Reversed-Phase Thin-Layer Chromatography of the Rare Earth Elements

R. Kuroda*/M. Adachi/K. Oguma

Laboratory for Analytical Chemistry, Faculty of Engineering, University of Chiba, Yayoi-cho, Chiba, Japan

Key Words

Thin-layer chromatography Reversed-phase chromatography Rare earth elements Bonded reversed-phase silica

Summary

Partition chromatographic behaviour of the rare earth elements on C18 bonded silica reversed-phase material has been investigated by thin-layer chromatography in methanol -- lactate media. The rare earth lactato com- plexes are distributed and fractionated on bonded silica layers without ion-interaction reagents. The concentration and pH of lactate solution, methanol concentration and temperature have effects on the migration and resolution of the rare earth elements. The partition system is particularly suited to separate adjacent rare earths of middle atomic weight groups, allowing the separation of gadolinium, terbium, dys- prosium, holmium, erbium and thulium to be achieved by development to 18cm distance.

I ntroduction

High-performance liquid chromatography (HPLC) has increasingly been used for the separation of the rare earth elements on a column containing microparticulate bonded- phase porous-silica ion-exchange resins [ 1 - 4 ] and chemical- ly modified porous polymer [5 -9 ] or a gel type strong cation-exchange resin [10]. Totally porous silica gel has also effected the HPLC separation of the rare earths [11-13] particularly in combination with a mobile phase system involving nitric acid. Development of dynamic chromato- graphic systems has also opened the way to give improved resolution of the rare earths [14--18]. Dynamic ion-ex- changers are formed when hydrophobic ions present in the mobile phase are adsorbed onto the hydrophobic surface of a reversed-phase material to produce a charged double layer at the surface where ion exchange can occur.

We found that the rare earth lactato-complexes are distribut- ed and fractionated chromatographically on a thin layer

between C18 bonded silica reversed-phase and lactate buffers without the hydrophobic ions that should modify the surface of bonded silica. Recent thin-layer chromato- graphy of the rare earths has been focused on separations conducted on silica gel plates in combination with develop- ing solvent systems involving (1) bis-(2-ethylhexyl)-phos- phate, ethers (or ketone), (tetrahydrofuran) and nitric acid [19-21 ] and (2) aqueous ammonium nitrate [22] or sulfate solutions [23]. Little is known about the partition chroma- tographic behaviour of the rare earth complexes on the bonded reversed-phase silica either in HPLC or thin-layer chromatography.

Experimental

Solutions

Appropriate amounts of the oxides or nitrates of scandium, yttr ium, the rare earths, thorium and uranium(Vl) were dissolved in concentrated hydrochloric acid and evaporated to dryness, followed by dissolution of the residue in 3M hydrochloric acid to give 0.1M metal solutions (M = mol dm-3). The 0.1M stock solution of Ce(l l l ) was obtained by dissolving an appropriate amount of CeCI3"7H20 in 3M hydrochloric acid. A test solution was prepared by taking an aliquot of the stock solution, mixing with lactic acid solution and adjusting the pH to a desired value with dilute ammonium hydroxide solution to yield 0.1M in lactate and 0.001 M in metal.

Thin-Layer Plate

Whatrnan KC 18 reversed-phase thin-layer chromatography plates (5cm x 20crn; layer thickness : 200/Jm) were employ- ed. To survey the chromatographic behaviour of each metal, plates of half size (5cm x 10cm) were prepared by proper- ly cutting the above plates.

Procedure

A 1-/JI aliquot of a test solution was applied to the plate 1.5cm from one edge using a micropipette and the plate was air-dried. The plate was then placed in the cylindrical glass tank (23cm high and 7cm in diameter) holding 10ml of the developing solvent ana allowed to stand for 30rain

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0009-5893/88/11 0989-04 ~ 03.00/0 �9 1988 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH

to equil ibrate wi th the tank atmosphere. Then the plate was immersed in the solvent and developed unti l the solvent

f ront had risen 7.5cm (for survey of behaviour) or 18cm (for separation) f rom the start.

Af ter development, the plate was dried under an I R lamp and the metals were detected by spraying the fo l lowing reagents:

- Thor ium and u ran ium(V I ) :0 .1% aqueous arsenazo III solution;

- Scandium, y t t r ium and the rare earths:O.1% aqueous xylenol orange fo l lowed by exposure to ammonia.

R e s u l t s a n d D iscuss ion

Lactic acid is a weak acid (pKa = 3.86 at 25~ reacting wi th the rare earths to form moderately strong complexes

wi th di f ferent stabilities. It has long been used as an effective reagent for the cation-exchange separation of the rare earths. However, there were no sufficient selectivity dif- ferences among the rare earth elements when developing

them in aqueous lactate media on C-18 bonded phase

thin-layers. Therefore, R F values of the rare earths were measured in methanolic lactate media.

The effect of methanol concentrat ion in the developing solvent on the retention of a selected group of the rare earths as well as thor ium and uranium is summarized in

Table I, where the overall concentrat ion of lactate was kept constant at 1M. Strong tai l ing occurred for a system containing 80% methanol. In the methanol concentration range of 0 to 50%, RF values tend to increase slightly wi th

increasing methanol concentration. Reversed-phases have been employed in combinat ion wi th ionic surface active reagents ( ion-interaction reagent) to establish the ion- exchange surface, so that dynamic ion-exchange system may result which wi l l give superior resolution of the rare

earths and greater f lex ib i l i ty wi th respect to the choice of separation condit ions. In this respect nonionic molecules such as methanol or acetonitr i le compete for adsorption sites on the reversed-phase wi th ionic surface active species (hydrophobic ions). Therefore, they are frequent ly used to control the concentrat ion of adsorbed hydrophobic ions.

As far as the thin- layer chromatographic condit ions are concerned, however, the behaviour of the rare earths was not affected by hydrophobic ions to any great extent either

in the presence or absence of methanol or acetonitr i le. This is perhaps because the actual capacity factor (k') range involved in effective HPLC separation of the rare earths gives rise to very low RF values as can be seen from the relationship.

R = l / ( l + k ' )

where R is the retention ratio, which is very close to the

R F value [24]. Low RF ranges do not reveal the datailed feature of dynamic ion-exchange.

In the absence of hydrophobic ions we can see the reversed- phase part i t ion chromatographic behaviour of the rare earth complexes. Increasing concentration of methanol favours the hydrophobic i ty of the mobile phase, thus extracting more and more the lactato complexes into the mobile phase and yielding high R F values (Table I).

The effect of pH on the R F values of a selected group of the rare earths as well as thor ium and uranium was examin-

ed for 1M lactate in 50% methanol system. The results are given in Table I1. Lighter rare earths, thor ium and uranium show dist inct ly decreasing R F values wi th increas- ing pH values, but scandium and heavier rare earths (except

for terbium) do not show a marked variation of F{ F wi th pH values.

R F values of the rare earths in lactate in 50% methanol system are plot ted in Fig. 1 as a funct ion of the atomic

number of the rare earths for the overall lactate concentra- tions of 0.1, 0.5 and 1.0M. Selectivity differences are not uni form throughout the rare earths; a large difference is there in 0.5M lactate for gadol inium through erbium and also in 1 M lactate for l ight to middle rare earths.

The effect of temperature on the R F values is shown in Fig. 2. As can be seen, R F values of scandium, the rare earth elements and thor ium decrease wi th increasing temperature which ranges from 15 to 50~ Generally,

the distr ibut ion coeff icient, K, may be given by the usual

equation [25] for thermodynamic equi l ibr ium:

K = e x p ( - AG~

Table I. R F values for metals on KC 18 as a function of methanol concentration in methanolic 1M lactate media

Methanol (%, v/v) Metal

0 25 50 80

Sc(I I I) 0.29~0.37 0.50-0,59 0.63-0.72 0.00-0.26 0.69--0.80

La (I II) 0.20-0.34 0.20-0.38 0.13-0.29 0.00-0.26 Pr (I I I ) 0.26-0.40 0.35---0.41 0.23-0.43 0.00-0.53 Sm(lll ) 0.33-0.44 0.38-0.46 0.37-0.53 0.00-0.68 Tb(l l l ) 0.49-0.59 0.60--0.70 0.73-0.83 0.00-0.83 Y b(I I I) 0.74--0.93 0.85--0.96 0.88--0.95 0.00--0.01

0.80--0.89 Th (IV) 0.01 --0.07 0.00~3.09 0.01 --0.06 0.00--0.04 U (V I) 0.00--0.02 0.00-0.04 0.00--0.02 0.00--0.02

The pH of aqueous lactate solution was adjusted to 6.35 before mixing with methanol. The R F values were obtained at 15~

990 Chromatographia Vol. 25, No. 11, November 1988 Originals

Table II. R F values for metals in 1M lactate in 50% methanol on KC 18 as a function of pH {15~

pH*

Metal 2.50 3.50 4.50 5.50 6.35 (3.65) (4.48) (5.18} (6.21) (6.91)

Sc(I I I) 0.61 -0.68 0.64-0.74 0.63-0.72 La (I ~ I) 0.54-0,69 0.50 -0.67 0.53-0.59 0.14 -0 .3 t 0.t 3--0.29 Pr (I I I) 0 .48-0 32 0.51 -0.67 0.59-0.61 0.30-0.50 0.23-0.43 Sm (I I I) 0.55-0.70 0.50 -0.68 0.59-0.61 0.51-0.58 0.37-0.53 Tb ( I I I ) 0.52--0.70 0.65-0.73 0.72-0.83 0.76-0.86 0.73-0.83 Y b(l I I) 0.86--0.94 0.87-0.94 0.89-0.95 0.89--0.96 0.88-0.95 Th (I V) 0.51 --0.66 0.53--0.63 0.41-0.49 0.04--0.10 0.01-0.06 U (V l) 0.48 -0.67 0.37-O.44 0.01 --0.05 0.OO--O.O t O,00--0_O2

Values are those for the aqueous lactate solutions before mixing with methanol. Values in parenthesis are apparent pH's obtained for 50% methanolic solutions, which are 1 M in lactate.

RF 1.OC

G5C

O.OC

z5

%. "o

o 50 ,~

c3

3.00 21 39" 57 58 59 60 61 62 63 64 65 66 67 6869 70 71 " 9092

Atomic number Fig. 1

R F values of metals on KC18 bonded phase in xM lactate in 50% methanol (15~ [] 0.1M lactate, z~ 0.SM lactate, o 1M lactate Aqueous lactate solutions adjusted to pH 6.35 before mixing with methanol.

where R is the gas constant, T is temperature and AG ~ is the standard Gibbs' free energy change of solute in the mobile phase passing into the stationary phase. The free energy change, AG ~ , of solute is generally negative when the solute goes from the mobile phase to the stationary phase so that the distribution coefficient, K, may be ex- pected to decrease with temperature. Since K is related to R, the retention ratio, by the equation [25]

R = V m / ( V m + KV,)

where V m and V s are the mob i le and s ta t ionary phase

vo lumes per un i t co lumn vo lume, R may be expec ted to

increase w i th temperature . However , this is no t the case

for the present pa r t i t i on ch romatograph ic system. The low

R F in higher temperature increases separat ion t ime and

does no t cause the apparent reso lu t ion.

Sc

Sm

RF

La Pr

t00 f Th

050 I

~176 Temperature/'c

Fig. 2

R F values of metals as a function of temperature. Eluent system: 1M lactate in 60% methanol (pH adjusted to 6.35 before mixing with methanol).

R~ 1,00

0.5C

O.OC

(a)

O, cs .

0~ OK 0K 0~, 0~o 0 G,

9 ~ OE,J 0Eu 05 Os~ OS~ 0s. OS~ Os-

Sa.mpIe n~tures

160

Fig. 3

E

~5

3.0

R~ (b) t0C

Lu

~'~Er Er

@o 0.o 0.5(

00, 0D,

07b 0S

0C~ 0 c~

000 ~ ~ ~_ Sarape m~tures

l&O

s

,a 0

100

Ioo

Separation of scandium, yttr ium, rare earths, thorium and uranium (at 15~

Eluent system: (a) 1 M lactate in 50% methanol (pH adjusted to 6.36 before mixing

with methanot). (b)O.5M lactate in 50% methanol (pH adjusted to 6.35 before

mixing with methanol)�9

Chromatographia Vol. 25, No. 11, November 1988 Originals 991

Results on the separation of scandium, yt t r ium, the rare earth elements, thor ium and uranium in 1 M lactate in 50% methanol medium are shown in Fig. 3a wi th the develop- ment distance of 18cm. It takes about three hours to accomplish the separations. It may be useful to see the rare earth distr ibut ion in thor ium and uranium, which retain near the start. Lighter rare earths, lanthanum to neodym- ium, exhib i t comet- l ike spots. The system is part icularly effective to separate adjacent rare earths of middle atomic weight group, as shown in Fig. 3b. As can be seen, the six adjacent rare earths gadol inium, terbium, dysprosium, ho lmium, erbium and thu l ium can be separated effectively by development to 18cm distance.

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Received: Aug. 17. 1988 Accepted: Aug. 25. 1988 A

992 Chromatographia Vol. 25, No. 11, November 1988 Originals