Sampling and storage of radioactive solutions

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Sampling and storage of radioactive solutions

Bowes, G. C.; Baerg, A. P.

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d

-National Research Council of Canada

Division of Physics

SAMPLING AND STORAGE OF RADIOACTIVE SOLUTIONS

G. C. BOWES AND A. P. BAERG

Ottawa

PXNR- 2288 _{NRC- 11513 JUNE, 1970}

NRC-11513

SAMPLING AND STORAGE OF RADIOACTIVE SOLUTIONS

by

G.C. Bowes and A.P. Baerg

Division of Physics National Research Council

Ottawa, Canada

Introduction

Basis of the Methods

Polyethylene Ampoules - Preparation Transfer from Glass Ampoules

Aliquoting by Weight Temperature Effects Static Charge

Buoyancy Corrections Di lution

St o rage of Radioactiv e Solutions Arresting Solvent Loss

Source Mounts and Source Preparation Source Mounts

Source Preparation Summary and Di scussion Tabl e I Re f erences Figures Page 1 2 3 4 6 6 7 7 9 11 12 14 14 1 5 1 6 1 8 2 0 22

The subdivision and sampling of radioactivity for

Ｚｾｾｾ､｡ｲ､ｩｺ｡ｴｩｯｮ＠ and other measurements are almost invariably =ade u s i ng aqueous solutions as carrier vehicles for the small,

ｯ ］ｾ･ｮ＠ submicrogram masses involved. In earlier work i t was cowmon practice to use microchemical techniques for volumetric

aliquoting. With improved accuracy requirements in the last

decade, most standards laboratories have adopted gravimetric = ethods to determine the solution sample masses.

Gravimetric sampling avoids the difficulties o f liquid density variations and modern balances readily permit masses of 10 - 100 mg samples commonly u sed to be determined to t he

4

order of 1/10 . The sampling accuracy may be seriously

ｾｭ ｰ｡ｩｲ･､＠ however unless stringent precautions are taken to

avoid solution mixing errors, weighing errors and liqu id losses due to evaporation.

The manipulation of s o lutions, where errors are most like ly to occur , are subj ec t to wide var iation and there is evidence from recent radi oactivity comparisons (1, 2) spon-sored by the International Bureau of Weights a n d Measures, th at sampling procedures may contribute significantly to t he di spersion of results Ｈ ｾ Ｑ Ｅ Ｉ Ｎ＠ The techniques used at N. R.C. h a ve been b riefly described previousl y {3) and found , over

ten years experience to give a precision of less than 0.1%. Several others have described similar sampling techniques

{4, 5) and investigated different weighing methods (6, 7, 8 , 9) . This report describes in greater detail the methods used at N.R.C. and i t is hoped that a combination of such information will lead to widely accepted methods, sufficiently reproducible

as not to limit the degree of agreement attainable in inter-laboratory comparisons.

Basis of the Methods

In principle, the mass of liquid samples may be determined either by weighing directly the sample delivered onto a source mount located on the balance pan (extrapolation method ) or by difference, weighing the ampoule containing the solution, before and after sample delivery ("pycnometer" method). In the extrapolation method a series of weighings is required to allow correction for sample evaporation on the balance pan. Several authors (6, 8, 9) have investigated and compared the two methods and concluded that, with care the attainable accuracy is comparable but that the pycnometer method is in-herently the more accurate.

aliquoting (by the pycnometer method), diluting, transfer as well as storage of radioactive solutions are centred around

the use of polyethylene ampoules or pycnometers. This approach

was first introduced at the A.W.R.E. Aldermaston laboratories and i t constitutes an important advance over the earlier

volumetric techniques using glassware for sampling and storage

(10). The supplier of the ampoules originally used however

declined to produce further quantities after 1964. Following

an unsuccessful search for an alternate supply N.R.C. com-missioned a Canadian plastics firm to construct a mold to

produce ampoules meeting certain design criteria. The new

*

ampoules became generally a v ailable in 1966 (11)

Polyethylene Ampoules - Preparation

The plastic ampoules as received from the manufacturer hav e a capacity of 5 ml and a wall thickne ss of about 3/ 4 mm.

(see 1, fig. 1). They are prepared for use by forming a capillary from the mass of plastic at the base of the neck

(2 and 3 of fig. 1). This is accomplished by slowly heating this thicker section of the neck by rotating near a micro-flame, then drawing the soft transparent plastic slowly in a

v ertical direction. After cooling, the capillary is cut with

*

Sold as "Virg in Polyethylene Bottles, 5 c.c." by Canus Equipment L i mited, 340 Gladstone Avenue, Ott a wa , Ca n a da.

t h e flame, sealing the end at the same time. It is good practice at this stage to apply a simple manual compression

test for possible pinhole leaks. Finally the capillary is

cut to length. Typically the capillary dimensions may be 8

ern long, 0.5 rom I.D., 1 rom O.D. The ampoule may now be weighed, before actual use, if i t is required to keep a

record of the amount of solution subsequently remaining in i t .

Transfer from Glass Ampoules

Despite the measurable solubility of glasses (12) i t i s fairly common to use glass ampoules for storage of

s tandardized solution and, more particularly, for s h ipping

radioactivity to be used in intercomparisons. The manipulations

involv ed in exposing and transferring the solution to poly-ethylene ampoules present serious potential sources of error but with care i t need not exceed 0.001%.

The glass ampoule should be near temperature equilibrium wi th the balance before opening. Then, in a l l subsequent o per a tions good thermal insulation a round the ampoul e s shoul d b e used to prevent warming of the solution which would lead to undesirable delay before accurate weig h i ng can b eg in.

in solution concentration gradients. The liquid should therefore be thoroughly mixed and then induced to drain com-pletely from the ampoule tip before i t is opened .

Opening of the ampoule is most safely accomplished by drawing a file mark near the top of the neck. The tip is then cracked by momentarily contacting the file mark with a

small bead of molten glass. Immediately after the glass

tip is cracked and removed, the liquid should be withdrawn

into a collapsed polyethylene ampoule. There is usually no

need to withdraw all of the solution and i t is much more important to remove an adequate sample (e.g. about 3 gms) quickly, in one operation without passing air through the solution. The time required for transfer need not e xceed a few minutes. Since the rate of evaporation from an open

ampoule is about 2 mg/hr the systematic error in this operation

may be reduced to less than 0.001%. After withdrawing the

solution the outside of the plastic capillary is wiped with a tissue and, if small droplets are found to adhere to the i nterior, a few drops of liquid are expelled to waste. The pressure on the ampoule is then released slowly so as to l eav e the capillary interior free of liquid dropl e ts.

Once temperature equilibrium with the balance chambe r is e stablished the ampoule can be weighed again to determine

the solution content. All manipulations for weighing and aliquoting should be made using forceps or thermal insulation about the ampoule.

Aliquoting by Weight

The mass of an aliquot (dispensed outside the balanc e) i s determined by the so-called pycnometer (or elephant)

method, i.e. as the difference in weight of the ampoule before and after dispensing the liquid. Weighings are made on a

Mettler MS balance. The precautions necessary for accurate

weighing have been discussed in detail by several authors (6, 7, 9).

Temperature Effects

The troublesome effects which result from air density gradi ents are largely avoided by regulating the balance room temperature to

+

O.l°C (at N 22°C) and the humidity to

+ 2% Ｈ｡ｴｾ＠ 40%). Adjacent preparatory laborato ries are not

so well regulated but are maintained at the same mean values to a voi d long delays in reaching equilibrium with the balance c h amber.

Static Charge

Static charge accumulation by the plastic ampoule can

cause serious instabilities in weighing. These difficulties

are avoided by use of commercially available radioactive sources designed for charge elimination in balance weighing chambers.

Buoyancy Corrections

The mass, m, of the sample, corrected for buoyancy effects, is given by

where M

1 and M2 are respectively the apparent ampoule weight before and after sample delivery, p is the air density, p

a w

the density of the balance weights and p the density of the

liquid sample. The correction term, for commonly used

solutions and balance weights, at atmospheric pressure is near 0.10% a nd this correction would seldom vary by more than a few percent. If more accurate correction is required , s olution

and air density measurements would be n ecessary. Variation

obtained by the following equation, shown graphically in fig. 2.

273.13

pa

=

T

rB -

_{[ 760} 0.378elx 1.2929 g/1

j

where B is the barometric pressure in mm Hg, T is the absolute temperature in °K and e is the vapor pressure of water in the air. ( mm Hg

Although forceps and thermal insulation are used for all manipulations while dispensing liquid, air turbulence, possibly some thermal gradients and electrostatic effects do develop in opening the balance chamber and dispensing

aliquots. An automatic dispensing apparatus, designed to

eliminate these difficulties and to be used with an MS balance in a precise temperature controlled enclosure, is currently being studied by Dr . R.J. Adams of these laboratories.

At present, the overall precision in aliquoting by weight has been found to be very near 0.03% for an aliquot of 30 mg. This precision is comparable with that found by others using similar techniques {4, 5).

Dilution

If dilution of the master solution is required, the aliquot is mixed with the diluent in a 10 ml Erlenmeyer flask fitted with a tapered stopper (4 of fig. 1). These are preferred to volumetric flasks often used because (1) the design minimizes splash during mixing by gentle swirling,

(2) the wide base provides good stability and (3) the capillary of the plastic ampoule may be lowered well into the flask to

dispense the liquid. This is important, to prevent drops

of either activ ity or diluent remaining on the upper part of the wall or neck of the flask where complete mixing into the

volume is very difficult. To avoid the risk of contaminating

t h e ampoule capillary with stopper grease, the ground glass surface is left dry. This also avoids sticky problems for

the unassisted operator. Tapered polyethylene stoppers are

readily available if dry glass joints are found objectionable. Both the master solution aliquot and the diluent are

delivered to the pre-weighed dilution flask from polyethylene ampoules to provide a combined volume not greater than 5 ml.

(all components should be at ambient temperature). Only the

master sol ution aliquot is weighed by difference, from the

dilution flask and contents may be weighed at a convenient later time to determine the total mass of the solution. This total mass together with the master solution aliquot

mass gives the dilution factor. If an interim storage of

the flask is required before i t is weighed, i t should be maintained at constant temperature to prevent condensation of water on the glass walls.

Immediately after weighing the dilution flask with i t ' s contents however, the solution is mixed by gentle swirling, ensuring that no drops on the wall escape mixing into the

total volume. Inverting the flask for mixing is regarded as

a potentially serious source of error in dilution, whether or not the stopper is greased. The evidence is that the mildest swirling provides for homogeneity of the bulk of solution (5} but that loss of liquid, which is not yet mixed, in the

stopper crevices or as droplets on the walls, can cause signi-ficant error.

Before any distillation phenomena and evaporation loss can cause concentration gradients, an adequate sample (e.g. 3 gm) is transferred to a pre-weighed polyethylene ampoule. The same precautionary approach is used here as for the

withdrawal of solution from an opened glass ampoule. The

< <

-rate of evaporation from an open flask, while depending on volume and other factors, is also about 2 mg/hour. With a solution volume of 5 ml, the error in this operation need not exceed 0.001%.

Storage of Radioactive Solutions

Accurate calibrations require the use of the highest available specific activity, consistent with chemical

stability of solutions. Serious problems in absolute counting

may begin to set in, especially with complex decay, when the source mass exceeds the order of 1 ｾ ｧＮ＠ Preferably, the solids content of a solution should therefore be less than the order of 50 ｾ ｧＯｧｭ＠ as a general rule. Unfortunately , the solubility of glasses indicates (12) that a comparable mass of foreign solids can be introduced during storage periods of several weeks in glass vessels.

This problem may be avoided by simply storing the solutions in the polyethyl e n e a mpoul es already used for

aliquoting. The plastic is however semi-permeable and water

(but not solute) is lost through the walls. Typically, the

loss rate is 250 ｾｧＯ､｡ｹ＠ in a l aborat ory atmosphere under

quiescent conditions. Loss rates actually observed may differ

ｾ ｾｩ､ｩｴｹ＠ and turbulence of the air surrounding the ampoule, =ut they are not dependent on the volume of solution. If

ｾ ｾ ･＠ capillary is open-ended the loss rate is greater (e.g.

oy 50 - 250 ｾｧＯ､｡ｹＩ＠ and depends on the length and diameter o f the tube as well as on atmospheric humidity and turbulence . • ｾ ｲ＠ temperature and pressure fluctuations may enhance this

:oss somewhat through a pumping action. For long-term storage

o f ampoules, the end of the capillary may be sealed, using

ｾ ｨ ･＠ heated tips of a pair of forceps to apply sealing pressure

at the end of the tube. Weighing is required before and after sealing and again before and after re-opening, to permit cor-rections for loss of plastic.

Arresting the Solvent Loss

Transfer of solvent through semi-permeable membranes

is explicable thermodynamically, in terms of the solvent v apor

pressure differential across the membrane. With zero

dif-ferential the system is in dynamic equilibrium and solvent

loss can, in principle, be completely arrested. Thus, by

placing the semi-permeable polyethylene ampoule in another v essel maintained at the required humidity, this condition

ampoule may be a glass bottle (screw cap with plastic gasket), together with an open vial containing an inactive solution of the same chemical composition as that in the plastic ampoule

(5 of fig. 1). (It would of course also stop loss through

an open capillary.) There is however reason to expect some

residual weight change due, for example, to reaction of the humidifier solution with the glass.

The results of this approach are shown in fig. 3 for

several different solutions enclosed in polyethylene ampoules. The loss rates for ampoules located in dry air or in a laboratory

atmosphere (somewhat drafty, ＳｾＯｯ＠ humidity) are comparable,

between 300 and 400 ｾｧＯ､｡ｹＮ＠ When enclosed in a glass bottle

with an appropriate humidifier solution the loss rate is reduced to the order of, or less than, a few ｾ ｧＯ､｡ｹＮ＠ The weight change may in fact be reversed, as shown by the data

for lN HCl stored in a bottle with pure water as a humidifier. It should be noted that when the ampoule is first removed from a humid atmosphere to the balance chamber (e.g., Ｔｾ Ｏｯ＠ rh) a weight loss of about 50 ｾｧ＠ occurs. The ampoules are found to attain equilibrium with the weighing room atmosphere in about 30 minutes.

Although i t is desirable to minimize solvent loss during storage, accurate humidity control is not necessary to maintain

a standardized sol ution. Provided a record is kept of the mass of solution remaining in the ampoule, then a correction

for concentration variation through solvent loss can be

easily made and the ampoules may be safely used for storage.

Source Mounts and Source Preparation

This section on source mounting while not directly re-lated to the main subject matter of this report, is included here because of the importance in accurate radioactivity measurement.

Source Mounts (for Ｔ ｾ＠ proportional counters)

Conducting VYNS films, now commonly used for source mounts (13), are supported on aluminum rings (3.8 ern O . D., 2.5 ern I.D. and 0.8 rnrn thick). If the source is to be covered with another film, the original mounting film can be transferred from the larger aluminum rings to support rings of smaller I.D., made of brass or steel (3.8 ern O.D., 1.9 ern I . D. and 0.025 rnrn thick).

The conduction coating for the films is vacuum evaporated

ａｵＭＱｾ Ｏｯ＠ Pd alloy which has superior properties (14, 15) relativ e to the gold coatings previously used. The films are usually

2

If sources are to be covered with one or more films, care is necessary to prevent pockets of air being trapped

between the laminations. These pockets appear often to

result in counting instabilities. This is easily avoided by

placing the laminated source in an atmosphere of acetone vapor for 5 to 10 sec., under an inverted petri dish wetted with

this solvent. With this treatment trapped air escapes quickly

through micropores and the slackened films adhere well, becoming taut when the acetone v apor is removed. The same technique

is useful to induce film collapse onto metal surfaces, for example in transferring films to support rings of smaller internal diameter.

Source Preparation

Techniques to produce thin sources of high counting

efficiency are not yet well defined. The following may be

noteworthy.

(1) Highest counting efficiencies appear to be corr e lated with selection of VYNS films which are free of

blemishes and show no wetting tendency when rinsed with water or as they are lifted from the water

(2) A drop of spreading agent solution (Catanac SN, 50-100 ｾｧＯｭｬＩ＠ (16) is added to the aliquot of

activity on the film. By manual rotation and

vibration the aliquot can then be spread to cover 2

an area up to 1 em . This method consistently yields source counting efficiencies of 96 -9 8% for 60

Co of mass less than 1 ｾ ｧＮ＠

(3) Infra-red heating to dry the sources is no longer used because of evidence that this type of drying has deleterious effects on the film conductivity.

Instead the sources are dried by passing turbulent warm air (40 C) over them. 0 The simple apparatus

used for this purpose is shown in the s elf-explanato ry diagram of fig. 4. For rapid drying the air flow

is adjusted to induce mild ripples on the drop

surface. Typically, the drying time required for

35 mg. aqueous solution (including wetting agent)

is 10 - 15 minutes with warm air, 25 - 40 minutes

0

at 20 C.

Summary and Discussion

The solution sampling procedures, d e scribed here and elsewhere by other workers, have been shown capable of a

precision of about 0.02% for samples of 25 mg mass. Inherent inaccuracies due to neglect of evaporation losses though small, are summarized in Table I. The estimated losses are based on manipulation of 4 ml. volumes in glass ampules, dilution

flasks and the polyethylene dispenser ampoules primarily used in this work.

Evidently systematic errors due to evaporation need not exceed 0.004% with normal care in sampling by these methods, even if the losses are ignored. This possible error is a factor of about 25 smaller than errors commonly associated with the source activity measurement and about a factor of 5

smaller than the statistical error in weighing the liquid samples by either the pycnometer or extrapolation methods.

The insolubility and inert, non-adsorptive propert ies of polyethylene commend this type of ampoule , as well, for storage

of aqueous radioactive solution standards. The minor

dis-advantage that the plastic is semi-permeable resulting in solvent loss (but not solute), is readily minimized by main-taining records of the total mass and the solution contents during its use, by storing in an atmosphere of controlled

TABLE I

Summary of Estimated Evaporation Losses in Sampling Procedures

Loss Rate Time Concentration

Operation mg/ hr Hours % (based on 4 ml)

Aliguoting

1. Open glass ampoule 2 0.03 0 . 0015

2. Th ermal equilibration

of polyethylene

ampoule 0.01 1.0 0.00025

3. Aliquoting (over a

period of one hour) 0.01 1.0 0.00025

0.002%

Dilution and Aliguoting

4. (1) Open glass ampoule 2 0.03 0.0015

(2} Thermal

equili-brat ion 0.01 1.0 0 . 00025

5 . Delivery of dilution aliquots (as in 3

above} 0.01 1.0 0.00025

6 . Open di lution flask after adding diluent

weighing and mixing 2.0 0.03 0.0015

7 . Thermal equilibration

(as in 2 above} 0.01 1.0 0.00025

8 . Aliquoting (as in 3

above} 0.01 1.0 0.00025

humidity or a combination of the two approaches.

Polyethylene ampoules of the type described here, with associated dispensing capillaries, would also appear to be well suited for use in comparison distributions, especially

if enclosed in humidity controlling outer vessels. With the

solution concentration chosen so that dilutions are not re-quired, a major potential source of error in transfer from open glass vessels could be avoided.

References

1. Rytz, A. "Report on the International Comparison of t he

60

4vB (PC)-'Y Method by means of Co." March/April, 1963, Bureau International des Poids et Mesures, Sev res, France, 1965.

2. Muller, J.W. and Rytz, A. "Report on the Intercomparison of Dilution and Source Preparation Methods by Means of

60c _0. ..

France,

Bureau International des Poids et Mesures, Sevres, 1967.

3. Baerg, A.P. Metrologia, 2 (1966) 23.

4. Merritt, J.S. and Taylor, J.G.V. "Gravimetric Sampling in the Standardizing of Solutions of Radionuclides." CRGP-1256, Atomic Energy of Canada Limited, March, 1967.

5. Rytz, A., Colas,

c.

and Veyrodier, C. "Some Experiments in the Dilution of Radioactive Solutions and the Uniformity of Mixing." Bureau International des Poids et Mesures ,

27, 1-1969.

6. Campion, P.J., Dale, J.W.G. and Williams, A. Nucl. Inst.

and Meth. 31 (1964) 253.

7 . LeGallic, Y., Grinberg, G. and Thenard, M. Symposium on

Standardization of Radionuclides, IAEA , Vienna , 1966, paper SM-79/41.

8. Oakley, A.E. and Lowenthal, G.C. ibid. paper SM-79/ 43. 9 . Vander Eijk, W. and Moret, H. ibid. paper SM-79/43.

10. Morgan, F. Metrology of Radionuclides, proceeding of a

symposium, October, 1959, page 372, IAEA, Vienna.

11. Rytz, A. Communication to BIPM comparison participants,

21 July, 1966 .

12. Preiss, I.L. and Fink, R.W. Nucleonics, 15 (1957) 108.

13. Pate, B.D. and Yaffe, L. Can. J. Chern. 33 (1955) 15.

14. Lowenthal, G.C. and Smith, A.M. Nucl. Inst. and Meth.

30 (1964} 363.

1 5. Roy, J.C. and Turcotte, J., Can. J. Chern. 45 (1967} 1379. 16 . Baerg, A.P. Meghir, S. and Bowes, G.C. Intl. J. Appl.

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