Perspectives on Detection Limits - ACS Symposium Series (ACS

Chapter 9

Perspectives on Detection Limits for Nuclear Measurements in Selected National and International Programs 1

2

Lloyd A. Currie and Robert M. Parr Downloaded by UNIV OF OKLAHOMA on April 3, 2013 | http://pubs.acs.org Publication Date: December 9, 1987 | doi: 10.1021/bk-1988-0361.ch009

1

Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899 International Atomic Energy Agency, A-1400 Vienna, Austria 2

Issues involving the definition and practical significance of Detection Limits are discussed in the light of US and international programs in which the concept plays a central role. The US program relates to the "Lower Limit of Detection" (LLD) which forms a part of the Technical Specifications for nuclear power reactors, as required by the US Nuclear Regulatory Commission for measurements of effluent and environmental radioactivity. The programs of the International Atomic Energy Agency, which are oriented toward coordinated research and technical cooperation, similarly require common understanding and use of the "Limit of Detection" (LOD) as a practical and meaningful performance characteristic for measurements of trace elements in bioenvironmental matrices. Efforts to meet the needs of these two programs to formulate realistic and practicable detection limits will be reviewed, with special focus on problems-in-common such as the treatment of the blank, decision criteria, algorithm and assumption dependence, and the reporting of subliminal results. Detection limits have p r a c t i c a l s i g n i f i c a n c e i n a number o f important s o c i e t a l contexts, where measurement p r o c e s s e s must p o s s e s s adequate d e t e c t i o n c a p a b i l i t y t o meet s p e c i f i c d i a g n o s t i c , r e g u l a t o r y , o r r e s e a r c h needs. Two such c o n t e x t s , where s p e c i f i c r e q u e s t s were made t o f o r m u l a t e m e a n i n g f u l and r e l i a b l e approaches to d e t e c t i o n l i m i t s , i n v o l v e d : a) t h e r e q u i r e m e n t s o f t h e U. S. N u c l e a r R e g u l a t o r y Commission [NRC] f o r t h e d e t e c t i o n o f e f f l u e n t and e n v i r o n m e n t a l r a d i o a c t i v i t y , and b) t h e r e q u i r e m e n t s o f t h e I n t e r n a t i o n a l Atomic Energy Agency [IAEA] f o r t h e d e t e c t i o n o f t r a c e elements i n a wide range o f b i o e n v i r o n m e n t a l matrices. The NRC p r o j e c t r e l a t e d d i r e c t l y t o t h e f o r m a l r e g u l a t o r y r e q u i r e m e n t , as set f o r t h i n the t e c h n i c a l s p e c i f i c a t i o n s f o r operating nuclear power r e a c t o r s , t h a t t h e r a d i o a c t i v i t y measurement p r o c e s s e s have d e t e c t i o n l i m i t s meeting s p e c i f i e d s t a n d a r d s f o r t h e purpose o f p r o t e c t i n g t h e p u b l i c from e x c e s s i v e r e l e a s e s . T y p i c a l r e q u i r e m e n t s 0097-6156/88/0361-0171 $06.75/0 © 1988 American Chemical Society

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

DETECTION IN ANALYTICAL CHEMISTRY

Downloaded by UNIV OF OKLAHOMA on April 3, 2013 | http://pubs.acs.org Publication Date: December 9, 1987 | doi: 10.1021/bk-1988-0361.ch009

172

f o r t h e "Lower L i m i t o f D e t e c t i o n " [LLD] a r e g i v e n i n T a b l e I (1) . The IAEA e f f o r t was f o c u s s e d on measurements o f e s s e n t i a l and t o x i c t r a c e elements i n b i o e n v i r o n m e n t a l samples g l o b a l l y , as found i n t h e i r l a b o r a t o r y i n t e r c o m p a r i s o n and c o o r d i n a t e d r e s e a r c h programs. D e t e c t i o n c a p a b i l i t i e s i n t h i s case r e l a t e d i r e c t l y t o t h e a b i l i t y to d e t e c t d e f i c i e n c i e s ( o f e s s e n t i a l elements) o r e x c e s s e s ( o f t o x i c elements) i n foods o r b i o l o g i c a l samples from d i f f e r e n t g e o g r a p h i c a l regions. F i g u r e 1, w h i c h i s drawn from t h e IAEA's c u r r e n t l i s t o f a v a i l a b l e r e f e r e n c e m a t e r i a l s , i n d i c a t e s some t y p i c a l m a t r i c e s and elements o f i n t e r e s t ( 2 . 3 ) . Concentrations v a r i e d widely with element and m a t r i x ; by way o f i l l u s t r a t i o n , c e r t i f i e d v a l u e s on a d r y w e i g h t b a s i s i n sample H-8 (horse k i d n e y ) ranged from 0.91 mg/kg f o r Hg t o 12.6 g/kg f o r C I . The two p r o j e c t s shared some f e a t u r e s i n common. R e l i a b l e and comparable measures o f d e t e c t i o n c a p a b i l i t i e s were r e q u i r e d on a c o n t i n u i n g b a s i s among many l a b o r a t o r i e s , and they h a d t o be d e v e l oped i n a manner t h a t would be s u i t a b l e f o r p r a c t i c a l a p p l i c a t i o n t o a b r o a d range o f a n a l y t e s ( t r a c e elements, r a d i o n u c l i d e s ) and sample matrices. D e t e c t i o n l i m i t s had t o be a b s o l u t e i n t h e sense t h a t a c t u a l c o n c e n t r a t i o n o r r a d i o a c t i v i t y l e v e l s may be l i n k e d t o s p e c i f i c n u t r i t i o n a l o r h e a l t h e f f e c t s . A t t h e o u t s e t o f each study it was o b s e r v e d t h a t v a r y i n g d e f i n i t i o n s and e v a l u a t i o n s o f d e t e c t i o n l i m i t s were i n use, w i t h t h e r e s u l t t h a t s t a t e d d e t e c t i o n l i m i t s were n o t o n l y non-comparable b u t i n some c a s e s i n e r r o r by o r d e r s o f magnitude. Such d i s c r e p a n c i e s a r e a l r e a d y most s e r i o u s from a s c i e n t i f i c o r m e t r o l o g i c a l s t a n d p o i n t , b u t they invite dangerous m i s i n t e r p r e t a t i o n and c o n f u s i o n when v i e w e d by t h e l a y p u b l i c o r i n t e r p r e t e d by s p e c i a l i n t e r e s t groups. Approach t o NRC and IAEA S t u d i e s I n i t i a l States. An o u t l i n e o f t h e approach t a k e n f o r each study, t o g e t h e r w i t h an i n d i c a t i o n o f t h e i n i t i a l d e t e c t i o n l i m i t d e f i n i t i o n s i n use by t h e two o r g a n i z a t i o n s i s g i v e n i n T a b l e I I . A p a r t from t h e d i f f e r e n c e i n t e r m i n o l o g y - - Lower L i m i t o f D e t e c t i o n [LLD], and L i m i t o f D e t e c t i o n [LOD] -- t h e f o r m u l a t i o n s d i f f e r i n c o e f f i c i e n t s and i n u n d e r l y i n g p r i n c i p l e s . ( F o r t h e purposes o f t h i s c h a p t e r we s h a l l r e p r e s e n t t h e d e t e c t i o n l i m i t by t h e s i n g l e symbol Lp . ) The i n i t i a l NRC e x p r e s s i o n e x p l i c i t l y r e c o g n i z e s t h e two h y p o t h e s i s t e s t i n g e r r o r s , (5%) f a l s e p o s i t i v e s and f a l s e n e g a t i v e s , though w i t h a t r i v i a l r o u n d i n g e r r o r [4.66 r a t h e r than 4.65]. The i n i t i a l IAEA f o r m u l a t i o n , w h i c h was t a k e n from K e i t h (4), e x p l i c i t l y t a k e s i n t o account o n l y t h e f a l s e p o s i t i v e e r r o r , but a t a g r e a t l y reduced ( 1 - s i d e d ) s i g n i f i c a n c e l e v e l (as l i t t l e as 0.13% i n contrast to 5%). T h i s i s a problem: ignoring false n e g a t i v e s does n o t make them go away! I n f a c t , n e a r l y a l l such f o r m u l a t i o n s i n v i t e f a l s e n e g a t i v e s a t a r a t e o f 50%. The i n i t i a l NRC and IAEA f o r m u l a t i o n s d i f f e r e d a l s o w i t h r e s p e c t t o t h e denominator. That i s , R e f e r e n c e 4 t r e a t s d e t e c t i o n i n terms o f " s i g n a l s " , t h e denominator w h i c h c o n v e r t s from s i g n a l u n i t s to concentration being only implied. I n c o n t r a s t , the denominator f o r t h e LLD shows e x p l i c i t l y f a c t o r s f o r d e t e c t o r E ( f f i c i e n c y ) , sample V(olume), c h e m i c a l Y ( i e l d ) , and r a d i o a c t i v e D(ecay). The e s t i m a t e d s t a n d a r d d e v i a t i o n o f t h e b l a n k ( s ) i s t h e B


CURRIE & PARR

Perspectives on Detection Limits

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MATRIX

ELEMENTS OR NUCLIDES REFERENCED

Trace elements: Br, Ca,Cl,Cu,Fe,K,Mg,Mn, P,Rb,Zn

Trace elements: Ba, Environmental levels Ca,CI,Cr,Cu,Hg,Mg,Μη, Mo,Na,Nl,Pb,Sr

Rye flour

Cotton cellulose

v-e

V-9

v

Milk ponder

A-14

Sr-90, Cs-137 Na K, Ca

Trace elements : Br,Ca, Normal levels Cu,Fe,K,Na,Rb,S,Se,Zn

Freeze dried ani mal blood

A-13

Environmental levels


5mBq/g (0.14 pCl/g) 55mBq/g (1.48 pCi/g)

Ra-226 Sr-90

Animal bone


A-12

v

Milk powder

Trace elements: Ca Cl,Cu,Pe,Hg,K,Mg,Mn, Na,P,Rb,Zn

CONCENTRATION OR ACTIVITY LEVEL

A-ll

BIOLOGICAL MATERIALS ( t e r r e s t r i a l )

SAMPLE CODE

25 g

50 g

250 g

25 g

80 g

25 g

SAMPLE SIZE (1 UNIT)

CRM

RM

CRM

CRM

CRM

RM

SAMPLE"

or

CLASS

REMARKS

Non-certlfled information values f o r : Al,Br,Cd,Pe,Ga, Hf,Ll,S,Sc,Se,Sm,Sn,Th,U,V

Non-certifled information values f o r : Al,Au,Ba,Cd,Co,Cs, Mo,Na,S,Sb

Non-certifled information value for Sr (total)

Non-certlfled information values f o r : Mg,Nl,P,Pb

Non-certified information values f o r : Ag, A l , As, Au, B,Ba, Br, Cd, Cr, Ce, F, I, L l , Pb, Pt, Sb, Se, S i , Sn, Sr, T l , U, V


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In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. Normal levels

Normal levels

Normal levels

See remarks

Trace elements: Br, Ca,Cl,Cs,Cu,Fe,Hg,K, Mg,Mn,Na,Rb,Se,Zn

Animal bone ( i n c l u - Trace elements: Ba, ding mineral and Br,Ca,Cl,Fe,K,Mg,Na, organic components) P,Pb,Sr,Zn

Trace elements: Br, Ca,Cd,Cl,Cu,Pe,Hg, K,Mg,Mn,Mo,Na,P,Rb, Se,Zn

Multielement

Animal muscle

Horse kidney

Mixed human diet

H-5

H-8

H-9

30 g

30 g

30 g (2 v i a l s )

20 g (2 v i a l s )

50 g

RM

RM

RM

CRM

CRM

Representative of diet consumed i n Finland. Available from March 1986

Cd concentration similar to human levels Non-certified information values f o r : Co,Cs,S,Sr

Non-certifled information values f o r : Al,Cs,Su,Hf,La, Mn,Sb,Se,Sm,Th

F i g u r e 1. B i o l o g i c a l R e f e r e n c e and C e r t i f i e d R e f e r e n c e M a t e r i a l s a v a i l a b l e f r o m t h e I n t e r n a t i o n a l A t o m i c Energy Agency. (Reproduced w i t h p e r m i s s i o n f r o m R e f . 2. C o p y r i g h t 1986, 1987 I n t e r n a t i o n a l A t o m i c Energy Agency.)

* RM for Reference Material, CRM f o r C e r t i f i e d Reference Material


H-4

t

Trace elements: Ag, As,Ba»Br,Ca Cd,Ce,Co Cr,Cu,F,Fe,Ga,Hg,I,K Mg,Mo,Na,Nl,P,Pb,Si Sn,U,V,Zn

V-10

Hay (powder)



176

Table I I .

Detection Limits:

P r a c t i c a l Needs

N u c l e a r R e g u l a t o r y Commission [NRC]; I n t e r n a t i o n a l Atomic Energy Agency [IAEA] APPLICATION:


NRC

IAEA

Detection limits specified in the Technical Specifications f o r n u c l e a r power r e a c t o r s f o r t h e d e t e c t i o n o f e f f l u e n t and e n v i r o n m e n t a l r a d i o a c t i v i t y . D e t e c t i o n l i m i t s needed f o r t r a c e elements [ e s s e n t i a l , toxic] i n bioenvironmental matrices. [Coordinated R e s e a r c h Programs]

OBJECTIVE: Method-independent definition samples. ["cookbook" manual]

INITIAL STATE

(a

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formulation;

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NRC -- LLD = 4 . 6 6 s /(E«V»Y^D) B

IAEA -- LOD - 3 s

B

APPROACH: NRC

L i t e r a t u r e r e s e a r c h ; s i t e v i s i t s t o a s s e s s problems and practices. [NRC r e g i o n a l o f f i c e s ; power r e a c t o r ; contracting labs]

IAEA -- M u l t i d i s c i p l i n a r y 1985]

consultants'

m e e t i n g [IAEA HQ,

Dec.

s represents t h e s t a n d a r d d e v i a t i o n o f t h e background o r b l a n k . Other symbols i n d i c a t e : Efficiency, Volume, Y i e l d , and Decay f a c t o r , resp. B


9. CURRIE & PARR


s c a l i n g parameter i n b o t h e x p r e s s i o n s , though l i t t l e guidance i s given f o r i t s determination. A l s o t h e NRC d e f i n i t i o n i s s l i g h t l y ambiguous i n s t a t i n g t h a t s i s "the standard d e v i a t i o n o f the background...or... o f a b l a n k sample as a p p r o p r i a t e " ( 1 ) . Neither approach explicitly t r e a t s p o s s i b l e non-normality, degrees o f freedom, u n c e r t a i n t i e s i n t h e denominator [overall calibration f a c t o r ] , s y s t e m a t i c e r r o r , o r t h e a p p l i c a t i o n t o multicomponent systems which e x h i b i t i n t e r f e r e n c e and m a t r i x e f f e c t s and o f t e n r e q u i r e more s o p h i s t i c a t e d c o m p u t a t i o n a l p r o c e d u r e s . E f f e c t s o f t h e r e l a t i v e l y s i m p l i s t i c f o r m u l a t i o n s a r e seen, f o r example, i n T a b l e I and F i g u r e s 2 and 3. I n the t a b l e , we see t h a t o n l y two o f t h e r a d i o n u c l i d e s l i s t e d a r e commonly measured by " s i m p l e c o u n t i n g " where a s i g n a l minus background o p e r a t i o n o b t a i n s . F a r more commonly LLD's must be determined f o r multicomponent gamma r a y s p e c t r o m e t r y , which may i n v o l v e r e l a t i v e l y c o m p l i c a t e d computat i o n s and a t t e n d a n t assumptions r e g a r d i n g i n t e r f e r i n g components, b a s e l i n e shapes, e t c . Even i n t h e b e s t o f c i r c u m s t a n c e s , w i t h i s o l a t e d gamma r a y peaks, e s t i m a t i o n and d e t e c t i o n a l g o r i t h m s a r e n o t always c o n s i s t e n t and erroneous d e t e c t i o n l i m i t s f o l l o w . One p r a c t i c a l i l l u s t r a t i o n o f the p i t f a l l s attending the a p p l i c a t i o n of the " s i m p l e c o u n t i n g " e x p r e s s i o n t o r e l a t i v e l y s i m p l e gamma r a y s p e c t r o m e t r y has been d e s c r i b e d by R e i c h e l ( 5 ) . I n t h i s i n v e s t i g a t i o n , commercial spectrum a n a l y s i s s o f t w a r e was employed t o c a l c u l a t e t h e d e t e c t i o n l i m i t s o f Hg-203 (279 keV) and Cr-51 (320 keV) on a Compton continuum from i n t e r f e r i n g Co-60. The recommended d e t e c t i o n l i m i t o p t i o n which "has a p r o b a b i l i t y o f 95% o f [a peak] b e i n g d e t e c t e d , " produced d e t e c t i o n l i m i t s o f 1557 and 1511 counts f o r Hg-203 and Cr-51, r e s p e c t i v e l y . Y e t when peaks f o r these two n u c l i d e s were added t o t h e continuum a t l e v e l s o f about 1580 and 1910 c o u n t s , r e s p e c t i v e l y , from a mixed s o u r c e , n e i t h e r was d e t e c t e d by t h e s o f t w a r e , though b o t h were c l e a r l y v i s i b l e t o even an u n t r a i n e d eye! See F i g u r e 2. The t h i r d peak a t 310 keV, d e r i v i n g from t h e Co-60 Compton i n t e r a c t i o n s , was a l s o u n d e t e c t e d . The peaks remained u n d e t e c t e d u n t i l t h e i r l e v e l s were a p p r o x i m a t e l y t w i c e t h e presumed d e t e c t i o n l i m i t s , a t which p o i n t a good r e s u l t was o b t a i n e d f o r Hg-203, b u t a poor one [ t o o s m a l l ] f o r Cr-51. This l a t t e r r e s u l t no doubt f o l l o w e d from an erroneous b a s e l i n e assumption, even though the program o p t i o n f o r "double peak e v a l u a t i o n " was chosen. F i g u r e 3 i l l u s t r a t e s t h e problem f a c e d b y t h e IAEA i n t h e b r o a d e r c o n t e x t o f t h e i r t r a c e element l a b o r a t o r y i n t e r c o m p a r i s o n program. These d a t a show t h e r e p o r t e d r e s u l t s o f 16 l a b o r a t o r i e s f o r measurements o f a r s e n i c i n t h e h o r s e k i d n e y i n t e r c o m p a r i s o n sample (H-8), based on v a r i o u s v e r s i o n s o f atomic a b s o r p t i o n spectrometry, o p t i c a l emission spectrometry, neutron activation a n a l y s i s , and i n d u c e d X-ray e m i s s i o n a n a l y s i s . The o b j e c t i v e o f t h e h o r s e k i d n e y i n t e r c o m p a r i s o n was t o a s s e s s (and r e f i n e ) a n a l y t i c a l methods f o r t h e d e t e r m i n a t i o n o f e s s e n t i a l and t o x i c t r a c e elements i n t h i s s u r r o g a t e f o r human k i d n e y ( 7 ) . K i d n e y , as the main t a r g e t organ which accumulates t o x i c elements, was o f s p e c i a l i n t e r e s t w i t h r e s p e c t t o cadmium. Horse k i d n e y , which c o n t a i n s s i m i l a r l e v e l s o f cadmium t o the human k i d n e y c o r t e x , was s e l e c t e d f o r t h e development and maintenance o f methods h a v i n g a demonstrated l e v e l o f q u a l i t y t o a s s u r e r e l i a b l e b i o l o g i c a l m o n i t o r i n g o f t h i s element. Participants were i n v i t e d t o a n a l y z e some 24 a d d i t i o n a l t r a c e elements, however, B


177



178

F. Reichel (IAEA, 1985)


95% Confidence Level

250

300

350

Channel (keV)

F i g u r e 2. Non-detected, y e t q u i t e v i s i b l e gamma r a y peaks o f Hg-203 and Cr-51 from IAEA p r a c t i c a l e x a m i n a t i o n o f s o f t w a r e performance ( 5 ) . The continuum, shown dashed b e n e a t h t h e Hg-203 and Cr-51 peaks, i s due t o Co-60.

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27 25 38 54 73 35 61

J

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12 10 29

Lab. Code No.

F i g u r e 3. I n t e r l a b o r a t o r y r e s u l t s f o r As, from t h e IAEA i n t e r c o m p a r i s o n o f cadmium and o t h e r elements i n h o r s e k i d n e y (H-8) ( 7 ) . F i l l e d c i r c l e s r e p r e s e n t q u a n t i t a t i v e r e s u l t s ( u n c e r t a i n t i e s n o t e x c e e d i n g c i r c l e d i a m e t e r ) ; open c i r c l e s correspond t o reported detection l i m i t s .



9.

CURRIE & PARR

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most o f w h i c h are shown i n F i g u r e 1. R e s u l t s f o r cadmium, a t a l e v e l o f about 190 mg/kg were q u i t e s a t i s f a c t o r y , b u t the much lower concentrations o f a r s e n i c p r e s e n t e d d i f f i c u l t i e s . The 5 filled c i r c l e s i n f i g u r e 3 r e p r e s e n t " q u a n t i t a t i v e " r e s u l t s -- i e , those whose r e p o r t e d u n c e r t a i n t i e s do not exceed the d i a m e t e r o f the circle. These are c l e a r l y i n c o n s i s t e n t , by more t h a n t h r e e o r d e r s o f magnitude. Open c i r c l e s r e p r e s e n t " d e t e c t i o n l i m i t s " f o r nond e t e c t e d [ND] r e s u l t s ; t h e i r s p r e a d exceeds f o u r o r d e r s o f magni tude. The c r o w n i n g i n c o n s i s t e n c y i s the f a c t t h a t a r s e n i c has been "measured" by some l a b o r a t o r i e s a t l e v e l s c o n s i d e r a b l y i n excess o f the d e t e c t i o n l i m i t s o f o t h e r s r e p o r t i n g ND -- c l e a r l y a l o g i c a l impossibility. To d e r i v e a c t u a l l e v e l s and e s t a b l i s h q u a l i t y measurement f o r t r a c e elements i n f o o d s t u f f s , m a t e r i a l s such as m i l k powder (A-11) and r e p r e s e n t a t i v e r e g i o n a l d i e t a r y b l e n d s (eg, USDIET-1) have been provided (8.9). Consistency o f i n t e r l a b o r a t o r y r e s u l t s among "independent" a n a l y t i c a l p r o c e d u r e s makes p o s s i b l e the c e r t i f i c a t i o n o f such m a t e r i a l s or the e s t a b l i s h m e n t of d i e t a r y intakes. The n a t u r e o f the i n i t i a l measurement problem i s i l l u s t r a t e d by the r e s u l t s o f the m i l k powder ( A - l l ) i n t e r c o m p a r i s o n , where the means, r e l a t i v e ranges (max/min, d i m e n s i o n l e s s ) and medians o f 17 atomic a b s o r p t i o n s p e c t r o s c o p y [AAS] and 7 r a d i o c h e m i c a l n e u t r o n a c t i v a t i o n a n a l y s i s [RNAA] l a b o r a t o r y r e s u l t s f o r manganese were as f o l l o w s .

[AAS] [RNAA]

mean (mg/kg)

max/min

median (mg/kg)

3.99 0.295

279 4.8

0.67 0.26

(95% (0.45 (0.12

-

CI) 1.27) 0.58)

I t i s i n t e r e s t i n g t o note t h a t the r a t i o o f means (AAS/RNAA) i s 13.5, whereas the r a t i o o f medians i s b u t 2.6. Reliability is i n c r e a s e d t h r o u g h the use o f the median as a r o b u s t statistic, e s p e c i a l l y when the number o f r e p l i c a t e s η (here, l a b o r a t o r i e s ) i s r e l a t i v e l y large. (When n>8, the 95% CI o f the median does not d i r e c t l y depend upon the extremes, so some a d d i t i o n a l , a u t o m a t i c o u t l i e r p r o t e c t i o n i s afforded.) I t i s n o t e w o r t h y , however, t h a t a l a r g e number o f r e p l i c a t e s does not guarantee q u a l i t y ! The r e l a t i v e l y few RNAA measurements y i e l d a more r e l i a b l e r e s u l t t h a n would a v e r y e x t e n s i v e s e t o f AAS measurements ( r e p r e s e n t a t i v e o f t h i s p a r t i c u l a r intercomparison). The b a s i c i s s u e i s one o f contamina t i o n and i n t e r f e r e n c e , as i m p l i e d f o r example by the skewness r e p r e s e n t e d i n the r a t i o o f mean t o median. T h i s "blank" i s s u e i s b r o u g h t out a t t h i s p o i n t i n our d i s c u s s i o n because i t i s p r o b a b l y the s i n g l e most i m p o r t a n t l i m i t a t i o n t o r e l i a b l e measurement and r e l i a b l e d e t e c t i o n f o r l o w - l e v e l , multicomponent measurements i n complex m a t r i c e s . As such, i t has been a major c o n s i d e r a t i o n i n a d d r e s s i n g the q u e s t i o n o f " r e a l sample" d e t e c t i o n l i m i t s f o r b o t h r a d i o l o g i c a l and t r a c e element measurements i n bioenvironmental samples. Experts and Concerned Groups. The foregoing observations i n v o l v i n g d i v e r s e f o r m u l a t i o n s and i n t e r p r e t a t i o n s f o r r a d i o l o g i c a l d e t e c t i o n l i m i t s , and d i v e r s e and d i s c r e p a n t i n t e r l a b o r a t o r y r e s u l t s



180


f o r low l e v e l t r a c e element measurements - - l e d the NRC and the IAEA, r e s p e c t i v e l y , t o q u e s t i o n the v a l i d i t y or a t l e a s t the u n i v e r s a l i t y o f d e t e c t i o n l i m i t f o r m u l a t i o n s as t h e n a p p l i e d . In o r d e r t o address the q u e s t i o n i n an e f f i c i e n t manner the two o r g a n i z a t i o n s employed problem s o l v i n g approaches o f demonstrated e f f e c t i v e n e s s : v i z . , (a) s i t e v i s i t s and f i e l d i n t e r v i e w s w i t h each o f the a f f e c t e d s e c t o r s [NRC], and (b) an i n t e n s i v e workshop i n v o l v i n g e x p e r t s r e p r e s e n t i n g each o f the r e s p e c t i v e a n a l y t i c a l disciplines [IAEA, ( 1 0 ) ] . The importance o f t h e s e approaches d e s e r v e s emphasis, f o r i t t r a n s c e n d s the p a r t i c u l a r s c i e n t i f i c problem under c o n s i d e r a t i o n . I n the case o f the NRC, f o r example, the v a r y i n g needs and p e r c e p t i o n s o f the u t i l i t i e s and their o p e r a t o r s , the c o n t r a c t ( r a d i o l o g i c a l ) l a b o r a t o r i e s , the c r o s s - c h e c k ( c o n t r o l ) l a b o r a t o r y , the i n s p e c t o r s , and NRC h e a d q u a r t e r s p e r s o n n e l c o u l d not o t h e r w i s e have been d i s c e r n e d . As a r e s u l t i t was p o s s i b l e t o i d e n t i f y and t o some e x t e n t s o r t out measurement from p o l i c y i s s u e s , and attempt t o c l a r i f y measurement and detection l i m i t c o n c e p t s and t e r m i n o l o g y w h i c h were s u b j e c t t o ambiguous i n t e r p r e t a t i o n . A summary o f the f i n d i n g s o f the NRC s i t e v i s i t s i s given i n Table I I I . Table I I I .

NRC

Findings

* LLD Manual Needed - - f o r d i v e r s e backgrounds * Wide r a n g i n g nomenclature and [LLD, MDA, ...]

formulations

* Socio-political-economic issues [ b i a s e d r e p o r t i n g ( p u b l i c p e r c e p t i o n s ) , LLD r e q u i r e m e n t s minor components when h i g h i n t e r f e r e n c e l e v e l s , . . . ] * Detection decisions: *

Blank:

ambiguity

f a l s e negatives in

o f t e n e x c l u d e d from

initial

draft

ranged from 5% t o

for

50%

document, b l a n k v a r i a t i o n s

LLD

* Non-counting e r r o r s g e n e r a l l y ignored,

e s p e c i a l l y sampling

* Simple c o u n t i n g f o r m u l a t i o n o n l y [ s i g n a l - b l a n k ] ; i n a p p l i c a b l e to many cases o f n u c l e a r s p e c t r o m e t r y * Gamma s p e c t r o m e t r y : multiple detection decisions; occasionally h i d d e n , c h a n g i n g a l g o r i t h m s ; erroneous parameters * Appropriate

[ l o w - l e v e l ] and double b l i n d QA

samples needed

The c a l l i n g together o f complementary or m u l t i d i s c i p l i n a r y teams o f e x p e r t s i s a t r a d i t i o n i n a number o f i n t e r n a t i o n a l o r g a n i z a t i o n s , such as the IAEA. S i n c e the IAEA o b j e c t i v e w i t h r e s p e c t t o d e t e c t i o n l i m i t s was e x p l i c i t y "method independent", such an approach was mandatory. That i s , e x p e r t knowledge c o n c e r n i n g the n a t u r e and s o u r c e s o f e r r o r , such as the b l a n k , m a t r i x e f f e c t s , and d e t e c t o r c h a r a c t e r i s t i c s f o r the methods o f i n t e r e s t (NAA, AAS,


9. CURRIE & PARR


181

ICP,...) was e s s e n t i a l so t h a t t h e approach t o d e t e c t i o n l i m i t s would a t l e a s t be b r o a d enough t o encompass t h e c h a r a c t e r i s t i c s o f t h e s e p a r t i c u l a r methods. A t h e o r e t i c a l f o r m u l a t i o n was n o t enough; a common, p r a c t i c a l approach t o d e t e c t i o n , s u i t a b l e f o r t h e s e s e v e r a l methods o f t r a c e a n a l y s i s was c a l l e d f o r . T a b l e IV g i v e s the c o m p o s i t i o n and o b j e c t i v e s o f t h e IAEA t a s k f o r c e .


Outcomes The b a s i c c o n c e p t s o f d e t e c t i o n d e c i s i o n s and d e t e c t i o n l i m i t s , based upon h y p o t h e s i s t e s t i n g as o u t l i n e d e a r l i e r i n t h i s volume, n e c e s s a r i l y s e r v e d as t h e f o u n d a t i o n f o r t h e NRC and IAEA programs. For t h e p r o b a b i l i s t i c p a r t o f t h e problem, f a l s e p o s i t i v e s and f a l s e n e g a t i v e s were t a k e n e q u a l t o 0.05 i n each case. T h i s means t h a t f o r normal random e r r o r s , t h e d e t e c t i o n d e c i s i o n i s made once t h e o b s e r v e d s i g n a l exceeds 1.645 times t h e s t a n d a r d e r r o r o f t h e n u l l s i g n a l [ s t a n d a r d e r r o r o f t h e b l a n k f o r a w e l l known b l a n k , o r Jl larger f o r paired observations]. When t h e number o f degrees o f freedom i s s m a l l , o f c o u r s e , t h e ζ-statistic i s r e p l a c e d by Student's-t. F o r s i m p l e measurements t h e d e t e c t i o n l i m i t i s j u s t twice t h i s c r i t i c a l l e v e l . In addition to this r e l a t i v e l y straight f o r w a r d n o r m a l , " s t a t i s t i c a l " t r e a t m e n t o f d e t e c t i o n , however, i t was n e c e s s a r y t o pay a t t e n t i o n t o t h e s p e c i a l c h a r a c t e r i s t i c s o f t h e r e a l measurement p r o c e s s e s i n v o l v i n g t h e d e t e c t i o n o f low l e v e l s o f mixed r a d i o n u c l i d e s , and t r a c e c o n c e n t r a t i o n s o f m u l t i p l e elements i n complex m a t r i x m a t e r i a l s . T h i s m a t t e r , w h i c h was an e x p l i c i t com ponent o f b o t h p r o j e c t s , cannot be overemphasized. I f one c a l c u l a t e s s t a t i s t i c a l d e t e c t i o n l i m i t s f o r i n t e r f e r e n c e - f r e e measure ments i n pure s o l u t i o n s , e s t i m a t e d d e t e c t i o n l i m i t s w i l l be t o o low, o f t e n by o r d e r s o f magnitude. S i m i l a r l y , d e t e c t i o n l i m i t s w h i c h a r e p r e d i c t e d f o r an a l t e r a t i o n o r o p t i m i z a t i o n o f a measurement p r o c e s s w i l l be u n r e a l i s t i c a l l y o p t i m i s t i c i f one a p p l i e s j u s t a s i m p l e s t a t i s t i c a l f o r m u l a t o c a l c u l a t e t h e change i n d e t e c t i o n l i m i t , eg, w i t h i n c r e a s e d r e p l i c a t i o n o r i n c r e a s e d c o u n t i n g time. Unless t e s t s are made w i t h known o r common m a t e r i a l s comparable i n c o m p o s i t i o n t o the samples o f i n t e r e s t , these erroneous e s t i m a t e s w i l l n o t be exposed. NRC S p e c i a l T o p i c s . The e s t i m a t i o n o f p r a c t i c a l , r e a l sample d e t e c t i o n l i m i t s r e q u i r e s a t t e n t i o n t o a l l o f the sources o f e r r o r t h a t must be f a c e d i n d e r i v i n g c o n f i d e n c e , o r more c o r r e c t l y , u n c e r t a i n t y i n t e r v a l s (11). I n t h e NRC s t u d y , s p e c i a l a t t e n t i o n was g i v e n t o t h e f o l l o w i n g : a) bounds f o r uncompensated s y s t e m a t i c e r r o r i n t h e background and c a l i b r a t i o n f a c t o r , b) n o n - P o i s s o n random e r r o r ( t h a t w h i c h exceeds " c o u n t i n g s t a t i s t i c s " ) , c ) d e c o n v o l u t i o n and model e r r o r connected w i t h a l p h a o r gamma-ray spectrum a n a l y s i s o f multicomponent m i x t u r e s , and d) s p e c i a l l i m i t a t i o n s from t h e nonnormality o f the Poisson distribution f o r extreme low-level c o u n t i n g , such as o c c u r s w i t h m o n i t o r i n g o f a c t i n i d e s o r o t h e r a l p h a emitters. Each o f these f a c t o r s r e q u i r e d extension o f the "simple-counting" expression o r i g i n a l l y found i n t h e d r a f t NRC Technical S p e c i f i c a t i o n s [see t a b l e I I ] . A full discussion, supplemented w i t h a p p r o p r i a t e f o r m u l a s , r e f e r e n c e s , and examples may be found i n Ref. 12; and one o f t h e more b r o a d l y a p p l i c a b l e r e s u l t s w i l l be t r e a t e d h e r e . I n t a b l e V we g i v e an e x t e n s i o n o f the


182


T a b l e IV. C o n s u l t a n t s ' M e e t i n g on L i m i t o f D e t e c t i o n Dates and P l a c e 2-4 December 1985;

IAEA H e a d q u a r t e r s ,

Vienna


Participants Austria:

W. Wegscheider

Belgium:

J.P. Op de Beeck R. Van G r i e k e n

F.R.G.

M. S t o e p p l e r

U.S.A.

L.A.

IAEA

R.M. P a r r ( S c i e n t i f i c F. R e i c h e l R. Schelenz H. V e r a - R u i z

Currie (Chair) Secretary)

METHODS Topics

· A c t i v a t i o n A n a l y s i s and gamma-ray ( J . P . Op de Beeck; F. R e i c h e l )

spectrometry

• XRF and PIXE (R. Van G r i e k e n ) • AAS (M. S t o e p p l e r ) • ICP-AES (W. Wegscheider) • Voltammetry and o t h e r methods (M. S t o e p p l e r ) GOALS General d i s c u s s i o n • Method-independent d e t e c t i o n (LoD)

definition

of

the

limit

of

• P r a c t i c a l d e t e r m i n a t i o n and use o f t h e LoD • Special problems a n a l y t i c a l methods • Decision making n e a r t h e LoD

and

associated

reporting

with

of

individual

data

P r e p a r a t i o n o f meeting r e p o r t


below

or

9. CURRIE & PARR


183

T a b l e V. D e s i g n and O p t i m i z a t i o n (LJJ ) V a r i a t i o n s w i t h background (B) and C o u n t i n g Time ( t ) ( a )

LLD

t«r Β < 1 1

Perspectives on Detection Limits - ACS Symposium Series (ACS

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