Chapter
2
Experimental Techniques for Inverse Gas Chromatography 1
2
3
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A. E. Bolvari, Thomas Carl Ward , P. A. Koning , and D. P. Sheehy
Department of Chemistry, Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Details for producing optimum performing packed, capillary, and fiberfilled columns in inverse gas chromatography experiments are discussed. Also, the crucial factors that might lead to instrumental error in this technique are evaluated and cautions are provided. In inverse gas chromatography (IGC), the interactions of gaseous probe molecules with a stationary phase contained within a column results in a characteristic retention time, t , which can be P translated into a number of important thermodynamic, kinetic, and surface properties. The theory and principles have been well developed and reviewed in other sources (1)• However, to obtain meaningful data, one must design and perform experiments with an awareness of the assumptions and limitations of both the theory and the measurements. There are many small numerical corrections and tedious technical requirements for success that individually seem insignificant; however, when taken as a whole, they can determine the success or failure of the research. A careful analysis of each of the areas of application, for example, diffusion constants, solubility parameters, and activity coefficients, reveals that the cumulative nature of the IGC inaccuracies makes careful technique imperative i f meaningful absolute results are to be obtained. Examination of Equations 1,2, and 3 reveal the origin of some of the possible errors that may enter the data. These are elaborated in the following text. The key parameter in the IGC measurements is the specific retention volume, V , or the amount of carrier gas required to elute a probe from 9 column containing one gram of interacting stationary phase material. The exact quantity in terms of experimental variables i s : Address correspondence to this author. Current address: Amoco Chemicals Corporation, Naperville, IL 60566 Current address: 3M Company, St. Paul, MN 55144 0097-6156/89/0391-0012$06.00/0 © 1989 American Chemical Society 1
2 3
In Inverse Gas Chromatography; Lloyd, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
2.
BOLVARI ET AL.
Experimental Techniques for Inverse Gas Chromatography
273.2
(t - t )
P T
W
°
P
J
c
(1)
where
C
(2)
1 -
and
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J
-
1.5[
]
(3)
In these e q u a t i o n s , T i s the temperature of the flowmeter, which measures t h e c a r r i e r gas f l o w r a t e F; W i s t h e weight o f polymer on t h e column; t i s t h e r e t e n t i o n time of t h e probe m o l e c u l e ; t i s t h e retention time o f a n o n i n t e r a c t i n g marker such as a i r ; J i s t h e c o r r e c t i o n f o r the p r e s s u r e d r o p a c r o s s t h e column; C i s t h e c o r r e c t i o n f o r t h e vapor p r e s s u r e o f water i n t h e soap b u b b l e flowmeter; P i s the p r e s s u r e o f the c a r r i e r gas a t t h e o u t l e t o f the column ( a t m o s p h e r i c p r e s s u r e ) ; P. i s t h e p r e s s u r e a t the i n l e t ; and PH^O i s t h e v a p o r p r e s s u r e o f water a t t h e t e m p e r a t u r e of t h e f l o w m e t e r . Thus, t h e p r e s s u r e s , t i m e s , w e i g h t s , and f l o w r a t e s a r e c r i t i c a l f o r p r e c i s e c a l c u l a t i o n s and a r e examined i n t h i s p a p e r . F i r s t , i t w i l l be u s e f u l t o d i s c u s s t h e t y p e s of s t a t i o n a r y phases b e f o r e p r o c e e d i n g w i t h t h e d e t a i l s o f proper experimental procedure. I n v e r s e gas chromatography has p r o v e n t o be a p a r t i c u l a r l y i m p o r t a n t t e c h n i q u e f o r t h e i n v e s t i g a t i o n o f p o l y m e r s , w i t h most s t u d i e s making use o f packed columns. IGC a l s o has been r e c e n t l y extended t o t h e i n v e s t i g a t i o n of f i b e r s and o f polymers c o a t e d on c a p i l l a r y columns. The p r e p a r a t i o n o f each of t h e s e columns i s very important to o v e r a l l success. Indeed, t h e most e s s e n t i a l p i e c e o f equipment i n IGC i s t h e c h r o m a t o g r a p h i c column. The f u n c t i o n o f t h e column i s t o encourage r e p e t i t i v e p a r t i t i o n i n g o f each s o l u t e m o l e c u l e between the gas and t h e l i q u i d o r s o l i d phase under c o n d i t i o n s t h a t m i n i m i z e t h e range o f r e t e n t i o n t i m e s e x h i b i t e d by i d e n t i c a l m o l e c u l e s of each s o l u t e (2). The a c c u r a c y o f t h e r e s u l t s i s d i r e c t l y i n f l u e n c e d by t h e degree t o which t h i s g o a l has been a c h i e v e d . I n t h i s r e g a r d , r e t e n t i o n diagrams, i n which t h e n a t u r a l l o g of t h e s p e c i f i c r e t e n t i o n volume i s p l o t t e d v e r s u s r e c i p r o c a l a b s o l u t e t e m p e r a t u r e , a r e e x c e l l e n t i n d i c a t o r s o f o v e r a l l column performance (1.) • Because o f t h e ease o f t e m p e r a t u r e c o n t r o l i n a gas chromatograph, one may e x p l o r e t h e polymer u s i n g t h e p r o b e above and below v a r i o u s p o s s i b l e phase t r a n s i t i o n s , w i t h a l i n e a r r e s p o n s e b e i n g r e v e a l e d by t h e r e t e n t i o n diagram when o p e r a t i n g under e q u i l i b r i u m conditions. Packed Columns Packed columns a r e s u i t a b l e f o r a wide range of i n v e s t i g a t i o n s , i n c l u d i n g low m o l e c u l a r weight m a t e r i a l s , homo- and c o p o l y m e r s , b l e n d s , and b l o c k copolymers ( 3 ) . I n t h e s e i n v e s t i g a t i o n s , t h e
In Inverse Gas Chromatography; Lloyd, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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14
INVERSE GAS CHROMATOGRAPHY
s t a t i o n a r y phase c o n s i s t s of a n o n i n t e r a c t i n g , f i n e l y d i v i d e d s o l i d t h a t was c o a t e d w i t h a t h i n f i l m o f the polymer o r polymer b l e n d . To a c h i e v e t h i s , t h e polymer i s d i s s o l v e d i n the s o l v e n t of c h o i c e ( d e s i r a b l y of h i g h vapor p r e s s u r e ) and f i l t e r e d . A q u a n t i t y of i n e r t s u p p o r t i s weighed t o a c h i e v e a 15 wt/wt-% s t a t i o n a r y p h a s e . An acid-washed, d i m e t h y l c h l o r o s i l a n e t r e a t e d s u p p o r t i s u s u a l l y chosen. One w i d e l y used s u p p o r t i s Chromasorb W, m a n u f a c t u r e d by Johns M a n s v i l l e . A l t e r n a t i v e l y , 60/80 mesh s i z e d g l a s s beads may be u s e d . S u c c e s s f u l l o a d i n g s range i n the 4 t o 13 wt-% c a t e g o r y f o r Chromasorb W and a t a p p r o x i m a t e l y 0.5 wt-% f o r g l a s s beads. The i n e r t s u p p o r t and f i l t e r e d polymer s o l u t i o n a r e p l a c e d i n t o a hedgehog f l a s k . The hedgehog f l a s k i s a 500 mL pear shaped f l a s k with vigreux f i n g e r s extending to i t s i n t e r i o r . Coating i s a c h i e v e d by slow e v a p o r a t i o n o f t h e s o l v e n t u s i n g a r o t a r y evaporator. The r o t a r y m o t i o n , i n c o m b i n a t i o n w i t h t h e hedgehog d e s i g n , a s s u r e s g e n t l e but adequate a g i t a t i o n needed f o r complete and u n i f o r m coverage of the s u p p o r t . A l - S a i g h and Munk have r e p o r t e d a n o v e l s o a k i n g t e c h n i q u e , r e q u i r i n g s e v e r a l hours of work, t h a t r e p o r t e d l y m i n i m i z e s l o s s of polymer and a i d s i n d e t e r m i n i n g p a c k i n g l o a d i n g s ( 4 ) • The c o a t e d s u p p o r t must be then d r i e d i n a vacuum o v e n . I n o r d e r t o e n s u r e maximum s o l v e n t removal, the oven temperature s h o u l d be a d j u s t e d t o j u s t below t h e polymer g l a s s t r a n s i t i o n temperature and d r y i n g m a i n t a i n e d f o r 48 hours. F o l l o w i n g t h i s t r e a t m e n t , the p a c k i n g becomes a f r e e f l o w i n g powder i f the f i l m i s t h i n enough, even f o r low m o l e c u l a r weight waxes. The s u p p o r t i s s i f t e d t h r o u g h a 60/80 mesh s i e v e t o e n s u r e even p a r t i c l e s i z e s . Any l a r g e agglomerates and f i n e s ( f r a c t u r e d s o l i d s u p p o r t ) t h a t a r e n o t removed i n t h i s s t e p would r e d u c e p a c k i n g e f f i c i e n c y , cause peak t a i l i n g , and expose u n c o a t e d , non-neutralized surfaces. To b e g i n p a c k i n g t h e column, s t a i n l e s s s t e e l t u b i n g i s s t r a i g h t e n e d (1 t o 2 m), s i l a n e t r e a t e d s t e e l wool i s used t o p l u g one end, and t h i s same end i s a t t a c h e d t o a water a s p i r a t o r . The c o n t a i n e r h o l d i n g t h e s t a t i o n a r y phase i s weighed. A few grams a r e put i n the columns and p a c k i n g i s a c c o m p l i s h e d w i t h t h e a i d of a mechanical v i b r a t o r . The s u p p o r t e d s t a t i o n a r y phase i s c o n t i n u a l l y added and packed u n t i l t h e column i s f i l l e d , a t which time t h e c o n t a i n e r i s weighed a g a i n . These w e i g h i n g s must be as p r e c i s e as can be a c h i e v e d so t h a t t h e e x a c t amount of s t a t i o n a r y phase i n t h e column i s known. The o t h e r end of t h e column i s s e a l e d w i t h g l a s s wool; t h e column i s c o i l e d and p l a c e d i n the chromatograph w i t h o n l y i t s i n l e t p o r t connected. A f t e r a 30 minute p u r g e w i t h h e l i u m gas, t h e column i s t a k e n t o 10 degrees above the g l a s s t r a n s i t i o n temperature (Tg) or t h e c r y s t a l l i n e m e l t i n g p o i n t (Tm), w h i c h e v e r i s h i g h e s t , of t h e p o l y m e r ( s ) and h e l d a t t h i s temperature f o r 12 hours. T h i s f u r t h e r a l l o w s f o r removal o f any r e s i d u a l s o l v e n t and s o l v e n t - i n d u c e d morphologies present i n the polymer. Complete s o l v e n t removal i s e s s e n t i a l . In any experiment t h a t i s s u b s e q u e n t l y c o n d u c t e d and shows a time-dependent o r n o n r e p r o d u c i b l e c h a r a c t e r , r e s i d u a l s o l v e n t s h o u l d be s u s p e c t e d . The w e i g h t of the polymer on the column i s d e t e r m i n e d most commonly by a s h i n g , u s i n g a t h e r m o g r a v i m e t r i c a n a l y s i s system (TGA)• A t y p i c a l experiment r e q u i r e s 10 mg o f c o a t e d s u p p o r t .
In Inverse Gas Chromatography; Lloyd, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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2.
BOLVARI ET AL.
Experimental Techniques for Inverse Gas Chromatography
T h i s i s l o a d e d o n t o a TGA sample pan and t h e n t h e TGA oven t e m p e r a t u r e i s r a i s e d t o 550°C i n an oxygen atmosphere* Measurements a r e made both on t h e c o a t e d and u n c o a t e d s u p p o r t t o c o r r e c t f o r weight l o s s of the i n e r t s o l i d s u p p o r t * Since almost a l l c a l c u l a t i o n s i n IGC r e q u i r e knowledge of the s p e c i f i c r e t e n t i o n volume, an a c c u r a t e d e t e r m i n a t i o n of t h e amount of c o a t i n g i s crucial. T h i s i s g e n e r a l l y r e g a r d e d as one of t h e most l i k e l y s o u r c e s of e r r o r i n the IGC e x p e r i m e n t * C e r t a i n polymers can not be s u c c e s s f u l l y b u r n e d o f f of t h e s u p p o r t and a l t e r n a t i v e methods must be used t o d e t e r m i n e l o a d i n g * For example, s o x h l e t e x t r a c t i o n s must be used f o r most s i l o x a n e c o n t a i n i n g p o l y m e r s . M u l t i p l e specimens must be t e s t e d t o g e t the e r r o r i n l o a d i n g t o l e s s t h a n 1% f o r most c a l c u l a t i o n s . The amount of l o a d i n g found by t h e d e s c r i b e d methods i s t y p i c a l l y 3% l o w e r than t h a t e x p e c t e d from the i n i t i a l w e i g h t of polymer and s u p p o r t . T h i s i s due t o c o a t i n g of t h e g l a s s f l a s k w i t h the polymer. The optimum l o a d i n g l i e s between 6 t o 15 wt/wt-%. S e v e r a l r e s e a r c h e r s (.5,6) have shown t h a t below 4 t o 6 wt/wt-% l o a d i n g , a c o n t r i b u t i o n t o r e t e n t i o n t i m e s due t o a d s o r p t i o n on u n c o a t e d s u p p o r t can be d e t e c t e d , w h i l e a t h i g h e r l o a d i n g s (
