Mass-spectrometric determination of iodine-129 using O2–CO2 mixed-gas reaction in inductively coupled plasma tandem quadrupole mass spectrometry

This paper presents a mass-spectrometric method for determining the radionuclide iodine-129 (129I) from the significant amount of interference in inductively coupled plasma tandem quadrupole mass spectrometry (ICP–MS/MS) using a dynamic reaction cell passing a mixture gas of O2 and CO2. Thus far, mass spectrometry analysis of trace amounts of 129I has been hampered by the presence of xenon-129 (129Xe) and the formation of polyatomic ions from excess amounts of stable isotope 127I. In this study, flowing a mixture gas of O2 and CO2 into the dynamic reaction cell (Q2) successfully removed both 129Xe interference and polyatomic interference (127IH2) in the analysis of 129I in ICP–MS/MS. The resulting ratio of (background noise of m/z 129)/127I was 4.6 × 10–10 ± 3.3 × 10–10, which enables the analysis of 10 mBq/L of 129I in the presence of 100 mg/L of stable 127I without chemical separation. The detection limit of this method was 0.73 mBq/L (= 0.11 ng/L) with an APEX-Q sample inlet desolvation device. For demonstration purposes, spike and recovery analysis of rainwater was performed, and good agreement between the spiked and recovered amounts was achieved.


Introduction
Radioactive iodine-129 ( 129 I), which has a long half-life of 1.57 × 10 7 years, is a remarkable indicator for the investigation of influence to the environment from nuclear power activities. Over the past 50 years, the inventory of 129 I on Earth has increased from the pre-nuclear-age level of 10 -12 -10 -15 to 10 -11 -10 -9 due to accumulation through artificial activities, such as discharge from nuclear fuel reprocessing plants, atmospheric nuclear weapons testing, and nuclear accidents (as a fission product of 235 U and 239 Pu) [1]. Monitoring anthropogenic 129 I as an indicator or tracer can help understand the transport and dynamics of 131 I, which has a short half-life of 8 days, in radioactive wastes discharged from nuclear facilities or in materials deposited by fallout from nuclear accidents [2][3][4][5]. The simultaneous determination of radioactive 129 I and natural 127 I (i.e., the ratio of 129 I/ 127 I) has contributed to determine the origin and age of seawater, porewater, groundwater, methane hydrate, and meteorites [6][7][8][9][10]. Thus, the determination of 129 I contributes to many fields, such as environmental science, nuclear forensics, and geological science; however, the determination of 129 I is challenging using mass spectrometry (MS).
Accelerator mass spectrometry (AMS) is often used to determine the ratio of 129 I/ 127 I due to the high mass resolution and low background ( 129 I/ 127 I = 10 -15 -10 -10 ) [11][12][13][14]; however, its cost is high, and the control of contaminants is difficult. Inductively coupled plasma mass spectrometry (ICP-MS) is a popular means to determine ultratrace elements and is employed by many institutions. The advantage 1 3 of ICP-MS over AMS is that it performs analysis with higher sample throughput; however, it must remove massspectrometric interferences (xenon-129 [ 129 Xe] and IH 2 + ), which tend to increase the background noise (BGN) intensity of m/z 129 and the detection limit. 129 Xe is the primary source of interference in MS analysis of 129 I and is contained in argon (Ar) plasma gas as an impurity. It can be almost completely removed by the charge transfer reaction using O 2 via the reaction cell in ICP-MS (Xe + + O 2 → Xe + O 2 + ) [15]. Ohno et al. reported suppression of the high BGN level caused by 129 Xe via the O 2 reaction using the Agilent triple quadrupole (Q) ICP-MS system. It should be noted that the MS configuration was m/z 129 (Q1) → O 2 reaction (Q2) → m/z 129 (Q3). Previous studies achieved a ratio of 129 I/ 127 I = 5 × 10 -9 by the O 2 (mono-gas) reaction [15,16]; however, a ratio of 129 I/ 127 I with an order of magnitude less than 10 -9 is required for the analysis of actual rainwater [18][19][20]. Thus, the ICP-tandem quadrupole MS (ICP-MS/MS) technique faces challenges achieving values less than 10 -9 .
The primary factor limiting the improvement of the ratio is the formation of hydrogen additives (i.e., 127 IH 2 + ) as polyatomic ions, which are created in Ar plasma and increase the ratio of 129 I/ 127 I. In current reaction techniques, such as dynamic reaction cell (DRC, i.e., Q2) or collision-reaction cell, the simultaneous removal of IH 2 and Xe has been challenging.
This paper presents a method for removing interferences (Xe + and IH 2 + ) using a mixture of O 2 -CO 2 gas in Q2 (DRC) in ICP-MS/MS. Analysis of actual rainwater was performed to evaluate the ability of the proposed method to remove IH 2 in Q2.

Reagents and preparation
A radioactive 129 I − stock solution (0.3 Bq/g [= 45.9 ng/g], radioactive purity > 99%, used as potassium iodide) obtained from Kaken Corporation Ltd. (Ibaraki, Japan) was diluted to the desired concentration using Milli-Q water (18.2 MΩ·cm; Merck KGaA, Darmstadt, Germany). A stable iodine ( 127 I − ) solution was prepared from ammonium iodide (purity 99.5%) obtained from FUJIFILM Wako Pure Chemical Co. (Osaka, Japan). Tetramethylammonium hydroxide (TMAH; Tama Chemicals Co. Ltd., Kawasaki, Japan) was spiked into the 127 I − solution as a stabilizer. Cesium (Cs) was used as an internal standard solution (1000 mg/L; atomic absorption spectrometry grade; SPEX Certiprep Inc., Metuchen, NJ, USA) and was spiked into each test solution. All other analytical-grade reagents were purchased from FUJIFILM and were used without further purification unless otherwise noted.

Instrumentation
A triple quadrupole ICP-MS/MS instrument (NexION 5000, PerkinElmer, Sheraton, CT, USA) equipped with a DRC (Q2) between two quadrupole mass-filters (Q1/Q3) and a four-channel reaction gas manifold was used for all experiments. Ultrapure-grade gases (O 2 > 99.99995%, CO 2 > 99.995%, NO > 99%, N 2 O > 99.999%) were mixed in the gas manifold and directly introduced into the DRC. The sample solutions were injected by a PFA concentric nebulizer and cyclonic spray chamber (300-350 μL/min) or by a high-efficiency sample-injection device (APEX-Q, Elemental Scientific, Omaha, NE, USA) into the ICP-MS instrument. Optimization of the parameters or conditions in ICP-MS/MS was obtained using 200 ng/L of mixed element standard solution (Li, Mg, In, Ce, and U) and the solution was flowed at above mentioned flow rate in the STD mode (no gas mode), while the sensitivity (intensity) of 115 In + was maximized during the maintaining less than 3% of CeO + / Ce + ratio. The resultant obtained parameters for ICP-MS/ MS analysis are listed in Table S1 in the Supplementary information (SI).

Calculations
The normalized intensity (N I ) values were calculated using the following equation: where the initial intensity (cps) is the original intensity when no gases flow into the DRC (i.e., flow rate = 0 mL/min), and the measured intensity (cps) is the actual intensity in the presence of gases in the DRC at an arbitrary flow rate (mL/ min). The subscript "f" represents the flow rates of gas in the DRC.

Sample collection and pretreatment
Rainwater was collected in Fukushima Prefecture, Japan. The samples were filtered using a hydrophilic polytetrafluoroethylene (PTFE) polymer membrane filter (0.45 µm × 47 mm). The rainwater was prepared to 3% TMAH aqueous solution before ICP-MS/MS analysis. Although an acidic media solution (e.g., HNO 3 ) causes a memory effect in the spray chamber of ICP-MS by the production of volatile species such as HI and I 2 , the use of an alkaline media (e.g., TMAH) can circumvent the interferences with short washout time [21][22][23][24]. The washout was carefully confirmed using Milli-Q water after measurement of 129 I solution which was coexistence of 0.0-3.0% TMAH as previously reported [25,26] [Note: the flowing 3% TMAH solution was effective on the removal of the memory in this study].

Results and discussion
Behavior of the BNG of m/z 129 in the presence or absence of mono-gas in the DRC (Q2) In the absence of any gases in the DRC (Q2), the average BGN of m/z 129 was 1.4 × 10 3 ± 9.5 × 10 1 cps (gas flow rate = 0 mL/min; Q1 → m/z 129 / Q3 → m/z 129). The values depended on the employed Ar gas cylinders; the intensity ranged from 1.4 × 10 3 to 5.0 × 10 3 cps for each cylinder. When the reaction gases flowed into the DRC (Q2), the BGN of m/z 129 was effectively removed using O 2 , as illustrated in Fig. 1a. The actual BGN was finally decreased to 0-1 cps. In addition, Fig. 1b demonstrates that the intensity of I + as the analytical target was almost maintained by using O 2 gas. For example, the signal intensity of 1.2 × 10 5 cps was maintained with the injection of 1.0 µg/L for 127 I, as illustrated in Fig.  S1 in the SI. These results reveal that the O 2 reaction in the DRC (Q2) is effective in determining 129 Xe + and 129 I + , and the results are similar to those of previous studies [15,16,20].   improved N I further, reducing it to 2.8 × 10 -4 . The contributions of the gases to the N I value were 5.4 × 10 -2 and 5.2 × 10 -3 for O 2 and CO 2 , respectively. Whereas the use of mono-gas in the DRC (Q2) had difficulty simultaneously removing the two interference states ( 129 Xe and 127 IH 2 ) and discriminating 129 I in the presence of a high concentration of 127 I, the use of a mixture gas resulted in the simultaneous elimination of the interference states. This method can be combined to the preconcentration of the sample water to enhance the analytical sensitivity of MS due to the allowance of the higher ratio of BGN (129)/ 127 I since the actual concentration of radioactive 129 I was at an ultra-low level.

Effect of the mixture of reaction gases on BGN suppression
The mechanism of the suppression of IH 2 + by CO 2 is discussed based on the experimental data as follows. Based on the experimental results that CO 2 oxidation was slower than that of O 2 and N 2 O, the oxidative reaction was unlikely to occur. Furthermore, because the ionization potential of CO 2 (13.79 eV) is much higher than that of I (10.45 eV) and IH (10.39 eV) (cf. no data found for IH 2 ), the reaction by the charge transfer effect was unlikely to occur. Although H 2 gas was intentionally introduced into the DRC, little or no IH 2 + was formed. In other words, even if a small quantity of I in O 2 gas existed as an impurity, it would not form IH 2 . In addition, based on the reaction degree illustrated in Fig. 3, the reaction suppressing the formation of IH 2 + was preferentially caused only for hydrogen additive (IH 2 + ) passing through Q1 (m/z 129), not for the I + ions. This suggests that the kinetic energy of IH 2 + was preferentially reduced by the collision reaction of CO 2 with polyatomic ions (IH 2 + ) due to the larger cross section of CO 2 than that of I + . Similar behavior has been reported, namely, that the kinetic energies of medium or heavy elements are more effectively reduced by inert gases with larger atomic sizes, such as neon (Ne) and krypton (Kr), in the collision reaction, than by gases with smaller atomic sizes, such as He [28][29][30].

Determination of 129 I/ 127 I ratio and analytical performance
A blank solution (0 Bq/L of 129 I) containing 0.5 w/w% of 127 I (as 127 I in NH 4 I) with 1.0 μg/L of 133 Cs as an internal standard was measured by the proposed method (O 2 : 1.0 mL/ min and CO 2 : 2.0 mL/min). The axial field technology voltage was adjusted to a negative value of -25 to suppress the BGN of m/z 129 (Fig. S2 in SI). An intensity ratio of (m/z 129)/ 127 I of 4.6 × 10 -10 ± 3.3 × 10 -10 was achieved. Table 1 presents a comparison between the proposed method and previous methods for obtaining the ratio of 129 I/ 127 I. The value of 129 I/ 127 I in the proposed method was relatively lower than other previous methods with high precision. The previous methods had difficulty measuring rainwater due to the relatively high 129 I/ 127 I ratio. The calibration curve of 129 I was shown in Fig. 4. The relative standard deviation (RSD) of 1.0 Bq/L of 129 I was 2.5% (45.8 ± 1.2 cps) and the detection limit (3σ) was 6.7 mBq/L (equal to 1.0 ng/L) (sample inlet: a PFA concentric nebulizer and a cyclonic spray chamber). In addition, the signal stabilities were confirmed under the flowing O 2 -CO 2 mixed gas for an hour in the presence of 1.0 Bq/L of 129 I or blank solution (3% TMAH only), as shown in Fig. 5. The intensity of 129 I was maintained within