Isobaric interference removal for selected radionuclides using nitrous oxide and ammonia with inductively coupled plasma tandem mass spectrometry

Abstract

The determination of long-lived radionuclides by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) is a well-established approach. However, such determinations can still be hindered by isobaric interferences from stable isotopes of neighbouring elements. As such, investigations towards novel gas cell approaches for removing interfering ions are required in order to improve the reliability of the analysis. Nitrous oxide (N₂O) is a reaction gas that has been well studied for stable isotope analysis. Studies towards its applicability to radionuclide analysis have so far been limited. Here, the use of N₂O, as well as a mixture with ammonia (NH₃), have been evaluated for determinations of 10 radionuclides of interest for nuclear decommissioning: ⁴¹Ca, ⁶³Ni, ⁷⁹Se, ⁹⁰Sr, ⁹³Zr, ⁹³Mo, ⁹⁴Nb, ¹⁰⁷Pd, ¹³⁵Cs, and ¹³⁷Cs. Single element solutions of stable isotope analogues of the radionuclides, as well as solutions of the interfering ions, were used to observe the reactions with the ICP-MS/MS reaction cell gases. Abundance-corrected sensitivities were used to assess the achievable separation factors and sensitivities for the determination of the radionuclides of interest. The N₂O/NH₃ gas mixture was found to provide a significant enhancement in the removal of isobaric interferences, as well as instrument detection limits (given in brackets), compared to N₂O alone for determinations of ⁴¹Ca (0.50 pg g⁻¹ (0.0016 Bq g⁻¹)), ⁷⁹Se (0.11 pg g⁻¹ (5.4 × 10⁻⁵ Bq g⁻¹)), ⁹⁰Sr (0.11 pg g⁻¹ (0.56 Bq g⁻¹)), ⁹³Mo (0.12 pg g⁻¹ (0.0044 Bq g⁻¹)), ¹³⁵Cs (0.1 pg g⁻¹ (7.5 × 10⁻⁶ Bq g⁻¹)), and ¹³⁷Cs (0.1 pg g⁻¹ (0.33 Bq g⁻¹)).

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Shaun T. Lancaster

Shaun Lancaster currently holds a permanent position at Montanuniversität Leoben, Austria. He earned his PhD in 2021 in conjunction with the University of Aberdeen, Scotland, and instrument manufacturer P S Analytical, England. His work focused on method development for mercury speciation and subsequent analysis of methylmercury in the livers of Scottish birds of prey, as well as the development of novel atomic fluorescence based instrumentation for continuous monitoring of mercury in effluent wastewater streams. Following this, he began his postdoc at Montanuniversität Leoben working on the development of analytical methodology using ICP-MS/MS and XRF for the analysis of complex electronic waste matrices to assist the recycling industry as part of the drive towards a circular economy. Since taking a permanent position, his future ambition is to utilize ICP-MS/MS reaction cell gas chemistry to unlock new analytical approaches to solve challenging applications. In particular, to remove spectral interferences on difficult-to-measure isotopes for stable isotope ratio and radionuclide determinations, as well as for isotope ratio determinations of oxygen, which have previously never been performed by ICP-MS/MS.

Introduction

Decommissioning of nuclear sites represents a high cost and long-term analytical challenge to return the site to a safe state for future reactor development or alternative use. This necessitates the accurate measurement of a range of radionuclides in complex and varied sample matrices. Of particular interest are the medium and long-lived radionuclides that represent the most significant contribution to the long-term waste inventory that must be safely stored or disposed of. Examples include waste products generated from nuclear fission of ²³⁵U, such as ⁷⁹Se, ⁹⁰Sr, ⁹³Zr, ¹⁰⁷Pd, ¹³⁵Cs, and ¹³⁷Cs, as well as activation products such as ⁴¹Ca, ⁶³Ni, ⁹³Mo, and ⁹⁴Nb formed by neutron capture in the concrete and steel casings of the nuclear reactor.¹ Efficient characterisation of radioactive waste streams is key for safe and cost effective waste management, and requires the use of rapid and robust analytical methodology.

Inductively coupled plasma mass spectrometry (ICP-MS) has been for decades a frequently applied method of choice for the determination of long-lived radionuclides, providing more rapid analysis of samples compared to decay counting instrumentation and thereby contributing to faster and more cost-effective decommissioning.^1,2 Accurate quantitative determinations of long-lived radionuclides by ICP-MS are hindered primarily by isobaric interferences from naturally occurring stable isotopes of other elements, as well as polyatomic ions and peak tailing interferences from neighbouring masses, often requiring time-consuming offline chromatographic separations to resolve. Modern quadrupole-based ICP-MS systems are equipped with a reaction cell that can be used to resolve isobaric interferences based on the differences in reactivity of different elements. Target analytes can be determined using either a mass-shift approach, where the analyte of interest is selectively reacted to form a product ion at a higher mass-to-charge ratio (m/z), or an on-mass approach, where the interfering ion is selectively reacted and the analyte of interest is measured without altering its m/z.³ With the advent of tandem mass spectrometry (MS/MS), only interferences at the m/z of the target analyte are of concern, as a quadrupole mass filter removes all other ions prior to entering the gas cell.⁴

The most commonly utilized cell gases for interference removal are hydrogen (H₂), helium (He), oxygen (O₂), and ammonia (NH₃).⁴ Of these, O₂ and NH₃ are commonly used for the removal of isobaric interferences for radionuclide determinations.^5,6 Both H₂ and He have high ionisation energies and have low reactivity with most elements, and are typically employed as collisional gases, which can only reduce polyatomic interferences (by kinetic energy discrimination). Reactive cell gases can be combined with He to improve the interference separation. The addition of an unreactive gas can enhance sensitivity and reactions through the collisional focussing effect, focussing the ions to the minimum of the effective potential of the quadrupole by reducing the radial kinetic energy of the ion.^7,8 Moreover, the addition of H₂ to NH₃ gas was observed to improve formation of M(NH₃)_n product ions, rather than splitting the sensitivity between product ions with a lower number of hydrogen atoms,⁹ demonstrating that combining two gases can lead to a different product ion formation. Uncovering new and improved interference removal pathways through ICP-MS/MS reaction cell chemistry can improve detection limits, sample throughput, and provide simpler methodology by minimizing the need for complex and time consuming chemical separations. Thus, further investigation into the use of alternative cell gases and cell gas combinations is necessary to expand options for end users.

Nitrous oxide (N₂O) is a reaction gas that has been extensively studied for stable isotope determinations,^10–13 however for radionuclides its use has been limited primarily to the removal of radioactive caesium isotopes from stable barium isotopes enabling ¹³⁵Cs/¹³⁷Cs source attribution, which is a valuable tool for long-term environmental monitoring and used following the Fukushima disaster.^14–16 Additionally, limited studies focus on mixtures of N₂O with other gases, such as He, H₂ and NH₃.^17,18 As such, this study aims to evaluate the use of N₂O, as well as gas mixtures of N₂O with NH₃, as ICP-MS/MS cell gases for the removal of isobaric interferences on 10 radionuclides that are of importance in waste characterisation for nuclear decommissioning: ⁴¹Ca, ⁶³Ni, ⁷⁹Se, ⁹⁰Sr, ⁹³Zr, ⁹³Mo, ⁹⁴Nb, ¹⁰⁷Pd, ¹³⁵Cs, and ¹³⁷Cs. To achieve this, elemental standards containing stable isotopes of the elements of interest were utilized as analogues to assess the reactivity of the elements compared to the reactivity of the isobaric interferences. Sensitivities and interference separation factors for the target radionuclides were calculated using the instrument response of the stable analogues and factoring the isotopic abundances.

Experimental

Reagents

Nitric acid (HNO₃, w = 65%, p.a. grade; Carl Roth GmbH, Karlsruhe, Germany) was purified using a sub-boiling distillation system (Savillex DST-4000, AHF Analysentechnik, Tübingen, Germany). Reagent grade I water (18.2 MΩ cm; MilliQ IQ 7000, Merck-Millipore, Darmstadt, Germany) was used for all acid dilutions. Vials and pipette tips were pre-cleaned by soaking overnight in diluted sub-boiled nitric acid (w = 3%) before use.

Single-element standards of caesium (Cs), copper (Cu), potassium (K), magnesium (Mg), molybdenum (Mo) (β = 1000 μg mL⁻¹; Certipur, Merck); aluminium (Al), barium (Ba), calcium (Ca), iron (Fe), niobium (Nb), palladium (Pd), strontium (Sr), zirconium (Zr) (β = 1000 μg mL⁻¹; Inorganic Ventures, Christiansburg, VA, USA); ytterbium (Y, β = 1000 μg mL⁻¹; High Purity Standards, North Charleston, SC, USA); selenium (Se, β = 1000 μg mL⁻¹; Peak Performance, CPI international, Santa Rosa, CA, USA); nickel (Ni, β = 1000 μg mL⁻¹; Alfa Aesar, Karlsruhe, Germany); and silver (Ag, β = 10 μg mL⁻¹; Inorganic Ventures) were used throughout this work. Potassium bromide (NORMAPUR grade; VWR, Vienna, Austria) salt was used to prepare standards for the analysis of bromine (Br).

Instrumentation

All work was carried out using a NexION 5000 ICP-MS/MS system (PerkinElmer, Waltham, MA, USA) equipped with a quadrupole-based dynamic reaction cell (DRC). The applied bandpass of the reaction cell quadrupole can be modified using rejection parameter “a” (RPa) and rejection parameter “q” (RPq). Instrument parameters for the different measurement modes applied are given in Table 1. Argon (purity 5.0 (≥99.999%); Linde Gas GmbH, Stadl-Paura, Austria) was used as the plasma gas. Nitrous oxide (medicinal grade; Linde Gas GmbH) and ammonia (purity 5.0 (≥99.999%); Linde Gas GmbH) were used as reaction gases.

Table 1 Operational parameters of the ICP-MS/MS system

Parameter	Standard mode	N2O DRC mode	N2O/NH3 DRC mode
Scan mode	MS/MS	MS/MS	MS/MS
Cell gas	None	N2O	N2O and NH3
RPa	0	0	0
RPq	0.25	0.45	0.45
Sample introduction	Self-aspiration	Self-aspiration	Self-aspiration
Nebulizer	PFA MicroFlow	PFA MicroFlow	PFA MicroFlow
Spray chamber	Peltier cooled SilQ cyclonic spray chamber	Peltier cooled SilQ cyclonic spray chamber	Peltier cooled SilQ cyclonic spray chamber
Spray chamber temperature	5 °C	5 °C	5 °C
Interface cones	Nickel	Nickel	Nickel
RF power	1600 W	1600 W	1600 W
Ar nebulizer gas flow	0.92–0.98 L min^−1	0.92–0.98 L min^−1	0.92–0.98 L min^−1
Ar auxiliary gas flow	1.2 L min^−1	1.2 L min^−1	1.2 L min^−1
Ar plasma gas flow	16 L min^−1	16 L min^−1	16 L min^−1
Hyperskimmer park voltage	5 V	5 V	5 V
OmniRing park voltage	−185 V	−185 V	−185 V
Inner target lens voltage	2 V	2 V	2 V
Outer target lens voltage	−7 V	−7 V	−7 V
Deflector exit voltage	−8 V	−8 V	−8 V
Differential aperture voltage	−3.5 V	−3.5 V	−3.5 V
Q1 AC rod offset	−6 V	−10 V	−10 V
Q1 rod offset	−2 V	0 V	0 V
Cell rod offset	−33 V	−5 V	−5 V
Axial field voltage	0 V	250 V	250 V
Cell entrance voltage	−5 V	−8.5 V	−8.5 V
Cell exit voltage	−2 V	−5.5 V	−5.5 V
Q3 AC rod offset	−2.5 V	−7 V	−7 V
Q3 rod offset	−2 V	−10 V	−10 V
Dwell time	50 ms	50 ms	50 ms

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Analytical measurement

Standards were diluted in nitric acid (w = 2%). The isotopes of stable analogues and interferences monitored in this study are listed in Table 2. Initial mass scans were carried out using 0.8 mL min⁻¹ N₂O, as well as a mixture of 0.8 mL min⁻¹ N₂O and 0.4 mL min⁻¹ NH₃, in order to identify product ions that formed for each element monitored. Further investigation was carried out by varying gas flow rates and monitoring selected product ions.

Table 2 List of radionuclides of interest and the isotope of the stable analogue and interferences measured in this study. Half-lives of the radionuclides of interest were sourced from the atomic and nuclear database of the Decay Data Evaluation Project.¹⁹ Abundances for the interferences are given as the isotopic abundance of the interfering element.²⁰ Abundances for polyatomic interferences are given as the product of the isotopic abundances of the elements it contains

Radionuclide	Half-life (years)	Stable analogue monitored	Spectral interference	Interference abundance (%)
^41Ca	100 200 ± 1700	^44Ca	^41K	6.73
			^40Ar^1H	99.59
			^40Ca tailing	96.94
^63Ni	98.7 ± 2.4	^60Ni	^63Cu	69.17
^79Se	356 000 ± 40 000	^82Se	^79Br	50.69
^90Sr	28.80 ± 0.70	^88Sr	^90Zr	51.45
			^89Y^1H	99.97
^93Mo	4000 ± 800	^94Mo	^93Nb	100
			^93Zr	n/a
^93Zr	1 610 000 ± 60 000	^94Zr	^93Nb	100
			^93Mo	n/a
^94Nb	20 040 ± 40	^93Nb	^94Zr	17.38
			^94Mo	9.19
^107Pd	6 500 000 ± 300 000	^105Pd	^107Ag	51.84
^135Cs	1 330 000 ± 190 000	^133Cs	^135Ba	6.59
^137Cs	30.018 ± 0.022	^133Cs	^137Ba	11.23

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Once the optimum gas flow rates had been determined for the removal of isobaric interferences, further interferences from elevated levels of Mg (as MgO), Al (as ArAl) and Fe (as ArFe) were assessed by introducing each interfering element as a 5 μg g⁻¹ single element standard and monitoring the instrument response.

Data processing. Sensitivities for radionuclides were determined from their stable analogues by scaling the isotopic abundances to 100% viaeqn (1):where k is the sensitivity in cps (ng⁻¹ g⁻¹), I_std and I_blk are the measured signal intensities (in cps) of the standard and the blank respectively, w_std is the mass fraction of the standard in ng g⁻¹, and x is the isotopic abundance (as isotope amount fraction) as stated by the IUPAC Commission on Isotopic Abundances and Atomic Weights.²⁰

The separation factor between a target radionuclide and its interference is given as the ratio of the sensitivity of the target analyte to the sensitivity of the interference (k_radionuclide/k_interference). The greater the separation factor, the better resolved the radionuclide measurement is. For stable isotopes, the sensitivity is based on the elemental concentration.

Product ion formation has been calculated by the ratio of the sensitivity of the product ion of an element to the sensitivity of the same element achieved on-mass in the absence of a cell gas (k_{product ion}/k_{on−mass, no gas}) and expressed as a percentage.

Instrument detection limits were determined as three times the standard deviation of 10 replicate determinations of the blank signal (using w = 2% HNO₃), divided by the sensitivity of the radionuclide. The sensitivity was determined by performing a calibration using 7 standards of the stable isotopes of each given element at the optimum cell gas conditions. The sensitivity was normalized to 100% abundance of the isotope to determine the sensitivity of radionuclide determinations, as in eqn (1). The concentrations of the standards were: 1–1000 pg g⁻¹ for Ni, Sr, Zr, Nb, Mo, Pd, and Cs, 10–10 000 pg g⁻¹ for Se, and 100–100 000 pg g⁻¹ for Ca (due to the low abundance of the ⁴⁴Ca isotope).

In order to convert the sensitivities and instrument detection limits from mass fractions to activities, the specific activity was calculated for each radionuclide viaeqn (2):

where  a is the specific activity in Bq g⁻¹ of pure substance, N_A is the Avogadro constant, t_1/2 is the half-life of the radionuclide, and m_a is the atomic mass of the radionuclide. The mass fraction units (in ng g⁻¹) were converted to units of activity, A (in Bq g⁻¹ of sample), viaeqn (3):

A = w × a × 10⁻⁹ (3)

where the factor of 10⁻⁹ is included to convert between g (in Bq g⁻¹) and ng (in ng g⁻¹).

Results and discussion

Calcium-41

⁴¹Ca (t_1/2 = 1.002 × 10⁵ years) is produced by neutron activation of ⁴⁰Ca (96.94% abundance) present in the concrete shield surrounding nuclear reactors, which makes up a considerable amount of waste during decommissioning. Determinations of ⁴¹Ca has primarily been be carried out using liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) following chemical separations.^21–23 Recently, the first determinations of ⁴¹Ca by ICP-MS/MS were conducted by Russell et al.,²⁴ where the authors suggests the use of an NH₃/H₂/He gas mixture for interference removal. By this approach, ⁴¹Ca⁺ does not react with NH₃ or H₂, while interferences of ⁴⁰Ar¹H⁺ and ⁴⁰Ar⁺ (from peak tailing) are removed by a charge transfer reaction with NH₃. The authors reported a detection limit of 99 pg g⁻¹ (0.32 Bq g⁻¹) and a sensitivity of 3700 cps (ng⁻¹ g⁻¹) (1150 cps (Bq g⁻¹)) for ⁴¹Ca.²⁴ However, the major drawback of the NH₃/H₂ approach is that the stable ⁴¹K isotope also does not react with either gas, therefore ⁴¹Ca determinations are still subject to interferences in matrices containing high K levels. In literature, N₂O has previously been demonstrated to remove argon-based interferences, as well as interferences of K, by mass-shifting to the CaO⁺ product ion.^25,26 However, group 2 elements are known to also form hydroxide product ions with N₂O due to impurities in the cell gas,²⁶ therefore peak tailing of stable ⁴⁰Ca may still be problematic.

In this study, a mixture of N₂O and NH₃ were tested for determinations of the ⁴¹Ca. The addition of NH₃ to the N₂O generally caused group 2 elements to primarily form the hydroxide product ion (+17 amu) instead of the oxide product ion, while no product ion formation was observed for the group 1 elements. The argon interference is also removed effectively, as both N₂O and NH₃ react with Ar and ArH by charge transfer. For the determination of ⁴¹Ca using the ⁴¹Ca¹⁶O¹H⁺ product ion, the optimum gas mixture was found to be 0.4 mL min⁻¹ N₂O and 0.2 mL min⁻¹ NH₃, achieving a sensitivity of 50 000 cps (ng⁻¹ g⁻¹) (15 500 cps (Bq g⁻¹)) for ⁴¹Ca (Table S1). This gas mixture allowed interferences from peak tailing of ⁴⁰Ca to be successfully removed, as the ⁴⁰Ca¹⁶O¹H₂⁺ product ion was not observed to form. This was in contrast to using N₂O alone and monitoring the ⁴¹Ca¹⁶O⁺ product ion (m/z = 57) as impurities in the N₂O gas caused a noticeable shift of ⁴⁰Ca⁺ to ⁴⁰Ca¹⁶O¹H⁺ (m/z = 57). Importantly, the interference from ⁴¹K⁺ was eliminated due to the selective reaction of ⁴¹Ca⁺, thus providing a distinct advantage over the current best available methodology. The instrument detection limit achieved for ⁴¹Ca using the N₂O/NH₃ gas mixture approach was 0.35 pg g⁻¹ (0.0011 Bq g⁻¹). Further investigation was also carried out to investigate the effectiveness of the optimised N₂O/NH₃ gas mixture approach for sample matrices that may contain high levels of Mg, as ²⁵Mg¹⁶O⁺ is an additional interference at m/z 41. Analysis of a 5 μg g⁻¹ Mg standard indicated a minor interference of 17 cps (μg⁻¹ g⁻¹) Mg. Thus samples with excessive Mg levels may require dilution.

Nickel-59/63. Neutron activation of stable Ni isotopes in steel reactor casings produce two long-lived Ni radionuclides: ⁵⁹Ni (t_1/2 = 7.6 × 10⁴ years) and ⁶³Ni (t_1/2 = 98.7 years). Following chemical separations, ⁶³Ni can be easily measured using LSC.^27,28 The ⁵⁹Ni radionuclide is more challenging to measure by LSC due to the more abundant beta emissions of ⁶³Ni, but has been measured using X-ray emission techniques.²⁹ For ICP-MS/MS analysis, the ⁵⁹Ni and ⁶³Ni radionuclide have isobaric interferences from stable ⁵⁹Co and ⁶³Cu isotope, respectively. Russell et al.³⁰ demonstrated previously that an NH₃/H₂/He gas mixture can be used to separate ⁶³Ni from the ⁶³Cu isobar. By this approach, ⁶³Ni reacts to form the ⁶³Ni(NH₃)₃⁺ product ion at a greater rate than the formation of ⁶³Cu(NH₃)₃⁺, achieving a ⁶³Ni/⁶³Cu separation factor of 100 and detection limits of 0.25 pg g⁻¹ (0.52 Bq g⁻¹). The addition of H₂ to the NH₃/He mixture allowed for greater formation of the M(NH₃)_n product ions. The authors additionally noted that the use of H₂ significantly improved the formation rate of the ⁶³Ni(NH₃)₃⁺ product ion, and reported an achieved sensitivity of 6100 cps (ng⁻¹ g⁻¹) (2.86 cps (Bq g⁻¹)).

To date, there has been no successful ICP-MS/MS reaction cell based separation of ⁵⁹Ni from ⁵⁹Co reported in literature. While determinations of ⁵⁹Ni using N₂O and a N₂O/NH₃ gas mixture were attempted in this study, it was found that Co and Ni behaved very similarly under both conditions. As such, no separation of ⁵⁹Ni and ⁵⁹Co could be achieved and ⁵⁹Ni was not investigated further here.

Although Ni and Cu are both relatively unreactive with N₂O, minor formation of oxide product ions have been previously observed.¹³ In this study, Ni and Cu formed oxide product ions (+16 amu) at 4.6% and 0.1% respectively, allowing for a potential interference separation route. The maximum separation factor of 13 600 was achieved using a N₂O flow rate of 1.4 mL min⁻¹, achieving a ⁶³Ni sensitivity of 3640 cps (ng⁻¹ g⁻¹) (1.71 cps (Bq g⁻¹)). The obtained instrument detection limit using N₂O was calculated to be 0.29 pg g⁻¹ (0.62 Bq g⁻¹). Slightly higher sensitivities could be achieved by reducing the N₂O flow rate, but result in a less efficient separation of ⁶³Cu from ⁶³Ni (Fig. 1). For example, applying 1 mL min⁻¹ N₂O achieved a ⁶³Ni sensitivity of 5310 cps (ng⁻¹ g⁻¹) (2.49 cps (Bq g⁻¹)) and an instrument detection limit of 0.20 pg g⁻¹ (0.42 Bq g⁻¹), but a lower separation factor of 4960. Despite the slightly lower sensitivity and higher instrument detection limits obtained compared to the existing NH₃/H₂/He approach, the factor of 10 to 100 times greater separation factors achieved using N₂O provides a significant advantage. Moreover, samples containing elevated levels of Al may pose an additional challenge, as ³⁶Ar²⁷Al⁺ interferes on m/z 63 and, in this study, was observed to form at a rate of 11 100 cps (μg⁻¹ g⁻¹) Al in the absence of a cell gas. When applying N₂O at the optimum conditions, the rate of interference became negligible at <0.1 cps (μg⁻¹ g⁻¹) Al.

Fig. 1 (A) Calculated sensitivity of the ⁶³Ni radionuclide determined as the ⁶³Ni¹⁶O⁺ product ion (blue, circle) and the obtained sensitivity of the ⁶³Cu interference determined as the ⁶³Cu¹⁶O⁺ product ion (orange, square), and (B) the separation factor of ⁶³Ni/⁶³Cu.

When applying an N₂O/NH₃ gas mixture, both Cu and Ni were observed to form product ions at +17 amu, +34 amu, and +51 amu (Fig. 2). While these 17 amu increments could correspond to either ¹⁶O¹H or ¹⁴N¹H₃ ligands, it is likely that the ¹⁴N¹H₃ is forming, as the product ion formation rates aligned with previous literature using only NH₃ as a cell gas.³¹ Cu formed the Cu(NH₃)₂⁺ at the highest rate (maximum of 7.4% at 0.4 mL min⁻¹ N₂O and 1.0 mL min⁻¹ NH₃), while Ni was observed to form Ni(NH₃)₃⁺ at the highest rate (maximum of 4.4% at 0.2 mL min⁻¹ N₂O and 1.0 mL min⁻¹ NH₃). By utilizing the Ni(NH₃)₃⁺ product ion with a gas mixture of 0.6 mL min⁻¹ N₂O and 0.4 mL min⁻¹ NH₃, a similar sensitivity of 3440 cps (ng⁻¹ g⁻¹) (equivalent to 1.62 cps (Bq g⁻¹)) could be achieved, however with a separation factor of 254 (Table S2) – approximately 50 times less efficient than when using N₂O alone. Moreover, at these flow rates, slightly higher instrument detection limits of 0.58 ng g⁻¹ (1.2 Bq g⁻¹) were obtained, and the removal of the ³⁶Ar²⁷Al⁺ interference was less efficient (1.9 cps (μg⁻¹ g⁻¹) Al) than when using N₂O alone. Hence, it would be recommended that the N₂O approach would be optimal for determinations of ⁶³Ni.

Fig. 2 Product ion formations of Ni (blue) and Cu (orange) with 0.6 mL min⁻¹ N₂O and 0.4 mL min⁻¹ NH₃ applied as a reaction gas mixture.

Selenium-79

⁷⁹Se (t_1/2 = 7.6 × 10⁴ years) is a product of ²³⁵U fission that is important to characterise during decommissioning due to its high environmental mobility. Given its long half-life, ICP-MS is well suited to the determination of the ⁷⁹Se radioisotope. In addition to isobaric interferences from the stable ⁷⁹Br isotope, argon-based polyatomic interferences (⁴⁰Ar³⁸Ar¹H⁺, ³⁸Ar₂¹H⁺, and peak tailing of ⁴⁰Ar₂⁺) present an additional challenge for the analysis of ⁷⁹Se by ICP-MS/MS. A recent publication utilized NH₃ gas with ICP-MS/MS to reduce ⁷⁹Br and argon-based interferences by charge transfer, following a chemical separation step. The authors achieved an instrument limit of detection of 1.2 pg mL⁻¹ (5.7 × 10⁻⁴ Bq mL⁻¹).³² The authors additionally reported a separation factor of 7 orders of magnitude using their combined chemical separation and ICP-MS/MS methodology.

Here, bromine was observed to react very efficiently with N₂O to form the BrO⁺ product ion, with a maximum product ion formation of 77% at 0.6 mL min⁻¹. Selenium was observed to react with N₂O to form the SeO⁺ product ion, however the maximum product ion formation was 25% at a cell gas flow rate of 1.8 mL min⁻¹. At lower flow rates, the Se signal remained primarily on-mass, with a maximum of 96% signal intensity (relative to no cell gas) at an N₂O flow rate of 0.6 mL min⁻¹. By utilizing the on-mass determination of Se, separation of ⁷⁹Se from the interfering ⁷⁹Br can be achieved. A separation factor of 188 000 was achieved in this study using 2.0 mL min⁻¹ N₂O, with a calculated sensitivity for ⁷⁹Se of 3140 cps (ng⁻¹ g⁻¹) (6670 cps (Bq g⁻¹)) (Fig. 3), with instrument detection limits of 0.51 pg g⁻¹ (2.4 × 10⁻⁴ Bq g⁻¹). Higher sensitivities could be achieved by reducing the N₂O flow rate, although at the cost of less efficient ⁷⁹Br interference removal. The argon-based polyatomic interferences were no longer observed at N₂O flow rates of 1.2 mL min⁻¹ and above. At 1.2 mL min⁻¹, the ⁷⁹Se sensitivity obtained was 7520 cps (ng⁻¹ g⁻¹) (16 000 cps (Bq g⁻¹)), with an instrument detection limit of 0.38 pg g⁻¹ (1.8 × 10⁻⁴ Bq g⁻¹). Thus, the use of N₂O provides an alternative cell-gas approach to the recently developed NH₃ approach.

Fig. 3 (A) Calculated sensitivity of the ⁷⁹Se radionuclide determined on-mass (blue, circle) and the obtained sensitivity of the ⁷⁹Br interference determined on-mass (orange, square), and (B) the separation factor of ⁷⁹Se/⁷⁹Br.

Applying the N₂O/NH₃ gas mixture improved interference removal at lower flow rates, allowing for determinations of ⁷⁹Se at greater sensitivities. By applying 0.4 mL min⁻¹ N₂O and 0.2 mL min⁻¹ NH₃, a calculated sensitivity of 13 100 cps (ng⁻¹ g⁻¹) (equivalent to 27 800 cps (Bq g⁻¹)) was achieved with an improved interference separation factor of 4 600 000 (Table S3) and a calculated instrument detection limit of 0.17 pg g⁻¹ (8.0 × 10⁻⁵ Bq g⁻¹). This is likely due to an efficient charge transfer reaction between ⁷⁹Br⁺ and NH₃. The use of NH₃ has been shown impede the sensitivity of Se at higher flow rates (by a charge transfer reaction).³³ In this case however, the low flow of NH₃ enhanced the removal of the ⁷⁹Br interference, but was not a high enough flow to impact the sensitivity of ⁷⁹Se. Moreover, it is possible that the N₂O acts to enhance the sensitivity of ⁷⁹Se on-mass through the collisional focussing effect.¹³ Therefore, the use of the N₂O/NH₃ mixture offers an alternative approach for improved sensitivity and lower instrument detection limits, as well as a high interference separation factor of 6–7 orders of magnitude that can compete with the chemical separation approaches.

Strontium-90

Another fission product of high concern is ⁹⁰Sr (t_1/2 = 28.80 years), which is also known to be mobile in nature. Although this radionuclide can be determined using LSC following chemical separation,³⁴ ICP-MS determinations are growing more common due to advances in instrument sensitivity. The standard approach for interference removal for ⁹⁰Sr determinations by ICP-MS or ICP-MS/MS is the use of oxygen as a reaction gas.^35,36 In this approach, oxygen reacts efficiently with the interfering ⁹⁰Zr⁺ (forming ⁹⁰Zr¹⁶O⁺ and ⁹⁰Zr¹⁶O₂⁺), as well as ⁸⁹Y¹H⁺, but does not react efficiently with ⁹⁰Sr, thus allowing for an on-mass determination of ⁹⁰Sr. This established method is reported to provide instrument detection limits of 4 fg g⁻¹ (0.02 Bq g⁻¹).³⁶

The use of N₂O provided separation of ⁹⁰Sr from ⁸⁹Y¹H⁺ interference by forming the ⁹⁰Sr¹⁶O⁺ product ion (m/z = 106), with the ⁸⁹Y¹⁶O¹H⁺ interference forming at <0.3 cps (ng⁻¹ g⁻¹) (of ⁸⁹Y). However, the ⁹⁰Zr interference also reacted with N₂O to form ⁹⁰Zr¹⁶O⁺. Higher flow rates of N₂O lead to a higher separation of ⁹⁰Sr from ⁹⁰Zr, however at the cost of lower sensitivity (Fig. 4). Using only N₂O, the maximum achieved separation factor for the removal of ⁹⁰Zr on ⁹⁰Sr was 10 600 at 3 mL min⁻¹, with a sensitivity of 2870 cps (ng⁻¹ g⁻¹) (equivalent to 0.562 cps (Bq g⁻¹)) for ⁹⁰Sr. The instrument detection limit under these conditions was calculated to be 0.27 pg g⁻¹ (equivalent to 1.4 Bq g⁻¹), which does not provide an advantage over the current best available methodology.

Fig. 4 (A) Calculated sensitivity of the ⁹⁰Sr radionuclide determined as the ⁹⁰Sr¹⁶O⁺ product ion (blue, circle) and the obtained sensitivity of the ⁹⁰Zr interference determined as the ⁹⁰Sr¹⁶O⁺ product ion (orange, square), and (B) the separation factor of ⁹⁰Sr/⁹⁰Zr.

Applying an N₂O/NH₃ gas mixture and using the ⁹⁰Sr¹⁶O¹H⁺ product ion (m/z = 107) provided a much greater separation from ⁹⁰Zr, with <0.01% product ion formation of Zr at +17 amu. The maximum separation factor of 334 000 was achieved at gas flow rates of 0.8 mL min⁻¹ N₂O and 0.5 mL min⁻¹ NH₃ (Table S4). Product ion scans indicated that Zr preferentially formed higher-order product ions (primarily at +82 amu to +84 amu) using the N₂O/NH₃ gas mixture, allowing for more effective removal on m/z 107. Moreover, interferences from polyatomic ⁸⁹Y¹H⁺ and peak tailing of ⁸⁹Y⁺ were observed to be more effectively reduced using the N₂O/NH₃ gas mixture. By reducing the NH₃ flow rate to 0.2 mL min⁻¹ and maintaining a N₂O flow rate of 0.8 mL min⁻¹, a sensitivity of 108 000 cps (ng⁻¹ g⁻¹) (equivalent to 21.1 cps (Bq g⁻¹)) for ⁹⁰Sr was obtained (SrOH product ion formation of 38%), while the ⁹⁰Sr/⁹⁰Zr separation factor remained >100 000 (Table S4), giving a much greater performance than using N₂O alone. The instrument detection limit for ⁹⁰Sr at these gas flow rates was calculated to be 0.015 pg g⁻¹ (0.076 Bq g⁻¹), which is similar to the detection limits achieved by the oxygen cell gas approach^35,36 and thus makes the N₂O/NH₃ gas mixture approach a viable alternative for interference removal.

Zirconium-93, molybdenum-93, and niobium-94

⁹³Zr (t_1/2 = 1.61 × 10⁶ years), ⁹³Mo (t_1/2 = 4000 years), and ⁹⁴Nb (t_1/2 = 2.004 × 10⁴ years) are produced through neutron activation of stable ⁹²Zr (17.16% abundance), ⁹²Mo (14.65% abundance), and ⁹³Nb (100% abundance), respectively, that are contained within steel casings of nuclear reactors. Additionally, ⁹³Zr is also a high-yield fission product in spent nuclear fuel. Following chemical separations, ⁹³Zr and ⁹³Mo can be determined by LSC,^37,38 whereas ⁹⁴Nb can be determined by gamma- or X-ray spectrometry.³⁹ Given the long half-lives, ICP-MS determinations are well suited to the analysis of these radionuclides. However, ⁹³Zr and ⁹³Mo share isobars with each other as well as ⁹³Nb, which provides an additional challenge as ⁹³Nb is the only stable isotope of Nb. ICP-MS/MS has previously been demonstrated to be an effective approach to analysing ⁹³Zr and ⁹³Mo. Petrov et al.⁹ reported that an NH₃/H₂/He gas mixture could be utilized to effectively remove interferences of ⁹³Mo and ⁹³Nb on ⁹³Zr by measuring the ⁹³Zr(NH₃)₆ product ion, with separation factors of 10⁵ achievable from both interferences. The authors report a sensitivity of approximately 20 000 cps (ng⁻¹ g⁻¹) (230 000 cps (Bq g⁻¹)) and a detection limit of 0.14 pg g⁻¹ (1.3 × 10⁻⁵ Bq g⁻¹) for ⁹³Zr. Russell et al.³⁰ subsequently demonstrated that the same NH₃/H₂/He gas mixture could be utilized to analyse ⁹³Mo using the ⁹³Mo(NH₃)₂ product ion. The authors reported a sensitivity of 1100 cps (ng⁻¹ g⁻¹) (31 cps (Bq g⁻¹)) and a detection limit of 45.6 pg g⁻¹ (1.6 Bq g⁻¹). The ⁹⁴Nb radionuclide has been much less studied using ICP-MS/MS compared to ⁹³Zr and ⁹³Mo and, to the authors knowledge, there is no literature on the best available reaction gas conditions. Nevertheless, as the ⁹⁴Nb radionuclide shares isobars with stable ⁹⁴Zr and ⁹⁴Mo isotopes, it has also been assessed using N₂O and a N₂O/NH₃ gas mixture in this study.

Zr and Nb react with N₂O to form primarily oxide (+16 amu) and dioxide (+32 amu) product ions, whereas Mo reacts very little with N₂O.¹³ Therefore, the removal of stable ⁹³Nb interference, as well as interferences from the ⁹³Zr radionuclide, using an on-mass determination of ⁹³Mo would be possible. Using a flow rate of 1 mL min⁻¹ N₂O, ⁹³Mo could be separated from ⁹³Nb and ⁹³Zr by factors of 750 and 1160 respectively, while providing a calculated sensitivity of 126 000 cps (ng⁻¹ g⁻¹) (equivalent to 3540 cps (Bq g⁻¹)) (Fig. 5). The instrument detection limit was calculated to be 0.017 pg g⁻¹ (6.1 × 10⁻⁴ Bq g⁻¹). Additionally, the ⁹³Zr and ⁹⁴Nb radionuclides can be separated from the ⁹³Mo radioisotope and the stable ⁹⁴Mo isotope respectively by mass-shifting and analysing the dioxide product ion. However, as Zr and Nb react very similarly with N₂O,¹³ these radionuclides cannot be separated from their stable isotope counterparts.

Fig. 5 (A) Calculated sensitivity of the ⁹³Mo radionuclide (blue, circle), the calculated sensitivity of the interfering ⁹³Zr radionuclide (grey, triangle) and the obtained sensitivity of the stable ⁹³Nb interference (orange, square), and (B) the separation factor of ⁹³Mo/⁹³Nb (blue, circle) and ⁹³Mo/⁹³Zr (orange, square).

Applying an N₂O/NH₃ gas mixture, Mo was, again, found to react relatively little with the cell gas and most of the ions were transmitted on-mass. The Mo(OH)₂ product ion was formed at the highest rate (maximum of 3.2% formation). Nb and Zr preferentially reacted to form higher order product ions, primarily around +82 amu to +84 amu (Fig. 6). Both ¹⁴N¹H₃ and ¹⁶O¹H shift the mass by +17 amu, and can form ligands with and without the hydrogen atoms present (i.e. as MO⁺ or MNH₂⁺), therefore it is difficult to predict exactly which higher-order complexes are being formed at these higher masses. Nevertheless, the distinct differences in reactivity between Mo and the interfering Nb and Zr isobars mean that separation is possible. However, the addition of NH₃ resulted in a slightly lower sensitivity. Using 0.6 mL min⁻¹ N₂O and 0.4 mL min⁻¹ NH₃ achieved equivalent separation factors compared to using 1 mL min⁻¹ N₂O, with a calculated sensitivity for ⁹³Mo of 105 000 cps (ng⁻¹ g⁻¹) (equivalent to 2950 cps (Bq g⁻¹)) (Table S5), 17% lower than using N₂O alone. However, the instrument detection limit was calculated to be 0.015 pg g⁻¹ (5.5 × 10⁻⁴ Bq g⁻¹), which was similar to that obtained by using N₂O alone. Further improvements to the removal of Zr and Nb could be achieved by varying the N₂O and NH₃ flow rates, but at the cost of lower sensitivities for ⁹³Mo (Table S5).

Fig. 6 Product ion formations of Zr (blue) and Nb (orange) under two different reaction gas mixtures: 0.2 mL min⁻¹ N₂O and 1.0 mL min⁻¹ NH₃ (left); and 1.0 mL min⁻¹ N₂O and 0.2 mL min⁻¹ NH₃ (right).

Separation of Zr and Nb was found to be possible using the N₂O/NH₃ mixture, however with low separation factors. The product ions of Nb and Zr at +82 amu and +84 amu were found to form at different rates depending on the composition of the N₂O/NH₃ mixture. For ⁹³Zr, the greatest separation factor achieved for the removal of ⁹³Nb was 80 by observing the product ion at +84 amu and using a cell gas mixture of 0.2 mL min⁻¹ N₂O and 1 mL min⁻¹ NH₃ (Table S6). The high ammonia gas flow rate resulted in a relatively low sensitivity of 7670 cps (ng⁻¹ g⁻¹) (86 700 cps (Bq g⁻¹)), equivalent to 2.6% product ion formation. The instrument detection limit for ⁹³Zr was calculated to be 0.27 pg g⁻¹ (2.4 × 10⁻⁵ Bq g⁻¹). For ⁹⁴Nb, the greatest separation factor achieved for the removal of ⁹⁴Zr was 25 by observing the product ion at +83 amu and using 1 mL min⁻¹ N₂O and 0.1 mL min⁻¹ NH₃ (Table S7). In this case, the flow rates also corresponded to the maximum observed product ion formation of 9.6%, equivalent to a sensitivity of 19 900 cps (ng⁻¹ g⁻¹) (2830 cps (Bq g⁻¹)) for ⁹⁴Nb. The instrument detection limit for ⁹⁴Nb was calculated to be 0.23 pg g⁻¹ (0.0016 Bq g⁻¹). An additional concern for determinations of ⁹⁴Nb is the ⁴⁰Ar⁵⁴Fe⁺ interference in samples with elevated Fe content. In the absence of a cell gas, the interference formed at a rate of 82 cps (μg⁻¹ g⁻¹) Fe. At the flow rates of 1 mL min⁻¹ N₂O and 0.1 mL min⁻¹ NH₃, the interference was reduced to 0.7 cps (μg⁻¹ g⁻¹) Fe. However, given the low separation factors and sensitivity achieved for ⁹³Zr and ⁹⁴Nb, the use of the N₂O/NH₃ gas mixture may have limited application for these radionuclides.

Palladium-107

¹⁰⁷Pd (t_1/2 = 6.5 × 10⁶ years) is a fission product found in spent nuclear fuel. Determinations by LSC are possible for ¹⁰⁷Pd, however given its very long half-life, ICP-MS determinations provide greater sensitivity compared to radiometric techniques.⁴⁰ For ICP-MS determinations, the radionuclide ¹⁰⁷Pd shares isobaric interferences with stable ¹⁰⁷Ag. Literature utilizing reaction gases for interference separation of ¹⁰⁷Pd is limited, with studies focussing on improving chemical separations prior to ICP-MS detection.⁴¹ In combination with chemical separation, Weller et al.⁴⁰ utilized a propane/He gas mixture to separate ¹⁰⁷Pd from its isobaric interference by monitoring the Pd(C₂H₂) product ion, resulting in detection limits of <2 pg g⁻¹ (<4 × 10⁻⁵ Bq g⁻¹). The product ion formation was reported to be approximately 4%, which limits the available sensitivity, however the authors also report that similar detection limits were achieved for ¹⁰⁷Pd in the absence of a reaction gas.

Both Pd and Ag react very little with N₂O, with less than 1% product ion formation.¹³ However, Ag does not form AgO⁺ product ions (instead, favouring AgN₂O⁺ formation), whereas Pd does form PdO⁺ product ions.¹³ Therefore, determinations of the ¹⁰⁷Pd radionuclide may be possible. However, in this study, the maximum formation rate of the PdO⁺ product ion was 0.15% at 0.7 mL min⁻¹ N₂O, equating to a sensitivity of 164 cps (ng⁻¹ g⁻¹) (8620 cps (Bq g⁻¹)) for ¹⁰⁷Pd. The calculated instrument detection limit was 14 pg g⁻¹ (2.7 × 10⁻⁴ Bq g⁻¹). N₂O shows potentially limited use for determinations of ¹⁰⁷Pd due to the low achievable sensitivities.

Applying the N₂O/NH₃ gas mixture, Pd and Ag reacted to form product ions at +17 amu, +34 amu, and +51 amu. While these 17 amu increments could correspond to either ¹⁶O¹H or ¹⁴N¹H₃ ligands, it is likely that the ¹⁴N¹H₃ is forming, as the product ion formation rates aligned with previous literature using only NH₃ as a cell gas.³¹ The Ag(NH₃)₃⁺ product ion formed at a lower rate than that of Pd(NH₃)₃⁺. Using cell gas flow rates of 0.6 mL min⁻¹ N₂O and 0.8 mL min⁻¹ NH₃, a separation factor of 84 could be achieved, with a sensitivity of 3300 cps (ng⁻¹ g⁻¹) (173 000 cps (Bq g⁻¹)) (Table S8 and Fig. 7). The calculated instrument detection limit under these gas conditions was 1.1 pg g⁻¹ (2.0 × 10⁻⁵ Bq g⁻¹). While the achieved sensitivity is 20 times greater than using N₂O alone, the use of N₂O alone achieved detection of ¹⁰⁷Pd (as PdO⁺) free from ¹⁰⁷Ag interference, since the AgO⁺ product ion was not formed. Therefore, depending on the mass fraction of Ag present in the matrix, it may be preferable to use the N₂O approach over the N₂O/NH₃ gas mixture approach.

Fig. 7 Product ion formations of Pd (blue) and Ag (orange) with 0.6 mL min⁻¹ N₂O and 0.8 mL min⁻¹ NH₃ applied as a reaction gas mixture.

Caesium-135/137

¹³⁵Cs (t_1/2 = 1.33 × 10⁶ years) and ¹³⁷Cs (t_1/2 = 30.018 years) are important fission products used for environmental monitoring. Specifically, ¹³⁵Cs/¹³⁷Cs ratios have been used as a forensic tool to identify sources of radioactive contamination, for example following the Fukushima nuclear disaster.¹⁶ ICP-MS/MS methodology is already established for this application, where N₂O has already been used as a cell gas to separate ¹³⁵Cs⁺ and ¹³⁷Cs⁺ from the stable isobaric interferences of ¹³⁵Ba⁺ and ¹³⁷Ba⁺ respectively.^14–16 By this approach, Ba is mass-shifted and the Cs can be determined on-mass. Additionally, Magre et al.¹⁸ recently described the use of an N₂O/NH₃ gas mixture for the determination of ¹³⁵Cs/¹³⁷Cs ratios with improved interference removal. The authors noted a sensitivity of 110 000 cps (ng⁻¹ mL⁻¹) (equivalent to 1 500 000 cps (Bq g⁻¹) for ¹³⁵Cs and 34 cps (Bq g⁻¹) for ¹³⁷Cs) under wet-plasma conditions, with a factor of 3 times improvement using an Apex Ω desolvating system. The authors reported instrument blank levels of <0.6 cps. Under optimum conditions, including the use of the desolvating system, the authors achieved detection limits of 1.66 fg g⁻¹ (1.2 × 10⁻⁷ Bq g⁻¹) for ¹³⁵Cs and 0.67 fg g⁻¹ (0.0022 Bq g⁻¹) for ¹³⁷Cs.

The N₂O/NH₃ gas mixture was applied to the separation of Cs radionuclides from isobaric Ba interferences. At cell gas flow rates of 1 mL min⁻¹ N₂O and 0.1 mL min⁻¹ NH₃, separation factors of 223 000 for ¹³⁵Cs/¹³⁵Ba and 131 000 for ¹³⁷Cs/¹³⁷Ba were obtained, with sensitivities of 101 000 cps (ng⁻¹ g⁻¹) for both radioisotopes (Table S9) (equivalent to 1 370 000 cps (Bq g⁻¹) for ¹³⁵Cs and 31.4 cps (Bq g⁻¹) for ¹³⁷Cs). The instrument blank was found to be 0.7 cps for both ¹³⁵Cs and ¹³⁷Cs. The instrument detection limit under these conditions was calculated to be 0.017 pg g⁻¹ (equivalent to 1.3 × 10⁻⁶ Bq g⁻¹ for ¹³⁵Cs and 0.056 Bq g⁻¹ for ¹³⁷Cs). Compared to the use of N₂O only (optimum flow rate = 1 mL min⁻¹), the N₂O/NH₃ provided a factor of 2 times greater interference separation, while maintaining an equivalent sensitivity. Given the achieved sensitivity and blank levels, the results in this study are in good agreement with those produced by Magre et al.,¹⁸ though with higher observed detection limits due to the absence of a desolvating system.

Conclusions

This study demonstrates the promising applicability of N₂O for the removal of isobaric interferences for radionuclides of interest in nuclear decommissioning, especially for ⁶³Ni, ⁷⁹Se, and ⁹³Mo. Moreover, the great power of combining two reactive cell gases to open new interference removal pathways using new product ions is highlighted. In particular, enhanced interference removal for the group 2 radionuclides (⁴¹Ca and ⁹⁰Sr), as well as for ⁷⁹Se, ⁹³Mo, and ¹⁰⁷Pd were achieved using a novel N₂O/NH₃ gas mixture approach. Hence, it can be recommended that both N₂O and NH₃ are employed during ICP-MS/MS analysis. Such findings may also be applicable to stable isotope analysis, therefore further investigation is required into the use of the N₂O/NH₃ gas mixture, as well as for other reactive cell gas mixtures, for a larger set of elements and additional radionuclides of interest, such as the actinides.

Author contributions

Shaun T. Lancaster: conceptualization, investigation, writing – original draft. Ben Russel: conceptualization, writing – review and editing. Thomas Prohaska: resources, writing – review and editing. Johanna Irrgeher: funding acquisition, resources, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5ja00254k.

Acknowledgements

The project (21GRD09 MetroPOEM) has received funding from the European Partnership on Metrology, co-financed from the European Union's Horizon Europe Research and Innovation Programme and by the Participating States. The authors thank Karl Andreas Jensen, Solutions 4 Science, and PerkinElmer for fruitful discussions and continuous support.

Notes and references

1. I. W. Croudace, B. C. Russell and P. W. Warwick, J. Anal. At. Spectrom., 2017, 32, 494–526.
2. X. Hou and P. Roos, Anal. Chim. Acta, 2008, 608, 105–139.
3. D. W. Koppenaal, G. C. Eiden and C. J. Barinaga, J. Anal. At. Spectrom., 2004, 19, 561–570.
4. L. Balcaen, E. Bolea-Fernandez, M. Resano and F. Vanhaecke, Anal. Chim. Acta, 2015, 894, 7–19.
5. P. E. Warwick, B. C. Russell, I. W. Croudace and J. Zacharauskas, Anal. At. Spectrom., 2019, 34, 1810–1821.
6. S. Diez-Fernández, H. Isnard, A. Nonell, C. Bresson and F. Chartier, J. Anal. At. Spectrom., 2020, 35, 2793–2819.
7. D. J. Douglas and C. J. B. French, J. Am. Soc. Mass Spectrom., 1992, 3, 398–408.
8. D. J. Douglas, J. Am. Soc. Mass Spectrom., 1998, 9, 101–113.
9. P. Petrov, B. Russell, D. N. Douglas and H. Goenaga-Infante, Anal. Bioanal. Chem., 2018, 410, 1029–1037.
10. G. K. Koyanagi and D. K. Bohme, J. Phys. Chem. A, 2001, 105, 8964–8968.
11. V. V. Lavrov, V. Blagojevic, G. K. Koyanagi, G. Orlova and D. K. Bohme, J. Phys. Chem. A, 2004, 108, 5610–5624.
12. K. Harouaka, C. Allen, E. Bylaska, R. M. Cox, G. C. Eiden, M. L. di Vacri, E. W. Hoppe and I. J. Arnquist, Spectrochim. Acta, Part B, 2021, 186, 106309.
13. S. T. Lancaster, T. Prohaska and J. Irrgeher, J. Anal. At. Spectrom., 2023, 38, 1135–1145.
14. M. Granet, A. Nonell, G. Favre, F. Chartier, H. Isnard, J. Moureau, C. Caussignac and B. Tran, Spectrochim. Acta, Part B, 2008, 63, 1309–1314.
15. J. Zheng, W. Bu, K. Tagami, Y. Shikamori, K. Nakano, S. Uchida and N. Ishii, Anal. Chem., 2014, 86, 7103–7110.
16. T. Ohno and Y. Muramatsu, J. Anal. At. Spectrom., 2014, 29, 347–351.
17. K. J. Hogmalm, T. Zack, A. K. O. Karlsson, A. S. L. Sjöqvist and D. Garbe-Schönberg, J. Anal. At. Spectrom., 2017, 32, 305–313.
18. A. Magre, B. Boulet, H. Isnard, S. Mialle, O. Evrard and L. Pourcelot, Anal. Chem., 2023, 95, 6923–6930.
19. Decay Data Evaluation Project, http://www.lnhb.fr/home/conferences-publications/ddep_wg/.
20. J. Meija, T. B. Coplen, M. Berglund, W. A. Brand, P. De Bièvre, M. Gröning, N. E. Holden, J. Irrgeher, R. D. Loss, T. Walczyk and T. Prohaska, Pure Appl. Chem., 2016, 88, 293–306.
21. X. Hou, Radiochim. Acta, 2005, 93, 611–617.
22. P. E. Warwick, I. W. Croudace and D. J. Hillegonds, Anal. Chem., 2009, 81, 1901–1906.
23. D. Hampe, B. Gleisberg, S. Akhmadaliev, G. Rugel and S. Merchel, J. Radioanal. Nucl. Chem., 2013, 296, 617–624.
24. B. Russell, H. Mohamud, M. G. Miranda, P. Ivanov, H. Thompkins, J. Scott, P. Keen and S. Goddard, J. Anal. At. Spectrom., 2021, 36, 845–855.
25. S. T. Lancaster, T. Prohaska and J. Irrgeher, Anal. Bioanal. Chem., 2022, 414, 7495–7502.
26. D. R. Bandura, V. I. Baranov, A. E. Litherland and S. D. Tanner, Int. J. Mass Spectrom., 2006, 255–256, 312–327.
27. X. Hou, L. F. Østergaard and S. P. Nielsen, Anal. Chim. Acta, 2005, 535, 297–307.
28. P. E. Warwick and I. W. Croudace, Anal. Chim. Acta, 2006, 567, 277–285.
29. I. Gresits and S. Tölgyesi, J. Radioanal. Nucl. Chem., 2003, 258, 107–112.
30. B. Russell, S. L. Goddard, H. Mohamud, O. Pearson, Y. Zhang, H. Thompkins and R. J. C. Brown, J. Anal. At. Spectrom., 2021, 36, 2704–2714.
31. N. Sugiyama and K. Nakano, Reaction Data for 70 Elements Using O2, NH3 and H2 Gases with the Agilent 8800 Triple Quadrupole ICP-MS, Agilent Technical Note, 2014, P/N:5990-4585EN, https://www.agilent.com/cs/library/technicaloverviews/public/5991-4585EN_TechNote8800_ICP-QQQ_reactiondata.pdf.
32. I. M. Banjarnahor, V.-K. Do, T. Furuse, Y. Ohta and K. Tanaka, J. Radioanal. Nucl. Chem., 2025, 334, 4997–5306.
33. S. D'Ilio, N. Violante, C. Majorani and F. Petrucci, Anal. Chim. Acta, 2011, 698, 6–13.
34. P. Xu, C. Ding, G. Yu and Z. Chen, J. Radioanal. Nucl. Chem., 2022, 331, 3269–3274.
35. B. Russell, M. García-Miranda and P. Ivanov, Appl. Radiat. Isot., 2017, 126, 35–39.
36. J. Feuerstein, S. F. Boulyga, P. Galler, G. Stingeder and T. Prohaska, J. Environ. Radioact., 2008, 99, 1764–1769.
37. Y. Luo, X. Hou, J. Qiao, L. Zhu, C. Zheng and M. Lin, Anal. Chem., 2022, 94, 11582–11590.
38. A. G. Espartero, J. A. Suárez, M. Rodríguez and G. Pia, Appl. Radiat. Isot., 2002, 56, 41–46.
39. S. Osváth, N. Vajda and Z. Molnár, Appl. Radiat. Isot., 2008, 66, 24–27.
40. A. Weller, T. Ramaker, F. Stäger, T. Blenke, M. Raiwa, I. Chyzhevsky, S. Kirieiev, S. Dubchak and G. Steinhauser, Environ. Sci. Technol. Lett., 2021, 8, 656–661.
41. M. Faure, C. Gautier, P. Fichet, A. Maubert, E. Debonnet, E. Laporte, C. Colin and L. Hélot, J. Radioanal. Nucl. Chem., 2025, 334, 4687–4707.

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