Anal. Chem. 2005, 77, 2544-2549
Cloud Point Extraction Coupled with Microwave or Ultrasonic Assisted Back Extraction as a Preconcentration Step Prior to Gas Chromatography Theodosios I. Sikalos and Evangelos K. Paleologos*
Department of Chemistry, Laboratory of Food Analysis, University of Ioannina, 45110, Ioannina, Greece
Cloud point extraction of nonionic and anionic surfactants was applied as a preconcentration step prior to gas chromatography. No cleanup step preceded chromatographic analysis. The obtained surfactant-rich phase was treated with water-immiscible solvents, and the target analytes were back extracted by short-term microwave application or ultrasonication. A mixture of six PAHs (naphthalene, acenaphthene, fluorene, anthracene, fluoranthene, pyrene) was used as test compounds. The obtained detection limits were in the microgram per liter area. Recoveries of spiked water and soil samples ranged between 92 and 105% while analysis of certified reference materials gave results in good agreement with the certified values. Under the optimum experimental conditions, there was no interference or blocking of the column. According to our results, this approach presents a convenient solution to the up-to-date problem of combining gas chromatography with micellar cloud point extraction. Application of the cloud point phase separation behavior of surfactants in aqueous media for the analytical determination of trace organic analytes has aroused growing attention in recent years.1-3 Cloud point phase separation is the phenomenon in which aqueous solutions of several surfactants undergo phase separation upon certain conditions such as temperature manipulation4-6 and addition of salts or acids.7,8 The result is the formation of two distinct phases: a surfactant-rich phase and an aqueous phase with concentration of surfactant close to the critical micellar * Corresponding author. Tel.: +3026510-98720. Fax: +3026510-98795. E-mail:
[email protected]. (1) Gullickson, N. D.; Scamehom, J. F.; Harwell, J. H. In Surfactant-Based Separation Processes; Schamehom, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989; pp 139-151. (2) Hinze, W. L.; Pramauro, E. CRC Crit. Rev. Anal. Chem. 1993, 24, 133177. (3) Carabias-Martinez, R.; Rodriguez-Gonzalo, E.; Moreno-Cordero, B.; Perez Pavon, J. L.; Garcia-Pinto, C.; Fernandez Laespada, E. J. Chromatogr., A 2000, 902, 251-265 (and references within) (4) Watanabe, H. In Solution Behaviour of Surfactants; Mittal, K. L., Fendler, E. F., Eds.; Plenum Press: New York, 1982; Vol. 2, p 1305. (5) Corti, M.; Minero, C.; De Giorgio, V. J. Phys. Chem. 1984, 88, 309-317. (6) Blankschtein, D.; Thurston, G. M.; Benedek, G. B. J. Chem. Phys. 1986, 85, 7268-7288. (7) Schott, H. J. Colloid Interface Sci. 1997, 192, 458-462. (8) Casero, I.; Sicilia, D.; Rubio, S.; Perez-Bendito, D. Anal. Chem. 1999, 71, 4519-4526.
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concentration.1,4 It has been demonstrated that the surfactant-rich phase, thus separated under the cloud point conditions, is able to extract and preconcentrate a wide range of organic compounds from the aqueous phase. Up to now, nonionic surfactants, such as Triton X series,9-12 PONPE series,4,8,13 and Brij series,15-17 are the most widely used amphiphiles for cloud point extraction (CPE). These surfactants have been successfully applied to extract polycyclic aromatic hydrocarbons (PAHs), polychlorinated compounds such as polychlorinated biphenyls, polychlorinated dibenzofurans, and dibenzo-p-dioxins, synthetic pesticides, hydroxyaromatic compounds, vitamins, hydrophobic membrane proteins, and pharmaceuticals from natural waters, soils, and sediments, as well as complex biological fluids. Recently, the cloud point extraction methodology has been extended to utilizing zwitterionic surfactants13,18 and anionic surfactants.8 The use of preconcentration schemes based on surfactantmediated phase separation, offers an interesting alternative to the conventional extraction systems. The small volume of the surfactant-rich phase stemming from this methodology and its compatibility with hydroorganic mobile phases have been exploited in the past few years for the extraction and preconcentration of organic compounds prior to high-performance liquid chromatography (HPLC)3, flow injection analysis,19 and capillary electrophoresis.3,20 In each case, a lot of experimental work was devoted to resolving problems emerging from the viscosity of the micellar phase, its adsorption on the analytical columns, and the broad overlapping peaks when UV detection is employed. (9) Sirimanne, S. R.; Barr, J. R.; Patterson, D. G.; Ma, L. Anal. Chem. 1996, 68, 1556-1560. (10) Fang, Q.; Yeung, H. W.; Leung, H. W.; Huie, C. W. J. Chromatogr., A 2000, 904, 47-55. (11) Garcia-Pinto, C.; Perez-Pavon, J. L.; Moreno-Cordero, B. Anal. Chem. 1995, 67, 2606-2612. (12) Wu, Y. C.; Huang, S. D. Anal. Chim. Acta 1998, 373, 197-206. (13) Saitoh, T.; Hinze, W. L. Anal. Chem. 1991, 63, 2520-2525. (14) Silva, M. F.; Fernandez, L. P.; Olsina, R. A. Analyst 1998, 123, 1803-1807. (15) Eiguren-Fernandez, A.; Sosa-Ferrera, Z.; Santana-Rodriguez, J. J. Anal. Chim. Acta 1998, 358, 145-155. (16) Sirimanne, S. R.; Patterson, D. G.; Ma, L.; Justice, J. B. J. Chromatogr., B 1998, 716, 129-137. (17) Eiguren-Fernandez, A.; Sosa-Ferrera, Z.; Santana-Rodriguez, J. J. Analyst 1999, 124, 487-494. (18) Saitoh, T.; Hinze, W. L. Talanta 1995, 42, 119-127. (19) Burguera, J. L.; Burguera, M. Talanta 2004, 64, 1099-1108 (and references within). (20) Carabias-Martinez, R.; Rodriguez-Gonzalo, E.; Dominguez-Alvarez, J. and Hernandez-Mendez, J. Anal. Chem. 1999, 71, 2468-2474. 10.1021/ac048267u CCC: $30.25
© 2005 American Chemical Society Published on Web 03/03/2005
On the other hand, the application of CPE as a preconcentration step prior to gas chromatography has found very little application due to the viscous nature of the surfactant, which endangered blocking of the capillary column. Even an extensive contemporary review by Carabias-Martinez et al.3 admits to this fact citing as an only work that of Froschl et al.21 However, in this work injection of the micellar extract into the gas chromatograph is made possible after extensive cleanup with two columns (silica and Florisil) in order for the surfactant (Triton X-100) to be completely removed. Analogous cleanup preceded GC analysis of phenothiazine tranquilizers after CPE recently reported by Ohashi et al.22 and of PAHs by Singh et al.23 In this work, we applied the conventional cloud point technique for nonionic surfactants and the acid-induced phase separation of anionic surfactants to preconcentrate a series of PAHsswhich are proven to produce almost quantitative recoveries with surfactant CPE systems3,8,23sfrom aqueous and soil samples. Those analytes were back-extracted from the surfactant-rich phase into a waterimmiscible solvent (isooctane), by applying microwaves or sonication and directly analyzed with GC-flame ionization detection (FID) without the need for any supplemental cleanup. Surprisingly, no interference from the surfactant appeared in the chromatograms, especially for anionic surfactants, while the column properties remained intact, allowing for the reproducible determination of all PAHs at concentrations of the microgram per liter level. EXPERIMENTAL SECTION Apparatus. For qualitative and quantitative analysis of the selected analytes, a gas chromatograph HP 5890 series (HewlettPackard, Waldronn, Germany) with a FID was used, equipped with a 30 m × 0.25 mm fused-silica capillary column (HP-5, J. & W. Scientific, Folsom, CA). The carrier gas was helium at 75 kPa. The determination was carried at split (10:1) mode. A Shimadzu CTO-10A oven was used for temperature control of the water bath in which the separator vessels were placed. A Hettich, Universal centrifuge was used for separation of the surfactant-rich phase. Reagents. Naphthalene, acenaphthene, fluorene, anthracene, fluoranthene, and pyrene and the internal standard (octadecane) were of analytical grade purchased from Sigma-Aldrich (Athens, Greece). Appropriate amounts were diluted with methanol to prepare 100 mg/L stock solutions, which were further diluted with doubly distilled water to prepare working solutions (10-1000 µg/ L). Thus, the methanol content, which usually hampers clouding, was less than 1%. All solvents used (isooctane, hexane, chloroform) were HPLC grade. Triton X-114 (Aldrich, Catalog No. 36,934-9) was used, without further purification, to prepare a 100 g/L aqueous solution. Sodium dodecanesulfonic acid (SDSA) from Fluka was used without further purification to prepare a 1 g/L standard solution. (21) Froschl, B.; Stangl, G.; Niessner, R. Fresenius J. Anal. Chem. 1997, 357, 743-746 (22) Ohashi, A.; Ogiwara, M.; Ikeda, R.; Okada, H.; Ohashi, K. Anal. Sci. 2004, 20, 1353-1357. (23) Singh, H. N.; Fu, Z. S.; Williams, R. W.; Kippenberger, D. K.; Morris, M. D.; Sadek, F. S. In Chemical Analysis of Polycyclic Aromatic Hydrocarbons; Vo-Dinh, T., Ed.; Willey and Sons: New York, 1989; Chapter 5, pp 151169.
Procedures. Cloud Point Extraction from Aqueous Samples. In a typical cloud point experiment of nonionic surfactants, 10 mL of the standard or sample solution was placed in a Hach centrifugal vial. An 80-µL sample of Triton X-114 stock solution, 100 µL of 1 M phosphate buffer pH 7.0, and 100 µL of saturated NaCl solution were consequently added, and the mixture was left to stand for 10 min in the water bath at 50 °C. Alternatively, for anionic surfactants, in 5 mL of sample were added 3 mL of concentrated hydrochloric acid and 2 mL of a 1 g/L SDSA solution, and the mixture was left to stand for 1 h in the water bath at 50 °C. Centrifugation for 10 min at 3500 rpm was adopted for separating the surfactant-rich phase. The vials were then placed in an ice bath for 5 min to increase viscosity of the micellar phase, and the aqueous supernatant was decanted by inverting the tube. In the case of anionic surfactant CPE, the surfactant-rich phase (∼50 µL) accumulated at the surface of the aqueous phase and it was carefully collected with the aid of a micropipet (Gilson, Minipuls, 10-100 µL) along with some remaining water to ensure complete collection of the surfactant-rich phase. A 150-µL aliquot of isooctane containing 0.1 mg/L of the internal standard (octadecane) was added to the surfactant-rich phase (150-200 µL), and the preconcentrated analytes were extracted by applying microwaves (700 W) for 2 min or by sonicating (400 W) for 5 min. Two distinct layers were formed: the surfactant-rich phase (lower) and the isooctane phase (upper). A 1-µL sample of this supernatant isooctane phase was injected into the chromatograph. Cloud Point Extraction from Soil Samples. A PAH-free soil sample spiked with 0.05, 0.1, and 0.2 µg/g of each PAH was used to test the applicability of the method to soil samples. The anionic surfactant CPE approach was used for this evaluation. A 0.5 ( 0.1 g sample was placed in a Hach centrifugal vial followed by the addition of 5 mL of doubly distilled water, 3 mL of concentrated hydrochloric acid, and 2 mL of a 1 g/L SDSA solution. The suspension was left to stand for 1 h in the water bath at 50 °C and was centrifuged for 15 min a 4000 rpm. The surfactant-rich phase was withdrawn with the aid of a micropipet and placed in a screw-capped tube along with 150 µL of isooctane. The mixture was subjected to microwaves for 2 min, and 1 µL of the supernatant was injected into the GC. When the nonionic surfactant CPE is applied to solid samples (e.g., soil, ash, etc.), 10 mL of the CPE solution is added and the mixture is left to stand for 6 h under agitation and 1 h under sonication for the target PAH to be extracted into the aqueous phase. The formed slurry is subsequently filtered, and the aqueous remaining solution is subjected to the same procedure described previously for nonionic surfactants. Gas Chromatographic Conditions. The GC analysis of the selected compounds was carried out according to the following conditions: The temperature of the injector was maintained at 100 °C. The column temperature remained at 100 °C for 4 min and then was raised to 280 °C at 10 °C/min. There it remained for 3 min. Finally, it was raised to 340 °C and remained for 5 min for column cleanup. RESULTS AND DISCUSSION Preliminary Tests. To verify the theory that back-extraction of a hydrophobic analyte into a water-immiscible solvent is a sufficient precaution for the elimination of the interference from Analytical Chemistry, Vol. 77, No. 8, April 15, 2005
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Figure 1. Chromatogram of (a) a blank and (b) a solution spiked with 0.1 mg/L naphthalene, acenaphthene, fluorene, anthracene, fluoranthene, and pyrene after nonionic surfactant-mediated CPE (1 g/L Triton X-114) and microwave-accelerated back-extraction into isooctane (200 µL). Other conditions as described in the text.
the surfactant, a preliminary experiment was set. The cloud point procedure was applied to a blank solution containing 1 g/L Triton X-114 (100 µL from 100 g/L stock solution in 10 mL of distilled water). The surfactant-rich phase was treated with 200 µL of isooctane, and 1 µL of the supernatant isooctane phase was injected in the GC. As can be seen from Figure 1a, there are four major peaks while the rest of the chromatogram is relatively clear, presenting a smooth baseline. GC/MS analysis of the isooctane phase revealed that these peaks correspond to octylphenol and octylphenyl ether fragments, which are typical of Triton X-114. In the case of anionic surfactants, some scattered peaks of small intensity corresponding to dodecanesulfonic acid fragments appeared. Those peaks were further diminished when the micellar extracted was mixed with a 6 M NaOH solution (100 µL) added prior to the addition of isooctane. Alkaline environment serves to neutralize excess HCl and allow dodecanesulfonic acid to be deprotonated and transformed into its anionic form (sodium salt), which is expected to be less extractable in the isooctane phase. The aforementioned cloud point procedure was consequently applied to a solution spiked with 0.1 mg/L of each PAH. It is obvious from Figure 1b that their peaks appear free from any interference by the presence of the surfactant, while an injection of a standard solution of PAH in isooctane (10 mg/L) showed that there is no differentiation in the obtained retention times. Repeated injections of the blank and spiked extract also gave no significant fluctuations of the retention times. 2546
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Following these observations, an optimization experiment was designed in order to demonstrate the effective determination of PAH in liquid and solid matrixes after cloud point extraction and microwave-accelerated back-extraction. Optimization Experiments. The parameters prone to optimization in a typical nonionic surfactant-based cloud point experiment are sample pH, added surfactant volume, heating temperature, and duration. With regard to the anionic surfactant CPE, no optimization was performed and the optimum experimental conditions were adopted from the paper by Casero et al.8 On the other hand, a substantial amount of attention has been paid to the microwave- or sonication-accelerated back-extraction parameters such as type and volume of organic solvent and duration for the application of microwaves or ultrasounds. (i) Effect of pH on Nonionic CPE. The extraction efficiency is almost impervious to the prevailing pH conditions for a pH range of 2-11 as was expected due to the nature of the target analytes. To ensure uniform conditions, the pH was adjusted to 7 with a phosphate buffer and 0.1 mL of it was added to all standard and sample solutions. (ii) Effect of Surfactant Concentration. The optimization of surfactant volume is an important parameter in the present work because its amount should be sufficient for the quantitative extraction of the target analytes but not excessive in order not to interfere with the back-extraction process. As can be seen from Figure 2 a volume of 50-150 µL of the 100 g/L stock solution, corresponding to 5-15 mg of surfactant in 10 mL of sample,
Table 1. Relative Intensities of 0.1 mg/L Naphthalene after Cloud Point Extraction into Triton X-114 or SDSA Micelles and Back-Extraction into Isooctane, Hexane, and Chloroform extraction technique
Figure 2. Effect of surfactant concentration on the cloud point extraction and microwave back-extraction of 0.1 mg/L naphthalene, acenaphthene, fluorene, anthracene, fluoranthene, and pyrene. (pH 7, isooctane volume 150 µL). Other conditions as described in the text.
produced optimum results when 150 µL of isooctane is used for back-extraction. Smaller amounts lead to a reduction of the analytical response due to incomplete partitioning of the analytes in the surfactant micelles, while larger amounts of surfactant lead to a incomplete separation of the surfactant and the isooctane layer (formation of slurry). A surfactant volume of 80 µL (8 mg, 0.8 g/L) was finally selected with a view of further optimizing the volume of the organic solvent used for back-extraction. In all cases (nonionic and anionic CPE), the volume of the surfactant-rich phase along with remaining water ranged between 130 and 160 µL. (iii) Effect of Incubation Temperature and Duration. In concurrence with the literature, the incubation temperature and duration as well as the centrifugation parameters posed no significant alteration in the obtained analytical signals for all PAHs. Therefore, an incubation temperature of 50 °C, applied for 10 min, was adequate for micelle formation and effective CPE, while centrifugation for 5 min at 4000 rpm ensured phase separation. Increased temperature was used not only to accelerate phase formation in anionic CPE but to eliminate any drawbacks deriving form the presence of methanol especially at high PAH concentrations. (iv) Effect of Organic Solvent. Three water-immiscible solvents (hexane, isooctane, chloroform) were applied in order to evaluate their efficiency for extracting 0.1 mg/L of the target analytes from 10 mL of aqueous solution. As can be seen from Table 1, they all perform adequately for both microwave- and ultrasonicassisted back-extraction from nonionic- or anionic-derived surfactant-rich phase. Isooctane was finally selected because hexane and chloroform have poor reproducibility, especially when the microwave approach is applied, due to their increased volatility. The volume of isooctane was finally optimized with a view to recover the target analyte from the surfactant-rich phase, yielding a high preconcentration factor. As can be seen from Figure 3, the optimum results are produced when 100 or 150 µL of isooctane is used. Larger volumes result in a gradual decrease of the analytical response due to subsequent dilution (reduction of the theoretical preconcentration factor) while smaller amounts produce slurries due perhaps to the formation of partially miscible ternary mixtures among water, surfactant, and organic solvent.
nonassisted microwave 1 min 2 min 5 min ultrasonication 1 min 2 min 5 min 10 min a
isooctane
relative intensitya hexane chloroform
115
115
115
595 600 602
605 607 603
590 593 599
264 345 598 601
270 340 595 600
260 339 599 599
Naphthalene/octadecane (internal standard) peak area 10-3.
Figure 3. Effect of isooctane volume on the microwave accelerated back-extraction of 0.1 mg/L Naphthalene, Acenaphthene, Fluorene, Anthracene, Fluoranthene and Pyrene after cloud point extraction. (pH)7, Triton X-114)0.8 g/L or SDSA 0.2 g/L). Other conditions as described in the text.
For this reason, a volume of 150 µL was finally selected as optimum because although 100 µL seems to yield better signals it often gave hazy solutions and slurries, thus resulting in poor reproducibility. (v) Effect of Microwave and Ultrasonication Parameters. Perhaps the most important task of this study was to evaluate the effect of the application of microwaves or ultrasounds to the quantitative back-extraction of the preconcentrated analytes from the surfactant-rich phase into the organic solvent. For that reason, the micellar extract along with isooctane (150 µL) was treated with microwaves in a microwave oven. It was found that when the power of microwaves reached 700 W the preconcentrated PAHs were quantitatively extracted into isooctane within 1 min, producing an analytical signal 5 times greater than the nonmicrowave-treated extracts. Further application of microwaves up to 4 min gave no significant difference in the analytical response. Therefore, a 2-min application was finally selected. In the case of ultrasonication, which power was fixed at 400 W, the increase in duration produced a gradual increase from the nontreated extract showing a maximum for 5 min of ultarsonication and a respective 5-fold increase. Analogous results were obtained for nonionic and anionic surfactant-rich phases, proving the applicability of the method for Analytical Chemistry, Vol. 77, No. 8, April 15, 2005
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Table 2. Figures of Merit of the Method for Both Nonionic and Ionic Extraction Approach
pah
retention time
regression equation
correl coeff (r2)
linear range (µg/L)
detctn limita (µg/L)
quant limitb (µg/L)
enhancement factorc
naphthalene acenaphthene fluorene anthracene fluoranthene pyrene
4.35 ( 0.06 10.62 ( 0.08 12.73 ( 0.08 15.84 ( 0.07 19.67 ( 0.10 20.32 ( 0.10
Ed ) (5.8(0.5) × 10-4 C(µg/L) + (3.5(0.3) × 10-3 Ed ) (5.2(0.4) × 10-4 C(µg/L) + (4.2(0.3) × 10-3 Ed ) (3(0.3) × 10-3 C(µg/L) + (0.5(0.1) × 10-3 Ed ) (1.8(0.2) × 10-3 C(µg/L) + (1.2(0.1) × 10-3 Ed ) (2.3(0.3) × 10-3 C(µg/L) + (1.5(0.2) × 10-3 Ed ) (2.2(0.3) × 10-3 C(µg/L) + (2.3(0.2) × 10-3
0.9985 0.9988 0.9990 0.9982 0.9986 0.9991
25-1000 25-1000 2.5-500 5-1000 2.5-500 2.5-500
9.3 9.9 0.9 1.6 1.0 1.1
27.8 29.7 2.7 4.9 2.9 3.3
70.1 69.8 67.5 65.8 70.5 71.2
a Defined as three times the signal-to-noise ratio. b Defined as 10 times the signal-to-noise ratio. c Defined as the ratio of method’s detection limit to that of nonextractive. d Relative peak area (analyte/internal standard).
Table 3. Recoveries of the Selected PAHs (a) Spiked River Water Sample after Nonionic CPE and Back-Extraction into Isooctane amount founda/recovery, % naphthalene acenaphthene fluorene anthracene fluoranthene
amt spiked (µg/L) 50 100 200
52.1 99.4 190.8
a
50.2 99.8 194.6
100.4 99.8 97.3
53.7 100.3 198.8
107.4 100.3 99.4
50.3 100.4 200.2
100.6 100.4 100.1
49.6 95.3 184.8
99.2 95.3 92.4
pyrene 49.8 100.4 191.6
(b) Spiked Soil Sample after Acid-Induced Anionic CPE and Back-Extraction into Isooctane amount founda/recovery, % naphthalene acenaphthene fluorene anthracene fluoranthene
amt spiked (ng/g) 50 100 200
104.2 99.4 95.4
47.2 98.4 193.4
94.4 98.4 96.7
46.8 99.8 204.4
93.6 99.8 102.2
48.4 100.3 199.6
96.8 100.3 99.8
47.9 98.5 199.8
95.8 98.5 99.9
49.1 97.5 198.4
98.2 97.4 99.2
99.6 100.4 95.8
pyrene 52.1 102.1 199.2
104.2 102.1 99.6
Average of five determinations.
Table 4. Analysis of Certified Reference Materials for PAH after Cloud Point Extraction into Triton X-114 or SDSA Micelles and Back-Extraction into Isooctane NIST 1647c PAH
certified amt (µg/L)
naphthalene acenaphthene fluorene anthracene fluoranthene pyrene
199.6 205.5 47.5 7.9 76.5 85.2
nonionic CPE amta
found (µg/L)
194.4 ( 6.8 202.4 ( 7.1 48.6 ( 1.2 6.8 ( 0.3 78.9 ( 1.8 84.8 ( 2.1
ERM-CC013
naphthalene acenaphthene fluorene anthracene fluoranthene pyrene a
anionic CPE % av recovery
found amt (µg/L)
% av recovery
97.4 98.5 102.3 86.1 103.1 99.5
195.2 ( 5.8 203.4 ( 5.5 48.2 ( 1.1 6.9 ( 0.2 77.2 ( 1.2 85.0 ( 1.4
97.8 99.0 101.5 87.2 100.9 99.8
nonionic CPE
anionic CPE
certified amt (µg/L)
found amta (µg/L)
% av recovery
found amt (µg/L)
% av recovery
2.80 1.59 2.57 3.70 15.9 12.8
2.67 ( 0.1 1.51 ( 0.1 2.41 ( 0.1 3.29 ( 0.1 15.87 ( 0.8 12.61 ( 0.7
95.2 94.9 93.8 88.9 99.8 98.5
2.72 ( 0.08 1.53 ( 0.08 2.45 ( 0.06 3.49 ( 0.07 15.9 ( 0.5 12.8 ( 0.5
97.3 96.2 95.4 94.2 100.0 100.0
Average of five runs.
both surfactant-mediated approaches. Therefore, so long as the extraction of PAH is quantitative, it is the back-extraction approach that regulates enhancement factors regardless of the nature of applied surfactant. This is further proved by the agreement in the results obtained for both CPE approaches. Analytical Performance of the Method. Under the selected optimum experimental conditions for cloud point extraction and 2548
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subsequent back-extraction of the selected PAHs, the methodology was applied to determine the deducted analytical advantages. Table 2 shows the figures of merit obtained by applying the proposed methodology for the determination of the selected PAHs starting from a 10-mL standard solution spiked with varying concentrations. It is obvious that all compounds can be determined satisfactorily, yielding detection and quantification limits of the
microgram per liter level, yielding enhancement factors of over 60 compared to nonextractive GC analysis. To verify the applicability of the method to real samples, we applied the method for nonionic surfactants to a river water sample and for anionic surfactants to a soil sample. Both samples were collected from virgin unpolluted territories in northwestern Greece (Epirus, Zagori); they were previously analyzed by purge-and-trap GC/MS and found free of PAHs (PAH level below 10 ng/L). Table 3 presents the recoveries obtained for performing the proposed methodology to the river water sample (nonionic CPE) and the soil sample (acid-induced anionic CPE) spiked with 0.05, 0.1, and 0.2 mg/L and 0.05, 0.1, and 0.2 µg/g of each PAH, respectively. As can be seen, the recoveries range from 92 to 105% even for the soil samples. To further fortify our results, the proposed experimental procedure was applied for the analysis of PAH in certified reference materials. NIST 1647c certified for the 16 EPA priority pollutant PAHs, in acetonitrile, was 100-fold diluted with the river water sample and extracted with both nonionic and anionic CPE procedures. Moreover, the ERM-CC013 reference soil, was also
used to assess the method’s applicability. Appropriate amounts of soil were subjected to both CPE approaches and GC analyzed after microwave back-extraction into isooctane. As can be seen from Table 4 the results obtained are satisfactory for all selected PAHs. CONCLUSIONS The application of microwaves or ultrasounds to assist the back-extraction of PAHs (as test compounds) from micellar extracts into water-immiscible solvents was proved as an efficient precaution step for the combination of cloud point extraction with gas chromatography. This notion presents an opportunity for further development in this so far neglected area of investigation and may even enable the injection of micellar extracts into mass selective detectors.
Received for review November 23, 2004. Accepted January 30, 2005. AC048267U
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