Analytical Supercritical Fluid Extraction ... - ACS Publications

Chapter 3 Analytical Supercritical Fluid Extraction Methodologies Bob W. Wright, John L. Fulton, Andrew J. Kopriva, and Richard D. Smith

Downloaded by FUDAN UNIV on January 3, 2017 | http://pubs.acs.org Publication Date: March 17, 1988 | doi: 10.1021/bk-1988-0366.ch003

Chemical Sciences Department, Pacific Northwest Laboratory, Richland, WA 99352 Off-line supercritical fluid extraction, ultrasonic supercritical fluid extraction, and on-line supercritical fluid extraction-gas chromatography methodologies that have been developed specifically for analytical sample preparation and analysis are described. These methods offer the potential for extraction rate increases of over an order of magnitude, and are compatible with online analysis which provides the basis for automated sample preparation and analysis. These methods are particularly useful for small sample sizes or trace levels of analytes, and have been demonstrated to operate quantitatively. Combined ultrasonic supercritical fluid extraction can further enhance extraction rates from macro-porous materials by inducing convection through internal pores. The apparatus and instrumentation are described in detail and several examples are presented illustrating the applicability of these methodologies. Until recently, the use of supercritical fluids for sample extraction was generally confined to chemical processing applications (1-3) . However, t h e use of s u p e r c r i t i c a l fluid e x t r a c t i o n (SFE) f o r a n a l y t i c a l purposes i s a t t r a c t i n g i n c r e a s e d a t t e n t i o n and t h e development o f s e v e r a l new t e c h n i q u e s has been r e p o r t e d (4-14). A number of p o t e n t i a l advantages a r e p o s s i b l e w i t h SFE compared w i t h c o n v e n t i o n a l e x t r a c t i o n methods. These advantages i n c l u d e more r a p i d e x t r a c t i o n r a t e s , t h e p o s s i b i l i t y o f more efficient extractions, increased selectivity, possible analyte f r a c t i o n a t i o n d u r i n g e x t r a c t i o n , and c o m p a t i b i l i t y w i t h o n - l i n e a n a l y s i s methods s u c h as c o n t i n u o u s s p e c t r o s c o p i c m o n i t o r i n g o r p e r i o d i c chromatographic analyses. The p o t e n t i a l advantages o f SFE a c c r u e from t h e p r o p e r t i e s of a s o l v e n t a t t e m p e r a t u r e s and p r e s s u r e s above i t s c r i t i c a l p o i n t . At e l e v a t e d p r e s s u r e t h i s s i n g l e phase w i l l have p r o p e r t i e s t h a t a r e i n t e r m e d i a t e between t h o s e o f t h e gas and l i q u i d phases and a r e dependent on t h e f l u i d c o m p o s i t i o n , p r e s s u r e , and t e m p e r a t u r e . The c o m p r e s s i b i l i t y o f s u p e r c r i t i c a l f l u i d s i s l a r g e j u s t above t h e c r i t i c a l t e m p e r a t u r e and s m a l l changes i n p r e s s u r e r e s u l t i n l a r g e changes i n d e n s i t y of the fluid . The d e n s i t y of a sup e r c r i t i c a l f l u i d i s t y p i c a l l y 100 t o 1000 t i m e s g r e a t e r t h a n t h a t 0097-6156/88/0366-0044$06.00/0 © 1988 American Chemical Society

Charpentier and Sevenants; Supercritical Fluid Extraction and Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


3.

WRIGHT E T AL.

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45

o f t h e gas and s o l v a t i n g c h a r a c t e r i s t i c s a p p r o a c h i n g t h o s e o f a l i q u i d are imparted. However, t h e d i f f u s i o n c o e f f i c i e n t s and v i s c o s i t y o f t h e f l u i d remain i n t e r m e d i a t e between t h o s e o f t h e gas and l i q u i d p h a s e s a t moderate d e n s i t i e s (3.) , t h u s a l l o w i n g r a p i d mass t r a n s f e r o f s o l u t e s compared w i t h t h e l i q u i d . Many f l u i d s a l s o have c o m p a r a t i v e l y low c r i t i c a l temperatures that allow e x t r a c t i o n s t o be c o n d u c t e d a t r e l a t i v e l y m i l d t e m p e r a t u r e s , e.g., 31 °C f o r c a r b o n d i o x i d e . In a d d i t i o n t o c o n t r o l l i n g t h e fluid p r e s s u r e and/or temperature t o r e g u l a t e the d e n s i t y or s o l v a t i n g power, one can use v a r i o u s f l u i d s o r f l u i d m i x t u r e s t h a t e x h i b i t d i f f e r e n t s p e c i f i c chemical i n t e r a c t i o n s to o b t a i n the desired s o l v e n t s t r e n g t h and s e l e c t i v i t y . The l i q u i d - l i k e s o l v e n t power and r a p i d m a s s - t r a n s f e r p r o p e r t i e s of s u p e r c r i t i c a l f l u i d s clearly provide t h e p o t e n t i a l f o r more r a p i d e x t r a c t i o n r a t e s and more e f f i c i e n t e x t r a c t i o n t h a n i s f e a s i b l e w i t h l i q u i d s due t o b e t t e r p e n e t r a t i o n of t h e m a t r i x . T h i s p a p e r d e s c r i b e s v a r i o u s a n a l y t i c a l s u p e r c r i t i c a l f l u i d ext r a c t i o n methodologies developed i n the a u t h o r s l a b o r a t o r y and summarizes selected investigations conducted to evaluate the a p p l i c a b i l i t y and e f f i c i e n c y o f t h e s e methods f o r a r a n g e o f applications. Described methodologies include off-line supercritical fluid extraction, ultrasonic supercritical fluid extraction, and on-line supercritical fluid extraction-gas chromatography. 1

Off-line

Supercritical Fluid

Extraction

Sample p r e p a r a t i o n i s o f t e n more d i f f i c u l t and t i m e - c o n s u m i n g t h a n the a c t u a l a n a l y s i s procedure. Furthermore, e x t r a c t i o n of analytes from t h e m a t r i x i s g e n e r a l l y t h e most t i m e - c o n s u m i n g s t e p o f sample preparation and i t can lead to r e l a t i v e l y i n e f f i c i e n t analyte r e c o v e r i e s . O f f - l i n e s u p e r c r i t i c a l f l u i d e x t r a c t i o n p r o v i d e s an a l t e r n a t i v e t o t r a d i t i o n a l Soxhlet or u l t r a s o n i c l i q u i d e x t r a c t i o n methods. S e v e r a l r e c e n t s t u d i e s have shown a n a l y t i c a l SFE p r o v i d e s comparable or better extraction efficiencies than Soxhlet e x t r a c t i o n (8,12 14) . More i m p o r t a n t l y , i n c r e a s e d e x t r a c t i o n r a t e s o f o v e r an o r d e r o f m a g n i t u d e were a c h i e v e d (12.) which o f f e r s i g n i f i c a n t time s a v i n g s . O f f - l i n e SFE i s a d a p t a b l e t o sample s i z e s ranging f r o m a few m i l l i g r a m s t o s e v e r a l grams and i s e q u a l l y a p p l i c a b l e t o t h e r e c o v e r y o f t r a c e a n a l y t e s o r complex m i x t u r e s . Examples o f b o t h t y p e s o f a p p l i c a t i o n s a r e d e s c r i b e d . r

r

Apparatus. The o f f - l i n e SFE i n s t r u m e n t a t i o n c o n s i s t e d of three main components: a h i g h - p r e s s u r e pump, a h e a t e d e x t r a c t i o n c e l l , and a d e p r e s s u r i z a t i o n and sample c o l l e c t i o n s y s t e m . A schematic d i a g r a m of t h i s i n s t r u m e n t a t i o n i s shown i n F i g u r e 1. A modified R a b b i t HPX s o l v e n t d e l i v e r y s y s t e m e q u i p p e d w i t h a 10 mL/min pump head and an e l e c t r o n i c p r e s s u r e m o n i t o r ( R a i n i n I n s t r u m e n t Co., Woburn, MA) was u s e d t o p r e s s u r i z e and d e l i v e r t h e extracting fluids. The pump head, check v a l v e s , and s e v e r a l i n c h e s of t h e i n l e t l i n e i m m e d i a t e l y p r i o r t o t h e pump head were c o o l e d by r e c i r c u l a t i n g a -15 °C e t h y l e n e g l y c o l - w a t e r m i x t u r e t h r o u g h c o p p e r b l o c k s t h a t were machined t o f i t t h e pump head geometry. The i n l e t c h e c k v a l v e c a r t r i d g e was also modified f o r d i r e c t connection to 1/8-in. s t a i n l e s s s t e e l t u b i n g to allow a l a r g e r supply of lowp r e s s u r e s o l v e n t t o e n t e r t h e pump. No o t h e r m o d i f i c a t i o n s t o t h e check v a l v e s o r p i s t o n a s s e m b l y were r e q u i r e d . High-pressure s y r i n g e pumps w i t h adequate volumes have a l s o been u s e d (1,14). The p r e s s u r i z e d f l u i d was t r a n s f e r r e d t o t h e 316 s t a i n l e s s s t e e l h i g h - p r e s s u r e e x t r a c t i o n v e s s e l (shown i n F i g u r e 2) t h r o u g h 1/16-in.


Charpentier and Sevenants; Supercritical Fluid Extraction and Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1988. Reciprocating Pump

Glass-Lined S.S. Extraction Vessel

Oven

1

1/16 in. S.S. Tubing

Η Ο

Dewar

χ

>

»

Η Ο Ο

>

ο

χ

ζ > ο

η

>

Η

w χ

d

>

Η

w w η

d

C/3

Liquid Nitrogen

Heated • Restrictor (50 μίτι S.S.)

Sealed Collection Vessel

Heated Transfer Line

F i g u r e 1. Schematic diagram o f t h e o f f - l i n e s u p e r c r i t i c a l f l u i d e x t r a c t i o n a p p a r a t u s . (Reproduced from Ref. 12. C o p y r i g h t 1987 American C h e m i c a l S o c i e t y . )

Fluid Supply

\

1/8 in. S.S. Tubing


3. W R I G H T E T A L .

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Fluid Extraction

Methodologies

47

From High-Pressure Pump


To Heated Transfer Line, Restrictor, and Collection Vessel

Borosilicate Glass Test Tube

5.4 cm Sample to be Extracted Teflon O-Rings

Extraction Chamber

7.6 cm

316 Stainless Steel Extraction Vessel

-6.3 cmF i g u r e 2. D e s i g n o f s u p e r c r i t i c a l f l u i d e x t r a c t i o n v e s s e l . (Reproduced from R e f . 12. C o p y r i g h t 1987 American Chemical S o c i e t y . )

American Chemical Society Library

1155 ISth St., N.W. Charpentier and Sevenants; Supercritical Fluid Extraction and Chromatography Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


48

SUPERCRITICAL FLUID EXTRACTION AND

CHROMATOGRAPHY

s t a i n l e s s s t e e l t u b i n g . The e x t r a c t i o n c e l l was c o n s t r u c t e d t o o p e r a t e s a f e l y above 400 b a r and 200 °C. The e x t r a c t i o n c e l l body and t o p were s e a l e d w i t h d o u b l e T e f l o n O - r i n g s . The t r a n s f e r l i n e from t h e pump e x t e n d e d t o t h e bottom o f t h e e x t r a c t i o n c e l l chamber (~5 mL volume) where t h e sample was p l a c e d i n a 5.4 χ 1.3 cm o.d. b o r o s i l i c a t e g l a s s t e s t tube. This design allowed the e x t r a c t i o n f l u i d t o sweep t h e sample f r o m b o t t o m t o t o p and t h e n e x i t the extraction vessel. A stainless steel f r i t w i t h 2.0-μιη p o r e s was p l a c e d i n t h e e x i t p o r t o f t h e e x t r a c t i o n v e s s e l t o p r e v e n t sample f r o m b e i n g f l u s h e d out i n c a s e o f sudden p r e s s u r e s u r g e s . Empty 1 / 4 - i n . o.d. HPLC columns o f t h e a p p r o p r i a t e l e n g t h t o g i v e t h e d e s i r e d v o l u m e h a v e a l s o be u s e d as extraction cells. The e x t r a c t i o n v e s s e l was maintained at e l e v a t e d temperatures i n a Hewlett-Packard 5700 gas chromatograph oven. The oven was c o n t i n u o u s l y p u r g e d w i t h n i t r o g e n when u s i n g flammable e x t r a c t i o n f l u i d s o r f l u i d m i x t u r e s . The e x t r a c t s t r e a m was t r a n s p o r t e d from t h e e x t r a c t i o n c e l l t o t h e e x t e r i o r o f t h e oven t h r o u g h 1 / 1 6 - i n . i.d. g l a s s - l i n e d stainless tubing. The t r a n s f e r l i n e was c o n n e c t e d t o a 10-12-cm l e n g t h o f 50-pm i . d . s t a i n l e s s s t e e l t u b i n g which was c r i m p e d a t t h e e x i t end t o c o n t r o l t h e f l o w r a t e o f t h e f l u i d and s e r v e as a d e p r e s s u r i z a t i o n zone f o r t h e e x t r a c t i o n s t r e a m . Short lengths (5-10-cm) o f 10-25-μιη i . d . f u s e d s i l i c a t u b i n g have a l s o been u s e d as r e s t r i c t o r s . The e x t r a c t i o n e f f l u e n t s were c o l l e c t e d by f r e e z i n g (or l i q u e f y ing) i n a s e a l e d round bottom f l a s k c o o l e d i n a l i q u i d n i t r o g e n bath. T h i s method e n s u r e d t h a t no a n a l y t e l o s s e s t o t h e atmosphere occurred during c o l l e c t i o n . C o l l e c t i o n can a l s o be done by b u b b l i n g t h e e x t r a c t i o n e f f l u e n t i n t o a v i a l c o n t a i n i n g a few mL o f a s o l v e n t o r i n t e r n a l s t a n d a r d s o l u t i o n (7 14) . f

T r a c e L e v e l E x t r a c t i o n . To p r o v i d e a c h a l l e n g i n g sample f o r e v a l u a t i o n o f o f f - l i n e SFE, a c t i v a t e d c a r b o n was s p i k e d a t 50 ppm with s e v e r a l p o l a r and h i g h e r m o l e c u l a r weight p o l y c y c l i c a r o m a t i c com pounds. A one-gram sample was s u b j e c t e d t o 16 h o f S o x h l e t e x t r a c t i o n u s i n g c a r b o n d i s u l f i d e and t h e n f o l l o w e d w i t h a s e c o n d s i m i l a r e x t r a c t i o n u s i n g m e t h y l e n e c h l o r i d e . A n o t h e r sample was extracted f o r 1 h w i t h s u p e r c r i t i c a l c a r b o n d i o x i d e a t 125 °C and 400 b a r . As shown i n T a b l e I , no d e t e c t a b l e l e v e l s o f t h e compounds were r e c o v e r e d i n t h e combined S o x h l e t e x t r a c t s . However, low l e v e l s o f t h e compounds were r e c o v e r e d w i t h s u p e r c r i t i c a l c a r b o n d i o x i d e e x t r a c t i o n o f t h e a c t i v a t e d c a r b o n . A l t h o u g h o n l y low l e v e l s o f t h e Table

I.

Compound Chrysene Benzanthrone 1-Nitropyrene Dibenzocarbazole Coronene

E x t r a c t i o n Comparison o f A c t i v a t e d Carbon by S o x h l e t and SFE Soxhlet

% Recovery 0 0 0 0 0

SFE

% Recovery 1 6 10 10 0.2

spiked analytes were r e c o v e r e d from the carbon, t h i s example i l l u s t r a t e s t h e p o t e n t i a l o f SFE f o r e x t r a c t i n g l o w - l e v e l a n a l y t e s from h i g h l y a d s o r p t i v e m a t r i c e s . I m p o r t a n t c o n s i d e r a t i o n s i n a n a l y t i c a l SFE a r e t h e p r o v i s i o n s t a k e n f o r c o l l e c t i n g t h e sample d u r i n g t h e d e p r e s s u r i z a t i o n p r o c e s s . D e p e n d i n g on t h e e x a c t c o n d i t i o n s , i t i s p o s s i b l e f o r t h e a n a l y t e s



3.

WRIGHT E T AL.

Supercritical

Fluid Extraction

49

Methodologies

t o n u c l e a t e and become e n t r a i n e d i n t h e e x p a n d i n g gas and f o r m an a e r o s o l which can be e a s i l y l o s t t o t h e atmosphere. Experiments have shown t h a t o v e r 90% o f t r a c e l e v e l a n a l y t e s can be l o s t under some c o n d i t i o n s when u s i n g open c o l l e c t i o n i n a narrow-necked f l a s k c o o l e d t o subambient t e m p e r a t u r e s (12.) . C o l l e c t i o n i n a s e a l e d v e s s e l e l i m i n a t e s l o s s e s due t o e i t h e r v o l a t i l i t y o r t o a e r o s o l f o r m a t i o n , but i n c r e a s e s t h e e x p e r i m e n t a l c o m p l e x i t y . C o l l e c t i o n by b u b b l i n g t h e e x t r a c t i o n e f f l u e n t i n a few m i l l i l i t e r s o f s o l v e n t i s a l s o e f f e c t i v e f o r low f l u i d f l o w r a t e s . The c o o l i n g due t o ex pansion of the f l u i d minimizes v o l a t i l i t y l o s s e s of both the analyte and s o l v e n t . However, t h i s c o l l e c t i o n method becomes i m p r a c t i c a l f o r the h i g h e r f l u i d f l o w r a t e s needed f o r r a p i d e x t r a c t i o n of l a r g e r sample s i z e s (e.g., gram r a n g e ) , s i n c e v e r y h i g h gas flow r a t e s would be r e q u i r e d making i t d i f f i c u l t t o c o n t a i n t h e f l o w i n a s m a l l volume o f s o l v e n t . The volume o f a f l u i d i n c r e a s e s by a p p r o x i m a t e l y t h r e e o r d e r s o f magnitude d u r i n g e x p a n s i o n t o a gas. E x p e r i e n c e has shown t h a t a t l e a s t t e n sample volumes (or e x t r a c t i o n v e s s e l volumes) o f compressed f l u i d a r e g e n e r a l l y needed t o a c h i e v e "exhaustive" e x t r a c t i o n f o r t y p i c a l matrices. Obviously, analytes with very low s o l u b i l i t y i n s u p e r c r i t i c a l f l u i d s c o u l d r e q u i r e l a r g e r e x t r a c t i o n volumes. Complex M i x t u r e E x t r a c t i o n . A n a l y t i c a l SFE can a l s o be u s e d f o r complex m i x t u r e sample p r e p a r a t i o n . T y p i c a l examples using h a z a r d o u s waste samples a r e d e s c r i b e d below. Sample A was a s o i l b o r i n g c o n t a m i n a t e d w i t h c o a l g a s i f i c a t i o n r e s i d u a l s and sample Β was f r o m a waste s t r e a m f r o m a t r e a t m e n t f a c i l i t y . The major o b j e c t i v e o f t h e s e s t u d i e s was t o compare t h e e x t r a c t i o n a b i l i t i e s ( e . g . , amount o f m a t e r i a l e x t r a c t e d ) of t h r e e d i f f e r e n t fluid systems u s i n g a p p r o x i m a t e l y f o u r - g r a m a l i q u o t s o f t h e samples. The specific fluid systems, the e x t r a c t i o n c o n d i t i o n s , and the percentage o f t h e t o t a l mass o f m a t e r i a l e x t r a c t e d from each sample are l i s t e d i n Table I I . Table

Fluid

II.

System

Carbon D i o x i d e Carbon D i o x i d e M e t h a n o l (80:20) Pentane-Ethanol (93:7 mol %)

S u p e r c r i t i c a l F l u i d E x t r a c t i o n Comparison Hazardous Waste Samples Extraction Conditions Temp. P r e s s u r e F l u i d Volume (Bar) (mL) (°C)

of

Percent E x t r a c t e d Sample A Sample Β

150

415

500

27

28

150

400

500

26

49

215

165

750

29

51

The p e r c e n t a g e o f m a t e r i a l e x t r a c t e d w i t h t h e t h r e e d i f f e r e n t f l u i d systems was v e r y s i m i l a r f o r sample A. However, t h e amount o f m a t e r i a l removed from sample Β was a p p r o x i m a t e l y 1.8 t i m e s g r e a t e r with m e t h a n o l - m o d i f i e d c a r b o n d i o x i d e and e t h a n o l - m o d i f i e d p e n t a n e than with pure carbon d i o x i d e . The h i g h e r e x t r a c t i o n t e m p e r a t u r e u s e d f o r t h e p e n t a n e s y s t e m c o u l d have c o n t r i b u t e d t o t h e i m p r o v e d efficiency for this fluid. I d e n t i c a l t e m p e r a t u r e s were u s e d f o r t h e c a r b o n d i o x i d e and c a r b o n d i o x i d e - m e t h a n o l f l u i d s y s t e m s , w h i c h s u g g e s t s t h a t t h e components were more s o l u b l e i n t h e more p o l a r f l u i d system. T h i s i s c o n s i s t e n t w i t h t h e sample c o m p o s i t i o n , s i n c e subsequent analyses of sample Β i n d i c a t e d t h a t i t contained


50

SUPERCRITICAL

FLUID EXTRACTION

AND

CHROMATOGRAPHY

phenolics, a m i n e s , e s t e r s , n i t r a t e d a r o m a t i c s and other polar compounds. Sample A, on the other hand, c o n t a i n e d neutral p o l y c y c l i c a r o m a t i c compounds t h a t would not be e x p e c t e d t o have s i g n i f i c a n t l y h i g h e r s o l u b i l i t y i n t h e more p o l a r f l u i d s y s t e m s . However, t h e gas c h r o m a t o g r a p h i c p r o f i l e s o f t h e c a r b o n d i o x i d e e x t r a c t and t h e c a r b o n d i o x i d e - m e t h a n o l e x t r a c t o f sample A were slightly different. As can be o b s e r v e d f r o m t h e chromatograms i n F i g u r e 3, t h e r e a r e g r e a t e r r e l a t i v e q u a n t i t i e s o f t h e higher m o l e c u l a r w e i g h t compounds i n t h e c a r b o n d i o x i d e - m e t h a n o l e x t r a c t than the pure carbon d i o x i d e e x t r a c t . T h i s may be a c c o u n t e d f o r by slightly greater solubility of these h i g h e r molecular weight compounds i n t h e more p o l a r f l u i d . These examples i l l u s t r a t e t h e p o t e n t i a l o f a n a l y t i c a l SFE f o r r a p i d and e f f i c i e n t complex m i x t u r e sample p r e p a r a t i o n .


Ultrasonic Supercritical Fluid

Extraction

The a p p l i c a t i o n o f u l t r a s o u n d d u r i n g s u p e r c r i t i c a l f l u i d e x t r a c t i o n c r e a t e s i n t e n s e s i n u s o i d a l v a r i a t i o n s i n d e n s i t y and p r e s s u r e which have t h e p o t e n t i a l o f i m p r o v i n g s o l u t e d i f f u s i o n and e n h a n c i n g o v e r a l l extraction rates. F o r a g i v e n power i n p u t , i t can be p r e d i c t e d t h a t t h e a m p l i t u d e o f t h e r e s u l t i n g d e n s i t y waves w i l l be approxim a t e l y t w i c e as g r e a t as i n a gas o r l i q u i d , whereas t h e pressure a m p l i t u d e w i l l be i n t e r m e d i a t e between t h a t o f a gas and liquid. T h e s e d e n s i t y waves w o u l d be e x p e c t e d t o i n d u c e c o n v e c t i o n of s o l u t e s from t h e i n n e r p o r e s o f a m a t e r i a l s i n c e t h e d e n s i t y waves i n t h e m a t r i x m a t e r i a l w i l l be much s m a l l e r t h a n t h a t o f t h e f l u i d i n the pores. Furthermore, a c o u s t i c streaming, the c h a r a c t e r i s t i c p a t t e r n o f s t e a d y m i c r o - v o r t i c e s i n s o n i c a t e d l i q u i d s (UL) , also o c c u r s i n s u p e r c r i t i c a l f l u i d s and w o u l d l i k e w i s e be e x p e c t e d t o d e c r e a s e e x t e r n a l mass t r a n s f e r r e s i s t a n c e , t h e c o n v e c t i v e t r a n s p o r t o f t h e s o l u t e from t h e s u r f a c e o f t h e m a t r i x i n t o t h e b u l k s o l u t i o n , and t o c r e a t e c o n v e c t i o n i n the macropores of the m a t e r i a l by r e c i r c u l a t i o n of the f l u i d . The a p p l i c a t i o n of u l t r a s o u n d to s u p e r c r i t i c a l f l u i d s also creates l o c a l i z e d heating that could enhance t h e d e s o r p t i o n r a t e o f s o l u t e s f r o m a m a t r i x . Sound e n e r g y i s a b s o r b e d i n a s u p e r c r i t i c a l f l u i d and n e a r t h e c r i t i c a l p o i n t , a t t e n u a t i o n o f t h e sound waves i n c r e a s e s d r a m a t i c a l l y b e c a u s e o f h i g h e r v i s c o u s d i s s i p a t i o n and s t r u c t u r a l r e l a x a t i o n o f m o l e c u l a r clusters (1£). I t s h o u l d be n o t e d t h a t c a v i t a t i o n (the c r e a t i o n and v i o l e n t c o l l a p s e of s m a l l vapor bubbles), the predominate e f f e c t of u l t r a s o u n d i n normal l i q u i d s , does not o c c u r i n s u p e r c r i t i c a l f l u i d s b e c a u s e no vapor-liquid surface e x i s t s to s u s t a i n the bubble structure. However, s o n i c a t i o n may a l s o improve e x t r a c t i o n s by a l t e r i n g or d i s r u p t i n g the i n t e r n a l s t r u c t u r e of a porous m a t e r i a l . Apparatus. The u l t r a s o n i c s u p e r c r i t i c a l f l u i d e x t r a c t i o n v e s s e l c o n s i s t e d o f a 25 mL h i g h - p r e s s u r e s t a i n l e s s s t e e l c e l l equipped w i t h two s e t s o f 0 . 7 5 - i n . d i a m e t e r windows ( f o r o b s e r v a t i o n during extraction) and a c o m m e r c i a l 20 kHz h i g h power u l t r a s o n i c h o r n (Branson W-350 S o n i f i e r w i t h a 102 c o n v e r t e r ) . The c e l l geometry was d e s i g n e d t o maximize t h e s o n i c e n e r g y p e r u n i t volume and t o c r e a t e r e s o n a n c e e f f e c t s t o b e t t e r t r a n s f e r e n e r g y from t h e s o n i c horn to the f l u i d . The u s a b l e c e l l volume u n d e r n e a t h t h e h o r n was a p p r o x i m a t e l y 12 mL. The u l t r a s o n i c t r a n s d u c e r was c o n n e c t e d t o t h e e x t r a c t i o n v e s s e l t h r o u g h an O - r i n g s e a l a t a wave node t o m i n i m i z e dampening o f t h e s o n i c e n e r g y . The e x t r a c t i o n v e s s e l was p l a c e d i n a t h e r m o s t a t e d water b a t h , and t h e f l u i d i n s i d e t h e v e s s e l was r e c i r c u l a t e d t h r o u g h a c o i l o f 1 / 8 - i n . o.d. s t a i n l e s s s t e e l tubing with a small magnetically


3. W R I G H T E T A L .

Supercritical

Fluid Extraction

51

Methodologies


Carbon Dioxide-Methanol (80:20)

0

10

20

30

40

50

60

70

Time (min.)

1 !

1

50 50

100

1 150

1 200

Temperature (°C)

Figure 3. C a p i l l a r y GC chromatograms o f t h e s u p e r c r i t i c a l c a r b o n d i o x i d e (bottom) and methanol m o d i f i e d c a r b o n d i o x i d e (top) e x t r a c t s o f a hazardous waste sample.


Γ~ 250


52


CHROMATOGRAPHY

c o u p l e d g e a r pump (183-346, Micro-Pump C o r p . ) . F l u i d r e c i r c u l a t i o n provided a convenient method o f o b t a i n i n g c o n t r o l l e d m i x i n g a n d a l s o s e r v e d t o d i s s i p a t e t h e heat g e n e r a t e d from t h e s o n i c energy. A s c h e m a t i c d i a g r a m o f t h e u l t r a s o n i c e x t r a c t i o n s y s t e m i s shown i n F i g u r e 4. The c o n c e n t r a t i o n o f t h e e x t r a c t e d a n a l y t e s was d i r e c t l y m o n i t o r e d by m e t e r i n g a low f l o w o f t h e e x t r a c t i o n e f f l u e n t t h r o u g h a 254 nm UV a b s o r b a n c e d e t e c t o r ( A l t e x , Model 153) e q u i p p e d w i t h a high pressure flow c e l l . The e x t r a c t i o n e f f l u e n t f l o w r a t e was r e g u l a t e d with a fused s i l i c a c a p i l l a r y r e s t r i c t o r attached t o the outlet of the detector. The f l o w r a t e t h r o u g h t h e d e t e c t o r was low enough so t h a t , w i t h c o n t i n u o u s a b s o r b a n c e measurements, o n l y a s m a l l p e r c e n t a g e o f t h e o v e r a l l s y s t e m f l u i d volume was l o s t . The extraction effluent was c o l l e c t e d b y i n s e r t i n g t h e c a p i l l a r y r e s t r i c t o r exit into a v i a l containing a suitable solvent. The e x t r a c t i o n s y s t e m was i n i t i a l l y p r e s s u r i z e d w i t h a m o d i f i e d V a r i a n 8500 s y r i n g e pump. By c o n t r o l l i n g t h e pump p r e s s u r e , a s m a l l make up s t r e a m o f f l u i d was i n t r o d u c e d t o compensate f o r t h e l o s s o f f l u i d t h r o u g h t h e UV d e t e c t o r . U l t r a s o n i c SFE o f A d s o r b e n t s . To e v a l u a t e u l t r a s o n i c SFE, t h e ex t r a c t i o n r a t e s o f chrysene from v a r i o u s porous adsorbent m a t e r i a l s u s i n g s u p e r c r i t i c a l c a r b o n d i o x i d e were compared b e f o r e a n d d u r i n g the a p p l i c a t i o n of u l t r a s o u n d . C h r y s e n e was c h o s e n as t e s t a n a l y t e b e c a u s e o f i t s low s o l u b i l i t y i n c a r b o n d i o x i d e ( •π

> Η Ο Ο

Ο

ο

>

> Ο Η Ο

w X H

*i r d

ο > f

Η

w

C

Ci a


ι

1

1

'

"

3fT

1/4" o.d. s.s. tubing

1/4" o.d. frit

t

Spacer (1/4" o.d. χ 3/32" i.d. χ 1/8" long s.s. tubing)

1/4" o.d. frit

t

m

2£1

1/4" Swagelok nut and ferrule assembly

F i g u r e 7. D e s i g n o f the m i c r o e x t r a c t i o n c e l l s . (Reproduced from Ref. 13. C o p y r i g h t 1987 American C h e m i c a l S o c i e t y . )

tubing

A

u

1/16" Swagelok nut and ferrule assembly

a

1/16" to 1/4" Swagelok column end fitting



58


CHROMATOGRAPHY

c o n t r o l t h e temperature o f t h e r e s t r i c t o r and expansion r e g i o n . T y p i c a l l y , t h i s r e g i o n was m a i n t a i n e d near t h e upper o p e r a t i n g t e m p e r a t u r e o f t h e c h r o m a t o g r a p h i c oven (between 250 and 280 °C) . A H e w l e t t - P a c k a r d 5890 gas chromatograph e q u i p p e d w i t h a flame i o n i z a t i o n d e t e c t o r was u s e d t o p e r f o r m c h r o m a t o g r a p h i c a n a l y s e s . F u s e d s i l i c a c h r o m a t o g r a p h i c columns were e i t h e r 15 m χ 0.25 mm i . d . o r 15 m χ 0.53 mm i . d . , coated with 0.25-μιη o r 5.0-μιη f i l m t h i c k n e s s , r e s p e c t i v e l y , o f DB-5 s t a t i o n a r y phase (J&W S c i e n t i f i c ) . A s h o r t r e t e n t i o n gap o f d e a c t i v a t e d f u s e d s i l i c a t u b i n g (30 cm χ 0.53 mm i . d . ) was c o n n e c t e d t o the i n l e t of the chromatographic column t o a i d s o l u t e f o c u s i n g and c o n c e n t r a t i o n o f t h e e x t r a c t i o n effluent. The oven was c o o l e d t o subambient t e m p e r a t u r e s u s i n g c a r b o n d i o x i d e d u r i n g on-column d e p o s i t i o n t o a i d f o c u s i n g and c o n c e n t r a t i o n o f t h e e x t r a c t i o n e f f l u e n t . Subsequent a n a l y s e s were p e r f o r m e d by t e m p e r a t u r e programming a t s e l e c t e d r a t e s . H e l i u m was used f o r the c a r r i e r gas a t l i n e a r v e l o c i t i e s o f a p p r o x i m a t e l y 40 cm/sec. Q u a l i t a t i v e E x t r a c t i o n Analyses. Qualitative characterization of a sample m a t r i x c a n be o b t a i n e d by p e r i o d i c s a m p l i n g and a n a l y s i s o f the e x t r a c t i o n e f f l u e n t at v a r i o u s p r e s s u r e s . T y p i c a l l y , a sample i s extracted at progressively higher pressures t o obtain s e l e c t i v e fractions. However, t o a c h i e v e maximum s e l e c t i v i t y , t h e sample must be e x h a u s t i v e l y e x t r a c t e d a t e a c h p r e s s u r e t o remove e s s e n t i a l l y a l l t h e m a t e r i a l t h a t i s s o l u b l e a t a g i v e n p r e s s u r e p r i o r t o t h e next higher e x t r a c t i o n pressure. Rigorous execution o f t h i s process can be d i f f i c u l t with the current instrumentation, so l e s s s e l e c t i v e f r a c t i o n a t i o n i s u s u a l l y adopted f o r q u a l i t a t i v e analyses. An example o f t h e a p p l i c a t i o n o f o n - l i n e SFE-GC f o r t h e q u a l i t a t i v e c h a r a c t e r i z a t i o n o f orange p e e l i s shown i n F i g u r e 8. Ap p r o x i m a t e l y 100 mg o f orange p e e l was e x t r a c t e d f o r 10 min i n t e r v a l s a t two p r o g r e s s i v e l y h i g h e r p r e s s u r e s u s i n g s u p e r c r i t i c a l c a r b o n d i o x i d e a t 40 °C. A f t e r each e x t r a c t i o n a t e m p e r a t u r e programmed GC a n a l y s i s was c o n d u c t e d . In t h i s p a r t i c u l a r case, t h e e x t r a c t i o n s were d i s c o n t i n u e d d u r i n g t h e GC a n a l y s e s , b u t i f more s e l e c t i v e f r a c t i o n a t i o n s h a d been d e s i r e d t h e y c o u l d have b e e n continued d u r i n g t h e a n a l y s e s and t h e e f f l u e n t directed to the c o l l e c t i o n v e s s e l . S i n c e o r a n g e p e e l was e x p e c t e d t o c o n t a i n v o l a t i l e f l a v o r compounds, a c h r o m a t o g r a p h i c column w i t h a t h i c k s t a t i o n a r y phase f i l m and subambient c o l l e c t i o n and f o c u s i n g o f t h e e x t r a c t i o n e f f l u e n t were u s e d i n t h e GC a n a l y s i s . The chromatogram o b t a i n e d at t h e lower e x t r a c t i o n p r e s s u r e has r e l a t i v e l y higher con c e n t r a t i o n s o f t h e e a r l i e r e l u t i n g compounds and t h e h i g h e r p r e s s u r e e x t r a c t i o n has r e l a t i v e l y h i g h e r c o n c e n t r a t i o n s o f t h e l a t e r e l u t i n g compounds. S i n c e t h e e n t i r e range o f compounds f r o m t h e orange p e e l e x h i b i t e d s u b s t a n t i a l s o l u b i l i t y i n t h e c a r b o n d i o x i d e a t t h e low pressure, i t was n o t p o s s i b l e t o o b t a i n a h i g h l y selective extraction. U t i l i z a t i o n o f a f l u i d h a v i n g l o w e r s o l v a t i n g power, s u c h a s SFg, Xe, o r Kr ( 1 7 ) , would p r o v i d e g r e a t e r p o t e n t i a l f o r s e l e c t i v e f r a c t i o n a t i o n o f v o l a t i l e and s e m i v o l a t i l e low m o l e c u l a r weight compounds. T h i s example i l l u s t r a t e s t h e p o t e n t i a l o f SFE-GC o f p r o v i d i n g n e a r l y automated sample p r e p a r a t i o n a n d a n a l y s i s f o r the qualitative characterization of sample m a t r i c e s . More s e l e c t i v e SFE-GC a n a l y s e s o f h i g h e r m o l e c u l a r weight compounds w i t h l o w e r c a r b o n d i o x i d e s o l u b i l i t i e s have been r e p o r t e d e l s e w h e r e (13.) . Quantitative Extraction Analyses. Exhaustive e x t r a c t i o n of a sample m a t r i x a t a p r e s s u r e where a l l o f t h e components o f i n t e r e s t are s o l u b l e provides t h e c a p a b i l i t y f o r a q u a n t i t a t i v e a n a l y s i s . T h i s method i s i l l u s t r a t e d w i t h t h e example shown i n F i g u r e 9. XAD-2



Figure 8. C a p i l l a r y GC chromâtograms o b t a i n e d from s u p e r c r i t i c a l c a r b o n d i o x i d e e x t r a c t i o n a t two d i f f e r e n t p r e s s u r e s o f orange p e e l .


SUPERCRITICAL FLUID EXTRACTION AND CHROMATOGRAPHY

C 0 , 50 °C, 325 b a r 2


CHRYSENE

ι

1

0

10

1

I

I

0

50

90

1

1

20 Time (min.)

30

I 150 Temperature (°C)

I 210

1 40

Γ 265

Figure 9. C a p i l l a r y GC chromatogram o b t a i n e d from t h e s u p e r c r i t i c a l c a r b o n d i o x i d e e x t r a c t i o n o f XAD-2 r e s i n .


3.

WRIGHT ET AL.

Supercritical

Fluid Extraction

61

Methodologies


r e s i n t h a t had been s p i k e d w i t h 7.5 ng e a c h o f s e v e r a l p o l y c y c l i c a r o m a t i c h y d r o c a r b o n s (PAH) was e x t r a c t e d w i t h c a r b o n d i o x i d e a t 325 b a r and 50 °C f o r 7 min. The f l o w r a t e o f d e c o m p r e s s e d carbon d i o x i d e was o v e r 250 mL/min, which a l l o w e d a s u f f i c i e n t l y l a r g e f l u i d volume t o be u s e d t o o b t a i n an e x h a u s t i v e e x t r a c t i o n . The c h r o m a t o g r a p h i c oven was h e l d a t 0 °C d u r i n g t h e e x t r a c t i o n t o a i d f o c u s i n g o f t h e a n a l y t e s and was t h e n t e m p e r a t u r e programmed a f t e r the extraction was completed to e f f e c t the chromatographic separation. The d e t e c t o r r e s p o n s e was c a l i b r a t e d f o r each compound immediately p r i o r t o the e x t r a c t i o n - a n a l y s i s . The s p e c i f i c compounds used, t h e s p i k e l e v e l s , and t h e r e c o v e r e d q u a n t i t i e s o f each compound a r e l i s t e d i n T a b l e IV. W i t h i n e x p e r i m e n t a l e r r o r (~15%), q u a n t i t a t i v e e x t r a c t i o n and r e c o v e r y o f a l l t h e compounds, e x c e p t b e n z o [ e j p y r e n e , was o b t a i n e d . B e n z o [ e j p y r e n e has l o w e r s o l u b i l i t y i n carbon d i o x i d e and was not c o m p l e t e l y e x t r a c t e d w i t h the f l u i d T a b l e IV. Compound

Fluorene Phenanthrene Pyrene Chrysene Benzo [e_] pyrene

Recovery

Spike L e v e l (ng)

7..6 7..4 7..5 7..3 7..4

o f PAH

from XAD-2 R e s i n

R e c o v e r e d Amount (ng)

7..6 7..1 6..6 6..4 4,.9

%

Recovery

100 96 88 88 66

volume u t i l i z e d . However, t h e r e m a i n i n g b e n z o [ a ] p y r e n e was r e c o v e r e d with a second e x t r a c t i o n . The e x t r a n e o u s p e a k s i n t h e chromatogram can be a t t r i b u t e d t o i m p u r i t i e s c o n c e n t r a t e d from t h e c a r b o n d i o x i d e o r from contaminants e x t r a c t e d from t h e XAD-2. Since several m i l l i l i t e r s of l i q u i d carbon d i o x i d e are decompressed t h r o u g h t h e c h r o m a t o g r a p h i c column w i t h q u a n t i t a t i v e e x t r a c t i o n s , i t i s e s s e n t i a l t o have v e r y p u r e c a r b o n d i o x i d e t o p r e v e n t s e r i o u s contamination. O n - l i n e SFE-GC has a l s o been u s e d as an a l t e r n a t i v e t o t h e r m a l desorption gas chromatography o f Tenax s a m p l i n g devices for q u a n t i t a t i v e a n a l y s i s o f v o l a t i l e o r g a n i c compounds (VOC). The SFEGC a p p r o a c h a l l o w s t h e a n a l y t e s t o be r e c o v e r e d from t h e a d s o r b e n t sampling devices at mild temperatures, which prevents thermal decomposition and other problems associated with high t e m p e r a t u r e d e s o r p t i o n . T h i s work i s d e s c r i b e d i n d e t a i l e l s e w h e r e (Wright,B.W.; Kopriva, A.J.; Smith, R.D. submitted for publication). Conclusions O f f - l i n e s u p e r c r i t i c a l f l u i d e x t r a c t i o n , simultaneous u l t r a s o n i c s u p e r c r i t i c a l f l u i d e x t r a c t i o n , and o n - l i n e s u p e r c r i t i c a l fluid e x t r a c t i o n - g a s chromatography have been d e s c r i b e d . These a n a l y t i c a l s u p e r c r i t i c a l f l u i d e x t r a c t i o n methods p r o v i d e t h e p o t e n t i a l f o r very rapid extraction rates and compatibility with on-line a n a l y t i c a l methods. E x t r a c t i o n r a t e i n c r e a s e s o f o v e r an o r d e r o f m a g n i t u d e compared t o S o x h l e t methods have been d e m o n s t r a t e d and even greater increases seem f e a s i b l e . O p t i m i z a t i o n of fluid s o l v a t i n g c o n d i t i o n s a l s o provides the p o t e n t i a l f o r s e l e c t i v e f r a c t i o n a t i o n of s p e c i f i c analytes. The a p p l i c a t i o n o f u l t r a s o u n d during supercritical fluid extraction p r o v i d e s an efficient


62

SUPERCRITICAL FLUID EXTRACTION AND CHROMATOGRAPHY

mechanism of s t i r r i n g a n d may e n h a n c e extraction rates i n macroporous m a t e r i a l s by i n d u c i n g c o n v e c t i o n t h r o u g h t h e p o r e s . On l i n e e x t r a c t i o n - a n a l y s i s methods combine sample p r e p a r a t i o n a n d a n a l y s i s and p r o v i d e t h e p o t e n t i a l f o r r a p i d a n d h i g h l y s e n s i t i v e analyses. A d d i t i o n a l s t u d i e s and f u r t h e r development o f a n a l y t i c a l SFE methods a r e needed f o r a more c o m p l e t e e v a l u a t i o n and t o a l l o w achievement o f t h e i r f u l l p o t e n t i a l .


Acknowledgment s A l t h o u g h t h e r e s e a r c h d e s c r i b e d i n t h i s a r t i c l e h a s been f u n d e d w h o l l y o r i n p a r t by t h e U n i t e d S t a t e s E n v i r o n m e n t a l P r o t e c t i o n A g e n c y t h r o u g h I n t e r a g e n c y Agreement DW 8 99930 650-01 t h r o u g h a R e l a t e d S e r v i c e s C o n t r a c t w i t h t h e U.S. Department o f E n e r g y under C o n t r a c t DE-AC06-76RLO 1830, i t has n o t been s u b j e c t e d t o Agency r e v i e w and t h e r e f o r e does n o t n e c e s s a r i l y r e f l e c t t h e views o f t h e Agency a n d no o f f i c i a l endorsement s h o u l d be i n f e r r e d . Mention o f t r a d e names o r c o m m e r c i a l p r o d u c t s does n o t c o n s t i t u t e endorsement o r recommendation f o r u s e . Literature 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Cited

Stahl, G.M.; Wilke, G. Extraction with Supercritical Gases; Verlag Chemie: Deerfield Beach, FL, 1980. Paulaitis, M.E.; Krukonis, V.J.; Kurnick, R.T.; Reid, R.C. Rev. Chem. Eng. 1983, 1, 179-250. McHugh, M.A.; Krukonis, V.J. Supercritical Fluid Extraction, Principles and P r a c t i c e ; Butterworths: Boston, MA, 1986. Unger, K.K.; Roumeliotis, P.J. J. Chromatogr. 1983, 282, 519-526. Smith, R.D.; Udseth, H.R.; Wright, B.W. In Supercritical Fluid Technology; Penninger, J.M.L.; Radosz, M.; McHugh, M.A.; Krukonis, V.J., Eds.; Elsevier: Amsterdam, The Netherlands, 1985; pp 191-223. Sugiyama, K.; Saito, M.; Hondo, T.; Senda, A.J. J. Chromatogr. 1985, 332, 107-116. Hawthorne, S.B.; Miller, D.J. J. Chromatogr. Sci. 1986, 24, 258-264. Schantz, M.M.; Chestler, S.N. J.Chromatogr. 1986, 363, 397-401. Kalinoski, H.T.; Udseth, H.R.; Wright, B.W.; Smith, R.D. Anal. Chem. 1986, 58, 2124-2129. Campbell, R.M.; Lee, M.L. Anal. Chem. 1986, 58, 2247-2251. Capriel, P.; Haisch, Α.; Khan, S.U. J. Agric. Food Chem. 1986, 34, 70-73. Wright, B.W.; Wright, C.W.; Gale, R.W.; Smith, R.D. Anal. Chem. 1987, 59, 38-44. Wright, B.W.; Frye, S.R.; McMinn, D.G.; Smith, R.D. Anal. Chem. 1987, 59, 640-644. Hawthorne, S.B.; Miller, D.J. Anal. Chem. 1987, 59, 17051708. Fogler, H.S. In Sonochemical Engineering; Fogler, H.S., Ed.; AIChE: New York, 1971; Vol.67, p 1. Chynoweth, A.G.; Schneider, W.G. J. Chem. Phys. 1951, 19, 1566-1569. Smith, R.D.; Frye, S.L.; Yonker, C.R.; Gale, R.W. J.Phys. Chem. 1987, 91, 3059-3062.

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November 13, 1987


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