# Methods of separation and quantification by ICP-MS for Pd-107 in radioactive waste

Marina Faure, Celine Gautier, Pascal Fichet, Adrien Maubert, Eloan

Debonnet, Elodie Laporte, Christèle Colin, Lucie Helot

► To cite this version:

Marina Faure, Celine Gautier, Pascal Fichet, Adrien Maubert, Eloan Debonnet, et al.. Methods of separation and quantification by ICP-MS for Pd-107 in radioactive waste. Journal of Radioanalytical and Nuclear Chemistry, 2025, 10.1007/s10967-025-10147-6. cea-05117637

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# Methods of separation and quantification by ICP-MS for Pd-107 in radioactive waste

Marina Faure¹, Céline Gautier¹*, Pascal Fichet², Adrien Maubert¹, Eloan Debonnet¹, Elodie Laporte¹, Christèle Colin¹, Lucie Hélot¹

¹Université Paris-Saclay, CEA, Service de Physico-Chimie, 91190, Gif-sur-Yvette Cedex, France

²Université Paris-Saclay, CEA, Service Gestion Opérationnelle des Filières de déchets, 91190, Gif-sur-Yvette Cedex, France

*E-mail: celine.gautier@cea.fr

## Abstract

The goal of this study was to improve the current method used to characterize radioactive waste for dismantling and decommissioning applications. In particular, a focus was made on the method for the separation of palladium from other radionuclides. A purification procedure based on extraction chromatographic Ni-resin® for Pd-107 analysis was investigated on non-radioactive commercial standard solutions. It was developed from a design of experiment (DoE) method using a screening design to recover the palladium entirely. An additional step with LN-resin® was introduced to achieve a better selectivity. The complete method consisting of sample digestion, filtration, and the combination of these two resins showed a good palladium recovery at 85%. The optimized procedure was applied on real radioactive waste (effluents and sludges). The specificity and efficiency of this new separation were compared with those obtained with the current reference procedure used in our laboratory. As a result, the new validated protocol successfully replaced the liquid-liquid extraction in chloroform and separation by anion exchange chromatography implemented in the existing method. This study also includes a
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comparison between two quantification methods by ICP-MS Q: one based on a standard
of natural Pd and the other one based on a Pd-107 standard.

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**Keywords**

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Palladium-107, inductively coupled plasma mass spectrometry simple quadrupole (ICP-MS Q), radiochemical separation, extraction chromatography, radioactive waste, design of experiment, Ni®- and LN®-resins.

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**Introduction**

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Achieving carbon neutrality presents a number of challenges, including tripling the nuclear energy capacity of the world by 2050 compared to 2020, in order to reduce dependence on coal and gas [1]. At the same time, numerous decommissioning and dismantling (D&D) projects are underway or will be launched to replace ageing nuclear facilities [2]. Consequently, managing nuclear waste is essential for the nuclear industry. The radionuclides present in radioactive waste need to be identified and their activity or mass must be quantified in order to manage long-term storage safety, enable their potential recycling and monitor the potential contamination of the environment. However, before their accurate quantification with a suitable detection technique, most of them have to be separated from their interfering elements. Laboratories performing these radiochemical purifications and quantifications are facing difficulties due to the wide variety of sample matrices and the large number of radionuclides to analyze.

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The measurement of palladium (Pd) is of great interest for a number of reasons in the nuclear industry. Firstly, the vitrification process is affected when platinum group metals (PGM) are present in excessive quantities [3, 4]. Secondly, nuclear fuel reprocessing plants are encountering problems with Pd aggregation in pipes during the Plutonium, Uranium, Reduction, EXtraction (PUREX) process [5, 6]. For these reasons, and because of the high price of PGM, several research programs are ongoing to quantify and eventually recover Pd from the nuclear cycle [7–9].
In different application areas such as the automotive industry, organic chemistry, health and environment, where only the total stable Pd concentration is important, several analytical techniques can be used to analyze this element: inductively coupled plasma mass spectrometry (ICP-MS), ICP with Optical Emission Spectroscopy (ICP-OES), spectrophotometry and graphite furnace Atomic Absorption Spectrometry (AAS)... [10, 11]. In the nuclear field, quantifying total Pd is interesting but more importantly, determination of Pd-107 is required for the long-term management of radioactive waste. Indeed, Pd has six stable isotopes and 36 known radioisotopes, including Pd-107. This radionuclide is a fission product with a half-life of 6.5 million years decaying to stable Ag-107 [12]. Besides, it is a pure beta emitter resulting mainly from the decay of Pu-239 and U-238 in low yields. As a beta emitter with a long period, Pd-107 can be quantified either by mass spectrometry (ICP-MS, AMS, TIMS...) or by liquid scintillation counting (LSC). However, for long-lived radionuclides with low abundance and low beta energy, such as Pd-107, MS is more sensitive than LSC and is often preferred [13, 14]. Thus ICP-MS Q is a promising detection technique to determine Pd-107.

The majority of articles dealing with the separation of Pd in the automotive industry are mainly focused on the separation of the PGM from each other [15–17]. In general, different separations were developed in the 50’s such as Pd precipitation with oximes, iodine, cyanides, thiols, diols and all their respective derivatives [18–22]. Studies published after the 50’s mostly involve liquid-liquid extractions (LLE) with different extracting agents, often not commercially available and requiring organic synthesis, which is not practical for regular analysis [23–28]. In addition, the organic solvents required to perform LLE are usually harmful to both operators and environment. Furthermore, the organic effluents generated during LLE are costly for laboratories involved in the radioactive waste characterization due to the difficulty of evacuating and treating them. Nevertheless, this method is still in widespread use, even three-phase extractions with ionic liquids have been reported [29, 30]. From the 1960s, separations based on ion exchange resins have been developed. Pd can be highly fixed on anion exchange resins using hydrofluoric, nitric or dilute hydrochloric acid [31–35]. However, its strong retention on anion exchange resins makes its elution complicated and incompatible with the subsequent measurement step, for example with ICP-MS. On the other hand, cation exchange resins are more commonly used
to separate cations of the sample matrix (such as alkali or alkali-earth metals) from Pd as this latter is not retained, particularly in the presence of chlorides [36]. In general, ion exchange resins are not selective enough to purify Pd completely. They are often combined with other separation methods [37–39]. More recently, extraction chromatographic resins have been implemented. They combine the specificity of the extracting molecules of a LLE with the ease of use of liquid chromatography. Ni-resin belongs to this category. It is based on dimethylglyoxime (DMG) well known for its affinity towards Ni and was originally developed to extract it from solutions at pH 8-9. Furthermore, the precipitation or extraction of Pd by this oxime is well documented [19, 40–42]. Several articles, such as Vogl et al. reported the implementation of Ni-resin for the purification of Pd with a good retention [39]. A year later, Dulanská et al. optimised an efficient separation protocol for Pd-107 on this resin while working on two types of radioactive waste [37]. As Pd-107 was analyzed by LSC, the authors were mainly concerned with the decontamination of beta emitters. 100% of Pd was recovered for a simulated solution when using a Ni-resin only, whereas recovery was about 70 to 89% for a radioactive evaporator concentrate and sludge after performing a precipitation, a Ni- and cation exchange resins. Weller et al. published another Pd-107 separation performed on simulated and pond sediment samples from Chernobyl [38]. Pd was measured by triple quadrupole ICP-MS after cation exchange and a Ni- resin to separate it from Y, Zr and some Ag. In addition, the collision cell was used with a mixture of propane and He gases to remove residual Ag. The recovery of Pd in the radioactive sample was not reported. However, it was estimated to be around 62-70% for simulated samples loaded on Ni-resin only and 47-53% after both resins. The concentration of Pd-107 could not be determined due to the lack of standard containing Pd-107. Recently, the extraction chromatographic LN-resin based on alkylphosphorus extractants, was investigated to recover Ag-111 from a Pd target [43]. Often used to separate rare earths and actinides, it can retain Ag in dilute HCl contrary to Pd. This option is interesting in the case of Pd-107 analysis by ICP-MS for which stable Ag-107 is an isobaric interference of Pd-107 [43]. Other studies were conducted on Pd-107 separations but analytical techniques and matrices were different from the one investigated in our work [42, 44–46].

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The aim of this study is to develop an alternative procedure to the current reference method (LLE and anion exchange resin) of our laboratory to comply with the European Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation. Based on the previously cited ref. [37, 38], Ni-resin was selected as a suitable option for the purification of Pd-107 in radioactive waste, compared to precipitation, LLE or ion exchange resins. Precipitation was not selected for potential lack of efficiency. As Pd is present in small quantities in the studied samples, precipitation will not occur easily and it will be very complicated to recover a precipitate of such a low mass. The addition of stable Pd in excess to induce precipitation was rejected to avoid Pd-107 dilution or saturation of the ICP-MS detector. Moreover, elements present in higher concentrations might be carried out with Pd precipitate and degrade the precipitation selectivity [47]. LLE was dismissed due to the implementation of organic solvents, which are generally not compatible with REACH regulation and may expose the operator to additional radiations.

In this article, the quantification of Pd-107 was performed by ICP-MS Q over LSC to obtain a better sensitivity. The collision cell available with our ICP-MS instrument was not used to preserve sensitivity. The contributions of potential interferents to the signals measured by ICP-MS Q at m/z 105, 107 and 108 during the analyses were evaluated to determine the selectivity required for the separation. A design of experiment (DoE) methodology was used to provide in-depth knowledge of the parameters influencing the radiochemical separation of Pd on Ni-resin by performing a minimum of experiments [48]. The development phase on the Ni-resin was performed on simulated non-radioactive samples containing significant stable interfering elements. The extraction chromatographic LN-resin was combined with the Ni-resin to eliminate residual traces of the interfering elements from Pd. The new separation was applied on radioactive sludges and effluents. By testing different radioactive samples with complex matrices, this work allows the robustness of the newly developed separation to be assessed. In addition, the recovery yields of Pd and remaining interfering elements after the new separation were compared with those obtained with the existing procedure. In contrast to the previous published studies, Pd-107 concentrations were determined by ICP-MS Q with and without a standard solution containing this radionuclide and the approaches will be compared.
142 **Experimental**

143 **1. Reagents and setup**

144 The chemicals used in all the experiments were of analytical grade. 69% HNO₃, 40% HF
145 37% HCl and 28% NH₄OH were purchased from J.T.Baker, Chemlab and VWR
146 Chemicals. Oxalic acid (OA) and 50% NaOH were bought from Merck. DMG and citric
147 acid (CA) were marketed by Acros organics, while chloroform came from Honeywell. 70%
148 Hydrogen peroxide and absolute ethanol were supplied by Fisher chemicals. Ascorbic acid
149 (AA) was purchased from Sigma-Aldrich. The water used for the experiments was of ultra-
150 pure quality from a purification system PureLab system (Véolia, France).

151 The development of the new separation method and the preparation of calibration curves
152 (CC) were performed on a mixture called QC28. It is a certified ICP standard containing
153 28 elements at 100 mg/L each (Ag, Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg,
154 Mn, Mo, Na, Ni, Pb, Sb, Se, Sr, Si, Ti, Tl, V, Zn). Zr, Y and Pd were added to this mix
155 using 3 different single element certified ICP standards at 1000 mg/L. To calculate the
156 recovery of Pd after separation from radioactive samples, a Pd tracer enriched to 93.8% in
157 Pd-105 (Eurisotop) was used: the metallic power was digested and diluted to a solution of
158 approximately 10 mg/L in 5M HCl for spiking.

159 The sample digestion step was performed in a SpeedWave DAK-100 microwave oven from
160 Berghof (Germany). Mechanical stirring during LLE was carried out in a Turbula T2F
161 three-dimensional stirrer from WAB (Switzerland).

162 Ni-resins® (100-150µm) and LN-resins® (50-100µm), marketed by Triskem, were
163 purchased in 2 mL cartridges for vacuum assisted flow. A vacuum box sold by Triskem
164 was implemented to perform the separation faster. AG1-X8 anion exchange resin (100-200
165 mesh) in HCl form was marketed by Bio-Rad. Polyethylene filters with a porosity of 20
166 µm from Triskem were implemented for filtration step.

167
Concentrations were measured using either a Spectro Ametek's SpectroBlue ICP-AES or an Agilent's 7900 ICP-MS Q, depending on the desired sensitivity. The ICP-AES was implemented in the development phase to determine the behavior of Pd and the different elements on Ni- and LN-resins and to optimize the elution steps, while the ICP-MS was used to determine lower concentrations of Pd and interfering elements. To simplify reading, ICP-MS Q is denoted as ICP-MS hereafter. CCs were established by measuring 6 standards prepared from dilution of certified ICP standards (SPEX Certiprep). A quality control (QC28 or Pd QC) and a blank (2% HNO₃) were analyzed every 10 samples during ICP concentration measurements. In this article, uncertainties are always given with a coverage factor k of 2. If samples were not digested, uncertainties for the measured concentrations were defined as 20%. However, with sample digestion, uncertainties were set to 30% for measurements greater than or equal to LOQ (Limit of Quantification). All the uncertainty values and LOQs were determined by interlaboratory results and a validation process in agreement with the ISO 11352 standard [49]. The LOQ for the following elements was 0.1 µg/L for undiluted samples analyzed by ICP-MS: Ag, Cd, Cu, Mo, Pd, Sr, Y, Zn and Zr. The LOQ for the same elements in undiluted samples analyzed by ICP-AES was 0.01 mg/L.

NemrodW software was used to establish a DoE using a screening-factor design to determine the factors influencing the separation method [48].

## 2. Studied samples

The current and the new separation method were tested and compared on three different samples. Some samples were spiked with Pd-107 standard or Pd-105 tracer. All details are given in Table 1.

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194 **Table 1:** Samples characteristics and solutions used to compare the current separation and quantification method for Pd-107 with the newly developed one.
195

<table><thead><tr><th>Sample types</th><th>QC28</th><th>Effluent</th><th>Sludge</th><th>Pd solution enriched in Pd-107 isotope</th></tr></thead><tbody><tr><td>Denoted in this study</td><td>Simulated sample</td><td>Sample 1</td><td>Sample 2</td><td>Pd-107 standard</td></tr><tr><td>Origin</td><td>Certified ICP solutions</td><td>Pool for nuclear waste</td><td>Evaporator concentrate located in a facility dedicated to the treatment of radioactive effluents [50]</td><td>Purified solution originated from radwaste [51]</td></tr><tr><td>Type of matrix</td><td>Simple: 2-5% HNO<sub>3</sub></td><td>Simple: effluent with a low content of metals and no suspended matters</td><td>Complex: suspended matters & diverse composition in metals and radionuclides</td><td>Simple: 2% HCl</td></tr><tr><td>Radioactivity</td><td>Non radioactive</td><td>Low</td><td>High γ & β ≈ 50 000 Bq/g α ≈ 1 500 Bq/g</td><td>Low</td></tr><tr><td>Composition</td><td>See part above for ICP standards</td><td>→Pd, Ag, Cd, Mo, Y, Tl, Zr &lt; LOQ of 0.1 µg/L → K<sup>+</sup>, Na<sup>+</sup> ≈ 0.6 mg/L → Si<sup>4+</sup>, Ca<sup>2+</sup> ≈ 0.06 mg/L</td><td>→Sludge contains: Pd, Ag, Cd, Cu, Mo, Sr, Zn, Tl, Zr →Na<sup>+</sup> ≈ 50 g/L → K<sup>+</sup> & Ca<sup>2+</sup> ≈ 3 g/L →NO<sub>3</sub><sup>-</sup> ≈ 100 g/L →Cl<sup>-</sup> ≈ 4 g/L →PO<sub>4</sub><sup>3-</sup> ≈ 2 g/L →B & Fe ≈ 1 g/L</td><td>See its isotopic composition and total Pd concentration determined by Shmeit et al. [51]</td></tr><tr><td>Quantity for the separation</td><td>→5 µg or 0.5 mg of Pd →0.5 mg of 29 other elements</td><td>10 g (no digestion needed)</td><td>2 g</td><td>/</td></tr><tr><td>Eventual spike</td><td>/</td><td>→Low spike: 0.27 µg of Pd (including 0.04 µg of Pd-107) →High spike: 2 µg of Pd-105 tracer + 0.27 µg of Pd (including 0.04 µg of Pd-107)</td><td>→No spike to determine the isotopy: sludge already contains Pd-107 → Spike to calculate separation yield: 2 µg of Pd-105 tracer</td><td>/</td></tr></tbody></table>

196

197 **3. Assessing Pd quantification interferences in ICP-MS Q**

198 The signal for m/z 105, 108 and 107 is required to quantify Pd-107 by ICP-MS for both current and developed methods. Therefore, assessing the possible interferences such as
200 oxides and recombinations created during the ICP-MS measurement for these m/z is of
201 high interest.

202 The Table S1 lists the possible interferences associated with the elements present in
203 Samples 1 and 2 [52]. Ag and Cd produce isobaric interferences for m/z 107 and 108
204 respectively. They bias the Pd analysis in proportion to their concentration. Therefore, their
205 contribution to the interference does not need to be assessed in this experiment. Single
206 solutions of the following elements were prepared in 2% HNO₃ at 0.1; 0.5; 1; 10; 50 µg/L
207 concentrations: Cu, Mo, Sr, Y, Zn and Zr. For Cu and Zn, 100 µg/L concentration was also
208 measured. The latter point was added because the concentrations of Cu and Zn can reach
209 100µg/L even after separation. Complementary experiments were done for Zn in 0.005M
210 HCl. The signal for m/z 105, 107 and 108 was recorded and compared to the blank signal.
211 The signal created by the interferences at the 3 studied m/z was considered significant when
212 higher than the signal of the LOD (Limit of Detection) knowing that the latter is defined
213 as follows in Counts Per Second (CPS): mean of the blanks (in CPS) + 3 times the standard
214 deviation of the blank signals.

215 **4. Pd-107 quantification methods on ICP-MS**

216 Two methods of Pd-107 quantification denoted as Methods A and B are available in our
217 laboratory. During all analysis on ICP-MS, the following m/z were acquired: 102, 104,
218 105, 106, 107, 108, 109 and 110.

219 4.1 Method A: Calibration curve averaging the slopes from Pd-106/108 using a
220 stable Pd standard

221 The CC was prepared between 0 and 100 µg/L with a certified ICP standard of naturally
222 isotopic Pd. The slope of the Pd-107 CC was calculated as the mean of the slope values of
223 the CCs obtained for the isotopes 106 and 108. It is assumed that the mass bias is not
224 significant and thus the sensitivities are the same for close m/z. The y-intercept of the CC
225 for Pd-107 is assumed to be zero.

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227 4.2 Method B: Calibration curve with a Pd-107 standard

228 A CC was prepared with a solution enriched in Pd-107 containing the following Pd isotopes
229 (see Table 1): 102, 104, 105, 106, 107, 108, 110 [51]. With this Pd-107 standard, it must
230 be noted that the m/z=102 isotope was not taken into account to determine Pd recovery
231 yield because of its very low abundance. Table S2 shows the concentration of the five
232 standard concentrations used for the CC of isotopes 105, 107, 108 for ICP-MS analysis.

233 4.3 Calculation of separation yields and Pd-107 concentrations

234 A solution in the same medium as the one obtained at the end of the separation, considered
235 as a blank, was analyzed to determine the signal corresponding to the LOD as defined in
236 paragraph 3. The concentrations of Pd-105 and Pd-108 in the tracer solution were measured
237 after each separation, at the same time as the purified fractions. Both purified fractions
238 obtained after separation with and without tracer were analyzed undiluted. Besides, the
239 purified fraction obtained after separation with tracer was also diluted 15 times before
240 analysis for m/z 105 determination.

241 For radioactive samples, Pd recovery yield was calculated from Pd-105. It corresponds to
242 the ratio between the mass of Pd-105 measured in the purified fraction obtained after the
243 separation with tracer and the sum of the mass of Pd-105 added from the tracer and the
244 initial amount of Pd-105 in the sample. The latest was calculated from the isotopic ratio
245 105/108 in the non-spiked fraction, the amounts of Pd-105 and 108 added from the tracer
246 and the final amounts measured after separation in the spiked fraction.

247 Once the yield of separation calculated, the signals (in CPS) at m/z 107 and 109 were
248 examined. Ag has only two stable isotopes at m/z 107 and 109 with similar abundances. If
249 a purified solution has a signal at m/z 109 that is lower than the signal corresponding to the
250 LOQ of 0.1 µg/L, then it confirms that Ag was successfully separated from Pd.
251 Nevertheless, if the signal at m/z 109 was significantly higher than the signal corresponding
252 to the LOD, it was subtracted from the signal at m/z 107 to determine Pd-107 concentration
253 accurately. This latter calculation assumed that the isotope composition of Ag in the sample
254 is analogous to that observed in nature. Additionally, it was verified that the decay of Pd-
255 107 is slow enough to not contribute significantly to the isotopic composition of Ag in the sample [51].

257 Subsequently, the signal corresponding to Pd-107 in the purified solutions collected after separation was converted in concentration through the CCs obtained from Methods A or B. Finally, the quantity of Pd-107 in ng/g originally present in the samples was calculated, taking into account the separation yield. The uncertainties of 30% related to the Pd-107 quantities were calculated by combining the uncertainties associated with the masses of digested samples, the standards and the ICP-MS measurements. The overall expanded uncertainties are given with a coverage factor k of 2.

264 4.4 Comparison of quantification methods A and B

265 The validity of Method A implemented currently in our laboratory was compared to Method B. Moreover, the impact of the method of separation on the quantification step was evaluated. Fig. S1 in the supplementary describes the strategy implemented to perform the comparison and validation of both methods.

269 First, slopes obtained for Pd-107 with both methods were compared by using a relative difference calculation defined below with equation (1).

$$271 \quad Relative \ difference \ (%) = 100\left(\frac{slope_{Pd(106\to108)} - slope_{Pd107}}{slope_{Pd107}}\right) \quad (1)$$

272 The two methods of quantification also needed to be compared by calculating Pd-107 concentrations in standards used for CC. Therefore, Pd-107 concentrations of the 5 standards used for the CC of Method B (Table S2) were determined using Method A. The calculation of the relative difference is defined below with equation (2).

$$276 \quad Relative \ difference \ (%) = 100\left(\frac{[Pd107]_a - [Pd107]_b}{[Pd107]_b}\right) \quad (2)$$

277 [Pd-107]a is the Pd-107 concentration measured by ICP-MS and calculated from CC with Method A, in the Pd-107 standard solution.
[Pd-107]~b~ is the theoretical Pd-107 concentration spiked and obtained from Pd-107 standard solution [51].

In addition, radioactive samples purified with the current and the newly developed protocols were analyzed. The Pd-107 concentrations were deduced from ICP-MS measurements using both Methods A and B. The normalized error (En) as defined in Ref. [50] was used to make comparisons. Additional details can be found in the supplementary below Fig. S1.

**5. Reference method on simulated and radioactive samples**

A separation and quantification method for Pd-107 was optimized in our laboratory several years ago. It is based on two main separation steps including LLE with DMG and anion exchange chromatography, which are detailed in the synopsis given in Fig. 1. This procedure generates a large volume of effluents (around 500 mL per sample) and requires the use of HF and chloroform, which are not optimal towards security and REACH regulations.

To characterize the selectivity of our current separation method, preliminary experiments were carried out on simulated samples without sample digestion with only the LLE or the resin step. First, they were performed with 0.5 mg of Pd as a single element. The same procedure was then applied to a simulated sample containing 0.5 mg Pd and all other elements present in the QC28, Y and Zr standards. The above quantities of elements were used to start the LLE and the same quantities were independently loaded onto the AG1-X8 resin. In parallel, two purified fractions obtained from the LLE were loaded on the AG1-X8 resin after spiking with 0.5 mg Ag, Y, Zn and Zr. In these experiments, the concentrations were measured by ICP-AES.

In addition, our current separation was tested on different matrices (see Table 1) to compare it with the newly developed one. This purification procedure was associated to the quantification method denoted as Method A to measure the Pd-107 concentrations in real samples: the overall method is considered as a reference in the present work. Details of the
306 overall procedure and the different types of matrices on which it was tested are given in
307 Fig. 1.

<figure><img src="image_2.png" /></figure>

308

309 **Fig. 1** Synopsis of the current Pd-107 method combining LLE and anion exchange resin
310 applied to different matrices. This method is designated as the reference method.

311 **6. New developed method based on Ni-resins and LN-resins**

312 The development of the new method based on the combination of both Ni- and LN-resins
313 was performed in 7 main steps detailed below. The first 5 steps are related to the
314 optimization of the Ni-resin separation on simulated samples, while step 6 corresponds to
315 the optimization on the LN-resin. Step 7 describes the validation of the new separation
316 method on different types of samples. The details of the experimental procedure are shown
317 in Fig. S2 in the supplementary. All the experiments based on extraction chromatographic
318 resins were performed in a vacuum box.

319 6.1 Optimization of the new method on simulated samples

320 • Step 1: Reproduction of separation developed by Dulanská et al.

321 The Ni-resin method developed by Dulanská et al. was reproduced with only 1 mg of Pd
322 [37] and an elution step in 8M HNO₃.

323 • Step 2: Addition of interfering elements to the separation method

324 The separation of Ref. [37] was tested on the Ni-resin with 69% HNO₃ as elution conditions
325 and simulated samples (Pd + 30 other stable elements).

326 • Step 3: Design of Experiment (DoE)

327 To improve the separation from step 2, a DoE methodology was applied. A screening
328 design was implemented to determine the influencing factors according to a Hadamard matrix
329 [53]. The latter was chosen because it is easy to implement when many factors are studied,
330 but it should be noted that it does not take into account interactions between factors. 7
331 factors associated to 2 different levels were selected and two sets of 8 experiments were
332 carried out to assess the effect of each factor on 5 different responses where the second set
333 is a replicate of the first one. The 7 factors corresponding to 7 chemical conditions were
334 either the addition of AA, CA, OA and HNO₃ in a second washing step (denoted as washing
335 2) or the addition of HCl, OA or the change in concentration of HNO₃ for the elution
336 condition (denoted as elution 1). The levels of each factor were defined by the presence or
337 absence of these additives or concentrations in washing 2 or elution steps. The responses
338 studied with NemrodW software were the recoveries of Pd, Ag, Zr, Tl measured by ICP-
339 MS in the elution fraction after separation on Ni-resin. Ag and Zr were studied because of
340 the potential interferences (cf. Table S1) they can generate at m/z 107 during ICP-MS
341 analysis. On the other hand, Tl was also monitored because it was retained on Ni-resin in
342 the same way as Pd and can hinder Pd retention due to its presence in radioactive sludges.
Moreover, if an LSC measurement is considered for Pd-107 analysis, Tl has to be eliminated due to its gamma emitters.

### Step 4: Complementary experiments for the Ni-resin on simulated samples

In parallel, several additional tests were performed in which the first elution conditions were set to 15 mL of 6M $HNO_3$ with 0.025M OA. Based on the results of the screening design and complementary tests, a filtration step was added before loading the sample on Ni-resin to recover the precipitate of AgCl. Moreover, the second washing was chosen to be 10 mL of 0.025M AA in water in the following experiments. A resin re-equilibration step had to be added after this washing to facilitate Pd elution [54]. Several re-equilibration conditions were tested: 10 mL of either 0.5M, 1M, 2M, 3M or 4M HCl. The quantitative elution of Pd can be performed with either 69% $HNO_3$ or 6M $HNO_3$ + 0.025M OA.

### Step 5: Enforcement of separation method with the Ni-resin

The following steps were applied to simulated samples and sample 2 (see Table 2) on the Ni-resin: sample digestion in microwave oven, conditioning, loading and washing in 0.5M HCl, washing in 0.025M AA, re-equilibration in 3M HCl and finally elution in 69% $HNO_3$ was applied.

### Step 6: Reproduction of Tosato et al. separation on LN-resin with interfering elements

As shown by Tosato et al., Ag and Pd can be separated on LN-resin in 0.005M HCl [43]. Unlike Ni-resin where Pd was retained by forming a complex with DMG, Pd pass through on LN-resin. The separation protocol developed by Tosato et al. was tested with 0.5 mg of Pd and the 30 other stable elements from simulated samples.
368 6.2 Validation of the developed method on simulated and radioactive samples

369 • Step 7: Validation of the new separation method

370 The final procedure consisting of digestion, filtration, separation on Ni- and LN-resins and 371 quantification by ICP-MS was performed on different samples (see Table 1) as shown in 372 Fig. 2.

373

<figure><img src="image_3.png" /></figure>

374

<caption>375 **Fig. 2** New Pd separation method combining Ni- and LN- resins on different matrices.</caption>

376 It was applied four times to simulated samples, two times without any element as blank of 377 separation to assess the background signal. In the latter case, both new and already used 378 but cleaned micro-wave reactors for samples digestion were tested to determine if new 379 reactors were necessary or not to achieve accurate Pd-107 analysis. Moreover, it was tested
on two aliquots of the radioactive effluents (Sample 1) and sludges (Sample 2) according to the spiking conditions described in Table 1. The main difference between the simulated samples and the radioactive ones was the evaporation step. Especially when the sample matrix was complex, evaporations must be stopped with more than 0.5 mL remaining in the beaker to avoid loss of Pd. In this case, the samples were solubilized and evaporated two to three times with either 3 mL of 0.5M or 0.005M HCl depending on the separation step.

## 387 Results and discussion

### 388 1. Assessing Pd quantification interferences in ICP-MS

The current quantification of Pd-107 denoted as method A requires the measurement of the isotope ratio 105/108 in the non spiked sample, as well as the determination of the concentrations of Pd-105, 107 and 108 in the sample spiked with Pd-105 tracer. Simplification of the quantification process could be considered by spiking the samples with the standard of Pd-107. However, this standard is scarce and was hard to obtain. Therefore, its use for spiking all samples was not selected as a viable option to preserve it at long term.

A list of the potential interferences at m/z 105, 107 and 108 was compiled in Table S1. The elements Ag, Cu, Sr, Y, Zn, Cd, Zr and Mo were identified due to their direct isobaric interferences, oxides or recombinations with Ar [52] and their presence in the studied radioactive waste (see Table 1). Ga and Ge, which can form recombinations with Ar, were excluded from the study. Indeed, samples analyzed in our laboratory did not contain these elements. Single solutions of concentrations between 0.1 and 50 µg/L or 100 µg/L of each of the aforementioned elements were analyzed by ICP-MS. The aim of these experiments was to determine their potential contribution on the signal at m/z 105, 107 and 108 and thus to deduce the decontamination factors required for an accurate measurement of Pd-107 on Sample 2, the most complex sample to be characterized. The results are presented in Table 2.
**Table 2** Expected interferences at m/z 105, 107 and 108, along with the threshold concentrations above which these interferences are detected. The highest concentration measured is 100 µg/L for Cu and Zn, and 50 µg/L for the other elements. The symbol „/” denotes that no interferences were tabulated in litterature for this m/z with this element

<table><thead><tr><th rowspan="2"></th><th colspan="2">m/z 105</th><th colspan="2">m/z 107</th><th colspan="2">m/z 108</th></tr><tr><th>Interference threshold (µg/L)</th><th>Expected interference</th><th>Interference threshold (µg/L)</th><th>Expected interference</th><th>Interference threshold (µg/L)</th><th>Expected interference</th></tr></thead><tbody><tr><td>Ag</td><td>/</td><td></td><td>x > 0</td><td><sup>107</sup>Ag</td><td>/</td><td></td></tr><tr><td>Cd</td><td>/</td><td></td><td>/</td><td></td><td>x > 0</td><td><sup>108</sup>Cd</td></tr><tr><td>Cu</td><td>&lt;LOD</td><td><sup>40</sup>Ar<sup>65</sup>Cu<sup>+</sup></td><td>/</td><td></td><td>/</td><td></td></tr><tr><td>Mo</td><td>/</td><td></td><td>/</td><td></td><td>1 &lt; x &lt; 10</td><td><sup>92</sup>Mo<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Sr</td><td>1 &lt; x &lt; 10</td><td><sup>88</sup>Sr<sup>16</sup>O<sup>1</sup>H<sup>+</sup>;<br/><sup>88</sup>Sr<sup>17</sup>O<sup>+</sup>;<br/><sup>87</sup>Sr<sup>18</sup>O<sup>+</sup></td><td>/</td><td></td><td>/</td><td></td></tr><tr><td>Y</td><td>0 &lt; x &lt; 0.1</td><td><sup>89</sup>Y<sup>16</sup>O<sup>+</sup></td><td>10 &lt; x &lt; 50</td><td><sup>89</sup>Y<sup>18</sup>O<sup>+</sup></td><td>/</td><td></td></tr><tr><td>Zn</td><td>&lt;LOD</td><td><sup>68</sup>Zn<sup>37</sup>Cl<sup>+</sup>;<br/><sup>70</sup>Zn<sup>35</sup>Cl</td><td>&lt;LOD</td><td><sup>40</sup>Ar<sup>67</sup>Zn<sup>+</sup></td><td>&lt;LOD</td><td><sup>40</sup>Ar<sup>68</sup>Zn<sup>+</sup></td></tr><tr><td>Zr</td><td>/</td><td></td><td>0.5 &lt; x &lt; 1</td><td><sup>90</sup>Zr<sup>16</sup>O<sup>1</sup>H<sup>+</sup>;<br/><sup>90</sup>Zr<sup>17</sup>O<sup>+</sup>;<br/><sup>91</sup>Zr<sup>16</sup>O<sup>+</sup></td><td>x > 0.1</td><td><sup>92</sup>Zr<sup>16</sup>O<sup>+</sup></td></tr></tbody></table>

It is imperative that Ag and Cd, which create direct isobaric interferences for the m/z of interest in this study, are fully eliminated by the separation process to obtain unbiased signals respectively at m/z 107 and 108. The presence of Mo and Sr started to contribute to the Pd m/z signal when their concentrations were between 1 and 10 µg/L, while the impact of Y and Zr was observed at concentrations as low as 0.1 µg/L. It is therefore essential that the separation is also able to remove Y and Zr effectively. Our experiments showed that some residues of Mo and Sr still present after separation do not impact accurate Pd quantification. Regarding Cu and Zn, which are commonly found in our samples, this study demonstrated that up to 100 µg/L, they did not contribute to the m/z signal monitored here. Consequently, the effectiveness of the separation in decontaminating both elements can be lower without affecting the accuracy of Pd-107 quantification. In conclusion, the main interfering elements at m/z 107 were identified as Ag, Y and Zr.
423

## 2. Quantification of Pd-107 by ICP-MS: comparison of methods A and B

424

425

426

427

428

Two CCs were prepared for Pd and analyzed by ICP-MS on the same day using identical ICP parameters. The first CC was prepared according to method A (natural isotope standard devoid of Pd-107), whereas the second was prepared based on method B (Pd-107 standard solution, characterized by Shmeit et al. [51]). This experiment was repeated over two days to assess the repeatability of the results.

429

430

431

Firstly, the mean of Pd-106/108 slope values (corresponding to Method A) was compared with the slope value obtained with the Pd-107 standard (corresponding to Method B). The graphs showing these CCs are presented in Fig. 3, S3 and S4.

<figure><img src="image_4.png" /></figure>

432

433

434

<caption>**Fig. 3** Calibration curves of Pd-107 obtained from Methods A and B, measured by ICP-MS on day 2 of the experiment.</caption>

435

436

437

438

439

440

The relative differences between the CC slopes of Pd-107 determined for Methods A and B over the three days of experiments were less than 3%. To assess further the validity of quantification methods, the expected Pd-107 concentrations of the five solutions used to establish the CC related to Method B were compared with the Pd-107 concentrations of these same solutions calculated using Method A. The relative differences in Pd-107 concentrations for the five standards are presented in Table S3. They are all less than 5%,
indicating that no significant difference is observed and thus, both quantification methods were consistent. These findings reinforced the validity of the quantification method employed by our laboratory so far. Consequently, Method A is valid to quantify Pd-107 by ICP-MS. It is possible to establish a Pd-107 calibration curve without a Pd-107 standard by using Pd-106 and Pd-108, albeit with a certain degree of uncertainty, which is acceptable for the characterization of Pd-107 in nuclear waste from D&D. Following this result, method A is used in the rest of the article to determine Pd-107 concentrations.

To complete these results, samples 1 and 2 were separated using the old and new separation methods (see Figs. 1 and 2). Their Pd-107 concentrations were then determined using method A or B and compared with the En values calculated (see Fig. S1) and presented in Tables S4 and S5. All the En were below 1, regardless of the sample, separation or quantification method employed to calculate Pd-107 concentrations. The two quantification methods gave comparable results in ng/g, within uncertainties of 30%. It can therefore be concluded that both quantification methods are validated on radioactive effluent and sludge matrices that have undergone a separation procedure.

### 3. Separation with the reference method

#### 3.1 Preliminary experiments on simulated samples with the reference method

The selectivity of the current reference method was first investigated so that comparisons could be made with the new developed one. LLE and resin were performed independently without sample digestion. Fig. 4 presents recovery yields after the LLE step and Fig. 5 after the AG1-X8 resin for simulated samples containing 0.5 mg of Pd, QC28, Y and Zr. For the results presented in this part 3.1, no sample digestion was performed.
<figure><img src="image_5.png" /><figcaption>**Fig. 4** Distribution in % of the elements from simulated samples during the different separation steps of the LLE with DMG. “OE” meaning “Other Elements” includes: Al, As, B, Ba, Be, Ca, Co, Cr, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Se, Ti and V. Concentrations were determined by ICP-AES.</figcaption></figure>

The LLE, conducted with Pd only, resulted in a recovery yield of around 84%. It is likely that some Pd was lost during the evaporation step or remained in the chloroform organic phase throughout the four re-extractions. Indeed, Pd was not quantified in the starting aqueous phase nor in the wash fraction and it was not possible to analyze the organic phase composed of chloroform to confirm this assumption.

Conversely, as illustrated in Fig. 4, the loss of Pd during LLE was around 25% when other elements, including 0.5 mg of QC28, Y and Zr were introduced. The slight decrease in Pd yield can be attributed to two factors: firstly, the unextracted Pd in the aqueous starting phase, which was estimated to be 3-4%, and secondly, the formation of a third milky white phase during LLE, which could trap Pd as observed in Ref. [55]. For the different types of simulated samples, the recovery in Ag varied from 50 to 100% in the residual aqueous phase of the extraction. These scattered results originated from the chemical medium of standards used for the LLE experiments. All the standards were added in a beaker, evaporated to dryness and solubilized in 4M HNO₃ as if a sample digestion had been
performed. However, the Pd standard used was manufactured in 2% HCl, which caused Ag precipitation when the QC28 was mixed with Pd. In fact, depending on Ag and HCl concentrations, a AgCl precipitation can occur and hinder the distribution of Ag between LLE phases [56]. To better monitor the behavior of Ag, a LLE was carried out with Ag only, which showed that 85% remained in the initial aqueous phase and was not extracted with Pd.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Distribution in % of the elements from simulated samples during the different separation steps of the AG1-X8 anion exchange resin. “OE” meaning “Other Elements” includes: Al, As, B, Ba, Be, Ca, Co, Cr, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Se, Ti and V. Concentrations were determined by ICP-AES.</figcaption></figure>

The anion exchange resin initially conditioned in HNO₃ medium was chosen to achieve a good selectivity towards Pd from the literature [57]. As shown by Macnevin et al. and Aardaneh et al. [31, 35], ammonia is highly efficient to elute Pd. Indeed, when loaded with Pd alone, the AG1-X8 anion exchange resin resulted in a yield of around 90% for Pd. Approximately 8% of Pd was lost during the loading process and the initial washing with HNO₃ on the resin. Similar results were obtained for Pd when stable elements were introduced with it. From ICP-AES analyses, only Tl is observed in the final fraction: a part of Tl (around 10%) was co-eluted with Pd. This result is in agreement with the distribution coefficients of Pd and Tl on anion exchange resin [57]. The fractions collected during the
separation process were found to be devoid of Ag. The absence of this element can be
attributed to the same phenomenon observed in the LLE process: it precipitated during the
mixing of the standards due to the presence of HCl in the Pd standard. To determine Ag
behavior, fractions collected following the LLE procedure were spiked with Ag, Y and Zr
prior their loading onto AG1-X8 resin. The initial loading and HNO₃ washing resulted in
the removal of 90 to 95% of Ag. On the other hand, Y exhibited a similar behavior, as
illustrated in Fig. 5. However, Zr behaved differently: only 50% of Zr was removed during
the first step and the remaining Zr was eluted with water whereas 95% was removed in the
first fraction when LLE was not conducted beforehand. This suggests a higher retention of
Zr on the resin when loading a solution obtained after LLE. This behavior could be due to
traces of organic molecules potentially present in the LLE fraction (chloroform, DMG) that
could modify its retention. Combining yields after LLE and AG1-X8 resin gave a 67%
value of recovery for Pd.

3.2 Reference method applied on different samples

The entire procedure described in Fig. 1 from part 5 in the Experimental was applied to
simulated samples, radioactive samples 1 and 2 (see Table 1). It included sample digestion,
LLE and AG1-X8 resin. Results obtained by ICP-MS are shown in Table 3.

519

520

521

522

523

524

525
**Table 3** Recoveries in % and concentrations in µg/L of chemical elements that could affect the Pd-107 quantification. Values were obtained after a sample digestion in HNO₃/HF and a separation by LLE and AG1-X8. “/” indicates that the sample does not contain the element.

<table><thead><tr><th rowspan="2"></th><th rowspan="2"></th><th colspan="9">Chemical elements</th></tr><tr><th>Pd</th><th>Ag</th><th>Cd</th><th>Cu</th><th>Mo</th><th>Sr</th><th>Y</th><th>Zn</th><th>Zr</th></tr></thead><tbody><tr><td>Simulated samples</td><td>Recovery (%) [remaining concentration] (µg/L)</td><td>45</td><td>&lt;0.01</td><td>&lt;0.01</td><td>0.004 [0.8]</td><td>&lt;0.01</td><td>&lt;0.01</td><td>&lt;0.01</td><td>0.01 [1.5]</td><td>&lt;0.01</td></tr><tr><td>Sample 1</td><td>Recovery (%) [remaining concentration] (µg/L)</td><td>Low Pd spike: 32 High Pd spike: 77</td><td>/</td><td>/</td><td>6 [0.5]</td><td>/</td><td>10 [0.11]</td><td>/</td><td>14 [1.5]</td><td>/</td></tr><tr><td>Sample 2</td><td>Recovery (%) [remaining concentration] (µg/L)</td><td>47</td><td>&lt;0.04</td><td>&lt;0.23</td><td>0.003 [0.4]</td><td>0.01 [0.1]</td><td>0.04 [0.1]</td><td>/</td><td>0.05 [0.4]</td><td>&lt;0.03</td></tr></tbody></table>

The sample digestion step with HF resulted in a Pd recovery yield of 45 %, which is below the 67 % yield obtained in 3.1 without digestion for simulated samples. The presence of HF probably hindered slightly the extraction of Pd during the LLE. It must be noted that HF was introduced in this protocol to complex Fe in order to avoid its co-extraction of Pd by DMG during LLE [58].

The reference method enabled to achieve comparable recoveries for Pd when the separation was conducted on both simulated sample and spiked Sample 2. For sample 1 spiked with a low or a high quantity of Pd, Pd yields were calculated for all isotopes and were consistent with each other, except for Pd-102, which is too low in abundance to be accurately
quantified. The recovery of Pd is 2.4 time higher when the separation was performed with the high spiked sample 1 (2.27 µg of Pd) than with the low spiked sample 1 (0.27 µg). The low quantity of Pd might be more difficult to extract quantitatively. This emphasizes the challenges associated with implementing this method on samples with low Pd quantity. The current separation method is efficient in decontaminating Ag, Cd, Y and Zr, which can create interferences during ICP-MS measurements and impede the quantification of Pd-107. Notwithstanding the complexity of Sample 2, none of the aforementioned four elements were quantified. However, the signal of the m/z corresponding to Ag, Cd and Zr was slightly above the LOD, indicating the presence of residual traces. To achieve accurate quantification of the Pd-107, it was necessary to retrieve the Ag-109 signal to the Pd-107 signal. The concentrations of Cu and Zn were quantified in all samples at concentrations lower than 50 µg/L. As demonstrated in Part 1 of the Results, it can be deduced that these elements do not affect the quantification of Pd-107. Conversely, both Samples 1 and 2 contained Sr at a concentration of 0.1 µg/L while only the purified Sample 2 exhibited quantifiable levels of Mo at the same concentration level. In part 1 of the Results, these concentrations were demonstrated to be insufficient to bias the quantification of Pd-107. As a conclusion, the current method is highly selective to obtain accurate measurements of Pd-107 in radioactive waste, including complex samples such as sludges. It can therefore be used as a reference for comparing with the new developed separation.

## 4. Separation with the new developed method

### 4.1 Optimization of the Ni- and LN-resins separation on simulated samples

As described in the part 6 from the Experimental, the development of the new method based on the combination of both Ni- and LN-resins was performed in 7 main steps. The different experimental conditions tested are detailed in Fig. S2. The results obtained in these 7 steps are given below.

According to the work of Weller et al. and Dulanská et al., Ni-resin was selected as a suitable option to replace the LLE step performed in the current separation method [37, 38]. In both cases, DMG is the extracting molecule impregnated on Ni-resin. This molecule is known to have a high stability constant (log K = 34.1 at 25°C) with Pd [59]. The stability
constants of the interfering elements with DMG are not tabulated in the literature, which makes impossible to foresee their behavior. Consequently, it is necessary to carry out experiments to determine the selectivity of Ni-resin towards them. Moreover, it has to be reminded that Dulanská et al. [37] evaluated the behavior of interfering elements for LSC analysis and not for ICP-MS analysis.

573 • Step 1: Reproduction of the separation developed by Dulanská et al..

The Ni-resin protocol from Dulanská et al. was applied: 83% of Pd was recovered when working with simulated samples, which is coherent with Ref. [37]. However, the high elution volume used, 40 mL of 8M HNO₃, induced lengthy separations, which was considered not to be optimal for our laboratory. So, it was decided to reduce it without reducing Pd recovery. Therefore, an elution step with 10 mL of 69% HNO₃ was subsequently implemented.

580 • Step 2: Addition of interfering elements to the separation method.

To complete the study of Dulanská et al. [37] dealing with few chemical elements, 27 others were loaded in significant amount (0.5 mg) with Pd on Ni-resin (see Fig. 6). Like Weller et al. [38], the selectivity of the resin was investigated with a low amount of Pd (5 µg) because the detection was performed by ICP-MS. The analysis with this instrument is incompatible with the addition of 0.5 mg Pd carrier as Dulanská et al. [37] did when using LSC. Indeed, the introduction of such quantity would lead to the analysis of a Pd concentration at around 25 mg/L, which is not possible for ICP-MS without a high dilution factor and consequently, the measurement of Pd-107 would be degraded finally.
<figure><img src="image_7.png" /><figcaption>**Fig. 6** Distribution in % of the elements from simulated samples during the different separation steps of the method with Ni-resin. “OE” meaning “Other Elements” includes: Al, As, B, Ba, Be, Ca, Co, Cr, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Se, Ti and V. Concentrations were determined respectively by ICP-AES for the loading and washing steps and by ICP-MS for the elution step.</figcaption></figure>

It can be observed that most of the studied elements were not retained on the Ni-resin under the loading and washing conditions. As for the reference method, it has been seen that Ag precipitated with HCl before its loading on the resin. As the precipitation was not totally quantitative, an average of 12% of Ag was measured in the loading & washing fractions. We can assume that the precipitated AgCl was also loaded onto the resin, adhered to the upper frit and was partially solubilized when concentrated HNO₃ was used for elution. Therefore, between 2-12% of Ag was recovered in the elution fraction.

In an attempt to use AgCl precipitation as a separation step, precipitation tests were conducted with only 100 µg or 600 µg of Ag in 0.5M HCl. The results demonstrated that 20 µg of Ag remained in solution regardless of the initial mass of Ag introduced. For precipitation to occur, the product of the reactant concentrations must be higher than the solubility constant of AgCl (pKs = 9.7 at 25°C) [56]. However, when the Cl⁻ concentration is significant, the chemical equilibrium is shifted, resulting in the formation of the soluble AgClₓ¹⁻ₓ species. In our case, the precipitation of AgCl is not quantitative and is therefore difficult to predict, as it depends on the respective concentrations of Ag⁺ and Cl⁻ in our samples. Consequently, this option was not chosen as a way to eliminate Ag.
On the other hand, Tl and Pd showed a strong affinity for the Ni-resin and behaved similarly with a 95% recovery yield. This result highlights the efficiency of the new elution step in a smaller volume but more concentrated $HNO_3$. Weller et al. reported that Tl was not retained on the Ni-resin when working in 0.01M HCl medium. It was decided not to change HCl concentration so as to favor Ag precipitation and facilitate its removal (this point will be discussed later). Moreover, 0.3% of residual Zr, equivalent to 1.5 µg/L, was determined by ICP-MS analysis of the elution fraction. This result is in agreement with Weller et al. [38] who addressed the problem of residual Zr in the purified fraction of Pd by adding a cation exchange resin upstream of the Ni-resin. From our preliminary experiments, it can be concluded that the Ni-resin is promising to isolate Pd but the separation procedure has to be improved to obtain better decontamination factors, in particular for Ag and Zr.

### Step 3: Design of Experiment (DoE)

A screening design from DoE methodology was implemented to improve the selectivity of the Ni-resin separation and to reduce the concentration of $HNO_3$ in the elution without increasing its volume [53]. The goal was to test numerous washing or elution conditions while limiting the number of experiments to be carried out. The screening design was used to determine whether the introduction of a washing step denoted as washing 2 containing OA, AA, CA or 1M $HNO_3$ had an impact on the recovery yields of Pd, Tl, Y, Zr, and Ag in the elution fraction. This screening design also aims at evaluating the impact of the addition of OA or 1M HCl in the elution step as well as the importance of the $HNO_3$ concentration during this same step. The studied complexing agents were selected due to their usual implementation in extraction chromatography. It can be noted that Tl was also included in the study to prevent its co-elution with Pd and thus enable the application of the developed method for Pd-107 analysis by LSC.

The experiments and results of the screening design are given in Table S6 and the data were processed with NemrodW software. Y was never quantified (<0.015%), while Zr recoveries were determined to be between 0.02 and 0.12%. No significant changes of recoveries were observed in relation to the different experimental conditions tested, highlighting their lack of effect on the removal of residual traces of Zr from the elution
fractions. From NewrodW processing, it can be inferred that AA is a complexing agent suitable to eliminate Tl from Ni-resin. So, a washing with 0.025M AA quantitatively eliminated Tl (>95%) without eluting Pd. This result is consistent with previous works of the literature. AA is known as a reducing agent towards metals and has already been used to reduce Tl [60, 61]. It can be observed that OA and CA have no significant impact on any element in a washing step. As expected, Pd elution was more efficient with high $HNO_3$ concentrations, but the combination of 1M HCl with $HNO_3$ also favored its elution. On the other hand, adding HCl to the elution solution increased the amount of Ag measured in this fraction, probably by contributing to the dissolution of the AgCl precipitate deposited on the upper frit of the Ni-resin.

From NewrodW processing, it can be deduced that the addition of OA to the elution step did not help in recovering Pd from Ni-resin. However, this was contradictory in appearance with the results obtained in preliminary experiments performed to support the definition of the screening design. Indeed, elution with 6M $HNO_3$ + 0.025M OA resulted in 90% Pd recovery. Thus, it appears that there are interactions between factors, which are not taken into account in the type of screening design applied. Indeed, OA probably only has an effect on Pd elution when the resin is equilibrated in HCl, or when the $HNO_3$ concentration for elution is within a certain range to be defined (>4M and <12M). If the $HNO_3$ concentration is too low, $Pd(DMG)_2$ complex does not dissociate, and if the $HNO_3$ concentration is too high, the OA loses its usefulness, perhaps because the pH is too low. A DoE with an optimization step could provide more information and define the effective range of OA as an elution additive, but this was not the approach considered here since satisfactory conditions can be found from the present experiments.

### Step 4: Complementary experiments for the Ni-resin on simulated samples

Based on the results of the screening design and the support experiments, a filtration step on 20 µm polyethylene filters was added to recover AgCl precipitate and thus prevent its loading on Ni-resin. This additional step also enables to avoid the dissolution of AgCl precipitate during washing and elution steps. In those conditions, the part of Ag, which is not precipitated is eliminated in the loading and washing 1 steps.
Moreover, a washing step with AA was used to remove Tl. Two options are available to elute Pd: either 5-10 mL of 69% HNO₃ or 15 mL of 6M HNO₃ + 0.025M OA. When the whole separation method was tested, although 95-100% of Tl was removed from the elution fraction, the Pd yields were disappointingly low whatever the elution conditions chosen. Elution with concentrated HNO₃ enabled to achieve a 50% recovery, while HNO₃ and OA yielded from 20 to 36% of Pd. The introduction of washing 2 with 0.025M AA inhibited Pd elution. Therefore, the resin had to be re-equilibrated with HCl prior to Pd recovery in agreement with works of literature [54].

Before adding this step to the separation, shorter options were investigated. It consisted of the addition of the AA in the loading solution. However, in the presence of AA, other elements precipitated prior loading and dragged Pd, significantly reducing its recovery yield [62]. On the other hand, AA was diluted with 0.5M HCl instead of water, but Tl was no longer removed from the resin. As neither of these tests worked, a re-equilibration step had to be added.

To re-equilibrate the Ni-resin after the washing with AA, HCl was chosen in comparison to HNO₃ (Ref. [37]). Several HCl concentrations were tested depending on the elution medium. The results are given in Table S7.

The experiments performed confirmed the importance to re-equilibrating the Ni-resin after implementing AA medium: for both elution conditions, it was possible to recover Pd with yields higher than 90% with a re-equilibrating step. From this experiment, there are two options available to elute Pd quantitatively. If the elution was carried out with 6M HNO₃ + 0.025M OA, the re-equilibration step was set to 10 mL of 0.5M HCl, whereas if the elution was performed with 69% HNO₃, the re-equilibration step was imposed at 10 mL of 3M HCl.

It was decided to elute Pd with 5-10 mL of 69% HNO₃ instead of the 15 mL of 6M HNO₃ + 0.025M OA so as to limit the use of complexing agents, which are monitored by the French national radioactive waste management agency (ANDRA) and thus simplify the current waste management of our laboratory [63]. Therefore, and in accordance with the results in Table S7, the chosen re-equilibration step was performed with 3M HCl for an expected recovery yield around 100% for Pd.
701

702 • Step 5: Enforcement of separation method with the Ni-resin

703 The new separation method was applied to simulated samples and Sample 2. It includes
704 sample digestion in 69% HNO₃, filtration, separation on Ni-resin and then quantification.

705 The recoveries obtained for the interfering elements and their associated concentrations in
706 the purified fractions are detailed in Table S8. Several conclusions can be drawn from this
707 experiment:

708 ➔ First, the filtration step was important to prevent resin clogging by removing the
709 AgCl precipitate and the silicates contained in Sample 2, which were not fully
710 dissolved due to the absence of HF during the sample digestion.

711 ➔ Second, interfering elements that bias Pd-107 quantification were still present in
712 the purified Pd fraction following a separation on Ni-resin for both simulated
713 samples and Sample 2 (see Table S8). With the exception of Cu and Y, the
714 remaining concentrations of the other elements studied were found to exceed the
715 limits from which interferences are generated at m/z 105, 107 and 108, as detailed
716 in Table 2. In particular, for Sample 2, Ag, Cd and Zr were quantified at
717 concentrations, which can bias Pd-107 quantification.

718 ➔ Third, compared to the simulated samples, a decrease in decontamination was
719 observed for Cd, Sr and Zr in sample 2. It may be attributed to the matrix effect of
720 Sample 2 because this sample has a concentration of NaNO₃ at around 2M. Indeed,
721 it is well known that high concentrations of salts can reduce the retention capacity
722 of extraction chromatographic resins [64].

723 In conclusion, some elements that could cause interferences at m/z 105, 107 and 108 were
724 quantified in the simulated samples and the Sample 2 after a separation on one single Ni-
725 resin. Consequently, an additional purification step was required to further minimize the
726 quantity of these elements and thus to achieve better decontamination factors for an
727 accurate Pd-107 quantification.
A combination of two successive separations on Ni-resins was tested on both simulated samples and Sample 2. The implementation of a second Ni-resin did not improve sufficiently the decontamination factors, in particular for Zr, which was still quantified at concentrations higher than 0.1 µg/L. Furthermore, the recovery yield of Pd was found to be 85% in simulated samples digested with HNO₃ after the first Ni-resin, whereas it decreased to 62% after the second Ni-resin, which is considered as too penalizing by our laboratory. Besides, the combination of two Ni-resins implies the use of twice the amount of reactants and time, which is detrimental to routine analyses. In conclusion, the option of two consecutive Ni-resins was eliminated for further study. Nevertheless, in the case of samples that differ in composition, this approach can be convenient.

738

739 • Step 6: Reproduction of Tosato et al. separation on LN-resin with interfering elements
740

741 A second chromatographic resin was selected for its capacity not to fix Pd while retaining
742 interfering elements such as Ag. From the work of Tosato et al. [43], it was decided to
743 investigate the efficiency of LN-resin for our issue. It must be noted that Tosato et al. [43]
744 only studied the behavior of Ag and Pd on this resin. The selectivity of this resin can be
745 partly foreseen from Ref. [65] but all the elements of interest are not reported. Prior
746 including the LN-resin in the developed method, a mixture of Pd and 29 other elements
747 was loaded onto it under the same conditions as those described by Tosato et al. [43], i.e.
748 in 0.005M HCl medium. This experiment was undertaken to identify whether other
749 chemical elements that interfere with the Pd-107 measurements were also removed in
750 addition to Ag.
<figure><img src="image_8.png" /><figcaption>**Fig. 7** Distribution in % of the elements from simulated samples during the loading step on the LN-resin in 0.005M HCl. “OE1” meaning “Other Elements of group 1” includes: Be, Fe and Ti. “OE2” meaning “Other Elements of group 2” includes: Al, B, Ba, Co, K, Li, Ni, Mg and Na. The concentrations were determined by ICP-AES.</figcaption></figure>

The results displayed in Fig. 7 were conclusive: almost all Pd was not fixed on the LN-resin (recovery yield of 92%) while Ag, Mo, Tl, Zn, Y and Zr were retained while using 0.005M HCl. Additionally, Cu, Cd and Sr were observed to co-elute with Pd. The results are in accordance with the yields reported by Tosato et al. [43] for their simulated samples. They achieved in recovering 99.96% of Pd in the loading fraction and eluted Ag with 1M HCl, resulting in a recovery of 95%. These preliminary tests suggest that the LN-resin may be an effective method for completing the removal of elements, such as Zr, that interfere with Pd-107 quantification while maintaining a high level of Pd recovery.

## 4.2 Validation of the new developed method on simulated and radioactive samples

* Step 7: Validation of the new separation method

It was decided to combine the Ni-resin with the LN-resin in this specific order. Indeed, the LN-resin requires a very dilute media in HCl and our samples usually contain high concentrations of Cl⁻ ions. If the LN-resin was implemented first, it is required to evaporate and solubilize the samples in 0.005M HCl following their digestion but this step will be
complicated to implement in such diluted acid. Sample 2 matrix would stick to the beaker, which may cause Pd losses. Therefore, to better control the composition of the loading medium on the LN-resin, an evaporation and change of media following the Ni-resin was performed. The separation protocol including sample digestion in HNO₃, filtration, Ni- and LN-resins before quantification by ICP-MS was applied to blanks, simulated samples, Samples 1 and 2 (see Table 1 and Fig. 2). Results are detailed in Table 4.

**Table 4** Recoveries and concentrations of chemical elements that could affect the Pd-107 quantification with ICP-MS. Values were obtained after a sample digestion in HNO₃ and a separation on one Ni-resin and LN-resin. "/" indicates that the sample does not contain the chemical element.

<table><thead><tr><th rowspan="2"></th><th rowspan="2"></th><th colspan="9">Chemical elements</th></tr><tr><th>Pd</th><th>Ag</th><th>Cd</th><th>Cu</th><th>Mo</th><th>Sr</th><th>Y</th><th>Zn</th><th>Zr</th></tr></thead><tbody><tr><td rowspan="2">Simulated samples</td><td rowspan="2">Recovery (%)<br/>[remaining<br/>concentration]<br/>(µg/L)</td><td>90</td><td>0.05<br/>[10]</td><td>&lt;0.02</td><td>0.06<br/>[16]</td><td>&lt;0.02</td><td>&lt;0.02</td><td>&lt;0.02</td><td>0.3<br/>[77]</td><td>&lt;0.02</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Sample 1</td><td rowspan="2">Recovery (%)<br/>[remaining<br/>concentration]<br/>(µg/L)</td><td>Low<br/>Pd<br/>spike:<br/>60</td><td>/</td><td>/</td><td>45<br/>[6]</td><td>/</td><td>35<br/>[0.45]</td><td>/</td><td>166<br/>[30]</td><td>/</td></tr><tr><td>High<br/>Pd<br/>spike:<br/>74</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sample 2</td><td>Recovery (%)<br/>[remaining<br/>concentration]<br/>(µg/L)</td><td>43</td><td>&lt;0.004</td><td>&lt;0.23</td><td>0.08<br/>[12]</td><td>0.006<br/>[0.1]</td><td>0.03<br/>[0.3]</td><td>/</td><td>0.53<br/>[75]</td><td>&lt;0.003</td></tr></tbody></table>
For the blanks: they were digested in HNO₃ in either a new reactor or one that had been used several times for radioactive sludges, alternating cleaning procedures. This step helps to assess the potential contamination caused by the retention of elements in previously used reactors. Following separation on Ni- and LN-resins, it was found that 5 µg/L of Cu, 0.2 µg/L of Sr, 30 µg/L of Zn and 0.1 µg/L of Zr and Mo were present in the blanks, irrespective of the initial digestion reactor selected. The other elements were below the LOQ. On the other hand, Y signal was at the LOD level while the Ag signal was observed to be three times higher than the LOD. Indeed, Ag can precipitate when using HNO₃/HCl for digestion step and adsorb on the walls of the digestion reactor. Despite the washing procedure between each sample, the Ag decontamination might be not complete. Consequently, it was decided to use new reactors for further analyses to minimize Ag contaminations. From the results obtained on blanks, it can be deduced that quantifying Pd-107 below 0.03 µg/L will be challenging. Indeed, the signal at m/z 109 was always between the LOD and the LOQ. However, this signal can be subtracted from the one of m/z 107 for isotope correction. It should be noted that, this approach is valid only when the signal at m/z 107 is significantly higher than the LOD.

For other sample types (see Table 1): According to Table 4, it can be highlighted that the residual concentrations of Cu, Sr, Zn and Mo that could affect the measurement of Pd-107 were below the thresholds given in Table 2: the obtained decontamination factors were sufficient for real samples. The remaining concentrations did not generate a signal at masses 105, 107 and 108 during the quantification step by ICP-MS. Consequently, it can be deduced that Pd-107 can be successfully quantified in Samples 1 and 2. With this new separation, Tl was co-eluted with Pd in simulated samples and Sample 2 with respective recovery yields of 0.44% and 3.35%, while they were below the LOQs of 0.01% and 0.7% with the reference method. On the other hand, Ag concentration is problematic for the simulated samples, thereby preventing the quantification of Pd-107. A poor Ag retention on the LN-resin was observed in the present study in agreement with Ref. [43]. In some tests carried out after the Ni-resin, all performed with 0.005M HCl, the Ag was not retained and co-eluted with Pd similarly to Tosato et al. who also noticed the co-elution for low concentrations of HCl (<0.01M). The cause of this phenomenon remains unknown. However, the real samples received in our laboratory exhibited an average Ag
concentration that was 50 times lower than the one of the simulated samples, which makes the present developed method applicable to complex samples such as sludges studied in our laboratory. It is thus crucial to validate the separation method on these matrices rather than on the simulated samples so as to achieve the decontamination factors adapted to the needs.

Furthermore, it is somewhat unexpected that Pd was occasionally retained on the LN-resin. In separations involving the combination of digestion, filtration and separation with Ni- and LN-resins on the simulated samples, the Ni-resin enabled a Pd recovery between 85% and 100%, and with the LN-resin, an average reduction of the total yield by 10% was noticed. This observation is consistent with the findings presented in Section 4.1. Conversely, the loss of Pd on the LN-resin for Samples 1 and 2 was found to be between 30 and 50% respectively, while the Pd yield on the Ni-resin remained similar, between 85 and 100%. This result was not in agreement with the results presented for the simulated samples described in Section 4.1 where almost no Pd was retained at all on LN-resin. Several potential explanations can be assumed for this phenomenon. First, it is possible that radioactive samples contain other compounds, which are co-eluted with Pd after the Ni-resin and which may change the speciation of Pd and thus favor its binding to the LN-resin. Alternatively, the radioactivity of the samples may trigger the elution of Pd from the Ni-resin as a complexed form with one potential DMG degradation product, rather than in the form of free Pd, and the chemical form created would then be retained on the LN-resin. Last, the retention of Pd can be enhanced when the element is present in a trace quantity.

## 5. Comparison between the current and the new methods for the Pd-107 separation

Based on Tables 3 and 4, comparisons between methods (see Fig. 1 and 2) were performed on different samples (see Table 1). Unlike the reference method, matrix effects appear to play an important role in Pd recovery yield with the new separation method. Indeed, the latter demonstrated a higher yield of Pd for the simulated samples in comparison to Sample 1 and Sample 2. With the new method, simulated samples exhibited Pd yields that were twice as high as those obtained with the current method, while the yields of Pd for Sample
2 were comparable for both methods. For Sample 1 with a high spike, the yield was comparable to the one obtained with the reference method. Conversely, Sample 1 spiked with a lower quantity of Pd had a yield that was nearly two times higher with the new method. This highlights the better performance of the developed method even though low quantities of Pd are present. As with the reference method, Pd yields calculated for all isotopes in Sample 1 were consistent with each other.

The final validation of the new separation method in radioactive matrices was conducted by comparing the quantity of Pd-107 measured after separation using both the new and current protocols. Quantification was carried out using Method A according to the calculation described in the Experimental part 4.3.

For Sample 1: The measured quantity of Pd-107 was compared with those calculated theoretically following the addition of the spike of Pd-107 standard. The theoretical and measured values for all spiked samples were found to be between 1.60 to 1.76 ng/g in Pd-107 with both separation methods (see Table S4). Measured Pd-107 quantities were within the 30% confidence interval around the theoretical Pd-107 values and their En (as defined in Section 4.4 of Experimental) were found to be below 1 (see Table S4). Consequently, both separation methods lead to comparable Pd-107 quantities, thereby substantiating the accuracy of the novel separation method for radioactive effluents.

For Sample 2: The measured quantity of Pd-107 after separation using the reference method was 3.24 (± 0.97) ng/g, whereas using the new method, it was measured at 3.62 (± 1.09) ng/g. The two Pd-107 quantities overlapped within the uncertainties of 30% regardless of the separation method applied. In addition, the En between the two values was calculated at 0.26, which means that no significant difference with the new procedure is observed. Therefore, the new separation method can also be validated on radioactive complex sludges.
868 **Conclusions**

869 A separation method to purify and quantify Pd-107 accurately in radioactive effluents and
870 sludges was successfully developed. The method included the digestion of the sample with
871 HNO₃, followed by filtration and separation on Ni-resin, and then LN-resin and a final
872 measurement by ICP-MS Q. This new separation method is an improvement of the current
873 reference one, which combines sample digestion with HF, LLE in chloroform, and anion
874 exchange resin. The developed procedure prevents the implementation of HF, a highly
875 corrosive and harmful acid, and chloroform, a Carcinogenic, Mutagenic and Reprotoxic
876 chemicals (CMR) organic solvent. The separation is one day faster and uses five times less
877 solvent than the current one, thereby facilitating the current management of waste and
878 respecting more the environment and the operators. Concurrently, the contribution of
879 specific elements to the signal measured for Pd using ICP-MS was evaluated in the present
880 work. The elements with the greatest impact on the measurement of Pd at m/z 105, 107
881 and 108 were determined as Ag, Cd, Y, Zr, Sr and Mo. Decontamination factors obtained
882 with the new procedure were sufficient to determine Pd-107 accurately for complex
883 samples analyzed in our laboratory, such as sludges. Besides, two quantification methods
884 were employed to determine Pd-107. The reference quantification method is based on using
885 the average of the slopes of the calibration curves obtained for Pd-106 and Pd-108 as the
886 slope for Pd-107. The second method requires the implementation of a standard solution
887 enriched in Pd-107. Both quantification methods were consistent on both radioactive
888 effluent and sludge samples. This result is of prime interest for laboratories that do not have
889 access to a Pd-107 standard. The quantification of Pd-107 can be performed with ICP-MS
890 by using a standard of natural Pd with an uncertainty of a few percent, which is suitable for
891 the characterization of nuclear waste in the field of D&D. This approach can be applied for
892 other long-lived radionuclides for which standards are not commercially available, such as
893 Zr-93 or Sn-126.

894

895
896 Acknowledgements

897 This work was funded by the LA2D/LABOS project for which the CEA LASE is grateful.

898 The authors would like to thank Isabelle Billard for her guidance and expertise, Jacques

899 Bubendorff, Thomas Grangeon and Yann Kergadallan for their contribution to the
900 radiological measurements and their sound advice. We would also like to thank Kimberly

901 Colas for her valuable proofreading and corrections in English, as well as the members of

902 the LASE team for their invaluable support in daily tasks.

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1094 64. Horwitz EP, Chiarizia R, Dietz ML, et al (1993) Separation and preconcentration of actinides from acidic media by extraction chromatography. Anal Chim Acta 281:361-372. https://doi.org/10.1016/0003-2670(93)85194-O

1097 65. Braun T, Ghersini G (1975) Extraction Chromatography, ISBN-10: 0444998780, ISBN-13: 9780080858029. Elsevier Scientific Pub.

1099

1100
**Supplementary information**

1101

1102

<table><caption>**Table S1 Interferences of measurements for Pd when using ICP-MS Q.**</caption><thead><tr><td>Isotopes</td><td>Interferences for Pd in ICP-MS</td></tr></thead><tbody><tr><td>Pd-102</td><td><sup>204</sup>Pb<sup>2+</sup> ; <sup>86</sup>Sr<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Pd-104</td><td><sup>40</sup>Ar<sup>64</sup>Zn<sup>+</sup> ; <sup>208</sup>Pb<sup>2+</sup> ; <sup>88</sup>Sr<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Pd-105</td><td><sup>40</sup>Ar<sup>65</sup>Cu<sup>+</sup> ; <sup>88</sup>Sr<sup>16</sup>O<sup>1</sup>H<sup>+</sup> ; <sup>88</sup>Sr<sup>17</sup>O<sup>+</sup> ; <sup>87</sup>Sr<sup>18</sup>O<sup>+</sup> ; <sup>89</sup>Y<sup>16</sup>O<sup>+</sup> ; <sup>68</sup>Zn<sup>37</sup>Cl<sup>+</sup> ; <sup>70</sup>Zn<sup>35</sup>Cl<sup>+</sup></td></tr><tr><td>Pd-106</td><td><sup>40</sup>Ar<sup>66</sup>Zn<sup>+</sup> ; <sup>106</sup>Cd<sup>+</sup> ; <sup>88</sup>Sr<sup>18</sup>O<sup>+</sup> ; <sup>90</sup>Sr<sup>16</sup>O<sup>+</sup> ; <sup>89</sup>Y<sup>16</sup>O<sup>1</sup>H<sup>+</sup> ; <sup>89</sup>Y<sup>17</sup>O<sup>+</sup> ; <sup>90</sup>Zr<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Pd-107</td><td><sup>107</sup>Ag<sup>+</sup> ; <sup>40</sup>Ar<sup>67</sup>Zn<sup>+</sup> ; <sup>91</sup>Y<sup>16</sup>O<sup>+</sup> ; <sup>90</sup>Zr<sup>16</sup>O<sup>1</sup>H<sup>+</sup> ; <sup>90</sup>Zr<sup>17</sup>O<sup>+</sup> ; <sup>91</sup>Zr<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Pd-108</td><td><sup>40</sup>Ar<sup>68</sup>Zn<sup>+</sup> ; <sup>108</sup>Cd<sup>+</sup> ; <sup>92</sup>Mo<sup>16</sup>O<sup>+</sup> ; <sup>92</sup>Zr<sup>16</sup>O<sup>+</sup></td></tr><tr><td>Pd-110</td><td><sup>40</sup>Ar<sup>70</sup>Zn<sup>+</sup> ; <sup>110</sup>Cd<sup>+</sup> ; <sup>94</sup>Mo<sup>16</sup>O<sup>+</sup> ; <sup>94</sup>Zr<sup>16</sup>O<sup>+</sup></td></tr></tbody></table>

1103

1104

1105

1106
1107 **Table S2** Concentration of Pd isotopes in µg/L for the calibration curve in ICP-MS
1108 prepared with the solution containing Pd-107 for quantification method B (see details of
1109 the solution in Table 1).

<table><thead><tr><th rowspan="2"></th><th colspan="5">Concentrations of Pd in µg/L</th></tr><tr><th>Std1</th><th>Std2</th><th>Std3</th><th>Std4</th><th>Std5</th></tr></thead><tbody><tr><td>Pd-105</td><td>0.06</td><td>0.31</td><td>0.61</td><td>3.06</td><td>6.12</td></tr><tr><td>Pd-107</td><td>0.03</td><td>0.16</td><td>0.32</td><td>1.60</td><td>3.20</td></tr><tr><td>Pd-108</td><td>0.02</td><td>0.11</td><td>0.22</td><td>1.08</td><td>2.15</td></tr></tbody></table>

1110

1111

1112

1113

1114

1115

1116

1117

1118

1119
<figure><img src="image_9.png" /></figure>

1120

1121

**Fig. S1** Experimental details: Strategy to compare and validate separation (current & new) and quantification (A&B) methods

1122

1123

**Additional details from the Experimental part on En calculation on radioactive samples, according to the formula given in Ref. [50] and Fig. S1**

1124

1125

* For sample 1: the En was calculated from the amount of Pd-107 (in ng/g) measured by Methods A or B in the purified sample (uncertainties of 30%) and the theoretical amount of Pd-107 spiked (uncertainties of 0.7%) [51]. The measurements were repeated on two different days.

1126

1127

1128

1129

* For Sample 2, no certified Pd-107 value was available, so the Pd-107 quantities (in ng/g) obtained from different separation methods have to be compared. The En was calculated between the quantity of Pd-107 (in ng/g) obtained with the current and new developed separations and both calculated by Method A. This En helps to compare the separation method with regards to the determination of final Pd-107 quantity in the sample.

1130

1131

1132

1133

1134
<figure><img src="image_10.png" /></figure>

1135

1136

<caption>**Fig. S2** Main steps and detailed methods followed for the development of the new separation method with Ni- and LN-resins</caption>

1137
1138 **Complementary details** on the new separation method applied to Sample 2 and simulated
1139 samples.

1140 The separation method was carried out on sample 2 and four stable mixtures containing 5
1141 µg Pd and 0.5 mg of the elements from QC28, Y and Zr standards. Samples were digested
1142 in a microwave oven with 69% HNO<sub>3</sub>. After digestion, the samples (simulated and 2) were
diluted with water and evaporated to 0.5 mL at 200°C before being dissolved in 5 mL of
1143 0.5M HCl. The samples were loaded onto polyethylene filters directly connected to the top
1144 of Ni-resins, which have been previously conditioned with 10 mL of 0.5M HCl. The
1145 beakers were rinsed with 5 mL of 0.5M HCl and the rinse solutions were loaded onto the
1146 filter. After this step, filters were removed and the Ni-resins were washed with 10 mL of
1147 0.5M HCl and 10 mL of 0.025M AA. Prior to eluting Pd with 7 mL of 69% HNO<sub>3</sub>, 10 mL
1148 of 3M HCl was used to facilitate its release from Ni-resin [54]. The eluted solutions were
1149 evaporated to 0.5 mL at 200°C and an aliquot was taken to determine Pd recovery. Samples
1150 were re-solubilized in 0.005M HCl before being loaded onto pre-conditioned LN-resins
1151 with 10 mL of the same medium. The fractions containing the loading and the 10 mL
1152 washing with 0.005 M HCl (final volume 20 mL) were collected and analyzed by ICP-MS.
1153 Separate solutions were prepared to establish CCs, one combining Ag, Cu, Mo, Zn, Zr,
1154 Y, Tl, Sr, Pb and another for Pd only.
1155

1156

1157

1158

1159

1160

1161

1162

1163
<figure><img src="image_11.png" /><figcaption>**Fig. S3** Calibration curves of Pd-107 obtained from quantification Methods A and B, measured by ICP-MS Agilent 7900 on day 1 of the experiment.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. S4** Calibration curves of Pd-107 obtained from quantification Methods A and B, measured by ICP-MS Agilent 7900 on day 3 of the experiment.</figcaption></figure>
1171 **Table S3** Relative difference between Pd-107 concentrations of diluted standards used to establish the calibration curve of Method B and the calculated concentrations of Pd-107 in the same solutions using Method A.

<table><thead><tr><th></th><th>Relative difference day 1 (%)</th><th>Relative difference day 2 (%)</th><th>Relative difference day 3 (%)</th></tr></thead><tbody><tr><td>Std 1</td><td>-3.67</td><td>4.03</td><td>-1.44</td></tr><tr><td>Std 2</td><td>3.65</td><td>-4.13</td><td>0.94</td></tr><tr><td>Std 3</td><td>3.13</td><td>-0.15</td><td>-2.43</td></tr><tr><td>Std 4</td><td>3.48</td><td>1.21</td><td>1.30</td></tr><tr><td>Std 5</td><td>2.95</td><td>2.37</td><td>3.19</td></tr></tbody></table>

1174

1175

1176

1177

1178

1179

1180

1181
1182 **Table S4** Quantity of Pd-107 in ng/g, with uncertainties of 30%, determined for Sample 1 depending on their spike, separation and quantification methods. Measurements were performed on day 1 / day 2. En values were calculated from theoretical values (uncertainties of 0.7%).

<table><thead><tr><th colspan="2">Sample 1: Effluent</th><th rowspan="2">Theoretical<br>(ng/g of Pd-<br>107)</th><th colspan="2">Reference separation<br>method</th><th rowspan="2">Theoretical<br>(ng/g of Pd-<br>107)</th><th colspan="2">Developed separation<br>method</th></tr><tr><th></th><th></th><th>Quantified<br>(ng/g of Pd-<br>107)</th><th>En</th><th>Quantified<br>(ng/g of Pd-<br>107)</th><th>En</th></tr></thead><tbody><tr><td rowspan="2">High<br>Pd<br>spike</td><td>Method<br>A</td><td rowspan="2">1.64</td><td>1.69 / 1.75</td><td>0.10 /<br>0.20</td><td rowspan="2">1.76</td><td>1.80 / 1.81</td><td>0.07 /<br>0.08</td></tr><tr><td>Method<br>B</td><td>1.66 / 1.70</td><td>0.04 /<br>0.12</td><td>1.77 / 1.77</td><td>0.01 /<br>0.01</td></tr><tr><td rowspan="2">Low Pd<br>spike</td><td>Method<br>A</td><td rowspan="2">1.60</td><td>1.60 / 1.61</td><td>-0.004<br>/ 0.01</td><td rowspan="2">1.75</td><td>1.72 / 1.77</td><td>-0.04 /<br>0.05</td></tr><tr><td>Method<br>B</td><td>1.58 / 1.59</td><td>-0.04 /<br>-0.02</td><td>1.70 / 1.73</td><td>-0.10 /<br>-0.02</td></tr></tbody></table>

1186

1187

1188

1189
1190 **Table S5** Quantities of Pd-107 (in ng/g) measured by ICP-MS in Sample 2 using Methods
1191 A or B and the current or the new separation procedure and associated En values. Quantities
1192 are given with uncertainties of 30%.

<table><thead><tr><th rowspan="2">Sample 2: sludge</th><th colspan="2">Reference separation method</th><th colspan="2">Developed separation method</th></tr><tr><th>Quantity of Pd-107 (ng/g)</th><th>En</th><th>Quantity of Pd-107 (ng/g)</th><th>En</th></tr></thead><tbody><tr><td>Method A</td><td>3.46</td><td>Theoretical value</td><td>3.52</td><td>0.04</td></tr><tr><td>Method B</td><td>3.50</td><td>0.03</td><td>3.42</td><td>-0.03</td></tr></tbody></table>

1193 For sample 2: These En was calculated from the quantities of Pd-107 (in ng/g) obtained
1194 using Methods A and B when implementing the same separation method. It allows to
1195 evaluate whether there are differences between the two methods of quantification on
1196 complex matrices already containing Pd and without adding Pd-107 standard.

1197 **Complementary information on method validation with sample 2 using Table S5**

1198 In complement to the measurements detailed in Section 3.4 of Experimental part, the
1199 purified solutions obtained from both separation methods on Sample 2 were re-analyzed
1200 by ICP-MS three months later. Recoveries of Pd decreased by 10% and 30% for
1201 purifications performed with the current and new methods respectively. However, the
1202 results were coherent with both quantification methods. It highlighted the instability of the
1203 separated fractions and enforced the need to consider the separation yields for each
1204 measurement. Table S5 presents the quantities of Pd-107 (in ng/g) measured in Sample 2
1205 using Methods A or B and the current or the new separation method.
1206 Quantities of Pd-107 were close to the one-measured three months earlier. The En values
1207 given in Table S5 were calculated by choosing 3.46 ng/g as a theoretical value because it
1208 corresponds to the method used by our laboratory since Pd-107 analysis was implemented.
1209 All En values were below 1 whatever the separation and quantification method employed
1210 to calculate Pd-107 concentrations. As a conclusion, quantities of Pd-107 in Sample 2 were
1211 comparable, which validated quantification methods A and B as well as the new
1212 purification procedure on purified radioactive sludges.

1213

1214

1215

1216

1217

1218

1219

1220

1221

1222

1223

1224

1225

1226
**Table S6** Experimental conditions for steps washing 2 and elution 1 for the DoE. Concentrations in OA, AA, CA are always set to 0.025M, HCl concentration in elution is set to 1M, HNO<sub>3</sub> concentration when not written is 1M. Results displayed were the average of the two replicated sets of 8 experiments. Y was always measured below the LOQ of 5 µg/L. Concentrations were measured by ICP-MS with uncertainties of 20% and recoveries were calculated by calculating a ratio between the mass recovered after separation and the initial mass of elements loaded on the resin.

<table><thead><tr><th colspan="2">Experiment number</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th></tr></thead><tbody><tr><td rowspan="2">Experimental conditions</td><td>Washing 2</td><td>OA</td><td>HNO<sub>3</sub><br>AA</td><td>HNO<sub>3</sub><br>OA<br>CA</td><td>HNO<sub>3</sub><br>OA<br>AA</td><td>OA<br>AA<br>CA</td><td>AA<br>CA</td><td>HNO<sub>3</sub><br>CA</td><td>/</td></tr><tr><td>Elution 1</td><td>12M<br>HNO<sub>3</sub><br>HCl<br>OA</td><td>4M<br>HNO<sub>3</sub><br>HCl<br>OA</td><td>4M<br>HNO<sub>3</sub><br>OA</td><td>12M<br>HNO<sub>3</sub></td><td>4M<br>HNO<sub>3</sub><br>HCl</td><td>12M<br>HNO<sub>3</sub><br>OA</td><td>12M<br>HNO<sub>3</sub><br>HCl</td><td>4M<br>HNO<sub>3</sub></td></tr><tr><td rowspan="5">Results: elements recoveries in elution 1 (%)</td><td>Pd</td><td>100.0</td><td>53.14</td><td>28.21</td><td>82.14</td><td>51.69</td><td>77.59</td><td>100.0</td><td>22.20</td></tr><tr><td>Ag</td><td>26.49</td><td>32.01</td><td>2.36</td><td>3.89</td><td>31.22</td><td>3.69</td><td>17.38</td><td>3.81</td></tr><tr><td>Zn</td><td>0.14</td><td>0.01</td><td>0.01</td><td>0.17</td><td>0.02</td><td>0.15</td><td>0.13</td><td>0.01</td></tr><tr><td>Zr</td><td>0.12</td><td>0.12</td><td>0.04</td><td>0.02</td><td>0.04</td><td>0.02</td><td>0.05</td><td>0.03</td></tr><tr><td>Tl</td><td>89.37</td><td>4.61</td><td>96.08</td><td>26.84</td><td>0.60</td><td>3.00</td><td>93.38</td><td>2.41</td></tr></tbody></table>
<table><caption>**Table S7** Effects of re-equilibration step on Pd recovery yields in % after separation with the developed method based on Ni-resin.</caption><thead><tr><th rowspan="2"></th><th rowspan="2"></th><th colspan="5">Re-equilibration conditions</th></tr><tr><th>Without</th><th>0.5M HCl</th><th>2M HCl</th><th>3M HCl</th><th>4M HCl</th></tr></thead><tbody><tr><td rowspan="2">Elution conditions</td><td>69% HNO<sub>3</sub></td><td>50%</td><td>88%</td><td>91%</td><td>100%</td><td>97%</td></tr><tr><td>6M HNO<sub>3</sub> + 0.025M OA</td><td>30%</td><td>97%</td><td>Not tested</td><td>Not tested</td><td>76%</td></tr></tbody></table>

<table><caption>**Table S8** Recoveries and concentrations of chemical elements that could affect the Pd-107 quantification. Values were obtained after a sample digestion in HNO₃ and a separation on one Ni-resin. LOQs are defined in Part 2 of Experimental.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Simulated samples</th><th colspan="2">Sample 2</th></tr><tr><th>Recovery (%)</th><th>Remaining concentration (µg/L)</th><th>Recovery (%)</th><th>Remaining concentration (µg/L)</th></tr></thead><tbody><tr><td>Ag</td><td>0.01</td><td>5.3</td><td>0.005</td><td>0.08</td></tr><tr><td>Cd</td><td>0.004</td><td>2.0</td><td>0.03</td><td>0.2</td></tr><tr><td>Cu</td><td>0.07</td><td>38.4</td><td>0.04</td><td>19.2</td></tr><tr><td>Mo</td><td>0.24</td><td>138.1</td><td>0.03</td><td>0.85</td></tr><tr><td>Sr</td><td>0.005</td><td>2.6</td><td>0.02</td><td>0.68</td></tr><tr><td>Y</td><td>&lt;0.02</td><td>&lt;12.5</td><td>&lt;LOQ</td><td>&lt;0.5</td></tr><tr><td>Zn</td><td>0.35</td><td>200.7</td><td>0.43</td><td>174</td></tr><tr><td>Zr</td><td>0.07</td><td>36.5</td><td>0.28</td><td>18.3</td></tr></tbody></table>