Validation of the Measurement Process

5 Optimization of Experimental Parameters in Chemical

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 31, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0063.ch005

Analysis STANLEY N. D E M I N G Department of Chemistry, University of Houston, Houston, TX 77004

The t i t l e of this symposium, "Validation of the Measurement Process," may be interpreted in two ways: In its usual usage, "validation" means "the determination of the degree of validity of a [measurement process]"(1). This definition suggests an activity that takes place after the measurement process has been developed. If the evaluation is successful, the process will receive o f f i c i a l sanction, confirmation, or approval. An alternate meaning of "validation" is "to make valid" in the sense of "producing the desired result" (2); that i s , making the measurement process meet the criteria against which i t is to be evaluated. This definition suggests activity that takes place while the measurement process is being developed. This latter interpretation has been emphasized by Youden (3) and is the interpretation I wish to stress here i f the i n i t i a l development of a measurement process is carried out with the goal of meeting the evaluation c r i t e r i a , then the probability that the process will receive rapid approval is greatly increased. SYSTEMS THEORY Figure 1 shows a systems theory view of the measurement process. The primary input to the system is a s amp1e. The measurement process abstracts the desi rë^inTormation from the sample and transforms the information into a number. This number, or result, is the primary output from the system. 162 In Validation of the Measurement Process; DeVoe, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Experimental Parameters in Chemical Analysis

163


Ideally, the numerical value o f t h e output s h o u l d be r e l a t e d o n l y t o t h e d e s i r e d i n f o r m a t i o n i n the sample. As an example, a " p e r f e c t " measurement p r o c e s s f o r t h e a n a l y s i s o f enzyme a c t i v i t y w o u l d b e sensitive t o t h e amount o f enzyme i n t h e s a m p l e a n d i n s e n s i t i v e t o a l l other variables. In practice, the numerical value o f the output i s i n f l u e n c e d by a host o f other f a c t o r s . Some a r e associated with t h e sample m a t r i x , w h i l e o t h e r s app e a r as a d d i t i o n a l i n p u t s t o t h e measurement p r o c e s s . T h e s e f a c t o r s may b e s y s t e m a t i z e d a n d a r e shown s c h e m a t i c a l l y i n F i g u r e 1. An o b v i o u s c a t e g o r i z a t i o n o f t h e f a c t o r s a f f e c t i n g a measurement p r o c e s s i s t h e d i v i s i o n i n t o one set of factors t h a t a r e known t o h a v e a n e f f e c t o n the p r o c e s s ( s o l i d arrows) and a second s e t o f f a c tors that do a f f e c t t h e r e s u l t s o f t h e m e a s u r e m e n t p r o c e s s b u t have n o t y e t been i d e n t i f i e d - - that i s , they a r e unknown ( d a s h e d a r r o w s ) . Another grouping d i v i d e s the f a c t o r s i n t o those that are controlled (represented by a d o t on t h e t a i l o f t h e arrow) and those that are uncontrolled. When f a c t o r s a r e c a t e gorized i n these two ways, f o u r d i s t i n c t t y p e s r e sult : A factor that i s known t o e x e r t a s i g n i f i c a n t i n f l u e n c e on t h e r e s u l t o f a measurement process i s usually controlled. This w i l l u s u a l l y improve t h e p r e c i s i o n o f t h e method i f v a r i a t i o n s i n t h e uncontrolled factor level appear as n o i s e (that i s , t h e v a r i a t i o n s a r erapid with respect to the frequency o f measurement); i t might a l s o improve t h e accuracy o f the method i f t h e f r e q u e n c y o f c a l i b r a t i o n i s long with respect to variations i nthe uncontrolled factor level. Some factors a r e known t o i n f l u e n c e t h e r e s u l t of a measurement p r o c e s s b u t a r e l e f t uncontrolled. For example, i f a f a c t o r i s d i f f i c u l t o r e x p e n s i v e t o c o n t r o l and i f t h e f u n c t i o n a l r e l a t i o n s h i p o f i t s i n fluence i s known, t h e l e v e l o f t h i s f a c t o r m i g h t be measured and a c o r r e c t i o n a p p l i e d t o t h e r e s u l t . Or it might b e known t h a t a f a c t o r s i n f l u e n c e o n t h e r e s u l t , though r e a l , i s n o t s i g n i f i c a n t ; i t would p r o b a b l y be u n n e c e s s a r y t o c o n t r o l s u c h a f a c t o r . 1

Factors t h a t a r e unknown a n d c o n t r o l l e d a r e n o t u s u a l l y a p r o b l e m u n l e s s t h e method o f c o n t r o l i s i n -

In Validation of the Measurement Process; DeVoe, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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164

THE

MEASUREMENT

PROCESS


advertently changed. A f a m i l i a r example of a f a c t o r t h a t i s u n k n o w n and c o n t r o l l e d i s an impurity in a reagent: because the r e a g e n t i s always added i n a f i x e d amount, the l e v e l o f i m p u r i t y i s a l s o constant and is controlled. The e f f e c t s o f t h e r e a g e n t and t h e i m p u r i t y a r e c o n f o u n d e d and a r e u s u a l l y n o t s e p a rated unless a change i n r e a g e n t l o t or s u p p l i e r i s made. Most unknown factors are u n c o n t r o l l e d . It is a s s u m e d t h a t f a c t o r s i n t h i s c a t e g o r y do n o t o r will not exert a significant influence. Whatever i n f l u e n c e t h e y do e x e r t i s a c c e p t e d as " n o i s e " o r impreci sion. The s a m p l e c a n categories (_4) .

contain

RUGGEDNESS OF

factors

from a l l of

these

MEASUREMENT PROCESSES

As M a n d e l has p o i n t e d o u t , "The d e v e l o p m e n t o f a method of measurement i s to a l a r g e e x t e n t the d i s c o very o f the most i m p o r t a n t e n v i r o n m e n t a l f a c t o r s and the s e t t i n g of t o l e r a n c e s f o r the v a r i a t i o n of each one o f t h e m " (4Γ) . T o l e r a n c e s make p o s s i b l e t h e o p e r a t i o n a l i m p l e m e n t a t i o n of the concept of c o n t r o l : i t is o f t e n i m p o s s i b l e or i m p r a c t i c a l to c o n t r o l a f a c t o r at a g i v e n l e v e l , but i t i s u s u a l l y p o s s i b l e and practical t o c o n t r o l a f a c t o r w i t h i n a s p e c i f i e d do main of f a c t o r l e v e l s - - t h a t i s , to c o n t r o l a factor around a given level, within specified tolerances. The s p e c i f i c a t i o n o f f a c t o r t o l e r a n c e s i s based upon the r e q u i r e d p r e c i s i o n o f t h e m e t h o d and a n s w e r s t h e q u e s t i o n , "To w h a t e x t e n t c a n a f a c t o r be a l l o w e d to vary before the output o f t h e s y s t e m c h a n g e s by v_ amount?" F o r a s p e c i f i e d v a l u e o f y, i t i s d e s i r a b l e that t h e s e t o l e r a n c e s be b r o a d so that the measurement p r o c e s s i s r e l a t i v e l y i n s e n s i t i v e to small v a r i a t i o n s in factor levels. To i l l u s t r a t e , c o n s i d e r t h e rela tionship b e t w e e n r e a c t i o n r a t e ( t h e r e s u l t o f a mea s u r e m e n t p r o c e s s ) as a f u n c t i o n o f pH (a known and controlled factor) f o r the k i n e t i c d e t e r m i n a t i o n of enzyme a c t i v i t y ( s e e F i g u r e 2 ) . In g e n e r a l , enzymes do n o t f u n c t i o n w e l l a t e x t r e m e s o f pH and e x h i b i t an o p t i m u m w i t h r e s p e c t t o pH. Let us assume that a method i s t o be d e v e l o p e d f o r m e a s u r i n g t h e a c t i v i t y o f an e n z y m e . A p e r f o r m a n c e c r i t e r i o n has b e e n spe-



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i ν γ ν SAMPLE-

M

SYSTEM

-> R E S U L T

Figure 1. Systems theory view of the measurement process

Figu/e 2. Reaction rate as a function of pH for the kinetic determination of enzyme activity


165

166

V A L I D A T I O N O F T H E M E A S U R E M E N T PROCESS


cified that requires within y reaction rate

an i n t e r l a b o r a t o r y agreement units.

If t h e i n i t i a t i n g l a b o r a t o r y d e v e l o p s a method i n w h i c h t h e pH l e v e l i s s e t a t p o i n t A i n F i g u r e 2, then t h e i n t e r l a b o r a t o r y c o n t r o l o f pH m u s t b e e x tremely t i g h t : s m a l l d i f f e r e n c e s i n pH b e t w e e n t h e laboratories e v a l u a t i n g t h e m e t h o d w i l l show up a s a large between-laboratory variance. Worse still, in the absence o f a d d i t i o n a l i n f o r m a t i o n , i t w o u l d be d i f f i c u l t t o s e p a r a t e pH a s o n e o f t h e c a u s e s o f t h i s variance. If, instead, the i n i t i a t i n g laboratory suggests a m e t h o d i n w h i c h t h e pH l e v e l i s s e t a t p o i n t Β in Figure 2, t h e n s m a l l d i f f e r e n c e s i n pH b e t w e e n t h e l a b o r a t o r i e s e v a l u a t i n g t h e method will contribute very l i t t l e to the between-laboratory variance. This method would have a h i g h e r p r o b a b i l i t y o f b e i n g ac cepted a f t e r i t s f i r s t i n t e r l a b o r a t o r y t e s t . Many o f t h e f a c t o r s a f f e c t i n g measurement pro c e s s e s e x h i b i t t h e b e h a v i o r shown i n F i g u r e 2. Other factors initially increase and then a s y m p t o t i c a l l y approach a p l a t e a u (e.g., t h e s u b s t r a t e dependence o f many enzymes). With f a c t o r s e x h i b i t i n g these types of behavior, a d j u s t i n g the f a c t o r l e v e l s t o improve the system o u t p u t w i l l a l s o improve t h e f a c t o r t o l e r a n c e s (5) .

DEVELOPMENT OF MEASUREMENT PROCESSES The development o f a measurement p r o c e s s s h o u l d involve three stages: obtaining a response, improv ing t h e r e s p o n s e , and u n d e r s t a n d i n g t h e r e s p o n s e . Many l a b o r a t o r i e s c a r r y t h e development through the f i r s t stage o n l y . Youden (3) has p o i n t e d o u t t h e potential l i m i t a t i o n s o f s u c h m e t h o d s a n d h a s empha s i z e d t h e i m p o r t a n c e o f a c q u i r i n g an o p e r a t i o n a l un derstanding o f t h e measurement p r o c e s s e s ; t h a t i s , i d e n t i f y i n g and c o n t r o l l i n g those f a c t o r s that exert a significant effect on t h e s y s t e m . T h i s i s espe c i a l l y c r i t i c a l i f t h e measurement p r o c e s s e s are to become w i d e l y u s e d b y a number o f l a b o r a t o r i e s . The i m p r o v e m e n t o f r e s p o n s e h a s b e e n c a r r i e d o u t i n f r e q u e n t l y , a l t h o u g h i t s importance has been recog nized f o r some time. I n 1952 , B o x (6J) p r e s e n t e d a



5.


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167

p a p e r i n w h i c h i t was p o i n t e d o u t t h a t s i n g l e - f a c t o r at-a-time s t r a t e g i e s a r e inadequate f o r optimizing most chemical processes (.7,8_) a n d t h a t m e a s u r e m e n t processes (conceptually v e r y " s i m i l a r to production processes) c a n be e f f e c t i v e l y i m p r o v e d b y t h e u s e o f sequential f a c t o r i a l designs, a technique that later became known a s " e v o l u t i o n a r y o p e r a t i o n , " o r EVOP (9_, 10). EVOP s t r a t e g i e s a r e w e l l s u i t e d t o t h e i n d u s trial environment--the production process i s being run c o n s t a n t l y and p r o v i d e s a continuous framework for t h e l a r g e number o f e x p e r i m e n t s r e q u i r e d b y t h e sequential f a c t o r i a l designs. In the developmental l a b o r a t o r y , however, e f f i c i e n c y o f i n i t i a l experiment a t i o n i s s t r e s s e d a n d EVOP s t r a t e g i e s a r e l e s s d e s i rable. I n 1962, S p e n d l e y , H e x t , and H i m s w o r t h (11) i n t r o d u c e d t h e f i x e d s i z e s i m p l e x a s a more e f f i c i e n t sequential experimental design for t r a d i t i o n a l evolut i o n a r y o p e r a t i o n s ; L o n g (12) a p p e a r s t o have been the f i r s t t o a p p l y f i x e d s i z e s e q u e n t i a l s i m p l e x designs t o t h edevelopment o f measurement processes. Nelder a n d Mead (13) m o d i f i e d t h e s e q u e n t i a l s i m p l e x method t o a l l o w a c c e l e r a t i o n i n d i r e c t i o n s that are f a v o r a b l e and d e c e l e r a t i o n i n d i r e c t i o n s t h a t are unfavorable . We h a v e f o u n d t h e v a r i a b l e s i z e s i m p l e x ( s l i g h t l y m o d i f i e d ) t o b e a r a p i d means o f i m p r o v i n g r e s u l t s i n t h e development o f a n a l y t i c a l c h e m i c a l measurement processes (14-17). F a c t o r i a l d e s i g n s (18) , c e n t r a l composite d e s i g n s (19) , a n d B o x - B e h n k e n d e s i g n s ( 2 0 ) are u s e f u l f o r understanding the v a r i o u s f a c t o r ëTf e c t s upon t h e r e s p o n s e i n t h e r e g i o n o f t h e optimum.

EXAMPLE The d e t e r m i n a t i o n o f f o r m a l d e h y d e i n an aqueous sample c a n be d e t e r m i n e d by t h e a d d i t i o n o f chromotopic acid (4,5-dihydroxy-2,7-naphthalendisulfonic acid) and s u l f u r i c a c i d (21-25); a c o l o r develops, and t h e a b s o r b a n c e i s r e a d a t 570 nm. I n t h i s s t u d y ( 1 4 ) , a s a m p l e s i z e o f 2.00 m l was chosen. T h e amount o T a q u e o u s 20 g l " chromotropic acid (CTA, f a c t o r χι) a l l o w e d t o v a r y between 0.00 a n d 1.00 m l ; c o n c e n t r a t e d s u l f u r i c a c i d (H S0 , f a c t o r x ) c o u l d v a r y b e t w e e n 1.00 a n d 10.00 m l . T h e o b j e c t i v e s o f t h e study were: (a) t o determine t h e amounts o f H S 0 a n d CTA t h a t p r o d u c e d t h e g r e a t e s t a b s o r b a n c e f o r a g i v e n amount o f f o r m a l d e h y d e (2 ppm) 1

w

a

s

2

2

2

4


4


VALIDATION OF T H E M E A S U R E M E N T

PROCESS

7

I

1

0.1

1

0.2

1

1

ml o f

1

1

CTA

1

0.7

1

0.8

1

0.9

1

Analytica Chimica Acta

Figure 3. Simplex progress in the chromotropic acid-concentrated sulfuric acid domain. See text and Table 1 for details (14).


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169

TABLE 1


Simplex

Vertex

progress

Retained

Chronotropic

Sulfuric

vertices

acid

a c i d (ml)

1

-

0. .200

2. .00

0. .221

2

-

0. , 7 8 0

2. .18

0. . 0 8 0

3

1,2

0. ,355

2. .68

0. .531

- 0 . , 224

2. .50

- 1 .. o o o

-

(ml)

Absorbance

4

1,3

0. . 5 2 9

2. .26

0. ,223

5'

-

0. . 684

2. .94

0. .197

5

3,4

0. .321

2. .23

0. . 3 2 5

6

3,5

0. .147

2, .65

0. .563

6'

-

- 0 . .044

2. .85

- 1 .. o o o

7

3,6

0. .182

3. . 0 9

0. .562

8'

-

- 0 . .027

3, .07

- 1 .. o o o

8

6,7

0. . 2 6 0

2, .77

0. . 5 7 3

9'

-

0. . 2 2 6

2. . 33

0. .502

9

6,8

0.. 1 9 3

2. . 9 0

0.. 5 9 9

10'

-

0. . 3 0 5

3, .03

0.. 5 7 0

11

8,9

0. . 1 8 7

2. .74

0. . 584

a

Primes indicate

rejected

b

b

b

vertices.

^Boundary v i o l a t i o n . R e p r i n t e d from r e f e r e n c e 14 w i t h p e r m i s s i o n

of

Elsevier

Scientific

P u b l i s h i n g Company.


VALIDATION OF T H E M E A S U R E M E N T


TABLE 2 Results

of factorial

experiments

Chronotropic

Sulfuric

acid

acid

(ml)

Absorbance

(ml)

0. 524

2., 50

0.,538 0.. 515

2..80

0., 516 0 . 526

3..10

0., 530 0..455

2.. 50

0.. 509 2..80

0..583

3..10

0., 534

0., 575

0..545 0.. 386

2.. 50

0..428 0.. 545

2..80

0..537 0..554

3,.10

0.. 551

Reiminted Elsevier

i n part

from r e f e r e n c e

Scientific

Publishing

14 w i t h p e r m i s s i o n o f

Company.


PROCESS


5.

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Figure 4.

Cell mean plot of factorial study (14)


171

172

VALIDATION OF THE MEASUREMENT PROCESS

TABLE 3


F a c t o r i a l analysis of variance

(ANOVA)

Source o f

Degrees o f

Sum o f

Mean

variation

freedom

squares

square

Chromotropic

2

0.00369

0.00179

6.27

98.0

2

0.01926

0.00963

33.66

99.9

4

0.01670

0.00417

14.59

99.9

0.00258

0.00029

F-ratio

Significance (%)

acid Sulfuric acid Interaction

Error

Reprinted

i n part

from r e f e r e n c e

Elsevier S c i e n t i f i c Publishing

14 w i t h p e r m i s s i o n o f

Company.


Figure 5. Absorbance response surface as a function of chromotropic acid volume, and concentrated sulfuric acid volume (14)


5.

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173

and (b) t o u n d e r s t a n d t h e e f f e c t s o f H S 0 a n d CTA upon t h e r e s p o n s e i n t h e r e g i o n o f t h e optimum so t h a t f a c t o r t o l e r a n c e s c o u l d be s p e c i f i e d .


2

4

Figure 3 shows t h e p r o g r e s s o f t h e s i m p l e x t o ward t h e optimum ( d a t a i n T a b l e 1 ) . The numbers i n the f i g u r e i n d i c a t e t h e sequence i n which t h e r e t a i n ed v e r t i c e s w e r e e v a l u a t e d ; r e j e c t e d v e r t i c e s a r e n o t shown. Table 2 contains ther e s u l t s o f a t h r e e - l e v e l two-factor, f u l l factorial study with replication c a r r i e d o u t i n t h e r e g i o n o f t h e s i m p l e x optimum; t h e r e s u l t s o f theanalysis o f variance are presented i n T a b l e 3. I n F i g u r e 4, c e l l means a r e p l o t t e d vs. CTA f o r each o f t h e three l e v e l s o f H2SO4. Other studies ( 2 5 ) h a v e shown t h a t t h e a b s o r bance response i sr e l a t e d t o t h e r a t i o ( s u l f u r i c a c i d volume)/(total volume); this i s p r o b a b l y an e f f e c t caused by t h e heat o f m i x i n g w h i c h d r i v e s the reac tion toward completion. Assuming t h i s r e l a t i o n s h i p t o be a p p r o x i m a t e l y G a u s s i a n i n t h e r e g i o n o f t h e op timum, a model o f t h e form Absorbance = k ^ x p

[-( ( ( x / ( x + x 2

*(2.0/(x +x 1

+ 2

1

+ 2

2

2

2 . 0) ) - k ) ) / ( 2 k ) ] 2

2.0)) ( l - e x p f - k ^ ) )

can be f i t a n d i s v i s u a l i z e d i n t h e p s e u d o - t h r e e - d i m e n s i o n a l p l o t shown i n Figure 5. The f a c t o r i a l p o i n t s a r e superimposed on t h e s u r f a c e . At l o w H2SO4 v o l u m e s , i n c r e a s i n g t h e volume o f CTA moves a c r o s s t h e " f r o n t " o f t h e r e s p o n s e surface with the r e s u l t that response decreases. A t an i n t e r m e d i a t e l e v e l o f H^SO^, i n c r e a s i n g t h e v o l u m e o f CTA moves f r o m " b e h i n d " t h e d i a g o n a l r i d g e t o t h e t o p o f i t a n d down a g a i n o n t h e f r o n t s i d e . At the high est l e v e l o f H2SO4 s t u d i e d , i n c r e a s i n g t h e volume o f CTA moves a l o n g t h e " b a c k " o f t h e r i d g e w i t h t h e re s u l t that response increases. With t h i s o p e r a t i o n a l understanding o f a process f o r m e a s u r i n g t h e amount o f f o r m a l d e h y d e i n a n a q u e ous sample, f a c t o r l e v e l s and f a c t o r t o l e r a n c e s can be s p e c i f i e d . Because c o n c e n t r a t e d H S0i+ i s a w o r r y some reagent, a n d b e c a u s e a q u e o u s CTA s o l u t i o n s o f accurate concentration aree a s i l y prepared and han dled, i t w o u l d b e a p p r o p r i a t e t o s p e c i f y a t i g h t CTA l e v e l o f 0.1 m l w h e r e t h e v o l u m e o f Η 5 0 has l e s s o f an e f f e c t . 2

2

4


174

VALIDATION

OF T H E MEASUREMENT

PROCESS


CONCLUSION The s t r a t e g y o f o b t a i n i n g a response, improving the response, and u n d e r s t a n d i n g t h e response i s a reasonable means o f o b t a i n i n g s o u n d m e a s u r e m e n t p r o cesses. We h a v e f o u n d t h e v a r i a b l e size sequential s i m p l e x d e s i g n t o b e a n e f f i c i e n t means o f o p t i m i z i n g the primary response from a measurement process. Established s t a t i s t i c a l d e s i g n s a l l o w an understand ing o f t h e f a c t o r e f f e c t s and t h e i r interactions i n the r e g i o n o f t h e optimum.

ACKNOWLEDGMENTS The f o l l o w i n g have c o n t r i b u t e d t o t h e i d e a s and work p r e s e n t e d h e r e : P. G. K i n g , S. L. M o r g a n , L. R. Parker, J r . , A. S. O l a n s k y , a n d L. A. Y a r b r o . T h e author acknowledges support from t h e N a t i o n a l Science Foundation t h r o u g h g r a n t s GP-32911 a n d M P S - 7 4 - 2 3 1 5 7 .

LITERATURE CITED

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5.

DEMiNG

14. 15. 16.


17. 18. 19. 20. 21. 22. 23. 24. 25.


175

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Validation of the Measurement Process

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