Fundamentals and Applications of Chemical Sensors - American

16 Chemical Sensing Using Near-IR Reflectance Analysis David L. Wetzel

Downloaded by CORNELL UNIV on August 4, 2016 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch016

Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506

Near-infrared reflectance analysis is particularly well suited to chemical sensing because it operates on "as is" samples and yet has chemical specificity. The absorptions observed originate from vibrations of a relatively few chemical groups whose overtones and combination bands appear in the near infrared region. These groups are commonly found in natural and synthetic materials and their quantitation is possible at major component levels. Chemical sensing by near-infrared techniques does not usually require complete scanning since information for quantitation is found in select wavelength responses incorporated into a mathematical analytical expression. Correlation transformation between laboratory pre-analyzed samples (training set) and the optical data collected allows statistical wavelength selection and assignment of regression coefficients. The result is a select calibration for an analyte in a particular matrix or commodity. Quantitative chemical sensing occurs when the built-in computer of the near-infrared analyzer simultaneously calculates multiple component concentrations by solving prediction equations which use multiple wavelength intensity ratioed data for individual samples. Similarly, qualitative sensing information by discriminant analysis is obtained by use of a discrete wavelength multiterm function. Preselected wavelengths and dedicated preprogrammed microcomputers are ideal for numerous sensing and ultimately control functions. Chemical

sensing applied to "as i s " material requires an analytical system which i s tolerant of the sample as i t e x i s t s . Thus, the analyte must be determined i n the matrix i n which i t i s found. This precludes concentration steps, d i l u t i o n steps, sample p u r i f i c a t i o n or workup of any sort and p a r t i c u l a r l y makes sophisticated a n a l y t i c a l separations prior to monitoring impractical. Direct absorption spectroscopic sensing, i n some cases, f u l f i l l s the above requirements. In many cases, however, lack 0097-6156/86/0309-0271$07.50/0 © 1986 American Chemical Society

Schuetzle and Hammerle; Fundamentals and Applications of Chemical Sensors ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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o f s p e c i f i c i t y f o r d e t e r m i n a t i o n o f i n d v i d u a l a n a l y t e s i n a complex m a t r i x would appear t o be an almost insurmountable o b s t a c l e t o t h i s approach. I f t h e sample "condition" t o be sensed has l i g h t s c a t t e r i n g and/or s p e c t r o s c o p i c a b s o r p t i o n c h a r a c t e r i s t i c s i t may be p o s s i b l e t o make m u l t i c h a n n e l ( m u l t i w a v e l e n g t h ) measurement work. Through s t a t i s t i c a l means and c o r r e l a t i o n t r a n s f o r m a t i o n o f the spectroscopic data, q u a n t i t a t i v e chemical sensing i s p o s s i b l e . This i s t r u e , however, o n l y i n cases which permit good m a t r i x (background) c o r r e c t i o n and o p t i c a l measurement c o n d i t i o n c o r r e c t i o n which a l l o w l i n e a r a n a l y t i c a l response i n a reasonable range. I n n e a r - i n f r a r e d r e f l e c t a n c e , t h e m a t r i x c o r r e c t i o n i s made p o s s i b l e by t h e a d d i t i v i t y o f each weighted c o n t r i b u t i o n f o r t h a t wavelength i n t h e spectrum b e i n g observed. The low a b s o r p t i v i t y i n the n e a r - i n f r a r e d c o n t r i b u t e s t o t h i s necessary " a d d i t i v i t y " and e l i m i n a t e s , a l s o , the n e c e s s i t y o f sample d i l u t i o n p r i o r t o s e n s i n g . U n l i k e c l a s s i c a l a n a l y t i c a l s p e c t r o s c o p y performed on l i q u i d s or d i l u t e s o l u t i o n s o f a n a l y t e s , d i f f u s e r e f l e c t a n c e measurement i n the n e a r - i n f r a r e d must d e a l w i t h a composite e f f e c t o f s p e c t r o s c o p i c a b s o r p t i o n and s c a t t e r i n g from t h e a n a l y t e and the m a t r i x i n which i t i s found. D i f f e r e n c e s i n r e f r a c t i v e i n d i c e s o f t h e sample m a t e r i a l , s p e c u l a r r e f l e c t i o n and observance o f r e l a t i v e l y s m a l l d i f f e r e n c e s are a l l d e a l t with i n t h i s technique. Flow through s p e c t r o s c o p i c c e l l s f o r l i q u i d s have been used f o r decades f o r u l t r a v i o l e t , v i s i b l e , i n f r a r e d and f l u o r e s c e n c e o n - l i n e spectroscopic monitoring. The same i s t r u e f o r m o n i t o r i n g gases. F l u i d s , as t h e name i m p l i e s , present o n l y minor sample t r a n s p o r t problems but d e a l i n g w i t h g r a n u l a r m a t e r i a l s p r e s e n t s a c h a l l e n g e . C o l o r meters have been used f o r g r a n u l a r m a t e r i a l s f o r decades f o r c l i n i c a l , i n d u s t r i a l and o t h e r a p p l i c a t i o n s . U n f o r t u n a t e l y there a r e l i m i t e d cases where samples can be a n a l y z e d d i r e c t l y w i t h o u t p r i o r r e a c t i o n w i t h s p e c i a l r e a g e n t s . The responses o f i n t e n s e e l e c t r o n i c s p e c t r a a r e not n e c e s s a r i l y l i n e a r even i f s p e c t r o s c o p i c r e s o l u t i o n provides s e l e c t i v i t y , hence quantitation i s d i f f i c u l t . S i m i l a r l y , autofluorescence, coupled w i t h s e l e c t i v e e x c i t a t i o n and o b s e r v a t i o n wavelengths has been useful. Fluorophores, s e l e c t i v e l y coupled t o a p o r t i o n o f t h e s a m p l e , h a v e a l l o w e d t h e a r t o f f l u o r e s c e n c e m i c r o s c o p y t o be r e f i n e d and add the d i m e n s i o n o f c h e m i c a l s e l e c t i v i t y ( 1 ) . A d d i t i o n o f reagents p r i o r t o measurement d e t r a c t s from the p r a c t i c a l i t y o f e i t h e r f l u o r e s c e n c e o r a b s o r b t i o n f o r s e n s i n g purposes. Autof l u o r e s c e n c e o f f i n e l y d i v i d e d s o l i d s i n a v i s c o u s suspension has been used. Q u a n t i t a t i o n , i n t h i s case i n v o l v i n g wheat m i l l i n g f r a c t i o n s , has been enhanced by s t a t i s t i c a l d a t a treatment methods (2). These i n c l u d e p a r t i a l l e a s t squares, f a c t o r a n a l y s i s , o r p r i n c i p l e component a n a l y s i s . Such d a t a f i t t i n g techniques p r o v i d e compensation f o r measurement o r sample m a t r i x c o n d i t i o n s which would o t h e r w i s e compromise the l i n e a r i t y o f t h e s e n s i n g response. To a c h i e v e q u a n t i t a t i v e s e n s i n g by n e a r - i n f r a r e d r e f l e c t a n c e t e c h n i q u e s t h e wavelengths most r e s p o n s i v e t o t h e a n a l y t e must be selected. The a n a l y t i c a l e q u a t i o n produced by r e g r e s s i o n should p r o v i d e a weighted term t o account f o r d i f f e r e n c e s i n r e l a t i v e absorptivities. Method development i n c l u d i n g wavelength s e l e c t i o n , t r a i n i n g s e t s e l e c t i o n and e q u a t i o n t e s t i n g w i l l be t r e a t e d i n a l a t t e r s e c t i o n o f t h i s paper. Sample p r e s e n t a t i o n and an example


16.

Use of Near-IR Reflectance Analysis

WETZEL

Table I .

Uses o f N e a r - I n f r a r e d

Sensing

Sensing F u n c t i o n


273

Action

on/off

switching

go/no go

accept/reject

semi-quant i t a t i o n

ingredient s p e c i f i c a t i o n s (blend

quantitation

product s p e c i f i c a t i o n s ( b u y / s e l l )

( a l a r m , TV channel change) ( t o o wet

to

store) formulation)

o f a working s o l i d sample o n - l i n e n e a r - i n f r a r e d s e n s i n g a l s o be d e t a i l e d as a case h i s t o r y . Comparison o f N e a r - I n f r a r e d S p e c t r o s c o p i c Techniques

Reflectance

Analysis

Characteristics specific to n e a r - i n f r a r e d a n a l y s i s vs c l a s s i c a l s p e c t r o m e t r y :

with

(diffuse)

system

will

Classical

reflectance

n e a r - i n f r a r e d compared to m i d - i n f r a r e d d i f f u s e r e f l e c t a n c e compared to t r a n s m i t t a n c e neat samples compared to s o l u t i o n s p e c t r o m e t r y c o r r e l a t i o n t e c h n i q u e s f o r wavelength s e l e c t i o n d a t a base f o r e m p i r i c a l a n a l y t i c a l c o e f f i c i e n t s

The n e a r - i n f r a r e d r e g i o n o f t h e s p e c t r u m t h a t u s e s a l e a d s u l f i d e d e t e c t o r s p a n s a p p r o x i m a t e l y 1100 t o 2500 n a n o m e t e r s . E x c e l l e n t c l a s s i c a l s p e c t r o s c o p i c e x p l o r a t i o n and r e v i e w o f t h i s r e g i o n preceeded i t s c u r r e n t p r a c t i c a l use ( 3 - 5 ) . The v i b r a t i o n a l o v e r t o n e and c o m b i n a t i o n a b s o r p t i o n bands i n t h i s r e g i o n are b r o a d , they are o v e r l a p p i n g w i t h each o t h e r e s p e c i a l l y f o r s o l i d s and the a b s o r p t i o n s are e x t r e m e l y weak. T h i s i s i n comparison to the mid i n f r a r e d which has sharp w e l l d e f i n e d bands which are q u i t e o f t e n r e s o l v e d from each o t h e r , at l e a s t f o r pure compounds, and which have a h i g h i n t e n s i t y . At f i r s t c o n s i d e r a t i o n , these d i s a d v a n t a g e s of i n s e n s i t i v i t y brought about by low a b s o r p t i v i t y and the l a c k o f r e s o l u t i o n t h a t one f i n d s w i t h b r o a d bands w o u l d seem t o be a d e v a s t a t i n g blow to the n e a r - i n f r a r e d t e c h n i q u e . The f a c t i s , however, t h a t the weak a b s o r p t i v i t y i n t h i s r e g i o n of the spectrum i s an advantage because the n e c e s s i t y t o d i l u t e a s o l i d sample i s avoided ( 6 ) . The absorbance of a major component i n a neat, g r a n u l a t e d sample w i l l , i n f a c t , appear i n the range o f 0.4 - 1.0 absorbance u n i t s and n o n - l i n e a r i t y due to e x t r e m e l y strong a b s o r b t i o n i s u s u a l l y not a problem. Another advantage o f the weak a b s o r b t i o n i s t h a t when d o i n g d i f f u s e r e f l e c t a n c e , i t becomes n e c e s s a r y to c o r r e c t f o r the s p e c u l a r r e f l e c t a n c e component. The



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low a b s o r p t i o n , coupled with a r e l a t i v e l y constant index of refraction among members of a sample set makes the mathematics of correction for the specular component of reflectance quite simple. A linear multiterm spectroscopic response as a function of the concentration of the analyte i s an achievable goal ( 7 ) . However, a d d i t i v i t y of the spectroscopic response at the same wavelength of one component to the other component of the sample matrix i s absolutely e s s e n t i a l . This is due to the fact that with a neat sample, i n f i n i t e d i l u t i o n does not exist as in solution spectrophotometry for which c l a s s i c a l laws were developed. Since the bands are broad, i s o l a t i n g an analyte band from the bands of the matrix i s v i r t u a l l y impossible. Therefore, a d d i t i v i t y i s essential to make quantitation possible. Unless one i s applying this technique to pure chemicals, there i s nearly always a serious background problem. Although the problem i s serious, i t can be dealt with readily. Spectral subtraction i s not used but rather a s t a t i s t i c a l technique is employed for this purpose. The c o e f f i c i e n t s on each term in the equation w i l l , in fact, include the background correction for other materials present in the sample and in a number of samples constituting a set, the magnitude of each of these terms and the r a t i o of one of these c o e f f i c i e n t s to another is determined by empirical means ( 8 ) . Multiple linear regression i s one of the primary tools. With the multiple wavelength expression, i t i s in fact possible to have a non-linear effect which would appear in a single term by i t s e l f , be offset by a non-linear effect in a different term in the opposite d i r e c t i o n ( 9 ) . Although perfect compensation cannot always be predicted, adherence to t h i s expression of the analyte concentration i s always tested for and standard e r r o r of d e t e r m i n a t i o n i s c a l c u l a t e d on the b a s i s of testing the equation, complete with a sample preparation and measurement technique, on a set of known samples. This test i s , in fact, a part of the analytical method development for any purpose. In granular solids or in analysis of liquids or s l u r r i e s in which a considerable amount of particulate material exists, the scattering effect attenuates the optical signal in addition to the absorption. Scattering back from the body of the sample toward the s u r f a c e produces the i n t e n s i t y to be measured as d i f f u s e reflectance. Scattering also controls the depth of penetration of the sample as well as does i t s absorptivity ( 1 0 ) . The complexity of these two factors acting at once is d i f f i c u l t to predict a p r i o r i . This i s another reason why the empirical method and the empirical equation c o e f f i c i e n t s produced by a training set are essential. Unlike transmission spectroscopic techniques involving true solutions or pure l i q u i d s , the diffuse reflectance becomes angle dependent for incident radiation and for purposes of c o l l e c t i n g d i f f u s e l y reflected radiation p r e f e r e n t i a l l y over the specularly reflected radiation. The s p e c u l a r component i s minimized by experimental design, i f possible, but the unavoidable specular contribution requires correction. Not only does the presence of the specular component simply increase the total amount of radiation h i t t i n g the detector, but i t obscures the finding of information from the i n t e n s i t y of the s i g n a l . As the s p e c u l a r component approaches metallic r e f l e c t i o n , i t can, i n fact, produce an intensity c o n t r i b u t i o n opposite i n s i g n to that of d i f f u s e


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reflectance. This needless to say completely r u i n s chemical quantitation or at least, adversely affects the desired linear relationship. The chemical sensing by near-infrared reflectance a n a l y s i s thus i n v o l v e s c o r r e l a t i o n spectroscopy as opposed to c l a s s i c a l , n e a r - i n f r a r e d i n s t e a d of m i d - i n f r a r e d and d i f f u s e reflectance scattering considerations instead of simple transmission.


Correlation Techniques For Wavelength Selection Diffuse reflectance d i f f e r s from c l a s s i c a l transmission in which no particulate matter exists to scatter the beam of radiation. It i s necessary to c o n t r a s t correlation spectroscopy ( c o r r e l a t i o n a n a l y t i c a l techniques based on spectroscopic measurements) to a c l a s s i c a l , one wavelength, monochromatic application of Beer's law. The use of multiple wavelengths produces a multiterm a n a l y t i c a l equation in reflectance R of the general type: % Analyte = z + a log 1/Rj + b log 1/R

2

+ c log I / R 3

+...

In such an equation, at least one term, i s used as an indicator wavelength. The i n d i c a t i o n evidenced by a b s o r p t i o n could be p o s i t i v e or n e g a t i v e . An equation may use the wavelength at which the analyte absorbs. A positive c o e f f i c i e n t w i l l exist on that term. In a closed system, such as wheat, which i s constituted of approximately 70% starch, with the other 30% divided between protein and moisture and a few minor components, a lower quantity of an absorber such as starch could be an inverse indicator of the presence of protein. It i s common when measuring protein i n a high starch system, to u t i l i z e a wavelength at which the carbohydrate absorbs and to employ a negative c o e f f i c i e n t on that term. Thus, those two terms have e s s e n t i a l l y a push p u l l effect and add to the s e n s i t i v i t y . It i s nearly always essential to have at least one reference term, to show the overall intensity level of the b a s e l i n e . Use of r e f e r e n c e wavelengths provides some mathematical assistance to avoid baseline s h i f t due to scattering effects from d i f f e r e n t p a r t i c l e size in the case of a granular sample or a d i f f e r e n t p a r t i c l e population in the case of a s l u r r y . Two wavelengths may be chosen close together which would indicate a slope. In such a case, the difference between absorbance at those two wavelengths would appear in the a n a l y t i c a l expression. In other cases, a baseline corrected peak height for a p a r t i cular absorber may be employed as a term i n the equation. In such a case, the wavelength difference on either side of a peak maximum w i l l affect the contribution of that complex term. That increment or gap, in fact, under such circumstances becomes a part of the calibration. It i s as important a contribution to the c a l i b r a t i o n as the c o e f f i c i e n t s on the wavelength terms. In this c o r r e l a t i o n spectroscopy, c l a s s i c a l band assignments are not always possible. L i t t l e s p e c i f i c near-infrared l i t e r a t u r e exists in advance of most applications and i t i s not always possible to predict which wavelengths w i l l produce the best l i n e a r i t y and the best s e n s i t i v i t y for a given a n a l y t i c a l problem. In the empirical approach a variety of s t a t i s t i c a l treatments have been attempted. By f a r the most



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commonly used i s m u l t i p l e l i n e a r r e g r e s s i o n . T h i s can be done w i t h l i m i t e d s p e c t r o s c o p i c d a t a o b t a i n e d from f i l t e r i n s t r u m e n t s o r w i t h hundreds o f data p o i n t s o b t a i n e d from scanning instruments. M u l t i p l e l i n e a r r e g r e s s i o n may be performed on a " s t e p up b a s i s " where one b e g i n s w i t h t h e wavelength most h i g h l y c o r r e l a t e d and s u b s e q u e n t l y adds o t h e r h i g h l y c o r r e l a t e d wavelengths as w e l l as a r e f e r e n c e wavelength. I n t h e s t e p up procedure when a d d i t i o n o f f u r t h e r wavelength d a t a does n o t improve t h e c o r r e l a t i o n o r reduce the s t a n d a r d e r r o r o f c a l i b r a t i o n , then no f u r t h e r wavelengths a r e used. Reverse s t e p w i s e m u l t i p l e l i n e a r r e g r e s s i o n has a l s o been used, p a r t i c u l a r l y when d e a l i n g w i t h a f i l t e r i n s t r u m e n t which i s c a p a b l e o f measurement a t a l i m i t e d , r e l a t i v e l y s m a l l , number o f wavelengths. An example would be t o perform a m u l t i p l e l i n e a r r e g r e s s i o n on a sample s e t which i s g r e a t e r i n s i z e than t h e number of f i l t e r s and t o repeat t h e p r o c e s s by d i s c a r d i n g t h e wavelength which had t h e l e a s t impact on t h e p r e v i o u s r e g r e s s i o n . Typically, the Student t - t e s t on each i n d i v i d u a l term would be t h e c r i t e r i o n f o r e l i m i n a t i n g t h a t term. A f t e r a s e r i e s o f s t e p s , f o r example, the 19 i n i t i a l wavelengths used c o u l d be reduced down t o the p o i n t at which t h e c o r r e l a t i o n degrades s i g n i f i c a n t l y , o r t h e s t a n d a r d e r r o r o f c a l i b r a t i o n i n c r e a s e s s i g n i f i c a n t l y , then t h e r e v e r s e s t e p w i s e p r o c e s s would be stopped. I t i s a l s o p o s s i b l e w i t h computers t o perform many r e g r e s s i o n s s e q u e n t i a l l y and a u t o m a t i c a l l y . With l e s s than 100 d a t a p o i n t s on a r a p i d computer, o r l e s s than 20 on a slow computer, i t i s p o s s i b l e to r o u t i n e l y r e g r e s s a l l p o s s i b l e c o m b i n a t i o n s o f t h r e e wavelengths from a f i e l d o f 100 o r a f i e l d o f 20, r e s p e c t i v e l y , and s e l e c t t h e best combination. Computer programs a r e a l s o a v a i l a b l e f o r a l l p o s s i b l e c o m b i n a t i o n s o f f i v e o r more. However, a t some p o i n t , s i n c e t h e r e i s a g e o m e t r i c p r o g r e s s i o n i n the number o f c o m b i n a t i o n s as one adds a d d i t i o n a l terms, t h e computer time r e q u i r e d i s not n e c e s s a r i l y w a r r a n t e d f o r t h e s m a l l improvement i n the r e s u l t s . I n a d d i t i o n t o the m u l t i p l e l i n e a r r e g r e s s i o n , o t h e r t e c h n i q u e s such as p a r t i a l l e a s t s q u a r e s , f a c t o r a n a l y s i s ( p r i n c i p a l components) and F o u r i e r t r a n s f o r m have been a p p l i e d t o n e a r - i n f r a r e d s p e c t r o s c o p i c data. Q u a l i t a t i v e s e n s i n g based on d i s c r i m i n a n t a n a l y s i s d a t a treatment a l s o shows promise ( 1 1 , 1 2 ) . I n s t r u m e n t a t i o n F o r N e a r - I n f r a r e d Sensors T u n g s t e n - q u a r t z - h a l o g e n sources and lead s u l f i d e d e t e c t o r s a r e common i n most n e a r - i n f r a r e d c h e m i c a l s e n s o r s . The e x c e p t i o n t o t h i s i s t h e use o f independent l i g h t e m i t t i n g d i o d e s as i n d i v i d u a l monochromatic sources ( T r e b o r I n d . , G a i t h e r s b e r g , MD) i n t h e v e r y n e a r - i n f r a r e d where s i l i c o n d e t e c t o r s c a n be u s e d . With the e x c e p t i o n o f wavelength s e l e c t i o n by a c h o i c e o f LED sources i n t e r f e r e n c e f i l t e r s a r e used i n the most r o u t i n e i n s t r u m e n t s t o s e l e c t the a p p r o p r i a t e wavelengths from t h e continuum. The main d i f f e r e n c e i n f i l t e r i n s t r u m e n t s a r e : 1. The mechanism f o r wavelength change, 2. The mechanism and t i m i n g a s s o c i a t e d w i t h r e f e r e n c i n g t h e s i g n a l i n t e n s i t y r e f l e c t e d from t h e sample t o t h e i n t e n s i t y o f f o f a standard r e f l e c t o r , 3. The way i n w h i c h d i f f u s e l y r e f l e c t e d radiation i s collected. W a v e l e n g t h ( f i l t e r s e l e c t i o n ) i s a c c o m p l i s h e d by m o v i n g a



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d i s c r e t e f i l t e r i n t o t h e beam ( p e r p e n d i c u l a r t o t h e beam) f o r accumulating r e f l e c t a n c e measurement a t t h a t wavelength before s e l e c t i n g the next wavelength w i t h another d i s c r e t e f i l t e r . The a l t e r n a t i v e approach i n v o l v e s a s e r i e s o f r a p i d l y t i l t i n g filters which accumulate d a t a a t numerous wavelengths (depending on the angle t o the beam) w i t h each r o t a t i o n c y c l e . With t h i s mechanism, a c c u m u l a t i o n o f d a t a r e q u i r e s summation o f i n t e n s i t i e s a t each wavelength from m u l t i p l e r o t a t i o n s ( F i g u r e 1 ) . I n p r a c t i c e , r e f l e c t a n c e i s the r a t i o o f i n t e n s i t y o f f o f the sample t o i n t e n s i t y o f f o f a s t a n d a r d r e f l e c t i n g s u r f a c e . A clean ceramic surface or a d i f f u s e gold m i r r o r are t y p i c a l standard reflectors. I n t e n s i t y r e f e r e n c i n g may be done a t each d i s c r e t e f i l t e r p o s i t i o n immediately f o l l o w i n g t h e sample intensity measurement u s i n g a moveable m i r r o r . The r e s u l t i n g r a t i o o f i n t e n s i t i e s i s s t o r e d i n memory f o r each p r e s e l e c t e d wavelength before measurement a t t h e n e x t w a v e l e n g t h . I n an a l t e r n a t e instrumental design the system s t o r e s r e f e r e n c e i n t e n s i t i e s a t m u l t i p l e wavelengths w i t h t h e s t a n d a r d r e f l e c t o r and c o l l e c t s sample i n t e n s i t i e s a t m u l t i p l e wavelengths on a separate o p t i c a l pass w i t h the sample i n p l a c e . G r a t i n g monochromators w i t h an f = 2.0 o r b e t t e r and i n t e r f e r o m e t e r s a r e used f o r l a b o r a t o r y e x p e r i m e n t a t i o n and method development i n near i n f r a r e d r e f l e c t a n c e a n a l y s i s but a r e u n l i k e l y t o be used as r o u t i n e c h e m i c a l s e n s o r s . C o l l e c t i o n and measurement o f d i f f u s e l y r e f l e c t e d r a d i a t i o n s c a t t e r e d back from the sample i s accomplished i n s e v e r a l ways. A c o l l i m a t e d beam o f r a d i a t i o n from t h e f i l t e r s t r i k e s t h e sample s u r f a c e normal t o the p l a n e o f the s u r f a c e . Reflected radiation i n t e n s i t y f r o m t h e s a m p l e w i l l v a r y d e p e n d i n g on t h e a n g l e o f observation. I t may a l s o d i f f e r w i t h d i r e c t i o n p a r t i c u l a r l y i f t h e s u r f a c e has t e x t u r e . A l l o p t i c a l d e t e c t i o n systems attempt t o m i n i m i z e s p e c u l a r r e f l e c t i o n and maximize d i f f u s e r e f l e c t i o n which reaches the d e t e c t o r . One system employs an i n t e g r a t i n g sphere t o c o l l e c t d i f f u s e r e f l e c t i o n a t a maximum s o l i d angle but e x c l u d e s the cone s u r r o u n d i n g the i n c i d e n t beam where s p e c u l a r r e f l e c t i o n has t h e g r e a t e s t i n t e n s i t y . R a d i a t i o n e n t e r i n g the i n t e g r a t i n g sphere i s c o l l e c t e d on one o f two d e t e c t o r s l o c a t e d near the base of t h e sphere. Another d e s i g n uses a s i n g l e d e t e c t o r a t 45 degrees from the s u r f a c e and r o t a t e s the sample t o average out d i r e c t i o n a l differences. M u l t i p l e d e t e c t o r s p l a c e d on f o u r s i d e s a t an e l e v a t i o n o f 45 degrees a r e a l s o used. W i t h a l l t h r e e systems the geometry o f the c o l l e c t i o n a n g l e and t h e r e s u l t i n g i n t e n s i t y i s h i g h l y d e p e n d e n t upon t h e v e r t i c a l p o s i t i o n i n g o f t h e s a m p l e r e l a t i v e t o t h e f i x e d d e t e c t o r and window. V a r i a t i o n i n sample h e i g h t on a non c o n t a c t o p t i c a l u n i t can be a major a n a l y t i c a l l i m i t a t i o n . There a r e a t l e a s t t h r e e American and t h r e e European n e a r - i n f r a r e d m o i s t u r e m o n i t o r i n g systems which use water a b s o r p t i o n a t 1940 nm w i t h one o r more r e f e r e n c e wavelengths. These a r e designed t o be mounted above a conveyor b e l t w i t h c l e a r a n c e between the o p t i c a l window and the sample s u r f a c e . S i n c e water i s a s t r o n g a b s o r b e r the c o e f f i c i e n t s used i n the a n a l y t i c a l e x p r e s s i o n a r e s m a l l and the u n c e r t a i n t y a s s o c i a t e d by m u l t i p l y i n g the sampling n o i s e may be a c c e p t a b l e . For analytes where absorbance d i f f e r e n c e s a r e l e s s , t h e p r e c i s i o n may not be sufficient.



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Figure 1

Optical systems for near-infrared reflectance, A) turret mounted discrete f i l t e r s , integrating sphere and moveable mirror referencing. B) turret mounted discrete f i l t e r s , single detector and rotating sample cup (Dickey-John, Anburn, I L ) . C) t i l t i n g f i l t e r mount (continuously moving) with detectors on four sides of sample cup.


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Presentation


Solids. Conventional near-infrared reflectance analyzers use a v a r i e t y of methods to p o s i t i o n the sample i n t o the i n c i d e n t collimated beam and c o l l e c t reproducibly the d i f f u s e l y reflected radiation to measure the absorption which takes place in the body of the sample traversed. Solid samples are ground with care to achieve r e p r o d u c i b l e and reasonably uniform g r a n u l a t i o n f o r c a l i b r a t i o n and analysis measurements. The overall scattering c h a r a c t e r i s t i c s of the sample which s h i f t the reflectance baseline and control the depth of sample penetration and opportunity for absorption become a part of the method and the empirical a n a l y t i c a l equation ( 6 , 9 ) . Solid sampling i s summarized on the following table.

Table

II.

Examples of Sample Presentation

Systems For

Solids

Closed quartz covered cup for powders Open cup for pastes such as processed meat Cavity adjacent v e r t i c a l window for powder Compacted cavity above inverted window for powder Film or sheet of polymer

Liquids. Liquid sample handling devices have been designed and supplied by instrument manufacturers in a variety of shapes and sizes. One approach for near-infrared measurement of liquids i s to place a detector opposite the source to use the transmission mode. This is done in one design, regardless of the t u r b i d i t y of the sample. Another design retains the use of the integrating sphere and i t s positioning for diffuse reflectance and uses a combination of absorption of radiation going in and back through the l i q u i d before i t is collected in the integrating sphere. This amounts to a succession of absorption by the sample, diffuse reflectance from a r e f l e c t o r on the bottom followed by a b s o r p t i o n of the sample. Needless to say, the thickness of the c e l l i s extremely important, i t i s temperature dependent and a good deal of control i s required. Such a system, and a sophisticated electronic control c i r c u i t r y , is part of a l i q u i d sample drawer which f i t s i n t o a commercial instrument and i s c o n t r o l l e d by the firmware b u i l t i n t o that instrument. This configuration of an instrument i s marketed for i n d u s t r i a l f l u i d s and the dairy industry. For l i q u i d milk samples, i t i s essential to employ a homogenizer prior to the introduction of the sample into the l i q u i d drawer accessory to the instrument. The drawer i s also thermostated and a pump i s placed into the system to furnish samples to the o p t i c a l system. The same system with a homogenizer has been used for other i n d u s t r i a l samples which have c h a r a c t e r i s t i c s s i m i l a r to that of m i l k . O i l s and previously melted fats can be measured neat in a heated, thermostatically controlled l i q u i d drawer. Other l i q u i d drawers have been fabricated without the programmable control for use with an external water bath



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or o t h e r thermostated system. Some o f these s p e c i a l l i q u i d c e l l s have been used a t ambient temperature f o r a l c o h o l i n wine and f o r v a r i o u s c o m p o n e n t s i n b e e r and i n w o r t d u r i n g t h e p r o c e s s o f f e r m e n t a t i o n . The l i q u i d c e l l volume i s r a t h e r s m a l l but t h e t u b i n g l e a d i n g i n t o t h a t can be t a i l o r e d f o r t h e p a r t i c u l a r purposes. L a r g e bore c e l l s have been produced which can accomodate v i s c o u s f l u i d s and those c o n t a i n i n g as much as 60-70% s o l i d s . Although d i f f u s e r e f l e c t a n c e i n t h e n e a r - i n f r a r e d was never i n t e n d e d as a m i c r o t e c h n i q u e , t h e n e c e s s i t y t o make measurements on s m a l l amounts o f samples has been d e a l t w i t h by s e v e r a l people. Small volume c e l l s i n v o l v i n g 200 m i c r o l i t e r s o f l i q u i d have been produced. One approach t o d e a l i n g w i t h a s m a l l sample has been t h e d e s i g n and f a b r i c a t i o n o f a f o c u s i n g cup ( 1 3 ) . I n t h i s case, t h e cup i s t r a n s p a r e n t and t h e bottom o f i t i s m i r r o r e d so t h a t l i g h t which e n t e r s from t h e t o p i s then focused back up toward the c e n t e r p a r t c o n t a i n i n g t h e sample. A reasonable percentage o f t h e r a d i a t i o n i n c i d e n t on t h e sample e x i t s t h e sample towards t h e c o l l e c t i o n o p t i c s o f an I n f r a A l y z e r 400. Process A n a l y s i s Grab samples c a n be p u l l e d a t s t r a t e g i c p o i n t s i n a process t o determine t h e q u a l i t y o f t h e i n t e r m e d i a t e p r o d u c t , o r from t h e raw material. O b v i o u s l y , f o r t h e purposes o f c o n t r o l , t h e i n t e r m e d i a t e product has t h e g r e a t e s t p o t e n t i a l f o r g i v i n g t h e p r o c e s s o r u s a b l e and t i m e l y i n f o r m a t i o n . I n s t e a d o f random s a m p l i n g , i t may be d e s i r a b l e t o have p e r i o d i c s a m p l i n g , a n a l y z e t h e s e s a m p l e s as r a p i d l y as p o s s i b l e i n an o f f - l i n e p r o c e d u r e , t r a n s f e r the i n f o r m a t i o n obtained d i r e c t l y t o the personnel o r t o the c o n t r o l d e v i c e used t o take t h e a p p r o p r i a t e p r o c e s s i n g a c t i o n . A few examples o f such p r o c e s s i n g q u a l i t y parameter m o n i t o r i n g would i n c l u d e m o i s t u r e p r i o r t o some p r o c e s s i n g s t e p , p r o t e i n r e t e n t i o n a f t e r a s e p a r a t i o n process has o c c u r r e d , p u r i t y o f t h e product o r i n t e r m e d i a t e from t h e m a t e r i a l f o r w h i c h s e p a r a t i o n has been performed. I n t h e case o f d r y corn m i l l i n g , one c o u l d u s e monitoring o f the corn g r i t s or f l o u r t o look f o r a residue o f o i l . S p e c i f i c a t i o n s c a l l f o r a l i m i t t o t h e o i l l e v e l i n the g r i t s . I t i s a l s o p o s s i b l e t o m o n i t o r t h e amount o f product which was n o t r e c o v e r e d from t h e b y - p r o d u c t . Such would be the case o f m o n i t o r i n g f o r s t a r c h l e f t on the f i b e r i n e i t h e r d r y wheat m i l l i n g o r wet c o r n milling. I n both c a s e s , t h e s t a r c h thrown out w i t h the by-product would be c o n s i d e r e d a l o s s and a mark o f reduced e f f i c i e n c y i n t h e p r o c e s s . F o r c o n d e n s a t i o n polmers t h e process o f p o l y m e r i z a t i o n c a n be f o l l o w e d i n some cases by t h e d i s a p p e a r a n c e o f t e r m i n a l groups found i n monomers which a r e i n c o r p o r a t e d then i n t o the polymer. The e x t e n t o f c r o s s l i n k a g e and end capping has a l s o been measured f o r p r o c e s s i n g c o n t r o l purposes. U n s a t u r a t i o n d u r i n g the h y d r o g e n a t i o n p r o c e s s i n g o f o i l has been f o l l o w e d and q u a n t i t a t e d by nearinfrared. F e r m e n t a t i o n b r o t h s used t o produce p h a r m a c e u t i c a l s have been monitored f o r a c t i v i t y d u r i n g t h e f e r m e n t a t i o n process by near-infrared. S t a r c h g e l a t i n i z a t i o n o r degree o f cook has a l s o been monitored i n an o f f - l i n e manner. There a r e o t h e r examples o f process m o n i t o r i n g a n a l y s e s , but they a r e t o o numerous t o mention here. 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 on e x a m p l e s o f a c t u a l


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on-line systems used since the timeliness of on-line analysis gives the operator a chance to perform a processing control function better than o f f - l i n e techniques.


On-Line Monitoring of Liquids and Slurries On-line l i q u i d systems operating at ambient temperature have been i n use for a few years for monitoring alcohol in the wine industry, p a r t i c u l a r l y in France, by near-infrared combined transmission reflectance techniques. Beer, as a finished product, has been monitored by pumping debubbled beer from a sidearm transfer pipe. This has been pumped through a flow c e l l and measurements taken on regular intervals from the sidearm sampling loop. In the dairy industry, milk has been pumped through a homogenizer into the l i q u i d c e l l and measured i n a stop flow mode. A l s o , i n the brewing industry, wort has been monitored by pumping samples into a l i q u i d sample drawer during the process of fermentation. In wet corn m i l l i n g , the protein in the liquor from the centrifuges can be determined by pumping i t through a l i q u i d sampling device. From a high temperature high pressure hydrogenation reaction vessel, in the edible fats and o i l s industry, i t i s potentially possible through a sampling loop, to introduce hot o i l into the l i q u i d sampling device even i n the presence of the catalyst p a r t i c l e s . More on-line l i q u i d and slurry measurement systems w i l l no doubt be devised as needed. Most of these use a stop flow procedure with a sample loop. Whereas the sampling i s intermittent and r e f l e c t s what is flowing through the pipe or c i r c u l a t i n g through the reaction vessel at the time, such systems involve, nevertheless, true on-line procedures. On-Line Monitoring of Granular Materials It i s d i f f i c u l t to generalize concerning the flow c h a r a c t e r i s t i c s of granular materials. Each material has i t s own transport charact e r i s t i c s based on cohesive and adhesive forces, density, size of the p a r t i c l e s , shape and perhaps other unknown c h a r a c t e r i s t i c s . For a material of a given composition, humidity represents a day to day or hour to hour variable which could supercede the other characteristics. For on-line sensing of granular materials, the main challenge i s a suitable transport system. It i s necessary not only to get the sample into the o p t i c a l beam, but to do i t reproducibly, and to reproduce the amount of compaction each time. Complete removal of the sample i s necessary so there w i l l be no memory effect and new material can continually progress i n the path of the light beam. In addition to the work at Kansas State University in our laboratory and p i l o t m i l l , which w i l l be described l a t e r , other adaptations of commercial benchtop instruments to on-line monitoring are noted below. There i s at least one system i n the Netherlands adapted for dealing with dry milk powder. In addition, there are several systems in Europe where an instrument i s placed at the end of a flour m i l l i n g process to examine the f i n a l flour for protein, moisture and purity (14,15). Based on the information available from these i n d u s t r i a l i n s t a l l a t i o n s , i n each case, the o p t i c a l sensing head i s tethered by e l e c t r i c a l cable s u f f i c i e n t l y short so that the mainframe instrument i s housed in the production f a c i l i t y .




282

In a d d i t i o n , i n each o f these c a s e s , t h e measurement i s done i n a s t o p f l o w mode where a d i s c r e t e f i l t e r i n s t r u m e n t i s i n v o l v e d and the sample i s removed by f o r c e u s i n g e i t h e r an a i r b l a s t o r s u c t i o n or b o t h . These i n v o l v e c o m m e r c i a l l y a v a i l a b l e o r m o d i f i e d t a b l e t o p instruments of Percon, Hamburg, West Germany and T e c h n i c o n , T a r r y t o w n , NY, USA. One c o n t a c t type o f system i n Hungary (Focus E n g i n e e r i n g ) uses a d i f f u s e r e f l e c t a n c e i l l u m i n a t i o n / o b s e r v a t i o n window i n t h e bottom o f a conveying spout t o monitor- p r o t e i n i n precrushed sunflower t o ensure optimum p r o t e i n l e v e l o f t h e r e s u l t a n t s u n f l o w e r meal a t t h e end o f t h e o i l e x t r a c t i o n p r o c e s s . The r e p e t i t i v e , a v e r a g i n g , r a p i d s c a n n i n g i n s t r u m e n t s o f P a c i f i c S c i e n t i f i c ( S i l v e r S p r i n g , MD) have been adapted t o o n - l i n e use i n the t r a n s m i s s i o n and the d i f f u s e r e f l e c t a n c e mode (16, 1 7 ) . I n t h e former, a sample f i l l s a chute between t h e f i l t e r e d source and t h e d e t e c t o r s . I n d i f f u s e r e f l e c t a n c e a f i b e r o p t i c probe has been used as an a l t e r n a t i v e t o a v e r t i c a l i l l u m i n a t i o n and d i f f u s e r e f l e c t a n c e d e t e c t i o n window where samples a r e p l a c e d i n c o n t a c t by a f o u r vane f i l l - r e a d - d u m p v e r t i c a l r o t a r y system. KSU System o f N e a r - I n f r a r e d R e f l e c t a n c e M o n i t o r i n g i n a_ P i l o t F l o u r Mill The f o l l o w i n g items need t o be c o n s i d e r e d f o r a s u c c e s s f u l o n - l i n e m o n i t o r i n g system and i t s economic use i n a p r o c e s s i n g f a c i l i t y : -

A t r a n s p o r t system Key q u a l i t y f a c t o r s e l e c t i o n An o p t i c a l s e n s i n g head I n t e r f a c i n g t o t h e computer module A custom d a t a h a n d l i n g system A design of potential action to affect the m o n i t o r i n g d a t a

t h e p r o c e s s based on

Hardware. The sample t r a n s p o r t system must be d e s i g n e d f o r t h e f l o w c h a r a c t e r i s t i c s o f the m a t e r i a l being handled. For m a t e r i a l with c e r t a i n g r a n u l a r i t y , w i t h c e r t a i n c o h e s i v e f o r c e s , one system w i l l work when another m a t e r i a l may r e q u i r e some s o r t o f a g i t a t i o n t o promote f l o w . The o p t i c a l s e n s i n g head must be mounted on t h e t r a n s p o r t system. The t r a n s p o r t system d e s c r i b e d here i s t o m o n i t o r samples i n t y p i c a l metal s p o u t i n g i n t h e process o f v e r t i c a l t r a n s p o r t w h i c h i s f e d by g r a v i t y a f t e r t h e m a t e r i a l h a s b e e n e l e v a t e d by a pneumatic l i f t . T h i s i s c o m p l e t e l y d i f f e r e n t than mounting an o p t i c a l head a number o f i n c h e s away from a moving conveyor b e l t . With a moving conveyor b e l t , one i s a p t t o have a d i f f e r e n t e l e v a t i o n o f the product and d i f f e r e n t d i s t a n c e s from t h e s e n s i n g head w i l l change t h e o p t i c a l geometry s i g n i f i c a n t l y . A s l i g h t v e r t i c a l s h i f t i n the p o s i t i o n o f t h e sample w i l l change t h e s o l i d angle o f c o l l e c t i o n and t o t a l l y change t h e s e n s i t i v i t y and t h e c a l i b r a t i o n constants. That p a r t i c u l a r f e a t u r e i s p o s s i b l y t h e b i g g e s t l i m i t a t i o n w i t h conveyor b e l t mounted m o n i t o r s . The remote o p t i c a l s e n s i n g head i s t e t h e r e d t o the parent computer i n t h i s case the Technicon I n f r a A l y z e r Model 400 mainframe. There a r e p e r i p h e r a l s t o t h e c e n t r a l computer connected through an RS232 i n t e r f a c e as w i l l be d e s c r i b e d l a t e r . I n a d d i t i o n t o the c e n t r a l computer and



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i t s p e r i p h e r a l s , a d a t a feedback system t o the p r o c e s s i n g p l a n t i s essential. I n the case o f the f l o u r m i l l , i t was n e c e s s a r y f o r the d a t a feedback system to be low v o l t a g e and e x p l o s i o n p r o o f . The t a s k r e q u i r e d o f the c e n t r a l hardware i n c l u d e s c o n t r o l l i n g the remote d a t a a c q u i s i t i o n . The c e n t r a l hardware must r e c e i v e a raw s i g n a l from a remote sensor and process the s i g n a l f o r each measurement. I t i s d e s i r a b l e a l s o f o r mass s t o r a g e o f d a t a , such as would be o b t a i n e d i n one day's run or the run o f one o p e r a t i n g s h i f t i n the p r o c e s s i n g p l a n t . Software i s n e c e s s a r y f o r a v e r a g i n g o f sequential process data p o i n t s . I n an o n - l i n e s y s t e m , i t i s i m p o s s i b l e t o have a b s o l u t e l y every s i n g l e r e a d i n g be a p e r f e c t one. There i s p o t e n t i a l f o r c a v i t a t i o n i n the c e l l . I t i s possible that we c o u l d get one c o m p l e t e l y erroneous r e a d i n g and r a t h e r than be o v e r l y i n f l u e n c e d by t h a t , i t i s n e c e s s a r y to do some a v e r a g i n g . A t y p i c a l a v e r a g i n g would be a r u n n i n g mean. We have chosen a r u n n i n g mean o f f i v e . F i v e r e a d i n g s are o b t a i n e d w i t h our system at the present time i n l e s s than 2.5 m i n u t e s . We p l a n t o reduce t h i s to a p p r o x i m a t e l y 1.25 m i n u t e s . Other t a s k s o f the c e n t r a l hardware i n c l u d e output o f the averaged d a t a t o a p r i n t e r and t r a n s m i s s i o n o f processed d a t a t o the c o n t r o l area f o r a c t i o n e i t h e r by a process c o n t r o l computer or by human o p e r a t o r s i n charge o f the p r o c e s s i n g system. F i g u r e 2 i l l u s t r a t e s the I n f r a l y z e r 400 mainframe c o n n e c t e d v i a an RS232 i n t e r f a c e t o a H e w l e t t P a c k a r d 85 d a t a a c q u i s i t i o n computer. From the HP 85 through an HPIL i n t e r f a c e l o o p , the d a t a i s sent back t o the p r o c e s s i n g p l a n t where i t i s r e c e i v e d by a handheld c a l c u l a t o r HP 41CX ( 1 8 ) . P e r i p h e r a l s : dual m i c r o f l e x i b l e d i s c s t o r a g e and T h i n k J e t p r i n t e r ( H e w l e t t Packard models 9121D and 2225A) are connected to the HP 85 by way o f an HPIB i n t e r f a c e c a r d . The t a s k s o f the remote hardware i n c l u d e t r a n s m i t t i n g power t o the lamp o f the s e n s i n g head and c o n t r o l o f the m e c h a n i c a l f u n c t i o n of the s e n s i n g head. I n t h i s case the m e c h a n i c a l f u n c t i o n i n v o l v e s a c t i o n o f a s t e p p i n g motor t o change f i l t e r s or t o move a m i r r o r and a l l o w r e f e r e n c i n g . Thermal c o n t r o l must a l s o be p r o v i d e d f o r the chamber c o n t a i n i n g the f i l t e r s . Other f u n c t i o n s o f the remote hardware are the i n - p l a n t readout system, which must communicate w i t h the c e n t r a l hardware system. The c e n t r a l hardware system w i l l t r a n s m i t r e a l time d a t a t o p l a n t c o n t r o l p e r s o n n e l o r t o a process c o n t r o l computer. I n the p l a n t , a c a l c u l a t o r or o t h e r p e r i p h e r a l d i s p l a y r e c e i v e s what i s b e i n g t r a n s m i t t e d from the c e n t r a l hardware u n i t i n the l a b . The o p t i c a l s e n s i n g head i s a t t a c h e d t o t h e t r a n s p o r t system. The custom b u i l t t r a n s p o r t system ( F i g u r e 3) i n v o l v e s a t h r e e i n c h aluminum s p o u t i n g t h a t i s t y p i c a l l y found i n flour mills. To t h i s has been welded a v e r t i c a l 2"x4 p i e c e o f extruded aluminum. The s p o u t i n g i s t y p i c a l l y at an angle o f repose of a p p r o x i m a t e l y 70° or more from ground l e v e l and the v e r t i c a l e x t r u s i o n i s dropped from t h a t s p o u t i n g . I t i s connected at the bottom t h r o u g h a round tube p e r p e n d i c u l a r to the r e c t a n g u l a r e x t r u s i o n which then feeds back t o the s p o u t i n g by way o f a screw conveyor ( 1 9 ) . B o l t e d t o the e x t r u s i o n i s the o p t i c a l s e n s i n g head, i t s q u a r t z window i s then f l u s h w i t h the h o l e m i l l e d i n the extruded aluminum t o accomodate t h a t o b s e r v a t i o n p o r t . O p p o s i t e the window of the s e n s i n g head i s an i n s p e c t i o n p o r t so i t i s p o s s i b l e t o make sure t h a t t h e r e has been no a c c u m u l a t i o n o f m a t e r i a l s on the o p t i c a l n



SIGNAL

"InfraAlyzer* 400 MAINFRAME

- 6 -

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POWER AND CONTROL

HP1B RS 232 MASS STORAGE


HP1L HP 85 DATA ACQUISITION

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Figure 2 Block diagram of central computer, associated peripherals and I/O.


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window o r i f c l e a n i n g w o u l d be n e c e s s a r y , i t c o u l d be reached w i t h o u t d i s a s s e m b l i n g the whole system. The s e l e c t i o n of the h e i g h t o f s t o c k above the o b s e r v a t i o n window depends upon the m a t e r i a l b e i n g observed. I t i s d e s i r a b l e to have enough s t o c k above the window to produce a r e l a t i v e l y c o n s t a n t compaction a g a i n s t the window but i t i s u n d e s i r a b l e t o have an u n n e c e s s a r i l y h i g h head o f s t o c k because t h i s produces a l a g between what i s b e i n g observed and what i s f l o w i n g above. The d i s t a n c e from the o b s e r v a t i o n p o r t t o the conveyor screw was chosen to be j u s t h i g h enough t h a t the f l o w i n the conveyor screw would not produce c a v i t a t i o n i n the window a r e a . A h y d r a u l i c motor i s used to d r i v e the screw. Remote Readout. The readout system i n the p l a n t , i n t h i s c a s e , on the m i l l f l o o r or i n the m i l l o f f i c e , i s as f o l l o w s . An HPIL loop which emanates from the HPIL i n t e r f a c e t o the HP85 c o n t r o l computer extends back t o the m i l l a d i s t a n c e o f 108 f e e t . I n the m i l l , a handheld HP41CX c a l c u l a t o r i s plugged i n t o the l o o p . An HPIL v i d e o i n t e r f a c e i s a l s o plugged i n t o the l o o p . From the v i d e o i n t e r f a c e c o a x i a l c a b l e l e a d s t o a v i d e o monitor l o c a t e d i n the m i l l o f f i c e . These m o n i t o r s can be m u l t i p l e x e d and can be i n as many l o c a t i o n s as deemed n e c e s s a r y . F u r t h e r m o r e , i t i s p o s s i b l e to have m u l t i p l e r e c e p t i c l e s f o r HP41CX d a t a d i s p l a y . T h i s handheld d e v i c e can be c a r r i e d on t h e o p e r a t o r ' s b e l t and c a n be p l u g g e d i n t o a p o r t on any f l o o r of the o p e r a t i n g m i l l to o b t a i n a c u r r e n t r e a d o u t . F i g u r e 4 shows a b l o c k diagram of the e n t i r e system, i n c l u d i n g the i n s t r u m e n t room, the o p e r a t i o n f l o o r o f the p i l o t f l o u r m i l l , and the l o c a t i o n of the v i d e o m o n i t o r i n the m i l l o f f i c e . An e x t e r n a l HP85 computer can be used to c o n t r o l the I n f r a l y z e r 400 mainframe cycle i f a patchbox i s p l a c e d between the I n f r a l y z e r 400 mainframe and the RS232 i n t e r f a c e . The patchbox i n c l u d e s c o n n e c t i o n t o the s t a n d a r d RS232 output and a second c o n n e c t i o n l o o p to an i n t e r n a l switching point. I t i s then p o s s i b l e t o program the HP85 to time c o n t r o l the sequence o f d a t a a c q u i s i t o n . Instead of being d r i v e n by the firmware of the I n f r a A l y z e r 400, the HP85 can be programmed f o r any a p p r o p r i a t e time f u n c t i o n , depending on the f l o w of product or on the n e c e s s i t y to a c q u i r e d a t a . With a 3 f i l t e r readout, f o r example the minimum time would be t h i r t e e n seconds. When d a t a i s a c q u i r e d at more than 3 w a v e l e n g t h s , w i t h an i n s t r u m e n t o f t h i s t y p e , a l o n g e r p e r i o d i s r e q u i r e d . Another v a r i a t i o n on hardware i s e l i m i n a t i o n of the HP85 and a d a p t a t i o n of the handheld HP 41CX c a l c u l a t o r f o r c o n t r o l , readout and d r i v i n g p e r i p h e r a l s . T h i s i s p o s s i b l e w i t h an RS232/HPIL i n t e r f a c e a t t a c h e d to the e x t e r n a l c o n t r o l i n t e r f a c e patchbox. T h i s handheld c a l c u l a t o r can a c t u a l l y be at the remote a r e a and s t i l l i n i t i a t e the measurement c y c l e o f the I n f r a A l y z e r 400 i n i t s c e n t r a l l a b o r a t o r y l o c a t i o n . Software. To d e v i s e an i n p l a n t , o n - l i n e system from t a b l e top l a b o r a t o r y equipment i n v o l v e s some s o f t w a r e as w e l l . The t a s k s o f the s o f t w a r e i n c l u d e the c o n t r o l of the s e n s i n g d e v i c e , t h i s i s d r i v e n by the Technicon I n f r a A l y z e r 400 firmware. I n i t i a t i o n o f the c y c l e can be operated i n one of t h r e e methods. Method A - would be to use the a u t o c y c l e firmware of the T e c h n i c o n 400. Method B would be e x t e r n a l HP85 programmed time c y c l e . Method C would be remote



FLOOR

OFFICE

MILL

MILL

SAMPLE SPOUT

HP 41CX

Figure 4

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HP 85 DATA AQUISITION

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B l o c k diagram o f remote o n - l i n e system.

VIDEO MONITOR

HP IL VIDEO INTER FACE

HP1L

. POWER AND CONTROL

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Instrument Room


HP IB

THINK JET PRINTER

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HP41CX programmed time c y c l e . Another t a s k o f the s o f t w a r e i s to process the raw d a t a . The I n f r a A l y z e r 400 firmware can be used as is for this task. For o n - l i n e m o n i t o r i n g , i t has been p r e v i o u s l y mentioned, i t i s n e c e s s a r y t o average the d a t a . The averaging ( r u n n i n g mean o f f i v e r e a d i n g s ) i s done on the HP85 which a l s o takes care o f s t o r a g e , l o c a l output and remote o u t p u t . The remote output d i s p l a y r e q u i r e d HP41CX programming i n o r d e r to access the s i g n a l coming from the HPIL loop and t o d i s p l a y i t as d e s i r e d . C a l i b r a t i o n o f an o n - l i n e system i s not a t r i v i a l endeavor. Ideally, c a l i b r a t i o n c o n s t a n t s and i f n e c e s s a r y , wavelength s e l e c t i o n should be performed on the a c t u a l s e n s i n g head w i t h i t s geometry, w i t h the m a t e r i a l i n a f l o w i n g mode or stop f l o w mode, depending on the d e s i g n o f the o n - l i n e system. U n f o r t u n a t e l y , i t i s not always p o s s i b l e t o t i e up the m a n u f a c t u r i n g f a c i l i t y t o do t h i s . I t may a l s o not be p o s s i b l e t o produce and run through the l i n e m a t e r i a l which v a r i e s s u f f i c i e n t l y enough t o g i v e one the b r e a d t h o f c a l i b r a t i o n needed. I f one i s u s i n g an o n - l i n e m o i s t u r e meter, the s e l e c t i o n o f wavelengths i s not a problem. The a n a l y t e i s d e f i n e d , i t s o b s e r v a t i o n wavelengths are well-known and the wavel e n g t h s e l e c t i o n can be done e n t i r e l y o f f - l i n e . The c o e f f i c i e n t s i n the a n a l y t i c a l e q u a t i o n w i l l be s u b j e c t somewhat t o the o p t i c a l geometry and to the p a r t i c u l a r f i l t e r s and t h e i r d e n s i t i e s i n the s e n s i n g head. I n t h i s c a s e , the p u r i t y o f a p a r t i c u l a r i n t e r m e d i a t e product stream was t o be m o n i t o r e d f o r s e v e r a l months of o p e r a t i o n . Samples were taken at the m o n i t o r i n g p o i n t and a l a r g e set was accumulated. By v a r i o u s c l a s s i c a l t e c h n i q u e s , i t was determined what t h e e x t r e m e s were t h a t had b e e n o b s e r v e d t h r o u g h many operations. The extremes from s e v e r a l runs were p o o l e d , the pooled ones o f one extreme were c a l l e d A, the pooled ones from another extreme were c a l l e d B and s y n t h e t i c m i x t u r e s o f A and B were made by w e i g h t . Such a procedure i s based on the assumption t h a t w i t h l i n e a r c o m b i n a t i o n s of the extremes i t w i l l be p o s s i b l e t o r e p r e s e n t the n a t u r a l i n t e r m e d i a t e s w h i c h f a l l i n between. Assumptions l i k e t h i s c a n be v e r i f i e d by u s i n g a calibration produced from a s y n t h e t i c set and a p p l y i n g i t back to the i n t e r m e d i a t e s and i f a b s o l u t e numbers are not a v a i l a b l e f o r the i n t e r m e d i a t e s , they can at l e a s t be ranked to see i f the r a n k i n g agrees w i t h r a n k i n g by another method. F i g u r e 5 shows t h a t i d e a l l y i t i s d e s i r a b l e to m o n i t o r the c a u s a l q u a l i t y parameters. However, i t may a l s o serve the same purpose to measure a parameter which runs p a r a l l e l or i s p r o p o r t i o n a l , even i f i t i s an i n v e r s e r e l a t i o n s h i p . There are a number o f o p t i o n s a v a i l a b l e . I n making s y n t h e t i c b i n a r y m i x t u r e s , one must be v e r y c a r e f u l t h a t the primary d i f f e r e n c e between A and B i s n o t m o i s t u r e c o n t e n t , w h i c h has s t r o n g a b s o r b i n g p e a k s , o r granulation. As f o r the m o i s t u r e c o n t e n t , i t i s p o s s i b l e t o a v o i d i n c o r p o r a t i n g m o i s t u r e wavelengths i n t o your a n a l y t i c a l e q u a t i o n . As f o r g r a n u l a t i o n , t h i s c a n be o b s e r v e d f r o m o p t i c a l d a t a o f the parent s t o c k s of A and B. Implementation. The r a t i o n a l e for c o n t r o l using monitoring of q u a l i t y between A, d e f i n e d as good q u a l i t y and B, d e f i n e d as poor q u a l i t y , i s as f o l l o w s . I f the s t o c k going through the m o n i t o r i s l i k e A (good q u a l i t y ) then the process would be r o u t e d by a c e r t a i n pathway or a c e r t a i n c o n t r o l o f a p a r t i c u l a r machine. I f the s t o c k



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Figure 5

Parameter v a r i a t i o n o f b i n a r y

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i s more l i k e B, then i t would go e i t h e r by an a l t e r n a t e pathway or a s l i g h l t y m o d i f i e d f u n c t i o n o f the next machine. I t i s necessary to choose the l i m i t s o f percent B f o r the c o n t r o l f u n c t i o n s and r a p i d l y a n a l y z e p e r c e n t B w i t h the remote o n - l i n e m o n i t o r . Timeliness of a n a l y s i s i s the key to p r o c e s s c o n t r o l . The system d e s c r i b e d i s a p o t e n t i a l means to t h a t end. From the economic and p r a c t i c a l i t y s t a n d p o i n t , i t should be obvious t h a t i t i s e s s e n t i a l to f i n d a key m o n i t o r i n g p o i n t . If there i s l i t t l e v a r i a t i o n of s t o c k at t h a t p o i n t , t h e r e i s l i t t l e r e a s o n to m o n i t o r . I f a c o n s i d e r a b l e amount o f v a r i a t i o n i s o b s e r v e d , whether t h i s comes about by n a t u r a l v a r i a b l e s i n the m i l l o p e r a t i o n or by n a t u r a l v a r i a b l e s brought about from d i f f e r e n t batches of raw m a t e r i a l s , or whether t h i s i s brought about by a p a t h o l o g i c a l c o n d i t i o n from something g o i n g wrong w i t h a p i e c e of machinery, then t h e r e i s a p o t e n t i a l f o r the system to be out of c o n t r o l and the time l a g f o r f i n d i n g t h i s may be l o n g , and the l o s s e s may be g r e a t . The p e n a l t i e s o f not d e t e c t i n g t h i s change may v a r y g r e a t l y from p r o d u c i n g a s m a l l amount of o f f - g r a d e product to having i n e f f i c i e n t production f o r the p e r i o d of out of c o n t r o l operation. I n a 24 hour p r o c e s s , t h i s c o u l d mean 10 to 16 hours o f i n e f f i c i e n t or o f f - g r a d e p r o d u c t i o n , i f no c o r r e c t i o n i s made. The second t h i n g to c o n s i d e r i n e v a l u a t i n g p r a c t i c a l i t y i s what can be done i f changes are observed? The obvious q u e s t i o n i s what would the s u p e r v i s o r do to the process i f he were aware of the change? What t h e s u p e r v i s o r w o u l d do can be programmed i n t o a p r o c e s s c o n t r o l c o m p u t e r w h i c h c o u l d a u t o m a t i c a l l y make a c o r r e c t i o n . C o n t r o l a c t i o n r e q u i r e s some k i n d o f a movement o f a l e v e r to d i v e r t stream of product, o p e n i n g o f a v a l v e , o p e n i n g o f an a i r j e t , c o n t r o l l i n g of a motor, c o n t r o l l i n g of the f i n a l adjustment of p i e c e s of equipment. The c o n t r o l a c t i o n course i s d i c t a t e d by the p a r t i c u l a r process and the economics a s s o c i a t e d w i t h implementing it. The economics a s s o c i a t e d w i t h payback must be c a l c u l a t e d on an individual basis. R a p i d , t i m e l y , o n - l i n e m o n i t o r i n g p r o v i d e s the p r o c e s s o r w i t h an o p t i o n . I t i s n a i v e to assume t h a t the e n t i r e e f f i c i e n c y of a complex p r o c e s s c o u l d be c o n t r o l l e d by j u s t one m o n i t o r i n g p o i n t and one automated c o r r e c t i v e a c t i o n . I t i s more l i k e l y t h a t m u l t i p l e m o n i t o r i n g p o i n t s w o u l d be n e c e s s a r y and m u l t i p l e i n t e r c o r r e l a t e d c o r r e c t i o n p r o c e s s e s would have to be driven. A l l of t h i s t r a n s l a t e s i n t o the p o t e n t i a l of c o n s i d e r a b l e e x p e n d i t u r e and the n e c e s s i t y f o r p a r a l l e l s e n s o r i n g systems n e t worked i n t o a c e n t r a l computer. There i s a l s o the p o s s i b i l i t y of m u l t i p l e x i n g the s e v e r a l s e n s i n g heads to one mainframe i n s t r u m e n t i n o r d e r to share the f u n c t i o n of t h a t mainframe. T y p i c a l economic d a t a and v a r i a b l e s observed i n a f l o u r m i l l are p r e s e n t e d i n a later discussion. Monitoring System Development. The sequence of d e v e l o p i n g the system j u s t d e s c r i b e d i n v o l v e d seven s t e p s . The f i r s t involved g a t h e r i n g f u n d a m e n t a l knowledge of the i n f r a r e d spectroscopic c h a r a c t e r i s t i c s of wheat components ( F i g u r e 6 ) . S e c o n d l y , e x p e r i e n c e was o b t a i n e d i n q u a n t i t a t i o n of c o n s t i t u e n t s o f i n d i v i d u a l streams i n s p i t e of t h e i r h e t e r o g e n i e t y and v a r i e d c h e m i c a l c o m p o s i t i o n . O p t i c a l s o r t i n g has enabled us to reduce the s t a n d a r d e r r o r of a n a l y s i s by a f a c t o r o f 2 by s o r t i n g i n t o s u b s e t s a c c o r d i n g to




Figure 6 N e a r - i n f r a r e d s p e c t r a o f wheat f r a c t i o n s ( W e t z e l , D. L.; Mark, H. " S p e c t r o s c o p i c D e t e r m i n a t i o n o f C e l l u l o s e as C r i t e r i o n o f F l o u r P u r i t y w i t h Respect t o Bran", American A s s o c i a t i o n o f C e r e a l Chemists 62nd Annual M e e t i n g , San F r a n c i s c o , CA 1977).



16.

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291

median p a r t i c l e s i z e . Where we have an o v e r a l l c a l i b r a t i o n which g i v e s us a s t a n d a r d e r r o r of 0.283% p r o t e i n f o r example, out o f r o u g h l y 12% p r o t e i n s t o c k , the s t a n d a r d e r r o r s are going to be reduced t o 0.15% by a s o r t i n g p r o c e s s . That i s to say, the range o f c a l i b r a t i o n i s b r o k e n up e i t h e r a c c o r d i n g t o t h e r a n g e o f t h e a n a l y t e or a c c o r d i n g to c h a r a c t e r i s t i c s o f the m a t r i x and i n d i v i d u a l c a l i b r a t i o n s are done then a s o r t i n g process can d i r e c t the computer t o a p p l y the most a p p r o p r i a t e c a l i b r a t i o n which w i l l g i v e you the t i g h t e s t f i t and the best a n a l y t i c a l r e s u l t s . With samples as d i v e r s e i n c o m p o s i t i o n and h e t e r o g e n i e t y as d i f f e r e n t f l o u r streams i n a m i l l , t h i s was r e q u i r e d f o r p r o t e i n a n a l y s e s , but i t was not required for moisture a n a l y s i s (20). For the p u r i t y m o n i t o r i n g which we d e s c r i b e d i n t h i s case h i s t o r y , one t e s t p o i n t and the r a n g e o f v a r i a b i l i t y a t t h a t t e s t p o i n t had t o be o b t a i n e d by c o l l e c t i n g samples under v a r i o u s c o n d i t i o n s o f o p e r a t i o n over a p e r i o d of time i n o r d e r to e s t a b l i s h the v a r i a b l e s i n g r a n u l a t i o n s i z e , c o m p o s i t i o n and p u r i t y . O f f - l i n e c a l i b r a t i o n was done i n v o l v i n g s p e c i f i c intermediate products. A t a b l e top i n s t r u m e n t was m o d i f i e d f o r remote o n - l i n e use by s e p a r a t i o n o f o p t i c a l and computing modules and e x t e n s i o n of c a b l e s . The t r a n s p o r t system was d e s i g n e d , f a b r i c a t e d and t e s t e d i n t h e m i l l . The o v e r a l l system, i n c l u d i n g the 400 mainframe, a l l the p e r i p h e r a l s and a l l the r e m o t e s y s t e m s was i n s t a l l e d , debugged and t e s t e d d u r i n g m i l l operation. F i n a l l y , s p e c i a l m i l l i n g processes were r u n w i t h the o n - l i n e m o n i t o r system i n p l a c e ( 2 1 ) . T h i s was used f o r r e g u l a r scheduled runs o f the m i l l . The economics of m o n i t o r i n g were t e s t e d by e x p e r i m e n t a l p i l o t m i l l runs where d e l i b e r a t e changes were made i n the m i l l and d e l i b e r a t e c o r r e c t i v e a c t i o n s were t a k e n . To a s s e s s the o v e r a l l e f f e c t , numerous samples were c o l l e c t e d o f 23 product ( f l o u r ) streams and a n a l y z e d by c o n v e n t i o n a l methods. The p r o f i l e o f the m i l l was determined to generate the economic d a t a . Examples of such p r o f i l e d a t a are shown on F i g u r e 7 where a maximum of low ash product i s d e s i r a b l e and F i g u r e 8 where the weighted summation of p r o d u c t s h a v i n g two v a l u e s ( o b t a i n e d from F i g u r e 7) d i f f e r s f o r p r o c e s s i n g w i t h and w i t h o u t c o n t r o l a c t i o n . F i g u r e 9 i l l u s t r a t e s r e s p o n s e as a f u n c t i o n o f t i m e . The parameter monitored i n t h i s case was p r o t e i n . In t h i s case, a d i f f e r e n t b a t c h o f m a t e r i a l was sent through to t e s t the system's a b i l i t y to respond to t h a t new m a t e r i a l . One economic n o t e , i n the case o f one of our f l o u r m i l l i n g experiments shows t h a t where i t i s p o s s i b l e t o p r o d u c e 58% o f t h e more v a l u a b l e product before adjustment a f t e r the c o r r e c t i o n o f p r o c e s s i n g , t h i s i n c r e a s e d to 63%. The v a l u e o f t h i s product ( p a t e n t f l o u r ) i s about 125% the v a l u e o f the secondary product ( c l e a r f l o u r ) so a s h i f t of 5% o f m a t e r i a l from the c a t e g o r y o f lower v a l u e to one o f h i g h e r v a l u e r e p r e s e n t s an important s t e p i n p r o c e s s i n g e f f i c i e n c y . For a f l o u r m i l l o p e r a t i n g at 1500/cwt per day, a 5% s h i f t r e p r e s e n t s 120 t o 165 d o l l a r s per day or 44-60 thousand d o l l a r s per y e a r . The s p e c t r a showed on F i g u r e 10 are f o u r s u c c e s s i v e r e a d i n g s at 9:15, 10:15, 10:50, 11:15 o f m i l l i n g o p e r a t i o n s the same day. These were samples c o l l e c t e d and scanned on a scanning l a b o r a t o r y i n s t r u m e n t . The s h i f t o f the b a s e l i n e i n d i c a t e s a d r a s t i c change i n g r a n u l a r i t y . Such d r a s t i c changes i n g r a n u l a r i t y over r e l a t i v e l y s h o r t p e r i o d s o f time show t h a t any c a l i b r a t i o n would have t o be s u f f i c i e n t l y r o b u s t



292


.2

1

1 10

0

1

1

20

30

1

1

40

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CUMULATIVE % TOTAL PRODUCT

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LOW QUALITY PRODUCT

.2

1

0

1 2

\ 4

1

1

6

B

1 10

112

1 14

| 16

CUMULATIVE % TOTAL PRODUCT Figure 7 M i l l e r ' s ( c u m u l a t i v e ash) c u r v e s f o r p r o d u c t i o n p u r i t y and e x t r a c t i o n w i t h c o n t r o l a c t i o n (more h i g h v a l u e , low v a l u e ) w i t h o u t c o n t r o l a c t i o n ( l e s s h i g h , more low v a l u e ) .


WETZEL



with control action

yield

without control action

yield

Figure 8 D i s t r i b u t i o n o f m i l l i n g p r o d u c t s by v a l u e and y i e l d ( a r e a denotes summation o f product v a l u e ) .

Figure 9 protein.

Response o f r u n n i n g mean o f sensor t o change i n



F i g u r e 10 N e a r - i n f r a r e d r e f l e c t a n c e s p e c t r a of p r o d u c t s at f o u r times i n the same r u n .

intermediate


Ο ?ο ζ/3

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η m η > r m Ζ

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Ζ

δ

r η

>

α

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16. WETZEL


295


to withstand this or would have to have built into it a warning to tell you that at a given time, the reading you are getting may be outside the calibration range. Such variations in sample were not encountered in our first five and one half months of operation but the results shown on the previous diagram occurred when the miller decided to try something different, such a simple thing as turning off the air which provided turbulence in a particular purifier resulted in gravity taking over where you normally had turbulence and after five and one half months of successful operation with a particular calibration, suddenly a change in physical characteristics of the sample put the product outside of our calibration range. When the unexpected happens, that has to be added to the experience bag and incorporated so there will be less unexpected things occurring. Contribution No. 86-178-B from the Kansas Agricultural Experiment Station. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15.

Fulcher, R. G. Food Microstructure. 1982, 1, 176-75. Jensen, SV. A.; Munk, L.; Martens, H. Cereal Chem. 1982, 59(6), 477-84. Kaye, W. Spectrochim. Acta. 1954, 6, 257. Kaye, W. Spectrochim. Acta. 1955, 1, 181. Whetsel, K. B. Appl. Spectrosc. Rev. 1968, 2(1), 1. Wetzel, D. L. Anal. Chem. 1983, 55, 1165A. Ben-Gera, I.; Norris, K. H. Israel J. Agr. Res. 1968, 18, 125. Honigs, D. E; Hieftje, G. M.; Hirschfeld, T. Appl. Spectroscopy. 1984, 38, 844. Stark, E. W. "Interaction Between Data Treatments and Calibration Techniques in NIRA". 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. New Orleans, LA. Feb. 1985; Paper 1096. Wetzel, D. L. "Physical Sample Characterization by Granulation Sorting from Diffuse Reflectance Measurement in the Near-Infrared". Proc. Third Ann. Users Conference for NIR Researchers. Pacific Scientific, Silver Springs, MD. Feb. 1984. Rose, J. R. "Discriminant Analysis of NIR Research", 10th Annual Federation of Analytical Chemistry and Spectroscopy Societies meeting, Philadelphia, PA, Sept. 1983; Paper 26. Mark, H. L. Anal. Chem. 1985. 57, 1449-56. Hirschfeld, T. "Near Infrared Trace and Microanalysis", 1985, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. New Orleans, LA, Feb. 1985; Paper 1093. Bolling, H.; Zwingelberg, H. Getreide Mehl u. Brot, 1984, 38, 3. Osborne, B. "Progress of NIRA in Milling and Baking". Proc. of the Seventh International Symposium on Near-Infrared Reflectance Analysis, Technicon Industrial Systems, Tarrytown, NY. July 1984.


296 16.

17.

18.


19.

20.

21.

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS Stokes, L. "An Economical and Practical Evaluation of an On-Line Instrument for controlling Protein in Soybean Meal", American Association of Cereal Chemists 64th Annual Meeting, Washington, DC, Oct. 1979; Paper 145. Cooper, P. J. "Developing and Testing On-Line NIR Methods for Process Control", 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. New Orleans, LA, Feb. 1985; Paper 852. Wetzel, D. L., Levin, J. A. and Posner, E. S. "On-Line Process Monitoring: Hardware Software and Economics". Proc. of the Eighth International Symposium on Near Infrared Reflectance Analysis, Technicon Industrial Systems, Tarrytown, NY, August 1985. Wetzel, D. L. "On-Line Monitoring for Control of Process". Proc. Seventh International Near-Infrared Reflectance Symposium, Technicon Industrial Systems, Tarrytown, NY, July 1984. Wetzel, D. L., Wehling, R. L. and Stalnaker, J. B. "Individual Millstream Analysis by Near Infrared Reflectance", Annual Meeting of the American Association of Cereal Chemists, Chicago, IL, Sept. 1960. Posner, E. S. and Wetzel, D. L. "NIR for control of a flour mill", Annual meeting of the Association of Operative Millers. Cleveland, OH, April 1985.

RECEIVED January 24, 1986


Fundamentals and Applications of Chemical Sensors - American

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