DIRECT COUPLING OF SUPERCRITICAL FLUID EXTRACTIONS U P E R C R I T I C A L F L U I D C H R O M A T O G R A P H Y FOR T H E DETERMINATION OF SELECTED POLYCYCLIC AROMATIC H Y D R O C A R B O N S IN A Q U E O U S E N V I R O N M E N T A L S A M P L E S C H Y E P E N G O N G , H I A N K E E L E E and S A M F O N G Y A U LI*
Department of Chemistry, National University of Singapore, 10 Kent Ridge Cresent, Republic of Singapore 0511
(Received March 1991) Abstract. A rapid and simple method is described for the quantitative determination of polycyclic aromatic hydrocarbons in aqueous environmental samples. A microscale on-line supercritical fluid extractionsupercritical fluid chromatography system using carbon dioxide is employed. The extract is analysed using capillary supercritical fluid chromatography with UV detection. Detection was carried out at 254 nm. An extraction efficiency of as high as 91% was obtained for the PAHs.
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are of great environmental concern due to their toxicity and wide distribution. The United States Environmental Protection Agency (U.S.E.P.A.) has listed some of these compounds as priority pollutants (Marr et al., 1983). The usual extraction procedure for PAHs in aqueous samples employ liquid-liquid extraction. This technique is often very tedious and time consuming. At the same time, due to the lack of selectivity in this method, it is often necessary to include an additional sample clean-up step prior to analysis. Consequently, the extraction efficiency of such a method is unsatisfactory. Furthermore, with involvement of the additional steps, introduction of impurities is inevitable (Ibrahim et al., 1988). An alternative extraction technique would be supercritical fluid extraction (SFE). In the past years, interest in SFE, and in hyphenated SFE systems in particular, has been rapidly increasing (McNally et al., 1988). This technique offers not only a rapid rate of extraction, but it is also often expected to produce higher extraction efficiencies. In our previous investigation (Lee et al., 1990), a rapid and simple technique for the extracting, identifying and quantitating cholesterol from human serum was described. The work involved direct coupling of SFE to the usual supercritical fluid chromatography (SFC) system using supercritical carbon dioxide. However, unlike most typical on-line SFE-SFC systems, the set-up utilizes, besides the usual supercritical fluid chromatograph, an additonal extractor. The extractor is a 'cartridge like' chamber filled with some packing material. The extractor is connected directly to the SFC system just after the sampling * Author to whom correspondence should be addressed.
Environmental Monitoring and Assessment 19: 63-71, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
64
CHYE
PENG
ONG
ET AL.
valve. Extraction efficiencies of as high as 93% was obtained for cholesterol in that study using the set-up. In the present work, attempts were made to implement this extraction scheme for the quantitation of PAHs in aqueous samples. In addition, an investigation is performed to compare the extraction efficiency of the microscale SFE-SFC extraction method with that of the usual liquid-liquid extraction procedure employing cyclohexane as the extraction solvent.
Experimental The extraction chamber, depicted in Figure 1, was constructed from a 1/4 inch OD, 35 m m long brass tubing with a 2 m m ID. Part of the extractor was filled with Partisil-5ODS-3 C~s (5/~m particle size) packing material (Whatmann International, Clifton, New Jersey, U.S.A.). The extractor was connected to the SFC system between the sampling valve and the analytical column. All the experiments were performed on a Model SFC-3000 supercritical fluid chromatographic system (Carlo Erba Strumentazione, Rodano, Milan, Italy). The system is equipped with a micro UVis detector (Carlo Erba Strumentazione, Rodano, Milan, Italy) with the wavelength set at 254 nm. The column employed was a SE-52 fused silica capillary column (9 m X 100 #m ID X 0.45 #m coating thickness). A tapered restrictor fabricated in our laboratory with a calibrated flow rate of 15 ml/min was connected downstream of the UV cell. The end of the restrictor was maintained at a temperature of 320~ Injections were made with an air actuated Valco VICI injection valve (Houston, Texas, USA) equipped with 1-#1 loop. The injection time was 500 ms. The chromatographic data were collected and analysed on a Hewlett Packard Model 3394A integrator (Hewlett-Packard, Avondale, U.S.A.). The temperature at the injection port was kept at 28~ throughout the analysis. All chemicals were of analytical reagent grade or better. To investigate the suitability of the system for aqueous samples, four PAHs were choosen as test compounds. The four PAHs and their structures are shown in Table I. The reason for the choice of these PAHs is that the World Health Organization (WHO) has listed them as indicator PAH in drinking water (WHO, 1984). Fluoranthene was obtained from Fluka Chemie A G (Buchs, Switzerland) and the other PAHs were supplied by Aldrich Chemical (Milwaukee, Wisconsin, U.S.A.). Standard solutions of the individual PAHs were prepared in AR grade methylene chloride (Merck, Darmstadt, F.R.G.). Carbon dioxide of 100% purity was purchased from the British Oxygen Company (London, U.K.). The waste water sample collected was preconcentrated using a Bfichi Model RE 111 Rotavap (Flawil, Switzerland). This is because preliminary analyses of the waste water without any pre-enrichment procedure using the S F E - S F C system failed to detect any PAHs. This is expected as the levels of PAHs in the sample are believed to be very low and effective detection of any of these compounds at such levels without any sample preconcentration is highly unlikely. The preconcentration also served to facilitate liquidliquid extraction procedures since smaller volumes would be used in this case. To ensure
SUPERCRITICAL FLUID-EXTRACTION CHROMATOGRAPHY OF PAHs
65
FROM INJECTOR
1 / 4 INCHES OD x 1 / 1 6 REDUCING UNION
INCHES
TEFLON SLEEVE I 0.4.
mm
ID x 2
mm
OD
1 / 4 INCHES x 2 mm BRASS TUBING
TEFLON SLEEVE 0.4
mm
ID x 2
mm
OD ~
C18 PACKING MATERIAL
FRIT Fll-I'ING
OLUMN
Fig. I. Schematic diagram of the extractor.
that there was no loss o f P A H s d u r i n g the p r e c o n c e n t r a t i o n step, extractions o f the waste water sample were carried out without the preconcentration step. The levels o f P A H s detected were in agreement with the results o b t a i n e d when the preconcentrated step was included. The extraction o f the P A H s from the liquid samples using S F E was carried out using a similar p r o c e d u r e as described in o u r previous investigation (Lee et al., 1990). The samples to be extracted a n d analysed as well as the s t a n d a r d solutions were injected directly into the system using the Valco injection valve. The extractions were carried out isobarically at a pressure o f 11 M P a a n d an oven t e m p e r a t u r e o f 60~ The p r o c e d u r e for the conventional liquid-liquid extraction is described elsewhere (Fresenius et al., 1988). The
66
CHYE PENG O N G ET AL. TABLE I Structures and some physical constants for the four PAHs studied in this work. COMPOUND
ABBREVIATION
STRUCTURE
I)F l u o r a n t h e n e
Ft
~
2) Benzolblf Iuoranthene
.BIbIF
~(~T(~T
3) Benzolal-
BIaIP
RMM
BOILING PT/*C
202
375
T(~]
252
48 I
[('-~ ~("~)T(-') ~
252
456
Lk._)J~._)J.~_)j
276
pyrene
4) B e n z o l g h i l perylene
B[ghilP
not a v a i l a b l e
samples were first extracted twice with cyclohexane as extracting solvent. In each extraction, the mixture was mechanically shaken for half an hour. Subsequently the extract was subjected to sample clean-up using Bond Elut C]8 cartridges supplied by Analytichem (Harbor City, CA, U.S.A.). The extract obtained after clean-up was then analysed by capillary SFC using the same conditions as those employed for the supercritical fluid extraction, except that the extraction chamber was removed from the SFC system during these analyses. Calibration plots for the four PAHs were obtained using the latter set-up (i.e. without the extractor). Extraction efficiencies for the two methods were subsequently obtained by comparing their results with those given in the calibration plots. Results and Discussion
The use of on-line SFE-SFC has been successfully employed in many applications for solid matrices (McNally et al., 1988). By contrast, on-line SFE-SFC of aqueous samples has yet to receive as much attention. The reason could be largely due to the numerous problems associated with this type of extraction which render the use of the usual on-line
SUPERCRITICAL
FLUID-EXTRACTION
CHROMATOGRAPHY
OF PAHs
67
SFE-SFC impossible for these applications (Hendrick et al., 1989). In our work, an on-line SFE-SFC system has been successfully developed for the determination of selected PAHs found in aqueous samples. The system was built using a typical supercritical fluid chromatograph and a 'cartridge like' extractor. One of the advantages of this system is that the usual off-line or on-line monitoring system found in other SFE-SFC systems is not required. This simplifies both the instrumental set-up as well as the experimental procedure. Besides, the extraction and the analysis of extracts could be carried out by introducing the sample directly into the system via the sampling valve. Furthermore, in this work, sample clean-up was not required since the extractor also functions as a pre-column to retain undesirable compounds. With the current arrangement where the extractor is placed just before the analytical column, quantitative amounts of the extract are transfered without any lossess. Therefore such a system will allow for microscale type of extraction which is another attractive feature of the system. With the possibility of microscale type of extraction, peak broadening would be minimized (see Figure 2). Hence there is no necessity to include an additional sample valve to concentrate the extract prior to analysis. The results obtained for the extraction of PAHs from spiked aqueous samples using the on-line S F E - S F C as well as the liquid-liquid extraction procedure are shown in Table II and their respective chromatograms are illustrated in Figures 2 and 3. From the figures,
la
~MIIM Fig. 2. Typical c h r o m a t o g r a m of P A H extract from the spiked a q u e o u s sample using the microscale S F E - S F C system. C h r o m a t o g r a p h i c conditions: 11 M P a isobarically with an oven t e m p e r a t u r e of 60~ Peak identification: A = Ft, B ----B[b]F, C = B[a]P a n d D = B[ghi]P.
68
CHYE
PENG
ONG
ET AL.
TABLE II Comparison of results of extraction using on-line SFE-SFC and liquid-liquid extraction. Efficency (%)* Extraction method
Ft
B[b]F
B[a]P
B[gh0P
On-line SFE-SFC Liquid-liquid extraction
91 68
89 58
90 62
88 67
* Figures were obtained from extracting spiked distilled water samples. Blank runs were also carried out for the distilled water. l o n g e r r e t e n t i o n times were o b s e r v e d for the P A H s using the on-line S F E - S F C system ( F i g u r e 2) as c o m p a r e d to those o b t a i n e d by l i q u i d - l i q u i d extraction ( F i g u r e 3). This is e x p e c t e d as the retention times given in Figure 2 w o u l d also include the e x t r a c t i o n time
C
B
A
I
,_./L_ I~1
JI~IMIN
Fig. 3. Typical chromatogram of PAH extract from the spiked aqueous sample using liquid-liquid extraction. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft, B z B[b]F, C = B[a]P and D = B[ghlqP.
SUPERCRITICAL
FLUID-EXTRACTION
CHROMATOGRAPHY
OF PAHs
69
for the PAHs. In spite of their difference in retention times, the capacity factors were found to be very similar. An important point to note is that at least one hour was required for the extraction procedure alone in the liquid-liquid extraction method compared to the thirty-five minutes needed for the complete analysis (extraction and chromatography) using the SFE-SFC system. The calibration plots for the four PAHs are given in Figure 4 and a typical S F E - S F C chromatogram of the PAH standard mixture is shown in Figure 3. The extraction efficiencies obtained using the on-line SFE-SFC system were generally higher than the liquid-liquid extraction method. The additional sample clean-up step required in the latter method seems to be the main cause of the low extraction efficiency. Therefore, it is possible to implement a liquid-liquid extraction system to be coupled to the existng S F E - S F C system where the extractor would now serve as a pre-column for sample clean-up as well as its original function for extraction. Such a system would be useful for samples that cannot be analysed directly by the existing on-line SFE-SFC system due to reasons which prevent direct injection of the sample into the sampling valve (e.g. viscous samples). From a comparison of the total time required for each method, it is worthwhile noting that the on-line S F E - S F C system provides a simpler method and more rapid rate of extraction. Therefore it seems that the on-line SFE-SFC system would be an attractive alternative to the conventional liquid-liquid extraction for PAHs in liquid samples. In our investigation, environmental samples were analysed for the four PAHs.
~,.J
A Ft
A
/ . / j
9 B[a]F
.-//"/"
/j
..-" ..-'-
B[alP
A/-""
ID
X t~ O m ~e
IZ
.I t,a ~e
1 I
I
I
I
CONCENIR~TION
Fig. 4.
I
I
/PP~
Calibration plots for the four PAHs.
I
I
tI,BO~,
70
CHYE
PENG
ONG
ET AL.
A
d,
I
6w
Fig. 5. Typical chromatogram of PAH extract from environmental aqueous sample using the microscale SFE-SFC system. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft.
A m o n g s t these P A H s , only fluoranthene was detected in the waste water samples analysed. The results are shown in Table III a n d the c o r r e s p o n d i n g c h r o m a t o g r a m s are illustrated in Figures 5 a n d 6. F r o m these results, it was noted that the levels offluoranthene were fairly similar using the different types of extraction methods. The collective a m o u n t of P A H s present in these water samples was found to be below the guideline r e c o m m e n d e d by the W H O (i.e. 200 ng/1). However, due to the absence of the other two P A H indicators namely benzo[k]fluoranthene and indeno[ 1,2,3-cd]pyrene in this investigation, no conclusive statement can be made regarding the level o f P A H s in the aqueous samples. Nevertheless the microscale S F E - S F C for the analysis of P A H s in aqueous environmental sample has been successfully demonstrated. It is believed that there is great potential for the system to be e m p l o y e d for the rapid analysis of other environmental samples. The unique features o f the system include simplicity in the set-up, ease of operation, r a p i d rate o f extraction a n d high extraction efficiency. These advantages o f the system m a k e it an attractive alternative to conventional extraction a n d analytical procedures. TABLE III Amount of PAHs extracted from environment aqueous samples. Method of extraction
Concentration / ng/1)* Ft
On-line SFE-SFC Liquid-liquid extraction
8.7 8.3
* The other three PAHs were not detected.
SUPERCRITICAL F L U I D - E X T R A C T I O N C H R O M A T O G R A P H Y OF PAHs
71
I 6
~OMIN
Fig. 6. Typical chromatogram of PAH extract from environmental aqueous sample using liquid-liquid extraction. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft.
Acknowledgement The authors wish to thank the National University of Singapore for financial support.
References Fresenius, W., Quentin, K. E., and Schnider, W.: 1988, Water Analysis: A practical Guide To Physico-Chemical, Chemical And Microbiological Water Examination And Quality Assurance, Springer-Verlag, Berlin & Heidelberg, FRG, p. 552. Hedrick, J., and T. Taylor, L.T.: 1986, 'Quantitative Supercritical Fluid Extraction - Supercritical Fluid Chromatography Of a Phosphonate From Aqueous Media', Anal. Chem. 61,989. Ibrahim, E. A., and Suffer, I. H.: 1988,'Freon FC- 113 as an alternative to methylene chloride for liquid-liquid extraction of trace organic from chlorinated drinking water', J. Chromatogr., 454, 217. Lee, H. K., Li, S. F. Y., and Ong, C. P.: 1990, 'Microscale Supercritical Fluid Extraction Directly Coupled With Supercritical Fluid Chromatography', Presented at Second International Symposium on High Performance Capillary Electrophoresis, San Francisco, California, U.S.A. pp. 29-31, January 1990. Mart, I. L., and Cresser, M. S.: 1983, Environmental ChemicalAnalysis, Blackie and Sons, Great Britain, p. 253. McNally, M.E.P., and Wheeler, J.R.: 1988, 'Increasing Extraction Efficiency In Supercritical Fluid Extraction From Complex Matrices', J. Chromatogr., 447, 53. World Health Organization: 1984, Guidelines For Drinking Water Quality, Macimillan, Belgium, Vol. 2 p. 182.
Department of Chemistry, National University of Singapore, 10 Kent Ridge Cresent, Republic of Singapore 0511
(Received March 1991) Abstract. A rapid and simple method is described for the quantitative determination of polycyclic aromatic hydrocarbons in aqueous environmental samples. A microscale on-line supercritical fluid extractionsupercritical fluid chromatography system using carbon dioxide is employed. The extract is analysed using capillary supercritical fluid chromatography with UV detection. Detection was carried out at 254 nm. An extraction efficiency of as high as 91% was obtained for the PAHs.
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are of great environmental concern due to their toxicity and wide distribution. The United States Environmental Protection Agency (U.S.E.P.A.) has listed some of these compounds as priority pollutants (Marr et al., 1983). The usual extraction procedure for PAHs in aqueous samples employ liquid-liquid extraction. This technique is often very tedious and time consuming. At the same time, due to the lack of selectivity in this method, it is often necessary to include an additional sample clean-up step prior to analysis. Consequently, the extraction efficiency of such a method is unsatisfactory. Furthermore, with involvement of the additional steps, introduction of impurities is inevitable (Ibrahim et al., 1988). An alternative extraction technique would be supercritical fluid extraction (SFE). In the past years, interest in SFE, and in hyphenated SFE systems in particular, has been rapidly increasing (McNally et al., 1988). This technique offers not only a rapid rate of extraction, but it is also often expected to produce higher extraction efficiencies. In our previous investigation (Lee et al., 1990), a rapid and simple technique for the extracting, identifying and quantitating cholesterol from human serum was described. The work involved direct coupling of SFE to the usual supercritical fluid chromatography (SFC) system using supercritical carbon dioxide. However, unlike most typical on-line SFE-SFC systems, the set-up utilizes, besides the usual supercritical fluid chromatograph, an additonal extractor. The extractor is a 'cartridge like' chamber filled with some packing material. The extractor is connected directly to the SFC system just after the sampling * Author to whom correspondence should be addressed.
Environmental Monitoring and Assessment 19: 63-71, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.
64
CHYE
PENG
ONG
ET AL.
valve. Extraction efficiencies of as high as 93% was obtained for cholesterol in that study using the set-up. In the present work, attempts were made to implement this extraction scheme for the quantitation of PAHs in aqueous samples. In addition, an investigation is performed to compare the extraction efficiency of the microscale SFE-SFC extraction method with that of the usual liquid-liquid extraction procedure employing cyclohexane as the extraction solvent.
Experimental The extraction chamber, depicted in Figure 1, was constructed from a 1/4 inch OD, 35 m m long brass tubing with a 2 m m ID. Part of the extractor was filled with Partisil-5ODS-3 C~s (5/~m particle size) packing material (Whatmann International, Clifton, New Jersey, U.S.A.). The extractor was connected to the SFC system between the sampling valve and the analytical column. All the experiments were performed on a Model SFC-3000 supercritical fluid chromatographic system (Carlo Erba Strumentazione, Rodano, Milan, Italy). The system is equipped with a micro UVis detector (Carlo Erba Strumentazione, Rodano, Milan, Italy) with the wavelength set at 254 nm. The column employed was a SE-52 fused silica capillary column (9 m X 100 #m ID X 0.45 #m coating thickness). A tapered restrictor fabricated in our laboratory with a calibrated flow rate of 15 ml/min was connected downstream of the UV cell. The end of the restrictor was maintained at a temperature of 320~ Injections were made with an air actuated Valco VICI injection valve (Houston, Texas, USA) equipped with 1-#1 loop. The injection time was 500 ms. The chromatographic data were collected and analysed on a Hewlett Packard Model 3394A integrator (Hewlett-Packard, Avondale, U.S.A.). The temperature at the injection port was kept at 28~ throughout the analysis. All chemicals were of analytical reagent grade or better. To investigate the suitability of the system for aqueous samples, four PAHs were choosen as test compounds. The four PAHs and their structures are shown in Table I. The reason for the choice of these PAHs is that the World Health Organization (WHO) has listed them as indicator PAH in drinking water (WHO, 1984). Fluoranthene was obtained from Fluka Chemie A G (Buchs, Switzerland) and the other PAHs were supplied by Aldrich Chemical (Milwaukee, Wisconsin, U.S.A.). Standard solutions of the individual PAHs were prepared in AR grade methylene chloride (Merck, Darmstadt, F.R.G.). Carbon dioxide of 100% purity was purchased from the British Oxygen Company (London, U.K.). The waste water sample collected was preconcentrated using a Bfichi Model RE 111 Rotavap (Flawil, Switzerland). This is because preliminary analyses of the waste water without any pre-enrichment procedure using the S F E - S F C system failed to detect any PAHs. This is expected as the levels of PAHs in the sample are believed to be very low and effective detection of any of these compounds at such levels without any sample preconcentration is highly unlikely. The preconcentration also served to facilitate liquidliquid extraction procedures since smaller volumes would be used in this case. To ensure
SUPERCRITICAL FLUID-EXTRACTION CHROMATOGRAPHY OF PAHs
65
FROM INJECTOR
1 / 4 INCHES OD x 1 / 1 6 REDUCING UNION
INCHES
TEFLON SLEEVE I 0.4.
mm
ID x 2
mm
OD
1 / 4 INCHES x 2 mm BRASS TUBING
TEFLON SLEEVE 0.4
mm
ID x 2
mm
OD ~
C18 PACKING MATERIAL
FRIT Fll-I'ING
OLUMN
Fig. I. Schematic diagram of the extractor.
that there was no loss o f P A H s d u r i n g the p r e c o n c e n t r a t i o n step, extractions o f the waste water sample were carried out without the preconcentration step. The levels o f P A H s detected were in agreement with the results o b t a i n e d when the preconcentrated step was included. The extraction o f the P A H s from the liquid samples using S F E was carried out using a similar p r o c e d u r e as described in o u r previous investigation (Lee et al., 1990). The samples to be extracted a n d analysed as well as the s t a n d a r d solutions were injected directly into the system using the Valco injection valve. The extractions were carried out isobarically at a pressure o f 11 M P a a n d an oven t e m p e r a t u r e o f 60~ The p r o c e d u r e for the conventional liquid-liquid extraction is described elsewhere (Fresenius et al., 1988). The
66
CHYE PENG O N G ET AL. TABLE I Structures and some physical constants for the four PAHs studied in this work. COMPOUND
ABBREVIATION
STRUCTURE
I)F l u o r a n t h e n e
Ft
~
2) Benzolblf Iuoranthene
.BIbIF
~(~T(~T
3) Benzolal-
BIaIP
RMM
BOILING PT/*C
202
375
T(~]
252
48 I
[('-~ ~("~)T(-') ~
252
456
Lk._)J~._)J.~_)j
276
pyrene
4) B e n z o l g h i l perylene
B[ghilP
not a v a i l a b l e
samples were first extracted twice with cyclohexane as extracting solvent. In each extraction, the mixture was mechanically shaken for half an hour. Subsequently the extract was subjected to sample clean-up using Bond Elut C]8 cartridges supplied by Analytichem (Harbor City, CA, U.S.A.). The extract obtained after clean-up was then analysed by capillary SFC using the same conditions as those employed for the supercritical fluid extraction, except that the extraction chamber was removed from the SFC system during these analyses. Calibration plots for the four PAHs were obtained using the latter set-up (i.e. without the extractor). Extraction efficiencies for the two methods were subsequently obtained by comparing their results with those given in the calibration plots. Results and Discussion
The use of on-line SFE-SFC has been successfully employed in many applications for solid matrices (McNally et al., 1988). By contrast, on-line SFE-SFC of aqueous samples has yet to receive as much attention. The reason could be largely due to the numerous problems associated with this type of extraction which render the use of the usual on-line
SUPERCRITICAL
FLUID-EXTRACTION
CHROMATOGRAPHY
OF PAHs
67
SFE-SFC impossible for these applications (Hendrick et al., 1989). In our work, an on-line SFE-SFC system has been successfully developed for the determination of selected PAHs found in aqueous samples. The system was built using a typical supercritical fluid chromatograph and a 'cartridge like' extractor. One of the advantages of this system is that the usual off-line or on-line monitoring system found in other SFE-SFC systems is not required. This simplifies both the instrumental set-up as well as the experimental procedure. Besides, the extraction and the analysis of extracts could be carried out by introducing the sample directly into the system via the sampling valve. Furthermore, in this work, sample clean-up was not required since the extractor also functions as a pre-column to retain undesirable compounds. With the current arrangement where the extractor is placed just before the analytical column, quantitative amounts of the extract are transfered without any lossess. Therefore such a system will allow for microscale type of extraction which is another attractive feature of the system. With the possibility of microscale type of extraction, peak broadening would be minimized (see Figure 2). Hence there is no necessity to include an additional sample valve to concentrate the extract prior to analysis. The results obtained for the extraction of PAHs from spiked aqueous samples using the on-line S F E - S F C as well as the liquid-liquid extraction procedure are shown in Table II and their respective chromatograms are illustrated in Figures 2 and 3. From the figures,
la
~MIIM Fig. 2. Typical c h r o m a t o g r a m of P A H extract from the spiked a q u e o u s sample using the microscale S F E - S F C system. C h r o m a t o g r a p h i c conditions: 11 M P a isobarically with an oven t e m p e r a t u r e of 60~ Peak identification: A = Ft, B ----B[b]F, C = B[a]P a n d D = B[ghi]P.
68
CHYE
PENG
ONG
ET AL.
TABLE II Comparison of results of extraction using on-line SFE-SFC and liquid-liquid extraction. Efficency (%)* Extraction method
Ft
B[b]F
B[a]P
B[gh0P
On-line SFE-SFC Liquid-liquid extraction
91 68
89 58
90 62
88 67
* Figures were obtained from extracting spiked distilled water samples. Blank runs were also carried out for the distilled water. l o n g e r r e t e n t i o n times were o b s e r v e d for the P A H s using the on-line S F E - S F C system ( F i g u r e 2) as c o m p a r e d to those o b t a i n e d by l i q u i d - l i q u i d extraction ( F i g u r e 3). This is e x p e c t e d as the retention times given in Figure 2 w o u l d also include the e x t r a c t i o n time
C
B
A
I
,_./L_ I~1
JI~IMIN
Fig. 3. Typical chromatogram of PAH extract from the spiked aqueous sample using liquid-liquid extraction. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft, B z B[b]F, C = B[a]P and D = B[ghlqP.
SUPERCRITICAL
FLUID-EXTRACTION
CHROMATOGRAPHY
OF PAHs
69
for the PAHs. In spite of their difference in retention times, the capacity factors were found to be very similar. An important point to note is that at least one hour was required for the extraction procedure alone in the liquid-liquid extraction method compared to the thirty-five minutes needed for the complete analysis (extraction and chromatography) using the SFE-SFC system. The calibration plots for the four PAHs are given in Figure 4 and a typical S F E - S F C chromatogram of the PAH standard mixture is shown in Figure 3. The extraction efficiencies obtained using the on-line SFE-SFC system were generally higher than the liquid-liquid extraction method. The additional sample clean-up step required in the latter method seems to be the main cause of the low extraction efficiency. Therefore, it is possible to implement a liquid-liquid extraction system to be coupled to the existng S F E - S F C system where the extractor would now serve as a pre-column for sample clean-up as well as its original function for extraction. Such a system would be useful for samples that cannot be analysed directly by the existing on-line SFE-SFC system due to reasons which prevent direct injection of the sample into the sampling valve (e.g. viscous samples). From a comparison of the total time required for each method, it is worthwhile noting that the on-line S F E - S F C system provides a simpler method and more rapid rate of extraction. Therefore it seems that the on-line SFE-SFC system would be an attractive alternative to the conventional liquid-liquid extraction for PAHs in liquid samples. In our investigation, environmental samples were analysed for the four PAHs.
~,.J
A Ft
A
/ . / j
9 B[a]F
.-//"/"
/j
..-" ..-'-
B[alP
A/-""
ID
X t~ O m ~e
IZ
.I t,a ~e
1 I
I
I
I
CONCENIR~TION
Fig. 4.
I
I
/PP~
Calibration plots for the four PAHs.
I
I
tI,BO~,
70
CHYE
PENG
ONG
ET AL.
A
d,
I
6w
Fig. 5. Typical chromatogram of PAH extract from environmental aqueous sample using the microscale SFE-SFC system. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft.
A m o n g s t these P A H s , only fluoranthene was detected in the waste water samples analysed. The results are shown in Table III a n d the c o r r e s p o n d i n g c h r o m a t o g r a m s are illustrated in Figures 5 a n d 6. F r o m these results, it was noted that the levels offluoranthene were fairly similar using the different types of extraction methods. The collective a m o u n t of P A H s present in these water samples was found to be below the guideline r e c o m m e n d e d by the W H O (i.e. 200 ng/1). However, due to the absence of the other two P A H indicators namely benzo[k]fluoranthene and indeno[ 1,2,3-cd]pyrene in this investigation, no conclusive statement can be made regarding the level o f P A H s in the aqueous samples. Nevertheless the microscale S F E - S F C for the analysis of P A H s in aqueous environmental sample has been successfully demonstrated. It is believed that there is great potential for the system to be e m p l o y e d for the rapid analysis of other environmental samples. The unique features o f the system include simplicity in the set-up, ease of operation, r a p i d rate o f extraction a n d high extraction efficiency. These advantages o f the system m a k e it an attractive alternative to conventional extraction a n d analytical procedures. TABLE III Amount of PAHs extracted from environment aqueous samples. Method of extraction
Concentration / ng/1)* Ft
On-line SFE-SFC Liquid-liquid extraction
8.7 8.3
* The other three PAHs were not detected.
SUPERCRITICAL F L U I D - E X T R A C T I O N C H R O M A T O G R A P H Y OF PAHs
71
I 6
~OMIN
Fig. 6. Typical chromatogram of PAH extract from environmental aqueous sample using liquid-liquid extraction. Chromatographic conditions: 11 MPa isobarically with an oven temperature of 60~ Peak identification: A = Ft.
Acknowledgement The authors wish to thank the National University of Singapore for financial support.
References Fresenius, W., Quentin, K. E., and Schnider, W.: 1988, Water Analysis: A practical Guide To Physico-Chemical, Chemical And Microbiological Water Examination And Quality Assurance, Springer-Verlag, Berlin & Heidelberg, FRG, p. 552. Hedrick, J., and T. Taylor, L.T.: 1986, 'Quantitative Supercritical Fluid Extraction - Supercritical Fluid Chromatography Of a Phosphonate From Aqueous Media', Anal. Chem. 61,989. Ibrahim, E. A., and Suffer, I. H.: 1988,'Freon FC- 113 as an alternative to methylene chloride for liquid-liquid extraction of trace organic from chlorinated drinking water', J. Chromatogr., 454, 217. Lee, H. K., Li, S. F. Y., and Ong, C. P.: 1990, 'Microscale Supercritical Fluid Extraction Directly Coupled With Supercritical Fluid Chromatography', Presented at Second International Symposium on High Performance Capillary Electrophoresis, San Francisco, California, U.S.A. pp. 29-31, January 1990. Mart, I. L., and Cresser, M. S.: 1983, Environmental ChemicalAnalysis, Blackie and Sons, Great Britain, p. 253. McNally, M.E.P., and Wheeler, J.R.: 1988, 'Increasing Extraction Efficiency In Supercritical Fluid Extraction From Complex Matrices', J. Chromatogr., 447, 53. World Health Organization: 1984, Guidelines For Drinking Water Quality, Macimillan, Belgium, Vol. 2 p. 182.
