Arab Journal of Nuclear Science and Applications, 46(2), ( ) 2013

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1 Arab Journal of Nuclear Science and Applications, 46(2), ( ) 2013 Theoretical Study of Rare Earths Interferences in Inductively Coupled Plasma-Mass Spectrometry A. M. Rashad Central Laboratory for Elemental and Isotopic Analysis, NRC, Atomic Energy Authority.P. O. Box , Egypt Received: 20/6/2012 Accepted: 1/8/2012 ABSTRACT A theoretical estimate of the thermal properties of polyatomic ions which can interfere with rare earth elements during the analysis process using the inductively coupled plasma mass spectrometer ICP-MS were studied. The ionization energies and heats of formation of the selected species at 0K and 298 K of the expected reactions to occur under the effect of plasma conditions in ICP-MS, are calculated using Gaussian 03 software. Key words: Rare earth elements/ Gaussian 03 /Interferences / ICP-MS. INTRODUCTION The rare earth elements (REEs) represent a group of elements from La to Lu which show very similar physical and chemical properties. Due to small differences in ionic radii fractionation between light REEs (LREEs, La to Gd) and heavy REEs (HREEs, Tb to Lu) within the geochemical cycle may occur (1). The knowledge of the REEs distribution in rocks, minerals and solutions is of great importance for most geochemical studies and especially the occurrence or lack of Eu and/or Ce anomalies may provide valuable information (2,3). Inductively coupled plasma-mass spectrometry (ICPMS) is a multi-element analytical method with detection limits which are, for the REE, superior to most conventional techniques. The REEs are in the mass region from 139(La) to 175(Lu), which are almost free of matrix interferences and method blank intensities. For each element in the REEs series at least one isotope exists which is free from isobaric interferences. In comparison with other techniques like instrumental neutron activation analysis (INAA) or inductively coupled plasma-atomic emission spectrometry (ICP-AES), ICP-MS usually allows the determination of all REEs in geological materials without any chemical preconcentration. The problem of REEs oxide ion interferences has been a prominent feature in many publications in the field of ICP-MS (4-12). The effect of operating conditions on the formation of REEs oxide ions has been discussed in detail by Longerich et al. (4). Vaughan and Horlick (5) investigated the influence of sampler and skimmer orifice size on the yield of analyte oxide signals. Alternative sample introduction systems have been applied by Shibata et al. (6) and Kawabata et al. (7). However, these publications do not allow to estimate the degree of potential interferences in the determination of REEs in geological samples under routine conditions. In conjunction with geochemical applications oxide ion formation of Ba and REEs is mentioned by many authors (10,12) as a potential, but seldom serious problem. This paper is an attempt to quantify interferences by polyatomic ions in the determination of REEs concentrations in geological samples under fixed instrument settings during measurement. Thermochemical data of these various species are necessary to understand their formation and dissociation in the mass spectrometer under plasma conditions and to provide theoretical estimates of the structures and thermal properties of the species which can interfere with rare earth elements as shown in (table 1). 180

2 Table (1): Polyatomic interferences of rare earth elements in ICP-MS Isotope Abundance interference Reference Isotope Abundance interference Reference 144 Nd Ru 16 O Ho Sm 16 O Nd Ru 16 O Gd La 16 O Nd Ru 16 O Gd Ba 19 F + Pr 16 O + 14, Nd Ru 16 O Dy Sm 16 O + 14, Sm Ru 16 O Er Nd 16 O Sm 16 O Sm Ru 16 O Er Eu 16 O Sm Ru 16 O Eu Ba 16 O + 14, Sm Ru 16 O Eu Ba 16 O + 17, Sm Ru 16 O Tm Eu 16 O Sm Ru 16 O Tb Nd 16 O + 14, Yb Gd 16 O Lu Tb 16 O + 14, Yb Gd 16 O + 14 RESULTS AND DISCUSSION The calculations in this work have been carried out using the GAUSSIAN 03. The level of theory used in this work is MP3 and the basis set is CEP-4G. Heats of formation (Δ fh ) at 0K were obtained from the computed atomization energies of the individual species combined with the experimental gas phase 0 K heats of formation of the constituent atoms. The Δ fh 0 and temperature correction to enthalpy, H H 0, is obtained using the theoretical correction calculated for the species of interest and the experimental contribution for the constituent elements (20-22) as shown in table (2). In table (3) the obtaind data from the program using method MP3 with basis set CEP-4G are ploted at the lowest energy states which depend on the multiplicity, charges, refers to the selected charge such that, if the charge of the molecule is (0), this means that the molecule is neutral but if the charge is (1), this means that the molecule is ionized and m is the multiplicity, which is the quantification of the amount of unpaired electron spin. Table (2): Δ fh 0 atom and ΔH(298-0) element for the constituent atoms Atom (ΔfH 0 atom) ΔH(298-0) element Sm Nd Eu Ba La Pr Tb Ru Gd O

3 Table (3): Summary of the H 0, H 298 and G 298 obtained at MP3/CEP-4G Species Charge multiplicity H 0 H 298 G 298 O F SmO NdO EuO BaO LaO BaF PrO TbO RuO GdO RuO O F SmO NdO EuO BaO LaO BaF PrO TbO RuO GdO RuO The thermochemistry data which used for the calculations of ionization energy and the heats of formation of the selected species are shown in table 3, where H 0 refers to sum of electronic and zeropoint energies and H 298, G 298 refer to sum of electronic and thermal enthalpies and sum of electronic and thermal free energies, respectively (23). The calculations were done at different multiplicities to get the lowest valus of energy which means mor stable case. The ionization energies of the ions of interest are calculated in (ev). Computed ionization energies of rare earths oxides as shown in (table 4) are found to be between ev (SmO + ) and ev (EuO + ). These calculations help us to decrease the probability of interfering ions in rare earths ICP/MS analysis. The heats of formations of the selected molecules at 0K and 298 K are calculated as shown in (table 5). 182

4 Table (4): Ionization energies of some selected species using MP3/CEP-4G method of calculations Species IE (ev) Species IE (ev) O PrO F TbO SmO RuO NdO NdO EuO EuO BaO GdO LaO RuO BaF Table (5): Heats of formation of some selected species at 0K and 298 K. Δ Species Ch., m fh 0 Δ fh 298 Δ Species Ch.,m fh 0 Δ fh 298 (kj/mol) (kj/mol) (kj/mol) (kj/mol) SmO 0, SmO+ 1, NdO 0, NdO+ 1, EuO 0, EuO+ 1, BaO 0, BaO+ 1, LaO 0, LaO+ 1, BaF 0, BaF+ 1, PrO 0, PrO+ 1, TbO 0, TbO+ 1, RuO 0, RuO+ 1, GdO 0, GdO+ 1, RuO3 0, RuO3+ 1, Where m is the multiplicity and Ch is the charge. The heats of formation of SmO, NdO, EuO, BaO, LaO, BaF, PrO, SmO +, NdO +, EuO +, BaO +, LaO +, BaF + and PrO + are calculated in table

5 14 12 G O2 F2 10 IE (ev) A + = SmO+ NdO+ EuO+ BaO+ LaO+ BaF+ PrO+ TbO+ RuO+ NdO+ EuO+ GdO+ RuO3+ Species Figure 1: Plot of adiabatic ionization energies IE(A) (vertical bars) and IE(G) (horizontal lines). Charge exchange reaction A+ + G A + G+ (A+ = analyte ion; G = reactant gas) is occurring if IE(G) < IE(A). (Figure 1) reports the calculated IE values of species relevant to ions A+ as bar graphs, and the corresponding values relevant to gas G as horizontal lines. This figure allows a simple comparison between values as taken by these fundamental thermochemical parameters. They also offer a support for predicting occurrence of a given reaction between ions A+ and gas G. Such the reaction (A + + G A + G + (A + = analyte ion; G = reactant gas) is occurring if IE(G) < IE(A). Finally the obtained heats of formation of the rare earth elements oxides in the molecular case and in the ionic case and the obtained ionization energies of it can help us to improve and development of the method of analysis of rare earths elements using ICP-MS technique by choosing the suitable parameters to decrease the probability of interferences. This by decreasing the RF power, and also by using the cold plasma condition to make the ionization energy of the plasma doesn't reach the ionization energy of O 2 and F 2 to prevent these interferences. REFERENCES (1) P. Möller, G. Morteani, In: Augustithis SS; Theophrastus, Athens, pp , (1983) (2) B. R. Lipin, G. A. McKay; Washington, pp 348,(1989) (3) P. Henderson; pp 510, (1984) (4) H. P. Longerich, B. J. Fryer, D. F. Strong and C. J. Kantipuly, Spectrochim Acta 42B : 75, (1987) (5) M. A. Vaughan and G. Horlick, Spectrochim Acta 45B : 1289, (1990) (6) N. Shibata, N. Fudagawa and M. Kubota Spectrochim Acta 48B : 1127,(1993). (7) K. Kawabata, Y. Kishi, O. Kawaguchi, Y. Watanabe and Y. Inoue, Anal Chem , (1991) (8) N. Shibata, N. Fudagawa and M. Kubota, Anal Chem; 63 : 636, (1991) (9) M. A. Vaughan, G. Horlick Appl Spectrosc; 40:434, (1986) (10) E. H. Evans, J. J. Giglio, J Anal At Spectrom; 8 : 1, (1993) (11) K. E. Jarvis, Chem Geol; 68:31, (1988). (12) K. E. Jarvis, J Anal At Spectrom; 4:563, (1989) 184

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