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Lanthanides is in the f block corresponding to the filing of the seven 4f orbitals, respectively. The lanthanides are all electropositive metals with a remarkable uniformity of chemical properties; the only significant difference between two lanthanides is their size. The lanthanide cations are prototypical hard acids, bonding preferentially with hard bases such as oxygen donor ligands. The coordination numbers of lanthanides are between 6 and 12. The chemistry of lanthanides is well documented. Lanthanides are extensively used in commercial applications, such as alloys for aeronautical components, permanent and superconducting magnets, catalysts, phosphors, lasers, batteries, chemicals, ceramics, glass, glazes, etc. In the early days of lanthanide chemistry, before ion-exchange chromatography was developed, tedious crystallizations were used to separate the elements.
The most attracting feature of lanthanide chemistry is the uniformity in the properties of the lanthanide ions, because to the small differences in the sizes of their hydrated lanthanide ions. The radii of the ln3+ ions contract steadily from 116pm for lanthanum to 98pm for lutetium owing to the well-known ''lanthanide contraction.'' The uniformity in properties makes the separation of pure lanthanide elements difficult; common separation techniques is not be applicable for individual lanthanides of very high purity. The advantages of high-performance liquid chromatography (HPLC) are their ability to provide high, rapid separations and the ability to extend the separations and purifications from laboratory scale to large scale. This paper will mainly focus on the ways for individual separation of lanthanides using dynamic ion-exchange chromatographic methods employing HPLC.
We can use the differences in the stability constants of metal ions with a particular ligand to achieve their separation. Lanthanides form complexes with weak organic acids, such as a-hydroxy isobutyric acid (α-HIBA), citrate, lactate, tartrate, and a-hydroxy-a-methyl butyric acid, to name a few. Lanthanide ions and HIBA form positive complexes with a single charge which lowers the affinity of the lanthanide for the cation-exchange resin. Reducing the charge of lanthanides (in complex form) causes a reduction in their retention on the stationary phase. The difference in stability constants for the lanthanide-HIBA complex implies that each lanthanide will spend more or less time in the eluent depending on its relative stability with HIBA. This difference in the stabilities of the complexes of lanthanides can be used to enhance the separation of the lanthanides in chromatography. In the presence of the complexing agent, e.g., α-HIBA, lutetium is eluted first and lanthanum last, because of the formation of complexes of low stability in the case of La as compared with Lu. Among the various organic acids which have been used, α-HIBA has been shown to be a successful eluent between adjacent lanthanides, and so is the most extensively used complexing reagent for separation of lanthanides.
The factors which may influence the stability of the complex formed would obviously affect the retention behavior of lanthanides in a cation exchange column. Thus, retention of lanthanides from a column will decrease when the mobile phase pH is increased. This effect occurs until the dissociation of the protonated ligand is complete, beyond which further increase in pH does not change retention times. Analogously
, the changes in concentration of the mobile phase containing the complexing reagent can also affect the retention time; Reduction the concentration of complex ant in the mobile phase generally results in increasing of retention time.
Dynamic Ion-Exchange Methods
The HPLC was used to separate lanthanides in the 1970s. The bonded phase ion-exchange columns that were subsequently developed were quite rigid and exhibited good mass transfer properties, compared with the PS-DVB cation exchange resins. Afterwards, dynamic ion-exchange chromatographic techniques using coated columns have been developed for the rapid separation of lanthanides and they show several advantages, compared to traditional ion-exchange techniques; they will be explained in detail in the following sections.
5. Principle of Dynamic Ion-Exchange Chromatography
A C18 column which is a hydrophobic stationary phase, is used with a suitable ion-pairing reagent (IPR) for metal ion separations. There are some examples of ion-pairing reagents used for cation exchange separation include pentanesulfonate, hexanesulfonate, and
octanesulfonate (Table 1). The SP (stationary phase) provides a neutral surface, which can be modified to form an ion-exchange support, with both cation as well as anion exchangers by the use of some suitable modifiers. The modifiers can be continuously passed through the column or coated ''permanently'' onto a column, depending on their aqueous solubility. For example, when passing a solution of a water-soluble modifier, octanesulfonate (10-2 to 10-3 M) through a C18 support results in the formation of a cation exchange surface. This method is generally referred to as a ''dynamic ion-exchange'' technique. Lanthanides can be subsequently separated by exchange with the hydrogen ions present in the modifier, similar to the exchange that takes place in the conventional cation exchange resins. Use of a suitable complexing reagent, e.g., α-HIBA, leads to the elution and separation of lanthanides. The water insoluble modifiers, e.g., dodecylsulfate (C12H25SO4-), are dissolved in methanol-water mixtures and are passed through a C18 column.
Relying on the concentration of the modifier, the methanol content is varied, generally between 50 and 70 vol. %. A solution of about 10-3 to 10-4 M of modifier is used for modifying the column by passing about a liter through the column, to establish an ion-exchange support. The volume required for the equilibration varies with the concentration of the modifier and the solvent composition. After this step, a mobile phase containing the complexing reagent is passed for achieving the separation. The ion-exchange columns prepared by these methods are generally referred to as ''permanently'' coated columns in which separation of lanthanides occurs through an ion-exchange mechanism.
The following factors on retention are observed in the dynamic ion-exchange separation: The concentration of the IPR adsorbed onto the stationary phase is dependent on 1) concentration of organic solvent such as methanol in the mobile phase (higher concentration of methanol results in lower concentrations of IPR on the stationary phase); 2) its concentration in the mobile phase; and 3) hydrophobicity of the IPR. However, for a given eluent composition, the concentration of the adsorbed IPR remains constant. The retention of lanthanides increases with an increase in the concentration of the ion-pairing reagent and retention of solute decreases with increasing content of methanol in the mobile phase.
Superiorities of Dynamic System
There are two important features of dynamically modified column(1) their high column efficiency(2) easily variable ion-exchange capacity. As result of thin layer of modifier being coated onto the surface of the stationary phase The high column efficiency can lead to significant reduction in stationary phase mass transfer. The ion-exchange capacity depends on the surface concentration of the sobbed modifier which can be quickly changed over a wide range, by changing the mobile phase concentration. This feature is not available with conventional ion-exchange resins. The variation of ion-exchange capacity can be effectively used to optimize the column efficiency and the selectivity. Another important advantage of this technique is that the column can be reused for other reverse phase applications after washing it with water (in the case of dynamic ion-exchange) or with methanol (in the case of ''permanently'' coated columns).
As the modifier is adsorbed onto the C18 support , this will create an ion-exchange surface in the dynamic ion-exchange mode. The adsorbed IPR imparts a charge to the stationary phase, causing it to behave as an ion exchanger. A constant interchange of IPR occurs between the eluent and stationary phase and the stationary phase can be considered to be a dynamic ion exchanger. The sample ions are then exchanged between the stationary phase and the mobile phase by an ion-exchange process. The ion-pair model leads to the formation of ion-pair complex between analyst ion and modifier, which is subsequently partitioned between stationary phase and mobile phase. Retention, therefore, results mainly as a consequence of interaction taking place in the eluent between solute and IPR and the subsequent partition of the complex to the stationary phase. The degree of retention of the ion-pair is dependent on its hydrophobicity, which in turn depends on the hydrophobicity of the ion-pairing reagent. An increase in the percentage of methanol in the eluent generally decreases the interaction of the ion-pairs with the stationary phase. The ion-interaction model may be viewed as an intermediate between the dynamic ion-exchange and ion pairing models. It incorporates both the adsorptive effects, which forms the basis of dynamic ion-exchange, and the electrostatic effects, which are the basis of the ion-pair model. The schematic of ion-pair, dynamic ion-exchange, and ion-interaction models for the retention of anionic solutes is shown in Fig. 1. Although many of these models define the solute retention under certain conditions, it is likely that the exact mechanism could be a combination of dynamic ion exchange, ion-pair formation, etc.
8. THE METHODS OF EXPERIMENTAL
Briefly, the HPLC employs columns which contain SP materials of small and uniformly sized particles, necessitating high operating pressures. These columns provide high efficiency, as well as faster and high-resolution separations. Some typical stationary phase and mobile phase systems used for lanthanide separations are given next.
9. SP(Stationary Phase)
The perform process of dynamic ion-exchange separations take place on a wide range of stationary phases, which include chemically bonded silica materials and PS-DVB copolymers. C18 supports are the most popular choice. Columns packed with 3 or 5 mm particles are used for analytical scale separation of lanthanides.
10. MP(Mobile Phase)
There are a lot of mobile phase materials. Aliphatic sulfonic acids and their salts (Table 1) are used as water-soluble ion-pairing reagents in dynamic ion-exchange chromatography, e.g., sodium octanesulfonate. The complexing reagent, e.g., α-HIBA, is dissolved along with the ion-pairing reagent in HPLC grade water. The pH of the mobile phase is generally adjusted using dilute ammonia/sodium hydroxide. The mobile phase solution is passed through the reverse phase column to establish a dynamic ion-exchange surface, after which samples are introduced into the HPLC system for separation. In the case of ''permanently'' coated columns, about 60 ml of the mobile phase containing the complexing reagent is passed through the coated column prior to the introduction of sample.
11. Injection of Lanthanide Samples
Normally, lanthanide samples in the form of their nitrates are injected into the HPLC system. To prepare a calibration, lanthanide samples over the concentration range of about 1-10 ppm (injected amount 20 ml) are introduced into the system, though the linear dynamic range exceeds well beyond this region.
There are a lot of ways detecting the lanthanides. Post column derivatization has been an extensively used technique for the detection of lanthanides. The lanthanide complexes are detected at 590, 658, and 520 nm, respectively. Arsenazo III is generally employed in the aqueous as well as in acetic acid medium. The post column reagents are added to the eluate with a reciprocating pump/peristaltic pump. The rapid and efficient mixing of color-producing reagent with eluent is essential and dead volume must be minimized for achieving better resolution and sensitivity. The molar absorptivity of the complexes is generally in the region of 30,000-60,000 L/mol/cm, which permits a detection limit as low as 10-20 ng for various lanthanides.
13. Separation of lanthanides using dynamic ion-exchange HPLC
Nowadays a rapid and high-resolution separation of mixtures of lanthanide ions using sodium octanesulfonate as the ion-pairing reagent and α-HIBA as the complexing reagent was reported. A 15 cm reverse phase column was employed with sodium octanesulfonate (0.01 M) and α-HIBA (0.05-0.4 M); the pH of the mobile phase was kept at 4.6. Complete separation of lanthanides was obtained before 9 min (Fig. 1).
Fig1. Gradient separation of the lanthanides. Supelco LC18 column; linear program at pH 4.6 from 0.05 mol/L a-HIBA to 0.4 mol/L a-HIBA over 10 min at 2.0 ml/min; modifier, 1-octanesulfonate at 1 X10-2 mol/L; detection at 653 nm after postcolumn reaction with Arsenazo III; sample 5 ml of a solution containing approximately 10 mg/ml of each lanthanide.
The lanthanides could be eluted with sharp symmetrical peaks, reflecting the rapid mass transfer characteristics with high column efficiencies, i.e., HETP of about 0.02-0.03 mm. An average detection limit of approximately 2.5 ng was obtained for lanthanides in this study. A dynamic ion-exchange technique using sodium octanesulfonate-α-HIBA has also been employed for the individual separation from a mixture containing the lanthanides yttrium, uranium, and thorium. It was also
Fig. 2 Separation of lanthanides using gradient elution. Experimental conditions: column, reverse phase C18;
mobile phase, camphor-10-sulfonic acid (0.05 M); a- HIBA varied from 0.07 to 0.3 M, pH 3.8; flow rate:2 ml/min. Postcolumn reagent: Arsenazo III(1.8X 10-4 M); flow rate: 1.5 ml/min; detection :655 nm.
demonstrated that the peak positions of Th(IV) and U(VI) in the lanthanide elution chromatogram could be optimized by appropriate adjustments in the concentration of octanesulfonate, methanol, and eluent pH. In another study, the performance of α-hydroxycarboxylic acids, such as α-HIBA, α-hydroxy-α-methylbutyric acid, and lactic acid were compared for the separation of lanthanides using octanesulfonate as the ion-pairing reagent. It is also understood that a longer alkyl group in the hydroxycarboxylic acids improves the resolution, particularly for the lighter lanthanides. In another study, mandelic acid was employed instead of α-HIBA, with octanesulfonate as the ion-pairing reagent for the separation. Using a mandelic acid gradient, all 14 lanthanides were separated. The Th(IV) and U(VI) were well separated from lanthanides. The separation of lanthanides obtained, however, is inferior to that obtained with α-HIBA. This is possibly because of the fact that, with α-HIBA, the lanthanides are retained mainly through an ion-exchange mechanism; however, with mandelic acid, lanthanides may be retained through a hydrophobic interaction mechanism in addition to an ion-exchange mechanism. A rapid separation procedure for the isolation of individual lanthanides using camphor-10-sulfonic acid (CSA) as the ion-pairing reagent has also been developed. A binary gradient with a mobile phase composition of 0.05 M CSA and 0.07-0.3 M α-HIBA (pH 3.8) was employed and the lanthanides could be separated in about 8 min (Fig. 2). Excellent peak profiles with baseline resolution for individual lanthanides have been achieved with this method
14. Using lastingly coated columns to separate lanthanides
Nowadays there is a separation of lanthanides on a reverse phase column modified with eicosylsulfate was reported, with α-HIBA (0.025-0.25 M, pH 3.8) as the mobile phase. The lanthanides could be eluted within approximately 32 min with this method (Fig. 3). In another study, a column coated with Di-(2-ethylhexyl)-phosphoric acid (HDEHP) was employed for the individual separation of lanthanides. A binary gradient in concentration of α-HIBA (0.07-0.3 M, pH 3.5) was employed in that study for the separation (Fig. 4).
Fig. 3 Separation of lanthanides by HPLC. Experimental conditions: reversed phase column, 4.6 X150 mm 5 mm Supelcosil LC-18 coated with 2.5 X 10-4 M C20SO4 - in 25% acetonitrile-water; eluent, 0.025 M a-HIBA (pH 3.8) for 9 min followed by linear gradient from 0.025 to 0.25 M a-HIBA (pH 3.8) over 20 min; detection, 658 nm after postcolumn reaction with Arsenazo III.
Fig. 4 Separation of lanthanides using HDEHP coated column. Experimental conditions: reverse phase C18 column coated with HDEHP (0.27 mM); mobile phase,a-HIBA varied from 0.07 to 0.3 M; pH 3.5; flow rate, 1.5 ml/min; postcolumn reagent, Arsenazo III; flow rate, 2 ml/min; detection, 655 nm; lanthanide, 5 mg/ml, 100 ml injected.
The separation of the entire lanthanide series was completed in about 20 min. It was reported that these coated columns are quite stable and can be employed for longer periods.
Many papers on lanthanide analysis indicate the importance of the need for the development of new and better techniques of separation. Nowadays more and more HPLC techniques have been developed for the separation of individual lanthanides using ion- exchange, dynamic, etc. The selection of gradient elution has been commonly known as a key factor for rapid and high-resolution separations.α-HIBA is being used as one of the most efficient complexing agent.