A spot test for the specific activity of molybdenum-99

A spot test for the specific activity of molybdenum-99

Appl. Radiat. Isot. Vol. 38, No. 2, pp. 149-150, 184mg dissolving was prepared by solution (NH,),Mo,0,,.4H,O in a litre of water. The molybdenum stoc...

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Appl. Radiat. Isot. Vol. 38, No. 2, pp. 149-150,

184mg dissolving was prepared by solution (NH,),Mo,0,,.4H,O in a litre of water. The molybdenum stock standard solution was serially diluted to give standards containing 1.25, 2.5, 5.0, 7.5 and 10.0 mg L-’ molybdenum.

1987

ht. J. Rodiat. Appl. Instrum. Part A

0 Pergamon 0883-2889187

Journals Ltd 1987. Printed $3.00 + 0.00

in Great

Britain

Dithiol reagent

A Spot Test for the Specific Activity of Molybdenum-99 K. J. FARRINGTON*

The dithiol reagent was prepared by dissolving 1 g of Merck dithiol in 200 mL of deaerated I % potassium hydroxide solution to which had been added 2mL of thioglycollic acid (Hobart and Hurley, 1962).

and R. K. BARNES Molybdenum-99

Australian Atomic Energy Commission, Research Establishment Lucas Heights Research Laboratories, Private Mail Bag, Sutherland, N.S.W. 2232, Australia (Received 4 September

The activity of the fission-based %Mo was measured in a lead-shielded container in a calibrated ion chamber. Samples of 99Mo were diluted to 925 MBq mL_’ (25 mCi mL_‘) calibrated to the generator loading time. A volume of 0.1 mL was sufficient for the spot test, which requires one drop of solution from a Pasteur pipette.

1986)

A spot test using dithiol reagent was developed for the semi-quantitative estimation of the specific activity of 99Mo solution. The test was designed to cover a range of specific activities from < 92.5 to > 740 TBq g-‘MO. Five batches of 99Mo, previously assayed by polarography, were tested by the spot test method and each fell within the expected specific activity range. The spot test is a simple method requiring a low level of radioactivity, which is readily disposed of at the completion of the test.

Dithiol spar test The test was carried out in a fume chamber behind a lead glass window. One drop of dithiol reagent from a Pasteur pipette was added to one drop of hydrochloric acid in a white plastic lid of 1 cm diameter. One drop of 925 MBq mL_’ wMo test solution, calibrated to the generator loading time, was added from a Pasteur pipette and the solution was gently mixed by moving the lid with forceps. The molybdenum standard solutions were treated in the same way as the %Mo test sample. A blank spot test was made using water. The “MO spot test was compared visually with the series of molybdenum standards. The molybdenum standard solutions containing 1.25, 2.5, 5.0, 7.5 and IOmg L-’ of molybdenum represent specific activities of 740, 370, 185, 122 and 92.5 TBq g-’ MO, and the blank represents values greater than 740TBq gg’ MO. The specific activity of the 99Mo test sample was estimated, within a range, by visual comparison with the developed series of molybdenum standards. At the completion of the test the lids used for the spot test are discarded into a lead-shielded waste container. The activity used in the spot test was determined by measuring ten single drops of 925 MBq mL-’ %Mo solution delivered from a Pasteur pipette into a plastic counting tube. The activity, measured in a calibrated ion chamber, was 48.8 If- 2.2 MBq per drop. The limit of detection of molybdenum by the spot test was estimated at 0.25 pg.

Introduction The specific activity of fission based %Mo produced in the AAEC’s reactor HIFAR at Lucas Heights has been reported (Barnes er al., 1983) as 11461998 TBq gg’ MO at the time of loading 99Mo/W”Tc chromatographic generators. The assays were performed by differential pulse polarography. Molybdenum analysis can readily be carried out by a variety of methods on 99Mo samples which have decayed. However, highly radioactive solution presents problems of operator dose, and the possible contamination of instruments and laboratory. During reactor maintenance !+‘Mo is imported; the imported material must meet a specification for specific activity of > 185 TBq gg’ at generator loading time. To test this requirement, a rapid semi-quantitative technique requiring minimal levels of readily disposible radioactivity has been developed. A spot test method was developed using a dithioi reagent which under acid conditions gives a green coloured precipitate with molybdenum ions. Dithiol (toluene-3,4-dithiol) has been widely used for the detection and assay of a number of metal ions including molybdenum (Welcher, 1948). It has also been used to separate *MO by precipitation from other fission products (Blagojevic ef al., 1985).

Spectrophotometric

Experimental The molybdenum standards were prepared from Merck GR ammonium heptamolybdate tetrahydrate which was assayed with 0.5 M lead nitrate solution using 4-(2-pyridylazo)resorcinol monosodium salt as an indicator (Merck, 1972). A 100 mg L-’ molybdenum stock standard

Polarographic molybdenum measurement The concentration of molybdenum in five imported batches of fission-based %Mo were analysed by the method

for correspondence. 149

A.R I 3812-E

molybdenum measurement

This method was used purely to show that the proportion of molybdenum standards and reagents used in the spot test gave colours that obeyed Beer’s law. This was shown to be true when the dithiol reagent, the hydrochloric acid and the molybdenum standard solutions were mixed in the proportions in mL of 1: 1: 1 and 1: 1:2. The dithiol/molybdenum complex in hydrochloric acid was extracted with 3 mL of chloroform by shaking vigorously for 15 s. The coloured extract was examined in a Varian DMS 100 double beam u.v.-visible spectrophotometer at 680 nm against a blank prepared from the chloroform extract of dithiol, hydrochloric acid and water mixed in the appropriate proportions.

Molybdenum standards

* Author

meawement

150

Technical Table

Sample

A B C D E Spot test (x).

I. Evaluation

Polarographic assay (TBq g-’ MO)

of spot test for specific activity > 740 TBq g-’ MO

573.9 206.5 759.3 311.3 291.6 Evaluation

Note of 9’Mo

< 74&>370 TBq .I-’ MO

<37&>185 TBq g-’ MO

x x x x x based on 3 tests.

of Barnes et al. (1983). A Princeton Applied Research polarographic analyser (model 174 A) having a saturated calomel reference electrode was used. The five imported batches of “MO were also examined by the spot test method. The values obtained for the specific activities by both methods are shown in Table I.

Discussion The dithiol spot test was found to be satisfactory for the semi-quantitative estimation of microgram quantities of molybdenum. By relating this test to a standard activity of 99Mo the specific activity was estimated within a range of specific activities. Spectrophotometric examination of chloroform extracts of the reaction product from dithiol reagent, hydrochloric acid and molybdenum standard solutions in the ratio 1: 1: 1 or 1: 1:2, respectively, showed that the systems obeyed Beer’s law. The spot test performed satisfactorily when one drop of 99Mo was tested, but if required the colour intensity could be increased by using two drops of the %Mo test solution. Five batches of imported 99Mo assayed by differential pulse polarography gave specific activities of 206.5 to 759.3 TBq-’ g-’ MO at generator loading time. When these batches were examined using the spot test method, semiquantitative estimations were made of the specific activities which fitted satisfactorily within a range of values, as shown in Table 1. The method readily served its intended purpose

of acting as a pass/fail method for imported batch of fission based 99Mo, which are required to have a specific activity of greater than 185 GBq g-’ MO at the generator loading time.

Conclusion The dithiol spot test for the estimation of the specific activity of “MO is a rapid, direct semi-quantitative method. It is also useful as a pass/fail method when a product is required to satisfy a specification limit. The test requires a low level of radioactivity which is readily disposed of on completion of the test. The method involves less risk to the analyst and to the work place than instrumental methods. Acknowledgemenf-The authors wish to thank Mr N. Blagojevic for helpful advice and discussions during the preparation of the paper.

References Barnes R. K., Hetherington E. L. R. and Ohkubo M. (1983) Int. J. Appl. Radial. Isot. 34, 603. Blagojevic N., Boyd R. E. and Hetherington E. L. R. (1985) Int. J. Appl. Radial. Isot. 36, 85. Hobart E. W. and Hurley E. P. (1962) Anal. Chim. Acta 27, 144. Merck Standards 70, E. Merck, Darmstadt, F.R.G. (1972). Welcher F. J. (1948) Organic Analytical Reagents Vol. 4, p. 193. D. Van Nostrand Company, New York.