Precise Measurement of Tellurium Isotope Ratios in Terrestrial Standards Using a Multiple Collector Inductively Coupled Plasma Mass Spectrometry.

Precise tellurium (Te) isotope ratio measurement using mass spectrometry is a challenging task for many decades. In this paper, Te isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC–ICP–MS) in terrestrial Te standards have been reported. Newly developed Faraday cup with 1012 Ω resistor is used to measure low abundance 120Te, whereas the 1011 Ω resistor is used to measure other Te isotopes. The relative standard deviation obtained for Te isotope ratio measurement by Faraday cups of 120Te/128Te [0.002907(05)], 122Te/128Te [0.079646(10)], 123Te/128Te [0.027850(07)], 125Te/128Te [0.221988(09)], 126Te/128Te [0.592202(20)], and 130Te/128Te [1.076277(30)] were 0.140%, 0.014%, 0.026%, 0.005%, 0.004%, and 0.004%, respectively. The measured isotope ratio results are compared with previous results obtained by thermal ionization mass spectrometry (TIMS), negative thermal ionization mass spectrometry (N–TIMS), and MC–ICP–MS, showing an improvement in the precision about one order of magnitude for 120Te/128Te ratio. The present study shows better precision for Te isotope ratios compared to earlier studies.


Introduction
The origin of elements suggests that a number of different nucleosynthesis processes produce the present-day abundances of the elements. It is assumed that the products of the nucleosynthetic processes have been injected into our solar system from stars and supernovae, a mixture of debris together with intergalactic hydrogen and helium. The basic processes such as rapid (r), slow (s) neutron-capture processes, and proton (p) capture processes are involved in heavy-element synthesis. The r-and p-process elements are formed in different environments compared to the s-process. Therefore, earlier researchers proposed the possibility of inhomogeneity or mixing may have occurred between the process, during, or after the period of supernova injection. The isotopic heterogeneity in terrestrial and meteoritic samples are either produced by isotopic fractionation or by incomplete mixing of material with different nucleosynthesis histories during the formation of the solar system [1]. Cameron [2] suggested that accurate isotopic composition measurement on heavy elements with even atomic numbers reveal the anomalies between p-, r-, and s-process nuclides. The elements which have a comparable abundance from p-and s-process are ideal for checking Cameron's hypothesis. Tellurium (Te) is an ideal element to check the hypothesis. It has eight naturally occurring isotopes with a mass number 120 (0.10% nominal natural abundances), 122 (2.60%), 123 (0.91%), 124 (4.82%), 125 (7.14%), 126 (19.0%), 128 (31.7%), and 130 (33.8%), exhibiting a large number of oxidation states (−2, 0, +4, +6). Te is unique; it is the only element that possesses one p-process isotope of 120 Te, three s-process isotopes of 122 Te, 123 Te, 124 Te, and r-process isotopes of 128 Te, 130 Te. The 125 Te, 126 Te isotopes Molecules 2020, 25, 1956 2 of 14 are produced by both s-and r-processes. The 126 Te is formed by the beta decay of 126 Sn with a half-life of 234,500 years. This decay system is also used as a possible chronometer to understand the processes which occurred during the early solar system [3,4].
Analysis of Te isotopic composition is primarily difficult due to its high first ionization potential (~9 eV), which decreases analytical sensitivity. Several researchers have attempted various methods to measure Te isotopic composition for the past five decades. Initially, Te isotope ratios were measured using thermal ionization mass spectrometry (TIMS) with Re filaments [5][6][7]. The low ionization efficiency, relatively low natural abundances, and high sample amount restricted the development of Te isotope measurement using TIMS. In 1992, Wachsmann and Heumann [8] investigated the formation of negative ions for Se and Te isotopes, as higher electron affinity will produce high negative thermal ions. Thus, Te isotopic composition measurement using negative TIMS with an Re double filament and Ba(OH) 2 activator achieved a better precision by a factor of 10 [8]. In 2014, Fukami and Yokoyama measured Te isotope ratios using N-TIMS, achieving a good sensitivity and uncertainties of less than 0.01%. However, N-TIMS requires a significantly higher concentration of Te than that is required for ICP-MS measurements [9,10]. Subsequently, multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) paired with a desolvating nebulizer is used to achieve high improvement in accuracy and precision for the Te isotope ratio measurement, due to the ionization of a greater portion of Te atoms [11][12][13][14][15][16][17].
MC-ICP-MS is a good option to improve the precision of the isotope ratio measurement. The key drawback of MC-ICP-MS is the use of relatively low sensitivity Faraday cup detectors compared to the ion counting systems. Some of the MC-ICP-MS are equipped with a multiple ion counting system. Due to the high intensity of ions, they are easily damaged by false operations. To achieve the high sensitivity, Faraday cups equipped with 10 12 Ω resistors in the feedback loop are used [18][19][20]. The new Faraday cup with a highly resistive 10 12 Ω resistor can enhance the signal intensity and background noise by approximately 10 and three times, respectively. It is very useful for determining a small number of samples or signals [21,22]. The application of the new 10 12 Ω resistor has enabled both high sensitivity and precision. The hydride generation (HG) technique with MC-ICP-MS for the Te isotopic measurement, significantly improves ionization efficiency, analytical sensitivity, and limits potential isobaric interferences [23,24].
In this contribution, the analytical performance of MC-ICP-MS with a desolvating nebulizer sample introduction system employing the new Faraday cup of 10 12 Ω resistors are used for the determination of low abundance 120 Te. To demonstrate the applicability of this method Te isotope ratios were measured on terrestrial Te standards from different parts of the world, to detect any isotope heterogeneity or incomplete mixing present among the terrestrial standards.

Isotope Ratios of Te in a Reagent Standard
The Kanto chemical Te standard solution was used to evaluate the instrument protocol and demonstrate significant improvements in the exemption of isobaric interferences during MC-ICP-MS measurement. Te isotope ratios of Te standard measured on seven different dates from October 2019 to January 2020 are summarized in Table 1 and analytical uncertainties are expressed in two mean standard deviations (2σ m ).
The stability of Te isotope ratio measurements is checked by one single measurement, which consists of 10 blocks, each block representing the mean of 20 individual ratios ( Figure 1). The typical instrument sensitivity was 50 and 700 V per µg·mL −1 (total Te) under wet and dry plasma modes, respectively. The background signals were higher for dry plasma compared to the wet plasma. We evaluated the external reproducibility (two standard deviations) of the δ 126/128 Te measurement using laboratory Kanto Te standard in wet and dry plasma modes and are found as 0.034% and 0.018% , respectively ( Figure 2). Similarly, four other terrestrial Te standards were measured in both plasma modes. It shows the reproducibility of average δ 126/128 Te was 0.03 ± 0.02 and 0.01 ± 0.02 for wet and dry plasma modes, respectively. Thus, illustrating that dry plasma mode of Te measurement gives better reproducibility compared to the wet plasma mode. Uncertainties are expressed in two sigma mean (2σ m ). All the isotopes are normalized to 124 Te/ 128 Te = 0.14853 using the exponential law.
In this study, 126 Te/ 128 Te and 130 Te/ 128 Te ratios were measured using two different normalizations and are shown in Figure 3. The Te isotope ratios measured here are in good agreement with previously measured Te isotope ratios using different mass spectrometry instruments [5,6,8,[10][11][12]23] (Table 3). However, there is a slight disagreement among data sets reported for 130 Te/ 128 Te ratios [17,23]. The measured 130 Te/ 128 Te isotope ratio was 1.076277 ± 57 and a little higher than previous studies (Table  3), but they are within the analytical error (95% confidence interval) except for the data of Fukami and Yokoyama [10] and Lee and Halliday [11] (Figure 3). The higher 130 Te/ 128 Te isotope ratio was earlier reported by correction of Xe interference with Te isotopes ( 124 Te, 126 Te, 128 Te, 130 Te) during measurement [12,23]. It was noticed that the 126 Te/ 128 Te ratio was in good agreement with earlier studies, especially compared with similar normalizations ( Figure 3). Especially, the introduction of a 10 12 Ω resistor for measuring 120 Te ions reproduced better results of one order of magnitude for the 120 Te/ 128 Te isotope ratio compared to the previous studies ( Figure 4). The measured 120 Te/ 128 Te and other Te isotope ratios in this study are within the analytical error range as shown in Table 3 (Brennecka et al. [23], Fukami and Yokoyama [10], Fehr et al. [12], Lee and Halliday [11], De Laeter [6], Wachsmann and Heumann [8], and Smith and De Laeter [5]). However, Loss et al. [7] reported all Te isotope ratios consistently lower values except the 130 Te/ 128 Te isotope ratio which could be due to the mass discrimination correction factor.  The typical instrument sensitivity was 50 and 700 V per µ g·mL −1 (total Te) under wet and dry plasma modes, respectively. The background signals were higher for dry plasma compared to the wet plasma. We evaluated the external reproducibility (two standard deviations) of the δ 126/128 Te measurement using laboratory Kanto Te standard in wet and dry plasma modes, which were found as 0.034‰ and 0.018‰, respectively ( Figure 2). Similarly, four other terrestrial Te standards were measured in both plasma modes. They show the reproducibility of average δ 126/128 Te was 0.03 ± 0.02 and 0.01 ± 0.02 for wet and dry plasma modes, respectively, illustrating that the dry plasma mode of Te measurement gives better reproducibility compared to the wet plasma mode. reported all Te isotope ratios consistently lower values except the 130 Te/ 128 Te isotope ratio which could be due to the mass discrimination correction factor.  . Figure 4. Data comparison between this study and recent studies of the Te isotope ratio of 120 Te/ 128 Te ratio normalization with 124 Te/ 128 Te ratio.

Stable Isotope Variation of Different Terrestrial Standards
The reproducibility is expressed in terms of delta (δ) notation. The δ X/128 Te values were calculated based on Equation (1) relative to the mean of Te isotope ratios of laboratory Te standard measured on seven different dates. X stands for Te isotopes (120, 122, 123, 125, 126, 130).

Stable Isotope Variation of Different Terrestrial Standards
The reproducibility is expressed in terms of delta (δ) notation. The δ X/128 Te values were calculated based on Equation (1) relative to the mean of Te isotope ratios of laboratory Te standard measured on seven different dates. X stands for Te isotopes (120,122,123,125,126,130).
The standard value, R standard , corresponds to the mean of laboratory standard Te isotope ratios, where R Sample is 120 Te/ 128 Te, 122 Te/ 128 Te, 123 Te/ 128 Te, 125 Te/ 128 Te, 126 Te/ 128 Te, and 130 Te/ 128 Te ratio individually measured in terrestrial Te standard.
Using the above-explained method, we measured Te isotope ratios of four terrestrial atomic absorption standards, Wako Chemical, Aldrich Chemical, Johnson Matthey and Spex CertiPerp. The Te isotope ratios of four terrestrial Te standards were measured six times on different dates from October 2019 to January 2020 and the mean values were compiled and given in Table 2. The analytical uncertainties are expressed in two standard deviations (2SD) to check the credibility of our measurements. Figure 5 shows the reproducibility of Wako Chemical, Aldrich Chemical, Johnson Matthey, and Spex CertiPerp which are in the range from -0.01 ± 0.06 to 0.04 ± 0.06 for δ 126/128 Te. There was no isotopic variation observed among the terrestrial Te standards with respect to laboratory Te standard for δ 126/128 Te. This implies that the atomic adsorption standard solutions measured in this current study from different parts of the world show similar isotope compositions and they do not show any anomaly for δ 126/128 Te. It implicates that the isotopes of Te are homogeneous in nature.

Precision and Reproducibility
The long-term reproducibility was evaluated from the data measured over a period of six There was no isotopic variation observed among the terrestrial Te standards with respect to laboratory Te standard for δ 126/128 Te. This implies that the atomic adsorption standard solutions measured in this current study from different parts of the world show similar isotope compositions Molecules 2020, 25,1956 7 of 14 and they do not show any anomaly for δ 126/128 Te. It implicates that the isotopes of Te are homogeneous in nature.

Precision and Reproducibility
The long-term reproducibility was evaluated from the data measured over a period of six months from October 2019 to January 2020. The reproducibility (2SD) observed for the Te measurement are 3.24% , 0.13% , 0.74% , 0.02% , 0.01% and 0.02% for δ 120 Te/ 128 Te, δ 122 Te/ 128 Te, δ 123 Te/ 128 Te, δ 125 Te/ 128 Te, δ 126 Te/ 128 Te, and δ 130 Te/ 128 Te for dry plasma mode, respectively. The significant difference between the measured Te isotope ratio among the Te standards with respect to laboratory standard results is checked, taking into account the measured results and their uncertainties. In this process, the absolute difference (AD) of the measured values are compared to the uncertainty of the absolute difference (U AD ) [25]. The AD and U AD are calculated by the following equation (Equations (2) and (3)). ppm, and 3 ppm, respectively. The internal and external precisions are almost similar.

Comparison with Previous Studies
Previously, many researchers have used terrestrial Te standards as an in-house standard for their studies using different mass spectrometric techniques. Therefore, we compared our present Te isotope ratio results with previously measured results to detect any variations among the isotope ratios measured through time. The data sets are compiled and given along with our present result in Table 3. The reproducibility of δ 120/128 Te, δ 123/128 Te, δ 125/128 Te, δ 126/128 Te, and δ 130/128 Te of the present study was compared with the previous studies for comparison.
The δ 120 / 128 Te plot in a range of −3.82-4.09% , δ 123/128 Te in range of −0.55-2.25% , δ 126/128 Te in range of −0.01-0.10% , and δ 130/128 Te in range of −0.30-0.02% ( Figure 6). The δ 126/128 Te matches with previous studies and δ 130/128 Te shows a slight deviation compared to previous studies, but the deviation is less and within the analytical error. δ 120/128 Te and δ 123/128 Te show a relatively smaller deviation compared to the previous studies. The 120 Te and 123 Te have a direct interference with Sn and Sb. Therefore, using a special resistor (10 12 Ω) for measuring 120 Te and 118 Sn and correcting for those interfering isotopes on 120 Te and 123 Te leads to the achievement of precise isotope ratio results in this study. The other reason could be either due to their very low natural abundance or because of two different nucleosynthesis processes such as p-process and s-process. However, the deviation of δ 120/128 Te and δ 123/128 Te among standard solutions is still unclear. Uncertainties are expressed in 2SD (two standard deviations).
Molecules 2020, 25, 1956 9 of 14 and within the analytical error. δ Te and δ Te show a relatively smaller deviation compared to the previous studies. The 120 Te and 123 Te have a direct interference with Sn and Sb. Therefore, using a special resistor (10 12 Ω) for measuring 120 Te and 118 Sn and correcting for those interfering isotopes on 120 Te and 123 Te leads to the achievement of precise isotope ratio results in this study. The other reason could be either due to their very low natural abundance or because of two different nucleosynthesis processes such as p-process and s-process. However, the deviation of δ 120/128 Te and δ 123/128 Te among standard solutions is still unclear.

Instrumentation
Te isotope ratio measurements were performed using a Nu Plasma 3D (Nu Instruments Ltd., Wrexham, UK) MC-ICP-MS at National Institute for Quantum and Radiological Sciences (QST), in low-resolution mode using nickel sampler cone and nickel wide-angle skimmer cone. MC-ICP-MS consisted of fixed 21 collectors; among these, 16 were Faraday cups and 5 were Daly collectors ( Table  5). The L6 and L7 Faraday cups were equipped with a fixed 10 12 Ω resistor. Ax, H2, H3, H4, H5, H6, H7, and H8 Faraday cups were equipped with fixed 10 11 Ω resistors. H1, L1, L2, L3, L4, and L5 Faraday cups were equipped with a switchable dual resistors setup of 10 11 Ω and 10 12 Ω, respectively. The switching of the resistor for Faraday cups was performed using the Nu Plasma software. Gain

Instrumentation
Te isotope ratio measurements were performed using a Nu Plasma 3D (Nu Instruments Ltd., Wrexham, UK) MC-ICP-MS at National Institute for Quantum and Radiological Sciences (QST), in low-resolution mode using nickel sampler cone and nickel wide-angle skimmer cone. MC-ICP-MS consisted of fixed 21 collectors; among these, 16 were Faraday cups and 5 were Daly collectors ( Table  5). The L6 and L7 Faraday cups were equipped with a fixed 10 12 Ω resistor. Ax, H2, H3, H4, H5, H6, H7, and H8 Faraday cups were equipped with fixed 10 11 Ω resistors. H1, L1, L2, L3, L4, and L5 Faraday cups were equipped with a switchable dual resistors setup of 10 11 Ω and 10 12 Ω, respectively. The switching of the resistor for Faraday cups was performed using the Nu Plasma software. Gain calibration of each Faraday cup resistors was performed every day using the software operated standard procedure by supplying 4 V. It is consecutively applied for all the Faraday cups with 10 11 Ω to 10 12 Ω resistors. The gain values were within 20 ppm error range.
Recently, researchers have used Faraday detection systems with amplifiers equipped with 10 12 Ω and 10 13 Ω resistors in the feedback loop for the measurements of low ion intensities [19,20]. The 10 12 Ω resistor provides 10 times higher voltage compared to 10 11 Ω resistor for a given ion beam, whereas the Johnson-Nyquist (JN) noise level of the resistor increases by a factor of √ 10. Therefore theoretical 3-fold improvement in the signal to noise ratio is expected but, in practice, this ratio improves only by a factor of two.
The response time of the 10 12 Ω resistors takes time for the signal on the resistors to reach their baseline value, which is slower compared to the 10 11 Ω resistors. The curve fitting decay time for each resistor was determined by measuring the signal after closing the second line of sight valve using the Nu Plasma software. The tau corrections were carried out for the 10 12 Ω resistor. Therefore, a relatively slow response does not affect the data quality of MC-ICP-MS analytical performance.

Te Isotope Ratio Measurement Protocol of MC-ICP-MS.
The operating conditions of MC-ICP-MS for Te isotope ratio measurements are given in Table 4. Prior to each measurement session, the instrument was carefully tuned to maximum Te signal intensity by adjusting the torch position, Ar gas flow, lens voltages, and deflector settings. 126 Te isotope mass is selected as monitoring isotope for peak centering before the measurement of each block. On-peak background subtraction was performed using beam intensities that were measured by introducing 2% HNO 3 before sample measurement. Kanto chemical Te standard was used as laboratory standard and Te isotope ratio measurements were carried out in both wet plasma and dry plasma mode. In wet plasma mode, the measurement was performed using a 200 ng·mL −1 Te concentration solution. Sample solutions were introduced into the plasma through a micromist nebulizer with an aspiration rate of 200 µL·min −1 . After each measurement, a washout was performed with 2% HNO 3 for 15 min. The isotope ratio measurement comprises 10 blocks of 20 cycles with an 8 s integration time. One block consists of 20 cycles (isotope ratios).
In dry plasma mode, Aridus-3 desolvating nebulizer (Teledyne CETAC Technologies, USA) was used where 10 ng·mL −1 Te concentration solution was aspirated at a rate of 100 µL·min −1 . The Ar sweep gas flow rate was typically 4.25 L·min −1 with a nebulizer gas flow rate of 0.9 L·min −1 . No additional N 2 gas was used for the measurement. The other instrumental operating settings are identical to wet plasma measurements. The washout was approximately 30 min using a 2% HNO 3 . Te standard with a concentration of 10 ng·mL −1 and 200 ng·mL −1 was analyzed before and after every terrestrial Te standard to confirm the absence of drift in dry and wet plasma mode of measurement, respectively. The amount of Te consumed in one measurement was approximately 27 ng and 1067 ng for dry and wet plasma mode, respectively.
The Te isotope ratios (all ratios) obtained from the Faraday cups were corrected for mass fractionation by normalizing with 124 Te/ 128 Te = 0.14853 [11] and 125 Te/ 128 Te = 0.22204 [1] using exponential fractional law. However, the previously published isotope results have used different normalization ratios to correct for internal normalization. 124 Te/ 128 Te normalization was carried out for TIMS and N-TIMS measurements. Due to ionization potential, isobaric interference of Sn and Xe could be controlled [6,10]. The direct comparison of data with the previously published data is possible [10][11][12]15,23]. Since there is no isobaric interference of Xe or Sn with 125 Te, it was preferred in MC-ICP-MS studies. The internal normalization of 125 Te/ 128 Te was selected for mass bias correction, because the mass-bias-corrected 126 Te/ 128 Te and 130 Te/ 128 Te isotope data do not require any Sn correction, which is very much important for the sample containing a significant amount of Sn.
The possible isotopic interference for Te isotopes was listed and the interference corrections for Te isotopes were carried out by the earlier workers [11,12,23]. A similar correction procedure was followed for the Te isobaric correction in this study. The major isobaric interferences on Te isotopes can be generated from Sn, Sb, and Xe. Therefore, the ion currents of 118 Sn + , 121 Sb + , and 129 Xe + were measured during the measurement to do interference corrections. Xenon is present in the Ar plasma gas and the typically reported yield of Xe/Te ratios are 4-9 × 10 −4 [12]. The correction of 118 Sn on 120 Te, 122 Te, 121 Sb on 123 Te, 124 Te, and 129 Xe on 124 Te, 126 Te, 128 Te, and 130 Te isotopes were applied online during the measurement using exponential law. The intensities of 118 Sn, 121 Sb, and 129 Xe were 2.3 × 10 −16 A, 2.05 × 10 −15 A, and 6.28 × 10 −16 A for wet plasma condition, and 1.5 × 10 −16 A, 8.8 × 10 −16 A, and 9.6 × 10 −15 A for dry plasma, respectively. The typical production rate of tellurium hydride (TeH) was 2 × 10 −4 and <1 × 10 −5 for wet and dry plasma, respectively. The TeH correction was not necessary for the dry plasma condition because of its low production rate [15]. As it is already mentioned by the earlier studies that Te standard solutions typically displayed Sn/Te concentration ratios of about 1-3 × 10 −4 and Sn/Te ratios of up to 1.5 × 10 −3 could be tolerated for samples without compromising the accuracy of the analytical results [12].

Conclusions
Analytical performance of MC-ICP-MS method employing a Faraday cup with a new 10 12 Ω resistor was examined. Use of a special resistor (10 12 Ω) Faraday cup for measuring 120 Te shows 10 times detection efficiency and improves the precision by one order of magnitude for 120 Te/ 128 Te isotope ratio compared to previous studies. The reproducibility (2SD) observed for the Te isotope ratio measurement using dry plasma conditions are 3.24% , 0.13% , 0.74% , 0.02% , 0.01% , and 0.02% for δ 120/128 Te, δ 122/128 Te, δ 123/128 Te, δ 125/128 Te, δ 126/128 Te, and δ 130/128 Te, respectively. Furthermore, the Te terrestrial atomic adsorption standards used in this study from different parts of the world are isotopically homogeneous and do not show any incomplete mixing in nature. In the future, the environmental and meteorite samples can be measured precisely using this protocol of MC-ICP-MS, which shows better precision compared to the previous studies.