The separation of carrier-free 85Sr from a rubidium chloride cyclotron target

Appl. Radial. hr. Vol. 37. No. 3. pp. 240-242. 1986 Inl. J. Rodiar. Appl. Insrrunt. Parr A sc Pergamon Press Ltd 1986. Printed in Great Britain. 0X83-2889186 $3.00 + 0.00

The Separation of Carrier-Free from a Rubidium Chloride Cyclotron

“Sr

Target

P. M. SMITH-JONES,’ F. W. E. STRELOW’ and R. G. BijHMER’ ‘National Accelerator Centre. Council for Scientific and Industrial Research. Pretoria, ‘National Chemical Research Laboratory. Council for Scientific and Industrial Research. Pretoria and jDepartment of Chemistry, University of Pretoria, Pretoria, South Africa

A method is presented for the separation of carrier-free “Sr from a rubidium chloride cyclotron target. The deuteron bombarded target 1s dissolved in 100 mL of 0. I M hydrochloric acid. The solution is loaded onto an 8 mL column of AGSO-Xl2 cation exchange resin and “Sr retained. The partially loaded rubidium is eluted with 250 mL of 0.5 M hydrochloric acid. Copper, iron and “Zn are eluted with 40 mL of 0.5 M hydrochloric acid-95% acetone. A final elution with 3.0 M nitric acid gives very high purity XSSrwith a nearly 100% yield.

Introduction Strontium-85 (1, ,64.9 days) is produced in the Pretoria cyclotron by deuteron bombardment of natural rubidium chloride (XSRb(d, 2n)xsSr). This radioisotope with its convenient :-ray energy (514 keV) has been employed in nuclear medicine for bone scanning.“’ More recently “5Sr has been employed in tracer studies of %Sr and in the production of standard sources. Several methods have been described for the isolation of carrier-free ‘“Sr from rubidium targets. Early procedures relied on the coprecipitation of XSSr with nonisotopic carriers such as ferric chloride.“’ lead nitrate’2’ and barium carbonate.“’ These were however time-consuming and required a number of remote manipulations. Inorganic exchangers of activated charcoal’i’ and alumina,‘h’ have been used, but in the latter case significant amounts of aluminium were present in the “Sr fraction. Sulphonic acid cation exchangers have been utilised.“x’ but large columns and elution volumes have been reported. In the ion exchange separations reviewed little or no attention has been paid to the decontamination of ‘?Sr from impurities such as copper. “)I In addition to this the rubidium chloride target used in the Pretoria cyclotron is considerably larger than any reported elsewhere. The ion exchange technique previously used in this laboratory is as follows: Rubidium chloride (IO g) is melted onto a copper backing plate in an argon atmosphere. The target consisting of the backing plate affixed to a water cooled copper plate. is bombarded with deuterons in the cyclotron. The rubidium chloride (and small amounts of copper) is washed off the target with 100 mL of 0.5 M hydrochloric acid. This solution is then loaded onto a 56 mL column (20 x 180 mm) of AGSO-X12, 100~200 mesh. Rubidium

is eluted with 1500 mL of 0.5 M hydrochloric acid. Slro17tium-85 is recovered by elution with 3.0 M nitric acid. The ‘“Sr fraction is reduced to dryness and taken up in 1.0 M hydrochloric acid. This method was cumbersome. so in response to an increased demand for ‘?Sr, a study was launched to improve both the separation and the chemical purity of the product. A systematic investigation of the distribution coefficients of strontium and rubidium was undertaken with resins of various % divinylbenzene (DVB) and in various molarities of hydrochloric acid. In addition to this the effect of rubidium chloride concentration on the distribution coefficient was studied. Furthermore data supplied by Strelow”“’ on distribution coefficients in hydrochloric acid acetone showed a simple method for the elution of traces of copper. iron and zinc.

Experimental Reagem

and apparatus

Only analytical reagent grade chemicals and double distilled water were used. Polystyrene based sulphonic acid cation exchange resins from the AG50 range marketed by Bio-Rad Laboratories of Richmond, California were used. The ion exchange columns consisted of borosilicate glass tubes (IO x 200mm) fitted with a fused in No. I porosity glass sinter and a burette tap at the bottom. The glass columns were thoroughly cleaned with chromic acid and washed with distilled water before use. Analytical work was carried out using simulated targets consisting of lO.OOg rubidium chloride. IOOmg of copper and I mg of iron, with and without 37 MBq “Sr. Determination of rubidium, copper and iron was performed with a Varian AA 775 series Atomic Absorption Spectrophotometer. A 4096 channel Canberra series 80 multichannel analyser coupled to a Ge(Li) detector was used to identify and measure the radioactivity. Determination

of distribution

coqficients

Distribution coefficients of strontium and rubidium were determined by a batch equilibration method. A mixture of 5 m-equiv of ion in 250 mL of solution with 2.50 g of dry resin was equilibrated by mechanical shaker for 24 h at 293 K. At the end of this period the quantity of ion in the resin and solution phases were evaluated. The distribution coefficients were evaluated by the relationship. D=--

amount

of ion in resin x volume of solution

amount

of ion in solution

(mL)

x mass of dry resin (g)

Table I shows the distribution coefficients with 4% DVB and 12% DVB forms of AGSO. Values given by Strelow”“,“’ for 8% DVB and AGMPSO ( = 25% DVB) are also shown for comparison. The trend for an increasing separation factor with increasing DVB crosslinkage is clearly shown.

Table

I.

Dlstributmn

coefficients

AC50

resin

in

of strontium

%

-

DVB

~~

wth

acid

0.5

I .o

~-

Sr

Rh

Sr

Rh

Sr

Rh

Sr

Rb

4

1464

XI

446

4x.7

21.4

27.0

I0.X

,’

8

4700

120

1070

77.5 217

3.1

60.2

I54

h

I2

6117

I31

I507

72.7

20.2

”

-

9300

50

’

25 “Thlr

work.

hStr&XV.““’ ‘Strelow.“”

240

0.2

ruhldlum

acid

Molarity of hydrochloric 0.1 ~~~~

and

hydrochlonc

72 X0.3 226

277 1320

39.Y Y7

320

Ref.

Technical

150

200

250 Eluate

241

Note

300 (ml1

Fig. I. Separation of “Sr on AGSO-X12 cation exchange resin. Column size: 8 mL (100 x 10 mm). Resin size: 100 200 mesh. Flow rate: I mL/min. Temperature: 293 K. Eluents: I, 0. I M HCI; 2, 0.5 M HCI; 3, 0.5 M HC1/95% acetone; 4, water; 5, 3.0 M HNO,.

Initial experiments with AGSO-X16 and AGMPSO showed a slow exchange rate. so as a compromise AGSO-Xl2 was chosen for further studies. Distribution coefficients for tracer amounts of “Sr in various molarities of hydrochloric acid and rubidium chloride were determined by a shallow bed method. One gram of dry resin was equilibrated with 37 MBq of “5Sr in 100 mL of solution by pumping the solution through the resin at IO mL/min. After 2 h the solution and resin phases were isolated, and the relative amounts of “Sr and hence D evaluated. The values shown in Table 2 show the expected increase tn distribution coefficients for tracer amounts of “SSr. The effect of rubidium chloride concentration is also shown. The results obtained show the loading stage of the separation to be most critical. with a broad loading band associated with a small loading volume. Irrctditrrio~i of rubidium

chloride targets

The target was bombarded with 16MeV deuterons at 45 it A current in the external beam of the cyclotron and cooled for a week to allow shortlived isotopes to decay. The rubidium chloride was washed off the target with 100 mL of 0 I M hydrochloric acid and transferred to a glass beaker ready for processing.

Table 2. Dnuihuuon coefficients of strontium with AG50-X I? in hydrochlonc xi&rubidium chloride solutions Molarity

of RhCl

Molarily HCI

0.00

0.207

0.413

0.827

I.653

0. I

15,400

497

IX2

56.5

17.6

0.2

3450

-

-

~

-

0.5

531

~

-

-

-

Preparation

qf 0.5M

hydrochloric

acid -95%

Five millilitres of IO M hydrochloric 95 mL of acetone and volume changes Cution exchange

separation

aretone media

acid was added ignored.

to

of “Sr

A slurry of AGSO-Xl2 was prepared in water and then loaded onto a column up to a mark at 8 mL (3.7 g). The resin was cleaned with 50 mL of 5.0 M hydrochloric acid and 50mL of water. The column was equilibrated with 20 mL of 0.1 M hydrochloric acid. The solution of rubidium chloride in 0.1 M hydrochloric acid was filtered and loaded directly onto the column and a flow rate of I mL/min observed. Excess rubidium was eluted with 250 mL of 0.5 M hydrochloric acid at 1.2 mL/ min. The complete elution of copper, iron and “Zn was obtained by elution with 40 mL of 0.5 M hydrochloric acid95% acetone. The dead volume of the acetone mixture and any dissolved organic material was removed by washing with IOmL water. Strontium-85 was finally eluted with 40 mL of 3.0 M nitric acid. This ‘?Sr fraction was reduced to dryness, taken up in 1.O M hydrochloric acid and then dispensed for market. Analyticul

determinution

qf rubidium.

copper and iron

The eluates taken were evaporated to dryness and macro amounts of rubidium calculated directly. The residue was dissolved in 0. I M hydrochloric acid and potassium nitrate added to give a final concentration of 2000pg/mL potassium. Iron, coppper and rubidium were analysed by atomic absorption spectrometry using the 248.3, 324.7 and 780.0 nm lines respectively. A simulated production run using no “Sr but only rubidium, copper and iron to simulate a target, was carried out. Twenty millilitre fractions of the first 260 mL of eluate were collected. and IOmL fractions were collected for the next l90mL. Another run was performed using the same matrix but containing ‘?jr. The loading. rubidium elution

Technical

242

and copper cluatcs were collected m 3 fractions and the “Sr fraction collected in IO mL portions. The results are shown in Fig. 1. The isolated “Sr was of a high chemical purity with only 80 /ig of rubidium present. Copper and iron levels were belolc I pg. Typical data from actual production runs show a high radionuclidic purity wtth no rubidium radioisotopes or “‘Zn detected. “Sr recovery is in excess of 90.9”/b

Discussion The new procedure for the separation of “‘Sr from a matrix of rubidium. copper, iron and “Zn on a small column is an improvement on previous ion exchange methods in terms of efficiency. Furthermore the introduction of the hydrochloric acid-acetone media has removed the tailing of copper. iron and zinc and reduced their levels in the ““Sr product to below detectable levels. In actual production runs efficiency levels of greater than 99.9% have been achieved.

Note References I. Ray R. 0. and Barmuda R. C.R.C. Handbook of Radioucriae N&ides p. 466 (C.R.C. Press, Cleveland, Ohio. 1969). 2. Garrison W. M. and Hamilton J. G. C‘hcwr. RN. 49, 237 (1951). 3. Gruverman I. J. and Kruger P. /,I/. J. .4/~/>/.Rr/ditr/. ho/ 5, 21 (1959). 4. Mellish C. E. and Payne J. A. Rc,mw~ AERE-I ,21-5.7 (Haruell. Oxon. U.K., 1959). 5 Micheev N. B.. Sawycv G. I and Popovich V. B. Rndiokltimr~~r 13, 3 I2 ( I97 I ). 6. Kopecky P. and Madrova B. /jr/. J. App/. Ror/itr/. /.w/. 25, 469 (1974). 7. Winchester J. W. and Pinson W. H. Amrl. Chin Ar,to. 27, 93 (I 962). 8. Pope L. I. and Arsenic I. fnsr. Ft. AI. (Rom). RC-71973 (1973).

IO. Strelow F. W. E.. Victor A. H. van Zyl C. R. and Elotf C. At~ul. Clwm. 43, X70 (1971). I I Strelow F. W. E. Anrrl. C/tint. Actor 160, 3 I (I 984).