Determination of polonium-210 in urine by tract counting

Intcmational Juurnel of Applied Radiation and Ilotopcs, 1964, Vol. 15, pp. 665-569.

Pergamon Press Ltd. Printed in Northern Ireland

Determination of Polonium-Z 10 in Urine by Track Counting P. TAYLOR*,

M.

P. HIBBERT*

and B. E. LAMBERT

Radiological Protection Service, Belmont, Surrey (First received 12 February 1964 and

in final form 25 March 1964)

.i\nautoradiographic technique is described, and has proved to be sensitive and useful in the estimation of very low concentrations of Poet0 in urine. The chemical separation of the polonium is described in detail. Although the overall time for the assay of a sample is long, the technique can be used to advantage when there are a considerable number of samples. Furthermore, for samples containing up to 1 ,u,uc/l of urine, the length of time taken, in the autoradiographic method described, is no more than that required to obtain comparable accuracy by conventional a-scintillation techniques.

DOSAGE

DU POLONIUM-210

DANS L’URINE

PAR COMPTAGE

DE TRACES

On decrit une technique autoradiographique qui s’est montrte sensible et utile a l’estimation des teneurs t&s faibles de PO210dans l’urine. On rend compte detail16 de la separation chimique du polonium. Quoique la durte totale du procedt d’essai d’un Cchantillon soit longue, la methode peut servir avantageusement lorsqu’il y a un nombre considerable d’echantillons. De plus, pour les tchantillons qui contiennent jusqu’a 1 p,uc/l d’urine, la ptriode de temps ntcessaire, dans la mtthode autoradiographique ici dtcrite, n’est pas plus longue que celle demand&e par les techniques conventionelles de scintillation a afin d’obtenir une precision comparable. OIIPEJIEJIEHIIE

B

IlO.~OHMfI-210

B MWIE IIOCPEJICTBOM II3MEPElIIIR CJIEaA Onwcaua auto-PaA~orpa~asecKaR Texmitta, notiaaaauran ~ytrcTaure~briocTbII uo.teauocrt.

BbwiCJleHWEI OYeHb HHBKOrO Co~epmaHIlrr POCKY B MOYe. HeTaJIbHO On%WaHO XHMIl'IeCKOe 0Tnezlenne nononnfl. HecmoTpfi Ha TO, ~TO no;lHoe OnpegeaeHae o6paaqa 6epeT A0.1rOe spemfl,a~aTexH~KanpsiMeH~eTc~cycnexoM.Kor~a~MeeTc~ 3KawiTe.lbKoe wwIooBpa3qots. IipOMe ~orO,~jl~ 06pa3qoB, CoAepmqllx 20 1 p&f ~Owi, UcTpaseHHoe, spemR npn aBTOl'pal+YeCKOM MeTOne He AZIHifHee,'deM Tpe6yeTCH 3 CbbNHbIX MeTOgaX anb~a-CqsHTa;r:IRI1MB. npsf ~oi-4He cTeneHK T~YH~CTII.

FESTELLUNG

VON

POLONIUM-210

IM HARN

MITTELS

SPURZAHLUNG

Eine autoradiographische Technik wird beschrieben, die sich als empfindlich und ntitzlich zur Feststellung sehr Kleiner Konzentrationen von Posra im Harn erwiesen hat. Die chemische Trennung des Poloniums wird ausfiihrlich beschrieben. Obgleich die Gesamtzeit fur die .4nalyse einer Probe ziemlich lang ist, wird die Technik im Vorteil sein wenn eine gr&sere Anzahl von Proben vorliegen. Ausserdem ist bei Proben die bis 1 p&l Harn enthalten, die Versuchsdauer bei der beschriebenen Autoradiographischen Methode nicht liinger als bei Gebrauch der tiblichen Szintillationstechnik bei vergleichbarer Genauigkeit. * Former members of the staff of the Radiological Protection Service. 3

665

666

M. P. Taylor, P. Hibbert and B. E. Lambert

INTRODUCTION a-emitting nuclide Po21s cannot as yet be measured directly in the human body, but an estimate of body burden may be obtained by urine analysis. Since this nuclide is widely distributed in the biosphere ivith the result that small amounts are found in all human beings,(‘p”) it was necessary to determine the normal urinary excretion by persons exposed only to Po210 from natural sources, so that the significance of retention data for persons occupationally or accidently exposed to the nuclide could be established. Several methods have been reported’s_s) for the estimation of polonium, all of which depend on the same basic chemistry but introduce various modifications; in all these methods the Po210 is determined by scintillation counting. Since the concentration of Po2io in the urine of normal unexposed persons was expected to be low, in view of the small amount in the body, the count rate would be slow and long observation time would be necessary for good statistical accuracy; subsequently it was found that about a week or longer was necessary to give an accuracy of &lo per cent. Such a long observation period creates problems due to the instability ofthe electronic equipment and, furthermore, for routine analysis of a number of samples to be carried out within a reasonable time, it would be necessary to install a large amount of expensive counting apparatus. A nuclear track technique has been described for the estimation of plutonium plated electrolytically on to a stainless-steel disk(9). This technique consists in recording the image of the track caused by the emission of an aparticle in a photographic emulsion and, after development of the photographic plate, counting the tracks under a microscope. Such a techalso seemed suitable for polonium, nique provided that the nuclide could be plated out on to a small area so that a sufficient number of a-tracks could be counted when the plate was In order to scanned under a microscope. obtain the necessary a-track density a microcell was designed which allows the polonium to be plated on to a pure nickel disk, radius 6 mm. Before plating, it is necessary to concentrate the polonium in the urine to a final volume of

Jet

Air outlet

THE

Polythene

woshcr

ti

FIG. 1. The deposition cell.

about 25 ml. Our method of doing this is based on the same chemical principles as the other methods but the modifications. introduced affect all stages of the procedure and are important in enabling the electrochemical deposition to be carried out efficiently from a small volume. The main changes we’ have made to the methods previously described in the literature are: (1) the wet ashing stage, where the organic material in the urine is destroyed by nitric acid alone [this prevents charring and also avoids loss of polonium due to temperatures in excess of 200°C]; (2) the technique of precipitation of the polonium with the tellurium carrier, which is achieved in one step resulting in considerable time saving; and (3) the use of nickel disks instead of silver as the former are cheap enough to be discarded after each assay.

EXPERIMENTAL Apparatus The microcell is shown in Fig. 1; it is demountable and is made of Perspex and has a The internal total height of about 9 cm.

Determinationof polmzium-210in urineby track counting diameter of the cell is 25 mm tapering to 12 mm. The agitation apparatus for bubbling air from a compressor through the liquid in the cells consists of a main supply line from which extends flexible side tubes, each of which ends in a jet dipping below the surface of the liquid in the cell. The flexibility of the side tube enables the position of the jet in the liquid to be adjusted. Taps on the jets and the main supply line allow the adjustment of the pressure so that the degree of agitation is the same in each cell. Procedure A twenty-four hour sample of urine is collected, preferably in a Polythene bottle to minimize adsorption of polonium which occurs on glass surfaces. The sample is also acidified with 20 ml of concentrated HCl as soon as possible after collection of the sample to minimize adsorption. After measuring the total volume of the sample plus acid, it is wetashed by warming with concentrated nitric acid in a large Erlenmeyer flask until the residue is just dry and white. Final traces of nitric acid are removed by dissolving the solids in HCl and taking the solution just to dryness again. The solids are dissolved by warming with 10 ml distilled water and 10 ml of a potassium tellurate solution containing 1 mg Te/ml and by adding concentrated HCl drop by drop; the final concentration of acid added must not exceed 6N (i.e. 20 ml of acid) at this stage. The solution is then transferred to an evaporating basin, 1 g of solid sodium hypophosphite is added and the solution is evaporated practically to dryness on a water bath. After addition of 50 ml of distilled water, the basin is covered with a watch glass and the mixture is boiled gently for a short time to dissolve the salts and coagulate the precipitate of tellurium and polonium. The precipitate is separated from its supernatant by centrifuging: A few drops of bromine and 7.5 ml of concentrated HCl are added to the precipitate and the centrifuge tube is then placed in a water bath until the precipitate dissolves; the solution is then returned to the original evaporating basin

667

where any adhering precipitate is now dissolved in the bromine-HCl mixture. After washing out the centrifuge tube with a few millilitres of water, thevolume of the solution is adjusted to 20 ml and the excess bromine is removed by gently warming on a water bath until the solution is colourless, when 5 ml of 15 per cent (w/v) hydra&e hydrochloride solution is then added to precipitate the tellurium, which is allowed to coagulate. The precipitate is filtered off through a sintered glass filter (porosity, No. 4) and the basin is thoroughly washed out with The filtrate and washings are warm water. then evaporated to about 10 ml on a water bath. Any further tellurium which separates out at this stage must be filtered off through the sinter, the precipitate again being washed with water ; this process is repeated until the final volume of 10 ml is completely free from tellurium. The solution containing the polonium is now transferred to the assembled, clean deposition cell, the base of which contains a nickel disk which has been cleaned in dilute nitric acid; the exposed area of the disk is 113 mms (radius The solution is made neutral to 6 mm). methyl red with 30 per cent (w/v) NaOH, the volume adjusted to 22.5 ml and 2.5 ml of concentrated HCl are added. The solution contained in the cell is agitated at room temperature for 16 hr by blowing air through it. With each batch of samples a reagent blank and a standard of radium-D, in equilibrium with its daughters, are included. The rate of flow of air bubbling through the liquid in the cell was fast enough to keep the solution in a state of rapid agitation, but was not so great that the deposit of PoslO on the disk was removed. Iteis difficult to define the actual air pressure used as this is dependent on the back pressure in the apparatus, the number of cells used, etc. After completion of the electrochemical deposition process, the liquid in the cell is poured off and the nickel disk is carefully washed with distilled water, removed from the cell. and allowed to dry at room temperature il air. The active surface of the disk is secured in contact with the emulsion of a photographic plate (Ilford K2 Nuclear Research Plates).

668

M. P. Taylor, P. Hibbcrt and B. E. Lumbert

The position of the sample is marked on the plate by outlining the disk with a stylus and the number of the sample is also marked in a similar manner. The plates are then rewrapped and stored at 5°C for one week, after which time the disk is removed and the plates developed at 22°C for 8 min in Kodak D19 developer and fixed in Kodak F5 acid fixer until the plates have cleared. After washing and drying, the plates are examined under a microscope, using an oil immersion objective (total magnification being x 1350). The a-tracks were counted in five 5 mm scans across that part of the photographic plate which had been in contact with the active area of the disk. The activity of the sample was then calculated by comparison with the standard, correction being made for radioactive decay of the PoslO between precipitation and deposition on the disk, and for the reagent blank. RESULTS

AND

DISCUSSION

The actual number of tracks seen under the microscope is dependent on the percentage yield of the processed samples and the efficiency There is of the electrochemical deposition. also the constant factor introduced by geometry, namely, that the film can only record half the number of a-particles emitted by the nuclide deposited on the disk. As a standard is run with each batch of samples, it is not necessary to know the efficiency of the electrochemical process. However, in order that some estimate of the limits of detection could be made, the efficiency of the process was determined for each of sixty samples containing nominally 1 ppc radium-D in radioactive equilibrium with Poelo. The efficiency is dependent on the deposition time, the temperature of the solution and the rate of agitation of the solution. However, the use of a long deposition time (16 hr) makes the last two conditions less critical, nevertheless these experimental conditions were controlled as closely as possible. Assuming that each sample contained exactly 1 ppc of PozlO, the efficiency of the deposition was found to be 73 per cent with a standard error of the mean (SE.) of 2 per cent. In addition to this error, an allow-

ante has to be made for the error incurred in preparing the standard from a more active, calibrated solution, together with the limits stated for the accuracy of calibration.* It was estimated that the overall error was about 510 per cent; the efficiency of deposition is therefore 73 per cent with an error of about A7 per cent. To determine the chemical yield, a series of solutions containing inorganic salts representative of those m urine were spiked with the radium-D/Po210 standard, taken through the chemical process and then compared with a standard of the same radium-D solution. The chemical yield was found to be 96 per cent with a S.E. of 2.5 per cent. The overall efficiency of the method is therefore 70 f 7 per cent. As the compressed air supply used for the agitation of the samples is generated direct from the air of the laboratory, the possibility of introducing PO*lo from radon in the atmosphere into the samples was ‘considered. When an experiment was run in which distilled water acidified with HCl was placed in the deposition cells and agitated in the usual way, ,this was found to be negligible. The daily excretion of Po210 in urine was determined from measurements on thirty adult hospital patients, who had had no occupational or therapeutic contact with polonium or any of its radioactive parents. The spread of the measured concentration of Po210 in the urine was found to be <0*003-0*22 cl&l, with a mean of 0.06 p,ucIl and a standard deviation of 0.06 lpc/l, or in terms of excretion per 24 hr, the range was <0*007-0.25 ,u,uc/day with a mean of 0.06 ppclday and a standard deviation

of 0.05 p&day.

Acknowledgments-The authors wish to thank Dr. COPPEN, M.R.C. Neuropsychiatric Research Unit, Woodmansterne Road, Carshalton, Surrey, for his help in procuring the samples which enabled these studies to be made.

A. J.

* See Ra-D solution (RBY) 1 pc/lOO ml-Radioactive Products of the Radiochemical Centre, Amersham, Bucks.

Determination of fwhmium-2!I.O in urine by track counting

REFERENCES HILL C. R. Health Phys. 8, 17 (1962). HURSHJ. B. Science13!2, 1666 (1960). RUNDOJ. Report AERE HP/R 627 (1959). FINK R. M. Biological Studies with Polonium, Radium and Plutonium, McGraw-Hill, N.Y. ( 1950). 5. Los Alamos Report LA-1858.

1. 2. 3. 4.

669

6. LAMBIE D. A. Determination of Polonium in Urine. Paper given at a conference on biological monitoring at AERE, Harwell, 1954. See ulso Report AERE-AM 60 (1953). 7. UKAEA Report IGO-AM/W-167 (1958). 8. G~MMP. J. Report AERE Med/M 22 (1958). 9. SCHWENDIMAN L. C. and HEALYJ. W. Nuckonics 16, 80 (1958).