Microchim Acta (2011) 172:395–402 DOI 10.1007/s00604-010-0516-9

ORIGINAL PAPER

Preconcentration of erbium(III) ions from environmental samples using activated carbon modified with benzoyl hydrazine Qihui Wang & Xijun Chang & Zheng Hu & Dandan Li & Ruijun Li & Xiaoli Chai

Received: 2 September 2010 / Accepted: 21 November 2010 / Published online: 9 December 2010 # Springer-Verlag 2010

Abstract A method was established for the preconcentration of trace concentrations of Er(III) ion using activated carbon modified with benzoyl hydrazine. Parameters affecting solid-phase extraction such as pH value, shaking time, flow rate, sample volume were systematically studied. At a pH of 3.0, the maximum static adsorption capacity of the sorbent is 59.8 mg g−1 for Er(III), and the time for quantitative adsorption (>95%) is as short as 2 min. The adsorbed Er(III) was quantitatively eluted with 2 mL of 1.0 M hydrochloric acid and then determined by inductively coupled plasma optical emission spectrometry. The limit of detection (3σ) is 73 ng g−1, and the relative standard deviation is <2.0% (n=8). The method was validated by analyzing certified reference materials and successfully applied to the determination of trace Er(III) in environmental samples. Keywords Activated carbon . Erbium . Benzoyl hydrazine . Solid-phase extraction . Inductively coupled plasma optical emission spectrometry

Introduction Rare earth elements (REEs) have been widely used in catalysts, diagnosis of magnetic resonance imaging in Q. Wang : X. Chang (*) : Z. Hu : D. Li : R. Li : X. Chai Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China e-mail: [email protected] Q. Wang : X. Chang : Z. Hu : D. Li : R. Li : X. Chai Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, People’s Republic of China

medicine, some fertilizers in agriculture, superconducting materials, permanent magnets, fluorescent materials, solidstate laser areas and so on [1]. Therefore, analysis and research of REEs is becoming more and more important [2]. Erbium belongs to the rare element group which possesses special physical and chemical features, especially optical properties. In recent years, most of the interest in luminescent rare-earth ions has concentrated on Er(III). In particular its emission band around 1.53 μm has a special significance. The reasons for this are plain to see if one considers the rapid growth in optical telecommunications and some of the materials limitations on this technology. In addition, erbium doping into the glasses makes them suitable for utilization in optical amplifiers or waveguide lasers [3]. Erbium has also dominated research in light emission from other solid hosts, most notably semiconductors. Therefore, sensitive, reproducible and accurate analytical methods are required for the determination of trace erbium in environmental samples. For determination of REEs in the real samples, many analytical techniques have been used. These include: neutron activate analysis (NAA) [4], X-ray fluorescence spectrometry (XRF) [5], inductively coupled plasma optical emission spectrometry (ICP-OES) [6] and inductively coupled plasma mass spectrometry (ICP-MS) [7]. However, NAA requires very expensive nuclear reactor or particle accelerator, while the sensitivity of XRF is lower than that of ICP-OES. The ICP-MS has been acknowledged as one of the most powerful techniques for REEs determination, but the equipment is still too expensive for most institutions. In comparison, ICP-OES is a timesaving, simple, and well-available technique for the determination of REEs [8]. However, the direct determination of REEs at trace level is limited due to their low concentrations and matrix effects.

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These limitations can be overcome using a separation and preconcentration step [9]. The most widely used techniques for the separation and preconcentration of trace REEs include coprecipitation [10], ion-exchange [11], solid-phase extraction (SPE) [12] and liquid-liquid extraction [13]. The SPE becomes the most common technique for environmental sample pretreatment because of its simple operation, high recovery, high enrichment factor, short extraction time, low cost, and the ability to combine different detection techniques [14] of on-line or off-line mode. In SPE procedure, the choice of appropriate sorbent is a critical factor. Numerous substances have been applied as solid-phase extractants, such as chelating resin [15], modified silica [16], activated carbon [17], nanometer sized zirconium dioxide [18] and chitosan [19]. Activated carbon is one of the most extensive sorbents due to its large surface area, porous structure and high purity standards. However, without any surface treatment, activated carbon does not adsorb metal ions quantitatively at trace and ultratrace levels [20], which prompt us to study the possibility of modifying traditional activated carbon with organics or inorganics. Such variations in the behavior of the modified activated carbon are mainly dependent on the presence of some donor atoms or groups such as O, N, P and S into the incorporated organic modifier structure [21]. In this work, a new sorbent was synthesized by a simple and fast method. This sorbent showed excellent adsorptive selectivity toward Er(III), and found high adsorption capacity and short equilibrium time. Common coexisting ions did not interfere with the determination of Er(III). Then, the method was applied to preconcentrate trace Er (III) from environmental samples with satisfactory results (recoveries: 96-103%).

Experimental Apparatus All metal ions were determined by an IRIS Advantage ER/S inductively coupled plasma emission spectrometer (TJA, Franklin, MA, USA, http://www.thermo.com). The instrumental parameters were recommended by the manufacturer. The wavelength range of erbium selected is 337.271 nm [22]. Fourier Transform Infrared spectra (4000–400 cm−1) in KBr were recorded on a Nicolet NEXUS 670 FT-IR spectrometer (Madison, WI, USA, http://www.thermo.com). A Vario EL element analyzer, Elementar Analysensysteme (Hanau Germany), was used for elemental analysis. A pHs3C digital pH meter (Shanghai Lei Ci Device Works, Shanghai, China, http://www.lei-ci.com) was used for the

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pH adjustments. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals, Beijing, China, http://www.grinm.com) was used in the column process and flow studies. A self-made glass column (50 mm×2.5 mm i.d.) was used in this study. Reagents and standard solutions Unless otherwise stated, all chemicals used were of analytical grade. Double-distilled water was used throughout. According to a published procedure [23], all glassware was soaking in 10% (v/v) HNO3 by 24 h and washed with double-distilled water before used. Activated carbon (AC, 100–200 mesh) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China, http://www. guangfu-chem.com). Benzoyl hydrazine (BH, Shanghai ruicong Analytical Instrument Co. Ltd., Shanghai, China, http://www.shrcsys.com) and N, N′-dicyclohexylcarbodiimide (DCC, Sinop-harm Chemical Reagent Co. Ltd., Shanghai, China, http://www.reagent.com.cn/corporation/ infoDetail.asp?dInfoId=136) were used in this work. The stock standard solutions of REEs (1.0 mg mL−1) were made by dissolving rare earth oxides in hydrochloric acid except CeO2 that was dissolved in a mixture of HNO3 and H2O2 and later converted to HCl medium. The working standard solutions were prepared by diluting stock standard solutions with 0.5 mol L−1 HCl according to a published procedure [8]. Sample preparation The reference material of soil (GBW07402) was purchased from National Research Center for Certified Reference Materials (Beijing, China, http://www.nrccrm.org.cn). Balsam pear leaves and orange leaves were obtained from Anning village (Lanzhou, China). According to literature [24], they were dried in an oven at 80 °C to constant weight. The sample (approximately 0.500 g) was weighted and transferred to polytetrafluoroethylene (PTFE) beaker before adding 5.0 mL of concentrated HNO3. It was left at room temperature for one night and slowly evaporated (<165 °C) to dryness. The HClO4 (1.3 mL) was added when the beakers had cooled down. The temperature was then raised to 210 °C until white fume begin to form. The volume was adjusted to 100 mL with double-distilled water after the beaker had cooled down. The reference material (GBW07308) was obtained from National Research Center for Certified Reference Materials (Beijing, China, http://www.nrccrm.org.cn). Yellow River sediments were obtained from Yellow River (Lanzhou, China). The sample (0.5000 g) was accurately weighed into a PTFE beaker and aqua regia (12.0 mL and 4.0 mL of concentrated HCl and HNO3 respectively) was added to the

Preconcentration of erbium(III) ions from environmental samples O C

O OH

H2N

NH

C

397

DCC

O C

O NH

NH

C

Scheme 1 Synthetic route of the AC-BH

sample. As in the literature [25], the beaker was covered with a watch glass and the mixture was evaporated on a hot plate at 95 °C almost to dryness. Then 8.0 mL of aqua regia was added to the residue and the mixture was again evaporated to dryness. After cooling, the resulting mixture was filtered through a 0.45 μm PTFE millipore filter. The sample was diluted to 50 mL with double-distilled water and analyzed using the new sorbent.

Preparation of benzoyl hydrazine modified activated carbon (AC-BH) AC-COOH (5.0 g) and benzoyl hydrazine (5.0 g) were suspended in 100 mL of ethanol, followed by DCC (5.0 g). This mixture was refluxed at 80 °C for 48 h. The final product (AC-BH) was filtered, washed with ethanol and dried under vacuum at 80 °C for 8 h. The synthetic route of AC-BH is illustrated in Scheme 1.

Preparations Characteristic of AC-COOH and AC-BH Preparation of carboxylic derivative of activated carbon (AC-COOH) Activated carbon powder was first kept in 10% (v/v) HCl solution by 24 h in order to remove adsorbed impurities. Then 10.0 g of purified activated carbon was immersed in 200 mL of 8 mol L−1 nitric acid under stirring and heating at 60 °C for 5 h to increase the oxygen functional groups on the surface. The mixture was filtered, repeatedly rinsed with double-distilled water until to neutral and dried under vacuum at 80 °C for 8 h. The product was carboxylic derivative of activated carbon (AC-COOH). Fig. 1 FT-IR spectra of ACCOOH and AC-BH

The FT-IR spectra were taken to observe the functional groups of AC-COOH and AC-BH, and the spectra were shown in Fig. 1. For the HNO3-oxidized activated carbon, the characteristic vibration of unionized and uncoordinted carboxyl is shown as a strong peak of COO − , stretching at 1703.21 cm−1 and a shoulder of OH deformation vibration at 1438.11 cm−1, it is presented the carboxylic derivative of activated carbon was prepared successfully. When ACCOOH was modified with benzoyl hydrazine, the peak of 1703.21 cm−1 disappeared and several new peaks appeared

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Procedures Batch procedure A series of standard or sample solutions were transferred into a 25 mL beaker, and the pH value was adjusted to the desired value with 0.1 mol L−1 HCl and 0.1 mol L−1 NH3·H2O. Then the volume was adjusted to 10.0 mL with double-distilled water. Amount of 30 mg of AC-BH was added, and the mixture was shaken vigorously for 30 min to facilitate adsorption of the metal ions onto the sorbent. The concentrations of the metal ions in the solution were directly determined by ICP-OES. Then the sorbent was eluted with 1.0 mol L−1 HCl. The concentrations of the desorbed metal ions were directly determined. Column procedure Thirty milligrams of AC-BH was packed in a self-made glass column (50 mm×2.5 mm i.d.) plugged with a small portion of glass wool at both ends. In order to equilibrate,

100

95

Recovery (%)

in the spectrum such as 3429.33, 1574.60, 1516.56 and 1444.50 cm−1. According to the literature [26, 27], the new peaks could be assigned as follows: the peak at 1574.60 cm−1 was due to C=O stretching vibration. The peaks around 1516.56 and 1444.50 cm−1 could be assigned to the benzene ring characteristic vibrations in benzoyl hydrazine. The peaks of N-H stretching vibration occurred at 3429.33 cm−1. Elemental analysis indicated 48.62% carbon, 1.22% nitrogen and 1.82% hydrogen in AC-COOH and 63.30% carbon, 4.39% nitrogen and 2.53% hydrogen in AC-BH. It could be calculated that 1 g activated carbon contained 0.17 g benzoyl hydrazine. Consequently, the above experimental results showed that activated carbon was modified by benzoyl hydrazine.

Er 85

80 0

5

10

15

20

25

30

Time (min) Fig. 3 Effect of time on adsorption of 1.0 μg mL−1 Er(III) on AC-BH. Using conditions: pH 3.0; sorbent mass 30 mg; temperature 25 °C

clean and neutralize the column, 10% HNO3 solution and double-distilled water were successively passed through it before using. Portions of sample solutions containing Er (III) were prepared, and the pH value was adjusted to 3.0 with 0.1 mol L−1 HCl and 0.1 mol L−1 NH3·H2O. Each solution was passed through the column at a flow rate of 2.0 mL min−1 controlled with a peristaltic pump. The metal ions retained on column were eluted with 1.0 mol L−1 HCl and the quantities of analytes in the eluent were determined by ICP-OES.

Results and discussion Effect of pH The solution pH is one of the most important variables which affect the species of metals in solution through hydrolysis, complexation and redox reactions during metal recovery. Therefore, solution pH value is the first parameter to be optimized. According to the batch procedure, several metal ions, such as Eu(III), Sc(III), Ho(III), La(III), Er(III),

100

100

95

80

Recovery (%)

Recovery (%)

90

60 40

Er

90 85

Er 80

20 75 0 1

2

3

4

5

6

7

pH Fig. 2 Effect of pH on adsorption of 1.0 μg mL−1 Er(III) on AC-BH. Using conditions: shaking time 30 min; sorbent mass 30 mg; temperature 25 °C

0

1

2

3

4

Flow rate (mL min-1) Fig. 4 Effect of solution flow rates on adsorption of Er(III). Using conditions: pH 3.0; sorbent mass 30 mg; volume 10 mL; temperature 25 °C

Preconcentration of erbium(III) ions from environmental samples Table 1 Elution recovery (%) for Er(III) adsorbed on AC-BH

Concentration(mol L−1) Er(III) Eluent volume(mL) Er(III)

399 10.0 mL HCl 0.01 0.05 57.42 74.33 1.0 mol L−1 HCl 2.0 3.0 98.18 98.93

0.1 89.13

0.5 92.45

1.0 97.47

2.0 98.17

4.0 99.22

5.0 99.49

6.0 100.08

8.0 100.12

Y(III), Ce(III), Gd(III), Tb(III), Sm(III), Dy(III) and Yb(III) were tested at different pH values (form 1.0 to 7.0). It could be seen in Fig. 2 that quantitative extraction (>95%) of Er (III) occurred at pH≥3.0. Thus a pH of 3.0 was selected as the optimum condition. In addition, other REEs had insignificant adsorption on AC-BH at pH 3.0. Therefore, AC-BH had excellent adsorptive selectivity towards Er(III).

The effect of elution volume on the recovery of Er(III) was also studied by keeping the HCl concentration of 1.0 mol L−1. Quantitative recovery could be obtained with 2.0 mL of 1.0 mol L−1 HCl. Therefore, 2.0 mL of 1.0 mol L−1 HCl was used as eluent in further experiments.

Effect of shaking time

To obtain a high enrichment factor, metal ions in the large sample volume has been quantitatively adsorbed and desorbed by a small stripping volume. The effect of sample volume on metal adsorption was studied by passing 50– 300 mL 1.0 μg mL−1 of Er(III) through the column at the optimum flow rate. As illustrate in Fig. 5, the results showed that the maximum sample volume could be up to 200 mL with the recovery >95%. The high enrichment factor of 100 was obtained because the eluent volume was 2.0 mL.

The shaking time is an important factor determining the possible discrimination order in the behavior of the new synthesized phases towards the metal ions. In this work, different shaking time (range from 2 to 30 min) was studied for the evaluating of extraction of Er(III) by AC-BH. As shown in Fig. 3, the results indicated that Er(III) binding was fast, occurring within the first 2 min of contact (recovery >95%), which suggests that AC-BH had rapid adsorption for Er(III).

Maximum sample volume and enrichment factor

Adsorption capacity Effect of flow rate The capacity of the sorbent is an important factor because it determines how much sorbent is required to quantitatively remove a specific amount of metal ions from the solutions. The determination method of adsorption capacity was adopted from the paper by Maquieira et al. [28]. According to the batch procedure, 30 mg of sorbent was equilibrated

100 90

Rccovery (%)

Another important parameter in adsorption is the flow rate of the sample solution, which influence the interaction of the analytes with the sorbent and the time of complete analysis. Therefore, the effect of the flow rate of sample solution was examined under the optimum conditions by passing 10.0 mL of sample solution through the column with a peristaltic pump. The flow rates were adjusted in range of 0.5–4.0 mL min−1. As shown in Fig. 4, it was found that quantitative recovery (>95%) of Er(III) was obtained in interval flow rate of 0.5–2.0 mL min−1. Over 2.0 mL min−1, the recovery of the analytes decreases with the increase of the flow rate, probably because the metal ions do not equilibrate sufficiently with the sorbent. Thus, a flow rate of 2.0 mL min−1 is selected in this work.

80 70

Er

60

Elution condition The elution concentration of acid was studied by using 10.0 mL of different concentrations of HCl for the desorption of retained Er(III) from the sorbent. As could be seen in Table 1, 1.0 mol L−1 HCl was sufficient for complete elution (recovery >95%).

50 50

100

150

200

250

300

Volume of sample solution (mL) Fig. 5 Effect of the sample volume on recovery of 1.0 μg mL−1 Er (III) at pH 3.0. Using conditions: sorbent mass 30 mg; temperature 25 °C

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with 10.0 mL of Er(III) of various concentrations adjusted to pH 3.0 for 1 h. In order to reach the “saturation”, the concentration of Er(III) was increased till the plateau value (adsorption capacity value) obtained. Therefore, the maximum adsorption capacity of AC-BH was found to be 59.8 mg g−1 for Er(III).

To test the stability, the sorbent material was subjected to several adsorption and elution operations. The adsorption conditions were referenced according to the above experiments. After adsorption, the sorbent could be reused after regenerated with 10 mL 1.0 mol L−1 HCl and 20 mL double-distilled water, respectively. Up to 10 cycles of adsorption-elution experiments, the recovery of Er(III) did not decrease obviously. Therefore, the sorbent showed better reusability and stability towards the studied ion. Effects of coexisting ions To explore the effects of common coexisting ions on the adsorption of Er(III), the following experiments were investigated. Solutions of 1.0 μg mL−1 of Er(III) containing the added coexisting ions were prepared according to the batch procedure. The tolerance limit was set as the amount

Table 2 Effect of foreign ions on adsorbing recovery of 1.0 μg mL−1 of Er(III) on the sorbent

Na+ K+ Ca2+ Mg2+ SO42− PO43− La3+ Yb3+ Sm3+ Eu3+ Nd3+ Ho3+ Dy3+ Ce3+ Y3+ Tb3+ Gd3+ Sc3+

Concentration of Er(III) (μg g−1)

Sample

GBW07402 GBW07308

Reusability

Coexisting ion

Table 3 Analysis results for the determination of Er(III) in standard reference materials

Concentration (μg mL−1)

Recovery of Er(III) (%)

5000 5000 1500 1500 1500 1500 50 50 50 50 50 50 50 50 50 50

99.95 98.76 98.93 99.57 98.34 97.13 98.26 97.79 100.15 98.73 97.82 100.09 99.04 100.21 100.13 100.04

50 30

100.17 97.65

t-Testb

Found by present methoda (μg g−1)

Certified value (μg g−1)

2.2±0.2 1.7±0.5

2.1±0.4 1.8±0.3

1.41 0.56

a x  s (n=8). x: average value for eight determinations; s: standard deviation b

t0.05,8 =2.31

of ions causing recoveries of the examined elements to be less than 90%. As shown in Table 2, in excess of 5000 μg mL−1 of K+ and Na+, 1500 μg mL−1 of Ca2+, Mg2+, PO43− and SO42−, 50 μg mL−1 of La3+, Yb3+, Sm3+, Y3+, Ce3+, Gd3+, Tb3+, Eu3+, Nd3+, Ho3+ and Dy3+, and 30 μg mL−1 of Sc3+ did not interfere with the separation and determination of Er(III). The results demonstrated that AC-BH showed a excellent adsorptive selectivity towards Er(III). The detection limit and analytical precision Under the selected conditions, eight portions of standard solutions were enriched and analyzed simultaneously following the recommended procedure. The limit of detection (3σ) according to the definition of International Union of Pure and Applied Chemistry of this method was found to be 0.073 μg g−1 for Er(III). The relative standard

Table 4 Analytical results for the determination of Er(III) in environmental samples Ion

Added (μg g−1)

Yellow River sediments Er(III) 0 0.5 1.0 Balsam pear leaves Er(III) 0 0.5 1.0 Orange leaves Er(III) 0 0.5 1.0

Founda (μg g−1)

Recovery (%)

1.72±0.10 2.25±0.05 2.69±0.09

101.7 98.3

– 0.49±0.07 1.03±0.12

98.0 103.0

– 0.48±0.05 1.01±0.08

96.0 101.0

– Not detected a

The value following “±” is the standard deviation (n=3)

Preconcentration of erbium(III) ions from environmental samples

deviation (RSD) was lower than 2.0% (Er(III): 1.82%), which revealed that the method present good precision for the determination of trace Er(III). Application of the method Certified reference materials (GBW07402 and GBW07308) were used for method validation. Applying the t-test, the t0.05, two values for certified reference materials of GBW07402 and GBW07308 were listed in Table 3. As could be seen, there did not exist statistical differences between determined values and certified ones (t0.05 <2.31). The analytical results for the standard materials were in good agreement with the certified values. The method has been also applied to the determination of trace Er(III) in environmental samples. As could be seen in Table 4, the recoveries of Er(III) were in range of 96–103%. The results demonstrated that AC-BH was used for determination of trace Er(III) in Yellow River sediments, orange leaves and balsam pear leaves samples with satisfactory results. Therefore, the method was reliable and successful. Comparison with other methods The analytical characteristics of separation and determination of Er(III) by solid phase extraction with AC-BH were compared with the other reported methods, for example, solvent extraction [29, 30], cloud point extraction [22], ion imprinted [31, 32], electrophoresis [33], extraction chromatography [34, 35] and liquid membrane separation [36] as well as their combined techniques. Solvent extraction [30] had provided effective separation methods for Er(III), but required a large number of stages and long separation times for complete isolation. The synthetic method of ion imprinted material was very complex [32]. The lower sensitivity of capillary electrophoretic [33] and high costs of extraction chromatography [35] indicated the two methods were limited. Liquid membrane separation of Er (III) [36] showed the reductions in the number of separation stages, consumption of reagents, and investment costs, but the surfactant of liquid membrane was indispensable for maintaining the membrane integrity, resulting in the lower separation factor. However, the present method possessed the advantages of excellent adsorptive selectivity toward Er (III), short analysis time, high enrichment factor, low consumption of organic solvents and low costs. In addition, the synthetic method of the sorbent was very simple.

Conclusions The adsorption behavior of trace Er(III) on AC-BH was studied systemically. A simple, sensitive and reliable

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method was established for the determination of trace Er (III) in environmental samples based on preconcentration with benzoyl hydrazine modified activated carbon. The important characteristics of AC-BH are its excellent adsorptive selectivity towards Er(III) over other ions, short extraction time, high adsorption capacity, high enrichment factor and easy elution for Er(III). This method has been validated by analyzing certified reference materials and successfully applied to the analysis of trace Er(III) in Yellow River sediments, orange leaves and balsam pear leaves samples. The recoveries of Er(III) were in range of 96–103% and the results were satisfactory. References 1. Wei Z, Yin M, Zhang X, Hong F, Li B, Tao Y, Zhao G, Yan C (2001) Rare earth elements in naturally grown fern Dicranopteris linearis in relation to their variation in soils in south-Jiangxi region (southern China). Environ Pollut 114:345 2. Zhu Y, Umemura T, Haraguchi H, Inagaki K, Chiba K (2009) Determination of REEs in seawater by ICP-MS after on-line preconcentration using a syringe-driven chelating column. Talanta 78:891 3. Kenyon AJ (2002) Recent developments in rare-earth doped materials for optoelectronics. P Quant Electron 26:225 4. Nyarko BJB, Akaho EHK, Fletcher JJ, Chatt A (2008) Neutron Activation analysis for Dy, Hf, Rb, Sc and Se in some Ghanaian cereals and vegetables using short-lived nuclides and Compton suppression spectrometry. Appl Radiat Isot 66:1067 5. Vito IE, Olsina RA, Masi AN (2001) Preconcentration and elimination of matrix effects in XRF determinations of rare earth elements by preparing a thin film through chemofiltration. J Anal At Spectrom 16:275 6. Kolibarska I, Velichkov S, Daskalova N (2008) Spectral interferences in the determination of traces of scandium, yttrium and rare earth elements in “pure” rare earth matrices by inductively coupled plasma atomic emission spectrometry: part VII-Terbium, Dysprosium, Holmium and Thulium. Spectrochim Acta Part B 63:603 7. Kulkarni P, Chellam S, Mittlefehldt DW (2007) Microwaveassisted extraction of rare earth elements from petroleum refining catalysts and ambient fine aerosols prior to inductively coupled plasma-mass spectrometry. Anal Chim Acta 581:247 8. Ramanaiah GV (1998) Determination of yttrium, scandium and other rare earth elements in uranium-rich geological materials by ICP-AES. Talanta 46:533 9. Prasada T, Kala R (2004) On-line and off-line preconcentration of trace and ultratrace amounts of lanthanides. Talanta 63:949 10. Lakshtanov LZ, Stipp SLS (2004) Experimental study of europium (III) coprecipitation with calcite. Geochim Cosmochim Acta 68:819 11. Jia Q, Kong X, Zhou W, Bi L (2008) Flow injection on-line preconcentration with an ion-exchange resin coupled with microwave plasma torch-atomic emission spectrometry for the determination of trace rare earth elements. Microchem J 89:82 12. Liang P, Fa W (2005) Determination of La, Eu and Yb in water samples by inductively coupled plasma atomic emission spectrometry after solid phase extraction of their 1-phenyl-3-methyl-4benzoylpyrazol-5-one complexes on silica gel column. Microchim Acta 150:15

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