UB RAS Institute of Metallurgy, Yekaterinburg, Amundsena 101, Russia. Ural Federal University, Yekaterinburg, Mira 19, Russia.

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1 SELECTION OF INTERNAL STANDARD FOR ICP-AES ANAL- YS OF ORES, CONCENTRATES AND SLAGS BY THERMO- DYNAMIC MODELING A. Mayorova 1, O. Evdokimova 1, N. Pechishcheva 1, K. Shunyaev 1, A. Shchepetkin 1, P. Zaytseva 2, A. Pupyshev 2 1 UB RAS Institute of Metallurgy, Yekaterinburg, Amundsena 101, Russia. 2 Ural Federal University, Yekaterinburg, Mira 19, Russia. shun@ural.ru ABSTRACT Theoretical method of internal standard selection for inductively coupled plasma atomic emission analysis was tested on two systems (solutions after sample preparation of a) iron and b) copper-molybdenum ores, concentrates, slags). It involved the thermodynamic modeling of process in plasma at spraying of the solution. The effectiveness of internal standards selected in the modeling was demonstrated experimentally - analysis error with their using decreased up to 2.5 times. INTRODUCTION Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is widely used in the analytical laboratories of metallurgical enterprises, as it allows to carry out the simultaneous multi-element analysis and possesses high reproducibility of results, large determining concentrations range and low detection limits (up to ng/ml). Nevertheless, as any other analysis method, ICP-AES is not entirely universal and has a number of constraints and the specific error sources including those relating the composition of the analyzed samples. The emission spectrum of metallurgical samples in inductively coupled plasma discharge is characterized by large number of lines, any component presenting in considerable amount may have the influence on their intensity. The heaviest effects on results of ICP-AES analysis are caused by easily ionized elements (Na, K, Li), that contained in reagent for fusion ores and slag samples. In the most cases the error sources are the differences in surface tension and/or viscosity of calibration solutions and solution of the samples, also fluctuation of operation parameters during the analysis, associated with the oscillations of plasma power, nebulizer flow rate, carrier gas (argon) flow rate, etc. The one of the popular methods of improving accuracy and reproducibility of the ICP- AES analysis results is the internal standardization, when intensity ratio of the analyte spectral line and the internal standard line is used as a signal. The element of internal standard () must be present in the same concentrations in the sample solutions, calibration solutions and blank solutions. Usually the internal standard is specially added. must meet the following requirements: its spectrum must have only a few spectral lines; its solution has to be stable; the chemical properties must be close to those of analytes. Selection of and its spectral lines is commonly made on the basis of similarity of the first ionization potential of the atoms and the excitation potential of the spectral line with similar characteristics of the analyte. 314

2 Experimental selection of the optimal internal standard requires a large amount of experimental data and researches. However, the costs of labor, time, reagents and money can be significantly reduced by theoretically researches carrying out using different methods of modeling. For practical purposes of atomic emission spectroscopy analysis such models are more appropriate that allow to set the operation parameters and initial chemical composition of the system, then obtain the concentrations of various particles in plasma as a result of the calculations and use them to calculate intensities of spectral lines of atoms and ions, for example, (1). The aim of the present work was evaluation using thermodynamic modeling the efficiency of internal standard selection for ICP-AES analysis a) of iron ores and slags main components, b) of rhenium in the copper and molybdenum ores and concentrates. THERMODYNAMIC MODELING For practical application of the method of thermodynamic equilibrium modeling it is necessary to select in non-equilibrium thermodynamic system the conditions that are close to thermodynamic equilibrium that will apply to them equilibrium calculations. It is thought that the deviation from the conditions of local thermodynamic equilibrium in the central channel of the torch P is not too significant. By taking pre-keeping out of the existence of thermodynamic equilibrium in this region, and using some of the average characteristics of the plasma (primarily temperature), you can simply describe the thermochemical processes of atomization, ionization and excitation of spectral lines. In this paper, we use the algorithm and the recommendations of the work (1), we calculated the equilibrium composition of the plasma at a variation of operation parameters of ICP-AES spectrometer (plasma temperature T, carrier gas (argon) flow rate V Ar and nebulizer flow rate V n ). For modeling we used TERRA software (2) calculating multi-component hightemperature equilibrium, which has an extensive database of thermochemical properties of individual substances. The calculations take into account all individual substances potentially presenting in the plasma discharge in analyzing model solution. We considered The composition of solution injected into the plasma discharge was numerically equal to mass flow rate of the components (g/min). T, V Ar и V n were similar to those typically used in the analysis. According to the results of thermodynamic modeling we found concentrations of atoms and ions of the considered elements, that allowed us to calculate the intensity of spectral lines of the analytes I analyte and internal standard elements I as follows: hp v Ag E I analyte( ) n exp( ) (1) Z ( T) k T where h p = 6, J s Planck's constant; k = 1, J/К Boltzmann constant; ν frequency of the spectral line, sec -1 ; с = 2, m/sec speed of light in vacuum; Т temperature, К; n - concentration of atoms (ions) in the plasma, m -3, obtained as a result of thermodynamic calculations; E - excitation potential of atomic 315

3 (ionic) line, J (3); A g - transition probability (4); Z(T) - partition function, which is calculated by formula: 2 3 T T T T T Z ( T) a b 3 c 3 d 3 e 3 f 3, (2) the coefficients a, b, c, d, e, f are from (1). In all the calculations it was assumed that the effectiveness of the ICP-AES spectrometer nebulizer is 2%. Three series of calculations were performed, in each of these the s of two operating parameters (T, V Ar, V n ) were fixed, and the third was varied. We plotted the calculated s of the line intensities of analytes I analyte depending on each of the variable parameters, approximated by linear dependencies and compared with similar dependencies for the ratio I analyte /I. The most effective are the internal standard, the using of which allows the best way to compensate the influence of T, V Ar и V n on the intensity of the analyte line (slope of I analyte /I =f(t, V Ar, V n ) is minimal). ESTIMATION OF INTERNAL STANDARDS EFFICIENCY FOR RHENIUM DETERMINATION IN ORES AND CONCENTRATES According to the procedure (5) to determine the rhenium concentration decomposition of molybdenum and copper ores and concentrates is carried out by sintering the samples with magnesium oxide and oxidative additive NaNO 3 ; obtained melts are leached, the solutions are acidified by HNO 3 and directed to ICP-AES. Therefore, when modeling we considered the system: Ar-H 2 O-HNO 3 -Re-NaNO 3 - MgO-. The concentration of rhenium was 3 mg/l, which corresponds to the average content of rhenium in typical samples analyzed in the analytical laboratory of IMET UB RAS. The concentration of the internal standard element was 3 mg/l, HNO 3-0,072 mg/l, NaNO 3-1,5 g/l, MgO - 2 g/l (typical concentrations contained in the sample solution after decomposition). Tantalum, ruthenium, germanium and gadolinium were considered as the possible elements of the internal standard (), as they have ionization potentials close to that of rhenium E ion (7.87 ev) and the excitation potentials of the lines close to that of rhenium line Re II nm (6.28 ev): Ta (7.88 ev) and Ta II nm (5.93 ev), Ru (7.36 ev) and Ru II nm (6.31 ev), Ge (7.88 ev) and Ge II nm (6.40 ev), Gd (6.14 ev) and Gd II nm (6.08 ev) correspondingly (3). The results of the calculation of the spectral lines intensities (normalized to the initial s of varied parameters) are presented in Table 1. Figure 1 shows the temperature dependencies of the intensity of Re II nm line and ratio I Re /I for all the studied internal standards. Dependencies of the intensity of Re II nm line and ratios I Re /I for all the studied internal standards on carrier gas flow rate and nebulizer flow rate are shown respectively in Fig. 2 and

4 Таble 1. The normalized intensities of the spectral lines at varying of operation parameters Variable parameter Temperature Т, К Carrier gas flow rate V Ar l/min Nebulizer flow rate, V n ml/min Fixed parameters V Ar = 0.8 l/min V n =1.5 ml/min Т=7000 К V n = 1.5 ml/min Т=7000 К V Ar = 0.8 l/min Variable parameter Normalized intensities of the spectral lines I Re I Re /I Gd I Re /I Ta I Re /I Ge I Re /I Ru I Re I normalized I Re /I Gd, I Re /I Ru I Re /I Ta T, K I Re /I Ge Fig. 1. Dependencies of normalized intensity I Re and ratios I Re /I Gd, I Re /I Ta, I Re /I Ge, I Re /I Ru on plasma temperature. V Ar = 0.8 l/min, V n = 1.5 ml/min. 317

5 Inormalized I Re /I Gd, I Re /I Ge, I Re /I Ta I Re /I Ru I Re V Ar, l/min Fig. 2. Dependencies of normalized intensity I Re and ratios I Re /I Gd, I Re /I Ta, I Re /I Ge, I Re /I Ru on carrier gas flow rate V Ar. Т = 7000 К, V n = 1.5 ml/min. 1.5 I normalized I Re I Re /I Ru V n, ml/min 318 I Re /I Gd, I Re /I Ge, I Re /I Ta Fig. 3. Dependencies of normalized intensity I Re and ratios I Re /I Gd, I Re /I Ta, I Re /I Ge, I Re /I Ru on nebulizer flow rate V n. Т = 7000 К, V Ar = 0.8 l/min. Table 1 and Fig. 1-3 show that the ratios I Re /I for all considered internal standards are much less affected by variations of the operation parameters than the intensity of the Re II nm line and all our selected elements could theoretically be used as internal standards for ICP-AES determination of rhenium. To quantify the theoretical choice of the internal standard for each of them sum of slope s b of linear plots I Re /I =f(t, V Ar, V n ). b =b T +b Ar +b n were calculated, where b T, b Ar, b n are respectively slopes of linear intensity dependence on the temperature, carrier gas flow rate and nebulizer flow rate (in Table 2).

6 Table 2. Slopes of linear dependencies I Re /I = const + bp, where P = T, V Ar, V n Slope Without Gd Ta Gе Ru b T b Ar b n b Table 2 shows that the smallest of b (which takes into account influence of the three operation parameters) is observed for ruthenium (line Ru II nm) as an internal standard. As for gadolinium, tantalum and germanium, these s differ slightly. The slopes of all the plots are less using internal standards than without using. So, application of all considered internal standards helps to compensate the operation parameters fluctuations. Experimental testing of applicability of internal standards in the determination of rhenium in copper-molybdenum ores and concentrates by ICP-AES was carried out. Following the procedure (5) rhenium concentration in certified reference materials 2889, 5910, 5914, 3587 was determined. The analysis was performed on a ICP-spectrometer «Optima 2100 DV» using the above internal standards, emission lines and following operation parameters: - plasma gas flow rate 15 l/min; - auxiliary gas flow rate 0.2 l/min; - carrier gas flow rate 0.8 l/min; - nebulizer flow rate 1.5 ml/min; - effectiveness of cross-flow pneumatic nebulizer - 2%; - Rf power 1,3 kw; - radial viewing, view height 15 mm. It was found that tantalum is inconvenient as internal standard because of need to add additional chemical agent (e.g., hydrofluoric acid, oxalic acid) to prevent precipitation of tantalum acids. Germanium spectral line (Ge II nm) is influenced by the matrix there is overlapping with line Fe II nm that makes impossible the use of germanium as the internal standard for rhenium determination in ore materials usually containing the iron. Using of ruthenium was abandoned due to its high cost. As for using gadolinium as an internal standard, selected line Gd II nm is free from matrix spectral overlapping. Presented in Table 3, the results of the determination of rhenium in the samples of ores and concentrates show that internal standardization has allowed to reduce error of the results. 319

7 Table 3. Results of rhenium determination in ore and concentrates samples, % Wt. Object of analysis 2889 Complex ore 5910 Concentrate of copper-molybdenum ore 5914 Concentrate of copper-molybdenum ore 3587 Molybdenum concentrate Certified of rhenium Found Without RSD, % N=5 Found With RSD, % N= ± ± ± ± Thus, the results of the thermodynamic calculations show the possibility of using Ta II nm, Ru II nm, Ge II nm, Gd II nm as internal standards for P-AES determination of rhenium in molybdenum concentrates by Re II nm line. A series of the using preference is Ru> Ge> Ta> Gd, but experimental verification shown that even with the using of gadolinium we can reduce the error of the analysis result. ESTIMATION OF INTERNAL STANDARDS EFFICIENCY FOR IRON ORES AND SLAGS ANALYS To determine Al 2 О 3, SiО 2, MnО, MgО, CaО, Fe content in iron ores, concentrates, slags by ICP-AES the samples have to be dissolved by fusing with mix of sodium carbonate and borax and leaching by hydrochloric acid (1:1) under heating. Therefore, at the modeling we considered a system: Ar-H 2 O-HСl- Al+Si+Mn+Mg+Ca+Fe-Na-. The concentration of HCl was 0.2 M; concentration of analyte - 20 mg/l, Na - 10 g/l, the internal standards (Sc and Y) - 2 mg/l. Excitation potentials of the used spectral lines of analytes and internal standards are presented in Table 4. The spectral lines were selected experimentally - they do not overlap the line of matrix components, the scatter of measurements results when using them is minimal. The results of the calculation of the spectral lines intensities (normalized to the initial of varied parameter) are shown in Table 5. Figures 4-6 show examples of dependencies of some analytes lines intensity and ratios I analyte /I (Sc I and Y II as ) on the temperature, the carrier gas flow rate and nebulizer flow rate. 320

8 Table 4. Excitation potentials of the used spectral lines (3) Line, nm Excitation potential Е, ev Al I Ca II Fe I Mg I Mn II Si I Y II Sc I Table 5. The normalized intensities of the spectral lines at varying of operation parameters Variable parameter Temperature Т, К Carrier gas flow rate V Ar l/min Nebulizer flow rate, V n ml/min Fixed parameters V Ar = 0.8 l/min V n =1.5 ml/min Т=7000 К V n = 1.5 ml/min Т=7000 К V Ar = 0.8 l/min Variable parameter Normalized intensities of the spectral lines I Al I Si I Mn I Mg I Ca I Fe I Sc I Y

9 Fig. 4. Dependencies of normalized intensity I Si and ratios I Si /I Sc, I Sc /I Y on plasma temperature. V Ar = 0.8 l/min, V n = 1.5 ml/min. Fig. 5. Dependencies of normalized intensity I Al and ratios I Al /I Sc, I Al /I Y on carrier gas flow rate V Ar. Т = 7000 К, V n = 1.5 ml/min. 322

10 Fig. 6. Dependencies of normalized intensity I Ca and ratios I Ca /I Sc, I Ca /I Y on nebulizer flow rate V n. Т = 7000 К, V Ar = 0.8 l/min. The examples on Fig. 4-6 show that the internal standardization using scandium reduces the dependence of analytes emission lines intensity on the operation parameters. This trend continues for the most other analytes, however for manganese and calcium the reducing of dependence on instrumental parameters is better using yttrium. This can be seen from Table 6, which shows b T, b Ar, b n b (respectively the slopes of approximated to linear dependencies of the emission lines intensity on temperature, carrier gas flow rate and nebulizer flow rate). Таble 6. Slope of linear plots I analyte /I = const + bp, where P = T, V Ar, V n Analyte Al Si Mn Slope without without without =Sc =Y =Sc =Y =Sc =Y b T b Ar b n b Analyte Mg Ca Fe Slope without without without =Sc =Y =Sc =Sc =Y =Sc b T b Ar b n b

11 Thus, according to thermodynamic modeling results, for the four of the six aim components it is preferred to use scandium as internal standard for ICP-AES determination of their concentration. Therefore, this element was used in the experimental verification of the applicability of internal standardization. As described above the solutions of iron ores, concentrates and slag certified reference materials were prepared. In these solutions the concentrations of Al 2 О 3, SiО 2, MnО, MgО, CaО, Fe were measured with and without the use of scandium as (Table 7). Таble 7. Results of ICP-AES analysis of certified reference materials of iron ores, concentrates and slag with and without internal standartization, %Wt Sample 2057 iron concentrate 1865 iron ore 1480 iron ore 1776 slag Sample 2057 iron concentrate 1865 iron ore 1480 iron ore 1776 slag Certified Fe Al 2 O 3 SiO 2 Found Certifiefied Found Certi- Found with without with without with without MnO MgO CaO Certifiefiefied Found Certi- Found Certi- Found with- with without with without with out

12 RSD of the analysis results and the relative deviations of mean results from the certified s were evaluated. Their s when using Sc I for internal standardization for ICP-AES determination of Al 2 О 3, SiО 2, MnО, MgО, CaО, Fe in iron ores and slag decreased by 2.5 times. CONCLUSIONS 1. For two popular in practice of the analytical chemistry laboratory of IMET UB RAS systems (iron and molybdenum-copper ores, concentrates, slags) theoretical algorithm of selection the internal standard to improve the quality of ICP-AES analysis was tested, as described in (1) (with a slight modification). With the help of thermodynamic modeling it was shown that the using of the internal standard compensates drift of three instrumental parameters. 2. Testing of selected by thermodynamic modeling internal standards is necessary to detects the factors that impede the using of the most attractive according the theoretical point of view internal standards in practice (as in the case of the = Ta, Ge for Re determination). 3. Experimental verification of the modeling results for the selected (Gd II for Re determination and Sc I nm for Al 2 О 3, SiО 2, MnО, MgО, CaО, Fe determination) with the using of certified reference materials of ores, concentrates, slags showed a reduction of RSD and the difference of mean results with the certified s. The decrease of the last was essential, if certified content was higher. Thus, the experience of the theoretical selection of internal standard for ores materials ICP-AES analysis using thermodynamic modeling it can be considered as successful. It should be noted that using emission lines of calcium, manganese and yttrium have the same nature (they are ionic lines), and the rest, as the line of scandium have the other one (these are the atomic lines). Perhaps this is due to the fact that yttrium is more suitable for calcium and manganese as the internal standard than scandium. The study is supported by Program of UD RAS, project No 12-P and carried out using equipment of Collective Center Ural-M. REFERENCES 1. Pupyshev A.A., Danilova D.A.: Thermodynamic modelling method for inductively coupled plasma atomic emission spectrometry. Yekaterinburg, Ural state university, (in Russian). 2. Trusov B.G.: TERRA program complex for calculating the plasma-chemical processes. 3 th Int. Symp. Theor. and Appl. Plasma Chem. Ples, (in Russian). 3. A. N. Zaidel, V.K. Prokofiev, S.M. Raiskii, etc.: Tables of spectral lines. Publishing House Nauka, Moscow, 1977 (in Russian). 4. C.H. Corliss, W.R. Bozman: Experimental transition probabilities for spectral lines of seventy elements. Nat. Bur. Stand. (U.S.), Monogr. Washington, O.V. Evdokimova, P.V. Zaytseva, N.V. Pechishcheva, K.Yu. Shunyaev, A.A. Pupyshev: Optimization of conditions of sample pretreatment of rheniumcontaining products and the conditions of AES ICP rhenium determination in copper 325

13 and molybdenum concentrates. Int. Scient. Res. and Appl. Conf. "Rhenium. Scientific research, process developments, industrial application", Moscow,