Vol. 2 (2013), S0021

Mass SPectrometrY DOI: 10.5702/massspectrometry.S0021

Extractive Electrospray Ionization Mass Spectrometry for Uranium Chemistry Studies Huanwen Chen,* Mingbiao Luo, Saijin Xiao, Yongzhong Ouyang, Yafei Zhou, and Xinglei Zhang Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China Institute of Technology, Nanchang, P.R. China

Uranium chemistry is of sustainable interest. Breakthroughs in uranium studies make serious impacts in many fields including chemistry, physics, energy and biology, because uranium plays fundamentally important roles in these fields. Substantial progress in uranium studies normally requires development of novel analytical tools. Extractive electrospray ionization mass spectrometry (EESI-MS) is a sensitive technique for trace detection of various analytes in complex matrices without sample pretreatment. EESI-MS shows excellent performance for monitoring uranium species in various samples at trace levels since it tolerates extremely complex matrices. Therefore, EESI-MS is an alternative choice for studying uranium chemistry, especially when it combines ion trap mass spectrometry. In this presentation, three examples of EESI-MS for uranium chemistry studies will be given, illustrating the potential applications of EESI-MS in synthesis chemistry, physical chemistry, and analytical chemistry of uranium. More specifically, case studies on EESI-MS for synthesis and characterization of novel uranium species, and for rapid detection of uranium and its isotope ratios in various samples will be presented. Novel methods based on EESI-MS for screening uranium ores and radioactive iodine-129 will be presented. Keywords: EESI, uranium, chemistry, rapid detection (Received August 31, 2012; Accepted October 23, 2012)

INTRODUCTION Uranium, as the heaviest naturally occurring element, displays a remarkable structure and chemical diversity in nature. The chemistry of uranium has attracted increased attentions due to its fundamental importance in the fields of chemistry, physical, energy, environments, and biology, etc.1–3) Substantial progress in uranium studies normally requires development of novel analytical tools. Extractive electrospray ionization mass spectrometry (EESI-MS) has been successfully established for sensitive detection of trace various analytes present in complex matrixes without any pretreatment.4–10) EESI-MS shows excellent performance for monitoring uranium species in various samples such as natural water and organic solutions with mineral at trace levels owing to its good tolerance for extremely complex matrices.4–8) Ion suppression is significantly reduced in EESI by distributing the matrixes over a relatively wide section in a large three-dimensional space. Therefore, EESI-MS is an alternative choice for studying uranium chemistry, especially when it combines ion trap mass spectrometry. In this review, examples of EESI-MS for uranium chemistry studies will be given, illustrating the potential applications of EESI-MS in synthesis chemistry, physical chemistry, and analytical chemistry of uranium. Therefore, case studies on EESI-MS for synthesis and characterization of novel uranium species is first introduced, and then the examples * Correspondence to: Huanwen Chen, Jigngxi Key Laboratory for Mass Spectrometry and Instrumentation, East China Institute of Technology, Nanchang, P.R. China, e-mail: [email protected]

© 2013 The Mass Spectrometry Society of Japan

on EESI-MS for rapid detection of uranium and its isotope ratios in various samples will be presented. Finally, novel methods based on EESI-MS for screening radioactive iodine-129 will be presented.

SYNTHESIS AND CHARACTERIZATION OF UO5− SPECIES A novel uranium anion species (UO5−) was synthesized in the gas phase through the reaction of the oxygen with a negatively-charged organic uranyl species (HOUO2OH), prepared with uranyl acetate using extractive electrospray ionization multiple-stage tandem mass spectrometry. The molecular structure of UO5− was confirmed using collision-induced dissociation (CID) mass spectrometry experiments. The most probable reaction pathway for the generation of uranium anion species in the gas phase was proposed with the aid of the EESI-MS, as shown in Scheme 1. The electronic structure and bonding properties of UO5− was characterized and explored by combination of EESI-MS (see Fig. 1) and Density Functional Theory calculation with the TPSSh/TZVP method. Results showed that the new synthesized compounds was stable and reasonable, indicating that the EESI-MS is a powerful tool for preparation and characterization of such new uranium compounds.

RAPID DETECTION OF URANIUM AND ITS ISOTOPE RATIO (235U/238U) Rapid detection and analysis of radioactive species plays an important role in different areas including public safety, environmental science, nuclear geo-science, nuclear plant management and maintenance, etc. Radionuclide forms Page 1 of 5 (page number not for citation purpose)

EESI-MS For UranIum CHemIstrY StuDIes

Vol. 2 (2013), S0021

Scheme 1.

Fig. 1.

Proposed reaction pathways for the generation of UO5−.

The EESI-MS/MS spectra for the preparation of UO5−.

Fig. 3.

Fig. 2.

EESI-MS for sensitive detection of radioactive species: (a) schematic diagram of the EESI source and (b) schematic illustration of ion formation in the EESI process. (Reprinted with permission from ref. 11.)

various complexes in the natural environment and chemical industry, and thus sensitive and reliable methods are required to detect them both qualitatively and quantitatively. EESI-MS has been established for rapid detection of radioactive inorganic species in natural water samples (Fig. 2). Under the experimental conditions, uranyl species of uranyl acetate in various natural water samples including river water, lake water, and well water were rapidly detected using multiple-stage EESI mass spectrometry (Fig. 3). More © 2013 The Mass Spectrometry Society of Japan

The intrinsic fragmentation pathways of uranyl acetate (m/z 447) was validated using 235U isotope labeled compounds: (a) MS2 spectrum of m/z 447; (b) MS3 spectrum of m/z 388; (c) MS 4 spectrum of m/z 346; (d) MS2 spectrum of m/z 444; (e) MS3 spectrum of m/z 385; and (f) MS4 spectrum of 343. (Reprinted with permission from ref. 11.)

importantly, negatively charged uranyl acetate (m/z 447) undergoes characteristic fragmentation in the gas phase to produce UO3− as the final product (Fig. 4), providing the fundamental chemistry for specific detection of uranyl acetate species in complex matrixes by EESI without sample pretreatment. The results show that EESI-MS, a typical method initially developed for organic compound analysis, has promising perspectives for real time, online monitoring of inorganic species such as uranyl species in natural water samples. Uranium mainly exists as 238U and 235U in nature, and is an important radioactive material with wide military and Page 2 of 5

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Fig. 4.

Fig. 5.

Vol. 2 (2013), S0021

Fragmentation pathways and fragments of [UO2(CH3COO)3]− in the gas phase. (Reprinted with permission from ref. 11.)

EESI-MS mass spectrum for the detection of uranyl nitrate. The inset shows the mass spectrum of isotopic peaks. (Reprinted with permission from ref. 12.)

civilian applications. The most common isotopes of uranium are 238U and 235U, although 234U is also of low abundance. This ratio is a natural rule marking the artificial nuclear activities which change the isotope ratio of uranium. Uranium is of great importance as a nuclear fuel. Driven by the ambitious motivation to solve the energy problem, the globally rapid development of nuclear power plants has demanded large amounts of uranium, which sparked the advancement of uranium mining and scientific research on uranium. The concentration of uranium isotopes in uranium ores is a principal indicator measuring the quality of ores. Therefore, the analytical and separation technologies of uranium are of significance in the uranium industry. Without minimal sample preparation, EESI-MS has been used for quantitative detection of uranium isotopic ratio (235U/238U) in uranyl nitrate solution samples (Fig. 5), which were prepared from various samples including natural water samples, uranium ore samples and soil samples using nitric acid (Fig. 5). The relative errors for uranium isotope measurements were in the range of 0.21–0.25% and the corresponding relative standard deviation (RSD) values were 1.54–1.81% for the ore samples. These findings demonstrated that EESI-MS can detect uranium isotopes in water, soils © 2013 The Mass Spectrometry Society of Japan

and uranium ores with minimal sample pretreatment. The fast detection of uranium isotopes shows potential applications of EESI-MS in nuclear research laboratories and the nuclear energy industry.

SCREENING RADIOACTIVE IODINE-129 Detection of radioactive iodine-129 (129I) is of essential significance to public safety. Radioactive iodine species, particularly 129I, are hardly detected above the safety limit in nature such as natural water, ambient air and other related samples. Once nuclear leak or nuclear explosion takes place, a huge amount of radioactive matters including 129I are likely to be released into our living environments. In such an emergency case, rapid quantification of trace 129I in ambient air or water samples is of paramount importance. EESI-MS is proposed for high throughput sensitive detection of 129I in various matrix samples. For the relative simple matrix samples such as nuclear industry water, gaseous aerosol, etc., the base peak at m/z 129 (129I−) can be recorded and adopted directly for quantification of 129I as long as no fragments were observed by collision-induced dissociation (CID) of the base peak at m/z 129 (129I−). By following this Page 3 of 5

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Fig. 6.

Vol. 2 (2013), S0021

Tandem mass spectra of uranyl nitrate prepared from uranium ore samples. a) EESI-MS2 experiments targeting the peak at m/z 456; b) EESI-MS3 experiments targeting the peak at m/z 410, and the inset shows the CID experiments on m/z 366; c) EESI-MS 4 experiments targeting the peak at m/z 348; d) EESI-MS/MS experiments targeting the peak at m/z 453; e) EESI-MS3 experiments targeting the peak at m/z 407, and the inset shows the CID experiments on m/z 363; f) EESI-MS 4 experiments targeting the peak at m/z 345. (Reprinted with permission from ref. 12.)

strategy, the calibration curve showed a good linearity for 129 I detection within a relatively wide concentration range of 6.5–208 ppt (R= 0.996). The limit of 129I detection (LOD, n=13) was 1.38 ppt, with the corresponding relative standard deviation (RSD) values of 4.4–6.8% (n=13). The recovery of 97.5–102.4% was achieved at a spiking concentration level of 22.8 ppt (n=9) in water samples. KI + I2 = KI3

(1)

For the complex matrix samples such as natural river water, ground water, etc., tandem MS is required to exclude false positive signals. In this case, therefore, KI water solution is simply used for effective trapping molecular iodine (as described by Eq. (1)) to form I3− ions, which produce monovalent anion (I−) by CID. Therefore, the actual samples such as river water, aerosol particles, etc. were directly analyzed by EESI-MSn, providing LOD at low ppt range for direct detection of 129I. The experimental data show promising utility of EESI-MS in public safety evaluations, especially in case of nuclear leakage or nuclear explosions. Furthermore, the same strategy has been applied to screening iodine in pharmaceutical preparations, resulting in satisfactory analytical results.

Acknowledgements This work is jointly supported by NNSFC (No.21175019) and MOST (No.2011QY14015008) of China.

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Vol. 2 (2013), S0021 13) L. Yu, B. Hu, M. Luo, X. Zhang, H. Chen. Density functional study on decomposition reaction of organic uranyl acetate in gas phase. Chem. J. Chin. Univ. 12: 2460–2463, 2009.

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Extractive electrospray ionization mass spectrometry for uranium chemistry studies.

Uranium chemistry is of sustainable interest. Breakthroughs in uranium studies make serious impacts in many fields including chemistry, physics, energ...
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