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A REAPPRECIATION OF ALUMINA IN HIGH PRESSURE LIQUID CHROMATOGRAPHY

BIBLIOTHEEK TU Delft P 1759 4072

A REAPPRECIATION OF ALUMINA

IN HIGH PRESSURE

LIQUID CHROMATOGRAPHY

PROEFSCHRIFT

ter verkrijging van de graad van doctor in de technische wetenschappen aan de Technische Hogeschool Delft,

op gezag van de Rector Magnificus, prof. ir. B. P. Th. Veltman, in het openbaar te verdedigen ten overstaan van

het College van Dekanen

op donderdag 22 december 1983 te 14.00 uur door

CLAUDE JEAN CHARLES MARCEL LAURENT

geboren te Périgueux (Frankrijk), ingénieur de l'Institut National Polytechnique

D i t p r o e f s c h r i f t i s goedgekeurd door de promotor

AVANT-PROPOS

C e t t e thèse e s t l a c o n c l u s i o n des q u a t r e années de r e c h e r c h e que j ' a i pu e f f e c t u e r au TH de D e l f t grâce aux i n i t i a t i v e s de mon p r o m o t e u r , l e p r o f e s s e u r L. de Golan. O b t e n i r qu'un Français p u i s s e e f f e c t u e r son s e r v i c e m i l i t a i r e dans une u n i -versité H o l l a n d a i s e e s t un e x e r c i c e de h a u t e v o l t i g e q u ' i l f a u t v o u l o i r e n t r e p r e n d r e . J ' a d r e s s e à 1'équilibriste mon p l u s p r o f o n d r e s p e c t . Au p r o f e s s e u r , j ' a d r e s s e mes r e m e r c i e -ments pour l ' a i d e q u ' i l m'a apportée e t q u i s ' e s t exprimée sous l a forme de réflexions s t i m u l a n t e s e t d'une c o n f i a n c e permanente.

On a beau être m i l i t a i r e , on n'en r e s t e pas moins homme, un homme s e n s i b l e à l ' a t t e n t i o n e t l'intérêt pour mes t r a v e a u x manifesté p a r Hugo B i l l i e t e t P e t e r Schoenmakers, deux collègues

i n d i s s o c i a b l e s dans ma r e c o n n a i s s a n c e .

Ces t r a v e a u x n ' a u r a i e n t pas pu être menés à b i e n sans l a c o -opération e n t h o u s i a s t e des p e r s o n n e s s u i v a n t e s :

Messieurs Heurins e t Shewsbury de l a société Pye Unicam (Grande B r e t a g n e ) ,

M e s s i e u r s Buytenhuys e t Van de Maeden des l a b o r a t o i r e s de r e c h e r c h e de l a société AKZO (Arnhem),

M o n s i e u r Hwizev des l a b o r a t o i r e s du ministère de l a J u s t i c e ( R i j s w i j k ) ,

H. van Dam e t F. Boudewign pour a v o i r m i s à ma d i s p o s i t i o n

l e u r s c o n n a i s s a n c e s en électronique e t i n f o r m a t i q u e , M a d e m o i s e l l e W. Eonders e t 0. Bekers pour t o u t e s l e s mesures e t expériences q u ' i l s o n t effectuées,

G. Kleier du s e r v i c e t e c h n i q u e ,

l e s d e s s i n a t e u r s F. Bolman e t A. Sahrïel,

e t e n f i n l e s p h o t o g r a p h e s C. Warnaav e t F. Hammere. A tous j ' a d r e s s e mes p l u s p r o f o n d s r e m e r c i e m e n t s .

CONTENT C h a p t e r 1 WHY ALUMINA?

C h a p t e r 2

THE BEHAVIOUR OF ALUMINA IN AN AQUEOUS MEDIUM

C h a p t e r 3

THE USE OF ORGANIC MODIFIERS IN ION EXCHANGE CHROMATOGRAPHY ON ALUMINA

C h a p t e r 4

THE SEPARATION OF HEROIN SAMPLES ON ALUMINA BY ION EXCHANGE IN MIXED AQUEOUS-ORGANIC MOBILE PHASES

C h a p t e r 5

THE SEPARATION OF PROTEINS ON ALUMINA

C h a p t e r 6

A COMPUTER-CONTROLLED SINGLE PUMP SOLVENT PROGRAMMER SUMMARY

SAMENVATTING RESUME

C H A P T E R 1

W H Y A L U M I N A ?

Chromatography i s an a n a l y t i c a l s e p a r a t i o n t e c h n i q u e based on t h e d i s t r i b u t i o n o f compounds t o be s e p a r a t e d between a m o b i l e phase and a s t a -t i o n a r y phase. Because -t h e d i f f e r e n -t compounds w i l l be d i s -t r i b u -t e d -t o a d i f f e r e n t degree between b o t h p h a s e s , t h e i r m i g r a t i o n v e l o c i t y a l o n g t h e c h r o m a t o g r a p h i c column w i l l be d i f f e r e n t r e s u l t i n g i n a s e p a r a t i o n . H i g h P r e s s u r e L i q u i d Chromatography (HPLC) has now r e a c h e d a l e v e l o f development which p e r m i t s some s t a n d a r d i z a t i o n . Columns o f 10 cm l o n g f i l l e d w i t h 5 urn p a r t i c l e s a r e commonly u s e d . Those columns have a o r e s -s u r e drop of -some 200 atm. T h e o r e t i c a l l y 50 compound-s can be -s e p a r a t e d i n l e s s t h a n 10 m i n u t e s . P r a c t i c a l l y , a s e p a r a t i o n o f 10 compounds i s

S t r a i g h t Phase L i q u i d Chromatography

When t h e s t a t i o n a r y phase i s more p o l a r t h a n t h e m o b i l e phase, we sneak of S t r a i g h t Phase Chromatography (SP-Chromatography). The concept o f pol a r i t y p pol a y s an i m p o r t a n t r o pol e i n chromatography. P o pol a r i t y can be r e pol a -t e d -t o -t h e c o h e s i v e energy r e q u i r e d -t o -t r a n s f e r one mole of a s u b s -t a n c e f r o m t h e i d e a l gas phase t o i t s l i q u i d s t a t e . A l k a n e s a r e c o n s i d e r e d as t y p i c a l l y a p o l a r and water i s t h e most p o l a r s u b s t a n c e . The l a r g e r t h e d i f f e r e n c e between t h e p o l a r i t y of t h e m o b i l e and t h e s t a t i o n a r y phase, t h e more s e l e c t i v e t h e system w i l l b e .

A l u m i n a as w e l l as s i l i c a a r e t y p i c a l p o l a r a d s o r b e n t s , and v e r v n o p u l a r i n SP—Chromatography. The r e l a t e d s e p a r a t i o n mechanisms a r e now w e l l un-d e r s t o o un-d ( 1 , 2 ) . As c o m p e t i t o r s , t h e y have v e r y s i m i l a r c h a r a c t e r i s t i c s . Both posses h y d r o x y l g r o u p s on t h e s u r f a c e , c a p a b l e t o i n t e r a c t v i a hy-d r o g e n bonhy-ding w i t h s o l u t e s ( 3 , 4 ) .

The more b a s i c n a t u r e of a l u m i n a f a v o u r s s e p a r a t i o n of a c i d i c compounds ( 5 ) , whereas s i l i c a i s t o be p r e f e r r e d f o r t h e s e p a r a t i o n o f b a s i c com-pounds. A c i d t r e a t e d and n e u t r a l a l u m i n a a r e a l s o used f o r b a s i c samples ( 2 ) . However, t h e y must be r e g a r d e d as s u b t y p e s of t h e bare a l u m i n a . The a c t i v i t y of s i l i c a v a n i s h e s w i t h t h e d i s a p p e a r a n c e of s u r f a c e hv-d r o x y l groupes a t h i g h t e m p e r a t u r e ( 6 ) . An i n c r e a s e i n t h e a c t i v a t i o n t e m p e r a t u r e of a l u m i n a y i e l d s a c i d i c o r p o s i t i v e l y charged s i t e s ( a l u m i -nium i o n s o r p r o t o n a t e d v a c a n c i e s ) and b a s i c o r p r o t o n a c c e p t o r s i t e s ( o x i d e o r hydroxyde i o n s ) ( 7 ) a s s o c i a t e d i n a v e r y s t r o n g e l e c t r o s t a t i c f i e l d ( 8 ) . Hence, t h e water c o n t e n t of a l u m i n a c a n be used t o m a n i p u l a t e r e t e n t i o n (4) and must t h e r e f o r e be r i g o u r o u s l y c o n t r o l l e d .

A l u m i n a has a l s o been p r e f e r e n t i a l l y used f o r t h e s e p a r a t i o n o f a r o m a t i c h y d r o c a r b o n s and n o n - s a t u r a t e d h y d r o c a r b o n s ( 9 ) .

R e v e r s e d Phase Chromatography

When t h e m o b i l e phase i s more p o l a r t h a n t h e s t a t i o n a r y phase, t h e term R e v e r s e d Phase Chromatography i s u s e d . With a s t a t i o n a r y phase c l o s e t o an a l k a n e and an aqueous m o b i l e phase, t h e p o l a r i t y d i f f e r e n c e , and hence t h e s e p a r a t i o n p o t e n t i a l w i l l be maximal.

Because water i s by f a r t h e most p o l a r s o l v e n t , t h e p o l a r i t y of a wa-t e r / o r g a n i c m o b i l e phase m i x wa-t u r e can v a r y over a broad r a n g e . By choos i n g t h e j u d i c i o u choos r a t i o , mochoost choos o l u t e choos c a n be e l u t e d w i t h o p t i m a l r e t e n -t i o n . The p r e c i s i o n i n -t h e p r e p a r a -t i o n of -t h e w a -t e r / o r g a n i c m i x -t u r e i s v e r y i m p o r t a n t i n o r d e r t o o b t a i n r e p r o d u c i b l e r e t e n t i o n . I n c h a n t e r 6, we p r e s e n t a s o l v e n t programmer t h a t can d e l i v e r a c c u r a t e and p r e c i s e s o l v e n t m i x t u r e s under c h r o m a t o g r a p h i c c o n d i t i o n s .

The a p o l a r s t a t i o n a r y phases a r e o b t a i n e d by bonding a l k y l c h a i n s onto a b a r e s i l i c a s u p p o r t . The c h e m i c a l bond i s g e n e r a l l y o b t a i n e d bv r e a c t i o n of a c h l o r o s i l a n e w i t h a s i l a n o l g r o u p . D e s p i t e u n c o n f i r m e d i n d i c a t i o n s i n t h e l i t e r a t u r e t o t h e c o n t r a r y ( 1 0 , 1 1 ) , we show i n c h a p t e r 2 t h a t i t i s not p o s s i b l e t o a c h i e v e a s t a b l e m o d i f i c a t i o n of t h e a l u m i n a s u r f a c e . We b e l i e v e t h a t t h i s n e g a t i v e r e s u l t i s t h e main r e a s o n f o r t h e s t e a d i l y d e c r e a s e d use of a l u m i n a i n l i q u i d chromatography.

Indeed t h e r e a c t i v i t y of t h e s i l a n o l groups has been w i d e l y used t o p r o duce a l a r g e number of m o d i f i e d m a t e r i a l s . B e s i d e s t h e h y d r o p h o b i c s u p -p o r t s , o b t a i n e d by r e a c t i o n o f a l k y l s i l a n e s (12) o r f l u o r o s u b s t i t u t e d a l k y l s i l a n e s ( 1 3 ) , a new g e n e r a t i o n of m o d e r a t e l y p o l a r phases was i n -t r o d u c e d ( n i -t r i l e - , d i o l - , a m i n o - ) . A l s o s i l i c a based i o n exchange ma-te- mate-r i a l s wemate-re d e v e l o p e d ( 1 4 ) . As mate-r e s u l t of t h e s e a c t i v i t i e s c h e m i c a l l y mo-d i f i e mo-d s i l i c a has p e n e t r a t e mo-d i n t o a l l a r e a s o f l i q u i mo-d chromatography anmo-d has become t h e m a t e r i a l of c h o i c e f o r t h e s t a t i o n a r y p h a s e .

D i f f i c u l t i e s e n c o u n t e r e d w i t h s i l i c a based m a t e r i a l s

The u s e r s of c h e m i c a l l y bonded phases a r e f a c e d w i t h two i n h e r e n t weak p o i n t s of t h e m a t e r i a l . The s i l i c a m a t r i x i s a t t a c k e d by a l k a l i n e s o l u t i o n s (pH > 7) and a s i g n i f i c a n t number of u n r e a c t e d s i l a n o l groups r e -mains a t t h e s u r f a c e .

The work of Berendsen (12) shows how t h e r e a c t i o n between s i l a n e and s i l a n o l groups can be o p t i m i z e d i n o r d e r t o m i n i m i z e t h e number o f r e s i -d u a l s i l a n o l s . However, t h e s t e r i c a l h i n -d r a n c e between t h e h y -d r o c a r b o n c h a i n s i s t h e l i m i t i n g f a c t o r f o r a c o m p l e t e c o v e r a g e .

The r e m a i n i n g s i l a n o l groups can i n f l u e n c e t h e r e t e n t i o n mechanism by i n t e r a c t i o n w i t h p o l a r s o l u t e s o r t h r o u g h e l e c t r o s t a t i c i n t e r a c t i o n s w i t h i o n i z e d s o l u t e s ( 1 5 , 1 6 ) . When t h e s o l u t e i s c a t i o n i c , t a i l i n g o r c h e m i s o r p t i o n i s o f t e n o b s e r v e d . I n o r d e r t o l i m i t t h e s e n e g a t i v e i n f l u -e n c -e s , d i f f -e r -e n t approach-es c a n b-e a d o p t -e d .

F i r s t t h e pH of t h e m o b i l e phase c a n be i n c r e a s e d s o t h a t t h e i o n i z a t i o n of b a s i c compounds would be s u p p r e s s e d . Such a n i n c r e a s e c a n o n l y be t o -l e r a t e d up t o pH = 7, as h i g h e r pH-va-lues wou-ld r e s u -l t i n s e v e r e damage of t h e column. T h i s t e c h n i q u e i s thus l i m i t e d t o weakly b a s i c compounds (pKa < 6 ) .

By d e c r e a s i n g t h e pH, t h e i o n i z a t i o n o f s i l a n o l groups c a n a l s o be s u p -p r e s s e d . A r e a l im-provement i n t h e -peak sha-pe i s o b s e r v e d f o r a -pH-value as low as 2, but a t o t a l s u p p r e s s i o n o f s i l a n o l i o n i z a t i o n c a n o n l v be reached w i t h a pH of 1.5 ( 1 7 ) . Such a low pH a l s o d i s s o l v e s t h e s i l i c a m a t e r i a l .

A n o t h e r p o s s i b i l i t y i s t h e a d d i t i o n of s a l t t o t h e m o b i l e phase ( 1 8 , 19) i n o r d e r t o l i m i t t h e e l e c t r o s t a t i c i n t e r a c t i o n s . T h i s t e c h n i q u e o f t e n r e q u i r e s c o n c e n t r a t i o n s c l o s e t o t h e l i m i t of t h e s o l u b i l i t y of t h e s a l t ( b u f f e r ) , even when s a l t s a r e chosen t h a t d i s s o l v e e a s i l y i n m o b i l e p h a -ses w i t h h i g h o r g a n i c c o n t e n t . Such m o b i l e pha-ses c a n be v e r v c o r r o s i v e towards t h e i n s t r u m e n t a t i o n .

A t h i r d a l t e r n a t i v e i s t h e i n c r e a s i n g l y p o p u l a r I o n P a i r i n g t e c h n i q u e , a l t h o u g h t h e e x a c t mechanism has not v e t been e l u c i d a t e d . I t i s w e l l a c c e p t e d ( 2 0 , 21, 22, 23) t h a t p o s i t i v e l y charged compounds ( e . g . q u a t e r -nary amines) s h i e l d t h e n e g a t i v e l y charged s i l a n o l g r o u p s . As a r e s u l t , t h e r e t e n t i o n of amine and amino a c i d s becomes more r e g u l a r ( i . e . governed by h y d r o p h o b i c i n t e r a c t i o n s ) . The peak shape i s t h e n g r e a t l y i m -p r o v e d .

When n e g a t i v e l y charged p a i r i n g i o n s a r e u s e d , t h e r e t e n t i o n o f b a s i c compounds s t e a d i l y i n c r e a s e s but t h e peak shape i s not d i r e c t l y i n f l u e n -c e d . However, when t h e m o b i l e phase i s not b u f f e r e d , t h e a d d i t i o n o f

s u c h a c i d i c agents r e s u l t s i n a d e c r e a s e of pH w h i c h i s f a v o u r a b l e f o r the peak shape. A d d i t i o n of t r i f l u o r o - a c e t i c a c i d o r h e p t a n e s u l p h o n i c a c i d i n a n o n - b u f f e r e d aqueous m o b i l e phase i n d u c e s a pH c l o s e t o pH = 2, where i o n i z a t i o n of s i l a n o l groups i s h i g h l y s u p p r e s s e d (24, 2 5 ) . A l t e r n a t i v e s f o r Reversed Phase S t a t i o n a r y Phases

The development of new r e v e r s e d phase m a t e r i a l s not based on s i l i c a has been a major i s s u e i n t h e l i t e r a t u r e s , t h e c h a l l e n g e b e e i n g t h e a c q u i s i t i o n of a h y d r o p h o b i c m a t e r i a l , r e s i s t e n t t o extreme p H c o n d i t i o n s , h a -v i n g a s u f f i c i e n t m e c h a n i c a l s t a b i l i t y and t h e n e c e s s a r y c o r e s t r u c t u r e , p a r t i c l e d i a m e t e r and a c t i v e s u r f a c e which i s r e q u i r e d f o r HPLC.

Carbon-based m a t e r i a l s have g a i n e d p o p u l a r i t y i n gas chromatography (GC) ( 2 6 ) . U n f o r t u n a t e l y t h e y a r e not d i r e c t l y s u i t a b l e f o r HPLC because of t h e i r e x t r e m e l y poor m e c h a n i c a l s t a b i l i t y . However t h e good p H - s t a b i l i t v has i n c i t e d some work towards t h e c o n s o l i d a t i o n of t h e s e m a t e r i a l s . E l e c t r o c h e m i c a l r e d u c t i o n of P o l y t e t r a f l u o r e t h y l e n e (PTFE) seems t o y i e l d a r e l a t i v e l y s t r o n g and a c t i v e c a r b o n m a t e r i a l ( 2 7 , 2 8 ) . Guiochon and co-workers have c o n s o l i d a t e d t h e g r a p h i t i z e d t h e r m a l c a r b o n b l a c k used i n GC, by d e p o s i t i o n of p y r o l y t i c c a r b o n from t h e vapour phase py-r o l y s i s of benzene f o l l o w e d by g py-r a p h i t i z a t i o n of t h e p py-r o d u c t ( 2 9 , 30, 3 1 ) . L a t e r ( 3 2 , 3 3 ) , p y r o l y s i s was p e r f o r m e d on s i l i c a . Knox (34) has r e c e n t l y p r e s e n t e d a g l a s s y c a r b o n m a t e r i a l w i t h h i g h p o r o s i t y and good m e c h a n i c a l s t a b i l i t y . However, a l l t h e s e m a t e r i a l s have an e n e r g e t i c a l l y n o n u n i f o r m , o f t e n h i g h l y a c t i v e s u r f a c e r e s p o n s i b l e f o r l o c a l and s p e -c i f i -c i n t e r a -c t i o n s r e s u l t i n g i n a s s y m m e t r i -c a l peak shape. I n terms of e f f i c i e n c y , t h e s e c h r o m a t o g r a p h i c s u p p o r t s cannot compete w i t h c h e m i c a l -l y bonded s i -l i c a p h a s e s .

P o r o u s n o n - p o l a r polymers a r e a l s o used i n RP-chromatography. The most p o p u l a r one i s t h e A m b e r l i t e XAD2, a s t y r e n e d i v i n y l b e n z e n e porous c o -polymer ( 3 5 , 3 6 ) . These r e s i n s a r e r i g i d and r e s i s t t o h i g h l y a l k a l i n e and a c i d i c s o l u t i o n s ( f r o m pH = 1 t o 1 3 ) . However t h e r e s u l t s o b t a i n e d w i t h s u c h columns a r e g e n e r a l l y i n f e r i o r t o t h o s e o b t a i n e d w i t h good q u a l i t y h y d r o c a r b o n a c e o u s bonded p h a s e s . Even i f no s i g n i f i c a n t macro-s c o p i c macro-s w e l l i n g o c c u r macro-s , t h e l e macro-s macro-s c r o macro-s macro-s - l i n k e d r e g i o n macro-s at t h e pore w a l l macro-s are l i k e l y t o s w e l l i n a n o n u n i f o r m f a s h i o n due t o a g r a d i e n t i n c r o s s -l i n k i n g norma-l t o t h e pore w a -l -l . S o -l u t e m o -l e c u -l e s of r e -l a t i v e -l y s m a -l -l d i m e n s i o n s can e x p l o r e t h e s w o l l e n pore w a l l r e g i o n where t h e d i f f u s i o n r a t e i s low. C o n s e q u e n t l y , t h e r a t e of phase exchange, and as a r e s u l t , the e f f i c i e n c y of t h e column i s l o w . F u r t h e r m o r e , column e f f i c i e n c y o f -t e n d e c r e a s e s upon a f r e q u e n -t change of s o l v e n -t . S h r i n k i n g and s w e l l i n g of t h e p a r t i c l e s i s p r o b a b l y l a r g e enough t o cause c h a n n e l i n g of t h e c o -lumn p a c k i n g . However, t h e i n t r o d u c t i o n of a new phase (PRP-1) w i t h 10 um p a r t i c l e d i a m e t e r i s p a r t i c u l a r l y p r o m i s i n g ( 3 7 ) .

A l u m i n a

A l u m i n a i s a l s o r e s i s t e n t t o pH between 1 and 13. I t s c r i s t a l i n e n a t u r e g i v e s i t an e x c e l l e n t m e c h a n i c a l r e s i s t a n c e and no s w e l l i n g can be e x -p e c t e d . A l t h o u g h t h e use of a l u m i n a i n chromatogra-phy has s t e a d i l y

de-c r e a s e d , i t has remained de-c o m m e r de-c i a l l y a v a i l a b l e i n forms and p a r t i de-c l e s i z e s s u i t a b l e f o r h i g h e f f i c i e n c y columns ( 3 8 ) . C o n s e q u e n t l y , a l u m i n a has p e r s i s t e n t l y been used f o r s p e c i a l s e p a r a t i o n p r o b l e m s .

In t h e l i g h t of t h e above d i s c u s s i o n a r e a p p r e c i a t i o n o f a l u m i n a a p p e a r s t o be a p p r o p r i a t e . I t w i l l be shown i n t h e f o l l o w i n g c h a p t e r s I I - V t h a t an u n d e r s t a n d i n g of t h e s u r f a c e p r o p e r t i e s of a l u m i n a p r o v i d e s t h e means t o m a n i p u l a t e t h e s e p a r a t i o n mechanism f o r a v a r i e t y of i o n i z e d s o l u t e s . When mixed o r g a n i c - a q u e o u s s o l v e n t s a r e used s e l e c t i v i t y c a n be g r e a t l y enhanced and a l u m i n a becomes an a t t r a c t i v e c o m p e t i t i v e s t a t i o n a r y phase f o r HPLC.

We a r e now i n a p o s i t i o n t o r e p h r a s e t h e t i t l e of t h i s c h a n t e r f r o m whv a l u m i n a ? t o why not a l u m i n a ?

R e f e r e n c e s

(1) H. E n g e l h a r d t and H. E l g a s s , "LC on S i l i c a and A l u m i n a " i n H i g h Performance L i q u i d Chromatography Advances and P e r s p e c t i v e s . V o l . 1. Academic P r e s s , New York ( 1 9 8 0 ) .

(2) L.R. Snyder and B.E. B u e l l , J . Chem. Eng. Data 11 (1966) 5 4 5 . (3) L.R. S n y d e r , J . P h y s . Chem. 72 (1968) 489.

(4) H. E n g e l h a r d t and H. Wledeman, A n a l . Chem. 45 (1973) 1641. (5) L.R. S n y d e r , J . Chromatogr. 23 (1966) 388.

(6) J.M. B a t h e r and R.A.C. Gray, J . Chromatogr. 122 (1976) 159. (7) J.M. B a t h e r and R.A.C. Gray, J . Chromatogr. 156 (1978) 2 1 . (8) J . K i n g and S.W. B a r s o n , J . Chem. P h y s . 44 (1966) 1007.

(9) L.R. Snyder, " P r i n c i p l e s of a d s o r p t i o n c h r o m a t o g r a p h v " . M. D e k k e r , New York 1968, p. 167.

(10) J.H. Knox and A. P r y d e , J . Chromatogr. 112 (1975) 1971. (11) Y. H i r a t a and M. Novotny, A n a l . Chem. 51 (1971) 1807. (12) G.E. B e r e n d s e n , Ph.D t h e s i s ( 1 9 8 0 ) , D e l f t , The N e t h e r l a n d s .

(13) H.A.H. B i l l i e t , P . J . Schoenmakers and L. de G a l a n , J . Chromatogr. 218 (1981) 443.

(14) R.E. M a j o r s , H.G. B a r t h , C.H. L o c h m u l l e r , A n a l . Chem. 54 (1982) 323 R-363 R.

(16) E. Papp and Gy. V i g h , P r e s e n t e d i n t h e V I Ie I n t e r n a t i o n a l Symposium

on column L i q u i d Chromatography. Baden-Baden (GFR) 1983.

(17) R . I . F l a n a g a n , G.C.A. S t o r e y , R.K. Bhamra and I . J a n e , J . Chromato-g r . 247 (1982) 15.

(18) W.E. R u d z i n k s k i , D. B e n n e t t , V. G a r c i a and M. Seymour, J .

Chromato-g r . S c i . 21 (1983) 57.

(19) W.R. M e i a n d e r , J . Stoveken and C . J . H o r v a t h , J . Chromatogr. 199

(1980) 35.

(20) J.H. Knox and R.A. H a r t w i c k , J . Chromatogr. 204 (1981) 3. (21) A. Mahum and C. H o r v a t h , J . Chromatogr. 203 (1981) 5 3 .

(22) K.E. B i j , C. H o r v a t h , W.R. M e l a n d e r and A. Nahum, J . Chromatogr. 203 (1981) 65.

(23) C.F. G e l i j k e n s and A.P. De L e e n h e e r , J . Chromatogr. 194 (1980) 305. (24) C. O l i e m a n , L. Maat, K. W a l i s z e w s k y and H.C. Beyerman, J .

Chromato-g r . 133 (1977) 382.

(25) W.M.M. Schaaper, D. Voskamp and C. Olieman, J . Chromatogr. 195

(1980) 181.

(26) A.V. K i s e l e v , i n Advance i n Chromatography, chap. 4, p. 113, M a r c e l Dekker, New Y o r k ( 1 9 6 7 ) .

(27) Z. P l z a k , F.P. Dousek, J . J a n s t a , J . Chromatogr. 147 (1978) 137. (28) E. Smolkova, J . Zima, F.P. Dousek and J . J a n s t a , J . Chromatogr. 191

(1980) 6 1 . (29) H. C o l i n , C. Eon, G. G u i o c h o n , J . Chromatogr. 119 (1976) 4 1 . (30) H. C o l i n , C. Eon, G. G u i o c h o n , J . Chromatogr. 122 (1976) 2 2 3 . (31) H. C o l i n , C. Eon, G. G u i o c h o n , J . Chromatogr. 137 (1977) 19. (32) H. C o l i n , G. G u i o c h o n , J . Chromatogr. 126 (1976) 4 3 . (33) H. C o l i n , N. Ward, G. G u i o c h o n , J . Chromatogr. 149 (1978) 149. (34) M . T . G i l b e r t , J.H. Knox, B. Kaur, Chromatographia 16 (1982) 138.

(35) D.J. P i e t r z y k , E.P. K r o e f f and T.D. R o t s c h , A n a l Chem. 50 (1978) 497.

(36) E.P. K r o e f f and D.J. P i e t r z y k , A n a l Chem. 50 (1978) 502. (37) D.P. L e e , J . Chromatogr. S c i . 20 (1982) 203.

(38) M.J. Holdoway, Report of t h e c h r o m a t o g r a p h i c d i s c u s s i o n group AERE M 2749 ( 1 9 7 6 ) .

C H A P T E R 2

THE BEHAVIOUR O F ALUMINA IN A N

A Q U E O U S MEDIUM

SUMMARY

The a p p l i c a b i l i t y of a l u m i n a as a s t a t i o n a r y phase i n l i q u i d chromato-graphy i s c o n s i d e r e d .

D e s p i t e t h e p r e s e n c e o f h y d r o x y l groups on w e t t e d a l u m i n a , t h e c h e m i c a l m o d i f i c a t i o n w i t h s i l a n e r e a g e n t s i n a n a l o g y t o s i l i c a i s not s u c c e s s f u l . A l t h o u g h d e p o l a r i s a t i o n c a n be a c h i e v e d w i t h h e x a m e t h y l d i s i -l a z a n e , t h e p r o d u c t i s not s t a b -l e a g a i n s t h y d r o -l y s i s i n aqueous m e d i a . The use of a l u m i n a as an i o n exchanger i s more p r o m i s i n g , e s p e c i a l l y because a l u m i n a appears t o be s t a b l e o v e r a v e r y b r o a d pH r a n g e . As a r e s u l t o f i t s a m p h o t e r i c c h a r a c t e r a l u m i n a c a n be used e i t h e r as an a n i o n exchanger o r as a c a t i o n e x c h a n g e r . The l a t t e r p o s s i b i l i t y i s more p r o f i t a b l e i n a l k a l i n e s o l v e n t s and a l l o w s t h e s e p a r a t i o n o f s t r o n g l y b a s i c compounds a t pH v a l u e s as h i g h as 12. R e t e n t i o n can be c o n t r o l l e d by j u d i c i o u s c h o i c e o f i o n i c s t r e n g t h , c o u n t e r i o n and pH.

INTRODUCTION

The use of a l u m i n a as s t a t i o n a r y phase m a t e r i a l i n l i q u i d c h r o m a t o g r a phy has s t e a d i l y d e c r e a s e d d u r i n g t h e l a s t t e n y e a r s . I n c l a s s i c a l a d -s o r p t i o n chromatography (LSC) i t mu-st compete w i t h -s i l i c a ( 1 ) and b o t h m a t e r i a l s have been l a r g e l y s u p e r s e d e d by c h e m i c a l l y m o d i f i e d , d e p o l a -r i z e d s i l i c a s .

I n d e e d , l i k e s i l i c a , a l u m i n a must be r e g a r d e d as a t y p i c a l p o l a r a d s o r b e n t and most s e p a r a t i o n s p r o c e e d i n t h e same way on t h e two o x i -d e s . However, whereas s i l i c a i s o n l y a c t i v e t h r o u g h i t s s u r f a c e hyd r o x y l g r o u p s , a l u m i n a p o s s e s s e s two a l t e r n a t i v e p o s s i b i l i t i e s f o r i n -t e r a c -t i o n w i -t h s o l u -t e s ( 2 ) .

( i ) Many s o l u t e s a d s o r b on t h e a c i d i c s i t e s of a l u m i n a t h r o u g h nu-c l e o p h i l i nu-c i n t e r a nu-c t i o n nu-c o r r e l a t e d w i t h t h e i r e l e nu-c t r o n d e n s i t y o r b a s i c i t y .

The same s i t e s may w e l l form charge t r a n s f e r complexes w i t h t y -p i c a l e l e c t r o n d o n o r s , such as a r o m a t i c s o l u t e s ( 3 ) .

( i i ) A l t e r n a t i v e l y , a c i d i c s o l u t e s may i n t e r a c t w i t h b a s i c s i t e s on the a l u m i n a s u r f a c e t h r o u g h t r a n s f e r of a p r o t o n l e a d i n g t o c h e -m i s o r p t i o n . One i -m p o r t a n t exa-mple i s w a t e r , w h i c h t h r o u g h che-mi- chemi-s o r p t i o n g i v e chemi-s r i chemi-s e t o two h y d r o x y l groupchemi-s on t h e a l u m i n a chemi-s u r f a c e ( f i g . 1 ) . F i g . 1. ChemI s o r p t i o n o f w a t e r on b a r e a l u m i n a . E x p l a n a t i o n I n t h e t e x t . I n g e n e r a l t h e n , a l u m i n a i n t e r a c t s s t r o n g l y w i t h p o l a r i z e d m o l e c u l e s , so much so t h a t i t s a p p l i c a b i l i t y i s o f t e n l i m i t e d by t h e a d v e r s e i n -f l u e n c e o-f c h e m i s o r p t i o n on t h e shape o-f t h e e l u t i n g peak.

Such n e g a t i v e e f f e c t s a r e l a r g e l y absent i n modern r e v e r s e d p h a s e l i q u i d chromatography where t h e i n t e r a c t i o n of h y d r o c a r b o n a c e o u s s t a t i o -n a r y phases w i t h s o l u t e s p r o c e e d s l a r g e l y t h r o u g h h y d r o p h o b i c f o r c e s . The s y m m e t r i c a l peak shape and t h e r e s u l t i n g h i g h - e f f i c i e n c y columns have s t r o n g l y c o n t r i b u t e d t o t h e d e c l i n e of c l a s s i c a l a d s o r p t i o n c h r o matography. The c o m b i n a t i o n of an a p o l a r s t a t i o n a r y phase w i t h a j u d i -c i o u s l y -chosen p o l a r m o b i l e phase ( 4 , 5) i s -c e r t a i n l y t h e p r e f e r r e d c h r o m a t o g r a p h i c system f o r t h e s e p a r a t i o n of n o n - i o n i c s p e c i e s .

I o n i c s o l u t e s r e q u i r e a s p e c i a l a p p r o a c h t o d i m i n i s h t h e i r i n t e r a c t i o n w i t h r e m a i n i n g s i l a n o l g r o u p s , which r e s u l t s i n t a i l i n g p e a k s . One p o s s i b i l i t y i s t o s u p p r e s s s o l u t e i o n i z a t i o n t h r o u g h pH c o n t r o l of t h e m o b i l e phase ( 6 , 7 ) . T h i s i s not p o s s i b l e f o r s t r o n g a c i d s and even m o d e r a t e l y s t r o n g b a s e s , because t h e r e q u i r e d pH would a t t a c k t h e s i -l i c e o u s s u p p o r t which i s o n -l y s t a b -l e o v e r t h e range 2 &-lt; pH &-lt; 8 ( 8 , 9 ) . An a l t e r n a t i v e s o l u t i o n i s o f f e r e d by t h e i o n p a i r i n g t e c h n i q u e , where t h e i o n i c s p e c i e s p o s s i b l y c o u p l e s w i t h a h y d r o p h o b i c c o u n t e r i o n and e l u t e s i n n e u t r a l form (10, 1 1 ) . I n t h i s r e s p e c t a l u m i n a a l s o o f f e r s i n t e r e s t i n g p e r s p e c t i v e s , because i t i s r e s i s t a n t t o b a s i c s o l v e n t s and thus p e r m i t s i o n i z a t i o n s u p n r e s -s i o n of -s t r o n g b a -s e -s . Moreover, -s e v e r a l a u t h o r -s c l a i m t o have m o d i f i e d a l u m i n a t h r o u g h r e a c t i o n w i t h s i l a n e s i n a n a l o g y t o t h e m o d i f i c a t i o n of s i l i c a ( 1 2 , 13, 14, 1 5 ) . However, i t might be s u s p e c t e d t h a t t h e r e s u l t i n g A l - C — S i - C c h a i n i s l e s s s t a b l e t h a n t h e e q u i v a l e n t S i - O - S i - C c h a i n . I t w i l l be t h e s u b j e c t of t h i s s t u d y t o a s s e s s t h e s u i t a b i l i t y of a l u m i n a f o r modern l i q u i d chromatography. STRUCTURE OF ALUMINA

The c r y s t a l l i n e a l u m i n a g e n e r a l l y used i n chromatography i s known as y - a l u m i n a and i s o b t a i n e d by d e h y d r a t i o n of t h e m i n e r a l B a y e r i t e a t e l e v a t e d t e m p e r a t u r e (200-600°C). The arrangement of t h e atoms i n y-a l u m i n y-a cy-an be d e r i v e d from t h e i d e y-a l s p i n e l MgA^O^, where t h e u n i t c e l l c o n t a i n s 32 oxygen atoms a r r a n g e d i n 64 tetraëders, 8 of which c o n t a i n a c e n t r a l magnesium atom, and 32 octaëders, 16 of which c o n -t a i n a c e n -t r a l a l u m i n i u m a-tom. To m a i n -t a i n -t h e c h a r g e b a l a n c e i n y-a l u m i n y-a , t h e 8 my-agnesium y-atoms y-a r e r e p l y-a c e d by 5 1/3 y-a l u m i n i u m y-atoms, w h i c h l e a d s t o v a c a n c i e s and h e t e r o g e n e o u s d i s t r i b u t i o n of t h e a l u m i -nium atoms over t h e tetraëders and octaëders of t h e u n i t c e l l . The 2 2/3 v a c a n t p o s i t i o n s per u n i t c e l l a c t as s t r o n g n u c l e o p h i l i c a d s o r p -t i o n s i -t e s . C o n v e r s e l y , when -t h e vacancy -t a k e s up a p o s i -t i v e i o n ( e . g . a p r o t o n ) , i t a c t s as a s t r o n g e l e c t r o p h i l i c s i t e . The o p p o s i t e l y charged s i t e s g i v e r i s e t o a s t r o n g l o c a l e l e c t r o s t a t i c f i e l d t h a t i n -t e r a c -t s w i -t h p o l a r m o l e c u l e s , e v e n -t u a l l y l e a d i n g -t o c h e m i s o r p -t i o n ( 1 8 , 19). The A l - 0 bond has a s t r o n g e r i o n i c c h a r a c t e r t h a n t h e S i - 0 bond ( T a b l e 1 ) . As a r e s u l t i t i s more s e n s i t i v e t o n u c l e o p h i l i c a t t a c k . Indeed, Stone (20) mentions t h a t t r i s - t r i m e t h v l s i l o x y l a l u m i n i u m , (Me-jSiOJ-jAl, decomposes at a r e l a t i v e l y low t e m p e r a t u r e (250*C) by r u p t u r e of t h e A l - 0 bond. From the n a t u r e of t h e A l - 0 bond and t h e e l e c t r o s t a t i c s u r f a c e b e h a v i o u r i t might be s u s p e c t e d t h a t i n s i l a -n i z e d a l u m i -n a t h e s u r f a c e A l - O - S i bo-nd i s l e s s s t a b l e t h a -n t h e S i - O - S i bond.

A l u m i n a i s h y g r o s c o p i c and i t s a d s o r p t i o n p r o p e r t i e s f o r c h r o m a t o g r a -p h i c s o l u t e s de-pend u-pon t h e amount and t h e form of water -p r e s e n t a t i t s s u r f a c e ( 2 1 ) . T h i s water i s c e r t a i n l y not bonded u n i f o r m l y t o t h e s u r f a c e . The heat of a d s o r p t i o n d e c r e a s e s from 400 k j / m o l f o r t h e f i r s t few water m o l e c u l e s a t t a c h e d t o d r y a l u m i n a t o 40 kJ/mol when t h e s u r f a c e i s c o m p l e t e l y c o v e r e d by a monolayer of water ( 2 2 ) .

Tab I e 1. I o n i c c h a r a c t e r o f t h e b o n d . X -X ( a ) P a u l I n g (b> o m Hannay ( c ) S.-O 1.7 51 37 A I - 0 2.0 6 3 46 (a) E l e c t r o n e g a t i v i t y d i f f e r e n c e b e t w e e n t h e m e t a l and o x y g e n . ( b ) P a u l i n g s c a l e ( 1 6 ) .

( c ) Hannay and Smyth s c a l e ( 1 7 ) .

At room t e m p e r a t u r e t h e I R - s p e c t r u m of h y d r a t e d a l u m i n a d i s p l a y s t h e two broad bands a t 3300 and 1650. cm-* c h a r a c t e r i s t i c f o r w a t e r . Water

i s t h e n adsorbed i n m o l e c u l a r form. When, f o r c h r o m a t o g r a p h i c u s e , a l u m i n a i s s u b j e c t e d t o a h i g h e r t e m p e r a t u r e , t h e above mentioned bands a r e r e p l a c e d by bands near 3700 cm"' c h a r a c t e r i s t i c f o r t h e hy-d r o x y l group ( 2 3 ) . A p p a r e n t l y , t h e water m o l e c u l e hy-decomposes t o form two h y d r o x y l groups as shown i n F i g . 1. The two d i f f e r e n t h y d r o x y l groups r e t a i n t h e a c i d i c o r b a s i c c h a r a c t e r of t h e u n d e r l y i n g s i t e and e x p l a i n t h e a m p h o t e r i c p r o p e r t i e s of a l u m i n a as an i o n e x c h a n g e r . I t can a c t as an a n i o n exchanger by d i s p l a c i n g t h e h y d r o x y l group a t -t a c h e d -t o -t h e a l u m i n i u m a-tom. And i -t c a n a c -t as a c a -t i o n exchanger t h r o u g h t h e p r o t o n a t t a c h e d t o t h e oxygen atom.

EXPERIMENTAL

Two t y p e s of a l u m i n a have been u s e d . The gas c h r o m a t o g r a p h i c and i o n exchange e x p e r i m e n t s were performed w i t h a l u m i n i u m o x i d e F l ( b a t c h 2 0 8 7 2 ) , s u p p l i e d by A p p l i e d S c i e n c e L a b o r a t o r i e s (U.S.A.). A c c o r d i n g t o t h e m a n u f a c t u r e r ' s s p e c i f i c a t i o n s t h e p a r t i c l e s i z e ranges from 75 t o 150 \im and t h e s p e c i f i c s u r f a c e a r e a i s 210 m^/g.

A t t e m p t s at c h e m i c a l m o d i f i c a t i o n and l i q u i d chromatography e x p e r i -ments were performed w i t h ALOX T 10 ( b a t c h EF 42) from Merck ( G . F . R . ) . The B.E.T. n i t r o g e n a d s o r p t i o n t e c h n i q u e u s i n g t h e c y l i n d r i c a l model d i s c u s s e d by De Boer ( 2 4 ) y i e l d s a s p e c i f i c s u r f a c e a r e a o f 58 m^/g and a pore s i z e d i s t r i b u t i o n w i t h a mean v a l u e of 6 nm.

A l l c h e m i c a l s used were of t h e h i g h e s t q u a l i t y c o m m e r c i a l l y a v a i l a b l e . Water was s p e c i a l l y t r e a t e d w i t h i o n exchange r e s i n s and c a r b o n f i l t e r s a f t e r d i s t i l l a t i o n . A d s o r p t i o n i s o t h e r m s f o r copper and d i s s o l u -t i o n o f a l u m i n a were d e -t e r m i n e d by i n d u c -t i v e l y c o u p l e d plasma e m i s s i o n s p e c t r o m e t r y ( I C P ) . T h e r m o g r a v i m e t r i c s t u d i e s were made w i t h a S a r t o -r i u s Type A 7073 b a l a n c e .

The c h r o m a t o g r a p h i c equipment c o n s i s t s b a s i c a l l y of an M 6000 pump, a U6K i n j e c t o r and a UV 440 d e t e c t o r , a l l from Waters A s s o c i a t e s M i l f o r d , Mas, U.S.A. NonUV a b s o r b i n g amines were d e t e c t e d by d.c. v o l -tammetry u s i n g a Metrohm t y p e 634 c e l l , two g l a s s y c a r b o n e l e c t r o d e s , a c a l o m e l r e f e r e n c e e l e c t r o d e and a PAR model 174 p o l a r o g r a p h .

C h r o m a t o g r a p h i c columns of s t a n d a r d d i m e n s i o n s (25 cm, 4.6 mm I.D.) were home packed w i t h ALOX T 10 u s i n g t h e e q u a l d e n s i t y t e c h n i q u e u n -der a d d i t i o n o f g l y c e r o l t o t h e s l u r r y . T y p i c a l p l a t e numbers a r e around 3000.

RESULTS AND DISCUSSION

C h e m i c a l m o d i f i c a t i o n of a l u m i n a .

The c h e m i c a l m o d i f i c a t i o n of s i l i c a t h r o u g h r e a c t i o n w i t h e.g. monoc h l o r o s i l a n e s p r o monoc e e d s s m o o t h l y ( 2 5 ) . Upon r e a monoc t i o n w i t h s u r f a monoc e s i l a -n o l groups a t r u e , s t a b l e c h e m i c a l bo-nd i s formed ( S i - O - S i - R ) . The a p o l a r c h a r a c t e r of t h e m o d i f i e d s u r f a c e i s e a s i l y d e m o n s t r a t e d by t h e b e h a v i o u r o f a d r o p l e t of w a t e r added t o t h e m a t e r i a l : t h e d r o p l e t r e -mains a s p h e r e and does not wet t h e m o d i f i e d s i l i c a .

The p r e s e n c e of h y d r o x y l groups on t h e a c t i v e a l u m i n a s u r f a c e ( f i g . 1) s u g g e s t s t h a t a s i m i l a r m o d i f i c a t i o n might be f e a s i b l e w i t h a l u m i n a . I n d e e d , m o d i f i c a t i o n of a l u m i n a t h r o u g h c h e m i c a l r e a c t i o n s has been d e s c r i b e d i n t h e l i t e r a t u r e . Knox (12) r e p o r t e d t h e r e a c t i o n o f a l u m i n a w i t h a m i n o s i l a n e , c y a n o s i l a n e and p y r i d y l s i l a n e . E l e m e n t a l a n a l y s i s showed t h e p r e s e n c e o f c a r -bon. I n a column e x p e r i m e n t , t h e p y r i d y l m o d i f i e d a l u m i n a a c t e d as a weak a n i o n e x c h a n g e r . On t h e b a s i s of t h i s r e s u l t . Novotny (14) r e a c t e d i n s i t u by p a s s i n g o c t a d e c y l c h l o r o s i l a n e t h r o u g h a m i c r o c a p i l l a r y column c o n t a i n i n g a l u mina p a r t i c l e s . I n c o n s e c u t i v e r e v e r s e d phase c h r o m a t o g r a p h i c e x p e r i -ments t h e m a t e r i a l was o b s e r v e d t o have become l e s s p o l a r .

Our e x p e r i e n c e has been r a t h e r c o n t r a r y t o t h e s e r e p o r t s . We o b s e r v e d f i r s t t h a t most c h e m i c a l r e a g e n t s t h a t have been used s u c c e s s f u l l y w i t h s i l i c a (26) show no r e a c t i v i t y towards a l u m i n a . Even t r i m e t h y l c h l o r o s i l a n e (TMCS), which r e a c t s r a p i d l y w i t h s i l i c a , i s q u i t e i n a c t i v e . However, i f we use h e x a m e t h y l d i s i l a z a n e (HMDS) t h e r e s u l t i s e n c o u r a g i n g . A f t e r a c o n t a c t t i m e of one hour a t 50°C and r e m o v a l of e x -c e s s r e a g e n t s t h e p r o d u -c t i s a p p a r e n t l y n o n p o l a r as d e m o n s t r a t e d by t h e w a t e r d r o p l e t t e s t d e s c r i b e d a b o v e .

The d i f f e r e n t r e a c t i v i t y of TMCS and HMDS towards a l u m i n a i s c o n f i r m e d by a n o t h e r e x p e r i m e n t . A gas chromatography column i s packed w i t h a l u -mina and s u b j e c t e d t o e x c e s s TMCS and HMDS i n j e c t e d a t e l e v a t e d tempe-r a t u tempe-r e (200°C). A f t e tempe-r each i n j e c t i o n t h e tempe-r e t e n t i o n o f notempe-rmal a l k a n e s , w h i c h i s a t t r i b u t e d t o t h e p r e s e n c e of a c i d i c s i t e s on a l u m i n a ( 2 7 , 28) was measured. S u c c e s s i v e i n j e c t i o n s of TMCS o r TMES ( t r i m e t h y l -e t h o x y s i l a n -e ) l -e a v -e t h -e r -e t -e n t i o n o f a l k a n -e s u n a l t -e r -e d , b u t subs-equ-ent i n t r o d u c t i o n of HMDS r e s u l t s i n a d r a s t i c d e c r e a s e o f a l k a n e r e t e n -t i o n . T h i s a g a i n shows -t h a -t -t h e h i g h l y n u c l e o p h i l i c d i s i l a z a n e , a f -t e r h a v i n g r e l e a s e d a d s o r b e d water by r e a c t i o n , i s f i r m l y a t t a c h e d t o t h e a l u m i n a s u r f a c e and remains s o even a t 200"C.

T h e r m o g r a v i m e t r i c s t u d y of t h e r e a c t e d m a t e r i a l e x t r a c t e d from t h e GC columns r e v e a l s t h a t t h e d i s i l a z a n e i s r e l e a s e d between 200 and 300°C.

By c o n t r a s t , a n o n - t r e a t e d a l u m i n a l o s e s weight much more g r a d u a l l y up t o 800°C t h r o u g h t h e g r a d u a l r e l e a s e of a d s o r b e d w a t e r . A l l t h e s e o b -s e r v a t i o n -s d e m o n -s t r a t e t h a t HMDS ha-s r e p l a c e d water m o l e c u l e -s and made the a l u m i n a s u r f a c e h y d r o p h o b i c .

U n f o r t u n a t e l y , t h i s does not mean t h a t t h e m o d i f i e d a l u m i n a c a n be used as a s t a t i o n a r y phase i n r e v e r s e d phase l i q u i d chromatography. When c o n t a c t e d w i t h p o l a r s o l v e n t s s u c h as methanol and e s p e c i a l l y wa-t e r , wa-t h e HMDS i s r a p i d l y d i s p l a c e d a g a i n .

ICP-AES a n a l y s i s o f water brought i n c o n t a c t w i t h HMDS a l u m i n a showed the r e l e a s e of 1.02 mg S i p e r 100 mg a l u m i n a a f t e r one d a y . Combined w i t h t h e s p e c i f i c s u r f a c e a r e a o f 58 m^/g t h i s f i g u r e c o r r e s p o n d s t o the o r i g i n a l p r e s e n c e o f 3.4 S i atoms p e r nm^ of HMDS t r e a t e d a l u m i n a . C o n s i d e r i n g t h e s i z e of t h e m o l e c u l e we c o n c l u d e t h a t HMDS has i n d e e d c o m p l e t e l y r e p l a c e d water from t h e a l u m i n a s u r f a c e and formed a com-p l e t e m o n o l a y e r . E v i d e n t l y , however, t h i s HMDS i s not c h e m i c a l l y bonded onto a l u m i n a , but o n l y s t r o n g l y a d s o r b e d , and hence i s r a p i d l y r e -moved upon subsequent c o n t a c t w i t h w a t e r . A p p a r e n t l y , t h e s i l a z a n e mol e c u mol e a c t i n g as a Lewis base i s a t t r a c t e d by t h e a c i d i c s i t e of a mol u -m i n a , but t h e A l N bond i s v e r y p o l a r and not r e s i s t a n t t o t h e a t t a c k of t h e more s t r o n g l y a t t r a c t e d water m o l e c u l e .

O t h e r , p o s s i b l y more b a s i c r e a g e n t s , such as t e r t i a r y amines, do not behave d i f f e r e n t l y . I n i t i a l l y , d e p o l a r i z a t i o n of t h e a l u m i n a s u r f a c e i s o b s e r v e d , but i n t h e p r e s e n c e of water t h e o r i g i n a l , p o l a r a l u m i n a s u r f a c e i s r e s t o r e d . We c o n c l u d e , t h e r e f o r e , t h a t c h e m i c a l m o d i f i c a t i o n of a l u m i n a l e a d i n g t o a non p o l a r m a t e r i a l s u i t a b l e f o r RPLC c a n -not be r e a l i z e d i n t h i s way. Ion exchange p r o p e r t i e s of a l u m i n a .

A l u m i n a a c t s as a t y p i c a l i o n exchanger ( 2 9 ) . But i t i s a l s o amphote-r i c , s o t h a t i t s i o n exchange p amphote-r o p e amphote-r t i e s a amphote-r e s t amphote-r o n g l y pH-dependent. T h i s i s c o n f i r m e d by a few e x p e r i m e n t s w i t h a l u m i n a F l from A p p l i e d S c i e n c e L a b .

A d s o r p t i o n i s o t h e r m s f o r C u ( I I ) i o n s a r e p r e s e n t e d i n F i g . 2 a . When the m a t e r i a l i s used as such i n n e u t r a l s o l u t i o n t h e amount o f c o p p e r a d s o r b e d onto t h e s u r f a c e i n c r e a s e s s l o w l y w i t h t h e c o n t a c t t i m e and w i t h t h e s o l u t i o n c o n c e n t r a t i o n of copper i o n s t o a maximum v a l u e o f about 0.6 mmol/g o r 1.6 atoms/nm^.

A f t e r washing w i t h 0.1 M HC1 t h e c a t i o n exchange c a p a c i t y d e c r e a s e s s i g n i f i c a n t l y and o n l y 0.3 copper atoms/nm^ can be a d s o r b e d onto t h e a c i d i c a l u m i n a . T h i s f i g u r e i s r e a c h e d r a p i d l y and does not i n c r e a s e w i t h p r o l o n g e d c o n t a c t . I n s t e a d we o b s e r v e t h a t i n c o n c e n t r a t e d c o p p e r s o l u t i o n s t h e s u p p o r t m a t e r i a l i t s e l f i s a t t a c k e d and a l u m i n i u m atoms are r e l e a s e d from t h e s u r f a c e . The amount of a l u m i n i u m d i s s o l v e d c a n be measured by ICP AES and c o n v e r t e d t o an e q u i v a l e n t number o f s u r -f a c e a l u m i n i u m d i s l o c a t e d .

F |g » 2. A d s o r p t i o n I s o t h e r m s o f C u C I I ) on a l u m i n a . 10 g a l u m i n a i n 200 mL o f e a c h s o l u

-t i o n .

a) C o m m e r c i a l l y a v a i l a b l e a l u m i n a F . I .

b) The same a l u m i n a washed w i t h a 0.1 M HCl s o l u t i o n .

The r e s u l t s a r e p r e s e n t e d i n F i g . 3 f o r a c i d i c a l u m i n a . I n i t i a l l y , a t low copper c o n c e n t r a t i o n s t h e a l u m i n a i s l e f t i n t a c t and copper i o n s are a d s o r b e d onto t h e s u r f a c e , p o s s i b l y by r e p l a c i n g p r o t o n s . W i t h i n -c r e a s i n g -copper -c o n -c e n t r a t i o n a l u m i n i u m atoms a r e g r a d u a l l y r e l e a s e d and when t h e maximum c a p a c i t y of about 0.3 c o p p e r atoms/nm^ i s r e a c h e d the d i s s o l u t i o n of a l u m i n i u m i n c r e a s e s s h a r p l y . W i t h t h e r e a s o n a b l e a s s u m p t i o n of charge b a l a n c e t h r e e copper atoms would r e p l a c e two a l u -minium atoms and t h e c o r r e s p o n d i n g asymptote i s i n s e r t e d i n f i s . 3. I t i n t e r s e c t s t h e o r d i n a t e at a v a l u e of 0.27 c o p p e r atoms/nm . T h i s would t h e n be t h e maximum c a t i o n exchange c a p a c i t y of a c i d i c a l u m i n a

A L d e s o r b e d a t o m / n m2'1 0.02 0.01 -a t o m / n m F l9 » 3. D e s o r p t l o n o f A K I I I ) on a c i d i c a l u m i n a by i n c r e a s i n g c o n c e n t r a t i o n s o f C u ( l l ) . The s u r f a c e c o n c e n t r a t i o n o f Cu i s t a k e n f r o m c u r v e b In f i g . 2. The same e x p e r i m e n t s w i t h o r i g i n a l a l u m i n a r e v e a l t h a t up t o a s u r f a c e coverage w i t h 1.6 copper atoms p e r nm^ o n l y a few a l u m i n i u m atoms a r e d i s s o l v e d even a f t e r 167 h . T h e r e f o r e , t h e h i g h e r c a p a c i t y f o r c a t i o n s of t h a t a l u m i n a does not r e s u l t from a replacement of a l u m i n i u m atoms, but i s a t r u e c a t i o n exchange phenomenon.

A f i n a l c o n f i r m a t i o n i s d e r i v e d from t h e t i t r a t i o n c u r v e i n F i g . 4, o b t a i n e d by m e a s u r i n g t h e pH of a s o l u t i o n c o n t a i n i n g v a r y i n g amounts of HC1 and NaOH a t c o n s t a n t i o n i c s t r e n g t h a f t e r a c o n t a c t t i m e of s e -v e r a l d a y s . A w e l l d e f i n e d b a s i c f u n c t i o n i s o b s e r -v e d t h a t r e q u i r e s 1.08 mmol H+/g a l u m i n a f o r c o n v e r s i o n t o a c i d i c a l u m i n a . F o r a s p e c i -f i c s u r -f a c e a r e a o -f 210 rn^/g t h i s c o r r e s p o n d s t o 3.1 protons/nm^ i n e x c e l l e n t agreement w i t h t h e maximum c a p a c i t y of 1.6 C u ( I I ) atoms/nm^ i n d i c a t e d i n F i g . 2. The a m p h o t e r i c c h a r a c t e r of a l u m i n a i s demonstra-t e d by demonstra-t h e l e s s pronounced a c i d i c f u n c demonstra-t i o n i n F i g . 4. The o b s e r v a t i o n s c a n be e x p l a i n e d by t h e p r e s e n c e of two d i s t i n c t t y p e s of h y d r o x y l groups on t h e a l u m i n a s u r f a c e ( F i g . 5 ) . On t h e one hand t h e r e w i l l be h y d r o x y l i o n s chemisorbed o n t o a c i d i c s i t e s , i . e . a l u m i n i u m i o n s . On t h e o t h e r hand t h e r e w i l l be p r o t o n s c h e m i s o r b e d o n t o oxygen atoms. When n e u t r a l a l u m i n a i s washed w i t h a NaOH s o l u -t i o n , as i s commonly done i n p r e p a r i n g c h r o m a -t o g r a p h i c a l u m i n a , -t h e s e p r o t o n s w i l l be n e u t r a l i z e d and r e p l a c e d by more l o o s e l y bound sodium

0L i 1 1 1 1 1 1

0.03 0.02 0.01 0 0.01 0.02 0.03

[ H+] - • [ O H-]

F i g . 4. T i t r a t i o n c u r v e o f a l u m i n a m e a s u r e d In d u p l i c a t e (+,A) by c o n t a c t i n g 0.3 g o f a l u m i n a t o 30 mL o f a s o l u t i o n c o n t a i n i n g v a r i a b l e f r a c t i o n s o f s o l u t i o n A ( 0 . 1 M NaOH + 0.1 M NaCI) and s o l u t i o n B (0.1 M HCI + 0.1 M N a C I ) .

i o n s . The sodium i o n s p r e s e n t on a l k a l i n e a l u m i n a can exchange w i t h o t h e r c a t i o n s and a r e r e s p o n s i b l e f o r the c a t i o n exchange p r o p e r t i e s of a l k a l i n e a l u m i n a . I n t h i s environment t h e h y d r o x y l groups a r e t o o f i r m l y a t t a c h e d t o t h e a l u m i n i u m atoms t o be e x c h a n g e a b l e w i t h o t h e r a n i o n s .

C o n v e r s e l y , when t h e a l k a l i n e a l u m i n a i s washed w i t h HCI, t h e p r o t o n s produce two e f f e c t s . They d e s o r b t h e h y d r o x y l g r o u p s , which a r e t h e n r e p l a c e d by c h l o r i d e i o n s t h a t g i v e r i s e t o t h e a n i o n exchange p r o p e r -t i e s of a c i d i c a l u m i n a . A l s o , -t h e p r o -t o n s r e p l a c e -t h e c a -t i o n s ( e . g . sodium) a t t a c h e d t o oxygen atoms ( F i g . 5 ) . P r o b a b l y , t h e s e p r o c e s s e s proceed s i m u l t a n e o u s l y . The d i f f i c u l t y i n o b t a i n i n g a bare a l u m i n a s u r f a c e shows t h a t i t i s u n l i k e l y t h a t t h e c a t i o n s a r e c o m p l e t e l y r e -p l a c e d by -p r o t o n s b e f o r e t h e h y d r o x y l Ions exchange w i t h c h l o r i d e i o n s .

V

C L A l

/

\

-AL a c i d i c a l u m i n a - A L O H H C L / O

\

• / H O

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H C L A l -N a O H V \ N a O H O - - H X e \ / © ~ O H — A l O b a s i c a l u m i n a

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A l -© / N a O

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F i g . 5. S u r f a c e b e h a v i o u r o f a l u m i n a In b a s i c and a c i d i c m e d i a , r e s p e c t i v e l y .

Ion exchange chromatography w i t h a l u m i n a .

The a m p h o t e r i c c h a r a c t e r of a l u m i n a p e r m i t s i t s use as e i t h e r a c a t i o n o r an a n i o n e x c h a n g e r . I n a c i d i c media t h e c a t i o n exchange c a p a c i t y i s s m a l l and a c i d i c a l u m i n a would be b e t t e r s u i t e d as a n i o n e x c h a n g e r . I n a b a s i c environment a l u m i n a would a c t p r e d o m i n a n t l y as a c a t i o n e x -c h a n g e r . However, t h e v a r i a t i o n w i t h pH i s g r a d u a l and e i t h e r -c a t i o n o r a n i o n exchange p r o p e r t i e s c a n be o b s e r v e d o v e r f a i r l y broad nHr a n g e s . A l u m i n a has pnHroven t o be v e nHr y s t a b l e at extnHreme pH. E x p e nHr i -ments have shown t h a t t h e a l u m i n i u m c o n t e n t (measured bv ICP) o f a 0.1 M NaOH s o l u t i o n p e r c o l a t e d t h r o u g h an a n a l y t i c a l column d i d not exceed 10 ppm. T h i s means l e s s t h a n 1% d i s s o l u t i o n o f column m a t e r i a l a f t e r 150 h (1 month) c o n t i n u o u s use a t pH = 13.

T h i s broad pH-range i s a v a i l a b l e f o r c h r o m a t o g r a p h i c e x p e r i m e n t s . Two t y p i c a l examples a r e shown i n F i g . 6.

The s e p a r a t i o n of f o u r weak o r g a n i c a c i d s i s an example of a n i o n e x -change. O b v i o u s l y , a t t h e i n d i c a t e d pH of 5.5 t h i s s e p a r a t i o n c o u l d a l s o be r e a l i z e d w i t h m o d i f i e d s i l i c a and t h e use of a l u m i n a o f f e r s no a d v a n t a g e . By c o n t r a s t , t h e s e p a r a t i o n of m o d e r a t e l y s t r o n g bases a t

4 t i m e , s F i g . 6. a ) . s e p a r a t i o n o f a n i o n i c compounds on a l u m i n a . M o b i l e p h a s e : s o d i u m a c e t a t e b u f f e r pH = 5.5, I o n i c s t r e n g t h • 4 1 0- 2 M. UV d e t e c t i o n a t 254 nm. 1. C a t e c h o l , 2. B e n z o i c a c i d , 3. C l n n a m l c a c i d , 4. m - h y d r o x y b e n z o i c a c i d , 5. 4 - a m i n o - b e n z o i c a c i d . b ) . s e p a r a t i o n o f c a t i o n l c compounds on a l u m i n a . M o b i l e p h a s e : T e t r a e t h y l a m m o n l u m b o r a t e b u f f e r pH = 9.2, I o n i c s t r e n g t h = 4 10 M, a m p e r o m e t r l c d e t e c -t i o n +1,4 v o l -t SCE. 1. 1,4 dI a m i n o b u t a n e , 2. t r l e t h a n o l a m i n e , 3. q u i n i n e , 4. e p h e d r l n e , 5. a l l y -l a m -l n e , 6. p i p e r i d -l n e , 7. p y r r o -l i d i n e , 8. m o n o e t h a n o -l a m i n e ;

pH = 9.2 would not be p o s s i b l e under s i m i l a r c o n d i t i o n s w i t h s i l i c a , because t h i s m a t e r i a l r a p i d l y d i s s o l v e s a t pH v a l u e s e x c e e d i n g 8. I t i s t r u e t h a t t h e use of a precolumn (30) o r o r g a n i c b u f f e r s ( t r i e t h y l or t r i b u t y l a m i n e ( 8 ) ) p r o v i d e s some p r o t e c t i o n f o r s i l i c a columns a t h i g h pH.

N e v e r t h e l e s s , b a s i c compounds s u c h as amines, and a l k a l o i d s a r e u s u a l -l y s e p a r a t e d on RP s i -l i c a by means of t h e i o n p a i r i n g t e c h n i q u e ( 1 0 ) . I t i s h e r e t h a t a l u m i n a o f f e r s an a t t r a c t i v e a l t e r n a t i v e . As a r e s u l t of i t s h i g h r e s i s t a n c e t o a s t r o n g l y b a s i c e n v i r o n m e n t , b a s i c compounds c a n be chromatographed w i t h o u t any m o d i f i c a t i o n . The c a t i o n e x -change mechanism t h a t c o n t r o l s t h e s e p a r a t i o n can be employed t o manu-p u l a t e t h e s e l e c t i v i t y i n t h e u s u a l way. F i g . 7 i l l u s t r a t e s t h e demanu-pen- depen-dence of s o l u t e r e t e n t i o n on t h e n a t u r e and t h e c o n c e n t r a t i o n of t h e b u f f e r i o n . A p p a r e n t l y , t h e l a r g e r o r g a n i c c o u n t e r i o n s , s u c h as t e -t r a e -t h y l a m m o n i u m (TEA) and e s p e c i a l l y -t e -t r a b u -t y l a m m o n i u m (TBA), a r e more r e a d i l y d i s p l a c e d by s o l u t e i o n s t h a n t h e s m a l l e r sodium i o n

( f i g . 7 a ) . I n t h e same way, s o l u t e r e t e n t i o n i n c r e a s e s w i t h d e c r e a s i n g i o n i c s t r e n g t h , i . e . d e c r e a s i n g c o n c e n t r a t i o n of t h e b u f f e r i o n . O b v i -o u s l y , h-owever, t h i s c -o n c e n t r a t i -o n cann-ot be made t -o -o s m a l l f -o r f e a r of d i m i n i s h i n g t h e b u f f e r c a p a c i t y of t h e m o b i l e phase. 0.01 0.02 Q03 0.04 Q05 0.1 » - [ C o u n t e r ion ] F i g . 7. a ) . I n f l u e n c e o f t h e n a t u r e o f t h e c o u n t e r Ion on t h e r e t e n t i o n o f p y r r o l i d i n e a t pH = 9.2 TBA = t e t r a b u t y l a m m o n i u m Ion TEA = t e t r a e t h y l a m m o n i u r n Ion Na = Sod I urn I o n .

[ T E A+]

f I g 1 hi. I n f l u e n c e o f t h e c o n c e n t r a t i o n o f t h e TEA c o u n t e r Ion (mo 1/1 I t e r ) on t h e r e -t e n -t i o n o f v a r i o u s s o l u -t e s a -t pH » 9.2

0 m o n o e t h a n o l a m l n e , A p y r r o l i d i n e , • p l p e r l d i n e A d l e t h a n o l a m i n e , • q u i n i n e , • 1,4 d l a m i n o b u t a n e .

A f i n a l and r a t h e r more s p e c i f i c way t o i n f l u e n c e r e t e n t i o n i s t o v a r y t h e pH of t h e m o b i l e phase. For an i o n exchange mechanism t o be o p e r a -t i v e , -t h e s o l u -t e s mus-t be p r e s e n -t i n i o n i c f o r m . A b a s i c compound would be l a r g e l y i o n i z e d a t pH v a l u e s up t o p Ka -l and t h e n r a p i d l y

be-come n e u t r a l at pH = p Ka+ l . C o n s e q u e n t l y , t h e r e t e n t i o n on b a s i c a l u

-mina would r a p i d l y d e c r e a s e when t h e pH i s i n c r e a s e d from one u n i t be-low pK t o one u n i t o v e r pKa.

Advantage can be t a k e n of d i f f e r e n c e s i n pKa v a l u e s f o r t h e s e p a r a t i o n

of s o l u t e s . A maximum s e p a r a t i o n f a c t o r a i s o b t a i n e d at a pH v a l u e i n t e r m e d i a t e between t h e two pK v a l u e s . F i g 8 shows t h e i n f l u e n c e of pH m a n i p u l a t i o n on t h e s e p a r a t i o n of p e n t y l a m i n e and d i m e t h y l p i p e r a -z i n e .

I t s h o u l d be remarked t h a t m a n i p u l a t i o n of r e t e n t i o n by p H - c o n t r o l i s not f r e e from p r o b l e m s . At t h e h i g h pH-values of i n t e r e s t t h e c h o i c e of b u f f e r i s r a t h e r l i m i t e d due t o t h e o b v i o u s r e q u i r e m e n t s of chroma-t o g r a p h i c d e chroma-t e c chroma-t i o n . V a r i a chroma-t i o n of pH may r e s u l chroma-t i n a d e c r e a s e of chroma-t h e b u f f e r c a p a c i t y and hence a d e c r e a s e i n r e p r o d u c i b i l i t y of t h e chroma-togram.

k 3 -s o d i u m -s o d i u m -s o d i u m b o r a t e b i c a r b o n a t e h y d r o x i d e a A p y r r o l i d i n e pK = 11,2 x p e n t y l a m l n e pK = 10,6 * d l m e t h y l p l p e r a z l n e pK = 9,6

The b o t t o m p a r t shows t h e s e p a r a t i o n f a c t o r o f p e n t y l a m l n e ( x ) and d l m e t h y l p l p e -r a z l n e (*) a s a f u n c t i o n o f pH.

Some I l l u s t r a t i v e c h r o m a t o g r a m s a r e I n d i c a t e d .

A l s o , t h e s o l u t e I t s e l f may i n f l u e n c e t h e l o c a l pH of a p o o r l y b u f -f e r e d m o b i l e phase w i t h a d e t r i m e n t a l i n -f l u e n c e upon t h e peak s h a p e . N e v e r t h e l e s s , t h e examples p r e s e n t e d i n t h i s p r e l i m i n a r y s t u d y demon-s t r a t e t h e u t i l i t y of a l u m i n a f o r t h e demon-s e p a r a t i o n of demon-s t r o n g b a demon-s i c compounds i n aqueous m o b i l e phases b u f f e r e d t o h i g h pH. C o n t r o l o f r e t e n -t i o n -t h r o u g h i o n i c s -t r e n g -t h , c o u n -t e r i o n and p o s s i b l y pH p r o v i d e s -t h e means t o o p t i m i z e s e p a r a t i o n s . A f u r t h e r a i d c o u l d be t h e i n t r o d u c t i o n of s p e c i f i c m o d i f i e r s t o i n f l u e n c e t h e s o l u t e a c t i v i t y . Such a p p l i c a -t i o n s of a l u m i n a w i l l be communica-ted i n a f u -t u r e p u b l i c a -t i o n .

ACKNOWLEDGEMENTS

We acknowledge f i n a n c i a l s u p p o r t and f r u i t f u l d i s c u s s i o n s w i t h D r . v a n der Maeden and D r . Buytenhuys from AKZO C.R. (Arnhem, The N e t h e r -l a n d s ) . We a r e much i n d e b t e d t o miss W. Honders f o r h e r e n t h o u s i a s t i c c o o p e r a t i o n i n t h i s work.

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C H A P T E R 3

THE U S E O F O R G A N I C MODIFIERS IN

ION E X C H A N G E C H R O M A T O G R A P H Y

ON ALUMINA

SUMMARY

I t i s shown t h a t t h e s e l e c t i v i t y o f i o n exchange chromatography on a l u -mina can be s u b s t a n t i a l l y improved by a d d i n g o r g a n i c s o l v e n t s t o t h e aqueous m o b i l e phase. W i t h i n c r e a s i n g m o d i f i e r c o n t e n t t h e r e t e n t i o n o f o r g a n i c c a t i o n s g e n e r a l l y passes t h r o u g h a maximum. T h i s i s a t t r i b u t e d t o t h e c o m p e t i n g e f f e c t s of enhanced s o l v a t i o n of t h e b u f f e r c a t i o n and reduced s o l u t e i o n i z a t i o n . The m u t u a l i n t e r a c t i o n of pH, b u f f e r c o m p o s i t i o n , s o l u t e pK and e l u e n t c o m p o s i t i o n produces a v e r y f l e x i b l e system t h a t c a n be o p t i m i z e d f o r t h e s e p a r a t i o n o f a p a r t i c u l a r s a m p l e . As an example of p r a c t i c a l s i g n i f i c a n c e , t h e s e p a r a t i o n of s i x b a s i c a l k a l o i d drugs a t pH = 6 i n a m i x -t u r e of a c e -t o n i -t r i l e and wa-ter i s p r e s e n -t e d .

INTRODUCTION

I t i s w e l l known t h a t a l u m i n a and m e t a l o x i d e s i n g e n e r a l have I o n change p r o p e r t i e s . The s e p a r a t i o n of i n o r g a n i c i o n s forms a c l a s s i c example [ 1 , 2 ] , T h e i r s i m i l a r p o t e n t i a l f o r t h e s e p a r a t i o n of i o n i z e d o r -g a n i c compounds has r e c e i v e d l e s s a t t e n t i o n [ 3 ] , I n d e e d , w i t h t h e advent of r e v e r s e d phase l i q u i d chromatography, t h e e l e c t r o s t a t i c i n t e r a c t i o n s of charged s o l u t e s w i t h t h e s o l i d s u p p o r t have come t o be r e c o g n i z e d as a n u i s a n c e r a t h e r t h a n as an a i d i n s e p a r a t i o n . I n t h e c h r o m a t o g r a p h i c l i t e r a t u r e on c h e m i c a l l y m o d i f i e d s i l i c a , r e s i d u a l s i l a n o l groups seem t o s e r v e as t h e u n i v e r s a l scapegoat f o r poor peak shapes and u n e x p e c t e d r e t e n t i o n b e h a v i o u r .

Y e t , i n a r e c e n t a r t i c l e B i d l i n g m e y e r [4] has shown t h a t b a s i c compounds can be s e p a r a t e d more e f f i c i e n t l y on pure s i l i c a a c c o r d i n g t o a mechan i s m t h a t he i mechan t e r p r e t s as i o mechan exchamechange. However, t h e proposed a p p l i c a -t i o n s , n o -t a b l y -t h e s e p a r a -t i o n of a l k a l o i d s and o -t h e r s -t r o n g b a s e s , a r e s e v e r e l y r e s t r i c t e d by t h e l i m i t e d s t a b i l i t y of s i l i c a i n a l k a l i n e me-d i a . B i me-d l i n g m e y e r a l s o p o i n t s out t h a t t h e s i l o x a n e g r o u p s , w h i c h have some h y d r o p h o b i c a c t i v i t y , i n t e r f e r e w i t h t h e e l e c t r o s t a t i c i n t e r a c t i o n s of t h e s i l a n o l g r o u p s .

I n a p r e v i o u s paper we have r e c o l l e c t e d t h e a t t r a c t i v e i o n exchange p r o -p e r t i e s of a l u m i n a [ 5 ] . I t i s s t a b l e o v e r a -pH range as wide as 2-12. The Al-O bond i s more p o l a r t h a n t h e S i - 0 bond. The a n a l o g u e of s i l a n o l groups cannot be formed, because h y d r o x y l groups a r e not c o v a l e n t l y bon-ded o n t o a l u m i n a . N e v e r t h e l e s s , t h e a d s o r p t i o n of w a t e r t o t h e a l u m i n a s u r f a c e g i v e s r i s e t o h y d r o x y l groups of two d i f f e r e n t p o l a r i t i e s i n agreement w i t h t h e a m p h o t e r i c n a t u r e of a l u m i n a . As a r e s u l t , I t a c t s as an a n i o n exchanger i n a c i d i c s o l v e n t s , whereas i n n e u t r a l or b a s i c s o l -v e n t s i t w i l l p r e d o m i n a n t l y i n t e r a c t w i t h c a t i o n s . The t r a n s i t i o n from one mechanism t o t h e o t h e r seems t o be g r a d u a l arid dependent on t h e na-t u r e of na-t h e b u f f e r u s e d . T h i s q u e s na-t i o n w i l l be na-t h e s u b j e c na-t of a f u r na-t h e r s t u d y [ 6 ] . For t h e moment we n o t e t h a t by a j u d i c i o u s c h o i c e of t h e b u f -f e r i t i s p o s s i b l e t o s u p p r e s s a n i o n exchange and hence emphasize c a t i o n exchange p r o p e r t i e s f o r pH > 5. T h i s a l l o w s r e t e n t i o n of b a s i c compounds w i t h pK - v a l u e s e x c e e d i n g 6. I t i s p r e c i s e l y f o r t h o s e compounds t h a t i o n i z a t i o n i s d i f f i c u l t t o s u p p r e s s i n RPLC on c h e m i c a l l y m o d i f i e d s i l i -c a .

We have a l s o shown p r e v i o u s l y t h a t t h e c a t i o n exchange r e t e n t i o n on a l u -mina can be c o n t r o l l e d i n s e v e r a l ways [ 5 ] . The n a t u r e of t h e b u f f e r i o n and t h e i o n i c s t r e n g t h of t h e s o l v e n t have a s t r o n g i n f l u e n c e on t h e o v e r a l l r e t e n t i o n , but no s p e c i f i c i n f l u e n c e on t h e r e l a t i v e r e t e n t i o n of two compounds. To a c e r t a i n e x t e n t s e l e c t i v i t y can be m a n i p u l a t e d by a j u d i c i o u s c h o i c e of t h e pH of t h e m o b i l e phase. Because o n l y t h e p r o -t o n a -t e d form of a b a s i c compound w i l l be r e -t a r d e d by c a -t i o n exchange, a v a r i a t i o n of t h e pH around t h e pK - v a l u e of a compound w i l l have a l a r g e i n f l u e n c e upon i t s r e t e n t i o n . T h e r e f o r e , as we have shown b e f o r e [ 5 ] , t h e r e l a t i v e r e t e n t i o n of two compounds can be m a x i m i z e d by s e l e c t i n g a