Separation/Preconcentration Techniques for Rare Earth Elements Analysis

2.1 Precipitation/coprecipitation

3.1 Affecting factors for LLE of REEs

LLE is commonly used for the separation of REEs from non-REEs, group separation of REEs and separation of single REE to improve the sensitivity and selectivity for REEs analysis. And the affecting factors for LLE of REEs are mainly the selection of extractants, extractant concentration, medium pH and salting-out agents.

3.2 Extractants for REEs

In practical analysis, an appropriate extractant is required to feature with good selectivity, low water solubility, high extraction capacity, good stability and safety; in terms of chemical property, low density, high surface tension, low viscosity and high boiling point and flash point are required. According to the structure and property, along with the extraction mechanism, the extractants for REEs can be divided into neutral phosphorus compounds, acidic phosphorus compounds, organic amines, neutral oxygen-containing compounds, chelates and others (including supramolecule, e.g., crown ethers and calixarene).

3.2.5 Chelate extractant for REEs

Chelate extractants can form stable electrically neutral complex with RE ions which can be extracted by organic solvents. Besides β-diketones (e.g., acetylacetone, thenoyltrifluoroacetone [TTA], 1-phenyl-3-methyl-4-benzoyl-pyrazolone [PMBP]), homologs of 8-hydroxyquinoline and ketoxime derivatives (Table 11) are also commonly used chelating extractants for REEs.

Table 11:

Some chelating agent for REEs extraction.

Name	Structural formula	Trade names and abbreviation	Solubility (H2O, g/L)
Lauryl hydroximic acid	i-C11H23CONHOH	H106	0.02
5,8-Diethyl-7-hydroxy-6-dodecanone oxime	CH8(CH2)3CH(C2H5CH(OH)C(NOH) CH(C2H5)(CH2)3CH3	N509, LIX-63
2-Hydroxy-5-dodecyl benzophenone oxime		3045, LIX64
2-Hydroxy-5-octyl benzophenone oxime		N510	0.005
2-Hydroxy-5-sec-octyl oxy benzophenoneoxime		N530	0.001
7-Dodecenyl-8-hydroxyl-quinoline		N601, Kelex100	0.003

TTA, colorless acicular crystal, is soluble in benzene, methylbenzene and other organic solvents. And RE³⁺ can be extracted by TAA at pH 3–4 (Table 12). TTA is usually employed to synergistically extract REEs with other solvents such as ethylenediamine, 2,2,2-terpyridine and neutral phosphorus reagents.

Table 12:

Extraction constants for REEs extraction by PMBP and TTA [13].

RE3+	La	Ce	Pr	Nd	Pm	Sm	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y
pKα (TTA)	10.51	9.45	8.85	8.58	8.06	7.68	7.57	7.51	7.03	7.25	7.2	6.96	6.72	6.77
pKα (PMBP)	7.19 4.40	6.56	5.81	5.67		5.00	4.77		4.42				3.89

PMBP [5] is also a good β-diketone reagent for REEs extraction, with higher extraction capacity, acidity and faster reaction dynamic compared with TTA, and it has become one of the main extraction agent for the separation and preconcentration of REEs. 1-pheny-3-methyl-4-acetyl-pyrazolone is a homolog of PMBP, and the extraction efficiency of heavy REEs could reach more than 90 % under pH3. Oxime and hydroxylamine compounds have been investigated for heavy REEs extraction. Haraguchi et al. [14] synthesized several N-phenyl hydroxamic acid, among them, 2-hexyl-N-phenyl-decyl hydroxyl oxime acid is considered as the most suitable agent for the extraction of heavy REEs; the extraction capacity of N-p-methoxybenzoyl, -p(n, iso, t)-butylbenzoyl, -propoxybenzoyl, -m-nitrobenzoyl, -m-cyanobenzoyl-N-phenylhydroxylamine for REEs was discussed, along with, the effects of substituent in extractant molecules and the acidity of the reagents on the separation factor between REEs; it was found that the smaller the pK value of the extractant is, the greater the extraction constant is [15, 16, 17, 18].

3.2.6 Supramolecular extractants and others

Crown ethers and calixarenes are supramolecular compounds which can be used for the complexing and extraction of REEs [19, 20]. In the presence of counter anions, 18-crown-6 and benzo15-crown-5 exhibit very high extraction efficiency for REEs; Inoue et al. [21] used dicrown ether to extract REEs-picric acid complexes, and the extraction efficiency was increased. Calixarene has also been studied for the extraction of REEs [22], and some supramolecular compounds as REEs extractants are listed in Table 13. Calixarene 1,3-bis(diethylamide)generation of calixarenes [4] exhibits a high extraction efficiency for REEs, which is 20–22 % higher than that obtained by crown ether and dicrown ether.

Table 13:

Some supramolecular as REEs extractants.

Name	Structural formula	RE3+	Counter anion	Solvent	Ref.
18-Crown-6		La–Lu La–Ga	ClO4− Picric acid	CH2Cl2 CH2Cl2	[20] [23]
Octanedione 1,8-bis(4-benzo-15-crown-5)		La–Gd	Picric acid	CH2Cl2	[21]
1,3-bis(ethylenedioxy amide) is substituted calixarene		La–Lu	Picric acid	CH2Cl2	[24]
Hexahomotrioxacalix [3]arene		Sc, La, Y and Lu	DMSO		[25]

Sulfoxide with S–O bond is also a good extractant for REEs. Among them, dimethylsulfoxide was one of the most commonly used, which can form ionic associates soluble in benzene in the presence of anions, such as ClO4− or SCN⁻. Besides, pentylsulfoxide, di-n-octylsulfoxide and tetramethylenesulfoxide were also effective in REE’s extraction. Bis (2-ethylhexyl) sulfoxide can extract REE-SCN⁻ complex into benzene.

Light REEs can be extracted by employing crown ether 18-crown-6 [26] and benzo-15-crown-5 [27]; with the existence of pentadecafluorinecaprilate, light REEs could be extracted by 18-crown-6 in ionic association form [28].

3.3 Extractant concentration and extraction equilibrium constant

Distribution coefficient (D) is proportional to nth power of free extractant concentration; if free extractant concentration is increased by two times, D is increased by 2ⁿ times. When P204 is used for the extraction of RE ions, D is proportional to 3 power of the concentration of P204 dimer; the increase of the concentration of the extractant is beneficial to the extraction of interesting ions.

The higher the stability constant for the complex is formed by REE ions and the extractant, the complex is more stable and more easily to be extracted. Given a specific extractant for the extraction of RE ions, the stability of the complex is related to the charge and radius of RE ions. For RE ions with the same valence, the radius decreases with the increase of the atomic number, and the formed complex stability and extraction equilibrium constant increase with the increase of atomic number (Table 14), while the separation factors for the neighboring RE ions are not increasing with the increase of atomic number (Table 15). Thus, the extraction efficiency of REE ions and the separation factors for neighboring RE ions should be considered simultaneously, and the latter is sometimes more important.

3.4 Medium pH

In acidic complex extraction system for REEs, the effect of pH on D of REE ions in two phases is very obvious. Within a specific acidic complex extraction system, D will increase by 10n times (n is the valence of RE ions) when pH is increased by 1 unit. If the volume of two phases are the same, when D=1, half of RE ions are extracted, and the pH is called half extraction pH value, denoted by pH1/2.

where K is the extraction equilibrium constant and (HL)_o is the concentration of extractant (HL) in organic phase. As can be seen, the higher K or [HL]_o is, the lower the pH1/2 is, indicating RE ions can be extracted more easily. So pH1/2 is used to show the extraction ability for metal ions with the same valence. The pH1/2 for Y extraction by naphthenic acid is the highest among other ions (Table 16), indicating D of Y is the lowest.

P204 and P507 are specific representatives of acidic phosphorus extractants. The extraction ability of P204 is increasing with the increasing of the atomic number of REE (Table 17), and the difference between light and heavy REEs is obvious. P204 form dimeric polymer through hydrogen bonding in nonpolar solvents (e.g., benzene, CCl4), REE ions coordinates with three dimers by replacing three H in it and form neutral six coordinate complex, which is easily dissolved in organic solvents, with the extraction mechanism of cation exchange. In the nitric acid or hydrochloric acid matrix, the extraction efficiency of P204 for REEs will decrease with the increase of the matrix acidity. The basic structure of P507 is similar to that of P204, the reaction for the extraction of REEs is similar along with the performance variation. Due to the C–P bond in P507, the extraction ability of P507 is lower than that of P204, while P507 is featured with easy back-extraction and uneasy emulsification.

3.5 Salting-out agent

In a neutral complex extraction system (e.g., REEs extraction by TBP), the extraction efficiency would be improved by adding salting-out agents (e.g., LiNO₃, NH₄NO3), because the concentration of NO3− would be increased with the addition of salting-out agents, benefiting the increasing of D. Additionally, the hydration of salting-out agents would decrease the concentration of free water molecules in the extraction system, reducing the concentration of hydrated RE ions and increasing the activity of RE ions.

Dimethyl methyl phosphate heptyl ester (P350) presents much better performance for REEs extraction than TBP, the effect of concentration of salting-out agents LiNO₃ on D and separation factor (α) in the separation of La and Pr is very significant (Table 18).

In the ion association complex extraction system, the addition of salting-out agents would also help the extraction of REE ions (Table 19). In the extraction of REE ions by N263, salting-out agents can participate the ion association complex formation as reactants, and increase the average activity coefficient of RE(NO3)n. On the other hand, salting-out agents as complexing agents can improve the complexing of REE, and decrease the distribution ratio.

3.6 Extraction systems for REEs and their application

4.1 Operation modes and mechanism

To extremely decrease the volume of the extractant phase and simplify the experimental apparatus and operation procedure, analysts have spent a lot of energy on the configuration of experimental apparatus and exploration of microextraction mechanisms. So far, the most representative techniques in LPME include SDME, HF-LPME, dispersive liquid–liquid microextraction (DLLME) and solidified floating organic drop microextraction (SFODME).

4.2 Single-drop microextraction

In this extraction mode (Figure 6), the extractant is suspended on the needle tip of the microsyringe, and immersed in the sample solution, contacting directly with target analytes and sample matrix. Since the extraction medium is in the form of a single drop, this type of microextraction is called SDME. Direct-SDME or direct immersion (DI)-SDME, in which the extraction medium without any special supporter, is simple and inexpensive.

In direct-SDME, the analyte (A) is distributed between aqueous sample solution and organic solvent [40]:

The distribution coefficient of A between two phases (Korg/d) can be defined as

where Ceq,org and Ceq,d are the concentration of A in organic phase (acceptor phase) and sample solution (donor phase), respectively, under equilibrium.

Then Ceq,ord can be calculated by eq. (11):

where Ci is the initial concentration of A in sample solution, Vd and Vorg are the volume of sample solution and organic solvent, respectively.

As can be seen from eq. (11), a high Korg/d is beneficial to the extraction.

The two-phase micro-extraction mode is only applicable to a strong or moderate lipophilic analyte (Korg/d>500) instead of strongly hydrophilic analyte [40]; for acidic or alkaline analyte, the analyte should be existing in deprotonated form for extraction by adjusting sample pH to increase the partition coefficient.

To achieve the LPME of REE ions, PMBP–benzene was used as the extraction solvent (10 μL), and a 500-fold EF for La was obtained [41]. In this microextraction procedure as shown in Figure 4, perfluoroalkoxy (PFA) tubing was used to connect the extraction chamber, and a constant-flow pump and a 10-L microsyringe were employed to introduce the aqueous solution and extraction solvent, respectively. The sample solution was continuously pumped “around” the drop, allowing the analytes to be extracted efficiently. It is called continuous-flow microextraction. With some modification to the continuous-flow SDME setup that the waste outlet of tubing was placed into the sample reservoir, cycle-flow SDME was developed and successfully applied in the determination of trace metals in biological samples [42]. It allows further reduction in the sample size and non-attended operation since there is no danger of the sample vial running out.

Figure 4:

The experimental setup for continuous-flow SDME [41, 42].

Besides the two-phase microextraction mode mentioned above, Ma and Cantwell [43] combined solvent microextraction with simultaneous back extraction into a single microdrop, which was called three-phase (liquid–liquid–liquid) microextraction. As shown in Figure 5, n-octane was employed as an organic liquid membrane, confined inside a Teflon ring over a buffered aqueous sample solution. With the help of a microsyringe, a microdrop was left suspended inside the organic liquid membrane. The target analytes were initially extracted into the organic liquid membrane (neutral at high pH) and back extracted simultaneously into the acidic microdrop (protonated at low pH).

Figure 5:

Schematic diagram of “Ma and Cantwell” type three-phase SDME [43].

In three-phase SDME, the analytes are extracted from an aqueous donor phase to an aqueous acceptor drop covered with an organic layer. Due to the large volume ratio between the sample donor phase and the acceptor drop as well as the thin organic layer, high EFs can be obtained with three-phase SDME in a short time. The convenience and efficiency of in-line coupling with some analytical instrument, e.g., CE, are additional advantages of three-phase SDME. Most acidic or basic analytes can be enriched by adjusting the pH to promote neutral forms of the analytes in the donor phase and their charged forms in the acceptor phase in three-phase SDME.

Besides the acidic or basic analytes, three-phase microextraction coupled with CE-UV was employed for the analysis of trace REEs by using 40 mmol/L PMBP as the extractant and 4 % (v/v) formic acid as back-extraction solution [44]. Under the optimized conditions, the LODs of REEs were in the range of 0.19–0.70 ng/mL. The developed method was successfully applied to the determination of trace amounts of REEs in water samples.

4.3 Hollow fiber liquid phase microextraction

SDME is simple, inexpensive, rapid and available for headspace extraction, whereas requires careful and elaborate manual operations to overcome the problem of drop dislodgment, which would deteriorate the reproducibility. Prolonged extraction times and faster stirring rates are unfavorable in SDME, since they typically result in drop dissolution and/or dislodgment. In order to solve this problem, in 1999, Pedersen-Bjergaard and coworkers [45] introduced a method named HF-LPME, in which the micro-extract is contained within the lumen of a porous hollow fiber. With the hollow fiber supporting/protecting the micro-extract, HF-LPME is much more robust than SDME with extra sample clean-up ability. Similar to SDME, HF-LPME can also be performed in both two-phase and three-phase microextraction modes. In the two-phase HF-LPME, the analyte is extracted from the aqueous sample into the organic phase immobilized in the pores and lumen of the hollow fiber. In the three-phase mode (hollow fiber liquid–liquid–liquid microextraction [HF-LLLME]), the analyte is extracted from the aqueous sample through the organic solvent immobilized in the pores of the hollow fiber into another aqueous phase (acceptor phase) presented inside the lumen of the hollow fiber.

4.4 Two-phase HF-LPME

Figure 6 presents the schematic diagram of two-phase HF-LPME. In this operation mode, the extractants immobilized in the pores and the lumen of the hollow fiber are the same organic solvent, and the analytes are extracted from the aqueous sample (donor phase) directly into the organic acceptor solution. The extraction mechanism is similar to that specified in the section of Direct-SDME. HF-LPME is the most used LPME mode and it can be applied for the analytes with solubility in an organic solvent immiscible with water substantially higher than that in an aqueous medium.

Figure 6:

Schematic diagram of two-phase HF-LPME.

Compared with direct-SDME, two-phase HF-LPME is much stable and simple in terms of operation due to the accommodation and protection of the organic solvents in the hollow fiber.

4.5 Three-phase HF-LPME

Figure 7 presents the schematic diagram of three-phase HF-LPME, also called HF-LLLME. The analyte is extracted from aqueous sample solution through a thin phase of organic solvent inside the lumen of the pores of a polypropylene hollow fiber and finally into an acidic/alkaline acceptor solution inside the hollow fiber. Up to now, this extraction mode is limited to basic or acidic analytes whose solubility can be adjusted in the donor and acceptor solution, without application on the analysis of REEs. For example, in order to extract basic compounds, pH of the sample has to be adjusted into the alkaline region to suppress the solubility of the analyte in aqueous sample solution (donor phase), whereas pH in the acceptor solution should be low to promote the solubility of analyte in the acceptor phase. In this manner, the basic compounds may easily be extracted into the organic phase and further into the acceptor phase. And for acidic analytes, the pH adjustment is just reversed. Following extraction, the aqueous acceptor solution is directly injected into CE, HPLC/LC–MS or other detection techniques for subsequent analysis. In this three-phase system, the acceptor phase is aqueous, which is compatible with HPLC or CE analysis. The acceptor phase in three-phase HF-LPME can also be organic, involving two immiscible organic solvents immobilized in the pores of the HF and filled in its lumen, respectively.

Figure 7:

Schematic diagram of three-phase HF-LPME.

In three-phase HF-LPME, the extraction process of analyte (A) can be specified as follows [40]:

The distribution coefficient of A between organic phase and donor phase (Korg/d) and that between acceptor phase and organic phase (Ka/org) can be expressed as:

where Ceq,a is the concentration of A in acceptor phase under equilibrium, and the total distribution coefficient of A between acceptor phase and donor phase (Ka/d) can be expressed as

In the three-phase HF-LPME system, the extraction efficiency is not only affected by Korg/d, but also Ka/org, both of which determine the whole extraction process.

Under equilibrium, the concentration of A in acceptor phase (Ceq,a) can be calculated by

where Ci is the initial concentration of A in sample solution (donor phase), and Vd, Va and Vorg are the volume of sample solution, acceptor phase and organic solvent, respectively.

4.6 Dispersive liquid–liquid microextraction

Based on ternary component solvent system, DLLME was introduced by Assadi and co-workers in 2006 [46]. The operation mode and required apparatus of DLLME are quite different from SDME and HF-LPME. Briefly, a mixture of a water-immiscible extraction solvent and a so-called dispersing solvent, miscible in both phases, is rapidly injected into the aqueous sample solution by a microsyringe, resulting in a cloudy solution which consists of fine particles of the extraction solvent dispersed in the aqueous phase. In this step, the disperser solvent could help the extraction solvent distributed homogeneously in the whole aqueous solution in a short time, resulting in a large contact area between extraction solvent and aqueous sample, and a consequent rapid attainment of extraction equilibrium. By centrifugation, a small volume of extraction solvent (containing the extracted target analytes) sedimented in the bottom of the conical test tube was obtained and then transferred for subsequent analysis (Figure 8).

Figure 8:

Schematic diagram of DLLME.

For DLLME, the extraction time is defined as the time from the injection of dispersant solution into the sample to centrifugation. Since the contact area between the extractants and the sample solution is quite large, the phase transition from water to the extraction phase is very fast. In DLLME, the EF is defined as the ratio of analyte concentration in the aqueous phase and extraction phase, and extraction recovery (R) (or extraction efficiency) is defined as the ratio of the total amount of analyte in the extraction phase and the aqueous solution. It can be expressed as

where Csed is the concentration of analyte in the extraction phase, Vsed is the volume of extraction phase, C0 is the initial concentration of analyte in sample solution, Vaq is the volume of sample solution and R is extraction recovery (efficiency).

DLLME is a modified solvent extraction method, in which the extraction time is greatly reduced comparing with the other two methods mentioned above. The apparatus involved in the DLLME operation procedure just includes a microsyringe, a centrifuge and some conical test tubes. There is no need for magnetic stirrer, magneton for agitation, hollow fiber and no trouble in the possibility of extraction drop dislodgment. DLLME has some advantages such as simplicity of the operation, rapidity, low sample consumption, low cost, high recovery and high EF. Nevertheless, the reproducibility and anti-interference ability of DLLME are not very satisfactory.

The disperser solvent is required to be miscible with water and extractants, and acetone, methanol and acetonitrile are the mostly used disperser solvents. The extractants are usually required to be immiscible with water with higher density than water and good extraction capacity for target analytes.

Mallah et al. [47] used ionic liquids, 1-hydroxy-2,5-pyrrolidinedione (HYD), in a DLLME for determination of trace REEs in uranium dioxide powder. In this process, an appropriate mixture of extraction solvent and disperser solvent is rapidly injected into an aqueous sample containing Sm Eu, Gd and Dy complexes with HYD, and consequently a cloudy solution is formed. After centrifugation of this solution, the whole enriched phase was determined by ICP-OES. The experimental results indicate that the hexafluorophosphate anions with a partial ion exchange mechanism in ionic liquid solvents play a key role in the electrical neutralization of the lanthanoid complex in DLLME.

Chandrasekaran [48] developed a method based on DLLME and ICP-QMS for the simultaneous extraction and preconcentration of 14 REEs in groundwater. In this method, the REEs are complexed with 2,6-pyridinedicarboxylic acid (2,6-PDCA) in the presence of Aliquat-336 (tricaprylmethylammonium chloride), which enhanced the hydrophobicity of the ion-association complex, resulting in its improved extraction into chloroform. The extraction was carried out after adjusting the pH of the water sample to 4. The REE ions were back extracted from the chloroform layer with nitric acid for determination by ICP-QMS. Under optimum conditions, an average EF of 97 was obtained for 50 mL of water sample for ICP-QMS determination.

DLLME would provide relatively high EFs (e.g., 100-fold), while is generally suitable for water sample analysis due to limited anti-interference capability. Since SPE has excellent sample cleanup ability, DLLME has high EFs, and the desorption solvent of SPE matches the DLLME well, the coupling of SPE and DLLME will endow the dual extraction technique both excellent anti-interference ability and high EFs. Guo et al. [49] proposed a simple and efficient two-step method based on dispersive (D)-SPE and DLLME for the separation and preconcentration of 15 REEs from environmental water and sediment samples, followed by electrothermal vaporization (ETV)-ICP-MS detection. With Chelex 100 as the adsorbent of D-SPE, target REEs were first extracted and the retained REEs were then desorbed by 0.1 mol/L HNO₃. After 125 mmol/L Tris and 40 mmol/L PMBP were added into the above elution solution, target REEs were further preconcentrated into CCl₄ by DLLME. The developed dual extraction technique exhibited high EFs (234–566-fold) and good anti-interference ability.

4.7 Solidified floating organic drop microextraction

SFODME is another new LPME technique introduced by Khalili Zanjani et al. [50], in which small volume of an organic solvent with a melting point near the room temperature (in the range of 10–30°C) is floated on the surface of aqueous solution. The aqueous phase is then stirred for a prescribed period of time, and the sample is transferred into an ice bath. When the organic solvent is solidified, it is transferred into a small conical vial and the melted organic solvent is used for subsequent determination (Figure 9). Compared with conventional solvent extraction, it also has the advantage of great reduction in the amount of organic solvent and thus great enhancement of the EF. It is featured with simple operation, easy batch operation; high stirring rate sustainable, avoiding the problem of unstable drop in SDME and easy separation of organic phase and water phase after solidification.

Figure 9:

Schematic diagram of SFODME apparatus [50].

SFODME is based on equilibrium extraction. The affecting factors found in eqs (19, 20, 21) present the calculation of EF [51].

where Co,f is the final concentration of analyte in organic phase, Caq,f and Caq,i are the final and initial concentration of analyte in sample solution (water phase), Vo and Vaq are the volume of organic phase and sample solution (water phase) at a certain time (t), Ai is the contact area between organic phase and water phase, β is the mass transfer coefficient, Co,f is the final concentration of analyte in organic phase obtained by the calibration.

In SFODME, the main affecting factor is the selection of organic extraction solvent. It should meet the following requirements: (1) immiscible with water; (2) melting point is around room temperature (10–30°C); (3) good extraction ability for target analytes; (4) the density is less than water; (5) compatible to the employed instruments (e.g., GC, HPLC, AAS and ICP-OES). The suitable organic solvents include n-undecyl alcohol, n-dodecanol, 2-dodecanol, 1-bromo-hexadecane, 1,10-dichloro-decane and 1-chloro-octadecyl, and n-undecyl and n-dodecanol are the most commonly used organic solvents in SFODME.

Chen et al. [52] employed a SFODME-based method for the determination of trace Dy and Y in biological and environmental water samples followed by ETV-ICP-MS detection. 1-(2-Pyridylazo)-2-naphthol (PAN) was used as both chelating reagent in SFODME and chemical modifier in ETV for separation, preconcentration and volatilization of the REEs prior to their determination.

4.8 Affecting factors in LPME

The affecting factors are different in different LPME extraction modes. While the main affecting factors include extraction solvent, sample pH, stirring rate, extraction time, extraction temperature and ionic strength. For the extraction of REEs, the selection of chelating agent is very important, which refers to the section of “LLE of REEs”.

In general, the selection of extractants is mainly based on “like dissolves like” principle. And the following properties are preferred practically: (1) insoluble in water or very low solubility in water; (2) high boiling point and low saturated vapor pressure, to reduce the evaporation loss during extraction; (3) the distribution coefficient of analytes between the organic and aqueous phases is high. Table 22 lists the common organic solvents with corresponding properties.

Sample pH is usually adjusted to make target analytes existing in non-ionic form reducing their solubility in water and promoting the extraction. For the REEs extraction system which is mainly based on chelation or complexation reactions, pH mainly affect the stability of chelates or complexes, thus affecting the extraction.

Stirring rate is a dynamic factor affecting the extraction. In LPME process, the mass transfer of analytes is determined by the diffusion rate and the thickness of diffusion layer between sample solution and the organic solvent. Based on the theory of convection–diffusion, efficient stirring can accelerate the diffusion rate of the analyte and reduces the thickness of the diffusion layer. Thus, increasing the stirring rate would generally accelerate the extraction, and reduce the extraction time. While, when the stirring rate is too high, some negative impact would occur, e.g., extraction single drops is unstable, bubbles adhered on the surface of the hollow fiber membrane appear and impede extraction, or the dissolving loss of the organic extraction solvent is accelerated.

MIBK: 4-methyl-2-pentanone [C₆MIM][PF₆] 1-hexyl-3-methylimidazolium hexafluorophosphate. LPME of target analytes is a process based on their distribution between the sample solution and the extraction phase, and the maximum extraction efficiency would be obtained when the extraction equilibrium is reached. For the analytes with high distribution coefficient and fast mass transfer speed, the extraction equilibrium can be achieved in a short time, which is an incomplete extraction under equilibrium. However, for analytes with relatively low distribution coefficient or low mass transfer speed, long time are needed to reach the extraction equilibrium, and in this case, the extraction is usually processed under non-equilibrium state, during which the extraction time is strictly controlled to ensure the reproducibility.

Extraction temperature has a double impact on the extraction process in terms of thermodynamics and kinetics. In general, increasing the sample temperature would increase molecular motion, the diffusion coefficient of the analyte, and the mass transfer rate of the analyte, thus speeding up the extraction. However, increasing the extraction temperature would reduce the partition coefficient of the analyte, increase the water solubility of the organic solvent, resulting in lower extraction efficiency. Moreover, when the temperature is too high, small bubbles would occur in the aqueous phase, which is attached to the surface of the extraction solvent, hindering the extraction.

The addition of some inorganic salt, e.g., NaCl, Na₂SO₄, would increase the ionic strength of the sample solution, decrease the solubility of analytes in sample solution due to salt-out effect, increasing the distribution of analytes in the extraction phase and thus the extraction efficiency. While sometimes the salt addition would inhibit the extraction which is explained by that the salt would change the physical property of the extraction film and thus the diffusion rate of target analyte in the extraction phase.

4.9 Cloud point extraction

Cloud point extraction (CPE) is a new LLE mode which is based on the fact that most nonionic surfactants can form micelles in aqueous solutions and become turbid when they are heated beyond a temperature called the cloud point temperature (CPT). Above the CPT the micellar solution separates into a small volume of surfactant-rich phase and a diluted aqueous phase, in which the surfactant concentration is close to the critical micellar concentration (CMC). Any analyte solubilized in the hydrophobic core of the micelles, will be separated and concentrated in the small volume (usually in the range of 50–250 μL) of the surfactant-rich phase. This process is reversible, and two phases would combine into one phase when the temperature is lower than CMC. When the temperature is increasing, the micelle size of the surfactant is increasing, causing the hydrogen bonding not strong enough to keep the water molecule connected to the O atom of ether.

4.9.2 Affecting factors

4.9.2.1 Surfactants

Surfactants generally consist of a hydrophobic and a hydrophilic moiety, and the CPT is related to the chain length of hydrophilic and hydrophobic moiety in the surfactants. With the same hydrophobic moiety, CPT would increase with the increase of hydrophilic chain length; on the contrary, CPT declines with the increase of the hydrophobic chain length. Besides nonionic surfactants, amphoteric ionic surfactant exhibits cloud point phenomenon. Some anionic surfactants (e.g., sodium dodecyl sulfate [SDS], sodium dodecyl benzene sulfonate [SDBS] and sodium dodecyl sulfonate [SDSA]) in aqueous high concentration HCl medium would also appear phase separation. Table 24 lists the frequently used surfactants in CPE, along with the structure, CMC and CPT.

Table 24:

Surfactants commonly used in CPE.

Surfactants		CMC (mmol/L)	CPT (°C)
Chemical name	Commercial name in series
Polyoxyethylene fatty alcohol	Brij 30	0.02–0.06	2–7
	Brij 35	0.06	>100
	Brij 56	0.0006	64–69
P-tert-octyl phenyl polyethylene glycol ether	Triton X-100	0.17–0.30	64–65
	Triton X-114	0.20–0.35	22–25
N-alkyl phenyl polyethylene glycol ether	PONPE-7.5	0.085	5–20
	PONPE-10	0.07–0.085	62–65
Amphoteric ionic surfactants	C9APSO4	4.5	65
	C8-lecithin		45

Increasing the surfactant concentration would improve the extraction efficiency, increase the ratio of two-phase volume, and simultaneously reduce the EF and distribution coefficient. In order to increase EF, a low surfactant concentration is preferred, while too low surfactant concentration would lead to difficult separation of the sedimented phase from sample solution, deteriorating the accuracy and reproducibility.

4.9.2.2 Sample pH and chelating agent

For the extraction of REE ions, a chelating agent is necessary to transform REEs into a hydrophobic form, which is then extracted into the surfactants. pH should be appropriate for the formation of REEs complex. However, most of the complexation system for REEs occurs in slightly acidic pH (Figure 10), which is incompatible with digested solid samples which is generally strong acidic. Ohashi et al. [58] show good extraction in a more acidic media (pH = 3) for La, Eu and Lu(III), while higher concentrations of surfactant were needed to counter the effect of the acidic media on the extraction. And the EF would be reduced and LOD would be deteriorated due to the low extraction efficiency.

Figure 10:

Extraction percentage of La(III), Eu(III) and Lu(III) vs.pH without chelating agents. Circle, La(III); triangle, Eu(III); square,Lu(III); closed, Aldrich (lot no. 13112LB); half closed, Aldrich (lot no.03615HC); open, Nacalai Tesque; Ln(III), 3.6×10−5 mol/L; Triton X-100,2.0 % (v/v); I=0.01 mol/L (pH > 2.0) and 0.1 mol/L (pH < 2.0)with (H, Na)ClO₄ [58].

The behaviors of La(III), Eu(III) and Lu(III) in CPE with and without di(2-ethylhexyl) phosphoric acid (HDEHP) as the complexant, Triton X-100 as the surfactant, were investigated [58]. It was suggested that the extraction of Ln(III) into the surfactant-rich phase without added chelating agent was caused by the impurities contained in Triton X-100. The extraction percentage more than 91 % for all test REEs was obtained using 3.0×10−5 mol/L HDEHP and 2.0 % (v/v) Triton X-100. From the equilibrium analysis, it was clarified that Ln(III) was extracted as Ln(DEHP)₃ into the surfactant-rich phase. Besides, Li et al. [53] proposed an on-line CPE system with ICP-OES detection with/without 8-HQ as complexant for trace analysis of REEs in biological samples. The experimental results indicated that: (1) the use of on-line CPE greatly simplifies the extraction procedure; (2) LODs for REEs obtained by on-line CPE-ICP-OES with 8-Ox as chelating agent were slightly better than that obtained by on-line CPE-ICP-OES without 8-Ox as chelating agent, and the RSDs of the two methods were comparable; (3) both on-line CPE-ICP-OES systems had good selectivity and could be used for the biological sample analysis.

4.9.2.3 Extraction temperature and time and centrifugation time

The extraction temperature and time are determined by the properties of surfactants and the extracted compounds. Generally, the extraction temperature is 15–20°C higher than the CPT of the employed surfactant. Increasing extraction time would increase the extraction efficiency, while too long extraction time hardly affect the extraction efficiency, and 30 min extraction is commonly used for a good extraction efficiency.

Increasing the centrifugation time would initially increase the extraction efficiency and distribution coefficient, which will be constant after a long extraction time. For a CPE system with relative high CPT, too long centrifugation would lead to the reversion of phase separation, decreasing the extraction efficiency.

4.9.2.4 Ionic strength

The ionic strength hardly affects the extraction efficiency and distribution coefficient. The addition of some inert salt would change the cloud point of the surfactant, favoring the separation of two phases.

5.1 Carbon nanotubes and graphene oxide

Carbon nanotubes (CNTs), which were first discovered by Iijima [123], have led research to new area in many interdisciplinary investigations as the advantages of CNTs are unique structural, electronic, optoelectronic, semiconductor, mechanical, chemical and physical properties. CNTs can be described as a graphite sheet rolled up into a nanoscale-tube (which are single-wall carbon nanotubes [SWCNTs]), or with additional graphene tubes around the core of an SWCNT (which are multi-wall CNTs (MWCNTs)). These CNTs have diameters in the range between fractions of nanometers and tens of nanometers and lengths up to several centimeters with both their ends normally capped by fullerene-like structures.

Although raw CNTs walls are not reactive, oxidation of CNTs (by using such as HNO₃, NaOCl and KMnO₄ solutions) can lead to surface functionalization with oxygen-containing groups (e.g., carboxylic, carbonyl and hydroxyl group), allowing retention of cations [124]. These functional groups cause a rise in negative charge on carbon surface and the oxygen atoms in functional groups donate single pair of electrons to metal ions, consequently increasing their cation exchange capacity [125]. It is commonly believed that the chemical interaction between the metal ions and the surface functional groups of CNTs is the major sorption mechanism.

The oxidation of MWCNTs with concentrated nitric acid lead to the surface functionalization with oxygen-containing groups, and the isoelectric point (IEP) of MWCNTs shifts to the lower pH values. When the pH of the solution is higher than the IEP of the oxidized MWCNTs, the negative charge on the surface provides electrostatic attractions that are favorable for adsorbing cations. The decrease of pH leads to the neutralization of surface charge, so the adsorption of cations onto MWCNTs decreases quickly. Liang et al. [126] investigated the adsorption behavior of REEs on MWCNTs and found that quantitative adsorption (>95 %) for the target REEs ions was obtained when the pH exceed 3.0, and pH 4.0 was selected as the optimum condition for the SPE of REE ions, followed by elution with 1.0 mol/L HNO₃ and ICP-OES detection. The detection limits of this method for REEs was between 3 and 57 ng/L, and the relative standard deviations for the determination of REEs at 10 μg/L level were found to be less than 6 % when processing 100-mL sample solution.

Su et al. [118] prepared a Fe₃O₄@SiO₂@polyaniline–graphene oxide composite (MPANI-GO) through a simple non-covalent method and applied it to magnetic solid phase extraction (MSPE) of trace REEs in tea leaves and environmental water samples followed by ICP-MS determination. The GO plays a key role in the adsorption of REEs on MPANI-GO and the function of MPANI was mainly to anchor and magnetically functionalize GO. The adsorption mechanism of the REEs on the MPANI-GO could be attributed to the chelation of the functional groups of GO containing hydroxyl, epoxide, carboxyl, and carbonyl groups with REEs. When 0.5 mol/L HNO₃ was used as the eluting agent, the MPANI-GO can be reused for more than 30 times without obvious decrease of the recoveries of REEs. It should be also noted that the prepared magnetic nanoparticles are superparamagnetic, which could be attracted to a magnetic field but retain no residual magnetism after the field is removed. During the extraction, the suspended superparamagnetic nanoparticles adhered to the target REEs can be removed very fast from a matrix using a magnetic field, and they do not agglomerate after removal of the field.

5.2 Silica-based materials

Modified silica gel merits good selectivity, high mass-exchange characteristics and good mechanical stability. Immobilization of organic functional groups on a siliceous surface has been successfully employed to produce varieties of modified silica gel. In this process, an organic reagent or a synthesized organic molecule containing the desired functional group is directly attached to the support, or to the original chain bonded to the support via a sequence of reactions to increase the main chain, where other basic centers can be added to ensure the enhancement of a specific adsorption.

Based on the hard–soft acid–base theory, chemically stable metal chelates can be formed between REEs and β-diketone reagents (e.g., acetylacetone, trifluoroacetylacetone and hexafluoroacetylacetone) under suitable experimental conditions. These reagents, as derivatization reagents, have been successfully applied in GC analytical systems. Zhang et al. [127] prepared an acetylacetone-modified silica gel (ACACSG) for the preconcentration of trace REEs in environmental and biological samples. The maximum adsorption percentages for target REEs were obtained at pH 6.0–8.0. Two possible mechanisms could be used for the explanation of the adsorption of the REEs on ACACSG. First, REEs were adsorbed through the affinity of =O of acetylacetone. Second, the REEs were adsorbed on ACACSG by electrostatic action between REEs and the charge on silica gel surface. In the acidic medium, the first mechanism plays a dominating role. When the pH increases, the –OH on the surface of silica gel provides the ability of binding cation. Both mechanisms work, so the adsorption of REEs onto ACACSG increases quickly.

5.3 Chelating resin and ionic-exchange resin

Various chelating agents have been immobilized on polymeric supports to produce chelating sorbents for determination of REEs by SPE in natural water samples. These functional groups include 8-HQ [128], iminodiacetate (IDA) [129, 130], hydroxamic acid [131], 2-amino-5-hydroxy benzoic acid [132], 2,6-diacetylpyridine [133] and fluorinated β-diketone [134]. Chemical immobilization offers the ability to customize the sorption medium with different functional groups to improve selectivity, sorption capacity and reusability.

Fu et al. [135] synthesized alkyl phosphinic acid resin (APAR) for on-line preconcentration of trace REEs, followed by ICP-MS determination. REEs in seawater could be on-line concentrated on the APAR packed column, and eluted from the column with 0.5 mL 0.1 mol/L HNO₃ within 30 s. An EF of nearly 400 can be achieved for all REEs when the seawater sample volume was 200 mL, while the matrix and coexisting spectrally interfering ions such as Ba, Sn and Sb could be simultaneously separated. Zeereen et al. [111] prepared a new chelating resin by immobilizing 4-(2-thiazolylazo) resorcinol (TAR) on Chromosorb 106 and investigated the adsorption behavior of REEs on it. It was found that the REEs were quantitatively retained from saline solutions on the Chromosorb 106-TAR resin at pH 5.0 and can be eluted with 1 % (v/v) HNO₃. The resin possesses large sorption capacity for REEs ranging from 81.1 μmol/g for Lu and 108 μmol/g for Nd. Gupte et al. [136] employed resorcin [4]calixpyrrole and its azo derivative impregnated Amberlite XAD-2 resin for the preconcentration of La³⁺ and Ce³⁺. The resin was found to be highly selective for these two REE ions in the presence of various electrolytes at a particular pH. Table 26 presents some adsorption capacity for La³⁺ and Ce³⁺.

Separation of REEs by ion chromatography on a Dionex Ion Pac CS3 column (sulfonic acid type) has been achieved by both isocratic and gradient elution with HIBA as the mobile phase [137]. It was shown that the plate height for Y was distinctly smaller than those for REEs with similar ionic radii. An improved combined isocratic-gradient elution procedure was devised for separation of Sc, Y, La and the lanthanides in one run. By using strongly basic anion-exchanger (QAE-SephadexA-25), Gd complexed with negatively charged humic substances (humic and fulvic acids) was collected from sample solution [138]. A Gd-based MRI contrast agent (diethylenetriamine-N,N,N′,N′′,N′′′-pentaacetato aquo gadolinium(III), Gd–DTPA2−) was simultaneously collected on the same column. The Gd–DTPA complex was desorbed by anion-exchange with 50-mM tetramethyl ammonium sulfate, leaving the Gd–humic complexes on the column. The Gd–humic complexes were subsequently dissociated with 1 M nitric acid to desorb the humic fraction of Gd. The 2-step desorption with small volumes of the eluting agents allowed 100-fold preconcentration for the fractionation analysis of Gd at low ng/L levels by ICP-MS. On the other hand, Gd(III) neither complexed with humic substances nor DTPA, i.e., free species, was not adsorbed on the column.

The details on relevant chromatographic separation of REEs can be referred from the section of “Liquid chromatographic separations of REEs”.

5.4 Metal oxide nanostructured materials

Nanoparticles are often defined as particles of less than 100 nm in diameter. The nanoparticles exhibit new or enhanced size-dependent properties compared with larger particles of the same material. Depending on the chemistry of the constituent atoms, nanoparticles are known to exhibit remarkable adsorption properties due to their enhanced surface area and large interface volume. Nano-sized inorganic oxides are widely used in SPE for the preconcentration/separation of metal ions because it can adsorb metal ions with high adsorption capacity. Different types of nanoparticles, such as nanometer-sized powder materials, mesoporous nanoparticles and magnetic nanoparticles, have been applied in SPE of REE ions.

The effect of pH on the adsorption characteristics of La³⁺ and La complexes of citric acid, 2-hydroxyisobutyric acid (HIBA), and humic acid on nanometer-sized TiO₂ was investigated [139]. It was found that there is an obvious difference between the adsorption of free La³⁺ and La complexes on nanometer-sized TiO₂ with pH changing from 4.0 to 6.5. Quantitative adsorption of lanthanum citrate, lanthanum HIBA and lanthanum humic complexes were achieved in a pH ranging from 4.0 to 8.0, whereas free La³⁺ cannot be quantitatively adsorbed until the pH value is up to 6.5. However, if the pH value was lower than 4.0 (pH 3.5 for lanthanum citrate), the adsorption of lanthanum citrate and lanthanum humate complexes declined remarkably due to incomplete complexing between La³⁺ and the organic ligands (the first acid dissociation constants [pKa] of the citric acid and the 2-hydroxyisobutyric acid are 3.13 and 3.72, respectively).

These nanometer-sized metal oxides are not selective and not suitable for samples with complicated matrices. In order to improve selectivity of the nanoparticles, the modification of the adsorption material is usually required. The most often used method is to load a kind of specific chelate reagent by physical or chemical procedure on the surface of this material. The former method is simple but the loaded reagent is prone to leakage. PAN, PMBP, 8-HQ and dithizone were commonly used as modifiers on metal oxides nanoparticles. Due to the chelating ability of the loaded reagent with the target analytes, selectivity of the nanoparticles could be improved. Jon et al. [140] employed PMBP physically immobilized nanometer Al₂O₃ as the adsorbents for the extraction of REE ions Sc³⁺, Y³⁺ and La³⁺. The interesting ions can be adsorbed quantitatively on the above material at pH = 4.5 and the analytes adsorbed on the column can be eluted with 0.5 mol/L HCl. The dynamic adsorption capacity of target REEs on the modified nanometer Al₂O₃ are 5.7 (Sc), 13.1 (Y) and 15.5 (La) mg/g, respectively. However, these materials prepared by the physical method are not stable, and the chemically bonded materials are more stable and could be used repeatedly.

Mesoporous nanoparticles are special kind of mesoporous materials. The particle size of the material was less than 100 nm, while it has mesoporous on the particles. The application of the material as adsorbent has been viewed in three perspectives: (1) As a new nanometer material with unsaturated surface atoms that can bind with other atoms, it possess highly chemical activity, very high adsorption capacity and selective adsorption of metal ions. (2) Due to its large surface area, mesoporous material provides more active sites, this results in achieving equilibrium adsorption in short time. (3) By virtue of mesoporous structure, fast adsorption and desorption has been obtained. Huang et al. [77] employed mesoporous TiO₂ nanoparticles as column packing sorbent for flow injection micro-column preconcentration on-line coupled with ICP-OES determination of trace metals (including REEs) in environmental samples. The results showed mesoporous TiO₂ possesses a high adsorption capacity toward the metal ions and the adsorbent was suitable for the preconcentration of trace metal ions in samples with complicated matrix, and is highly stable with long column life time, enabling more than one hundred of load and elution cycles to be performed without loss of analytical performance.

5.5 Ion-imprinted materials

Ion-imprinted polymers (IIPs) are similar to molecular-imprinted polymers (MIPs), but they can recognize metal ions after imprinting and retain all the virtues of MIPs. IIPs have outstanding advantages such as predetermined selectivity in addition to being simple and convenient to prepare. Traditional IIP exhibits poor site accessibility to the target ions because the functionality is totally embedded by high cross-linking density in the polymer matrices. In order to obtain IIP with high selectivity and easy site accessibility for target ions, efforts have been made to deal with the issue of accessibility by imprinting on the matrix surfaces. Recently, surface imprinting technique attracted extensive research interest due to good accessibility to the target species, low mass-transfer resistance, and completed removal of templates, fast adsorption kinetics and ease of preparation.

Meng et al. [141] prepared a novel Ce³⁺ ion imprinted silica based on the supported material of ordered mesoporous silica SBA-15 by surface imprinting technique via reversible addition-fragmentation chain transfer polymerization. The results showed that the synthesized material possessed high-ordered mesoporous structure and showed high selectivity and satisfied adsorption capacity for adsorption of Ce³⁺. Zhang et al. [142] synthesized Dy³⁺-imprinted TTA modified silica gel sorbent by surface imprinting technique and employed it as a selective SPE sorbent for on-line extraction and subsequent ICP-OES determination of Dy³⁺. The largest selectivity coefficient for Dy³⁺ in the presence of competitive ions such as La³⁺, Nd³⁺ and Gd³⁺ was above 350. The static adsorption capacity and selectivity coefficient of the imprinted sorbent are higher than those of the non-imprinted sorbent and the imprinted TTA modified silica gel sorbent offered a fast kinetics for the adsorption and desorption of Dy³⁺. With a sample loading flow rate of 2.5 mL/min for 48 s preconcentration and elution flow rate of 1.5 mL/min for 8 s elution, an EF of 10 and the sample frequency of 40 samples/h were obtained.

5.6 Metal-organic frameworks (MOFs)

MOFs are a new class of hybrid inorganic–organic microporous crystalline materials self-assembled straightforwardly from metal ions with organic linkers via coordination bonds. Due to their unique physical and chemical properties, (e.g., permanent nanoscale pore structure, uniform pore size, large specific surface area and good thermal stability), MOFs demonstrated great application prospect in hydrogen storage, gas separation, the catalytic, sensors and biological imaging. The inherent high-specific surface area (appr. 5,000 m²/g) indicates a large adsorption capacity; the uniform pore structure with certain pore size allows a selective adsorption of molecular with less pore size; the functionalization in their pores or outer surface can be achieved easily. While the application of MOFs-based sorbents is mainly limited in simple water samples or simulated samples (in organic solvents) presently.

Ln³⁺@bio-MOF-1 were synthesized via postsynthetic cation exchange of bio-MOF-1 with Tb³⁺, Sm³⁺, Eu³⁺ or Yb³⁺, and their photophysical properties were studied [143]. It is demonstrated that bio-MOF-1 encapsulates and sensitizes visible and near-infrared emitting lanthanide cations in aqueous solution. The development and application of MOFs in trace REEs is greatly expected although it is relatively scarce presently.

5.7 Restricted access materials

RAMs are a class of biocompatible adsorbent particles enabling the direct extraction of analytes from biological fluid (e.g., plasma and urine). The separation mechanism is based on the hydrophilic effect and size exclusion. To be brief, a hydrophilic barrier enables the small molecules to permeate through the inner part of the material; meanwhile, it excludes the macromolecules by physical or chemical means on the surface of the material. They have been successfully used as separation media for the analysis of environmental and biological samples in recent years. In LC pre-column, the commercialized alkyl-diol silica (ADS) was frequently applied as RAM for the clean-up of complex biological samples, such as blood, serum, urine and milk. And the mechanism was explained by that the outer surface (hydrophilic diol group) repelled macromolecules (such as proteins, nucleic acids), while the internal surface, comprising hydrophobic reversed-phase (C₄, C₈ or C18), retained analytes that penetrate through the porous layer.

Yan et al. [144] prepared restricted accessed magnetic nanoparticles by self-assembly of a nonionic surfactant (Tween-20) onto the PAR functionalized magnetic nanoparticles (MNPs). It was found that the as-prepared restricted accessed Fe₃O₄@SiO₂@PAR nanoparticles have a porous structure with BET surface area of around 99.4 m²/g, average pore size of about 6.14 nm and pore volume of 0.47 cm³/g, respectively. Besides, the prepared restricted accessed Fe₃O₄@SiO₂@PAR showed a good size exclusion property toward protein, providing an application potential for the direct analysis of biological samples. Based on it, a novel method of restricted accessed MSPE combined with ICP-MS was developed for the direct determination of trace metal ions in human fluids. Under the optimum conditions, the adsorption capacity of La(III) and Nd(III) on the as-prepared restricted accessed Fe₃O₄@SiO₂@PAR were 33.7 and 36.9 mg/g, respectively.

5.8 Capillary microextraction

In 1990, Pawliszyn et al. proposed solid phase microextraction (SPME) [145] as a new pretreatment technique. It integrates extraction, preconcentration and sampling into one step, featuring with simple operation, low consumption of solvents/sample, good reproducibility, high efficiency and can be hyphenated with various detection techniques. It was commercialized by Supelco Corporation in 1993 and has been applied extensively in a variety of fields [146, 147, 148]. While the application of SPME in the analysis of trace elements and their species is relatively scarce [149].

Eisert and Pawliszyn [150] proposed another operation mode for fiber-SPME in 1997 – capillary microectraction (CME) – also called in-tube SPME. Figure 11 presents a schematic diagram of in-tube SPME combined with LC–MS system [151].

Figure 11:

Schematic diagram of the in-tube SPME/LC/MS system [151]. (A) load position (extraction phase) and (B) injection position (desorption phase).

The main difference between fiber-SPME and CME is the coating which is coated on the surface of the fiber for the former one while on the inside surface in the capillary for the latter. Compared with fiber-SPME, CME can be easily online combined with HPLC or CE, improving the producibility; the problem of coating easily swelling or falling off in some organic solvents is overcome, lowering the analytical cost; it can be used for strong polar analytes analysis, widening the application field; the thickness of the extraction coating in CME (0.1–1.5 μm) is far less than that in fiber-SPME (10–100 μm), and the contacting surface between the coating and analytes is larger than that in fiber-SPME, shortening the required time to attain the equilibrium. CME needs that the sample solution should be clean, otherwise the particles would block the capillary column or the flow channel.

CME can be divided into three operation modes [152], which include open-tubular CME, packed CME and monolithic CME. The application of CME in trace REEs analysis is scarce presently.

Wu et al. [153] prepared a Congo red (CR) modified SWCNTs coated fused-silica capillary for CME of trace La, Eu, Dy and Y in human hair followed by fluorinating assisted (F)ETV-ICP-OES determination. The maximum adsorption capacities of the CR-SWCNTs coated capillary evaluated from the breakthrough curve were 0.8, 0.7, 0.7 and 0.7 μg/m for La, Eu, Dy and Y, respectively. The proposed method was applied to the analysis of real-world human hair sample, featuring with simplicity, high sensitivity and less sample/reagents consumption. Zhang et al. [154] synthesized an IDA modified poly(glycidyl methacrylate-trimethylolpropane trimethacrylate) [poly(IDAGMA-TRIM)] monolithic capillary and used it for the selective extraction and preconcentration of REEs. Based on it, a novel method of poly(IDA-GMA-TRIM)-based CME on-line coupled to microconcentric nebulization (MCN)-ICP-MS was developed for the determination of ultra-trace REEs in biological samples (urine and serum). Under the optimal conditions, the LODs for REEs were in the range of 0.08 (Er)–0.97 ng/L (Nd) with a sampling frequency of 8.5/h. The method is featured with high sensitivity and sample throughput, low sample consumption and contamination risks, easy operation, wide pH application range and good tolerance to the matrix interferences.

A novel method of TiO₂ nanoparticles (NPs) functionalized monolithic CME on-line coupling with ICP-MS was developed for the sequential determination of Gd³⁺ and Gd-based contrast agents in human urine samples [155]. The monolithic capillary was prepared by embedding anatase TiO₂ NPS in the poly(methacrylic acid–ethylene glycol dimethacrylate) (MAA–EDMA) framework. The Gd³⁺ and Gd-based contrast agents (such as gadolinium–diethylene triamine pentaacetic acid (Gd–DTPA) and Gd–DTPA–bismethylamide (Gd–DTPA–BMA)) display different adsorption behaviors on the prepared monolithic capillary which possesses the adsorption properties of both anatase TiO₂ NPS and poly(MAA–EDMA) monolith. As shown in Figure 12, in the presence of PBS, Gd³⁺ was adsorbed completely on the poly(MAA–EDMA–TiO₂ NPs) monolithic capillary in a relatively wide pH range (2–9), whereas Gd–DTPA could be retained quantitatively on the monolith in the pH range of 2–3 and a sharp decrease of adsorption percentage of Gd–DTPA was observed when pH was higher than 4. It was speculated that in the presence of PBS, Gd–DTPA first reacted with PO43− to form a ternary complex (structure shown in Figure 12) because the interaction between Gd–DTPA and phosphate is stronger than that between Gd–DTPA and TiO₂ NPs. This ternary complex could be easily adsorbed by the prepared poly(MAA–EDMA TiO₂ NPs) monolithic capillary due to the good affinity of TiO₂ NPs to phosphate group at pH 2–3.

Figure 12:

Effect of pH on the adsorption behavior of Gd³⁺, Gd–DTPAand Gd–DTPA–BMA in the absence (a) or presence (b) of PBS, andthe formation of the deduced ternary complex (c) [155].