Anal. Chem. 1998, 70, 2221-2224
Iodine Determination in Food Samples Using Inductively Coupled Plasma Isotope Dilution Mass Spectrometry Gunther Ra 1 dlinger and Klaus G. Heumann*
Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University, Becherweg 24, D-55099 Mainz, Germany
Two different sample treatment methods are used in connection with inductively coupled plasma isotope dilution mass spectrometry for accurate and precise determinations of iodine traces in food samples. 129I-enriched iodate is applied as a spike compound for the isotope dilution step. Extraction of iodine by tetramethylammonium hydroxide (TMAH) solution at high temperatures in a closed vessel is one of the sample treatment methods. The other one is a complete decomposition of the sample with a mixture of perchloric acid and nitric acid using microwave assistance. By analyzing different certified reference materials (three milk powders with different iodine levels, BCR CRM 63, 150, and 151; bovine liver BCR CRM 185), the accuracy of ICP-IDMS with both sample treatment methods could be demonstrated. The relative standard deviation was typically in the range of 0.6-2.8% for iodine concentrations between 0.1 µg g-1 and 5 µg g-1. The detection limit was 8 ng g-1 using sample weights of 0.8 g. In a round robin test, using two different types of infant food samples, the results of the two ICP-IDMS methods and of an ICP-MS method without the isotope dilution technique, but applying the TMAH extraction procedure, agree very well with the mean of results of all participating laboratories also using ICP-MS/TMAH. However, the ICP-IDMS method is faster, more precise, widely independent of matrix effects, and, therefore, relatively accurate, which makes this method especially attractive for use as a routine method. Iodine is an essential trace element for humans and other biological species. As it is well known, iodine deficiency in human nutrition results in goiter formation, which is an enlargement of the thyroid gland. Humans, therefore, need a daily uptake of about 180-200 µg of iodine. To ensure sufficient iodine uptake, many food products are enriched in iodine; iodinated table salt is the most prominent example. The determination of iodine is always an analytical problem, especially at the relatively low natural concentration level, which is normally below 1 µg g-1. With respect to low detection limits, neutron activation analysis is a suitable analytical method.1 More than 10 years ago, methods of negative thermal ionization isotope (1) Palomares, J.; Travesi, A.; Dominguez, G. Radiochem. Radioanal. Lett. 1970, 3, 357-364. S0003-2700(97)01308-5 CCC: $15.00 Published on Web 04/16/1998
© 1998 American Chemical Society
dilution mass spectrometry (NTI-IDMS) were developed by Heumann et al. for the accurate determination of iodine traces in food and environmental samples.2-4 However, sample preparation for the thermal ionization technique is always very time-consuming, and TIMS instruments are not very often available in analytical laboratories, so that NTI-IDMS did not become a routine method. On the other hand, the much more time-efficient ICP-MS allows relatively sensitive detection of iodine, even when applying a quadrupole instrument, because there are no significant spectroscopic interferences in the corresponding mass rangesexcept by 129Xe from possible xenon impurities in the plasma gassas a number of different analyses have demonstrated.5,6 ICP-IDMS methods with different sample treatment procedures are developed in this work to use the high accuracy and precision of the isotope dilution technique as well as the advantages that this type of internal calibration is easy to handle, independent of matrix effects, and that loss of substance during the sample preparation process has no influence on the analytical result.7-9 EXPERIMENTAL SECTION Chemicals. Water was purified using a Millipore system (Milli-Q, 18 MΩ). HClO4 (supra pure) and HNO3 (pro analysis), both from Merck (Darmstadt, Germany), and tetramethylammonium hydroxide (Wako Chemicals, Neuss, Germany) were used without further purification. The iodate standard solution for the determination of the content of the 129I-enriched iodate spike solution by inverse IDMS was prepared from a concentrated KIO3 standard solution (Merck) by dilution. Samples. Four different certified standard reference materials from the Standard Reference Bureau of the European Union, three milk powder samples with different iodine levels (CRM 63, CRM 150, and CRM 151) and bovine liver CRM 185, were analyzed to (2) Schindlmeier, W.; Heumann, K. G. Fresenius Z. Anal. Chem. 1985, 320, 745-748. (3) Heumann, K. G.; Seewald, H. Fresenius Z. Anal. Chem. 1985, 320, 493497. (4) Reifenha¨user C.; Heumann, K. G. Fresenius J. Anal. Chem. 1990, 336, 559563. (5) Nirel, P. M. V.; Lutz, T. M. J. Trace Microprobe Technol. 1991, 9, 95-106. (6) Cox, R. J.; Pickford, C. J. J. Anal. At. Spectrom. 1992, 7, 635-640. (7) Heumann, K. G. Isotope Dilution Mass Spectrometry. In Inorganic Mass Spectrometry; Adams, F., Gijbels, R., van Grieken, R., Eds.; Wiley: New York 1988; pp 301-376. (8) Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1992, 118/119, 575592. (9) Heumann, K. G. Mass Spectrom. Rev. 1992, 11, 41-67.
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Figure 1. Sample treatment for the determination of iodine in food samples by ICP-IDMS using extraction of iodine by TMAH.
test the accuracy and precision of the ICP-IDMS methods. Two other samples, normally used in this composition as infant food (one sample is a milk final product made of milk powder, and the other one is a balanced diet without milk powder but containing soybean flour), were prepared and homogenized by the “Bundesinstitut fu¨r gesundheitlichen Verbraucherschutz und Veterina¨rmedizin (BGVV)” in Erlangen, Germany, to determine within a round robin test the suitability of ICP-MS for iodine analyses in food. This work describes the results obtained during this interlaboratory study by the ICP-IDMS methods and by the standard procedure of ICP-MS with TMAH extraction, which was also applied by all participating laboratories. Isotope Dilution Technique. Because iodate is the most stable chemical form of iodine in solutions, an 129I-enriched iodate solution was applied as a spike for the isotope dilution step. The preparation of the iodate spike solution, which was also used for corresponding iodate speciations by IDMS, is described elsewhere.4 However, an iodide spike solution can also be applied if the stability of this solution is checked from time to time. The isotopic composition of the iodate spike solution was determined to be 127I ) (15.11 ( 0.06)% and 129I ) (84.89 ( 0.06)%, and its content was determined to be (8.930 ( 0.015) × 1015 iodate ions/g of solution by an inverse isotope dilution technique, using an iodate standard solution of natural isotopic composition. Because of the long half-life of 129I (1.6 × 107 years), the prepared spike solution shows only little radioactivity, which is far below the licencing limit. Possible interferences on 129I by 129Xe from xenon contaminations in the argon plasma gas were corrected by always measuring the noninterfered 131Xe intensity. More details about the principles of IDMS can be obtained from the literature.7-9 Instrumentation. All measurements were carried out with a quadrupole ICP-MS (Elan 5000, Perkin-Elmer SCIEX) equipped with a cross-flow nebulizer (AHF Analysentechnik, Tu¨bingen, Germany) and a Scott spray chamber made of quartz (AHF). The nebulizer was operated at a sample flow rate of about 1 mL min-1 by using a peristaltic pump (Gilson Minipuls 3, Abimed, Langenfeld, Germany) and tubes made of Isoversinic (Abimed). The microwave-assisted digestion of samples was carried out with a high-performance microwave digestion unit (type mls 1200 2222 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
Figure 2. Sample treatment for the determination of iodine in food samples by ICP-IDMS using microwave digestion with a HClO4/ HNO3 mixture. Table 1. Blank Values and Detection Limits for the Determination of Iodine in Food Samples by ICP-IDMS
method
no. of determinations
blank (ng)
detection limit (ng g-1)a
TMAH extraction HClO4/HNO3 digestion
10 6
5.3 ( 2.2 4.6 ( 2.3
8.3 8.6
a
Detection limit refers to 0.8 g sample weight.
mega, MLS GmbH, Leutkirch im Allga¨u, Germany) equipped with a rotor for six samples (HPR 3000/6). The Teflon-PTFE-TFM vessels (SVS-160/60, 160 mL) are suitable for pressures up to 60 bar and temperatures up to 280 °C. When using HClO4 as an acid for sample decomposition, it is important that, for safety reasons, the microwave digestion unit has a blow-off valve, which opens at least at pressures of 60 bar. The optimized program for the microwave digestion was a stepwise increasing power of 200 and 400 W for a 5-min each, interrupted by 5 min cooling period without microwave power, adding three further periods of 600 W for 2, 3, and 4 min, respectively, also interrupted by 5 min. The iodine extraction by TMAH solution was carried out in closed Teflon-PFA vessels of 15 mL (AHF). All other vessels used for the sample treatment were made of PE or PP and were precleaned by shaking several times with purified water. Sample Treatment. The TMAH extraction method and the procedure of digestion with a mixture of perchloric acid and nitric acid are schematically represented in Figures 1 and 2. In the case of the TMAH extraction method, filtration of undissolved particles may be necessary after the extraction step, depending on the type of nebulizer used. If the introduction system of the ICP-MS is not clogged by this suspension, it can be directly analyzed. This was possible with the cross-flow nebulizer system used in this work, so that all TMAH extracted samples were measured without any filtration step. Contrary to the TMAH extraction procedure, a complete dissolution was obtained for all
Table 2. Iodine Concentrations (in µg g-1) Determined in Standard Reference Materials by ICP-IDMS Compared with NTI-IDMS Data and the Certified Values ICP-IDMS
a
sample
TMAH
HClO4/HNO3
NTI-IDMS
milk powder CRM 63 milk powder CRM 150 milk powder CRM 151 bovine liver CRM 185
0.288 ( 0.008 1.33 ( 0.02 5.33 ( 0.03 0.125 ( 0.001
0.283 ( 0.003 1.31 ( 0.02 5.39 ( 0.04 0.124 ( 0.003
0.285 ( 0.008 1.32 ( 0.01 5.34 ( 0.01
certified value 1.29 ( 0.09 5.35 ( 0.14 (0.105)a
Indicated value, not certified.
Table 3. Iodine Concentration (in µg g-1) Determined in Two Different Infant Food Samples by ICP-IDMS and ICP-MS within a Round Robin Test ICP-IDMS sample
TMAH
HClO4/HNO3
ICP-MS (TMAH)
mean of round robin test
milk final product balanced diet
0.72 ( 0.05 1.31 ( 0.01
0.68 ( 0.03 1.32 ( 0.01
0.68 ( 0.06 1.23 ( 0.03
0.73 ( 0.15 1.28 ( 0.08
investigated samples by the HClO4/HNO3 digestion method (see Figure 2). Blank Determinations and Detection Limits. Blank determinations were carried out parallel to all sample analyses by applying exactly the same procedures represented by Figures 1 and 2, respectively, but without sample material. The results are shown in Table 1. In the case of these ICP-IDMS analyses the variation of the blank valuesand not the precision of the isotope ratio measurementslimits the detection limit. The detection limit is, therefore, calculated as 3 times the standard deviation of the blank value, which results in 8.3 ng g-1 for the TMAH extraction and 8.6 ng g-1 for the HClO4/HNO3 digestion method using 0.8 g as a sample weight. RESULTS AND DISCUSSION Determination of Iodine in Standard Reference Materials. The ICP-IDMS results (mean with standard deviation of 5-8 independent analyses) obtained by the TMAH extraction method and the HClO4/HNO3 digestion are summarized in Table 2. The results by NTI-IDMS2 and the certified values10,11 are also listed for comparison. The results of ICP-IDMS with both sample treatment methods, TMAH extraction and HClO4/HNO3 digestion, agree very well with the NTI-IDMS and certified values so far as data are available. The precision of the ICP-IDMS determination is between 0.6 and 2.8% relative standard deviation, even at concentration levels of about 0.1 µg g-1. Standard reference material CRM 63 is a milk powder sample with natural iodine content, and the other two standard reference materials of milk powder are iodine-spiked samples. During the corresponding certification campaign in 1983, the unspiked milk powder could not be certified for iodine because of a lack of reliable analytical methods. However, with the present ICPIDMS results and the NTI-IDMS data, an iodine content of 0.285 ( 0.003 µg g-1 can now be assumed to be the correct value for (10) BCR Information, The Additional Certification of the Content (Mass Fraction)of Iodine in Two Spiked Samples of Skim Milk Powder, CRM No. 150-151; Report EUR 10364 EN; Brussels, 1983. (11) BCR Information, Bovine Liver, CRM No. 185; Report EUR 10618 EN; Brussels 1986.
CRM 63. The iodine content of the standard reference material bovine liver CRM 185 was only indicated to be 0.105 µg g-1, due to the same analytical problems. The values determined by ICPIDMS with the two different sample treatment methods are identical within the relative standard deviation of 2.4% but differ by about 20% from the indicative value. The indicative value of 105 µg g-1 was the mean obtained from the results of two laboratories (0.096 µg g-1 and 0.114 µg g-1, respectively) using radiochemical neutron activation analysis.11 Iodine certification of this standard reference material should now also be possible by the different ICP-IDMS and ICP-MS methods available today. Round Robin Test with Different Infant Food Samples. The results of a round robin test with two infant food samples different in composition, where 15 laboratories with ICP-MS and TMAH extraction took part, are summarized in Table 3. This interlaboratory study was carried out in Germany in 1997 in order to prove the suitability of an ICP-MS method to give accurate results in routine iodine analysis for food samples. The mean of all laboratory means with standard deviation is listed in the last column of Table 3; the contribution of our laboratory to this interlaboratory study with ICP-MS (TMAH) is given in the column before. In addition, ICP-IDMS with the two different sample treatment techniques, corresponding to Figures 1 and 2, was also applied. As expected, the uncertainty of the mean of all laboratories is higher than those by the single ICP-IDMS and ICP-MS methods. However, all data agree well within the given standard deviations, which shows that ICP-MS in connection with a reliable sample treatment procedure is a suitable analytical method for iodine determinations in food samples. Comparison of Different ICP-MS Methods. Analytical methods for routine analysis should be fast and not too expensive but precise and accurate as well. These conditions are extensively fulfilled by ICP-MS in connection with the isotope dilution technique. Because IDMS is a method of proven high precision and accuracy, it is internationally accepted as a definitive method.7-9 The 127I/129I isotope ratio of the isotope-diluted sample is the only number which must be experimentally determined for each iodine analysis by IDMS. This isotope ratio is not influenced by matrix Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
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effects or by the isolated amount of iodine, which are both great advantages compared with an ICP-MS analysis without the isotope dilution technique. In addition, ICP-MS data must be externally calibrated, whereas IDMS is an internal “one-point” calibration, which is also less time-consuming if a characterized 129I spike solution is available or was once prepared. The amount of 129I spike should be of the same order as the amount of iodine in the sample, which means that, at most, some micrograms of 129I are necessary per analysis so that the corresponding spike costs can be neglected. However, also in the case of ICP-IDMS, a sample treatment is necessary prior to the mass spectrometric measurement. Comparing the TMAH extraction method with the HClO4/HNO3 digestion method (see Figures 1 and 2), the first one is, in fact, simpler with respect to the instrumentation but normally more time-consuming. An extraction time of about 2-3 h is necessary for total iodine extraction, whereas the microwave-assisted digestion needs 45 min. Furthermore, food samples are totally dissolved by a HClO4/HNO3 mixture, whereas the TMAH method cannot really guarantee a 100% iodine extraction in all cases. On
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the other hand, both sample treatment procedures are easier to handle and less susceptible to analytical errors than oxygen combustion followed by the collection of the combusted residue in an amine solution.12 It can, therefore, be concluded that ICPIDMS in connection with either iodine extraction by TMAH or digestion of the sample with HClO4/HNO3 is a powerful, accurate, and relatively fast method for trace iodine determinations in food, even as a routine method at natural iodine levels. ACKNOWLEDGMENT We thank Dr. P. Fecher from the Landesuntersuchungsamt Nordbayern in Erlangen, Germany, for good cooperation during the round robin study. Received for review December 3, 1997. February 26, 1998.
Accepted
AC971308K (12) Ge´linas, Y.; Krushevska, A.; Barnes, R. M. Anal. Chem. 1998, 70, 10211025.
