Radioactive Isotope Detection Using Triple Quadrupole ICP-MS

Certified for Thermo Scientific™ iCAP™ TQe ICP-MS APPLICATION NOTE 44409 Accurate determination of 129I in environmental samples using triple quadrupole ICP-MS Author: Simon Lofthouse; Thermo Fisher Scientific, Hemel, Hempstead, UK Keywords: 129I, interference removal, oxygen, reaction gases, triple quadrupole ICP-MS Goal To demonstrate the ultratrace determination of 129I in aqueous samples using triple quadrupole ICP-MS. To demonstrate that triple quadrupole technology can be used in oxygen reaction mode to attenuate potential interference problems associated with the measurement of 129I. Introduction The most common regulatory methods for environmental analysis focus on highly toxic contaminants such as As, Cd, Hg or Pb, which are essentially present as stable isotopes only. In some cases however, the analysis of radioactive isotopes (or nuclides) is required to accurately assess potential hazards for example after incidents associated with power generation using nuclear technology. The most commonly observed product nuclides of nuclear fission of uranium are found in the mass range of about 90 amu and 140 amu. Due to their short half-lives, many direct fragments undergo further decay reactions and therefore, the isotopes causing the most concern when released to the environment are 90Sr, 133Cs or radioactive nuclides of iodine. Iodine is especially critical as it is incorporated into the thyroid gland of humans and can cause severe damage. For iodine, two isotopes are of most concern: 131I, which has a half-life of only 8 days and decays relatively quickly and 129I, which is a long lived radionuclide with a half-life of 15.7 million years. The presence of radioactive iodine in the environment is primarily due to emissions from nuclear fuel reprocessing plants alongside potential nuclear weapons testing and nuclear power plant accidents. 2 Concentrations of 129I in the environment are normally very low and this, together with the relatively low sensitivity of iodine, can make it challenging to measure accurately by ICP-MS. Most of the radionuclides of interest can be determined by radiometric techniques, but in most cases, long exposure times are required to ensure sufficient counting statistics. The opportunity to apply ICP-MS can therefore help to decrease sample turnaround times dramatically and speed up official responses to potential sources of contamination. ICP-MS is prone to interferences, as are all analytical techniques, which may bias the results. Whereas socalled polyatomic interferences are of most concern in the majority of applications and can be effectively eliminated using kinetic energy discrimination (KED), there are other interferences that require the use of reactive gases and subsequent chemical reactions in the cell. However, the selection of a reactive gas is not straightforward and may lead to adverse effects for other analyte ions. Triple quadrupole instrumentation typically employs a quadrupole before the collision/reaction cell in order to allow only specific ions into the cell, while eliminating others. This pre-filtering of the ion beam prior to the cell provides improved interference removal by reducing the range of side of reactions that can occur in the cell as well as allowing selective reaction chemistries to be applied using reactive cell gases. Challenges that need to be overcome for the measurement of 129I include the potential of 129Xe isobaric interference from impurities in the argon gas, potentially high levels of naturally occurring 127I in the samples and possible polyatomic interferences from 127IH2 +, Mo+ dioxide and dioxide hydride species and Sn+ hydroxide species. Single quadrupole ICP-MS using oxygen in the QCell can be used to significantly reduce isobaric interference from 129Xe+. However, potential problems with peak tailing from 127I, 127IH2 and polyatomic interferences from Mo+ and Sn+ based species still remain. To overcome these issues triple quadrupole ICP-MS operating in O2 reaction mode has been investigated to demonstrate the potential of this technique for measuring ultratrace levels of 129I in aqueous environmental samples. Instrumentation All measurements were performed using a Thermo Scientific™ iCAP™ TQ ICP-MS. The instrument was operated using the Thermo Scientific™ Qtegra™ Intelligent Scientific Data Solution (ISDS) Software and was tuned daily using a mixed standard solution. The typical operation parameters are shown in Table 1. Parameter Value RF Power 1,550 W Nebulizer Gas Flow 1.12 L·min-1 Interface High sensitivity QCell Settings SQ-O2 TQ-O2 Gas Flow 0.6 mL·min-1 0.6 mL·min-1 CR Bias -7.2 V - 7.2 V Q3 Bias -12 V -12 V Table 1. Typical instrumental parameters In order to select the appropriate instrument settings (e.g. choice of reaction gas and the mass to be transmitted to the analyser quadrupole (Q3)), the Reaction Finder Method Development Assistant was used. Reaction Finder allows analytical methods to be set up without prior detailed knowledge of potential reaction pathways caused by other components in the sample. For this work O2 was automatically selected by Reaction Finder for the analysis of 129I in TQ mode. A second measurement mode using oxygen and single quadrupole operation (SQ-O2 ) was also manually added to compare differences between these two modes alongside single quadrupole no gas (SQ-STD) and single quadrupole KED (SQ-KED) modes. The scan settings for the first quadrupole were optimized using intelligent Mass Selection (iMS). As can be seen from the results, iMS was able to remove all ions potentially causing interferences whilst assuring optimized transmission of the analyte. The use of high resolution (less than 1 amu) on Q1 was found to not be required for this application. 3 Signal on m/z 129 [cps] BEC 129I [ng·L-1] Result Autotune (0.34 mL·min-1) 30 1.3 Optimized O2 Flow (0.6 mL·min-1) 2 0.13 Mechanisms of iodine interference removal using O2 reaction gas with triple quadrupole ICP-MS In O2 mode, the 129Xe interference on 129I is extremely efficiently removed by a charge exchange reaction between Xe+ and O2 , while 129I doesn’t react. The presence of Mo and Sn in the samples leads to the formation of MoO2 +, SnO+ and SnOH+ product ions in the cell, which interfere with 129I. In single quadrupole mode, there is no way to prevent Mo and Sn entering the cell together with Xe and I, so MoO2 +, SnO+ and SnOH+ interference on 129I can’t be prevented. In fact, if Mo and Sn are present in the samples, SQ-O2 leads to worse results than can be achieved in either standard or He KED modes. In contrast, triple quadrupole operation allows Mo and Sn in the samples to be completely rejected from the ion beam before it enters the cell, preventing them from forming additional interference on 129I. MoO2 +, SnO+ and SnOH+ are present in the plasma, but at very low levels (particularly SnO+) and furthermore, MoO2 + reacts further with O2 in the cell to form higher Mo oxides which don’t interfere with 129I. Reaction of SnO and SnOH to SnO2 and SnO2 H also appears to occur, ensuring complete removal of these low-level interferences. The mechanism of interference removal on 129I using a triple quadrupole ICP-MS system is shown in Figure 1. 129I + 129I + O2 +, 127I +, 97Mo16O3 + 129Xe+ + O2 → 129Xe + O2 + 127IH2 + + O2 → 127I + + H2O 97Mo16O2 + + O2 → 97Mo16O3 + + O Rejected and cannot form 112Sn17O+, 112Sn16O1 H+ or 97Mo16O2 + at m/z 129 129Xe+, 127I 1 H2 +, 97Mo16O2 + 112Sn+, 97Mo+, 97Mo16O+ Figure 1. Interference removal mechanism for analysis of 129I Sample preparation 129I calibration solution solutions were prepared in a 0.5% v/v tetra methyl ammonium hydroxide (TMAH) solution rather than dilute HNO3, to prevent the severe memory effects caused by oxidation of iodide to iodine by nitric acid. The same matrix was used for the preparation of all blanks, calibration solutions, samples and rinse solutions for iodine analysis by ICP-MS. Interference solutions containing I, Mo and Sn were also prepared in the same matrix to demonstrate the interference removal and highlight the potential problems associated with typical environmental samples. A sample matrix intended to simulate the typical composition of soil or surface water samples was prepared containing Ca, Mg, Na (at 20 mg·L-1) and Al, Fe, Mn, Cu (at 5 mg·L-1), with Mo and Sn (at 1 mg·L-1) also added to provide sources of interference on 129I. Aliquots of this solution were spiked with known concentrations of 129I to assess spike recovery. Results The iCAP TQ ICP-MS parameters were initially optimized for maximum sensitivity using the supplied tune solution. The QCell oxygen gas flow was then optimized to maximize removal of the 129Xe+ interference whilst maintaining sensitivity for iodine. As can be seen in Table 2, the oxygen gas flow rate determined by the instrument’s autotune procedure still showed a low signal at m/z 129, which resulted in a slightly elevated blank equivalent concentration (BEC, the apparent concentration of 129I caused by the background signal) for 129I. In order to reduce the BEC further (whilst compromising signal to noise ratio as little as possible), the High gas flow setting was used with an offset value of 0.3 mL·min-1 in addition to the default flow rate determined by the autotune routine, giving a total gas flow rate of approximately 0.6 mL·min-1. This flow rate provided optimum 127I signal intensity versus the 129I background resulting in an ultralow BEC for 129I (Table 2). Table 2. Signal intensities at autotuned and optimised O2 flow 4 Figure 2 shows the calibration curves for 129I in triple quadrupole mode with oxygen cell gas (TQ-O2 ) and single quadrupole mode with oxygen (SQ-O2 ) after optimisation of the O2 flow. Figure 2. Calibration curves for 129I (0–100 ng·L-1). On mass mode using TQ-O2 (top) and SQ-O2 (bottom). Table 3 contains the calculated Instrument Detection Limits (IDL). These values are shown for both the single quadrupole in standard mode, He KED and O2 in single quad mode and triple quadrupole mode using O2 as the reaction gas. Table 3. Comparison between SQ and TQ modes for the analysis of 129I Component SQ-STD SQ-KED SQ-O2 TQ-O2 BEC (ng·L-1) 40 30 2.2 0.13 IDL (ng·L-1) 2 2 0.2 0.001 Interference test solution SQ-O2 apparent concentration at 129I TQ-O2 apparent concentration at 129I Improvement factor 20 µg·L-1 127I 45 ng·L-1 0.09 ng·L-1 500 1 mg·L-1 Mo 560 ng·L-1 0.07 ng·L-1 8,000 1 mg·L-1 Sn 30 ng·L-1 0.07 ng·L-1 430 In a second step to further elucidate the potential of using triple quadrupole ICP-MS for this analysis, solutions containing elements directly interfering on m/z 129 were analysed. In many environmental samples, like soils or plant digests, it is not unusual that higher amounts of Mo and Sn can be found. Table 4. Interference effects in SQ-O2 and TQ-O2 modes for the analysis of 129I Figure 3. Full mass scan performed on a sample containing 20 µg L-1 natural iodine (127I) in SQ-O2 mode. Xe based interferences are eliminated, but 127I 1 H2 + causes a significant false positive signal on m/z 129. Table 4 shows the apparent concentrations at 129I obtained when analyzing solutions containing 20 µg·L-1I, 1 mg·L-1 Mo and 1 mg·L-1 Sn respectively. The comparison between SQ-O2 and TQ-O2 modes shows clearly that by controlling the ions that enter the QCell using Q1, the background associated from interfering species is significantly reduced. As can be seen in Figure 3 and Table 4 respectively, even moderate amounts of natural iodine (127I) lead to a significant contribution to the signal observed at m/z 129 due to the formation of 127I 1 H2 + in single quad mode if 127I is not removed before the ion beam enters the collision/ reaction cell. Utilizing triple quadrupole functionality attenuates IH+ and IH2 + and eliminates problems with 97Mo16O2 +, 112Sn17O+ and 112Sn16O1 H+ ions that arise from the Mo and Sn present in typical environmental samples. Finally, the spike recovery results for a range of 129I concentrations in the simulated environmental matrix are shown in Table 5. These results show that 129I can be quantitatively measured at ultratrace concentrations despite the presence of potential interferences using TQ-O2 mode on the iCAP TQ ICP-MS. For Research Use Only. Not for use in diagnostic procedures. © 2020 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific and its subsidiaries. This information is presented as an example of the capabilities of Thermo Fisher Scientific products. It is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others. Specifications, terms and pricing are subject to change. Not all products are available in all countries. Please consult your local sales representatives for details. Not all products are available in all countries. Please consult your local sales representatives for details. AN44409-EN 0620C Find out more at thermofisher.com/TQ-ICP-MS Sample Spike recovery in TQ-O2 [%] 10 ng·L-1 105 20 ng·L-1 98 50 ng·L-1 97 100 ng·L-1 101 Table 5. 129I spike recoveries Conclusion Triple quadrupole technology can help to tackle difficult sample matrices, where significant contributions from matrix components lead to the formation of severe polyatomic interferences on the target analyte. In this work, the background signal on 129I caused by the presence of 129Xe and the formation of Mo+ and Sn+ species was significantly attenuated while maintaining high detection sensitivity, leading to ultralow detection limits for 129I. Excellent spike recoveries were also obtained for 129I in simulated environmental matrix samples.