · 100 ml1 Goats’ milk, g · 100 ml1 Egg, g/egg Chicken dishes, g/portion; average mass 240 g Meat dishes, g/portion; average mass 250 g Fish dishes, g/portion; average mass 222 g Legume dishes, g/portion; average mass 300 g Greek soft cheese, g · 100 g1 Bread, g · 100 g1

n

Endemic area

Mean

Range

12

0.47 0.35–0.77

12

4.15 7.50–12.60

n

Mean

Range

163

–

–

–

59

9.4

1.5–35.3

–

–

–

56

2.2

ND–15.7

1.8–48.8 2.7–597.0

19 16

1.9 23.8

0.5–6.0 ND–151.0

15 16

10

13.4 125.5

6.5

9

63.9

14

3.0

15

12

15.1

ND–18.0

2.4–158.0

ND–7.6

16

–

16

3.0

ND–12.3

–

–

2.0

ND–14.3

6.7–33.0

15

8.5

3.6–17.5

1.56 ND–14.5

21

0.54

ND–3.7

Note: ND, not detectable.

The iodine content present in the upper crust of the earth is leached by glaciation and repeated flooding, and is carried to the sea. Therefore, seafood is the richest source of iodine. A great variation in iodine levels occurs in ­seafood due to an inherent biological capacity of the individual species to accumulate iodine from the sea (Koutras et al., 1985). Thus, due to biological and geographical variability the data shown in Table 2.5 cannot be used universally, and cannot replace more detailed food composition databases. An excerpt from a recent Norwegian food composition table is given for illustration in Table 2.7. Table 2.8 presents another example of marked differences of iodine levels in daily diet samples in different countries, using the results achieved within a WHO and IAEA coordinated research project on dietary intake of several important minor and trace elements in diets consumed in a number of developed and developing countries (Dermelj et al., 1990). To review the present status of usage of various analytical techniques for the iodine determination in foodstuffs,

Table 2.7  Iodine content of Norwegian foods (g·kg1, fresh basis) according to The Norwegian Food Composition Table (2006) Milk and milk products Milk and milk-based beverages Milk Yoghurt Cream, sour cream Cheese Cheese, full fat Cheese, whey, goat’s milk Cheese, reduced fat Cheese, low fat Cheese, whey, cow’s and goat’s milk, fat reduced Egg Egg white Egg yolk Poultry and meat, raw Chicken breast and thigh, meat and skin Lamb, beef, pork Liver of lamb, beef, pork Dishes with poultry or meat Hamburger, double, with bread, cheese, lettuce,   dressing, etc., fast food restaurant Hamburger, extra large, with bread, cheese, etc.,   fast food restaurant Chicken burger, breaded, fried, with bread,   lettuce, dressing, etc., fast food restaurant Lasagne, with minced meat, frozen, industry   made Pasta dish, with turkey and cheese sauce, frozen,   industry made

170–220 160–240 120 190–1400 3050 180–660 200–1410 2000 390–490 30 1200 0a 200 20–40 20 40 10 50 160

Fish and shellfish Fatty fish, raw Lean fish, raw Shellfish, fish offal Fish products, sandwich fish

330–500 500 100 100–2340

Cereals Flour Crisp bread, crackers, etc. Cookies, sweet biscuits, rusks

0a–10 10 10–220

Potatoes, vegetables, fruits, and berries Potatoes, storage, raw Vegetables, raw and frozen Fruits and berries, raw/flesh

0a 0a–50 0a

Margarine, butter, oil, etc.

60

Other dishes, products, and ingredients Pizza, with meat balls and ham, frozen, industry   made Quiche, with ham and cheese, frozen, industry   made

50 180

a

 Determined with insufficient detection limit, which is not reported.

it may be stated that the catalytic spectrophotometric methods, namely those based on the Sandell–Kolthoff reaction, ICP-MS, and NAA methods are most widely used. ICP-IDMS and NAA procedures, being the primary analytical methods, have the highest metrological value

Assay of Iodine in Foodstuffs: Methods and Applications  25

Table 2.8  Values for iodine in IAEA daily diet samples from ­different countries (mg · kg1, dry mass) according to Dermelj et al. (1990) Country

Number of diet samples

Brazil

9

China Italy Japan Spain Sudan Thailand Turkey

11 19 5 20 5 8 6

Range

Median

0.282–0.633 0.045–6.33 0.062–0.456 0.088–7.65 0.150–1.96 0.084–0.540 0.057–0.187 0.065–0.337

0.49 0.16 0.18 0.55 0.46 0.16 0.11 0.16

and are therefore most frequently employed for certification of iodine levels in food-related reference materials and for checking accuracy of other methods. Hyphenated methods are usually needed for iodine speciation.

Conclusions Total diet studies and the duplicate portion method provide the most adequate data for the assessment of iodine intake from diet. A low-cost assay of iodine in foods and urine can be performed using catalytic spectrophotometric methods, namely those based on the Sandell–Kolthoff reaction. These methods, however, are not optimal for determination of the lower range of iodine levels occurring in foodstuffs, because of possible interference. NAA provides a possibility of nondestructive performance of reliable determination of total iodine in foodstuffs and diets at very low levels. The lowest detection limit of all analytical techniques can be achieved if RNAA or PS-NAA is used. PS-NAA is also suitable for iodine speciation analysis. Of the spectrometric techniques, ICP-MS is wellestablished and very frequently used for reliable determination of total iodine at very low levels, especially if ICP-IDMS is employed. Coupling of ICP-MS with chromatographic separation procedures is especially useful for the determination of iodine speciation in foodstuffs. Electrochemical techniques are inexpensive, but their limited specificity requires elimination of possible interference in the determination of even moderately low iodine levels in foodstuffs. They are well-suited for the detection of iodine species separated by chromatographic techniques. Chromatographic techniques are a well-established tool for iodine speciation when coupled with other detection methods. Results with the highest metrological value and the lowest uncertainty can be obtained using NAA and ICP-IDMS

l

l

l

l

l

l

l

procedures. They are therefore best suited for certification of iodine content in food-related reference materials, and for checking the accuracy of less expensive methods of iodine determination. The choice of analytical method(s) should take account of a wide range of iodine levels in foodstuffs and diets, the best criterion being fitness for purpose. One method may not be optimal and/or capable for iodine determination at all iodine levels occurring in foodstuffs and diets.

l

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