2. QUALITY CONTROL IN MASS SPECTROMETRY

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1 IAEA-SM-367/5/03 Evaluation of Uncertainties for Pu and U Measurements Achieved in the On-Site Laboratory by Thermal Ionisation Mass Spectrometry during Two Years of Operation E. Zuleger, K. Mayer, L. Duinslaeger, K. Casteleyn European Commission - JRC Institute for Transuranium Elements P.O. Box Karlsruhe Germany Abstract The quality control concept of the On-Site laboratory is based on a nested design of independent quality checks. It makes use of method intercomparison, subsample comparison, measurement of working standards and measurement of certified reference materials. With respect to method intercomparison, thermal ionization mass spectrometry is used as reference basis for the radiometric methods. This is applied for both isotope analysis and isotope dilution analysis. The paper discusses the experience gained with the quality control system in thermionic mass spectrometry measurements based on two years of operational experience. It describes the precisions and accuracies observed for the different types of measurements (isotope abundance, minor isotope abundance, isotope dilution) and shows the performance the technique has exhibited over two years of routine operation. The main sources of uncertainty of the analytical result will be discussed as well. 1. INTRODUCTION The On-Site laboratory (OSL) is a safeguards analytical laboratory, which performs the verification measurements on all the samples taken by Euratom inspectors at the Sellafield site. The operational aspects have been described earlier [1]. The concept for a stringent quality control was presented in another paper [2]. A multi-level approach was implemented in order to assure a constantly high level of quality of the measurements and a high degree of reliability of results. It has been pointed out in Ref. [2] that Thermal Ionisation Mass Spectrometry (TIMS) is at the heart of this quality control concept. In other words: permanent method intercomparison, using TIMS as a reference is an essential part of routine quality control. The key role of TIMS is justified by the fact that this method has been attributed the status of a primary method of measurement due to its short traceability chain and due to its potential for high precision and accuracy [3]. In order to fully exploit this potential, trained operators and well-established and validated analytical procedures are required. Precision and Accuracy of the results obtained by TIMS need to the monitored with great care. The present paper describes the experience gained in two years routine operation in the OSL. In the following sections we discuss in detail how instrumental control is archived, how process control is established and how sample precision is monitored. The quality control data

2 are used to estimate the measurement uncertainty of an individual sample. Finally, we describe how the measurement uncertainty is estimated and compare it to a fully orthodox uncertainty calculation according to the ISO Guide for the expression of Uncertainty in Measurement (GUM). Thermal Ionisation Mass Spectrometry is basically a technique for measuring isotope ratios. It requires a relatively pure sample for carrying out the actual measurement. Hence, a chemical purification step precedes the measurement. If the sample has been processed the relative isotope abundance, expressed in amount per cent (formerly called atom % or mole %) or in weight per cent (formerly called mass %), are derived from the measured isotope ratios. If a spike (a known amount of an isotope of the same element) has been added to the sample at the beginning of the analytical process, the concentration of the element of interest is determined from the measured isotope ratios. This is known as Isotope Dilution Mass Spectrometry (IDMS). 2. QUALITY CONTROL IN MASS SPECTROMETRY The On Site Laboratory (OSL) in Sellafield, UK has been in operation for two years. It is equipped with two thermal ionisation mass spectrometers (MAT 261 and MAT 262), set up for plutonium and uranium isotope ratio measurements. The samples are chemically separated by liquid-liquid extraction using TBP (tri n-butyl phosphate) as extractant, where purified fractions of uranium and plutonium are obtained. The radiochemical purity and the approximate concentration of the U (only 233 U spiked) and Pu fractions are checked by alpha spectrometry. Typically, about 4 ng Pu or 40 ng U are loaded onto a Re filament and subjected to measurement. The measurements are performed using the total evaporation method with signal integration throughout the measurement [4]. The quality control system which has been implemented is based on three steps: instrument control (level 1), process control (level 2) and sample precision (level 3). Once the analyst has confirmed the proper operation of the instrument, he proceeds with sample measurements. A computerized quality control system checks the results and suggests the analyst to accept or reject the result. The decision and responsibility, however, remains with the analyst Instrument control level 1 Instrumental performance verification is covered by measuring certified reference materials of U and Pu. This is achieved by comparing experimental results to the of e.g IRMM 183, 184, 185, 186, 187, 199 for U as well as IRMM 047 and the IRMM 290 serie for Pu. These reference materials are directly loaded without any chemical pre-treatment. In the sections below we will show typical examples, illustrating the quality control procedures and the performance achieved under routine conditions. The results are combined in table 1. Table 1. Certified and measured ratios of IRMM 185, IRMM 290 F1 and IRMM Standard level Number of measurements Measured n( 235 U)/n( 238 U) Long-term repeatability 2s (%) Bias (%) Certified value n( 235 U)/n( 238 U) ±2s IRMM 185 (U) ± IRMM 185 (U) ± n( 242 Pu)/n( 239 Pu) n( 242 Pu)/n( 239 Pu) IRMM 290F1 (Pu) ± IRMM 1575 (Pu) ±

3 Plutonium isotope ratio measurements IRMM 290 F1 is used as long-term instrument control sample (Fig.1). This reference material is certified to 0.01 % (rel.) for the n( 242 Pu)/n( 239 Pu) ratio. The repeatability standard deviation for n( 242 Pu)/n( 239 Pu) ratio measurements is 0.03% (2s) and hence only slightly higher than the certified uncertainty of the reference material. The value of the ratio n( 242 Pu)/n( 239 Pu), based on 25 measurements, is achieved over a period of 14 months and shows a good agreement with the. No significant bias was observed. The minor isotopes are not certified. However, the standard deviation gained for the ratio n( 240 Pu)/n( 239 Pu) is about 4.5 % OSL IRMM 290F level OSL IRMM 290F level 1 n(242pu)/n(239pu) n(240pu)/n(239pu) Fig. 1 A) n( 242 Pu)/n( 239 Pu) ratio of IRMM 290 F1, B) n( 240 Pu)/n( 239 Pu) ratio of IRMM 290 F1, gained over a period of 14 months (, = upper and lower control limit = 3s;, = upper and lower warning limit = 2s; triangle = certified ratio) Uranium isotope ratio measurements For uranium the standards IRMM 183 to 187 and 199 were used in the starting phase, however for long-term stability only IRMM 185 (Fig. 2) is used. Also here the value for the ratio n( 235 U)/n( 238 U) gained over a period of two years shows a good agreement with the. The average bias is 0.025% and therefore not significant. The repeatability standard deviation of our measurements over this period is 0.08% (2s)(Fig.2A). The achieved repeatability standard deviation of the minor isotope ratio n( 234 U)/n( 238 U) is inferior to the certified uncertainty (Fig. 2 B) and the value of the measurements is in good agreement with the. n(235u)/n(238u) OSL IRMM 185 level n(234u)/n(238u) OSL IRMM 185 level Fig. 2 A) n( 235 U)/n( 238 U) ratio of IRMM 185, B) n( 234 U)/n( 238 U) ratio of IRMM 185, gained over a period of 2 years (, = upper and lower control limit = 3s;, = upper and lower warning limit = 2s, triangle = certified ratio) 3

4 For level 1 (instrument control) the data collected over a period of two years clearly show good repeatability and no significant bias. We can therefore conclude that the instrument and the measurement procedure proved to be reliable and provide results of constantly high quality Process control level 2 After having verified the proper operation of the actual measurement instrument, the second level of quality control comprises also chemical separation and in case of IDMS spiking. Therefore each set of samples is accompanied throughout the entire analytical process by a working standard acting as quality control sample for process control. It is subjected to the same chemical treatment as the real samples. Again the experimental result is compared to the certified reference value. Additionally, the repeatability standard deviation of the method is derived from the last 25 measurements of the QC sample. The most recent QC result is then compared to the long-term average and the respective standard deviation. The measurement result of the QC sample is accepted if the following criteria are fulfilled: 1. Minimum one value is inside of the control limits (, = 3s) 2. two of three consecutive values are inside the warning limits (, = 2s) 3. no seven consecutive values with decreasing or increasing tendency 4. no ten consecutive values on one side of the central line Results are listed in table Plutonium isotope ratio measurements IRMM 1575 was used as process control sample (Fig.3). This material is a secondary reference material, which had been used in the early 1990 s for a REIMEP campaign (Regular European Interlaboratory Measurement Evaluation Programme). Almost 90 measurements OSL IRMM 1575 level 2 n(242pu)/n(239pu) Bias = Fig. 3. n( 242 Pu)/n( 239 Pu) ratio of IRMM 1575, chemically treated as the samples for process control have been performed with a long-term repeatability standard deviation of 0.12% (2s) for the n( 242 Pu)/n( 239 Pu) ratio. The value shows a small bias of However, the certified ratio n( 242 Pu)/n( 239 Pu) is inside the control limits of the value. The long-term repeatability increases as compared to the level 1 measurements. This is due to the chemical treatment, which may introduce minute cross contamination, operator dependent effect, and variations in chemical recovery and reagent quality Uranium isotope ratio measurements As an example for process control of uranium the results of IRMM 185 of the last two years are shown in table 1 and figure 4. The results are obtained by processing the standard in the same way as samples. The observed repeatability standard deviation for the ratio 4

5 n( 235 U)/n( 238 U) is 0.20 % (2s) and for the ratio n( 234 U)/n( 238 U) 4%, respectively. Both ratios of the IRMM 185 show a good agreement with the. The observed average bias of % is insignificant. OSL IRMM 185 level 2 OSL IRMM 185 level n(235u)/n(238u) n(234u)/n(238u) Fig. 4. A) n( 235 U)/n( 238 U) ratio of IRMM 185 B) n( 234 U)/n( 238 U) ratio of IRMM 185, treated as the samples for process control treated as the samples for process control As expected also the long-term repeatability for the uranium isotope ratio measurement of the standards increases compared to the level 1 measurement. Again, this is due to the chemical treatment, which may introduce minute cross contamination, operator dependent effects, variations in chemical recovery and reagent quality. However, the results are still in the range of the Plutonium concentration Similar to the process control for isotope ratio measurements, process control for isotope dilution mass spectrometry (IDMS) is introduced as well. This covers the spiking procedure as well as the stability of the spike and the spike/sample equilibration. A synthetically prepared solution (IDMS-Input standard) containing U and Pu in an elemental proportion of 100:1 is used for that purpose. This solution was prepared on gravimetrical basis from high purity element reference materials (i.e. CETAMA MP2 and IRMM EC 101). As can be seen from figure 5 the long-term repeatability is larger than the calculated uncertainty of the standard. In addition figures 5 and 6 show clearly clusters of results. This grouping correlates with the periods of analysis. concentration (mg/g) OSL Input Standard IDMS Pu level concentration (mg/g) OSL Input Standard IDMS U level Fig. 5. Long-term repeatability of the Pu assay of the OSL input standard Fig. 6. Long-term repeatability of the U assay of the OSL input standard 5

6 Still the obtained standard deviation of 0.07 % (2s, f=24) is smaller than the ESARDA target value (Tab. 2). However, the value of the measurement ( mg/g) corresponds to the calculated value. The bias of 0.02% is insignificant Uranium concentration The same material as described in the previous section is used for process control of uranium IDMS. From figure 6 it becomes evident that as already seen for Pu the uncertainty of the U measurement is depending on the analysis date. The long-term repeatability is 0.10 % (2s, f=24). The value, mg/g, shows a good agreement with the calculated value. The average bias is 0.06 % and therefore negligible. As shown for level 1 also for isotope ratio and concentration measurements at level 2 no bias occurs. However, long-term repeatability increases for both methods. For the element assay clearly a time dependent effect is visible Sample Precision level 3 In order to estimate the repeatability on the sample level each sample is run in duplicate. The duplicates are treated independently. The results obtained on the two subsamples are compared and the difference is subjected to a statistical test comparing it with the precision of the method as obtained in level 2 (i.e to the standard deviation obtained from 25 repeat measurements). The warning and control limits (,,, ) are calculated by the standard deviation of the standard accompanying the sample run. The solid line represents the zero-line. The same out of control criteria as for the process control are used to decide if a sample is accepted. However, as the standard deviation is depending on the concentration, this chart can only be used for samples within the same concentration range as the standard. The samples analysed by IDMS are diluted to a concentration of 100 to 400 µg U/g prior to chemical treatment. The working standard used for process control (level 2) is at the same concentration Plutonium and uranium isotope ratio measurements Figure 7 shows an example of a typical sample run in the OSL daily routine procedure. Five samples and a standard, all run in duplicate, are processed together. Each dot represents the difference between two sub-samples, typically for isotope ratio measurement in the 10 5 range. Fig. 7. A) difference chart for n( 242 Pu)/n( 239 Pu) B) difference chart for n( 235 U)/n( 238 U), compared to the differences gained by the compared to the differences gained by the standard IRMM level 3. standard IRMM 185 level 3. (, = upper and lower control limit = 3s;, = upper and lower warning limit = 2s) Plutonium and Uranium concentration The plutonium and uranium concentrations obtained on samples by isotope dilution (IDMS) are likewise checked by evaluating the difference of two sub-sample measurements relative to repeatability of a standard. The examples shown in figure 8 are compared to the IDMS-Input standard. 6

7 Fig. 8. A) difference chart for Pu concentration B) difference chart for U concentration compared to the differences gained by the compared to the differences gained by the standard IDMS Input - level 3. standard IDMS-Input - level 3. (, = upper and lower control limit = 3s;, = upper and lower warning limit = 2s) 3. Evaluation of the Combined Uncertainty In the previous chapters we described how the quality of measurement results is assumed by strict control of the measurement instrumentation and of the entire analytical procedure. This multi-level approach offers at the same time another important feature: from the repeatability standard deviation as derived from process control we can estimate the combined uncertainty. This procedure has been described earlier [5]. As mentioned earlier, thermal ionisation mass spectrometry provides primarily information on the isotope abundance ratio. The measured ratios may be subjected to a number of corrections (e. g. baseline, amplifier gain, detector efficiency, and isotopic fractionation (K-factor)). The corrected ratios are then used to calculate the isotope abundance or if a spike was added before the isotope concentration is calculated. In either case a reliable uncertainty statement has to be attributed to the final result. This uncertainty interval should contain the true value. In the OSL a robust method was established to estimate the combined uncertainty from the repeatability standard deviation (last 25 repeat measurement of a quality control sample) and the other main sources of uncertainty (e. g. spike concentration). The ISO Guide for the Expression of Uncertainty in Measurement [6] proposed a component by component approach, where the estimated individual uncertainty contributions of the individual steps in the analytical process are combined using standard rules of uncertainty propagation. This has been implemented in a commercially available software programme, the GUM Workbench [7], which uses a numerical differentiation method. Both approaches are compared for TIMS, IDMS and for isotope abundance calculation. Table 2. Comparison of the relative expanded uncertainties of isotope abundance and IDMS (U c = k*u c, with k = 2) of the robust approach implemented in the OSL and the GUM method. OSL GUM ITV 2000 [8] U content 0.15 % 0.20 % 0.36 % Pu content 0.20 % 0.20 % 0.36 % 235 U 0.20 % 0.23 % 0.28 % n ( 240 Pu)/n( 239 Pu) 0.12 % 0.11 % 0.22 % 7

8 A more detailed analysis has shown [5] that the main contributions in the uncertainty budget for the U and Pu content arise from the spike material used (NBS SRM 996, MP2, 233 U Harwell) and the measured n( 240 Pu)/n( 239 Pu) ratio in the spiked and unspiked sample, respectively. The main uncertainty contribution, more than 90 %, for the isotope ratio measurement by TIMS arises from the measurement itself (repeatability standard deviation). The same is valid for the abundance calculation. Here about 97% of the uncertainty contribution arises from the measurement, 2.6 % from the instrument calibration. As can be seen from table 2 the robust approach to estimate the uncertainties in the OSL is more or less the same as achieved by the detailed analysis via the GUM workbench. 4. Conclusion A valid, reliable and pragmatic quality control method is in operation in the On Site laboratory. In two years of field experience it could be demonstrated that a useful tool for analysts to evaluate their results is available. Taking into account the operational conditions (e.g. weekly changing operators, irregular analysis of certain sample types) and the routine character of the laboratory work, excellent repeatability standard deviations for U and Pu are achieved. The achieved accuracy is at a similarly high level. The introduced three levels of quality control in mass spectrometry not only serves for identifying individual results of poor quality, it is also used to derive measurement uncertainty statements and to monitor long term effects. With respect to the latter we could clearly identify time dependent effects in IDMS analysis. The source of this effect is being investigated. The observed uncertainties are well below the requirements of the International Target Values. REFERENCES [1] Blohm-Hieber, U., Mayer, K., Ottmar, H., Schneider, H.G., Revised Analytical Strategy for the Euratom On-Site Laboratory at Sellafield 21st ESARDA Symposium on Safeguards and Nuclear Material Management, 3-5 May 1999, Sevilla, Spain. [2] Mayer, K., Duinslaeger, L., Cromboom, O., Ottmar, H., Wojnowski, D., van der Vegt, H., Analytical Quality Control Concept in the Euratom On-Site Laboratories, Symposium on International Safeguards: Verification and Nuclear Material Security, Vienna, Austria, 29 October-1 November [3] P. De Bièvre, Fresenius J. Anal. Chem., 337, (1990). [4] Romkowski, M., Franzini, S., Koch, L.; Proc. ESARDA Symp. London (1987). [5] Casteleyn, K., Mayer, K., Cromboom,O., Implementation of Uncertainty Estimation according to ISO Guide in an Analytical Laboratory. 41 st INMM Annual Meeting, New Orleans, USA, July [6] Guide to the Expression of Uncertainty in Measurement ISO, Genua, [7] GUM Workbench, Metrodata GmbH, [8] H. Aigner et al., International Target Values (ITV) 2000 for Measurement Uncertainties in Safeguarding Nuclear Materials. IAEA STR-327 8