Anal. Chem. 1995, 67, 2064-2069
Experimental Design Approach for the Extraction of Polycyclic Aromatic Hydrocarbons from Soil Using Supercritical Carbon Dioxide Ian J. Bamabas, John R. Dean,* William R. Tomlfnwm, and Susan P. Owed Depaltment of Chemical and Life Sciences, Ellison Building, University of Notthumbria at Newcastle, Newcastle-upon-Tyne, NE1 8ST U.K.
Polycyclic aromatic hydrocarbons (PAHs) have been extracted from contaminated land samples by supercritical fluid extraction (SFE) with both pure and modified carbon dioxide. An experimental design approach, based on a central composite design, was used to determine which SFX operating variables affect the total recovery of 16 PAHS. Four parametes were chosen for evaluation: pressure, temperature, extradon time, and percent methanol modifier addition. Accessible levels of each parameter were dependent on instrumental constraints. A statistical treatment ofthe results indicated that extraction lime and percent modifier addition were the only variables to siguificantly affect PAH recovery. The levels of these variables were then set at their maximum values, while the pressure and temperature were maintained at their midpoint values in the design. These conditions were used in a repeatability study (n = 7), which extracted an average of 458.0 mg kgltotal PAHs from the contaminated land sample with an RSD of 3.1%. Sequential extractions on three of these samples, using identical operating conditions, did not show the presence of any PAHs. The result was compared with Soxhlet &action and microwave-assisted-action of the sample, which recovered an average of 297.4 (RSD 10.0%)and 422.9 mg kg-l (RSD 2.4%), respectively. Extraction of an interlaboratory test soil yielded high RSD values because of the presence of elemental sulfur. Supercritical fluid extraction (SFE)has increasingly been used to extract low-polarity pollutants from environmental samples because of both its potential to reduce the time required for extraction and the absence of large amounts of organic solvents.112 Supercritical COZas an extracting fluid has several advantageous features: low cost, low toxicity, and low critical parameters. It has the ability to dissolve a variety of nonpolar and moderately polar substances. These characteristics have meant that SFE using COZhas been extensively used to extract nonpolar analytes and thus has prompted much research into the extraction of polycyclic aromatic hydrocarbons (PAHs) by supercritical C02.3 ’Analyticaland Environmental Services, Northumberland Dock Road, Wallsend, Tyne and Wear, NE28 OQD U.K. (1) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59, 1705. (2) Lopez-Avila, Dodhiwala, N. S.; Beckert, W. F. J. Chromatogr. Sci. 1990,28, 468. (3) Barnabas, I. J.; Dean, J. R; Owen, S. P.Analyst 1994,119. 2381. 2064 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
Although PAHs are sufficiently soluble in supercritical COZ, their efficient recovery from environmental solids is greatly dependent on the nature of the matrix. Often, organic modiiiers are required in conjunction with C024,5to effectively remove the PAHs, which can vary substantially in their molecular weights. Other supercritical fluids have been used to aid PAH extraction, including N20, CHClF2,6 and water,’ but they are often more dficult to use than conventional Con, having additional safety requirements. High extraction temperatures have also been used to increase the efficiency of PAH extraction from environmental solids.8 These factors add to the complex nature of SFE,even for low-polarity analytes, and make the elucidation of the most efficient extraction parameters a difficult task. The variables which affect the efficiency of SFE are still poorly understood and perhaps can be investigated best by using an experimental design approach?JO This allows several variables to be studied simultaneously and uses subsequent statistical analysis to validate the procedure and results. This approach also allows a reduction in the number of experiments required to fully understand a system and to ascertain the existence of any interactions between variables. This, in turn, can greatly simplify the optimization procedure. The resultant procedure requires far fewer experiments to be carried out which are now capable of giving more information than the conventional “alter one factor at a time” evaluation. This paper discusses the extraction of native PAHs from samples of contaminated soil, which can represent one of the most difficult environmental matrices from which to extract, because of the often diverse and heterogeneous nature of the sample. The use of a “real” contaminated soil sample is favored over normal spiking methods to assess the efficiency of the extraction procedure, as recoveries obtained from spiked samples do not effectively represent the effects of the matrix.ll A central (4) Hill, J. W.; Hill, H. H. J. Chromatogr. Sci. 1993,31, 6. (5) Dankers, J.; Groenenboom, M.; Scholtis, L. H. A; van der Heiden, C. J. Chromatogr. 1993,641,357. (6) Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992,64, 1614. (7)Hawthorne, S . B.; Yang, Y.; Miller, D. J. Anal. Chem. 1994,66, 2912. (8) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993,65, 338. (9) Brereton, R G. Chemometrics: Applications of Mathematics and Statistics to Laboratoy Systems; Ellis Horwood: Sussex, England, 1990; Chapter 2. (10) Kane, M.; Dean, J. R; Hitchen, S. M.; Dowle, C. J.; Tranter, R L. Anal. Chim. Acta 1993,271, 83. (11) Burford, M. D.;Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1993,65, 1497. 0003-2700/95/0367-2064$9.00/0 0 1995 American Chemical Society
composite design12J3has been used to elucidate the optimum operating conditions for the removal of the maximum amount of PAHs from the contaminated soil sample. The combination of statistically designed experimentswith the versatilityof SFE allows a signiscant reduction in extraction time and method development
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Samples. All of the contaminated land samples used are native and contain relatively high levels of total PAHs (several hundred mg kg-l). The preparation of the bulk sample used throughout the central composite design is described in detail elsewhere14and consists of a drying stage followed by removal of stones and other large aggregates and blending of the sample in a commercial blender. The CONTEST soil sample used was supplied by the Laboratory of the Government Chemist (LGC), Department of Environment,U.K., and is part of an interlaboratory study. This soil was used as received without any pretreatment Supercritical Fluid Extraction. All of the extractions were performed on a Jasco SFE (Mettler-ToledoLtd., Halstead, Essex) instrument which possesses a second pump for modifier addition. The back-pressure regulator with the system allows independent control of both the flow rate and the extraction pressure (details described elsewhere15). SFC grade C02 (299.995%purity) was purchased from Air Products Ltd. (Sunderland, U.K.) and combined with methanol modifier by mixing in a pressure relief valve. The C02 flow rate was kept constant at 1 mL min-l throughout the experimentation. The sample (1 f 0.01 g) was accurately weighed into 4.6 mm i.d. stainless steel extraction cells having a volume of 2.5 mL (Phase Separations Ltd., Clwyd, U.K.) and placed inside the extraction oven, where the cells were allowed to equilibriate at the set temperature before sample extraction. Once the cell had reached the set extraction temperature and pressure, the pumps were switched off, and each sample was extracted for 5 min in the static mode, after which the pumps were switched on and the dynamic extraction time period was begun. All extracts were collected by inserting the end of the back-pressure regulator through a PTFE-lined septum into a vial containing 5 mL of methylene chloride. The collection vial was placed in an icewater batch to aid trapping. In addition, the escaping C02 was vented via a hypodermic needle into a c18 solid-phase extraction (SPE) cartridge (Bond-Elut, Phase Separations Ltd.), which prevented any of the PAHs from escaping in the aerosol formed by the C02 (Figure 1). To obtain quantitative recoveries, the SPE cartridge was back-flushedinto the collectionvial using methanol. The combined extract was made up to a final volume of 25 mL in methylene chloride to await analysis.
I>,::::: w :....:.::.o ; F) (at the 95% significance level) and represents the effect of adding the linear, quadratic, and cubic terms sequentially to the intercept and block terms.20 The summary statistics indicate that the quadratic model should be further investigated since the quadratic terms are significant ( F value in the final column of the table again indicates any significant (95%) lack offit of each model. In this case, none of the models show any significant lack of fit at the 95% confidence level to the experimental data points, but the quadratic model exhibits the least significant lack of fit. The quadratic model was then fully evaluated using ANOVA to determine which variables, if any, had a significant effect on PAH recovery. This was achieved by dividing each coefficient by its associated standard error to obtain a Student's value t, which tests whether the coefficient is different from zero, the null hypothesis. The associated p values (prob > It/) are interpreted as the probability of getting a coefficient as large as that observed when the true coefficient equals zero. Therefore, small values of p (
