An Assesment of the Selection Criteria for an Analyical Method for Radium-226 in Environmental Samples

8/3/2019 An Assesment of the Selection Criteria for an Analyical Method for Radium-226 in Environmental Samples

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Jointly published byElsevier Science S. A., Lausanne andAkad#miai Kiad6, BudapestJournal of Radioanalytical a nd Nuclear Chemistry, Articles,Vol. 193, No. 2 (1995) 329.-336

A N A S S E S S M E N T O F T H E S E L E C T I O N C R I T E R I A F O R A NA N A L Y T I C A L M E T H O D F O R R A D I U M - 2 2 6 I N

E N V I R O N M E N T A L S A M P L E SR. A. TINKER*, J ' . D. SM ITH*, M. B. C OOPER **

*School of Chemistry, Un iversi~ o f Melbourne, Parkvil le 305 2, Victoria, Australia.**Australian Radiation Laboratory, Yallambie 3085 , Victoria, Australia.

This papea"examines he process of making a decis ion on the optimum technique for the measurermm.to f l o wconcentrations of 226Ra in environmental materials. The available counting techniques are alphaspectrometry, high resolution gamm a spectrometry and liquid scintillation counting. The properties o f theanalytical technique; sensitivity, lower limit of detection (LLD) and precision are considered. Methodselection is also restricted by the available sample size and activity. The intlucner of procedure backgrounds,geonaetrie efficiency, chemical recovery, counting time, sam ple size and activity on th e precision an d LI Dare investigatod. The pro cess of method sclection, applicable to a wide ra nge of samples , is ilhis tratod byreference to sedim ents, waters and tissues.

Radium-226 o ccurs in the uranium-238 decay ser ies. I t is an a-e m itter (4.8 M eV ) with a half-l i f e of 1620 years . I t i s measured in a r ange of sample types for a var ie ty o f reasons . Inenvi ron men ta l mate r ia l s Z26Ra s measured in methods for d a t ing sediments and for es t imat ingthe age of f i sh2. There are guide- l ine values on Z26Ra n pota ble w aters and efficient m etho ds arerequi red wh en m any samples a re to be sc reened. 3

The imm ediate decay product radon-222 ( tn 3.8 days) is volati le and loss o f 2n gn oR en ca usesdisequi l ibr ium be tw een 226Ra and i t s decay pro duc ts (Table 1). The mo st d i rec t procedu re form easu r ing Z26Ra s by p repara t ion of a pure , th in source a nd co unt in g by a lpha spec t rometry .W he r e t he y c a n be a ppl ie d ga mm a spe c t r ome t ry a nd l i qu i d s c in t i ll a ti on c oun t i ng ma y beprefe rable because they requi re less sample prepara tion. Gam m a spec t rometry and l iquidsc i n ti l la t ion c oun t i ng de pe nd on m e a sur eme n t o f ~ R a de c a y p r oduc t s a nd e s t i ma t i on o f ~ R aby inference. Th e behaviour o f zz:'gngas i s central to these procedures . In gamm a spec t rometryi t is necessary tha t znP,.n s r e ta ined quan t i ta tive ly a l lowing measurem ent of produ c ts o f fur therdecay. In liquid scintillation counting the v olatility of 222Rnma y be u t il is e d by a l l ow i ng c ompl e t et ransfer f rom an aqueous solut ion conta ining 22~Ra o an over lying organic phase conta in ing thescintillant. +W hen gam m a spectrometry is applied direct ly to env ironm ental samples the 2~SRa 186 .1 keVpeak i s over lapped by the uranium-235 185 keV peak, and th is prec ludes accura te m easurem ento f the r~R a activi ty. Th e 222Kn510 keV y-p eak i s no t used in es t imat ing ZZtRabe c a use o f i tslow ab un dan ce (< 0.1 % ). Prov ided that a l l the z22P, is physically retained in the sample, ~z~gac a n be i n f er r ed f r om me a sur e me n t o f the m ul t ip l e y - pe a ks o f l e a d - 214 a nd b i smut h - 214(Table 1). After sealing the sample, ingro wth o f these d ecay prod ucts is contro lled by the half-l ife of 22ZRn equ ir ing abo ut 4 w eeks to at tain secu lar equil ibr ium.

W hen us ing l iquid sc int il la tion count ing, the quant i ta t ive t r ansfe r of 22 'Rn f rom th e aqu eou ssolu tion con tainin g Z2~Ra o a n imm iscible orga nic solv ent con tainin g the scinti llant is a bene fi tand i s the bas is of a method tha t has h igh cou nt ing ef ficiency. Transfe r of the 2z2Kn andattainment o f secular equil ibr ium requires abo ut 4 weeks. Be cau se o f the s hort half-l ives o f thedecay products, three a-d eca ys and two [3--decaysare observe d for each e2+Ra decay. The se can

0236-5731/95/US $ 9.50Copyright 9 1995 Akadt~miai Kia dt, BudapestAll rights reserved


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R. A. TINKER et a l .: AN ASSESSMEN T OF THE SELEC TION CRITERIA

b e c o u n t e d in co m b i n a t i o n g i v in g a c o u n t i n g e f f i c i e n c y a p p r o a c h i n g 4 0 0 % . W h e r e l o w l i m it s o fd e t e c ti o n a r e r e q u ir e d , a /1 3 d i s c r im i n a t i o n c a n b e u s e d . T h e a b a c k g r o u n d i s l o w a n d c o u n t i n ge f f i ci e n c y a p p r o a c h e s 3 0 0 % .

Table 1Table o f Radioactive Isotopes of he z~sUdecay series.Values obtained from the Tables of Radioactive Isotopes, E. Brown & R. B. Firestone. , Editor, V. S. Shirley. ,Published by John W iley & Sons, a W iley-IntersciencePublication , (1986).A l p h a D e c a y

Nu cl ide Ha~L~e Energy Abun dance(MeV) (%)

n6Ra 1620y 4.784 94.454.801 5.552nRn 3.823 d 5.490 99.924.987 0.08218Po 3.1 0m 6.003 1002~4Pb 27m

2~Bi 19.9m 5.513 0.01

214Po 164 ~ts 7.6 87 99.9 96.904 0.01~~ 22.3 y

2t~ 5.01 d 4.648 < 0.00121~ 138 .4 d 5.304 1002~pb

B e t a D e c a yEnergy r Abundan ce(keV) (%)

G a m m a D e c ayEnergy Abundance(keY) (%)186.1 3 .28262.4 0.01510 0.07

~22 0

510


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R. A, T INKER et al . : AN ASSESSMENT OF THE SELECTION CRITERIA

W h e n t h e n u m b e r o f s a m p l e c o u n t s i s f a r g r e a t e r t h a n b a c k g r o u n d c o u n t s t h e p r e c i s io n i sd e t er m i n e d b y t h e s a m p l e c o u n t s. W h e n t h e n u m b e r o f sa m p l e c o u n t s a p p r o a c h e s b a c k g r o u n dt h e p r e c i s i o n i s i n f lu e n c e d b y b o t h a n d t h e L L D f o r t h e in s t r u m e n t b e c o m e s i m p o r t a n t i n t h ep r oc e s s o f de c is i on .

L o w e r L i m i t o f D e t e ct i on : T he l ow e r li m it o f de t e c t ion ( L L D ) , f o r t he pu r pos e s o f t h i s r e po r t ,i s de f i ne d a s t he S ma ll es t c on c e n t r a t i on o f t he r a d i oa c t i ve ma t e r i a l i n a s a m p l e t ha t w i l l y i e l d an e t co u n t , a b o v e s y st e m b a c k g r o u n d t h a t w i ll b e d e t e c t e d w i t h 9 5 % p r o b a b i l i ty~. T h e L L D f o ra r a d i oa na l y t i c a l t e c hn i qu e i s de f i ne d a s :

L L D = 2 K v - - . -E V R N

w h e r e : BtKEVRN

i s t h e t o t a l n u m b e r o f b a c k g r o u n d c o u n t s.i s t he c oun t i ng t i me i n s e c onds .w i t h a no r m a l d i s t r ibu t i on a s s um e dK = 1 .6 4 5 f o r 9 5 % c o n f i d e n c ei s t he e f f i c i enc y f r a c t i on o f t he de t e c t o r .i s t h e v o l u m e o r m a s s o f s a m p l e .i s the c he mi c a l r e c ove r y f r a c t i on .i s th e b r a n c h i n g f ra c t i o n o f t h e e n e r g y d e c a yo f t h e r a d i o n u c li d e b e i n g m e a s u r e d(e .g . for gam m a spec trometry214Bi a t 609 .3 ke V

ha s a 0 . 46 f r a c ti on a bunda nc e ) .

Th e lower l imit of de tec t ion i s l imi ted by geom et r i c e f f i c iency, s am ple s i ze , chem ica l reco ver ya nd e ne r gy a bundanc e . T yp ic al l im i ti ng va l ue s f oun d i n ou r l a bo r a t o r i e s f o r a l pha s pe c t r om e t r y ,l i q u id s c i n ti ll a ti o n c o u n t i n g a n d g a m m a s p e c t r o m e t r y a r e g i v e n in T a b le 2. T h e L L D i sc a lc u la te d fr o m t h e n u m b e r o f c o u n t s in th e p r o c e d u r e b l an k . T h e s a m e c o u n t i n g ti m e i s u s e df o r s a m p l e a n d p r o c e d u r e b l a n k .

Table 2Table of typical values ound for detector efficiency,sample size, chemical ecoveryand energy abundance or eachcounting echnique.

LimitingParameters Alpha Spectrometry L iq ui d cintillationCounting Gam ma SpectrometryGeom etric Efficiency 0.25 3.2 0.04Sample Size 1 g 1 g 50 gChem ical Recovery 0.80 0.9 0.95Ene rgy Ab und ance 226Ra 1.00 2Z~Rn1.00" 2t4pb0.45 **

* 2nRn and each of its daughtersare measuredwith a energyabundanceof 1.00 (=100 %).** ~t53iat 609.3 keV is the most abundantof the y -peaks measuredwhen estimating226Rausing gam ma spectrometry.

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In s t rumen t a l

The following count ing systems were used wi th appropr ia te m uki -cha nne l ana lyser s (MCA s)and c om puter sof tware;Alpha Spectrometry: Ca nbe r r a, Mod e l 7404 qua d a l pha spe c tr ome t e r, c on t a i n i ng f ou rpass ive ly implanted planner s i l icon (PIPS) de tec tor s (300 mm 2 active area). 226Ra prepa red asa thin so urce b y electrod epo sit ion onto a si lver disc (17 m m diameter). 6 '7Liqu id Scintillation C ounting: LK B 1217 Rackbe ta l iquid sc int i l la t ion cou nte r us ing 22 mLsc int il la tion vial s wi th a to lue ne based sc int il la t ion cockta i l (160 g naphth a lene and 4 g of [2- (4-biphenyl ) -6-pheny l -benzoxazole ] n 1 .0 L o f to lue ne) .Gamm a Spectrometry:Canberra Model GR 2518, r everse e lec t rode coaxia l ge ranium 50 m mdiam eter de tec to r wi th a r e la t ive e f ficiency of 25%. Samples were prepared in a s tandardge ome t r y ( 50 mL ) e mbe dd e d i n e poxy r e s i n .

Discuss ion

Alpha spec t rometry has low b ackgro und leve ls (


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R . A . TIN K ER e t a l .: A N A SSESSM EN T O F TH E SELEC TIO N C R ITER IA

10~00C o u n t i n gT i m e ( s )

0 P t ~ i s i o n( % R S D )

80 6 ~ 4 ~ 2 ~(mm0

Figure 3Var ia t ion o f precis ion (%RS D) observed forgamm a spect rometry.

Table 3P r e c i s i o n g i v e n b y t h e t h r e e t e c l m i q u e s f o r asample act ivity of 60 m Bq counted for 10000 s .

Technique Precis ion (%RSD )Alpha Spect rometry 2Liquid Scintil lation Coun ting lGamm a Spect rometry 25

10000.0

+ 1000.0

100.0

+ G a m m a- m - - L S C

9 A l p h a

10.0 I I I I ~ "0 200 400 600 800 1000

C oun t ing T im e (ks)Figure 4Variation of the low er limit of detection with coun ting time for alph a spectrom etry, liquid scintil lation counting(LSC) and g amma spect rometry.

F o r a g i v e n a c t iv i ty t h e L L D o f e a c h te c h n i q u e i m p r o v e s w i t h i n c re a s i n g c o u n t i n g ti m e( F i g u r e 4 ). A t a ll c o u n t i n g ti m e s a l p h a s p e c t r o m e t r y g i v e s t h e b e s t L L D w i t h l iq u i d s c i n t i ll a t io nc o u n t i n g a f a c t o r o f a b o u t t w o l e s s se n s i ti v e .

S e l e c t i o n C r i t e r i a

Sample Size: S a m p l e s i z e i s o f t e n li m i te d . C o n t r i b u t i n g r e a s o n s m a y b e t h e s iz e a n da v al ia b iU t y o f o r g a n i sm s , s e d i m e n t c o r e d i a m e t e r s o r v o l u m e o f w a t e r s a m p l e s . S a m p l e s m a y b eu s e d a s c o l l ec t e d f o r g a m m a s p e c t ro m e t r y . T h e r e i s a m a x i m u m s iz e t h a t c a n b e u se d f o r l o wl e v e l w o r k a n d t h i s i s u s u a l l y a b o u t 5 0 m L . F o r a l p h a s p e c t r o m e t r y a n d l i q u id s c i n ti l la t i o n

3 3 3


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R. A . TINKER e t a l. : AN ASSESSMENTOF THE SELECTION CRITERIA

c o u n t i n g l o w l e v e l s a m p l e s a re p r o c e s s e d t o p r e p a r e t h e s o u r c e f o r c o u n t in g . T h e a m o u n t o fs a m p l e u s e d c a n b e v a r i e d a n d t h e 2 26 Ra m a y b e c o n c e n t r a t e d f r o m l a r g e w a t e r s a m p l e s. W h e ns a m p l e s u n d e r g o e x t e n s i v e p r o c e s s in g t h e a d v a n t a g e o f m i n im a l s a m p l e p r e p a r a t i o n f o r g a m m as p e c t r o m e t r y i s l o s t .

Activity Estimation: T h e s e l e ct i o n o f t h e o p t i m u m t e c h n iq u e r e q u i r e s p r e l im i n a r y e st i m a t io no f t h e s a m p l e a c t iv i t y , e it h e r f ro m t h e l i t e r a t u r e o r b y t r i a l a n a l y s i s. A p p r o x i m a t e e s t im a t e s o f2 26 Ra a c t i v i t ie s ( o r a l o w e r l i m i t ) c a n b e a c h i e v e d c o n v e n i e n t l y u s i n g g a m m a sp e c t r o m e t r y .

Precision a nd L ower Lim i t o f Detec tion: T h e p r e c i s i o n t o w h i c h t h e s a m p l e a c t i v i ty m u s t b em e a s u r e d d e p e n d s o n t h e r e a s o n f o r a n a ly s is . T h e p r e c i s i o n r e q u i re d , c o m b i n e d w i t h s am p l ea c ti vi ty , d i c t a te s t h e c o u n t i n g t im e . C o u n t i n g t i m e s b a se d o n t h e e s t i m a t e d a c t i v i t y a n d r e q u i r e dp r e c i s i o n a r e sh o w n i n t h e 3 - d i m e n s i o n a l p l o t s i n F i g u r e s 1 - 3 . A s t h e a c t i v i t y d e c r e a se s a n dr e q u i r e d p r e c i s i o n in c r e a se s , t h e c o u n t i n g t im e i s i n c r e a se d . T h i s c o u n t i n g t im e b e c o m e si m p o r t a n t w h e n e v a l u a ti n g s a m p l e th r o u g h - p u t a n d t h e c o s t o f e a c h a n a ly s is .

< 2 5 g " vI SampleVolume

Gamm a SpectrometryExcluded

GeometricEfficiency IChem ical Recovery - -Nucl ide Abundance

I

IBackgroundCount ing Time I =

- - ~ I C a l c u l at io n f L L D ]

I Cost Effectivness I

Sample Act ivi tyEstimation [

Preselectiunof Precision [q ,Set Sample Count ing Time Ifo r

Alph a Spectrometry ILiquid S cintillationCount ing

Gamm a Spectrometry

I Sele ct ion f Technique [Figure 5Flow diagram or the technique selection or the determ ination f low 226Ra oncentrations.

Cost-effectiveness: C o s t m u s t b e c o n s i d e r e d w h e n s e l e c ti n g a m e t h o d . T h e c o s t o f an a l y s isi nc lu d e s t h e c a p i t a l c o s t o f th e e q u i pm e n t, m a i n t e n a n c e c o st s , l a b o r c o s t , o v e r h e a d s a n d e x p e c t e dd e m a n d f o r th e se r v i c e . S k i l l e d p e r so n n e l a r e r e q u i r e d . Costs v a r y g r e a t l y f r o m laboratory tol a b o r a t o r y a n d c o u n t r y t o c o u n t r y a n d c o s t e s t im a t e s a r e n o t c o v e r e d i n t h is p a p e r .

A r a t io n a l p a t h w a y ( F i g u r e 5 .) a l l o w s s e l e c ti o n o f t h e o p t i m u m t e c h n i q u e f o r a r a n g e o fs a m p l e s o f d i f f e r e n t a c t i v it i e s . T h e p r o c e s s i s i ll u s t r a t e d i n t h e f o l l o w i n g w o r k i n g e x a m p l e s o fm e t h o d s e l e c ti o n u s i n g s e d i m e n t , w a t e r a n d t i s su e s a m p l e s .

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R. A . TINKER et al . : AN ASSESSMENTOF THE SELECTIONCRITERIA

Working ExamplesMethod Selection -Marine Sediment: Le a d- 210 da t ing o f m a r ine se d im e n t s r e l ie s on themeasurement of unsupported and supported 21~ Sup ported 21~ is in equilib r ium w ith i ts lon glived pare nt 226Ra. 226Ra s used to infer sup ported 21~ Z26Rac onc e n t r a t ions a r e c om m only

in the o r de r o f 20 m B q/g . I n ou r l a bo r a to ry se d im e n t s a m ple s i ze is r e s tr i c te d by the d i a m e te ro f t he s a m ple c o r e r e su l ti ng in vo lum e s o f a bou t 2 g . Le a d - 21 0 da t ing o f s e d im e n t s de m a ndshigh prec is ion. Cou nt ing t imes required for 5% prec is ion us ing a lpha spec t rom etry and l iquidsc int il la t ion count ing a re shown in the 3-dimen siona l p lo ts show n in F igu res 1 and 2 . Th e LL Dfor gamma spec t rometry and l iquid sc in t i l la t ion count ing a re ca lcula ted us ing the es t imatedcount ing t im es and prede te rm ined l imit ing paramete rs (T able 4).

Table 4Counting imes required o achievea 5% precision or 40 mBq in 2 g o f sediment.Techniques Counting Tim e(s) LLD (rnl3q/g)Alpha Spectrometry 100000 0.6Liquid S sintillationCounting 40000 2.9Gamma Spectrometry > 1000000 N/A

Ga m m a spe c t rom e t r y i s e xc lude d be c a use o f the im pr a c t ic a l c oun t ing t im e r e qu i r e d . L iqu idsc int il la tion coun t ing is considered opt im al because the cou nt ing t im e is shor t fo r the requiredprec ision. Fo r low er ac t iv i t ies a lpha spec t rom etry is u l t imate ly required .

Met ho d selection - Potable Waters: Inges t ion of 226Ra in dr inking w ate r i s a publ ic con cernbecause i t can accumulate in bon e tissue which may result in an inc rease r isk o f fatal cancer. Th eprop osed guide- l ine va lue for 22~Ra concen tra t ion in potab le wa te r in A ust ra l ia i s 250 mBq ]L. 9This is measured routinely to a precis ion of 10% with 1 .0 L v olumes. Chem ica l preco ncentra t ionof 226Ra n wate r enables the use of any of the three coun t ing techniques . Th e requ ired co unt ingt im e s f o r 10% p r e c is ion , and c o r r e spond ing LL D f o r 1 L wa te r s a m ple s a r e shown in Ta b le 5 .

Table 5Counting imesrequired o achievea 10 % precision or 250 mBq in 1.0 L of water.

Techniques Counting Tim e s) LLD (mBq/L)Alpha Spectrometry 5000 5.1Liquid ScintillationCounting 800 19.8Gamma Spectrometry 100000 13.0

Al l t he t e c hn ique s a r e su it ab le w i th the LL D we l l be low the gu ide - l ine va lue o f 250 m B q /L .Liquid sc in ti lla tion count ing, w i th the lowest cou nt ing t ime , i s the pre fe r red techn ique .

33 5


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R. A. TINKER e t a l. : AN ASSESSMEN T OF THE SELECTION CRITERIA

Method selection - Skeletal Tissue: T h e c o n c e n t r a t i o n s o f 2 ~ R a i n t h e s k e l e ta l m a t e r ia l o fA u s t r a l ia n c a rt i l a g in o u s fi s h i s b e i n g s t u d i e d i n o u r l a b o r a t o r i e s . P r e l i m i n a r y m e a s u r e m e n t s u s i n gg a m m a s p e c t r o m e t r y i n d i c at e 2 ~ R a c o n c e n t r a t io n s o f a p p r o x im a t e l y 1 - 5 m B q / g . S a m p l e s i ze sa r e a b o u t 1 . 0 g a n d a t th i s a ct iv i ty a 1 0 % p r e c i s i o n r e q u i r e s l o n g c o u n t i n g t i m e s . C o u n t i n g t i m e sa n d t h e L L D f o r e a c h t e c h n iq u e a r e s h o w n i n T a b le 6 .

Table 6Counting imes required to achieve a 10 % precision for 5 m Bq in 1.0 g o f skeletal material.Techniques Counting Time (s) LLD (mBq/g)Alpha Spectrometry 130000 1.0Liquid ScintillationCounting 120000 2.0Gamm a Spectrometry > 1000000 N/A

A l p h a s p e c t r o m e t r y w i t h a L L D l o w e r t h a n l i q u id s c i n ti l la t io n c o u n t i n g i s t h e p r e f e r r e dm e t h o d f o r t h i s m a t e r ia l .

JDS thanks the AustralianResearch C ouncil for financial support to purchase equipment for or-spectrometry.

R e f e r e n c e s

1. J.D . SMIT H, T. F. HAM ILTON, Australian Journal o f Marine and FreshwaterResearch, 36 (1985) 15.2. J .T. BEN NETT, G. W. BOEHLERT, K. K. TUREKIAN,Marine B iology, 71 (1982) 209.3. M .B. COO PER, M. J . WILKS,Report ARI.fI'R040, Australian RadiationLaboratory,Yallam bie, Australia, (1993).4. M .B. C OO PER Report ARL/ISOfl'C147/SC3/WG6/N16, AustralianRadiation Laboratory,Yallamb ie, Australia, (1993).5. L.A . CU RRIE, Analytical Chemistry,40-3 (1968) 586.6. N.E . WHITEHEAD, R. G. DITCHBURN, W. J . McCA BE, R. VAN DER RA AIJ ,Journal o f Radioanalytical and Nuclear Chemistry,Articles, 160-2 (1992) 477.7. T.F. HAM ILTON, V. M. MeRA E, J. D. SMITH, Journal of Radioanalytieal and Nuclear Chemistry,177-2 (1994) 365.8. G. HAN COO K, P. MA RTIN, AlligatorRivers Region Resenreh Institute,Jabiru, Au stralia, (1992).9. Draft Guidelines or drinking wa ~r quallity n Australia, AW RCfNHM RC, (1994).

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An Assesment of the Selection Criteria for an Analyical Method for Radium-226 in Environmental Samples