On-line and off-line preconcentration of trace and ultratrace amounts of lanthanides

Talanta 63 (2004) 949–959

On-line and off-line preconcentration of trace and ultratrace amounts of lanthanides T. Prasada Rao∗ , R. Kala Regional Research Laboratory (CSIR), Trivandrum 695 019, Kerala, India Received 30 September 2003; received in revised form 19 December 2003; accepted 7 January 2004 Available online 11 March 2004

Abstract The need for preconcentration of trace and ultratrace amounts of lanthanides from environmental, geological and biological samples is brought out in introductory part. Both on-line and off-line preconcentration procedures developed for lanthanides since 1980 are reviewed. The preconcentration techniques covered in this review include liquid–liquid extraction (LLE), ion-exchange, co-precipitation, and solid phase or solid–liquid extraction. Separate sections are devoted to each of the preconcentration techniques employed for enrichment of individual or mixtures of lanthanides. Future trends in singular or multielement preconcentration of lanthanides are also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Preconcentration; On-line; Off-line; Lanthanides

1. Introduction The inner transition elements of periodic table wherein 4f shell of electrons gets sequentially filled up, are generally known as rare earths (RE). The term “rare earths” is a misnomer as they are neither “rare” nor do they conform to the description of “earths”. The 15 rare earths account for nearly 1/4 of the metal content of the earth’s crust. Rare earth enrichment in earth’s crust could have taken place in the melts formed by partial melting of the mantle as they could not be accommodated in the common rock forming mineral phase owing to their large ionic radii and charge. Because of such a segregation of these elements, they could not be considered as “earth”. The elements with atomic numbers 57–71 starting with lanthanum are also known as lanthanides. 1.1. Occurrence and minerals Economically workable rare earth deposits were believed to be confined to two types of geological environments namely (i) alkaline rocks and carbonatites, and (ii) beach ∗

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and inland placers. However, studies on the large rare earth deposits in China have resulted in new concepts on factors that caused the geochemical environment. Normal weathering process have caused the accumulation of rare earths in sea water, and precipitation was influenced by earth’s atmosphere. It is believed that during the archaean period, the earth’s atmosphere was still rich in CO2 and N2 with conditions promoting the appearance of soluble phosphates. During the proterozoic age (∼2000 million years ago) the carbonates started forming, with corresponding decrease in CO2 in the atmosphere. The relative terrestrial abundance of Rare Earth Elements (REE) in earth’s crust is given in Table 1 [1]. The principal rare earth minerals are bastnasite, monazite and xenotime. Table 2 shows the relative abundance of REE in the earth’s crust and the proven global distribution of the principal minerals along with the individual RE contents in terms of their sesquioxides. However, the economics of monazite production depends on beneficiation of heavy minerals from placer deposits. This depends to a large extent on the marketing of major constituents like Ilmenite, Rutile, Zircon sand, Cassiterite, etc. which co-exist in raw sand. The monazite producing countries, namely India, Australia, China, USA (Florida), work under this constraint. But Bastnasite production by USA (California) is largely free from

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T. Prasada Rao, R. Kala / Talanta 63 (2004) 949–959

Table 1 Terrestrial abundance of REE and yttrium

China (80.00%)

) %

So

Ru

s

h ut

Af

ric

0 a(

1.1

1%

)

.90

this handicap. The capacity of USA Bastnasite processing facility thus exceeds the total world Monazite facility [2]. The country-wise split up of the world’s RE metal resources are given in Fig. 1 [3].

( sia

) India

(4.93%

(1 0.8 9% ) US A

18.00 46.00 5.50 24.00 6.50 1.00 6.40 0.90 4.50 1.20 2.50 0.20 2.70 0.80 28.00

Ca na d a A (0 u str .4 ali % a ( 0.4 ) %) 1.36 %)

La2 O3 CeO2 Pr6 O11 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Tb4 O7 Dy2 O3 Ho2 O3 Er2 O3 Tm2 O3 Yb2 O3 Lu2 O3 Y2 O 3

ers

Terrestrial abundance(%) [1]

Oth

RE oxide

Fig. 1. Country-wise split up of world’s RE resources.

1.2. Demand and economic importance

superconductors are well known, their importance became greatly enhanced after the discovery of the high temperature superconductors with rare earth and cupric oxide as major constituents [6]. RE-based ecological pigments consisting of metals like Ce, La and Nd are replacing the traditionally used inorganic pigments of high commercial value as they contain toxic metals like cadmium, lead and chromium, the use of which is being strictly controlled or banned in many countries [7,8]. These pigments are used in bulk quantities

The REE find varied applications in many different fields. High purity individual REE are increasingly used as major components in lasers, phosphors, magnetic bubble memory films, refractive index lenses, fibre optics and superconductors [4,5]. Recent innovations in the area of phosphors are trichromatic and superdeluxe lamps. Both of them employ RE ions such as Eu2+ , Eu3+ , Ce3+ and Tb3+ as activators in an oxide, aluminate or borate lattice. Although RE-based

Table 2 Rare earth and yttrium contents of major source mineralsa Element

La2 O3 CeO2 Pr6 O11 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Tb4 O7 Dy2 O3 Ho2 O3 Er2 O3 Tm2 O3 Yb2 O3 Lu2 O3 Y 2 O3

Bastnasite

Monazite

Xenotime

California

China

Eastern Australia

Western Australia

Florida

India

China

Malaysia

32.00 49.00 4.40 13.50 0.50 0.10 0.30 0.01 0.03 0.01 0.01 0.02 0.01 0.01 0.10

27.00 50.00 5.00 15.00 1.10 0.20 0.40 – – – 1.00 – – – 0.30

20.20 45.30 5.40 18.30 4.60 0.10 2.00 0.20 1.15 0.05 0.400 Traces 0.20 Traces 2.10

23.90 46.03 5.05 17.38 2.53 0.05 1.49 0.04 0.69 0.05 0.21 0.01 0.12 0.04 2.41

17.47 43.73 4.98 17.47 4.87 0.16 6.56 0.26 0.90 0.11 0.04 0.03 0.21 0.03 3.18

23.00 46.00 5.50 20.00 4.00 – – – – – 1.50 – – – –

23.35 45.69 4.16 15.74 3.05 0.10 2.03 0.10 1.02 0.10 0.51 0.51 0.51 0.10 3.05

0.50 5.00 0.70 20.20 1.90 0.20 4.00 1.00 8.70 2.10 5.40 0.90 6.20 0.40 60.80

Total REO in marketable 60–70%, 55–60%, 42.51%, concentrates. a Expressed as wt.% of RE oxide.

M

Cat che alysts mic / als (39

%)

T. Prasada Rao, R. Kala / Talanta 63 (2004) 949–959

Ph

Gl

ass

/ce

ra

p os

ho

cs mi

rs

/E

(25

c le

%)

tro

ni

( cs

6%

)

)

%

30

y(

rg

llu

eta

Fig. 2. Consumption of RE in different industrial sectors.

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that popular for the determination of REE in real samples. Spectrography was extensively used in 1970’s for determination of REE in real samples but requires elaborate and cumbersome sample preparation. Energy dispersive, wavelength dispersive and total reflectance X-ray fluorescence spectrometric techniques are multielement techniques but requires elaborate sample preparation in addition to being not sensitive for liquid samples. Neutron activation analysis (NAA), inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP-mass spectrometry (ICP-MS) are the techniques sought after for individual determination of lanthanides. However, the analysis of environmental and geological samples by using the above techniques is still difficult because of the very low concentrations of REE and presence of complex matrix. The main problem with biological samples is the availability of sample. One way of solving this problem as pointed out earlier [13] is by the development of coupled chromatographic–atomic spectrometric techniques. An alternative approach is the use of on-line and off-line (batch or column) preconcentration procedures in conjunction with analytical techniques as shown in the case of natural water samples [14]. 1.4. Preconcentration

in paints as these come under “Green Technologies”. REE have low solid solubility and similar electronic structure to that of aluminium and forms intermetallics with some of the alloying elements present in aluminium matrix. This results in strengthening of aluminium matrix without affecting ductility and improves the electrical conductivity, which is other wise brought down by alloying elements [9,10]. Similarly, REE additions to steel results in cleaner steels with the alteration of the shape and distribution of sulphides and oxysulphides [11,12]. Recently, the use of RE-based catalysts in petroleum refining industry brought forward many an interesting applications. The distribution of REE demand in different sectors is presented in Fig. 2. 1.3. Analytical chemistry of lanthanides Prasada Rao and Biju [13] have reviewed various analytical methodologies developed for lanthanides in metallurgical, environmental and geological samples. Molecular absorption spectrometry and molecular fluorescence has similar absorption or fluorimetric spectra when they are reacted with a chromogenic reagent. Hence, it is difficult to determine traces of lanthanides when they are present together. Flame atomic absorption spectrometry (FAAS) though provides successive determination of lanthanides but is not that sensitive and has been used only scarcely. Graphite furnace atomic absorption spectrometry (GFAAS) is useful for singular element determination of REE and is sensitive unlike FAAS. On the other hand, the interference due to matrix is pronounced and this technique is not

Preconcentration is a process in which the ratio of the amount of a desired trace element to that of the original matrix is enhanced. Preconcentration improves the analytical detection limit, increases the sensitivity by several orders of magnitude, enhances the accuracy of the results and facilitates easy calibration [15]. In general, it can be referred to as the enrichment process consisting of either removing the major component from minor ones or transfer of analyte from a large volume of one phase into a second phase of lesser volume. Enrichment is attained by the use of various preconcentration techniques based on physical, physico-chemical and chemical principles. The techniques generally employed in analytical chemistry are liquid–liquid extraction (LLE), ion-exchange, co-precipitation, and solid phase extraction (SPE). In 2001, Buchmeiser [16] has reviewed developments in off-line and on-line enrichment and quantification of REE with the main emphasis on ion-exchange high performance liquid chromatography and capillary electrophoresis. 1.5. Off-line preconcentration techniques Prasada Rao and Mary Gladis [17] and Prasada Rao and Preetha [18] have reviewed various enrichment techniques in which quinoline-8-ol or its derivatives and naphthols respectively are used as preconcentrating agents in inorganic trace analysis which include lanthanides also. As it is clear from the literature and above two review articles, off-line preconcentration techniques can be classified into batch and column modes.

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1.5.1. Liquid–liquid extraction (LLE) LLE is based on the distribution of a solute between two immiscible solvents. The distribution ratio (D) of a solute is defined as the ratio of the total concentration in the organic phase to that in the aqueous phase at equilibrium. This depends greatly on chemical reactions including chelation, ion-association, dissociation, polymerization and solvation in both the phases. Higher recovery of trace element and higher enrichment factors are obtained when the D value for trace element is large and that of the matrix is small. When the matrix is removed by extraction, the reverse is required for the successful enrichment. The proper choice of extraction agents and their concentration in organic solvents, pH or acidity of the aqueous phase, masking and salting out agents are important. After extraction, the scrubbing of matrix elements selectively from organic phase into the aqueous phase also enhances the enrichment factor [19–21]. 1.5.1.1. Solvent extraction. Crown ethers such as 12crown-4, 18-crown-6 and 15-crown-5 were employed to extract lanthanides from benzoate-benzoic acid solutions into various immiscible solvents, viz. heptanol, octanol, ethyl acetate, dichloromethane, chloroform and 1,2-dichloroethane prior to the determination by thermal lensing detection [22]. The intensity of thermal lens signal in the organic phase could be enhanced to 24-fold which is quite low. Furthermore, the applicability of this procedure to synthetic and real samples were not tested. RE impurities in sodium chloride sample were determined by ICP-AES after preconcentration with ethanol [23]. The detection limits range from 0.78 (for Yb) to 230 ng ml−1 (for Tb) which are quite good. However, the recovery of various lanthanides, yttrium and scandium lie in the range 85.0–95.2% which is rather poor but is not unexpected in view of the varying miscibilities of ethanol in water with change in concentration of sodium chloride. In general, solvent extraction, though often used for separation of RE is not that popular for preconcentration in view of the mutual solubility of phases at higher aqueous to organic phase volumes, formation of emulsion and also cost and toxic nature of extractants and immiscible organic solvents. 1.5.1.2. Cloud-point extraction. Traces of Er ion was preconcentrated using micelles of non-ionic surfactant polyethylene glycol mono-p-nonylphenyl ether from pH 8.5 borax solution by equilibrating for 10 min at 313 K. The mixture was centrifuged for 5 min at 2000 rpm and cooled at 255 K for 5 min [24]. The enriched Er ion was determined spectrophotometrically. However, the applicability of the developed method was tested only for synthetic samples of permanent magnet and a superconducting material. A similar cloud-point extraction preconcentration procedure developed by same researchers determined free and total Gd in urine samples [25]. However, this method requires prior separation of Gd from concomitant ions by cation-exchange

column separation. Jin et al. [26] preconcentrated REE by cloud-point extraction using polyvinyl alcohol condensed p-formylchlorophosphonazo and determined total RE content spectrophotometrically. This procedure finds application in determining the total RE content in ductile iron and micronutrient fertilizers. In all the above procedures, UV-Vis spectrophotometry was employed as the analytical detection technique after enrichment which enable the determination of singular REE only and total REE if more than one REE is present. 1.5.1.3. Liquid membranes. Ce was concentrated from RE metal mixtures using liquid membranes composed of 13% polybis(succinimide) or 5% bis-2-ethylhexylhydrogen phosphate or 2-ethylhexylhydrogen-2-ethylhexyl phosphonate prior to determination by spectrophotometry [27]. A very high enrichment factor of ∼170 was reported by these researchers which is quite good. However, as reported by authors, the recoveries are very poor, i.e. ∼90%. Kopunec and Benitez [28] have preconcentratively separated individual elements such as of Y, Ce, Eu and Tm and their binary mixtures, viz. Ce–Y, Ce–Eu and Ce–Tm with bis(2-ethylhexyl) phosphoric acid or n-tributyl phosphonate as carriers in supported liquid membranes. However, the above two procedures which adopt liquid membrane preconcentration were not applied to real or synthetic samples. 1.5.2. Ion-exchange Ion-exchange involves exchange of ions of like sign between a solution and a solid of highly insoluble body in contact with it. Analyte of interest can be exchanged from dilute solution with resin either in batch or column mode of operation. Subsequently, the analyte is eluted with a small volume, thus resulting in preconcentration. The ion-exchange resin can be regenerated with a proper electrolyte solution. The capacity of ion-exchangers is defined in terms of the number of exchangeable counter ions in the resin [29]. 1.5.2.1. Cation-exchange resins. REE were preconcentratively separated by passing through a strongly acidic polystyrene cation-exchange resin column (12 cm × 1.5 cm) from Zr, Ti, Fe and Al [30]. The recovery of 0.5–200 ␮g of REE (total) in Zr containing mineral is quantitative on determination by using Arsenazo III spectrophotometric method. However, the individual determination of REE is not possible as UV-Vis spectrophotometry was employed as the analytical detection technique. Rare earth metals and Y [31] preconcentrated on 20 cm bed of Biorad AG 50X-8 cation-exchange resin were eluted with 6 M HNO3 followed by 8 M HNO3 after discarding the eluents obtained on passing 2 M HCl followed by 2 M HNO3 . ICP-AES determination values of various standard reference materials subjected to above preconcentration agreed well with those of NAA and isotope dilution analysis. Furthermore,

T. Prasada Rao, R. Kala / Talanta 63 (2004) 949–959

the developed procedure is virtually free from interference due to Be, Co, Cr, Fe, Mn, Th, U, V and Zr. REE, viz. Gd, Sm and Eu [32] were preconcentrated in batch mode onto carboxyl cation-exchange resin by stirring for 2 h and determined by AES by packing into a cavity of carbon electrode. This procedure can readily be used for the determination of above mentioned REE in complex real metallurgical sample such as steel. Matterny and Macejko [33] employed Dowex 50W cation-exchange column to preconcentrate Y and lighter REE, viz. Nd, Eu and Sm. The above elements were separated by linear gradient or isocratic elution with 2-hydroxy-2-methylpropionic acid and determined by using Arsenazo III spectrophotometric method. The developed procedure has not been applied to real or synthetic samples. Sodium form of Chelex-100 resin (200–400 mesh) was used for preconcentration of radionuclides of Ce, Eu, Gd and Y from highly saline solutions and determined by ␥-ray spectrometry [34]. The preconcentration of REE was achieved by heating the perchloric acid digested human blood serum sample solution with 0.5 g of Chelex-100 resin at 80 ◦ C for 3 h after adjusting the pH to ∼6.0 [35]. ICP-MS determination of nitric acid eluted REE allow their quantification in human blood serum samples which lie in the range 0.82–214 pg ml−1 . 1.5.2.2. Chelating resins. Traces of REE present in CASS-3 sea water were preconcentrated onto CETAC-DSX100 chelating resin and eluents were subjected to ICP-MS analysis [36]. This procedure results in enrichment factors in the range 40–48 which are reasonably good. Yang et al. [37] employed Arsenazo III chelating resin to preconcentrate trace REE in GSD-2 reference material. The main drawbacks of the procedure are: (i) long preconcentrative separation procedure (∼1 h) and (ii) the recoveries are very poor (i.e. 94.4–103.3%). Ce and La present in coal and other energy related materials were preconcentrated using poly(dithiocarbamate) chelating resin and determined by ICP-AES [38]. The recoveries were generally >96% which are reasonably good at the concentrations involved in this procedure. Trace REE was preconcentrated onto Thorin loaded chelating resin (Amberlite XAD-7) by mechanically shaking 10 ml of sample with 10 mg of resin for 2 h [39]. The resin was filtered off on a Millipore membrane filter and subjected to determination by wavelength dispersive X-ray fluorescence spectrometry. The developed preconcentration procedure allows an impressive enrichment factor of ∼500. Further, the accuracy of the method was good as the results obtained on analysis of certified reference geological material, viz. USGS-G2 compare well with certified values. Masi and Olsina [40] have developed analogous procedure for preconcentration and determination using Amberlite XAD-4 or XAD-7 resins loaded with Quinoline-8-ol and 2-[2-(5-chloropyridylazo)-5 dimethylamino] phenol. However, this procedure has not been tested for its applicability to any real or synthetic samples.

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Trace REE was preconcentrated onto P 507 extraction resin packed onto 12 cm × 0.8 cm column and determined by emission spectroscopy [41]. This procedure, in addition to providing impressive enrichment factors of ∼105, allows successful analysis of REE in ores. REE impurities in 5 N Gd2 O3 were determined by carbon powder spectrography after P 507 extraction resin [42]. An impressive enrichment factor of >4000 was reported by employing the above preconcentration procedure. 1.5.3. Co-precipitation Co-precipitation is the process of carrying down of analyte along with the co-precipitant under the conditions of preconcentration or surface adsorption. Mixed crystal formation and occlusion mechanisms are considered to be responsible for co-precipitation [43]. Precipitation from homogenous solution, prolonged digestion, re-precipitation and washing with an appropriate solution to avoid peptisation are some of the means to minimize the loss of the desired element. Co-precipitation is used mainly to preconcentrate the analytes present at trace and ultratrace level. Das [44] has described two procedures for preconcentration of REE in geological materials: (i) by co-precipitation with calcium oxalate, separation on NH4 + form of Dowex 50X-8 resin and subsequent co-precipitation with calcium fluoride; and (ii) by double co-precipitation of fluorides of REE onto calcium fluoride, respectively. Both the procedures are rather laborious. In 1982, Kramer and Davies [45] have preconcentratively separated via co-precipitation of 90 Y, 147 Pm and 144 Ce with calcium oxalate and sequentially separated by LLE using bis(2-ethylhexyl) phosphate as extractant. This method has been applied to urine samples. The recoveries of 90 Y, 147 Pm and 144 Ce were 65, 90 and 87% which are very poor. Selective preconcentration of REE was achieved by Iwata et al. [46] by substoichiometric precipitation of calcium oxalate. This procedure in conjunction with NAA has been successfully employed for analysis of REE in citrus leaves. The reasonably good coefficient of variation value of <5%, was reported by these authors. Recently, Shen et al. [47] have adopted calcium oxalate co-precipitation procedure for the preconcentration of REE during the subsequent determination by spectrophotometry, spike isotope analysis and NAA. Though, authors claim no interference from extraneous ions, the developed procedure has not been tested for any real sample analysis. Ce present in iron and steels [48] was co-precipitated with lanthanum fluoride and determined spectrophotometrically with o-toluidine as chromogenic reagent. The precision of the developed procedure is quite impressive as the coefficient of variation was ∼2% during the determination of 0.01–0.05% of Ce in steels. Trace and ultratrace amounts of REE were co-precipitated with Ti(OH)4 –Fe(OH)3 and analysed by ICP-MS [49]. This procedure has been successfully tested for analysis of six Chinese soils and sediment reference materials as the relative errors between found and

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certified values were below 10%. Bhagavathy et al. [50] have developed a co-precipitative preconcentration procedure wherein individual REE are enriched onto iron(III) hydroxide from weakly alkaline solutions (pH > 9.0). The preconcentrated REE along with iron(III) hydroxide was pelletized and determined by first order derivative energy dispersive X-ray fluorescence spectrometry. The developed procedure offers very good recoveries for all REE. Microgram amounts of REE were separated from large amounts of Re, Zr, U, Be and Al by co-precipitation with MnO2 as carrier from fluoride solutions and determined by UV-Vis spectrophotometry [51]. The developed procedure has not been tested for real sample analysis. Wang et al. [52] co-precipitated 144 Ce from weakly alkaline solutions (pH 8–10) using MnO2 as carrier. Though the recovery is good (∼96%), application to real or synthetic samples has not been tested. Diantipyrinyl methane [53] was used for co-precipitation of mg amounts of lanthanoids such as Nd, Sm and La present in industrial waste waters for subsequent spectrographic analysis. The precision, as expected is rather poor as the coefficient of variation was ∼10% during the determination of 0.05–0.25 ppm of Nd, Sm and La. 1.5.4. Solid phase extraction (SPE) The basic principle of SPE or solid–liquid extraction is the transfer of analyte from aqueous phase to the active sites of the adjacent solid phase. This transfer is stimulated by the selection of appropriate operational conditions in the system of three major components water (liquid phase)–analyte–sorbent [54]. The analyte after sorption on solid phase is either desorbed with a suitable eluent or the analyte along with the sorbent is dissolved in a suitable solvent and further analysed. Hitherto, LLE is most often used due to its simplicity, rapidity, ready adaptability to scale up and easier recovery of analyte and solvent. SPE is replacing LLE due to several advantages offered by the former technique [55–57]. These include 1. 2. 3. 4. 5. 6.

speed and simplicity in the off-line mode; absence of emulsion which preclude addition of modifier; low costs due to low consumption of reagents; flexibility; safety with respect to hazardous samples; easier incorporation into automated analytical techniques and; 7. more importantly environmental friendly. SPE is based on the distribution of analyte between a solution and a solid sorbent by mechanisms such as physical sorption, complex formation and other chemical reactions on or in the sorbents. 1.5.4.1. Naphthalene. 1-(2-Pyridylazo)-2-naphthol (PAN) modified naphthalene was prepared by dissolving the named reagents in the ratio of 1:9 respectively in ace-

tone and adding dropwise to the aqueous solution while stirring [58]. REE (>50 mg) present in 500 ml of aqueous sample solution, whose pH was adjusted to >9.0, were preconcentrated onto PAN modified naphthalene, pelletized and determined using EDXRF. The developed procedure has been successfully utilized for the determination of individual REE in xenotime samples. A similarly prepared 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone modified naphthalene was used for preconcentration of La and Eu [59]. The sample solutions (adjusted to pH 4.7) were passed through a PTFE microcolumn (20 mm × 1.4 mm (i.d.)) containing chelate modified naphthalene at a rate of 1 ml min−1 . The solids in the column were dissolved by pumping acetone at 40 ◦ C through the column and La and Eu contents were determined by Electrothermal analysis (ETA)-ICP-AES. The method finds application for the determination of La and Eu in environmental and biological materials. Quite impressive detection limits (160 and 30 pg) and coefficient of variation (4.2 and 3.1%) respectively were reported by Xiong et al. [59] for La and Eu respectively. Cai et al. [60] developed a sensitive and rapid method for the determination of La, Eu and Yb by ICP-AES after solid phase extraction with tribromoarsenazo-cetylpyridinium bromide modified microcrystalline naphthalene and elution with 3.0 M HCl. The detection limits of this method for La, Eu and Yb were 1.3–8.6 ng ml−1 and the coefficient of variation for nine replicate determinations of 0.5 ␮g ml−1 of REE were 1.4–2.2% which are extremely good. This method has been applied to the analysis of La, Eu and Yb in vehicle exhaust particulates (NIFS CRM no. 8) and bush branches and leaves (GBWO 7602 GSV) and La in citrus leaves (NIST SRM 1752). The results obtained were found to be in good agreement with certified values. 1.5.4.2. Activated carbon. REE-Quinoline-8-ol complexes formed in weakly alkaline solutions (pH > 9.0) were preconcentrated onto activated carbon and dried at 80 ◦ C, pelletized and were subjected to first order derivative EDXRF [61]. The recoveries were found to be good for various REE as they lie in the range 98–100%. 1.5.4.3. Cellulose. Trace REE present in sea water were preconcentrated onto a microcolumn (10 cm × 6 mm i.d.) packed with Quinoline-8-ol-5-sulphonic acid cellulose [62]. The preconcentrated REE were eluted with 5 ml of 3.0 M HNO3 resulting in enrichment factors of ∼200 which is very good. The recoveries of the above method in conjunction with ICP-MS for 15 REE were in the range 94–103% which are reasonable at the concentration levels involved in this procedure. 1.5.4.4. Silica gel. Ionic and colloidal forms of Eu(III) were found to adsorb quantitatively on MnO2 deposited silica gel [63] in < 0.01 M HNO3 solutions and desorbed by using 0.5–1.0 M HNO3 . Silica gel loaded with PAN

T. Prasada Rao, R. Kala / Talanta 63 (2004) 949–959

[64] was used for preconcentration of REE from weakly alkaline solutions (pH 8.0–9.5) and determination of REE was done subsequently using EDXRF. Impressive detection limits of 0.26, 0.06 and 0.61 mg l−1 and coefficient variation values of <5% were reported for Pr, Nd, Sm and Y. However, these two procedures have not been tested for their applicability to either synthetic or real samples. Silanized microgranular silica gel (0.07–1 mm) coated with petroleum sulphoxide was used for extraction chromatographic preconcentration of REE impurities in high purity yttrium oxide [64]. The eluate was precipitated as oxalate and dissolved in aqueous 50% HCl. This procedure resulted in enrichment factors of the order of 40 which are rather low. In addition, very poor recoveries of 65–120% for 0.1–2.0 ␮g g−1 of REE with coefficient of variation ranging from 9 to 19% were reported by Peng et al. [65]. 1.5.4.5. Titanium dioxide. Geological reference materials containing REE were mineralized and subjected to preconcentration onto nanosized TiO2 after adjusting the pH to ∼7.0 [66]. The samples on analysing by electrothermal vaporization (ETV)-ICP-AES gave rather poor recoveries, viz. 96.3–109%. 1.5.4.6. C18 cartridge. Bis(2-ethylhexyl) hydrogen phosphate and 2-ethylhexyl dihydrogen phosphate adsorbed on a Sep-Pak C18 cartridge were used for preconcentration of trace REE present in sea water [67]. An impressive 29–1000-fold preconcentration of REE was achieved prior to ICP MS quantitation. The results were comparable to those obtained by ICP-MS coupled with solvent extraction preconcentration and NAA after Chelex-100 preconcentration. The coefficient of variation of all analytes were 2.7 and 1.0% (n = 4) for 1 and 5 l samples, respectively which are extremely good. 1.5.4.7. Polyurethane foam. Cerium(IV) in glasses were determined by NaI(Tl) well type crystal and counting device after preconcentration with tri-n-butyl phosphate and thenoyl trifluoroacetone loaded polyurethane foam [68]. A detailed procedure for the preparation of loaded polyurethane foam cylinder (4.5 cm diameter × 2 cm height) was given by the authors. Furthermore, the presence of extraneous ions like Ca, Mg, Na, Al, K, Co, Ni and Sr did not interfere greatly during the solid phase extractive preconcentration of Cerium(IV). 1.5.4.8. Polymer supports. Polyarsenazo-N fibrous chelating sorbent packed in a column bed of 2 cm diameter × 7 mm height was used for preconcentration of ␮g amounts of lanthanoids and Y present in 0.5 M NaCl sample at a flow rate of 10 ml min−1 [69]. The retained analytes are eluted with >30 ml of 6 M HCl or 5 M H2 SO4 providing moderately good enrichment factors of around ∼70. Though

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fulvic acids present in natural waters do not interfere; U, Th, Zr, Hf, Nb, Ta, Pa, Am and Cm were reported to coextract with lanthanides. Polyacrylonitrile fibres were modified in four steps to produce poly(acrylaminophophonic dithiocarbamate) chelating fibres [70]. These fibres were cut into <0.25 mm and slurry packed in a column (5 cm × 4 mm i.d.) to preconcentrate REE present in sea water at a flow rate of 7.0 ml min−1 . The preconcentrated REE were eluted with 5 ml of 0.01 M ammonium citrate at a flow rate of 1.0 ml min−1 and determined by ICP-MS. This procedure provides very good enrichment factors of the order of ∼200. Very impressive detection limits (0.2–2.0 ng l−1 ) and coefficient of variation (<5%) were reported by these authors. REE present in sea water (1.5 l) was passed through a column of Quinoline-8-ol immobilized on polyacrylonitrile hollow fibre membrane at a flow rate of 10 ml min−1 [71]. The preconcentrated REE were eluted with 5.0 ml of 1.0 M HCl/0.1 M HNO3 and analysed by ICP-MS. Though the detection limits (0.21–2.7 ng l−1 ) are quite good, the recoveries are rather poor as they lies in the range 91–107%. Non-ionic polymeric adsorbents, viz. styrene-divinyl benzene, Amberlite XAD-4 functionalized with o-vanillin semicarbazone were used for the column separation/preconcentration of La(III), Ce(III), Th(IV) and U(VI) [72]. The resins exhibited good chemical stability, reusability and faster rate of equilibrium for their determination by spectrophotometry and the simultaneous confirmation of the results by ICP-AES and ET-AAS. The developed method allows sequential chromatographic separation of binary or ternary mixtures of the above inorganics. Further, the method was also applied to monazite sand and some standard geological materials. Impressive recoveries of 96–98% were reported by these authors [72]. Dysprosium ion imprinted polymer (IIP) particles were prepared by thermal co-polymerization of styrene and divinyl benzene monomers in the presence of Dy(III)-5,7dichloroquinoline-8-ol-4-vinyl pyridine ternary complex and 2,2 -azobisisobutyronitrile (AIBN) as initiator [73]. The solid phase extraction using Dy IIP particles results in enrichment factor of ∼10 but gave selectivity coefficients of 60–180 for Dy with respect to Y, Nd, La and Lu which are much higher in case of Y and Lu compared to the best and commercially used liquid–liquid extractant, viz. di-(2-ethylhexyl) phosphoric acid. Same authors on post ␥-irradiation of Dy IIP particles observed an enhancement of selectivity coefficients by two to four times compared to the unirradiated Dy IIP particles [74]. It is pertinent to mention here that post ␥-irradiation did not affect the preconcentration efficiency of IIP particles. Kala et al. [75] compared the enrichment efficiency of Erbium IIP particles formed by thermal, photochemical and ␥-irradiation polymerization by copolymerization of Er(III)-5,7-dichloroquinoline-8-ol-4-vinyl-pyridine template in presence of methyl methacrylate (functional) and ethylene glycol dimethacrylate (crosslinking) monomers and AIBN (in case of thermal and photochemical only). It

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was shown that the enrichment efficiency increases in the order of radiochemical < photochemical < thermal. 1.5.5. Other Off-line enrichment techniques 1.5.5.1. Chemically modified electrodes. A W wire chemically modified electrode (CME) was prepared by treating with 2-thenoyltrifluoroacetone and 1,10-phenanthroline [76]. This CME preconcentrates Eu(II) in pH 5.2 hexamine– HCl buffer solution. The method has been applied to the analysis of CRM, viz. Mussel sample. The coefficient of variation for 10 determinations of 2.9 nM of Eu was 5.26% which is rather poor. 1.5.5.2. Donan dialysis. Dinunzio et al. [77] have developed a preconcentration procedure for REE involving Donnan dialysis method. Enrichment factors of 16–22 were reported for La, Eu and Lu which are quite low. The presence of Al in the receiver electrolyte increased the enrichment factor and reduced the interfering effect of phosphate. High values of coefficient of variation (2–6%) were reported during the determination of La, Eu and Lu. 1.5.5.3. Chemofiltration. De Vito et al. [78,79] evaluated the chemofiltration preconcentration methods involving Thorin or Arsenazo III as complexing agents for the REE. The resulting thin film specimens were then analyzed by XRF. The latter procedure alone with detection limits of 17.5 (for Eu) to 34 ng ml−1 (for Gd) was applied to the determination of REE in NIST standard reference glass sample. 1.6. On-line preconcentration techniques Various on-line flow injection preconcentration procedures developed for inorganics were reviewed by us elsewhere [80,81]. The advantages of on-line flow injection analysis (FIA) techniques include high efficiency, simple on-line operation, low sample and reagent consumption, relatively simple and compact hard ware and freedom from contamination. FIA techniques offer improvement in efficiency and in automation of different separation and preconcentration processes [82]. Pretreatment steps performed using FIA are rapid and require less than 1 min for pretreatment and analysis including chemical conversions [83]. More interest in using the FIA is to implement on-line derivatization reactions [84], separation processes [85,86] as well as a number of sample handling modes to fit the initial sample conditions and suitable for single or multielement detection [87–89] or implementation of flow through (bio)chemical sensors [90–92] among others. 1.6.1. FIA-ICPMS A flow injection on-line preconcentration procedure based on collection onto Knotted reactor as hydroxides of REE

and elution with 1.0 M HNO3 for 80s at a flow rate of 1.2 ml min−1 [93]. The resulting solution was analyzed by ICP-MS. Under these conditions, an enrichment factor of 55–75 were achieved which are moderate. Quite impressive detection limits (0.06–0.27 ng l−1 ) and coefficient of variation (1.8–4.2%) for 16 determinations were reported. The results obtained by this method for geological reference materials BIR-1, BHVO-1 and MGRI agree well with certified reference values. Further, the method has been applied for the determination of REE in four pore water samples. Flow injection-ICP-MS [94] procedure was developed based on the collection onto iminodiacetate type chelating resin disc (5 mm diameter, 10 ␮m pore size and 0.5 mm thickness). A quite impressive 100-fold preconcentration was achieved during the determination of REE in natural water samples. Quantitative recoveries were obtained for 1 pg ml−1 of REE with coefficient of variation in the range 4.7–9.7% which are quite impressive in view of the concentration levels involved. Subparts per trillion levels of REE in natural water samples were determined by ICP-time of flight (TOF)-MS after preconcentrative separation on Knotted reactor [95]. With 30 s preconcentration time and a sample flow rate of 4.4 ml min−1 resulted in enrichment factors in the range of 15–22 for the different REE which are rather low. On the other hand, the detection limits are highly impressive as they lie in the range 3–40 pg l−1 . Willie and Sturgeon [96] have developed on-line preconcentration procedure using a column containing iminodiacetate based resin. The preconcentratd REE from sea water samples were determined by ICP-TOF-MS with impressive detection limits of 20–50 pg l−1 . The precision of the method is also quite good (5–6% for n = 5) in view of the very low concentrations involved. Very low sample throughputs (5 h−1 ) were reported by using this method. 1.6.2. FIA-ICP-AES REE were preconcentrated onto carboxyamino and carboxy imino group carrying sorbents and determined by ICP-AES [97]. Quite impressive enrichment factors (∼99) were reported by the authors. Reasonably good detection limits (0.1–4 ␮g l−1 ) and coefficient of variation (3–5%) were reported by Gribneva et al. [97]. Spirulina plancensis bacteria immobilized controlled pore glass packed into a methylmethacrylate microcolumn (2.5 cm × 2 mm i.d.) was used for preconcentration of La and Nd in high purity Ce(III) samples [98]. The determination by ICP-AES resulted in reasonably good detection limits of 9 and 0.54 ng ml−1 of La(III) and Nd(III) respectively. Lee et al. [99] developed a FIA-ICP-AES method based on the preconcentration of Sm and Nd onto a minicolumn of PTFE tubing (4 cm × 0.25 mm o.d.) filled with Dowex 50X-8 resin. The method was applied to the determination of Sm in coal fly ash with a coefficient of variation of <4% which is quite good. A microcolumn packed with immobilized 1-phenyl-3-methyl-4-benzoyl-5 pyrazolone on microcrystalline naphthalene was used for on-line preconcentration

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of La, Nd, Eu, Dy and Y [100]. This procedure gave results in good agreement with the reference values during the analysis of certified geological materials. Reasonably good detection limits in the range 0.06–0.32 mg l−1 of REE were reported by Huang et al. [100]. Nanometer sized TiO2 sorbent was used by Liang et al. [101] for on-line preconcentration of La, Y, Yb, Eu and Dy and determined by ICP-AES. This method has been successfully applied for the determination of trace REE in some environmental samples. 1.6.3. FIA-spectrophotometry REE were preconcentrated and separated from complex matrices by being adsorbed at pH 5.0 onto a microcolumn packed with aminophosphonic carboxylic acid resin [102]. The adsorbed analytes were desorbed with 1.0 M HCl and reacted with Arsenazo III in a reactor coil during spectrophotometric detection at 649 nm. Reasonably good coefficient of variation value of <4% was reported by these authors for 0.2 ␮g ml−1 of La. However, this procedure is useful only for determining total RE content in a geological and Al alloy samples. 1.6.4. FIA-fluorimetry Cerium(III) at concentrations of 0.3–10 ng ml−1 was preconcentrated onto alkaline treated Teflon tubing column [103]. The adsorbed species were dissolved in 58 ␮l of 1.5 M HCl and determined by using spectrofluorimetric technique. However, the applicability of this procedure to synthetic or real samples was not tested.

2. Future trends Despite being regarded by many analytical chemists as time consuming additional chemical step introducing contamination and precision problems, “preconcentration” still enjoys important role to solve certain critical analytical problems that arise during the determination of lanthanides by various analytical techniques. Thus, demands for ultratrace level determination of lanthanides for purposes of exploration, biological evaluation and monitoring of environmental impact will stimulate the development of newer and newer preconcentration procedures in conjunction with analytical techniques. Co-precipitation, though, traditionally used for preconcentration of rare earths, is not that popular now-a-days as the determination procedure has to be applicable in the presence of large amounts of co-precipitant. Ion-exchange and LLE still find increasing use for preconcentration/separation of lanthanides. The main drawback of the former technique is the need for frequent replacement of ion-exchange resins required for preconcentration. LLE is simple, rapid and ready to scale up and recovery of analyte and extractant is easier. However, LLE technology has drawbacks such as low enrichment factors, emulsion formation necessitating the

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addition of modifiers, higher consumption of reagents, waste water problems, etc. On the other hand, SPE eliminates the above drawbacks of LLE during preconcentration of inorganics, in general and lanthanides in particular. The use of on-line flow injection analysis preconcentration is preferred over off-line techniques wherever applicable as the number of samples that can be analysed per hour is as high as ∼30 with 1 min loading time and offer good precision as these are computer controlled. The kinetics of extraction also play a role in obtaining higher selectivity coefficients for analyte in presence of other co-existing metal ions when they are present together in admixtures. Future direction in the fields of preconcentration of lanthanides lies with increasing use of different solid phase extractants prepared via. new synthetic strategies and their utilization in on-line mode prior to determination by using single or multielement analytical techniques.

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