Precise measurement of tellurium isotope ratios in terrestrial standards using a multiple collector inductively coupled plasma mass spectrometry

3.1. Standards and Reagents

Five commercially available terrestrial Te standards have been used for the measurement of Te isotope ratios. Atomic absorption spectrometry standard solutions were obtained from Kanto Chemical (Lot no.103G9089, Kanto Chemical Co. Inc., Tokyo, Japan), Wako Chemical (Lot. No. JCF9875 Wako pure chemical industries, Ltd., Osaka, Japan), Aldrich Chemical (Lot no.12329LR, Aldrich Chemical Company, Inc., Milwaukee, WI, USA), Spex CertiPerp (Lot No. 6-250TE, Spex CertiPerp Company, Inc., Metuchen, NJ, USA), and Johnson Matthey Chemicals (Lot No. 801141G, Royston, UK). All standard solutions were concentration at 1000 µg·mL−1. The dilution was carried out in a class 100 laminar flow hood using deionized water (>18 MΩ·cm−1) produced with a Milli-Q system (Merck Millipore, Burlington, MA, USA) and Tamapure AA-100 ultrapure (Tama Chemicals, Kawasaki, Japan) HNO3.

3.2. 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 1012 Ω resistor. Ax, H2, H3, H4, H5, H6, H7, and H8 Faraday cups were equipped with fixed 1011 Ω resistors. H1, L1, L2, L3, L4, and L5 Faraday cups were equipped with a switchable dual resistors setup of 1011 Ω and 1012 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 1011 Ω to 1012 Ω resistors. The gain values were within 20 ppm error range.
Recently, researchers have used Faraday detection systems with amplifiers equipped with 1012 Ω and 1013 Ω resistors in the feedback loop for the measurements of low ion intensities [19,20]. The 1012 Ω resistor provides 10 times higher voltage compared to 1011 Ω resistor for a given ion beam, whereas the Johnson–Nyquist (JN) noise level of the resistor increases by a factor of 10−−√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 1012 Ω resistors takes time for the signal on the resistors to reach their baseline value, which is slower compared to the 1011 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 1012 Ω resistor. Therefore, a relatively slow response does not affect the data quality of MC–ICP–MS analytical performance.

3.3. 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. 126Te 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% HNO3 before sample measurement.
Table 4. MC–ICP–MS operating and measurement conditions.
RF Power 1300 W
Acceleration Potential (V) 6000
Sampler cone Ni cone
Skimmer cone Ni wide-angle cone
Resolution Low
Cool gas 13.4 L·min−1
Auxiliary gas 0.90 L·min−1
Wet Plasma Dry Plasma
Sample Conventional Spray chamber Desolvating Nebulizer
Nebulizer Micromist, 200 µL·min−1 C-Flow PFA, 100 µL·min−1
Nebulizer gas 1.14 L·min−1 0.90 L·min−1
Sweep Ar Gas —- 4.2 L·min−1
N2 gas —- 0 L·min−1
Sample Concentration 200 ng·mL−1 10 ng·mL−1
Typical Sensitivity 50 V per µg·mL1 700 V per µg·mL−1
Washout time 10–15 min 30 min
130Te Beam intensity 1.60 V 1.46 V
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% HNO3 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 N2 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% HNO3. 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.
There were 11 Faraday cups used for the simultaneous collection of ion beams. The L5 Faraday cup resistor setup was changed from 1011 Ω to 1012 Ω resistor. The measurement of Te isotope ratios was carried out using Faraday cups with a mixed resistor of 1011 Ω and 1012 Ω in a static cup configuration mode. The 118Sn, 120Te, were collected using L6 and L5 1012 Ω resistor Faraday cups, and 121Sb, 122Te, 123Te, 124Te, 125Te, 126Te, 128Te, 129Xe, and 130Te were detected using L4, L3, L2, L1, Ax, H1, H3, H4, and H5, 1011 Ω resistor Faraday cups, respectively (Table 5).
Table 5. Configuration of collectors in MC–ICP–MS.
Detectors L7 D4 D3 D2 D1 L6 D0 L5 L4 L3 L2 L1 Ax H1 H2 H3 H4 H5 H6 H7 H8
Monitored Isotopes 120Te 122Te 123Te 124Te 125Te 126Te 128Te 130Te
Isobaric Interference 118Sn 121Sb 129Xe
Faraday Cup resistors 1012 Ω 1012 Ω 1012 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω 1011 Ω
L refers to lower side Faraday cups, H refers to higher side Faraday cups with respect to the axial (Ax) Faraday cup, and D stands for Daly detectors.
The Te isotope ratios (all ratios) obtained from the Faraday cups were corrected for mass fractionation by normalizing with 124Te/128Te = 0.14853 [11] and 125Te/128Te = 0.22204 [1] using exponential fractional law. However, the previously published isotope results have used different normalization ratios to correct for internal normalization. 124Te/128Te 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 125Te, it was preferred in MC–ICP–MS studies. The internal normalization of 125Te/128Te was selected for mass bias correction, because the mass-bias-corrected 126Te/128Te and 130Te/128Te 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 118Sn+121Sb+, and 129Xe+ 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 118Sn on 120Te, 122Te, 121Sb on 123Te, 124Te, and 129Xe on 124Te, 126Te, 128Te, and 130Te isotopes were applied online during the measurement using exponential law. The intensities of 118Sn, 121Sb, and 129Xe 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].

4. Conclusions

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