United States Environmental Protection Agency Environmental Monitoring Systems - Laboratory Las Vegas NV 89114 Research and Development EPA-600/S2-83-039 July 1983 &EPA Project Summary Test Procedure for lodine-131 in Drinking Water: Interlaboratory Collaborative Study W. H. Yanko, C. T. Bishop, and A A. Glosby An interlaboratory collaborative study was conducted on a test procedure for measuring the iodine-131 concentra- tion in drinking water. The purpose of the study was to determine the esti- mated precision and accuracy of test results from participating laboratories using this test procedure and analyzing drinking water samples. Drinking water samples containing iodine-131 at concentrations on the designated collection date of 7.8,25.9, and 78.3 picocuries per liter were an- alyzed in triplicate by 11 collaborators (10 laboratories) using the test pro- cedure. A statistical analysis of the test re- sults showed coefficients of variation for repeatability (within-laboratory pre- cision) of 8.7, 10.5, and 7.7 percent respectively for the three samples for an average repeatability precision of 9.0 percent The analysis also gave coefficients of variation for reproduci- bility (combined within-and between- laboratory precision) of 17.4,15.5, and 15.7 percent for the respective samples for an average reproducibility precision of 16.2 percent The average carrier iodine recoveries were 76.4, 79.3, and 79.8 percent respectively for the three samples, giving an overall average of 78.5 percent. A comparison of the grand average test results for the three samples with the known values for those samples shows accuracy indexes of 97.4, 97.3, and 84.8 percent respectively, for an aver- age accuracy for the test method of 93.2 percent. This Project Summary was developed by EPA's Environmental Monitoring Systems Laboratory, Las Vegas, NV, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction This report is submitted as partial fulfill- ment of an Interagency Agreement, EPA- IAG-79-D-X0736, between the Environ- mental Protection Agency and the Depart- ment of Energy (DOE). Work was uone at the Mound Facility of the Monaanto Re- search Corporation under DOE Contract Number DE-AC04-76-DP00053. The work covered the period of September 1, 1979 to June 15, 1980. The National Interim Primary Drinking Water Regulations (NIPDWR) require the use of approved test procedures for an- alyzing public drinking water supplies for contaminants. The Nl PDWR contains pro- visions for the use of alternate test pro- cedures with precision and accuracy equivalent to or better than the approved test procedure. This report describes a multilaboratory test of a method selected for the analysis of drinking water samples for iodine-1 31 concentrations. The pur- pose of the study was to determine what precision and accuracy could be expected in the test results from laboratories using the method for analyzing drinking water supplies for iodine-131 contamination. The NIPDWR requires a sensitivity (lower detection limit) of 1 picocurie per liter (pCi/l) for measuring iodine-131 concen- trations in drinking water supplies, lodine- 131 decays by emitting a beta particle and a gamma photon. Gamma spectroscopy has good specificity for gamma emitters, but its sensitivity will meet the required sensitivity (1 pCi /I) only if the iodine-131 ------- is first separated from multiliter size sam- ples. The method tested, and described here, requires separating the iodine-1 31, using carrier iodine, from one or more liters of sample and then counting the beta activity produced by the separated iodine, in a low beta background counting system. Procedures /. Analytical Test Procedure The analytical procedure used in this study consists of detailed steps in which the iodine-1 31 with added iodate carrier is separated from the sample as palladium iodide. The iodine-131 (plus carrier) is reduced to the iodide state with sodium sulfite, precipitated as silver iodide, and purified with zinc powder and by two precipitations as palladium iodide. The final palladium iodide is collected on a filter and then counted for beta activity. The recovery of the added carrier is determined for each sample by weighing the palladium iodide precipitate. The counting efficiency is determined using the same amount of iodate carrier as is used in the procedure and standard iodme-131 activity, precipi- tated as palladium iodide, and beta counted. 2. Collaborative Test Procedure A copy of the test procedure was sent to potential participant laboratories; those laboratories stating their willingness to participate were used in this study. Three reference sample solutions were prepared using tap water, sodium thio- sulfate and sodium carbonate preservatives, known quantities of iodine-131 activity, and lithium nitrate (for batch homogeneity testing). The three reference solutions contained iodine-131 concentrations of 7.8, 25.9, and 78.3 pCi/l respectively (activity calculated for the collection date to which analytical results were to be normalized). The first, middle, and last 3.5-liter fractions of each sample batch were used for homogeneity tests. Each participant was sent an instruction sheet, a data report sheet, and iodme-1 31 standard solution for calibrations, and a 3.5-hter portion of each of the three sample solutions. 3. Data Processing Procedures A statistical evaluation of the test results was done by the procedures described in E-691, E-177, and E-178 of the ASTM Standard Part 41, 1 980, to determine the repeatability precision (within-laboratory variation); the reproducibility precision (combined within- and between-laboratory variation); and the accuracy of the test procedure. The standard deviations and equations for their calculations are listed below. Standard deviation of replicate test re- sults within Lab i, for sample j, S,j ra l_h-i •r (1) where: Xl)h = the result reported for the h replicate of the j sample material by Lab i X,j = the mean of the individual results of sample j for Labi n,j = the number of replicates of sample j reported by Labi Repeatability (within-laboratory) standard deviation for sample j, Sr Since the number of replicates is the same (3) for all participants for all three samples, the equation can be given as follows: P Sr = [ 1/P2S,2 'l I M (2) where: P = the number of participants in the study. Standard deviation of individual labora- tory average from grand average for the j sample material, S* i= 1 _ where: X, = the average of the test re- ' suits for sample material j by Lab i Xj = the grand average for sample material j Standard deviation of between-labora- tories for the j sample material, SL. SL= (4) Reproducibility (combined within- and be- tween-laboratory) standard deviation for the ] sample material, SR SR)= Srj2 The percent coefficient of variation for repeatability (within- laboratory precision) (also called repeatability index) for sample J. Vr% Vr%= 100Sr/X~ J ' (6) The percent coefficient of variation for between- laboratory precision for sample j, V, % VL%= 100 SL/X, (7) The percent coefficient of variation for reproducibility (combined within- and be- tween-laboratory precision) (also called reproducibility index) for sample j, VR% VR%=100SR)/X, (8) Accuracy index, a percent relationship of the grand average to the known value for the j sample material, Aj% •i- = 100^- O) where: Y, = the known value for the j sample material (pCi/l) t-test to determine significant differences or systematic error for sample j, t t.= (5) , (P-1) degrees of freedom Sxj/(P)1/2 (10) where: P = number of participants Y, = known value of the sam- ple j iodine-131 concen- tration tc = 2.23, critical value for 11 participants, values for t greater than 2.23 are significantly differ- ent and show a system- atic error. Results and Discussion A summary of the statistical evaluation of the test results for the three samples is given in Table 1, which lists the following statistical parameters and values for the three iodine-131 concentrations: 1. The known value, Y,, for each of the iodine-131 concentrations^in pCi/I. 2. The grand average value, X,, for each iodine-131 concentration (from 11 participants) in pCi/l. ------- Table 1. Summary of Collaborative Study Results - Precision and Accuracy lodine-131 (pCi/l) Parameter e Yj (pCi/l) X, fpd/lj A°/0 S, (pCi/l) Sr(pCi/l) Si (pCi/l) c ' /*r;/n &p \pL,l/l/ vr<% v'% Vfr% 1 1 7.8 7.6 97.4 1.19 0.66 1.14 1.32 8.7 15.0 17.4 2 25.9 25.2 97.3 3.25 2.64 2.87 3.90 10.5 11.4 15.5 3 78.3 66.4 84.8 9.54 5.14 9.07 10.43 7.7 13.7 15.7 Average 93.2 9.0 13.4 16.2 a Terms are defined in the text. 3. The accuracy index, A,% (from 11 participants) for each iodine-131 concentration, and the average ac- curacy index over the concentration range of 7.8 to 78.3 pCi/l. 4. The standard deviation of the grand average values for the three iodine- 1 31 concentrations, S ^.. 5. The repeatability (withirl-laboratory) standard deviation, Sr, for each iodine-131 concentration. 6. The between-laboratories standard deviation, SL, for each iodine-131 concentrator!. 7. The reproducibility (combined with- in- and between-laboratory) standard deviation, SR, for each iodine-131 concentration. 8. The coefficients of variation for re- peatability, Vr%; for between-labora- tory precision, VL %; and for repro- ducibility, VR%, fdreach iodine-1 31 concentrator1! and the averages of each for the concentration range of 7.8 to 78.3 pCi/l. The 66.4 pCi /I grand average compared to the 78.3 pCi/l known value showed a significant difference or systematic error. A t-test gave a t( value of4.13 compared to the critical value (tc) of 2.23 for 11 partici- pants Therefore, 66.4 pCi/l is significantly different than 78.3 pCi/l and shows a low bias for the method for that level of iodine- 1 31 concentration. Conclusions The repeatability precision (within-la- boratory precision), reproducibility preci- sion (the combined within- and between- laboratory precision), and accuracy have been determined by a multilaboratory test of the method. Criteria by which to evaluate an alternate test procedure for equivalency have been established. Accuracy indexes of 97.4, 97.3, and 84.8 percent for the 7.8, 25.9, and 78.3 pCi/l iodine-131 concentration samples respectively, and an average accuracy index of 93.2 percent were obtained with this test procedure. The study did not reveal an explanation for the lower 84.8 percent accuracy for the sample with the highest iodine-131 concentration. The estimated repeatability (within-lab- oratory), single-operator, single-machine, same-day, relative precision of the test procedure for the determination of iodine- 131 concentrations in drinking water at the 7.8 pCi /I level (averaged over the 11 participants) is ±1.3 pCi/l(17.4 percent 2Spercent); atthe25.9 pCi/l level(aver- aged over the 11 participants) is ±5.3 pCi/l (20.9 percent, 2S percent); and at the 78.3 pCi/I level (averaged over the 11 participants) is±10.3 pCi/l(1 5.5 percent, 2S percent), for an average 2S percent of 18.0 percent for the range of iodine-131 concentrations of 7.8 to 78.3 pCi/l. The estimated reproducibility (combined within- and between-laboratory), multiop- erator (multilaboratory), single-machine, same-day, relative precision of the test procedure in the determination of iodine- 131 concentrations at the 7.8 pCi/llevel (averaged over the 11 participants) is ±2.7 pCi/l (35.8 percent, 2S percent); at the 2 5.9 pCi/I level (averaged over the 11 participants) is ±8.2 pCi/l(32.6 percent, 2S percent); and at the 78.3 pCi/l level (averaged over the 11 participants) is ±21.5 pCi/l (32.3 percent 2S percent), for an average 2S percent of 33.6 percent for the range of iodine-131 concentra- tions of 7.8 to 78.3 pCi/l. The variations observed in the iodine carrier recoveries by the 11 participants for the three samples showed a depen- dence on analyst technique for the method. Also, the similar average iodine carrier recoveries for the three samples (76.4, 79.3, and 79.8 percent respectively) does not offer any possible explanation for the low bias observed for the sample with the 78.3 pCi/l iodine-131 concentration. The sensitivity of the test procedure is not limited as much by the chemistry of the procedure (iodine recovery) as it is by the background and the counting efficiency of the counting system used. With a beta background of <5 cpm and a counting efficiency of >20 percent, the sensitivity of measurement (lower detection limit) required in the NIPDWR (1 pCi/l) can easily be met All participants whose test results were used in the precision and accuracy analysis of this study used count- ing instruments with beta backgrounds <5 cpm and counting efficiencies >20 percent. Recommendations It is recommended that the method tested in this study be investigated for procedure steps that result in analyst tech- nique dependence (as demonstrated by the variations in the iodine carrier re- coveries ranging from 48 to 100 percent). One possible weakness in the procedure is in the instructions for the reaction of solid zinc powder with solid silver iodide in an aqueous system. The procedure should be modified to include a more detailed procedure for the preparation of a palladium iodide-131 counting standard and for the determina- tion of the counting efficiency. A revision of section 6.5, the standardization of po- tassium iodate, is recommended. The method should be re-tested to de- termine if the low bias is real for the 78.3 pCi/l iodine-131 concentration and higher levels. ------- W. H. Yanko, C. T. Bishop, and A. A. Glosby are with Monsanto Research Corp., Miamisburg, OH 45342. Earl L. Whittaker is the EPA Project Officer (see below). The complete report, entitled "Test Procedure for Iodine-131 in Drinking Water: Interlaboratory Collaborative Study,"(Order No. PB 83-207589; Cost: $10.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency P.O. Box 15027 Las Vegas, NV 89114 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 PS «>0?912PROTECTION saaRfcs- ------- |