Anal. Chem. 2005, 77, 7062-7066
Extraction and Quantitative Analysis of Iodine in Solid and Solution Matrixes Christopher F. Brown,* Keith N. Geiszler, and Tanya S. Vickerman
Pacific Northwest National Laboratory, Richland, Washington 99352
129I
Due to its long half-life (15.7 million years) and relatively unencumbered migration in subsurface environments,1-3 129I has been recognized as a long-term risk driver at numerous federal and privately owned facilities. At the U.S. Department of Energy’s Hanford Site, located in southeastern Washington state, 129I was produced as a byproduct of nuclear fission in the site’s nine plutonium production reactors. Inadvertent release of waste solutions to the environment has resulted in 129I being the secondmost widespread radionuclide in the Hanford groundwater system.4 To understand the long-term risk associated with 129I at the Hanford Site, all possible sources must be examined. The three
primary sources include the following: 129I contained within the residual waste in underground storage tanks, 129I that currently resides in the vadose zone, and 129I already in the groundwater system. The ability to quantitatively assess the 129I contribution from these sources requires specialized extraction and analytical techniques, which are complicated and not always quantitative. Several techniques have been identified to extract iodine from solid matrixes;5-10 however, all of them rely on two fundamental approaches: liquid extraction or chemical/heat-facilitated volatilization. The majority of the liquid extraction techniques utilize either an alkaline leaching solution (typically tetramethylammonium hydroxide) or fusion (sodium hydroxide or potassium hydroxide) followed by sample dissolution in deionized water. Pyrohydrolysis techniques typically utilize temperatures in excess of 1000 °C to volatilize iodine from the solid. A carrier gas, such as oxygen, transports the volatilized iodine to a trap containing an alkaline preservation solution (tetramethylammonium hydroxide with sodium bisulfite). Chemical volatilization can be performed using nitric acid and 30% hydrogen peroxide. The offgas is then pushed into a stripping solution containing either carbon tetrachloride or sodium bisulfite. While these methods are typically chosen for their ease of implementation, they may not totally dissolve the solid sample or release all the iodine from the sample. As mentioned previously, some of the iodine partitions onto the soil; therefore, extraction methods that do not result in total sample dissolution could underestimate the total iodine content. Several analytical techniques are available for the quantitative analysis of iodine in solution. Ionic species of iodine are most commonly measured using ion chromatography (IC). IC analysis of iodine is particularly useful due to its ability to discern speciation: iodide versus iodate. However, one drawback of this analytical technique is the inability to differentiate between various isotopes of the same element. Another drawback of using IC to measure iodine is the inability to quantify the element at trace levels. Typical iodine detection limits using IC are in the range of 1-5 µg/L.11 Recent work by Cataldi et al.12 using chromatographic
* Corresponding author. Phone: (509) 376-8389. Fax: (509) 376-4890. E-mail:
[email protected]. (1) Bird, G. A.; Schwartz, W. J. Environ. Radioact. 1996, 3, 261-279. (2) Cantrell, K. J.; Serne, R. J.; Last, G. V. Hanford Contaminant Distribution Coefficient Database and Users Guide; PNNL-13895 Rev. 1, Pacific Northwest National Laboratory, Richland, WA, 2003. (3) Um, W.; Serne, R. J. Radiochim. Acta 2005, 93, 57-63. (4) Hartman, M. J.; Morasch, L. J.; Webber, W. D. Hanford Site Groundwater Monitoring for Fiscal Year 2003; PNNL-14548, Pacific Northwest National Laboratory, Richland, WA, 2004.
(5) Marchetti, A. A.; Rose, L.; Straume, T. Anal. Chim. Acta 1994, 296, 243247. (6) Schnetger, B.; Muramatsu, Y. Analyst 1996, 121, 1627-1631. (7) Gelinas, Y.; Krushevska, A.; Barnes, R. M. Anal. Chem. 1998, 70, 10211025. (8) Moran, J. E.; Fehn, U.; Teng, R. T. D. Chem. Geol. 1998, 152, 193-203. (9) Radlinger, G.; Heumann, K. G. Anal. Chem. 1998, 70, 2221-2224. (10) Hou, X.; Dahlgaard, H.; Rietz, B.; Jacobsen, U.; Nielsen, S. P.; Aarkrog, A. Anal. Chem. 1999, 71, 2745-2750. (11) Ito, K. Anal. Chem. 1997, 69, 3628-3632.
is a contaminant of interest in the vadose zone and groundwater at numerous federal and privately owned facilities. Several techniques have been utilized to extract iodine from solid matrixes; however, all of them rely on two fundamental approaches: liquid extraction or chemical/heat-facilitated volatilization. While these methods are typically chosen for their ease of implementation, they do not totally dissolve the solid. We defined a method that produces complete solid dissolution and conducted laboratory tests to assess its efficacy to extract iodine from solid matrixes. Testing consisted of potassium nitrate/ potassium hydroxide fusion of the sample, followed by sample dissolution in a mixture of sulfuric acid and sodium bisulfite. The fusion extraction method resulted in complete sample dissolution of all solid matrixes tested. Quantitative analysis of 127I and 129I via inductively coupled plasma mass spectrometry showed better than (10% accuracy for certified reference standards, with the linear operating range extending more than 3 orders of magnitude (0.005-5 µg/L). Extraction and analysis of four replicates of standard reference material containing 5 µg/g 127I resulted in an average recovery of 98% with a relative deviation of 6%. This simple and cost-effective technique can be applied to solid samples of varying matrixes with little or no adaptation.
7062 Analytical Chemistry, Vol. 77, No. 21, November 1, 2005
10.1021/ac050972v CCC: $30.25
© 2005 American Chemical Society Published on Web 09/30/2005
separation and amperometric detection has pushed the detection limit into the sub-part per billion range (0.5 µg/L). Under most circumstances, detection limits are not a great concern when analyzing solutions for stable iodine. For solutions containing radioactive isotopes of iodine, namely 129I, instrument detection limits become critical. The current maximum contaminant level for 129I in drinking water is 1 pCi/L, which equates to a solution concentration of 5.7 ng/L (parts per trillion). Therefore, it is usually necessary to perform a series of separations followed by sample preconcentration to achieve accurate quantitative measurements. This can be accomplished by performing repeated solvent extractions using carbon tetrachloride and nitric acid. The final step involves the precipitation of iodine using silver nitrate. At this point, the sample is ready for 129I counting using low-energy photon spectrometry. A significant advantage of this technique is that it allows for the preconcentration of large quantities of sample, which can greatly improve the limit of quantification for the respective analysis. One weakness of this technique is the technique itself; the multiple steps that are required are tedious and can lead to laboratory error. Furthermore, care must be taken during several steps of the procedure to ensure volatilization of iodine does not occur. Finally, large volumes of sample are not always available to allow significant preconcentration. Technological advancements in the field of inductively coupled plasma mass spectrometry (ICPMS) have enabled this instrument to become an important resource in the analysis of iodine.13,14 Unlike IC, ICPMS instruments can measure multiple isotopes of the same element simultaneously. Two drawbacks of ICPMS are that (1) it provides no information on the speciation of analytes and (2) it cannot differentiate directly between elements with isotopes at the same mass (i.e., 238U and 238Pu). To its credit, ICPMS instrumentation is capable of measuring many elements, primarily trace metals, at the part per quadrillion level (pg/L). Additionally, most samples can be analyzed without performing the specialized separations required by other techniques. However, iodine, being composed of anionic aqueous species, does require a special protocol in order to be analyzed via ICPMS. Care must be taken during the analysis of samples to ensure that iodine does not sorb to instrument glassware, creating “memory” effects during the analytical run. This issue can be remedied by analyzing the samples in an alkaline matrix. The end result is an analytical method capable of measuring iodine, both 127I and 129I, in solution at the low nanograms per liter range. When ultratrace analytical detection limits are necessary or isotopic ratios of 129I/127I are desired, accelerated mass spectrometry (AMS) is the preferred analytical instrumentation. While AMS can provide quantitative analysis of 129I/127I ratios in the range of 10-10-10-12, it requires sophisticated instrumentation that is costly and not readily available to the general scientific community.14 Therefore, this type of instrumentation is not suitable for routine iodine analysis. We defined a method that produces complete solid dissolution and conducted laboratory tests to assess its efficacy to extract (12) Cataldi, T. R.; Rubino, A.; Ciriello, R. Anal. Bioanal. Chem. 2005, 382, 134141. (13) Haldimann, M.; Eastgate, A.; Zimmerli, B. Analyst 2000, 125, 1988-1982. (14) Izmer, A. V.; Boulyga, S. F.; Zoriy, M. V.; Becker, J. S. J. Anal. At. Spectrom. 2004, 19, 1278-1280.
iodine from solid matrixes.15 Testing consisted primarily of potassium nitrate/potassium hydroxide fusion of the sample followed by sample dissolution in a mixture of sulfuric acid and sodium bisulfite. The sulfuric acid and sodium bisulfite solution was added to dissolve/reduce the residual solids and to prevent the volatilization of dissolved iodide. Once dissolved, the solution was analyzed directly on a Perkin-Elmer DRC II ICPMS using reaction cell technology. This paper highlights the success of this fusion protocol as well as the accuracy and precision of our analytical technique. The two techniques, when combined, provide an efficient and cost-effective approach to quantitate total iodine (127I, 129I) in both solid and solution matrixes. EXPERIMENTAL SECTION Chemicals. All chemicals and solutions were prepared using distilled water further treated using a Millipore system (Billerica, MA) with a minimum resistivity of 18 MΩ‚cm. Spectrasol CFA-C (Spectrasol Inc., Warwick, NY) and H2SO4 (Allied Chemical, Hollywood, FL) were used without additional purification. KI (Fisher, Fairlawn, NJ), KIO3 (Fisher), KNO3 (Fisher), KOH (Fisher), and NaHSO3 (Sigma-Aldrich, St. Louis, MO) were all purchased direct from the manufacturer as reagent grade chemicals. The 127I certified stock standard and Sb used for instrument calibration were supplied by High Purity Standards (Charleston, SC). An independent 127I calibration verification standard was purchased from VHG Labs (Manchester, NH). The 129I certified stock standard used for instrument calibration was supplied by the National Institute of Standards and Technology (Gaithersberg, MD). An independent 129I calibration verification standard was purchased from Amersham Biosciences (Piscataway, NJ). Samples. Four types of material were used to develop and test extraction methods. The first materials tested were iodine salts, in the form of either potassium iodate or potassium iodide. Both an iodide and iodate salt were used to ensure the form of iodine present in solid matrixes did not have a negative impact on extraction efficiencies. The second test material, soil-1, consisted of the National Institute of Standards and Technology San Joaquin Soil, Standard Reference Material (SRM) 2709, which is an agricultural soil that has been oven-dried, sieved, and blended to create a homogeneous material suitable for use in testing analytical techniques and procedures for soils and sediments. The weight of San Joaquin Soil (SRM 2709) used in the method development tests was varied between 0.05 and 0.3 g in order to identify the optimum sample weight to be extracted/dissolved using this procedure. The third material to be analyzed, glass-1, was a vitrified glass containing stable iodine (127I), with a primary composition of 46% SiO2, 20% Na2O, and 10% Al2O3. Again, the weight of aliquots of the glass-1 material was varied between 0.1 and 0.3 g in order to identify the optimum sample weight of glass material to be extracted/dissolved using this procedure. The final material used to validate this method, samples 404D and 405D, were radiologically contaminated residual tank sludge retrieved from tank C-106 at the Hanford Site.16 Tank waste sample weights (15) Nishiizumi, K.; Elmore, D.; Honda, M.; Arnold, J. R.; Gove, H. E. Nature 1983, 305, 611-612. (16) Deutsch, W. J.; Krupka, K. M.; Lindberg, M. J.; Cantrell, K. J.; Brown, C. F.; Schaef, H. T. Hanford Tank 241-C-106: Residual Waste Contaminant Release Model and Supporting Data; PNNL-15187, Pacific Northwest National Laboratory, Richland, WA, 2005.
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were targeted to be ∼0.2 g to minimize worker exposure to radiological dose. Extraction Technique. Up to 300 mg of the solid material was mixed with 6 mL of a 30% KOH and 3% KNO3 solution as a fluxing agent in a zirconium crucible. Several process spike samples, which were used to determine extraction efficiencies and test for iodine volatilization, were prepared by adding either 1 µg of 127I or 0.05 µg of 129I to the crucible containing KOH-KNO3 solution (blank spike) or sample and KOH-KNO3 solution (matrix spike). The crucible was then placed in a 90 °C oven and allowed to evaporate to dryness, after which it was covered and transferred to a muffle furnace preheated to 550 °C. Fusion was accomplished by heating the sample-flux mixture for 60 min at 550 °C. After 60 min, the crucible was removed from the furnace and allowed to cool to ambient room temperature. The fused solid was then dissolved in deionized water (18 MΩ‚cm). The dissolution solution was transferred to a 50-mL volumetric flask. The crucible was then triple-rinsed with deionized water, and these rinses were also added to the volumetric flask. The resulting solution was diluted to a total volume of 50 mL with deionized water. The solution was decanted and centrifuged to collect the residual solids. The supernatant was collected and submitted for iodine analysis as the “liquid” sample fraction. The residual solids were dissolved using up to 2 mL of a 1:1 solution of concentrated sulfuric acid and 1 M sodium bisulfite. The solution was brought to volume (typically 50 mL) using 1% Spectrasol CFA-C in water. An aliquot of this solution was submitted for iodine analysis as the “solid” sample fraction. Due to the high dissolved metal content of the tank sludge, the above procedure was modified slightly after fusion of the sample was completed. The fused sample was allowed to cool and then was dissolved in ∼10 mL of deionized water (18 MΩ‚cm). This solution was transferred to a 50-mL centrifuge tube, and up to 20 mL of the 1:1 solution of concentrated sulfuric acid and 1 M sodium bisulfite was used to reduce the metals in solution and dissolve the residual sample material. The volume of solution used to extract the sample was calculated gravimetrically, and an aliquot of the final solution was submitted for 129I analysis. Instrumentation and Analysis of Iodine. Direct analysis of the resultant dissolved/fused sample solutions was performed via inductively coupled plasma mass spectrometry (Elan DRC II, Perkin-Elmer, Wellesley, MA) using a tertiary amine carrier solution (Spectrasol CFA-C). Spectrasol CFA-C, which is a surfactant that coats the glassware and prevents the buildup/memory of iodine in the ICPMS introduction system, was diluted in deionized water (18 MΩ‚cm) to create a 1% working solution. Calibration standards were prepared by diluting the 100 mg/L 127I and 1 mg/L 129I certified stock standards into appropriate volumes of the 1% Spectrasol CFA-C solution containing 5 ng/ mL 121Sb as the internal standard; internal standards are chosen based on their proximity (atomic mass) to an element of interest and are used to verify instrument performance and to correct for instrument drift. An independent calibration check standard was prepared from the 10 000 mg/L 127I and 1 mg/L 129I independent certified stock standards in 1% Spectrasol CFA-C. One percent Spectrasol CFA-C was used to prepare instrument blanks and was also used as the rinse solution throughout the run. 7064 Analytical Chemistry, Vol. 77, No. 21, November 1, 2005
Table 1. 127I Content of Salt Samples sample ID
weight (g)
KI-1 KI-2 KI-3 KIO3-1 KIO3-2 KIO3-3
0.0512 0.0519 0.0512 0.0668 0.0649 0.0646
127I
(recovery, %) 97.4 96.9 95.7 95.0 96.9 97.9
Perkin-Elmer’s Elan DRC II is a state-of-the-art ICPMS that uses Dynamic Reaction Cell technology to eliminate polyatomic and mass interferences. This technology is particularly useful for the analysis of 129I, which has 129Xe as a mass interference. 129Xe has a natural relative abundance of 26.4% and is present at low levels (˜20 ppb) in the atmosphere. Although the Elan DRC II uses argon as the sample introduction/carrier gas, small amounts of atmospheric gases typically contaminate the introduction gas stream. The reaction cell on the Elan DRC II can be used to react 129Xe with O , preventing it from entering the mass analyzer 2 quadrupole. This results in a dramatic reduction in background signal at mass 129, which ultimately leads to an improvement in analytical detection limits. Typical instrument detection limits for 129I using the Dynamic Reaction Cell are in the range of 5-10 ng/L. 127I does not have any natural mass interferences; therefore, it was analyzed on the Elan DRC II instrument in standard analytical mode. The instrument detection limit for 127I varied slightly from day to day but was always within the range on 10-25 ng/L. Several attempts were made to measure the 129I content of the tank sludge samples using the Dynamic Reaction Cell technology; however, an unknown interference created during injection of the O2 reaction gas prevented quantitative analysis of the element. Therefore, 129I was also analyzed on the Elan DRC II instrument in standard analytical mode. The 129Xe contribution was removed from the subsequent analytical data using background subtraction. The instrument detection limit for 129I analyzed on the PerkinElmer Elan DRC II in standard analytical mode was 10 ng/L. RESULTS AND DISCUSSION Iodine Salts. The percent recoveries for the three replicate analyses of the iodine salts were all in excess of 95% (Table 1). The high recoveries achieved indicate that the extraction method was successful and that the form of iodine present does not affect extraction efficiency. Additionally, the extraction method was essentially 100% efficient, with overall average recoveries of 96.7 and 96.6% for the KI and KIO3 salts, respectively. Although the average recoveries were similar, the range of percent recoveries measured during this study was more consistent than the 85118% range reported by Anderson and Markowski.17 Further, the precision of the extraction and analytical methods was superb, with standard deviations of 0.9 and 1.5% for the three replicates of KI and KIO3 salts, respectively. These results highlight two key findings: (1) iodine in the form of either iodate or iodide is stable throughout the extraction process, and (2) our ICPMS analytical method can be utilized to directly quantitate dissolved iodide in postextracted solutions. (17) Anderson, K. A.; Markowski, P. J. AOAC Int. 2000, 83, 225-230.
Table 2. 127I Content of SRM 2709 sample ID
weight (g)
SJS-1C SJS-2C SJS-3C SJS-4C SJS-5C
0.0528 0.1051 0.2051 0.1506 0.3152
Table 4. 129I Content of Tank Samples 127I
(µg/g)
sample ID
weight (g)
3.77 4.55 5.21 5.05 4.79
404D-A 404D-B 405D-A 405D-B 405D-C blank spike
0.2189 0.2053 0.2039 0.2686 0.3862 na
129I
(µg/g)
0.592 0.632 0.807 0.654 0.416 na
129i
(recovery, %) na na na na na 98.7
Table 3. 127I Content of Glass-1 sample ID
weight (g)
glass-1A glass-1B glass-1C glass-1D glass-1E
0.1069 0.2056 0.3092 0.1067 0.2010
127I
(µg/g)
0.31 0.56 0.66 0.64 0.69
Standard Reference Material 2709. The total iodine concentration (127I) measured for the five replicates of San Joaquin Soil (SRM 2709) varied between 3.77 and 5.21 µg/g. (Table 2). The reported data have been blank corrected and combine results from both the liquid and solid extract fractions. The italicized data reported for samples SJS-4C and SJS-5C (both matrix spike samples) indicate that the reported values were corrected for the iodide spike contribution (i.e., the iodide spike contribution has been removed from the reported data). Overall, the data indicate that the extraction method was successful. There was excellent reproducibility in all of the replicates that contained greater than 0.1 g of solid mass. The results imply that the fusion extraction technique requires at least 0.1 g of soil when the solid contains 5 µg/g iodine. The average iodine concentration of the four samples that met this criterion was 4.9 µg/g with a relative standard deviation of less than 6%. Comparison of this result with the noncertified iodine content of the San Joaquin Soil resulted in a 98% recovery, which compared quite well with the results reported by Resano et al.18 and was considerably higher than the 89% recovery reported by Marchetti et al.5 for the same standard reference material. Glass-1 Sample Material. The five replicates of glass-1 material had 127I contents ranging from 0.31 to 0.69 µg/g (Table 3). The reported data have been blank corrected and combine results from both the liquid and solid extract fractions. The italicized data reported for samples glass-1D and glass-1E (both matrix spike samples) indicate that the reported values were corrected for the iodide spike contribution. As with the SRM 2709 material, the data indicate that the extraction method was successful. There was excellent reproducibility in all replicates containing greater than 0.2 g of glass sample. This result implies that the fusion extraction technique requires greater than 0.2 g of glass sample material when the total iodine content of the sample is lower than 0.5 µg/g. The average iodine concentration of the three samples that met this criterion (glass-1B, glass-1C, glass-1E) was 0.64 µg/g with a relative standard deviation of less than 11%. The larger degree of variability in the analytical results of the glass versus the San Joaquin Soil sample was likely an artifact of the total iodine content of the respective samples; the (18) Resano, M.; Garcia-Ruiz, E.; Moens, L.; Vanhaecke, F. J. Anal. At. Spectrom. 2005, 20, 81-87.
glass sample contained ∼1 order of magnitude less iodine than the SRM 2709 soil sample. 404D and 405D Tank Waste Material. Table 4 contains the blank-corrected total iodine (129I) concentration of each tank sludge sample as a function of sample weight. The italicized data reported for sample 405D-C (matrix spike) indicates that the reported value was corrected for the 129I spike contribution. The duplicate tank sludge samples from 404D had an average 129I concentration of 0.612 µg/g with a difference between the two samples of 6.6%. The three replicate aliquots from tank sludge sample 405D highlight a greater degree of bulk sample heterogeneity. The three samples had an average 129I concentration of 0.625 µg/g with a relative standard deviation of 17.3%. Although the measured relative standard deviation between the three replicate 405D tank sludge samples was less than optimal, it was similar to that measured for other “mobile” constituents in the tank waste material using similar extraction and analytical techniques.16 The outstanding recovery of the 129I blank spike sample (98.7%), when coupled with the known heterogeneity associated with the respective tank samples, has made it possible to assume that the fusion was complete and quantitative for 129I in the tank sludge material. Although it was not the emphasis of this study, 127I was also measured in the tank waste samples. Due to the suspected presence of a significant amount of fission product 129I in the tank waste material, it was presumed that the 129I/127I ratios in the samples would be well within the analytical limitations of ICPMS instrumentation (greater than 10-6).19 129I/127I ratios in the tank sludge material ranged from 1.28 × 10-1 to 2.88 × 10-1. CONCLUSIONS Results from this study have shown that potassium hydroxide/ potassium nitrate sample fusion coupled with extraction and dissolution using sulfuric acid and sodium bisulfite is a simple, reliable, and quantitative method for the extraction of iodine from solid matrixes. The greatest benefit of this extraction technique is the total dissolution of the solid sample without volatilization of iodine. The method has been shown to be effective using a standard reference material in addition to well-characterized glass and Hanford tank sludge samples. Additionally, this extraction technique can be applied to other matrixes with little or no adaptation. This research has also shown the applicability of ICPMS as a rapid and quantitative technique for use in the analysis of iodine. The primary benefit of analyzing samples using ICPMS is the (19) Izmer, A. V.; Boulyga, S. F.; Becker, J. S. J. Anal. At. Spectrom. 2003, 18, 1339-1345.
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exceptional analytical detection limits provided by this instrumentation. The ability to directly analyze the fused/extracted solution samples without the need for elaborate and time-consuming separation and preconcentration steps significantly increases productivity while minimizing the potential for laboratory-induced error. This analytical tool, when coupled with sample preparation
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via fusion, makes a simple and cost-effective method to quantitate total iodine (127I and 129I) in solid samples. Received for review June 2, 2005. Accepted August 29, 2005. AC050972V
