Ion-Exchange Chromatography of Glycolatocomplexes of Rare Earths

R. Kuroda* / T. Wada / G. Kishimoto / K. Oguma Laboratory for Analytical Chemistry, Faculty of Engineering, University of Chiba, Yayoi-cho, Chiba 260, Japan

Key Words IOn-exchange chromatography I~ chromatography Sc, y and Lanthanides Giycolatocomplexes

Summary The ion-exchange behavior of glycolatocomplexes of the rare earths (Sc, Y and lanthanides) with 1-octanesul- fonate as the hydrophobic ion has been investigated in aqueous glycolate media. The system is capable of separat- Ing adjacent light intra rare earths, La-Ce-Pr-Nd-Y-Sm, and heavy rare earths, Ho-Er-Tm-Yb-Lu-Sc, from each Other with good resolution by gradient elution at room temperature. Intermediate rare earths, Dy-Tb-Gd-Eu-Sm, are difficult to separate from each other. The position of u in the eluate is different from that found with most Other eluent systems used for rare earth separations, be- ing between Nd and Sin.

Introduction ~igh-performance liquid chromatography (HPLC) has Increasingly been applied to difficult intragroup separation and quantitation of the rare earths [1]. Particularly cation- eXChange chromatography with polymer-based resin [1- 10] and silica-based ion-exchangers [11-17] and ion-inter- action chromatography (IIC) [18-28] have been extensively used for this purpose with post-column reaction detectors. On account of the adjustable low ion-exchange capacity nature of IIC systems they afford more rapid separation than cation-exchange systems mostly with gradient etution techniques. With respect to IIC, ~x-hydroxyisobutyric acid (HIBA) has been used extensively as a ligand for the rare earths. Lactatocomplexes [3, 6, 27, 28] are also used for the Separation of the rare earths by HPLC. A disadvantage of the use of HIBA and lactate is that Y appears in the vicinity f By and Ho on elution and interferes with their detection. ith respect to rare earth separations, capillary isotacho-

Phoresis has an excellent capability, permitting the rare earths, including Pm, to be separated in less than 20 rain

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0009-5893/91/7 0065-04 $ 03.00/0

[29]. In this approach, however, studies like the influence of foreign metal ions on rare earth detection need be made.

The present paper reports an ion-exchange system involv- ing ODS-reversed phase, l-octanesulfonate and glycolate, a simple ct-hydroxycarboxylic acid, as a ligand where Y elutes between Sm and Nd. The HPLC system also gives excellent separation of adjacent heavy rare earths as well as of light rare earths, a disadvantage being an inability to separate some intermediate members of the rare earths from each other at room temperature.

Experimental Stock solutions of the rare earths were prepared from their oxides (99.9 % purity) to yield 0.1 M metal solutions in 3 M HC1. Rare earth mixed solutions were 0.002 M in each metal and 3 M in HCI. Sample and eluent solutions were filtered through a 0.45 lam membrane filter [Nippon Mill- ipore Kogyo, Yonezawa, Japan] and the eluent filtrate was degassed by a vacuum pump - ultrasonic cleaner (Branson Cleaning Equipment Co., Conn., USA). The hydrophobic ion reagent used for the reversed phase column was 1- octanesulfonate [Tokyo Kasei Kogyo Co., Ltd., Tokyo]. Glycolic acid was used as received.

The HPLC system consisted of two Model 880 PU intelli- gent pumps [Japan Spectroscopic Co., Ltd., Tokyo], a Rheodyne Model7125 samplingvalve [Rheodyne, Berkeley, Ca, USA] with a 20 btl sample loop and a Model 870 UV variable-wave length UV-Vis absorbance detector [Japan Spectroscopic Co., Ltd., Tokyo] with a 8 gl flow through cell of 10 mm optical path length. The detector signal was recorded with a Chromatocorder [System Instruments, Tokyo, Japan] computing integrator. Detection was by postcolumn reaction with arsenazo III [2,7-bis(2-arson- ophenylazo) l,8-dihydroxynaphthalene-3,6-disulfonicacid]. The arsenazo III solution [0.0001 M, pH 2.5] was delivered by a double plunger type reciprocating pump [DM7M- 1074, Sanuki Industry Co., Ltd., Tokyo] at 1 ml rain- 1 to join the effluent (flow rate 1 ml rain-1) via a T joint. The de- tection wavelength was 650 nm. The column was a 5 lxm Hitachi ODS No. 3056, 150 x 4 mm i.d.

The sample solution was 0.000l M in each rare earth, the glycolic acid was 100 times the molar concentration of the total rare earth elements and 0.01 M in 1-octanesulfonate,

Originals 65

�9 1991 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH

being adjusted to pH 3.5 with ammonium hydroxide. A 10 gl sample was injected via an injection valve. A linear solvent program was run for the elution from 0.15 to 0.4 M glycolic acid (pH 3.5) usually over 20 rain with the 1- octanesulfonate concentration at 0.01 M at a flow rate of 1 ml min-~. Prior to the run the column was conditioned with 0.15 M glycolate solution containing 0.01 M 1-oc- tanesulfonate [pH 3.5] for 2 h.

Results and Discussion Ion-interaction chromatography is one of the most effective means for achieving the separation of the rare earths in a very short time. Use of hydrophobic ions to modulate the apparent exchange capacity allows reduction of k' values to practical levels. Selectivity differences in the complexes of the rare earth elements with HIBA and lactic acid have been extensively utilized for their effective intragroup separation. Glycolic acid has been considered as less im- portant in low resolution cation-exchange chromatogra- phy [30]. Interaction of glycolatocomplexes with hydro- phobic ions is of interest to find further potential of IIC for separation of the rare earths.

The effect of the glycolicacid concentration of the eluent on the retention of glycolatocomplexes is illustrated in Figure 1, where isocratic elution at pH 3.5 was run by keeping the concentration of 1-octanesulfonate in the eluent constant at 0.01 M and metal concentrations at 0.0001 M. The results

20

1 5

.,u 1 0

o

a...J

0

0.

Y

I Ce

La

s c

, I , I 3 i I 0.2 0. 0.4

Glycolic acid concn. / M Figure 1 Retention time of several rare earths as function of glycolic acid concentration (pH 3.5) (20 ~ Concentration of each rare earth: 0.0001 M, 1-octanesulfonate concentration: 0.01 M.

L 0.5

for La, Ce, Pr, Y, Sin, Tin, Lu and Sc are given. As can be seen, retention depends greatly on glycolate concentration and greater differences in selectivity becomes apparent with decreasing concentration of glycolate. The retention times are adequate to separate lighter rare earths so that the concentration gradient will be effective in achieving high resolution. The linear gradient elution program was therefore tested at various times (min) from 0.15 to 0.4 M glycolic acid. Elutior~s were run over 10, 20 and 30 rain, their results being partly shown in Figure 2. Resolution between Er and Ho was somewhat lowered with 10 rain elution, so a 20 rain gradient was run in the following experiments.

In order to examine the interaction of hydrophobic ionS with the glycolatocomplexes the effect of 1 -octanesulfonate concentration was tested on the retention time during the gradient etution run. The result is shown in Figure 3. The retention time increases with increasing concentration of the sulfonate in accordance with the view that the capacity of the pseudoexchanger may be modulated by altering the concentration of the hydrophobic ion reagent in the mobile phase. The effect is more profound for lighter rare earthS. Since the clear-cut separation of adjacent Er and Ho waS

(A) 1 0 min Dy+Tb+Gd+Eu+Sm

(B)

Hc Y Er Pr

Tm

Yb

o.o ~io ~o'.o Retention time / min

30 ra in

Dy+Tb~Gd+Eu§

0.05 ABU

0.05 ABU I

Tm E

5'.0

Sc Y L ~ Nd Pr ga

0 . 0 10 ' . 0 1 5 . 0 2 0 ' . 0

Retention time / min

Figure 2

Gradient elution runs. Concentration gradient elution run over 10 and 30 min with glycolate (0.15-0.4 M, pH 3.5).

66 Chromatographia Vol. 32, No. 1/2, July 1991 Originals

2t

15

4A 10

O

4J

r 4~

_-----OLa /

f/f_----oSm / / ~ Tm

/

o o o Sc

0 i I l I 0.0 0.005 0.010 0.015 0.020

Figure3 1-Octanesulfonate concn. / M

Effect of 1-octanesulfonate concentration of apparent retention time of rare earths. Gradient elution run over 20 rain with glycolate (0.15- 0.4 M, pH 3.5). Rare earth concentration: 0.0001 M.

20

15

E

0A .i -~ 10

o -w C

L a

I I 5 1'0 ] 5 2'0

Methanol concn., % (v/v) Figure 5

Gradient elution run over 20 min at pH 3.5 of aqueous phase before mixing with methanol (overall glycolate concentration change from 0.15 to 0.4 M) . Rare earth concentration: 0.0001 M. 1 - O c l a n e s u l f o n a t e

concentration: 0.01 M.

20

15

E

0) E

10

',-t

4J 0A

Sm

0 I I I i I I I

3.0 4.0 5.0 6.0

Figure 4 pn

Effect of pH on retention time of rare earths. Gradient elution run Over 20 rain with glycolate concentration (0.15-0.4 M). 1-Octanesul- [Onate concentration: 0.01 M. Rare earth concentration: 0.0001 M.

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feasible at 0.01 M sulfonate, its concentration in the eluent was adjusted to 0.01 M taking into consideration the reten- tion time and selectivity difference.

The effect of pH of the eluent on retention during the gradient elution run for some representative rare earths is shown in Figure 4. As can be seen, the effect of pH is marked at pH values less than 5, retention reaching a constant value for pH values greater than 5 for each ele- ment. The general trend may be accounted for by the predominance of cationic glycolatocomplexes at lower pH values and rather rapid formation of stable anionic glycola- tocomplexes with increasing pH. pH gradient elution may be used, but it takes longer to achieve separation because of longer retention times for each rare earth. Irregular selec- tivity of Sc against the atomic number found at higher pH and limited use of ODS columns at higher pH ranges are also disadvantages of the pH gradient technique.

In IIC the use of organic modifiers to modulate capacity is a convenient means to reduce k' values to practical levels. The effect of methanol as modifier on the retention time of glycolatocomplexes is illustrated in Figure 5 as a function of its concentration. As can be seen, retention of glycolato- complexes decreases rapidly with increasing methanol concentration. Methanol helps to elute the glycolatocom- plexes, but does not improve resolution. Gradient elution in aqueous media is sufficient to afford practical levels of k' values for eluted rare earth complexes, so that only aque- ous eluent was used in this study. However, washing the

67

Dy+Tb+Gd+Eu+Sm

S o

A 0.0

0.05 ABU

I1r Y 'I'm

u ba

5'.0 16.0 15'.0 20'.0 R e t e n t i o n t ime ] , t in

Figure 6 Separation of 16 rare earths by gradient elution. Glycolate 0.15-0.4 M, pH 3.5 (20 rain elution run). Each rare earth concentration: 0.0001 M. I-Octanesulfonate concentration: 0.0I M.

column with methano l is an adequate means of regenera t ing the O D S columns.

P o s t - c o l u m n der iva t iza t ion reac t ions for de tec t ion is someth ing of a l imitation of IIC. Owing to low stabilities of g lycola tocomplexes arsenazo II l reacts with the rare ear ths in the presence of glycolic acid to produce a stable color. In order to produce a constant base line during gradient elution, the difference in p H be tween the arsenazo I I I solution and the glycolate e luent should be minimal , their values being held at 2.5 and 3.5, respectively. De tec t ion limits of a ng or less was general ly ob ta ined with arsenazo I I I for the rare earths. In Figure 6 ch roma tog rams of 16 rare earths is separa ted by gradient elution over 20 rain. As can be seen, adjacent heavy rare earths, Sc -Lu -Yb-Tm-Er -Ho- Dy, can be resolved with ease. The separat ion of adjacent rare ear th elements , La-Ce-Pr-Nd-Sm, is also easy. A dis- advan tage is that the in te rmedia te e lements Dy, Tb, Gd, Eu and Sm elute together. The position of Y in the elution sequence is different f rom that found with most o ther e luent systems used for rare ear th separat ions. Y behaves like a light rare ear th and is somewhere be tween Nd and Sin. De te rmina t ion of Y may be feasible without interfer- ence f rom the heavier rare earths.

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Received: Apr. 19,199I Revised manuscript received: May 22, 1991 Accepted: May 25,1991 D

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