Isotope Measurement of Uranium at Ultratrace Levels Using Multicollector Inductively Coupled Plasma Mass Spectrometry

Mass spectrometric analysis was carried out using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the precise and accurate determination of the isotope ratios of ultratrace levels of uranium dissolved in 3% HNO3. We used the certified reference material (CRM) 112-A at a trace level of 100 pg/mL for the uranium isotopic measurement. Multiple collectors were utilized for the simultaneous measurement of uranium isotopes to reduce the signal uncertainty due to variations in the ion beam intensity over time. Mass bias correction was applied to the measured U isotopes to improve the precision and accuracy. Furthermore, elemental standard solution with certified values of platinum, iridium, gold, and thallium dissolved in 3% HNO3 were analyzed to investigate the formation rates of the polyatomic ions of Ir 40Ar+, Pt40Ar+, Tl40Ar+, Au40Ar+ for the concentration range of 50–400 pg/mL. Those polyatomic ions have mass-to-charge ratios in the 230–245 m/z region that it would contribute to the increase of background intensity of uranium, thorium, plutonium, and americium isotopes. The effect of the polyatomic ion interference on uranium isotope measurement has been estimated.


Introduction
Over the past few decades, inductively coupled plasma mass spectrometry (ICP-MS) has been increasingly used for ultratrace elemental analysis owing to its advanced capabilities such as high sensitivity and short measuring time. 1−3 ICP-MS is superior to thermal ionization mass spectrometry (TIMS) in that it provides higher ionization efficiencies for a given sample size; this allows ICP-MS tools to be used widely for uranium(U) isotope ratio measurements at ultratrace analyte levels. 4,5The ICP-MS technique is based on the concept that plasma ions, which indicate the presence of an analyte in a sample, are driven by intense electromagnetic fields, and are then directed toward their own intrinsic paths by electric and magnetic sector analyzers. 6,7The ion currents measured by ion collector systems provide valuable information on the isotopic compositions in a sample solution.For ultratrace levels of the analytes of interest, the simultaneous measurement of different isotopes with a multicollector (MC) system is preferable to avoid the uncertainty caused by fluctuation of the ion beam intensity over time. 5,8It is due to the fact that the variations in signal intensity during the measurement period have an almost equal impact on the entire detector.Previous analytical studies demonstrated that MC-ICP-MS is a convenient and accurate approach for measuring U isotope ratios at ultratrace levels of analyte. 7,9owever, the mass bias effect, in which the transmission efficiencies of the ion beam depend on the ion's mass, poses a challenge to an accurate measurement of ion currents for the analytes of interest. 4Hence, mass bias correction is necessary to accurately determine the isotope ratios of a sample.The matrix elements in plasma are susceptible to a combination with the most abundant argon and atmospheric gases, which results in the production of polyatomic ions such as .The interference effect due to these polyatomic ions affects the precision and accuracy of isotopic measurements. 10The polyatomic ions of Ir 40 Ar + , Pt 40 Ar + , Tl 40 Ar + , Au 40 Ar + whose mass-to-charge ratios are around the 230-245 m/z region would contribute to the increase in background intensity of actinide isotopes such as 233 U, 236 U, 243 Am, and 231 Th.Consequently, it might cause the overesti-mation of these actinide isotopes at ultratrace levels.
In this study, the U isotope ratio was measured using CRM 112-A at a trace level of 100 pg/mL obtained from the US Department of Energy, New Brunswick.The standard reference material (SRM) U-005 was used as a bracketing standard for mass bias correction of the measured U isotope ratios from CRM 112-A.In addition, the production rates of the polyatomic ions were investigated using elemental standard solutions of Ir, Au, Pt, and Tl diluted in 3% HNO 3 .

Experiment
An MC-ICP-MS (NEPTUNE Plus, Thermo Scientific Inc.) instrument equipped with multi-collectors, an array of moveable detectors with nine Faraday cups, and five multiion counters, was used to measure the ultratrace U isotopes.The ion counters consist of three secondary electron multipliers (SEMs) and two compact discrete dynodes (CDDs).The ion counters were positioned on the low mass side.We employed the ion counters for measuring minor isotopes 234 U and 235 U of CRM 112-A.The instrument was operated in low-resolution mode (m/∆m = 400).The sample introduction system (CETAC Aridus II) on the ICP-MS instrument contained a membrane desolvator and a spray chamber coupled with a perfluoroalkoxy (PFA) nebulizer with a nominal uptake rate of 50 µL/min.The nebulized aerosol of argon and a sample diluted with 3% HNO 3 were fed continuously into the spray chamber heated at 110 o C, which maintained the sample in the vapor phase.The membrane desolvator was operated at 160 o C. The argon gas swept the solvent vapor into the porous wall of the heated membrane desolvator.Consequently, the sample aerosols had reduced the amount of solvent vapor, and they were aspirated into the argon plasma.This resulted in the enhancement of the sensitivity of the analyte isotopes.The argon plasma was driven into the argon gas flow by intense radio-frequency fields (27.12 MHz).We used high-purity nitric acid (Merck, Ltd.) and Milli-Q de-ionized water (~18 MΩ•cm) to dilute the reagents in our experiments.The background intensities of 3% HNO 3 measured by the ion counter were typically 233 U, 236 U, 234 U < 1cps, 235 U < 50 cps, and 238 U < 3000 cps.The tuning windows for controlling the torch position, the amount of instrumental gas, and the ion lenses were routinely adjusted to optimize the sensitivity and stabilize the ion beam intensity.We used the SRM U-005 solution (5 pg/mL) and collected its 238 U beam intensity in a Faraday cup to tune the MC-ICP-MS system.The typical operating conditions are given in Table 1.

Polyatomic ion interference effect
Polyatomic ions were measured using elemental standard  194, 195, 196, and 198. 12 This interference might lead to the overestimation of the amounts of these U isotopes at ultratrace levels.For instance, natural U has no 236 U isotope.The signal intensity of 196 Pt 40 Ar and its standard deviation measured at 400 pg/ mL were (1.270 ± 0.024) × 10 3 cps.This corresponds to ~54.5 fg/mL of 236 U.Under our measurement conditions, the average values of intensity ratios of 194  We measured the signal intensity ratios of 197 Au 40 Ar/ 197 Au to be (6.981± 0.099) × 10 −4 .Typical environmental samples contain various matrix elements, and hence, it is necessary to evaluate the influence of the possible interfering polyatomic ions on uranium isotope measurement at ultra trace level.

Uranium isotope ratio measurement
U isotope ratios were measured using the standard solution (CRM 112-A, 100 pg/mL) with the isotopic composition of natural U. Mass bias correction was carried out for the measured isotope ratios to enhance the accuracy and precision of the measurements.The mass bias effect in ICP-MS is caused by instrumental mass discrimination, which is largely attributed to the mass-dependent transmission efficiencies in the plasma/vacuum interface region. 13,14We used the following equation to correct the isotope ratios measured from CRM 112-A: 4,14 where Here, R corr and R obs are the corrected and measured isotope ratios of the CRM 112-A sample, respectively, f is the mass bias factor, and r true and r obs are the true and measured isotope ratios of the reference solution.The SRM U-005 solution, whose U concentration is similar to that of CRM 112-A, was used as a reference solution to derive the mass bias factor f. Prior to the isotope ratio measurement, the cup configuration was spatially fitted, and the gains of the Faraday cups and ion counters were then calibrated for the simultaneous measurement of multiple U isotopes.The cup configuration for the simultaneous measurement of U isotopes is shown in Table 2.In data acquisition, a time of 60 s was typically needed, as the signal became stabilized after the sample introduction. 8The integration time, i.e., the length of time used to integrate the ion current signals, was set to 4.194 s.The idle time needed for the magnetic field and Faraday cup detectors to settle was set to 3.0 s.Each measurement was performed over 40 acquisition cycles.For this measurement, a nebulizer with an uptake rate of 50 µL/min was used, and the concentration was 100 pg/ mL.The total amount of U consumed per measurement was thus deduced to be ~29 pg. Figure 2 shows the corrected U isotope ratios of CRM 112-A.The certified values are within the error bars of the measured isotope ratios.In the sampling sequence, 112-A and U-005 were measured in turn.The isotope ratios of 112-A were then corrected for each measurement.Table 3 presents the average values of the 234 U/ 238 U and 235 U/ 238 U ratios from 15 measurements.According to reference 16, the expanded uncertainty values of the certified values of 234 U/ 238 U and 235 U/ 238 U in Table 3 are 8.2 × 10 −8 and 4.0 × 10 −6 .The expanded uncertainty is expressed as k•u c , where k is the coverage factor (k = 2) and u c is the combined standard uncertainty.The expanded uncertainties for the measured isotope ratios were calculated according to the Joint Committee for Guides in Metrology (JCGM) Guide to the Expression of Uncertainty in Measurement. 16

Conclusion
The U isotope ratio was measured using CRM 112-A at ultratrace levels, and mass bias correction was performed.Without mass bias correction, the relative errors in the measured 234 U/ 238 U and 235 U/ 238 U ratios to the certified values were 1.67% and 1.52%, respectively.On the other hand, the relative errors in the measured 234 U/ 238 U and 235 U/ 238 U ratios to the certified values were reduced to be 0.34% and 0.14% after mass bias correction as shown in Table 3.Furthermore, we investigated the effect of polyatomic ions that may interfere with the U isotopic measurement.Since natural U contains no 236 U and very little of the abundant 234 U, the polyatomic ions 196 Pt 40 Ar + and 194 Pt 40 Ar + were shown to affect the accuracy of the isotopic measurement of ultratrace levels of uranium.It is important to estimate the formation rates of those polyatomic ions with mass-tocharge ratios in the 234-238 m/z region, which might lead to the overestimation of the amounts of the U isotopes.Table 3. U isotope measurement for CRM 112-A.In the "Measured" results, the average values and standard deviation of Pt 40 Ar/ 194 Pt and 196 Pt 40 Ar/ 196 Pt in the concentration range 50-400 pg/mL were (4.278 ± 0.158) × 10 −4 and (4.220 ± 0.171) × 10 −4 .Ir forms the polyatomic ions 191 Ir 40 Ar and 193 Ir 40 Ar with m/z = 231 and 233, which might interfere with the measurements of 231 Th and 233 U. 12 The 191 Ir 40 Ar/ 191 Ir and 193 Ir 40 Ar/ 193 Ir intensity ratios were (1.186 ± 0.022) × 10 −4 and (1.181 ± 0.013) × 10 −4 .Tl exists as 203 Tl and 205 Tl, and the polyatomic ions 203 Tl 40 Ar + and 205 Tl 40 Ar + have the same mass numbers as 243 Am and 245 Am.The 203 Tl 40 Ar + / 203 Tl and 205 Tl 40 Ar + / 205 Tl intensity ratios were (1.474 ± 0.034) × 10 −5 and (1.473 ± 0.033) × 10 −5 . 197Au 40 Ar + ions are the same m/z as 237 Np.

Figure 2 .
Figure 2. (a) 234 U/ 238 U and (b) 235 U/ 238 U isotope ratios measured for CRM 112-A.The solid lines represent the certified values of the isotope ratios 234 U/ 238 U and 235 U/ 238 U.

Table 1 .
11,12ting parameters for MC-ICP-MSMass Spectrom.Lett.2012Vol.3,No.2, 54-57 solutions of Ir, Au, Pt, and Tl diluted in 3% HNO 3 (Anapex Co., Ltd.) to investigate the polyatomic ions that might affect the accuracy and precision of the isotopic measurement of actinide elements.The polyatomic interference arising from the cluster ions generated in the plasma source depends on the sample composition.Heavy matrix elements (Ir, Au, Pt, and Tl) combined with the most abundant atoms ( 40 Ar) in the plasma have mass-to-charge ratios in the 230-245 m/z region.11,12Consequently, the Ir 40 Ar, Pt 40 Ar, Au 40 Ar, and Tl 40 Ar polyatomic ions produced in the plasma might interfere with the measurement of the actinide elements (U, Pu, Am, and Th).Figure1shows the intensities and intensity ratios of the 196 Pt 40 Ar, 193 Ir 40 Ar, 197 Au 40 Ar and 203 Tl 40 Ar polyatomic ions for the concentration range 50-400 pg/mL.The signal intensities of the polyatomic ions grow steadily with the increase of the matrix (Ir, Au, Pt, and Tl) concentration.In contrast, the intensity ratios of Ir 40 Ar/ Ir, Pt 40 Ar/Pt, Tl 40 Ar/Tl and Au 40 Ar/Au tend to be independent of the matrix element concentration.Pt 40 Ar interferes with 234 U, 235 U, 236 U, and 238 U because Pt has isotopes with the mass numbers Figure 1.Squares represent the calibration curves of the poly-atomic ions 196 Pt 40 Ar, 193 Ir 40 Ar, 197 Au 40 Ar, and 205 Tl 40 Ar in the concentration range 50-400 pg/mL.Open circles represent 196 Pt 40 Ar/ 196 Pt, 193 Ir 40 Ar/ 193 Ir, 197 Au 40 Ar/ 197 Au, and 203 Tl 40 Ar/ 203 Tl ratios.

Table 2 .
Cup configuration designed for the simultaneous measurement of U isotopes.IC#2 and IC#3 refer to ion counters.L4 refers to the Faraday cup detector