Accred Qual Assur DOI 10.1007/s00769-015-1178-4

PRACTITIONER’S REPORT

Are analytical standards and reagents really reliable? Ce´line Gautier1 • Perrine Wund1 • Elodie Laporte1 • Je´roˆme Vial2 • Didier Thie´baut2 • Ce´dric Rivier3 • Marielle Crozet4 • Florence Goutelard5

Received: 19 May 2015 / Accepted: 11 August 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract Quality assurance is one of the major challenges in analytical chemistry, whatever the scope of application. The quality of analytical standards is very seldom questioned; however, sometimes odd results are obtained, and all the other potential sources of discrepancies are eliminated. So, we investigated the reliability of three analytical standards and reagents implemented for radiochemical and chemical characterizations of nuclear waste. In particular, this work examined the purity of a source of tritiated dodecane, the trueness of a certified concentration value and the purity for a diethylenetriaminepentaacetic acid (DTPA) reagent and the trueness of a certified concentration value for a multi-anion standard used in an interlaboratory comparison exercise. It was shown that the source of tritiated dodecane contains 60 % of tritiated impurities. The trueness of the DTPA concentration certified by the supplier was questioned due to the & Ce´line Gautier [email protected] 1

CEA/DEN/DANS/DPC/SEARS/LASE, Atomic Energy Commission, CEA Saclay, PC 171, 91191 Gif-sur-Yvette, France

2

Analytical and Bioanalytical Sciences and Miniaturization Laboratory, UMR CNRS CBI 8231, ESPCI ParisTech, PSL Research University, 75005 Paris, France

3

CEA/DEN/MAR/DRCP/SERA/LAMM, Atomic Energy Commission, CEA Marcoule, Building 166, BP 17171, 30207 Bagnols-sur-Ceze, France

4

CEA/DEN/MAR/DRCP/CETAMA, Atomic Energy Commission, CEA Marcoule, Building 400, BP 17171, 30207 Bagnols-sur-Ceze, France

5

CEA/DEN/MAR/DEIM/Nuclab, Atomic Energy Commission, CEA Marcoule, Building 109, BP 17171, 30207 Bagnols-sur-Ceze, France

presence of impurities in the solution. It was proven that the long-term stability of the multi-anion standard was not guaranteed for nitrite. The results clearly demonstrated that, despite the certificates delivered by the suppliers, caution has to be taken toward the reliability of the analytical standards and reagents. Keywords Analytical standards  Certified reference materials  Reagents  Stability  Quality assurance  Reliability

Introduction The issue of quality assurance (QA) in the analytical chemistry laboratory has become of great importance for many years [1–5]. Quality control (QC) and QA are particularly essential for pharmaceutical sciences [6], environmental monitoring [5, 7, 8], food safety [9] and also nuclear industry [10–12]. Radioactive waste management is a challenging task faced by nuclear power countries and is a prime concern for the public and therefore for analytical laboratories. In France, the National Radioactive Waste Management Agency (ANDRA) is in charge of the management of radioactive waste. It requests radiochemical and chemical characterizations of nuclear waste and specifies acceptance criteria for packages that have to be respected by waste producers [12]. Consequently, analytical laboratories devoted to nuclear waste qualification must apply validated methods and provide accurate and reliable results [10, 11]. Accuracy is defined as the closeness of agreement between a measured quantity value and a true quantity value of a measurand, whereas trueness refers to the closeness of agreement between the average of an infinite number of replicate

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measured quantity values and a reference quantity value [13]. All the radiochemical and chemical analyses necessary to nuclear waste characterization require the implementation of different laboratory instruments, such as liquid chromatography systems (high-performance liquid chromatography coupled to mass spectrometry HPLC–MS, ion chromatography IC) or liquid scintillation counters (LSC). Those instruments must be calibrated with reference materials and standards of known compositions [13]. Calibration standards, certified reference materials (CRMs) and reagents must be reliable in terms of trueness and precision, namely accuracy, purity and long-term stability [1–5, 11, 14–17]. Their availability is often limited or even missing, especially for radioactive matrices [16, 17]. Among the list of nuclear waste characterizations to be investigated, many radionuclides such as tritium have to be quantified [12]. As a pure beta emitter with a low energy with its maximum at Emax = 18.6 keV, tritium is favorably measured by LSC [18]. The liquid scintillation analyzer is generally calibrated using tritiated water standards. For nuclear waste and effluent samples, tritium is usually extracted from the matrices and isolated from the potential interfering radionuclides using a combustion/pyrolysis step prior to LSC [18, 19]. The optimization and the validation of the combustion conditions also require the use of tritiated water standards [19]. For the analysis of radioactive materials such as organic solvents and oils, it is necessary to implement tritiated organic compounds as reference molecules [19]. In the nuclear fuel reprocessing cycle, liquid–liquid extraction processes generally involve dodecane as solvent. Consequently, for tritium determination in radioactive waste after a combustion step, it is crucial to have access to a reliable and pure source of tritiated dodecane. Moreover, in the framework of nuclear waste management, waste producers are requested to quantify organic ligands such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) [12]. Actually, polycarboxylate molecules have been widely used in the nuclear industry for decontamination purposes. Hence, they may be present in effluents and waste samples [20]. They may form complexes with radionuclides, which can facilitate their mobility and as a consequence their potential leaching in the environment [20]. They can be analyzed by highperformance liquid or ion chromatography [21] coupled with conductimetric [22] or mass spectrometry [23–25] detection. Our group previously reported the quantification of EDTA [25] in radioactive effluents by applying a HPLC–MS method. For all chromatographic separations [21–25], it is necessary to establish calibration curves based on standards to quantify these analytes of interest. The chemical composition in terms of inorganic anions has also to be characterized in the radioactive effluents

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[12]. Among the inorganic anions to be analyzed, chloride, fluoride, sulfate, nitrate and nitrite must be quantified because of their complexing and corrosive properties [12]. Common inorganic anions are widely determined by IC coupled with conductimetric detection [26–28]. Actually, this technique has become a reference method for the measurement of inorganic ions in water and wastewater [28], including radioactive effluents [29]. The anions quantification relies on calibration curves obtained from the analyses of IC standards and/or CRMs. Therefore, the characterizations of nuclear waste imply the use of unassailable standards or CRMs of tritiated dodecane, DTPA and anions. For more than 10 years, investigations have been conducted on the comparability of CRMs and their qualities when several CRMs are available or a new CRM is in the certification’s process [30–33]. For instance, Duewer et al. [31] reported comparability studies for potassium and cholesterol reference materials of clinical relevance. Interlaboratory and key comparisons have been organized for the measurements of tritiated water and anions standards. Makepeace et al. [34] gave the results of an interlaboratory comparison, which addressed the measurements of the specific activity of tritiated water under the auspices of the International Committee for Radionuclide Metrology (ICRM). The Consultative Committee for Amount of Substance—Metrology in Chemistry (CCQM) has also carried out many key comparisons to test the principal techniques and methods and to establish degrees of equivalence between national metrology institutes in the field of amount of substance, more particularly for the assessment of the purity of organic materials [35]. As an example, Mariassy et al. [36] described the investigations realized on anion calibration solutions of nitrate and nitrite in the key comparison CCQM-K59. The obtained results were questionable partly because of the IC standards used by the institutes, which should prompt to pay attention to the verification of certified concentration values. Although works have been reported [36] and several CRMs are commercially available for multi-anion standards, no comparative study has been undertaken and only a few suppliers exist for tritiated dodecane and DTPA solutions. Recently, Wahl et al. [37] investigated solvent purity using comprehensive 2D gas chromatography. They highlighted the presence of impurities in common organic solvents such as commercial acetone brands. This work pointed out that caution must be taken toward the purity of solvents used for trace analysis, which strengthened the need to evaluate the confidence level in reagents and standards used in the nuclear waste field. In the present study, we investigated the reliability of three standards and reagents implemented for radiochemical and chemical characterizations of nuclear waste. In particular, this work examined the purity for a source of

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tritiated dodecane, the trueness of a certified concentration value and the purity for a DTPA reagent and the trueness of a certified concentration value for a multi-anion CRM used in an interlaboratory comparison exercise.

Materials and methods All reagents [nitric acid purchased from Merck (Darmstadt, Germany)] and solvents [hexane purchased from Carlo Erba Reagents (Sainte Clotilde, France), ethyl acetate and ethanol purchased from Sigma-Aldrich (Saint Quentin Fallavier, France)] used were of analytical grade. For the radiochemical analysis of tritium, a source of tritiated dodecane was purchased from a US provider (no analytical standard or certified reference material is commercially available). According to the technical data sheet, its specific activity was specified at 40 kBq/g without any stated uncertainty, whereas the radiochemical purity and chemical purity were specified at 99 % from thin-layer chromatography (TLC). The recommended storage temperature was 0–5 °C. It was noted that no expiration date was indicated. In this work, the chromatographic separations were carried out on pre-packed FinisterreTM Si SPE cartridges (Teknokroma, Sant Cugat del Valle´s, Spain) containing 1 g of silica with average particle size of 50 lm. The dead volume of the cartridge was around 1.2 mL. The Si SPE cartridges were conditioned with 12 mL hexane. All fractions obtained after separation were collected to quantify tritium. All tritium measurements were performed with a Tri-Carb liquid scintillation counter (PerkinElmer, Villebon-sur-Yvette, France). The instrument was calibrated for tritium analysis using a certified tritiated water standard (CERCA LEA, Pierrelatte, France). The accuracy of the tritium analyses was checked annually with proficiency tests organized by the LNHB laboratory (Laboratoire National Henri Becquerel, Gif-sur-Yvette, France). All organic samples were diluted in 5 mL ethanol and then mixed with 10 mL Ultima GoldTM LLT scintillation cocktail (PerkinElmer, Villebon-sur-Yvette, France) in 20 mL polyethylene vials. It was checked that the organic molecules of the samples did not induce any quenching effects. The DTPA solution was purchased as a general purpose reagent (GPR) from an international supplier (no analytical standard or CRM is commercially available). This chemical product was a DTPA pentasodium solution in water, which should have a purity grade higher than 0.950 g/g. According to the certificate of analysis, the pH of the solution was certified at 11.7 and the amount concentration was certified at 1.02 mol/L (for a GPR, no uncertainty is given). In our group, the experimental pH was measured with a SevenMultiTM pH meter (Metrohm, Villebon-sur-

Yvette, France), which was calibrated with certified pH buffers (CertiPURÒ, Merck, Darmstadt, Germany). For IC experiments, single- and multi-anion standards were purchased as IC CRMs from Spex companies (NJ, USA). The interlaboratory comparison (ILC) exercise called ‘‘EQRAIN Ions 2014’’ was performed using a multianion CRM solution (the uncertainties of the assigned concentrations were given with a coverage factor of 2). The nitrite mass concentration was certified at (0.330 ± 0.016) mg/L for 6 months. All IC standards except the interlaboratory CRM solution were diluted prior to injection in ultra-pure water (resistivity 18.2 MX cm) obtained from a Milli-Q purification system (Millipore, Molsheim, France). The IC separations were conducted using an ICS-4000 capillary-scale instrument (Thermo Scientific, Sunnyvale, CA, USA). The capillary-scale system consisted of an AS-AP autosampler, a capillary single pump, an EG online eluent generator module, a conductivity detector (which was thermally controlled at 15 °C) and an IC cube. The latter integrated a capillary-scale inline eluent degasser, a separately controlled capillary column oven (which temperature was fixed at 30 °C), a capillary 4-port injection valve fitted with a 0.4 lL internal loop and an anion capillary eluent suppressor (ACES). The ICS-4000 system was equipped with capillary AG15/AS15 columns (Thermo Scientific, Sunnyvale, CA, USA) working at 0.012 mL/min with 38 mmol/L KOH eluent.

Results and discussion Source of tritiated dodecane For the determination of tritium in nuclear waste, it is requested to perform a pyrolysis/combustion prior to LSC [18, 19]. A commercialized source of tritiated dodecane was chosen as an organic reference molecule of nuclear processes. Before using it as a quality control standard, its purity was examined. For that purpose, liquid chromatography with gravitational elution was implemented. Due to the nonpolar property of dodecane, normal-phase chromatography based on silica as stationary phase was selected [38]. In those chromatographic conditions, dodecane must be quantitatively eluted with a nonpolar solvent (such as hexane), whereas polar compounds must be eluted with a polar solvent (such as ethyl acetate) [38]. After the loadings of the tritiated source, the Si SPE cartridge was successively flushed with hexane and ethyl acetate. Figure 1 shows the elution diagram of tritium contained in the tritiated dodecane source. Two elution peaks were, respectively, detected in hexane and ethyl acetate fractions, which was not consistent with the presence of a single tritiated molecule in the source. The first peak should

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correspond to the elution of tritiated dodecane. The second peak may be related to more polar tritiated impurities contained in the source, whereas the source purity was specified as 99 %. Besides, each elution peak corresponded to 40 % of tritium activity. The total loss of tritium activity was thus quantified at 20 %, which should correspond to polar impurities fixed on the Si SPE cartridge. To assess the presence of impurities, the fractions corresponding to the first elution peak were gathered and a second chromatographic separation was carried out with this purified tritiated dodecane solution. The corresponding elution diagram is depicted in Fig. 2. Only one elution peak was observed, and tritium activity was quantitatively recovered in hexane fractions. As a consequence, the commercialized source of tritiated dodecane contains 60 % of tritiated impurities. This leads to the conclusion that the applied TLC procedure was probably not sufficiently selective to purify the synthesized product, or/and the longterm stability of the tritiated molecule could not be guaranteed by the US supplier.

Fig. 1 Elution diagram of the non-purified tritiated dodecane source (each A corresponds to the flushing of 1 mL non-purified tritiated dodecane source, each B corresponds to the flushing of 2 mL hexane and each C corresponds to the flushing of 2 mL ethyl acetate)

General purpose reagent (GPR) of DTPA We previously developed a method to quantify EDTA in radioactive effluents by HPLC–MS [25]. Recently we proved that the same procedure can be applied for the analysis of DTPA. It can be noted that no commercialized CRM is available for EDTA and DTPA (however, a reagent conform to European Pharmacopoeia requirements is commercially available for EDTA). As for EDTA, a calibration curve was established for DTPA using a commercialized salt dissolved in diluted ammonia. A GPR of DTPA was purchased to be used as a quality control solution. It was decided to control its certified pH and amount concentration. DTPA (denoted here as H5DTP) has five acidic functions: the corresponding pKa values are as follows: pKa(1) = 2, pKa(2) = 2.6, pKa(3) = 4.3, pKa(4) = 8.6 and pKa(5) = 10.5 [39]. Prior to the experiments, the theoretical speciation of DTPA in water was modeled using JChess software (Ecole des Mines ParisTech, Fontainebleau, France). For that purpose, the database of the software (chess.tdb) was enriched with specific stability constants for DTPA obtained from Smith and Martell [39]. Figure 3 shows the theoretical titration curve of the DTPA solution by 5 mol/L HNO3 (triangles). The initial pH of the solution should be 11.7, as specified in the supplier certificate. Due to the closeness of the 5 pKa values, only one equivalence point should be expected during the titration, which corresponds to the conversion of DTP5– into H2DTP3–. To determine the DTPA amount concentration, 50 mL of the solution was experimentally titrated using 5 mol/L HNO3. The result of this experimental titration is presented in Fig. 3 (squares). The initial experimental pH of the solution was found to be higher than 14. Furthermore, the titration curve exhibited two equivalence points. Those experimental results were not in agreement with the theoretical simulations. To explain those differences, the presence of impurities can be assumed, such as NaOH impurities originating from DTPA synthesis. Based on this

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Experimental titration Theoretical titration without impurities Theoretical titration with impurities

14 12

pH

10 8 6 4 2 0

Fig. 2 Elution diagram of the purified tritiated-labeled dodecane source (each A* corresponds to the flushing of 1 mL purified tritiated dodecane source, each B corresponds to the flushing of 2 mL hexane and each C corresponds to the flushing of 2 mL ethyl acetate)

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0

5

10

15

20

25

30

Volume of added 5 mol/L HNO3 (mL)

Fig. 3 Experimental and theoretical titrations of the DTPA solution

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hypothesis, the first equivalence point was associated with the reaction of OH- with H? and the second one to the conversion of DTP5– into H2DTP3–, which led to respective amount concentrations of 0.28 mol/L for NaOH and 0.93 mol/L for DTPA. The theoretical titration curve corresponding to this composition was modeled with JChess software (diamonds in Fig. 3). The theoretical titration curve with NaOH impurities fits perfectly the experimental curve. Consequently, the DTPA solution contains impurities, which induces a bias of 9 % to the DTPA amount concentration. Multi-anion standard with NO22 Our group recently tested the new technology of capillaryscale IC systems for the measurements of common inorganic anions in radioactive effluents. To evaluate the analytical performance of this novel instrument, our group participated in an interlaboratory comparison exercise called ‘‘EQRAIN Ions’’ organized in 2014 by the CETAMA committee [40]. To quantify the different anions, calibration curves were established from a multianion CRM standard containing F-, Cl-, NO3-, SO42-, PO43- and mono-NO2- and Br- CRM standards. The performances of the laboratories were evaluated by means of z-scores and zeta-scores according to the ISO 13528 standard [41]. For each anion, the z-scores and the f-scores were calculated from the following equations [41]: xX r xX f ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2x þ u2X

z¼

where x is the laboratory’s own result, X is the assigned value, r is the standard deviation for proficiency assessment (this term was determined from the robust standard deviation according to ISO 13528 algorithm A [41]), ux is the laboratory’s own estimate of the standard uncertainty of its result x, and uX is the standard uncertainty of the assigned value X. For all the anions analyzed by our group in the ‘‘EQRAIN Ions’’ solution, the absolute values of z-scores and f-scores were lower than 2 (Table 1). The laboratory performance was considered as satisfactory for all the anions, except for NO2-. The authors show the results of the interlaboratory comparison exercise ‘‘EQRAIN Ions’’ for NO2- in Fig. 4. It should be noted that the robust mean value of the interlaboratory comparison exercise (0.243 ± 0.038) mg/L differed significantly from the assigned value (0.330 ± 0.016) mg/L for NO2-. The relative difference of 26 % between the robust mean value and the assigned value can be attributed to the instability of

NO2- due to its conversion to NO3- at long term. Actually, NO2- is known to be quite unstable, even in reagent-grade water, especially at low concentrations [42–44]. Comparing the reference value assigned by the interlaboratory comparison organizer with the results from our group and the participants in ‘‘EQRAIN Ions’’ confirmed an abnormal drift of the NO2- mass concentration in the solution. Therefore, the hypothesis of an erroneous reference value was confirmed. There is no guarantee that this drift is the same in each of the bottles sent to laboratories. The supplier recognized that the long-term preservation of 6 months was unsuitable for NO2- in this concentration range. So, caution should be taken toward to the long-term stability of the analytical standards to prevent biased results. For future EQRAIN ions comparisons, NO2should be separated from the other anions to ensure its stability at low concentrations.

Table 1 Results of our group obtained for the intercomparison test ‘‘EQRAIN Ions’’ organized in 2014 (the uncertainties are given with a coverage factor k = 2) Anion Assigned mass concentration (mg/L)

F-

Measured mass concentration (mg/L)

Relative f-score z-score difference to the assigned value (%) -0.9

-0.1

Cl-

3.72 ± 0.19

3.67 ± 0.04

-1.3

-0.5

-0.3

Br-

0.740 ± 0.037

0.701 ± 0.025

-5.3

-1.7

-1.0

8.64 ± 0.43

8.52 ± 0.51

-1.4

-0.4

-0.2

2.30 ± 0.12

2.22 ± 0.11

-3.5

-1.0

-0.6

6.34 ± 0.32

6.21 ± 0.81

-2.1

-0.3

-0.2

NO3SO42PO43-

0.1700 ± 0.0080 0.1659 ± 0.0050 -2.4

Fig. 4 Graph derived from the results of the interlaboratory exercise ‘‘EQRAIN Ions 2014’’ obtained for NO2- mass concentration (the horizontal solid line indicates the assigned value, the broken lines limit its standard uncertainty range, the diamonds correspond to the results of the 13 laboratories participating to the interlaboratory exercise, and the error bars indicate the reported standard uncertainty)

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Conclusion The reliability of the analytical standards and reagents is of prime importance. Their purity and the trueness of their certified values are a key determinant for the validity of the calibration procedures and therefore for the quality of the end results. In this work dealing with nuclear waste characterization, the reliability of three standards and chemical products was clearly questioned in terms of purity and trueness of the certified concentration values. This study obviously demonstrated that caution has to be taken toward the quality of analytical standards and reagents. Unlike what is commonly admitted, analytical standards and reagents can also be affected by errors. Nonetheless, suppliers of CRMs and standards increasingly attempt to put products on the market that are in conformity with international guidelines such as ISO Guide 34. Efforts have to be made to produce more unassailable standards and reagents for the nuclear waste field, which is essential to give confidence in nuclear waste management. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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