Detection in Analytical Chemistry - American Chemical Society

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Chapter 6

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Comparison of Detection Limits in Atomic Spectroscopic Methods of Analysis Michael S. Epstein Inorganic Analytical Research Division, National Bureau of Standards, Gaithersburg, MD 20899

The comparison of detection limits is a fundamental part of many decision-making processes for the analytical chemist. Despite numerous efforts to standardize methodology for the calculation and reporting of detection limits, there is still a wide divergence in the way they appear i n the literature. This paper discusses valid and invalid methods to calculate, report, and compare detection limits using atomic spectroscopic techniques. Noises which limit detection are discussed for analytical methods such as plasma emission spectroscopy, atomic absorption spectroscopy and laser excited atomic fluorescence spectroscopy. The c o m p a r i s o n o f d e t e c t i o n l i m i t s i s a fundamental p a r t o f most decision-making processes f o r the a n a l y t i c a l chemist. whether t h e d e c i s i o n i n v o l v e s t h e purchase o f a new i n s t r u m e n t o r t h e d e s i g n o f a t r a c e a n a l y s i s p r o t o c o l , t h e f i g u r e - o f - m e r i t [1] w h i c h i n f l u e n c e s the c h o i c e w i l l most l i k e l y be t h e d e t e c t i o n l i m i t . S i n c e one o r more o f t h e t e c h n i q u e s b e i n g compared i s o f t e n u n f a m i l i a r , t h e d e c i s i o n w i l l be b a s e d on i n f o r m a t i o n t h a t can be r e t r i e v e d from t h e literature, both from m a n u f a c t u r e r a d v e r t i s i n g and t h e open scientific literature. Unfortunately, despite the e f f o r t s of o r g a n i z a t i o n s such as t h e I n t e r n a t i o n a l U n i o n o f Pure and A p p l i e d C h e m i s t r y (IUPAC) t o s t a n d a r d i z e methodology t o c a l c u l a t e and r e p o r t d e t e c t i o n l i m i t s [ 2 ] , t h e r e i s s t i l l a wide d i v e r g e n c e i n t h e way t h a t d e t e c t i o n l i m i t s appear i n p r i n t . W h i l e t h e r e i s h o p e f u l l y no d e l i b e r a t e attempt on t h e p a r t o f a u t h o r s and m a n u f a c t u r e r s t o b i a s d e t e c t i o n l i m i t s towards a p a r t i c u l a r technique, t h e manner o f calculating and r e p o r t i n g c a n l e a d t o a m i s i n t e r p r e t a t i o n o f d e t e c t i o n l i m i t s by the c a r e l e s s o r u n f a m i l i a r reader. I f the d e t e c t i o n l i m i t methodology i s n o t w e l l documented, a c o m p a r i s o n c a n be b i a s e d by s e v e r a l o r d e r s o f magnitude. I t i s impossible t o completely eliminate bias i n detection limit comparisons, particularly when comparing detection capabilities i n r e a l sample m a t r i c e s . However, i f t h e b a s i c

This chapter not subject to U.S. copyright Published 1988 American Chemical Society

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

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p r i n c i p l e s b e h i n d t h e t e c h n i q u e s t o be compared a r e u n d e r s t o o d and we a r e aware o f t h e common ways i n w h i c h d e t e c t i o n l i m i t comparisons can be m i s i n t e r p r e t e d , r e a s o n a b l y v a l i d c o n c l u s i o n s c a n be drawn. Thus, t h i s d i s c u s s i o n w i l l c o n c e n t r a t e i n g e n e r a l on v a l i d and i n v a l i d ways t o compare d e t e c t i o n l i m i t s and i n s p e c i f i c d e t a i l about l i m i t i n g n o i s e s w h i c h determine those d e t e c t i o n l i m i t s u s i n g several o f t h e most common atomic spectroscopic techniques, i n c l u d i n g flame and plasma e m i s s i o n s p e c t r o s c o p y , atomic a b s o r p t i o n s p e c t r o s c o p y , and l a s e r - e x i t e d atomic f l u o r e s c e n c e s p e c t r o s c o p y . B e f o r e d e t e c t i o n l i m i t s a r e d i s c u s s e d i n any d e t a i l , i t i s n e c e s s a r y t o d e f i n e t h e scope o f t h e p r o c e s s t o w h i c h t h e d e t e c t i o n l i m i t a p p l i e s . F o r example, t h e d e t e c t i o n l i m i t d e t e r m i n e d f o r an element i n t h e absence o f c o n c o m i t a n t s (i.e., i n pure water s o l u t i o n ) i s l i k e l y t o be s i g n i f i c a n t l y l e s s t h a n t h e d e t e c t i o n l i m i t d e t e r m i n e d f o r a complete a n a l y t i c a l p r o t o c o l w h i c h i n c l u d e s s a m p l i n g , sample p r e p a r a t i o n , and a n a l y s i s . The former, w h i c h i s the type o f d e t e c t i o n l i m i t most o f t e n r e p o r t e d i n t h e l i t e r a t u r e , may be r e f e r r e d t o as fundamental i n t h a t i t r e f l e c t s o n l y the i n s t r u m e n t a l n o i s e sources w h i c h a r e i n h e r e n t i n t h e a n a l y t i c a l i n s t r u m e n t used. Fundamental d e t e c t i o n l i m i t s a r e o f t e n o f l i m i t e d v a l u e t o t h e p r a c t i c i n g a n a l y t i c a l chemist who must determine t h a t element i n r e a l and o f t e n v e r y complex m a t r i c e s . The l a t t e r type o f d e t e c t i o n l i m i t , r e f l e c t i n g t h e e n t i r e a n a l y t i c a l p r o t o c o l , may be r e f e r r e d t o as m e t h o d o l o g i c a l . Methodological d e t e c t i o n l i m i t s are a l s o o f l i m i t e d v a l u e s i n c e they i n c l u d e many v a r i a b l e s w h i c h cannot be e a s i l y reproduced. The d e t e c t i o n l i m i t s t o be d i s c u s s e d here w i l l be c a l l e d i n s t r u m e n t a l and w i l l be d e f i n e d as f a l l i n g between fundamental and m e t h o d o l o g i c a l i n t h a t they w i l l c o n s i d e r v a r i a t i o n s i n d u c e d by t h e i n s t r u m e n t a l o n e and by t h e i n t e r a c t i o n o f t h e sample w i t h the instrument, but w i l l not consider the e n t i r e a n a l y t i c a l scheme w h i c h i n c l u d e s b l u n d e r s and c o n t a m i n a t i o n i n t h e s a m p l i n g and sample p r e p a r a t i o n p r o c e s s . I t i s noteworthy t h a t i n s t r u m e n t a l d e t e c t i o n l i m i t s w i l l approach fundamental d e t e c t i o n l i m i t s when t h e sample m a t r i x i s s i m p l e o r when n o i s e r e d u c t i o n methods s p e c i f i c t o sample-matrix-induced noises are a p p l i e d . While the d i s c u s s i o n w i l l d e a l w i t h atomic rather than m o l e c u l a r s p e c t r o s c o p i c methods, many o f t h e p o i n t s t o be made w i l l a p p l y t o b o t h atomic and m o l e c u l a r methods. The major d i f f e r e n c e between t h e n o i s e c h a r a c t e r i s t i c s o f t h e two methods i s u s u a l l y t h e dynamic o r f l o w i n g s t a t e o f an atomic system, such as a h i g h temperature flame o r plasma, compared t o t h e s t a t i c s t a t e o f a m o l e c u l a r system i n w h i c h t h e sample u s u a l l y i s p l a c e d i n a s m a l l transparent cuvette. The dynamic s t a t e o f t h e atomic system generates an a n a l y t e s i g n a l - c a r r i e d n o i s e w h i c h i s p r o p o r t i o n a l t o a n a l y t e s i g n a l magnitude and thus becomes l i m i t i n g a t h i g h a n a l y t e concentrations. (A s i g n a l - c a r r i e d n o i s e i s one whose magnitude i s a c o n s t a n t p e r c e n t a g e o f t h e a m p l i t u d e o f a s i g n a l , w h i c h may be due to background o r t o t h e a n a l y t e . Thus, an a n a l y t e s i g n a l - c a r r i e d n o i s e i s a f l u c t u a t i o n i n t h e phenomenon caused by t h e a n a l y t e , where t h e phenomenon i s used as a measure o f t h e a n a l y t e c o n c e n t r a t i o n , such as a b s o r p t i o n o r e m i s s i o n o f e l e c t r o m a g n e t i c radiation). The s t a t i c s t a t e o f t h e m o l e c u l a r system l i m i t s t h e magnitude o f a n a l y t e s i g n a l - c a r r i e d n o i s e s , e x c e p t where t h e s t a t i c s t a t e i s d i s t u r b e d ( i . e . , v i b r a t i o n , c e l l p o s i t i o n changes, e t c . ) o r where r a d i a t i o n source f l u c t u a t i o n s a r e s i g n i f i c a n t a t h i g h a n a l y t e concentrations ( i . e . , molecular fluorescence spectrophotometry).

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

6. EPSTEIN

Comparing Detection Limits

111

B i a s In D e t e c t i o n L i m i t Comparisons

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There a r e s e v e r a l ways t h a t d e t e c t i o n l i m i t i n f o r m a t i o n can be presented i n o r d e r t o b i a s the o b s e r v e r . Again, i t must be emphasized t h a t i n most cases s u f f i c i e n t i n f o r m a t i o n w i l l be p r e s e n t e d i n a f i g u r e or i n the accompanying t e x t t o a l l o w the knowledgeable reader to properly interpret detection limit comparisons. R e a l or A r t i f i c i a l D e t e c t i o n L i m i t s . C e r t a i n l y , one o f the most common ways t o r e p o r t d e t e c t i o n l i m i t s i s i n "pure aqueous s o l u t i o n . " whether the a n a l y t i c a l c o n d i t i o n s or the i n s t r u m e n t a t i o n used i s c a p a b l e o f those d e t e c t i o n l i m i t s when r e a l samples are analyzed i s another question. T h i s source o f b i a s i s most o f t e n e n c o u n t e r e d when a new a n a l y t i c a l technique i s developed. An example i s the e a r l y development o f flame a t o m i c fluorescence spectroscopy (FAFS), where cadmium was "detected" with a t o t a l consumption b u r n e r i n an oxy-hydrogen flame a t 1 pg/mL [ 3 ] . C e r t a i n l y no one would attempt r e a l sample a n a l y s i s i n such a flame because o f i t s t u r b u l e n t f l o w and poor d i s s o c i a t i o n c h a r a c t e r i s t i c s . More r e a l i s t i c d e t e c t i o n l i m i t s are on the o r d e r o f 200 pg/mL i n an a i r - a c e t y l e n e flame [4] . I n flame atomic a b s o r p t i o n s p e c t r o s c o p y (FAAS), t i n d e t e c t i o n l i m i t s are s i g n i f i c a n t l y b e t t e r («4x) in a cool air-hydrogen flame t h a n i n h o t t e r flames as a r e s u l t o f i n c r e a s e d s e n s i t i v i t y and lower flame background e m i s s i o n [ 5 ] . The use o f the s a m p l i n g b o a t [6] i n FAAS a l s o improves d e t e c t i o n l i m i t s f o r many elements by an o r d e r o f magnitude because o f i n c r e a s e d sample t r a n s p o r t e f f i c i e n c y . However, n e i t h e r o f these t e c h n i q u e s i s w i d e l y used i n FAAS, s i n c e b o t h e x h i b i t s i g n i f i c a n t chemical i n t e r f e r e n c e s w i t h r e a l samples. Recently developed techniques, such as i n d u c t i v e l y - c o u p l e d plasma mass s p e c t r o m e t r y (ICP-MS) [7] and l a s e r enhanced i o n i z a t i o n s p e c t r o s c o p y (LEIS) [8] e x h i b i t s i m i l a r sample-related degradation of d e t e c t i o n l i m i t s . C e r t a i n l y , most t e c h n i q u e d e t e c t i o n l i m i t s s u f f e r somewhat when r e a l samples are a n a l y z e d and n o i s e s induced by the sample m a t r i x become limiting. The e x t e n t o f t h i s e f f e c t w i l l v a r y , however, from t e c h n i q u e t o t e c h n i q u e , and w i l l u s u a l l y d i m i n i s h as the method reaches m a t u r i t y . Detection Limit Criterion. The c r i t e r i o n used t o d e f i n e the d e t e c t i o n l i m i t , or perhaps as i m p o r t a n t , the p r o t o c o l used t o measure i t , can be c r i t i c a l i n e s t a b l i s h i n g a v a l i d d e t e c t i o n l i m i t . C u r r i e [9] has d e s c r i b e d the wide v a r i a t i o n i n d e t e c t i o n l i m i t definitions for radiochemical measurements reported in the literature. IUPAC [2] recommends the d e t e c t i o n l i m i t , c , be defined as the concentration of an analyte equal to a background-corrected s i g n a l , x-^ - xg, t h r e e times the estimated s t a n d a r d d e v i a t i o n o f a s i n g l e d e t e r m i n a t i o n u s i n g 20 measurements o f the b l a n k . L

X

L

c

L

=

X

B

+

k s

B

= ks /m B

where

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

W (2)

DETECTION IN ANALYTICAL CHEMISTRY

112 x xg sg c L

L

— — *

uncorrected signal blank measure estimated standard deviation of the blank measure detection l i m i t , which i s the concentration derived from the smallest measure ( x ) that can be detected with reasonable confidence. - numerical factor chosen i n accordance with the confidence l e v e l desired. - analytical sensitivity L

k

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m

As pointed out by Long and Winefordner [10], the use of k-3 allows a confidence l e v e l of 99.86% for a normal d i s t r i b u t i o n of xg, or an 89% confidence l e v e l for a non-normal d i s t r i b u t i o n . While xg w i l l often be normally d i s t r i b u t e d when instrumental noise limits detection, the presence of analyte contamination i n the blank, either i n the sample preparation process or as a series of discrete events ( i . e . , Na or Fe airborne particulates) i n the instrumental measurement process, w i l l r e s u l t i n a non-normal d i s t r i b u t i o n . Such a d i s t r i b u t i o n may be bimodal or skewed depending on the source and c h a r a c t e r i s t i c s of the contaminant. Long and Winefordner [10] have also presented several examples of the influence of measurement protocol on c-^. The use of values of k < 3 or the use of the standard deviation of the mean or pooled standard deviation rather than the standard deviation of a single measurement, can lead to C L values which deviate by an order of magnitude from the IUPAC model. Measurement protocols which include the error i n the a n a l y t i c a l s e n s i t i v i t y as well as the error i n the blank can also cause C L to deviate s i g n i f i c a n t l y from the IUPAC model, which assumes a well-defined s e n s i t i v i t y . F i n a l l y , the presence of very low frequency noise or d r i f t may not be incorporated into the IUPAC d e f i n i t i o n of detection l i m i t [11]. The c a l i b r a t i o n scheme used for r e a l samples may be spread out over a longer time period than was used for the determination of the detection l i m i t and thus noises which were i n s i g n i f i c a n t during the detection l i m i t measurement may be encountered. Ideally, a technique detection l i m i t should be determined using the measurement protocol employed for r e a l sample analysis. A n a l y t i c a l Blank. I f the detection l i m i t i s not measured from the true a n a l y t i c a l blank, a c r i t i c a l part of the detection l i m i t determination has been ignored. Since the emphasis i n this discussion i s on "instrumental" rather than "methodological" detection l i m i t s , only blanks r e s u l t i n g from the instrumentation w i l l be considered. Although method blanks can c e r t a i n l y be l i m i t i n g , p a r t i c u l a r l y for elements such as Fe, Na, and Ca which are common i n the laboratory environment, they are not as predictable as instrumental contamination blanks, since variations i n laboratory procedures and design w i l l be much greater than variations i n instrument design. For example, i n FAFS one can s i g n i f i c a n t l y improve a detection l i m i t i n a s i t u a t i o n l i m i t e d by flame scatter of source r a d i a t i o n by making measurements with no water being introduced into the flame. By eliminating the scattering species, unvaporized water droplets, the detection l i m i t i s improved. S i m i l a r l y , one can measure a graphite furnace atomic absorption

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

Downloaded by UNIV OF TENNESSEE KNOXVILLE on August 17, 2015 | http://pubs.acs.org Publication Date: December 9, 1987 | doi: 10.1021/bk-1988-0361.ch006

6. EPSTEIN

Comparing Detection Limits

113

s p e c t r o s c o p y (GFAAS) d e t e c t i o n l i m i t w i t h o u t a c t u a l l y a t o m i z i n g a b l a n k sample, assuming the n o i s e t o be independent o f the a t o m i z e r . T h i s i s c e r t a i n l y an i n v a l i d assumption when d e t e r m i n i n g an element whose most s e n s i t i v e a b s o r p t i o n l i n e l i e s i n the v i s i b l e r e g i o n o f the spectrum, such as barium, where t h e r m a l e m i s s i o n from the g r a p h i t e tube i s s i g n i f i c a n t , or where c o n t a m i n a t i o n i n the tube i s l i m i t i n g , such as when z i n c i s determined. For t h e s e elements, p u b l i s h e d d e t e c t i o n l i m i t s may be i n v a l i d , u n l e s s t h e y were measured under a c t u a l a n a l y s i s c o n d i t i o n s . The m o r a l i s thus t o measure the b l a n k under c o n d i t i o n s as s i m i l a r as p o s s i b l e t o the a n a l y s i s c o n d i t i o n s used. Instrument N o i s e C h a r a c t e r i s t i c s . Depending on the f r e q u e n c y domain spectrum o f the s i g n a l from the a n a l y t i c a l i n s t r u m e n t , t h a t i s , i f the n o i s e i s w h i t e ( s h o t ) o r 1/f ( f l i c k e r ) i n n a t u r e [ 1 ] , the i n t e g r a t i o n time o r time c o n s t a n t used f o r the d e t e c t i o n l i m i t d e t e r m i n a t i o n may have a s i g n i f i c a n t e f f e c t on the d e t e c t i o n l i m i t . I n cases where s h o t n o i s e i s most o f t e n l i m i t i n g , such as i n FAFS a t a l l wavelengths o r FAAS above 230 nm, o r ICP e m i s s i o n s p e c t r o s c o p y (ICP-ES) below 250 nm, the d e t e c t i o n l i m i t can be improved as the s q u a r e - r o o t o f the i n t e g r a t i o n time. In f l i c k e r noise l i m i t e d c a s e s , t h e r e may be l i t t l e or no improvement i n the d e t e c t i o n l i m i t w i t h an i n c r e a s e i n the i n t e g r a t i o n time. [12,13] The improvement i n d e t e c t i o n l i m i t f o r an i n c r e a s e o f i n t e g r a t i o n time w i l l be u l t i m a t e l y l i m i t e d by the s i g n i f i c a n c e o f v e r y low f r e q u e n c y d r i f t and the a v a i l a b i l i t y o f l a r g e volumes o f sample s o l u t i o n . Measurement U n i t s . Perhaps the most o b v i o u s y e t c o n f u s i n g a s p e c t o f many d e t e c t i o n l i m i t comparisons i s the use o f " r e l a t i v e " v e r s u s "absolute" u n i t s . R e l a t i v e u n i t s r e f l e c t a mass p e r u n i t volume, such as micrograms per m i l l i l i t e r , w h i l e a b s o l u t e u n i t s r e f l e c t a mass o n l y , such as micrograms. O b v i o u s l y , " r e l a t i v e " and " a b s o l u t e " u n i t s s h o u l d n o t be d i r e c t l y compared. However, a b s o l u t e u n i t s can be c o n v e r t e d i n t o r e l a t i v e u n i t s and v i c e v e r s a , employing the volume o f s o l u t i o n u t i l i z e d by a p a r t i c u l a r t e c h n i q u e . Nonetheless, how that conversion i s done or how i t i s documented can s i g n i f i c a n t l y b i a s the o b s e r v e r . Table I i l l u s t r a t e s s e v e r a l examples, t a k e n from the s c i e n t i f i c l i t e r a t u r e , o f the use o f d e t e c t i o n l i m i t v a l u e s i n a t a b l e f o r comparison purposes. I n each case the a u t h o r p r o v i d e s adequate i n f o r m a t i o n f o r the i n f o r m e d reader t o make an accurate comparison. Nevertheless, the c o n c l u s i o n s drawn by the c a r e l e s s o r u n i n f o r m e d r e a d e r who does not r e a d o r u n d e r s t a n d the f o o t n o t e s or the t e x t w h i c h d e s c r i b e s the t a b l e , can be b i a s e d by s e v e r a l o r d e r s o f magnitude. T a b l e l a , p r e s e n t s a comparison o f FAAS and GFAAS d e t e c t i o n l i m i t s [14]. Without r e a d i n g the t e x t which r e f e r s t o the t a b l e , one i s impressed by the s i g n i f i c a n t , 3 t o 4 o r d e r s o f magnitude, improvement u s i n g the g r a p h i t e f u r n a c e . However, the t e x t c l e a r l y i n d i c a t e s t h a t b o t h flame and g r a p h i t e f u r n a c e d e t e c t i o n l i m i t s assume a 1 mL sample volume which, w h i l e c e r t a i n l y v a l i d i n a flame, i s n o t v a l i d f o r a g r a p h i t e f u r n a c e s i n c e the maximum sample volume i s u s u a l l y about 50 t o 100 /xL. Some systems, l i k e the c a r b o n r o d a t o m i z e r [ 1 5 ] , can o n l y accommodate 1 t o 2 /iL o f s o l u t i o n . Thus, f o r a v a l i d comparison, the g r a p h i t e f u r n a c e d e t e c t i o n l i m i t s must be degraded by one t o two o r d e r s o f magnitude.

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

114

DETECTION IN ANALYTICAL CHEMISTRY Table l a .

D e t e c t i o n L i m i t s R e p o r t e d f o r Atomic A b s o r p t i o n

D e t e c t i o n L i m i t (zxg/mL) Nonflame Flame 6 x 10" 0.02 4 x 10" 0.002 1 x 10" 0.004 2 x 10" 0.0008

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Element Ba Ca Fe Mn

6

7

5

7

"...the l i m i t s are based on a s i g n a l - t o - n o i s e r a t i o = 2 c r i t e r i o n and the assumption t h a t a volume o f 1 mL i s the minimum r e q u i r e d f o r a d e t e r m i n a t i o n . For example, i f an a b s o l u t e d e t e c t i o n l i m i t i s g i v e n ( e . g . , nonflame a t o m i z e r ) as 10"^ g, t h i s i s e x p r e s s e d as a c o n c e n t r a t i o n a l d e t e c t i o n l i m i t o f 0.001 /xg/mL. One must b e a r i n mind t h a t most c u r r e n t nonflame a t o m i z e r s cannot h a n d l e samples l a r g e r t h a n , say, 0.1 mL, and t h a t most flame a t o m i z a t i o n systems cannot h a n d l e samples ( f o r a r e l i a b l e r e a d i n g ) o f much l e s s t h a n 1 mL. The 1 mL c r i t e r i o n . .. i s thus more f o r the purpose o f d i r e c t comparison than f o r the v e r y l o w e s t p o s s i b l e d e t e c t i o n l i m i t s . . . " R e p r i n t e d from [14] by p e r m i s s i o n o f J o h n W i l e y and Sons, c o p y r i g h t 1976. Source: Wiley.

Reproduced w i t h p e r m i s s i o n from Ref. 14.

Copyright

1976

I n T a b l e l b [16] a b s o l u t e r a t h e r t h a n r e l a t i v e d e t e c t i o n l i m i t s are compared f o r s e v e r a l t e c h n i q u e s . U n l e s s one l o o k s a t the f o o t n o t e s however, i t i s not obvious t h a t the d e t e c t i o n l i m i t f o r one method i s based on a 1 /xL sample s i z e , a n o t h e r on a 5 /iL sample s i z e and another on a 1 mL sample s i z e , thus b i a s i n g the c a r e l e s s o b s e r v e r o f t h i s t a b l e by 2 t o 3 o r d e r s o f magnitude. Table l b . Absolute D e t e c t i o n L i m i t s U s i n g Atomic F l u o r e s c e n c e S p e c t r o m e t r y and S e v e r a l Other Methods

Element Ag Cd Mg Ni

AFS 0.4 0.0015 1 5

D e t e c t i o n L i m i t s (pg) AAS 0.2 0.1 0.06 10

AEICP 200 70 3 200

AFS = Atomic f l u o r e s c e n c e s p e c t r o m e t r y - 1 /xL sample s i z e AAS = Atomic a b s o r p t i o n s p e c t r o m e t r y - 5 /xL sample s i z e AEICP = Plasma e m i s s i o n u s i n g the ICP - 1 mL sample s i z e [16] R e p r i n t e d from [16] by p e r m i s s i o n o f Pergamon J o u r n a l s L t d . Source: Reproduced w i t h p e r m i s s i o n from Ref. 16. C o p y r i g h t 1979 Pergamon P r e s s . F i n a l l y , i n Table l c [17] a comparison i s made of d e t e c t i o n l i m i t s f o r carbon f u r n a c e atomic e m i s s i o n s p e c t r o s c o p y (CFAES), flame e m i s s i o n s p e c t r o s c o p y ( F E S ) , and CFAAS. Note t h a t s e n s i t i v i t i e s are used as p s e u d o - d e t e c t i o n l i m i t s f o r CFAAS. These are not r e a l l y s e n s i t i v i t i e s as d e f i n e d by IUPAC [ 1 8 ] , but are c h a r a c t e r i s t i c c o n c e n t r a t i o n s , s i n c e they r e p r e s e n t a c o n c e n t r a t i o n e q u i v a l e n t to an absorbance of 0.0044. Furthermore, n o i s e In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6.

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115

measurements are n o t made i n the c a l c u l a t i o n o f t h i s parameter, so the t r u e d e t e c t i o n l i m i t w i l l l i k e l y be much s m a l l e r , p a r t i c u l a r l y in the case o f CFAAS, where t r a n s m i s s i o n flicker noise is negligible. T a b l e l c . D e t e c t i o n L i m i t s U s i n g Carbon Furnace Atomic E m i s s i o n S p e c t r o m e t r y and Other Techniques

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Element

Mo Si Be

CFAES

0.016 0.088 0.46

D e t e c t i o n L i m i t s (ttg/mL) Flame e m i s s i o n

0.03 10 10

CFAAS

0.005 0.01 0.0002

CFAES = Carbon f u r n a c e atomic e m i s s i o n s p e c t r o m e t r y - 20 /xL a l i q u o t of s o l u t i o n CFAAS = Carbon f u r n a c e atomic a b s o r p t i o n s p e c t r o m e t r y - S e n s i t i v i t y i n /xg/mL/0.0044 A - based on a 20 fih a l i q u o t o f s o l u t i o n R e p r i n t e d from [17] by p e r m i s s i o n o f E l s e v i e r S c i e n c e P u b l i s h e r s Source: Reproduced w i t h p e r m i s s i o n from Ref. 17. Copyright 1978 Elsevier Scientific. Now, l e t us summarize the q u e s t i o n s t h a t s h o u l d be c o n s i d e r e d when comparing d e t e c t i o n l i m i t s . F i r s t , what i s the n o i s e bandwidth ( d e f i n e d by the i n t e g r a t i o n time o r time c o n s t a n t f o r each measurement) o f each instrument? Were the measurements made under s i m i l a r c o n d i t i o n s and, when u s i n g a method such as ICP-ES [12,13], does i t make any d i f f e r e n c e ? Second,' are we d e a l i n g w i t h a b s o l u t e o r r e l a t i v e u n i t s and have the u n i t s been c o r r e c t l y c o n v e r t e d t o a l l o w a v a l i d comparison? T h i r d , does sample-induced n o i s e , t h a t i s n o i s e r e s u l t i n g from components i n the sample m a t r i x , s i g n i f i c a n t l y degrade d e t e c t i o n limits? T h i s may be more s i g n i f i c a n t f o r some t e c h n i q u e s than others. For example, s c a t t e r or m o l e c u l a r a b s o r p t i o n i n FAAS, when compensated f o r by a background c o r r e c t i o n method such as Zeeman s p l i t t i n g o r a continuum s o u r c e , w i l l u s u a l l y r e s u l t i n o n l y a s m a l l i n c r e a s e i n shot n o i s e due to a t t e n u a t i o n of primary source i n t e n s i t y and no s i g n i f i c a n t change i n d e t e c t i o n l i m i t s w i l l o c c u r . The same m a t r i x components i n an ICP-ES system, w h i c h i s f l i c k e r n o i s e l i m i t e d , may show a f a r more s i g n i f i c a n t d e g r a d a t i o n of d e t e c t i o n l i m i t when f l i c k e r i n the sample m a t r i x e m i s s i o n becomes the l i m i t i n g n o i s e . F o u r t h , does the s e n s i t i v i t y of the t e c h n i q u e d e c r e a s e i n the p r e s e n c e of the sample m a t r i x ? O f t e n c o n d i t i o n s w h i c h f a v o r the b e s t d e t e c t i o n l i m i t s , such as low b a c k g r o u n d e m i s s i o n or h i g h sample i n t r o d u c t i o n r a t e s a l s o r e s u l t i n r e d u c e d sample d i s s o c i a t i o n and t h u s d e c r e a s e d a n a l y t e s e n s i t i v i t y when a complex sample m a t r i x i s present. Are d e t e c t i o n l i m i t s d e t e r m i n e d under u n r e a l i s t i c c o n d i t i o n s or w i t h a p p a r a t u s u n s u i t a b l e f o r r e a l sample a n a l y s i s ? F i f t h , a r e we d e a l i n g w i t h c o n d i t i o n s o p t i m i z e d f o r a s i n g l e element or m u l t i e l e m e n t a n a l y s i s ? Compromise c o n d i t i o n s degrade d e t e c t i o n l i m i t s b u t improve the i n f o r m i n g power of the method ( i . e . , the t o t a l amount of i n f o r m a t i o n about a sample t h a t can be o b t a i n e d from an a n a l y t i c a l method).

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

DETECTION IN ANALYTICAL CHEMISTRY

116

Sixth and f i n a l l y , what c r i t e r i a were detection l i m i t and how was i t calculated?

used

to

define

the

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Noises Which Limit Detection Let us now look b r i e f l y into the three major classes of a n a l y t i c a l spectrometric methods: emission, absorption, and fluorescence. Noises w i l l be defined, and examples of how and when they l i m i t detection w i l l be given. Table II l i s t s the major noises which l i m i t detection for the three atomic spectroscopic techniques to be discussed. Detailed d e f i n i t i o n s of these noises may be found i n the paper by Epstein and Winefordner [1]. TABLE I I . Noises Which Limit Detection i n Atomic Spectroscopic Methods

EMISSION PMT shot noise induced by dark current, atomizer background emission, or sample matrix emission. Electronics noise (including RF) Atomizer background intensity fluctuations induced by atomizer gases, sample matrix components, or contamination.

ABSORPTION PMT shot noise induced by the radiation source, atomizer background emission, or sample matrix emission. Electronics noise Radiation source intensity fluctuations Atomizer transmission fluctuations induced by flame or furnace gases, sample matrix components, or contamination.

FLUORESCENCE PMT shot noise induced by dark current, atomizer background emission, sample matrix emission, or scattered r a d i a t i o n source intensity. Electronics noise (including RF) Radiation source intensity fluctuations carried by scatter, contamination fluorescence, or broadband fluorescence from flame and furnace gases or from sample matrix components. Atomizer background intensity fluctuations induced by flame or furnace gases, sample matrix components, or contamination.

Every spectrometric system consists of four of the components shown i n Figure 1: (a) a source of atoms; (b) a spectrometer to isolate the radiation whose intensity and frequency contains information about the analyte; (c) a photodetector to convert photons to e l e c t r i c current; and (d) a signal processing scheme to

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

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

(Absorption)

SOURCE

RADIATION

CELL

Figure 1.

DETECTION

NON-OPTICAL

VAPOR

ATOMIC

(Fluorescence)

SOURCE

R ADI A T I O N

DETECTOR

OPTICAL

Components of an a n a l y t i c a l spectrometric system.

SPECTROMETER

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PROCESSING

S IGNAL

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118

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decode the i n f o r m a t i o n s t o r e d i n the r a d i a t i o n . In emission methods, the h i g h temperature o f the atom s o u r c e p r o v i d e s e x c i t a t i o n energy t o promote e l e c t r o n t r a n s i t i o n s , w h i l e i n a b s o r p t i o n and f l u o r e s c e n c e methods, e x t e r n a l r a d i a t i o n s o u r c e s a r e u s e d t o induce the e l e c t r o n t r a n s i t i o n s o f the a n a l y t e atoms. A l l o f these i n s t r u m e n t components can r e s u l t i n n o i s e w h i c h l i m i t s d e t e c t i o n . S p e c t r o s c o p i c t e c h n i q u e s which use n o n - o p t i c a l d e t e c t i o n , such as LEIS o r p h o t o a c o u s t i c s p e c t r o s c o p y , a r e c h a r a c t e r i z e d by n o i s e s o u r c e s s i m i l a r t o f l u o r e s c e n c e , s i n c e the i n f o r m a t i o n - c a r r y i n g phenomenon i s energy r e l e a s e f o l l o w i n g a b s o r p t i o n . R a t h e r than r a d i a t i o n a l d e a c t i v a t i o n o f the e x c i t e d s t a t e , i n LEIS the energy release mechanism i s flame ion current g e n e r a t i o n , and in photoacoustic spectroscopy, it is thermal or collisional deactivation. E m i s s i o n N o i s e Sources. N o i s e s i n the e m i s s i o n t e c h n i q u e a r e the s i m p l e s t to understand. The i n d u c t i v e l y c o u p l e d plasma (ICP) and d i r e c t c u r r e n t plasma (DCP) are b o t h e m i s s i o n s o u r c e s w h i c h have become p o p u l a r i n r e c e n t y e a r s . The n o i s e s w h i c h l i m i t d e t e c t i o n u s i n g t h e s e e m i s s i o n s o u r c e s are e a s i l y c h a r a c t e r i z e d . W i t h v e r y low o p t i c a l throughput, such as when narrow s l i t w i d t h s a r e used i n the f a r UV, p h o t o m u l t i p l i e r dark c u r r e n t n o i s e may be s i g n i f i c a n t . However, i n most c a s e s , s h o t n o i s e i n d u c e d by the s o u r c e background r a d i a t i o n , o r f l i c k e r n o i s e c a r r i e d by the s o u r c e background are l i m i t i n g . The background i n t e n s i t y may r e s u l t from argon e m i s s i o n i n the s o u r c e o r may be i n d u c e d by i n t e r a c t i o n o f the s o u r c e w i t h the sample m a t r i x . I n the case o f f l i c k e r n o i s e , t h a t i s , the f l u c t u a t i o n i n the background i n t e n s i t y , the n o i s e u s u a l l y r e s u l t s from t e m p o r a l v a r i a t i o n s i n the sample t r a n s p o r t system o r the e x t e r n a l gas f l o w s . The major q u e s t i o n when comparing d e t e c t i o n l i m i t s u s i n g e m i s s i o n t e c h n i q u e s i s whether the s i g n a l - t o - b a c k g r o u n d r a t i o (SBR) o r the s i g n a l - t o - n o i s e r a t i o (SNR) was used as the measure o f d e t e c t i o n l i m i t . The SBR r e q u i r e s a measure o f the c o n c e n t r a t i o n c o r r e s p o n d i n g t o a m u l t i p l e o f the background i n t e n s i t y , r a t h e r than the n o i s e , and thus r e q u i r e s o n l y one measurement o f background. The measurement o f background i s u s u a l l y made i n the presence o f the a n a l y t e - c o n t a i n i n g sample by measuring a t a w a v e l e n g t h slightly o f f s e t from the wavelength o f the a n a l y t e i n t e n s i t y maximum. I n a m u l t i e l e m e n t system, i t i s thus much s i m p l e r t o m o n i t o r i n s t r u m e n t performance by measuring the SBR f o r each c h a n n e l , r a t h e r t h a n the SNR, w h i c h would r e q u i r e m u l t i p l e measurements. The d e t e c t i o n l i m i t i s t h e n c a l c u l a t e d by assuming t h a t the b a c k g r o u n d - c a r r i e d f l i c k e r n o i s e i s l i m i t i n g and t h a t t h e r e i s a c o n s t a n t r e l a t i v e s t a n d a r d d e v i a t i o n o f the background e m i s s i o n , u s u a l l y about one p e r c e n t . The l i m i t a t i o n t o t h i s procedure has been c l e a r l y p o i n t e d out by Boumans [ 1 9 ] , who d e s c r i b e s the r e l a t i o n s h i p o f the relative s t a n d a r d d e v i a t i o n o f the background t o the f l i c k e r n o i s e and shot n o i s e components by the f o l l o w i n g e q u a t i o n : (RSD)

B

where (RSD)

B

= (a

2 B

+ g/Vx )^

2

B

= observed r e l a t i v e s t a n d a r d d e v i a t i o n o f the background e m i s s i o n .

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

O)

6.

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ag = f l i c k e r f a c t o r i n d u c e d by v a r i a t i o n s i n i n s t r u m e n t a l components such as n e b u l i z e r o r gas flow controls. xg — measured background s i g n a l i n u n i t s o f anode current. g - photomultiplier gain f$ — c o n s t a n t c o e f f i c i e n t w h i c h i n c l u d e s components due to the e f f e c t i v e system n o i s e b a n d w i d t h , the e l e c t r o n i c charge, and g a i n f l u c t u a t i o n s due t o secondary e l e c t r o n e m i s s i o n . The v a l i d i t y o f assuming a c o n s t a n t s t a n d a r d d e v i a t i o n o f the background e m i s s i o n depends on the dominance o f background f l i c k e r noise. W i t h t h a t n o i s e , w h i c h Boumans p o i n t s out i s l i m i t i n g a t w a v e l e n g t h s g r e a t e r t h a n 300 nm, the SNR and thus the d e t e c t i o n l i m i t can be c h a r a c t e r i z e d by the SBR d i v i d e d by a f l i c k e r f a c t o r , ag the f i r s t term i n e q u a t i o n 3. The f l i c k e r f a c t o r w i l l be a f u n c t i o n o f a p a r t i c u l a r i n s t r u m e n t and w i l l have a magnitude w h i c h depends on the s t a b i l i t y o f v a r i o u s i n s t r u m e n t components. Thus, as l o n g as f l i c k e r n o i s e i s l i m i t i n g and the f l i c k e r f a c t o r does not change, the a p p r o x i m a t i o n i s v a l i d . D e v i a t i o n s from the a s s u m p t i o n occur a t s h o r t e r w a v e l e n g t h s , where the spectrometer optical t h r o u g h p u t and plasma background i n t e n s i t y d e c r e a s e . The background s h o t n o i s e i n t e n s i t y , r e p r e s e n t e d by the second term i n Boumans' e q u a t i o n , ( g ^ / x g ) ^ , w i l l make a s i g n i f i c a n t c o n t r i b u t i o n t o the v a r i a t i o n o f the background i n t e n s i t y , and the s i m p l e r e l a t i o n s h i p o f f l i c k e r f a c t o r t o SNR mentioned p r e v i o u s l y b r e a k s down. Thus, d e t e c t i o n l i m i t s c a l c u l a t e d from SBR's w i t h o u t c o n s i d e r a t i o n o f shot n o i s e may be i n e r r o r . A b s o r p t i o n N o i s e Sources. N o i s e s i n atomic a b s o r p t i o n s p e c t r o s c o p y a r e more complex than i n e m i s s i o n . When a source o f r a d i a t i o n i s i n t r o d u c e d , whose a t t e n u a t i o n c a r r i e s the a n a l y t e i n f o r m a t i o n , s e v e r a l new l i m i t i n g n o i s e s o u r c e s are i n t r o d u c e d . F l i c k e r noise due t o e m i s s i o n from the h i g h temperature atomic v a p o r c e l l i s not as s i g n i f i c a n t as i t i s i n e m i s s i o n t e c h n i q u e s , because atomic a b s o r p t i o n uses source m o d u l a t i o n t o d i s c r i m i n a t e a g a i n s t such n o i s e by encoding the a n a l y t e i n f o r m a t i o n s i g n a l a t a h i g h frequency. Shot n o i s e i s s t i l l observed as a r e s u l t o f background e m i s s i o n from the flame o r from sample m a t r i x components, b u t no s i g n i f i c a n t f l i c k e r n o i s e i s measured. However, new n o i s e s a r e s h o t and f l i c k e r from the r a d i a t i o n s o u r c e , flame t r a n s m i s s i o n f l i c k e r n o i s e w h i c h becomes l i m i t i n g a t wavelengths l e s s t h a n 230 nm, and m o l e c u l a r a b s o r p t i o n o r s c a t t e r n o i s e from sample m a t r i x components. A l l o f the f l i c k e r n o i s e s can be e f f e c t i v e l y e l i m i n a t e d by the use of double-beam o p t i c s i n c o n j u n c t i o n w i t h a background c o r r e c t i o n system such as Zeeman s p l i t t i n g or a w e l l - a l i g n e d ( o r wavelength-modulated) continuum s o u r c e . Thus the u l t i m a t e l i m i t i n g n o i s e i n atomic a b s o r p t i o n i s source shot n o i s e , w h i c h can be r e d u c e d ( r e l a t i v e t o t o t a l source i n t e n s i t y o r I ) by i n c r e a s i n g the source i n t e n s i t y , up t o the p o i n t o f o p t i c a l s a t u r a t i o n . Table I I I presents some examples o f l i m i t i n g n o i s e s in d i f f e r e n t atomic a b s o r p t i o n d e t e r m i n a t i o n s . These measurements are a c o m p i l a t i o n o f i n f o r m a t i o n from s e v e r a l s o u r c e s , b u t p r i m a r i l y from the work o f I n g l e [20,21] u s i n g a v e r y s i m p l e , single-beam Q

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

DETECTION IN ANALYTICAL CHEMISTRY

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atomic absorption spectrometer. Aluminum in a nitrous oxide-acetylene flame i s l i m i t e d by flame transmission f l i c k e r and source f l i c k e r . The flame transmission f l i c k e r r e s u l t s from absorption by molecular species (e.g., OH). Barium i n a similar flame, but with a double-beam instrument, i s l i m i t e d only by source induced shot noise. The double-beam system reduces the source f l i c k e r noise component. Calcium i n an air-acetylene flame i s l i m i t e d by both source f l i c k e r and shot noise, while i n the hotter nitrous oxide-acetylene flame, flame emission shot noise becomes greater than the source shot noise. The flame emission shot noise results from the intense molecular emissions of CN and CH i n the higher temperature flame. Copper i s l i m i t e d by both source f l i c k e r and shot noise at a one second integration time, but l i m i t e d by only f l i c k e r noise at a ten second integration time, a r e s u l t of the reduction of the shot noise component. An increase i n integration time w i l l improve the detection l i m i t i n a shot noise l i m i t e d case. Table I I I . Dominant Noises i n Atomic Absorption Spectroscopy

Element

Wavelength (nm)

Flame type

Limiting noise (Absorbance