JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Determination of Rare Earth Elements in Mineral Waters by Inductively Coupled Plasma Atomic Emission Spectrometry* Jana Kubova Faculty of Natural Sciences Comenius University 842 15 Bratislava Slovakia Vladislav Nevoral Reference Laboratory for Natural Healing Springs of the Ministry of Health 353 01 Marianske Lazn6 Czech Republic Vladi mir StreS ko Faculty of Natural Sciences Comenius University 842 15 Bratislava Slovakia 241 A procedure for selective preconcentration of all rare earth elements (REE) in mineral waters on a Dowex cation exchanger has been developed. The precision and accuracy of the procedure were checked on a synthetic standard solution as well as on real samples using inductively coupled plasma atomic emission spectrometry (ICP-AES).The efficiency of the preconcentration procedure was checked by recovery tests and the reliability of the ICP-AES determination of REE established by comparing the results obtained with those from spectrophotometric analysis. Keywords Mineral waters; rare earth elements; preconcentration; cation exchange; inductively coupled plasma atomic emission spectrometry Information about the rare earth elements (REE) contents in solid natural materials (minerals and rocks) enable some important geochemical problems to be solved.' However obtaining reliable results presents a complex analytical task. Instrumental neutron activation analysis ( INAA),2-4 spark source mass spe~trometry,~ isotope dilution mass spec- trometry,' inductively coupled plasma atomic emission spec- trometry ( ICP-AES)7-'o and more recently inductively coupled plasma mass spectrometry ( ICP-MS)11-13 are the most important methods that have been used for the determination of low levels of REE.Owing to its selectivity and the comparatively low initial cost and subsequent running costs as compared with for example ICP-MS or INAA ICP-AES was chosen for the present determination of REE in mineral waters. The REE contents in mineral waters are extremely low as compared with the main components and also show a large variation in their chemical composition. Therefore for the deter- mination of REE in mineral waters the use of a separation- preconcentration procedure is inevitable. Coprecipitation of Fe(OH)3,14 A1(OH)3,15,16 CaC204 or CaF2,17*18 separation on ion exchangers such as AG 50W'9,20 or Dowex 50W21-23 and liquid-liquid extraction2&26 are the most frequent procedures that have been described for the preconcentration of REE.The proposed procedure for the determination of REE in mineral waters uses a separation-preconcentration procedure with an ion exchanger Dowex 50W X12. Experimental Apparatus The instrumentation used for the AES measurements was as follows sequential atomic emission spectrometer (Plasmakon S 35 Kontron Germany) with a grating of 2400 lines mm-'; a concentric glass nebulizer (Type B Meinhard); an Ar-Ar plasma power 1.5 kW frequency 27.12 MHz flow rate of outer gas 14.5 dm3min-' of intermediate gas 1.0 dm3min-' of aerosol carrier gas 1.0 dm3rnin-l; sample uptake rate 1.5 cm3 min-' controlled by a peristaltic pump; and inte- gration time 5 s.For the spectrophotometric measurements a prism spectro- photometer (Spektromom 204 MCM Hungary) was used. Analytical Lines The wavelengths of the spectral lines and positions for the measurement of the background chosen in order to ensure the lowest mutual spectral interference^,^^ are listed in Table 1. All measurements were performed with blank and background correction. Chemicals Standard solutions of the REE were prepared by appropriate dilution of 1000 ppm atomic absorption spectrometric (AAS) standards (Alfa Products Ventron). The acids used were of analytical-reagent grade (Merck). The cation exchanger (Fluka Bucks) Dowex 50W X12 (50-100 mesh) was washed with 1moldm-3 NH4Cl and Table 1 Measured spectral lines Detection limit (3s) Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Spectral line/nm 361.38 371.03 333.75 413.76 390.84 430.36 359.26 381.97 342.25 350.92 353.17 345.60 349.91 346.22 328.94 261.54 Background/nm 361.50 370.90 333.84 413.87 390.90 430.41 359.40 382.06 342.35 350.96 353.21 345.69 349.95 346.26 328.99 26 1.42 A*/ Btl pg drn-3 ng dm-3 0.5 0.3 0.8 0.4 2.6 1.3 10.1 5.1 5.6 2.8 5.9 2.9 6.1 3.1 1 .o 0.5 3.1 1.6 3.9 1.9 2.0 1 .o 1.2 0.6 2.5 1.2 1.8 0.9 0.4 0.2 0.4 0.2 * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.*A Without the preconcentration procedure. ?After the preconcentration procedure (from 50 dm3 to 25 cm3).242 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.2 mol dm-3 ammonium citrate [(NH4),HC6H307] solu- tion pH 4.2 and then converted into the Hf form with 6 mol dmP3 HCl.Preconcentration of REE by Cation-exchange Separation Aliquots of 25-100 dm3 of mineral water were taken in polyethylene vessels (pre-washed with hot 3 mol dmd3 HC1) and immediately acidified with 6moldm-3 HCl to a pH The same amounts of each sample were placed in glass vessels ( 5 dm3) and boiled to remove CO followed by any volatile organic compounds that could be present. After partial cooling the sample was allowed to pass (at a flow rate of 4-5 cm3 min-') through a 16 mm diameter column packed with 302 mm of Dowex 50W X12 (50-100 mesh). The empty sampling vessels were washed with 250 cm3 of hot 3 mol dm-3 HC1 and SO0 cm3 of de-ionized water and the washings were added to the remaining 4-5 dm3 of sample.After the exchange process the cation exchanger was washed with 200cm3 of 40% acetone and 200 cm3 of 80% acetone which removed the majority of the organic compounds retained from the sample. Through the cation exchanger were then passed 100cm3 of 40% acetone 100 cm3 of distilled water and the main compo- nents of the mineral waters i.e. Na+ K' Mg2+ Ca2+ and Fe3+ were then eluted with 1500 cm3 of 1.6 mol dm-3 HC1 (at a flow rate of 1 cm3min-'). The REE were eluted with 1100 cm3 of 6 mol dm-3 HCl (at a flow rate of 0.5 cm3 min-I). The eluate was then evaporated to dryness in a quartz dish. To the remaining portion (about 100 cm3) 10 cm3 of concen- trated. HN03 were added to decompose any organic material present due to the ion exchangers.To the dry residue 10 cm3 of 6moldm-3 HC1 were added and again evaporated to dryness. The final residue was diluted with 2 cm3 of 6 mol dm-3 HCl the solution was transferred into a 25 cm3 calibrated flask diluted to the mark with de-ionized water and then used for the ICP-AES measurements. For the spectrophotometric determinations the dry residue was treated according to the procedure described in detail in ref. 21 i.e. after its dissolution in 0.1 mol dm-3 HCI it was passed through a chromatographic column and the separated REE were eluted with a-hydroxyisobutyric acid of various concentrations. The spec- trophotometric determinations were performed at 540 nm using XyIenol Orange in the presence of cetyfpyridinium bromide.of 1.5-1.8. Results and Discussion Precision and Accuracy of the Procedure The precision and accuracy of the proposed procedure were determined by the analysis of a synthetic standard solution spiked with known contents of the macrocomponents and REE. The composition of the synthetic sample without the REE data is given in Table 2. To the synthetic sample (as shown in Table 2) the REE were added in the amounts given in Table 3. A 25 cm3 aliquot of the synthetic sample with known REE contents (Table 3) was diluted to a volume of 12 dm3 using the synthetic sample free from REE (Table 2) and acidified with 60cm3 of 6moldm-3 HC1. The REE were preconcen- trated by the described procedure to the original volume of 25 cm3 and determined by ICP-AES.A total of 15 synthetically prepared samples of the same composition were preconcentrated by this procedure. Their mean REE contents as well as relative standard deviations (RSDs) are listed in Table 3 (column A). The data in column B of Table 3 are the results obtained for the same original synthetic samples containing the REE but which were analysed without the application of the separation-preconcentration procedure. It is evident from Table 3 that results obtained by Table 2 Composition 01' the synthetic mineral water Component Li + Na+ K+ NH Rb+ c s + cu2 + Mg2+ Ca2 + Sr2 + Ba2 + Zn2+ Cd2+ ~ 1 3 + Concentration/ mg dm-3 1.632 589.7 56.2 2.40 1.65 1.58 0.016 38.42 74.67 0.360 0.040 0.008 0.006 0.008 Component Ti4 + Pb2+ v4 + Cr3 + Mo6+ Mn2+ Fe2+ Fe3+ co2 + Ni2 + F- Br- I- HB02 Concentration/ mg dm-3 0.0008 0.0008 0.0008 0.0008 0.0008 0.024 2.173 7.120 0.016 0.016 0.156 1.546 0.193 28.900 Table 3 Recoveries of REE from synthetic sample Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Added/ mg dm-3 4.8 8.0 4.8 8.0 4.8 8.0 4.8 1.6 4.8 1.6 4.8 1.6 4.8 1.6 4.8 1.6 Found RSD A*/ Bt/ (procedure A) mgdm-3 mgdm-3 (%) 3.8 4.9 9.0 8.2 8.2 2.0 4.8 4.9 7.7 8.5 8.5 6.1 4.9 4.8 5.1 8.7 8.8 4.4 4.9 5.0 4.7 1.8 1.8 7.8 4.9 4.9 3.2 1.7 1.7 3.5 4.8 5.0 4.6 1.7 1.7 3.6 4.9 5.0 2.8 1.8 1.8 3.4 4.7 4.7 1.8 1.5 1.5 4.8 ~~~ - *A With cation exchanger (n = 15). B Without cation exchanger (n= 10).both procedures are in good agreement (with the exception The blank valiies were [with the exception of Eu (0.012 pg dm-3) and Nd (0.018 pg dm-3)] below the detection limit and were determined after passing the synthetic standard solution free of FLEE through the cation exchanger (ten replicates).The accuracy of the proposed ICP-AES method was checked spectrophotometrically with real samples of mineral waters. As an illustration examples of the determination of REE with the use of the both methods are presented in Table 4. of SC). Conclusion The method described enables a reliable ICP-AES determi- nation of all REE in mineral waters to be carried out. The possibility of treating a fairly large volume of mineral water ensures that very low REE contents can be determined. In order to obtain the high preconcentration factors necessary for the reliable determination of all REE present in the mineral waters at extremely low concentrations the cation exchanger Dowex was chosen.The separation-preconcentration pro- cedure is fairly timt consuming but passing the sample through the ion-exchange column does not need any particular atten- tion. Owing to the low abundance of REE in the laboratory environment the danger of contamination during the whole procedure can be neglected. The reliability of the separation-preconcentration procedureJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 243 Table 4 Accuracy of REE determination in natural mineral waters; concentrations are expressed in pg dm-3 n = 3 Element sc Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu c Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu c Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Sample 1 SP* ICP-AES 0.385 0.338 3.600 3.210 0.360 0.413 1.160 1.410 0.190 0.220 1.305 1.280 0.400 0.458 0.122 0.120 0.578 0.598 0.081 0.086 0.500 0.460 0.120 0.130 0.387 0.331 0.057 0.056 0.466 0.413 0.073 0.072 9.784 9.595 Sample 4 Sample 2 Sample 3 SP ICP-AES 0.280 0.243 2.750 2.860 0.191 0.188 0.565 0.550 0.099 0.091 0.740 0.710 0.219 0.210 0.068 0.069 0.318 0.328 0.056 0.060 0.318 0.320 0.081 0.081 0.281 0.282 0.049 0.044 0.380 0.386 0.058 0.054 6.453 6.476 Sample 5 SP ICP-AES 0.560 0.496 3.030 2.990 0.224 0.228 0.780 0.765 0.122 0.106 0.960 0.986 0.290 0.263 0.082 0.082 0.433 0.397 0.062 0.065 0.382 0.386 0.100 0.090 0.320 0.302 0.049 0.050 0.380 0.400 0.070 0.066 7.844 7.672 Sample 6 SP ICP-AES 0.148 0.115 1.004 1.053 0.451 0.487 0.671 0.719 0.091 0,095 0.231 0.257 0.116 0.124 0.030 0.031 0.114 0.131 0.019 0.021 0.130 0.136 0.027 0.020 0.078 0.069 0.012 0.012 0.083 0.079 0.012 0.012 3.217 3.361 Sample 7 SP ICP-AES 0.125 0.103 0.370 0.336 0.054 0.064 0.088 0.085 0.012 0.016 0.034 0.047 0.010 0.012 0.005 0.006 0.020 0.028 0.007 0.007 0.026 0.030 0.007 0.008 0.020 0.024 0.002 0.003 0.020 0.028 0.002 0.002 0.802 0.799 Sample 8 SP ICP-AES 1.010 0.983 2.080 1.960 0.099 0.101 0.339 0.334 0.060 0.058 0.477 0.476 0.166 0.141 0.050 0.048 0.246 0.229 0.045 0.043 0.316 0.309 0.047 0.047 0.190 0.174 0.028 0.026 0.224 0.219 0.031 0.029 5.408 5.177 Sample 9 SP ICP-AES 0.037 0.965 0.282 0.652 0.064 0.265 0.065 0.021 0.080 0.01 5 0.079 0.014 0.089 0.012 0.068 0.010 0.039 0.970 0.280 0.652 0.065 0.308 0.067 0.020 0.09 1 0.012 0.083 0.015 0.065 0.013 0.072 0.009 SP ICP-AES 0.562 2.720 0.075 0.029 0.042 0.226 0.101 0.048 0.330 0.056 0.404 0.085 0.210 0.03 1 0.162 0.017 0.520 2.690 0.080 0.03 1 0.030 0.260 0.114 0.040 0.380 0.060 0.380 0.088 0.220 0.029 0.170 0.016 c 2.718 2.761 5.098 5.108 Sample Source Locality Ambroi I Ambroi I1 Ambroi 111 Excelsior Lesni Karolina 11-sano Vincen tka Richard Marien bad Marien b ad Marienbad Marien bad Marienbad Marienbad Dolni Kramolin LuhaEovice Lazne Kynivart SP ICP-AES 0.092 0.081 0.820 0.862 1.980 1.933 0.518 0.559 0.066 0.066 0.427 0.416 0.102 0.112 0.035 0.033 0.122 0.118 0.022 0.026 0.080 0.082 0.036 0.033 0.060 0.060 0.031 0.023 0.068 0.066 0.009 0.009 4.468 4.479 Original volume of sample/dm3 38.0 65.0 90.0 62.0 62.0 62.0 44.0 63.0 105.0 50.0 50.0 50.0 42.0 42.0 42.0 65.0 69.0 69.0 100.0 94.0 92.0 25.0 25.0 25.0 99.0 105.0 100.0 was checked by a recovery test and by comparison of the results obtained with ICP-AES with those obtained by spectro- photometry.The REE contents in the mineral waters deter- mined by the described procedure correspond approximately to their contents in the Earth’s crust but the light REE are less abundant in the waters analysed. 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