Anal. Chem. 2002, 74, 74-79
Extraction Solvent Selection in Environmental Analysis Lisa J. Fitzpatrick and John R. Dean*
School of Applied and Molecular Sciences, University of Northumbria at Newcastle, Ellison Building, Newcastle upon Tyne, NE1 8ST, U.K.
A method for the prediction of a suitable solvent for the extraction of pesticides is outlined. The procedure is based on the Hildebrand solubility parameter, δt. The solubility parameter is broken down into three individual components, which are calculated by the addition of group contributions. To demonstrate the applicability of the approach pressurized fluid extraction was used to extract 4,4′-DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] and its metabolites, 4,4′-DDD [1,1-dichloro-2,2-bis(pchlorophenyl)ethane] and 4,4′-DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene], from an historically contaminated soil from the United States and pentachlorophenol from a certified reference material (CRM524) using various solvents. Visual representation of the individual parameters predicted the ideal extraction solvent to be DCM for 4,4′-DDT and its metabolites and a mixture of acetonitrile and dichloromethane (1:1, v/v) for PCP. These findings were confirmed by the experimental results. The extraction of organic pollutants from solid environmental matrixes is important because of the potential risks to health that may result. For example, the use of the pesticide DDT has been banned in the developed world for at least 25 years, but it is still widely used in developing countries, as it is very effective.1 However, DDT is highly persistent with a half-life in soil of 150 years. Phenolic compounds are widely used in the chemical industry for the manufacture of polymers, textiles, resins, and dyes, petroleum refining, pulp processing, and coal coking and as pesticides and herbicides. For example, pentachlorophenol can be used as an insecticide to control termites.1 To determine the level of contamination of industrial land sites requires, after appropriate sampling, extraction of the pollutants from the soil. A variety of techniques are available for extraction of organic pollutants from solid environmental matrixes.2 Techniques available range from the traditional (e.g., Soxhlet extraction, shake flask, and sonication) to instrumental extraction techniques (e.g., supercritical fluid extraction, microwave-assisted extraction, and pressurized fluid extraction). However, irrespective of the sophistication of the technique, each approach has a common feature, i.e., choice of solvent. The choice of solvent is largely dependent (1) Tomlin, C., Ed. The Pesticide Manual, 10th ed; The Royal Society of Chemistry: Cambridge. U.K., 1994. (2) Dean, J. R. Extraction methods for environmental analysis; John Wiley & Sons, Ltd.: Chichester, U.K., 1998.
74 Analytical Chemistry, Vol. 74, No. 1, January 1, 2002
upon past experience, manufacturers’ guidelines, or recommended standard methods, such as those compiled and promulgated by the U.S. Environmental Protection Agency (U.S. EPA) or the American Society for Testing and Materials (ASTM). Typically, as is the case with Soxhlet extraction, a large volume of organic solvent is required. Often the solvents recommended are chlorinated as in the use of dichloromethane. Prediction of the optimum solvent would therefore be advantageous. There are a few approaches used to try and predict the best solvent for chromatography. Rohrschneider3 classed a gas chromatography column stationary phase on the basis of the retention time of a similar n-alkane in the system. This retention index is independent of flow rate and the physical dimensions of the column. Snyder4,5 extended the work of Rohrschneider and developed a polarity index (P ′), where P ′ is used to describe the properties of the solvent. This technique involves the experimental determination of the distribution coefficient, Kg, for the test solutes, ethanol (e), n-octane (o), dioxane (d), and nitromethane (n), in various solvent systems. Correction for the solvent and solute molecular weight gives Kg′ and log Kg′′, respectively. The polarity index (P′) is then calculated by adding the log Kg′′ values for ethanol, dioxane, and nitromethane. The contribution to proton acceptor ability, the extent of dipole moment, and the proton donation ability (xe, xn, and xd, respectively) are calculated from the ratio log Kg′′/P ′ for each solute. The approach attempts to mirror the particular interaction properties that are peculiar to that solvent. The solvent prediction scheme used in this paper is based on the Hildebrand solubility parameter (δt).6,7 Previous work from this group has applied the Hildebrand solubility parameter to steroid solubility in supercritical carbon dioxide.8 It was found that δt in combination with a hydrophobicity term (Log P ) gave an approximate indication of steroid solubility in supercritical carbon dioxide. The solubility parameter is a measure of the internal (3) Schoenmakers, P. J. Optimisation of Chromatographic Selectivity; A Guide to Method Development; Journal of Chromatography Library 35; Elsevier: Amsterdam, 1986. (4) Snyder, L. R. J. Chromatogr. Sci. 1978, 16, 223-234. (5) Snyder, L. R.; Kirkland, J. J.; Glajch, J. L. Practical HPLC Development, 2nd ed; John Wiley & Sons, Ltd.: New York, 1997. (6) Barton, A. F. M. The Handbook of Solubility Parameters and other Cohesion Parameters; CRC Press Inc.: Boca Raton, FL, 1983. (7) Burke, J. Solubility Parameters: Theory and Application. In AIC Book and Paper Group Annual; Jensen, C., Ed.; American Institute for Conservation (AIC): New York, 1984; Vol. 3, pp 13-57. (8) Dean, J. R.; Kane, M.; Khundker, S.; Dowle, C.; Tranter, R. L.; Jones, P. Analyst 1995, 120, 2153-2157. 10.1021/ac001336u CCC: $22.00
© 2002 American Chemical Society Published on Web 11/30/2001
energy of cohesion in the solvent/solute. Solvents with similar solubility parameter form mixtures;7 hence, an analyte and a solvent that have similar solubility parameters should also form mixtures. δt is defined as the square root of the cohesive energy density6,7 or
δt ) (∆Ev/V)1/2
(1)
where δt is the total Hildebrand solubility parameter, ∆Ev is the energy of vaporization at a given temperature, and V is the molar volume of the molecule. Calculations of this sort require knowledge of the heat of vaporization at various temperatures as well as the molar volume of the substance. However, these values are not widely available for pesticides. Several groups have also developed quantities comparable to the Hildebrand solubility parameter including the addition of group contributions.6,9 However, these procedures do not give the contribution of each type of interaction commonly found in matter,for example, the polarity, dispersion, and hydrogenbonding ability of the solvent, each of which can be vital in the extraction of pesticides. The total solubility parameter has been divided by Small, van Arkel, and Prausnitz6 into two portions, a polar contribution and a nonpolar contribution. This procedure does not address the induced and hydrogen-bonding ability of the liquid. Hansen10 took this work further and assumed the total cohesive energy is a linear addition of three components: δh, hydrogen-bonding ability contribution; δd, dispersion coefficient contribution; and δp, polarity contribution. They are linked by the following equation.
δt2 ) δh2 + δp2 + δd2
(2)
Hansen based this work on semiempirical equations describing the entropy and enthalpy of mixing of polymers and solvents in solution.10 Null and Palmer also used this approach,11 although their work was based on the findings of Wiehe and Bagley,12 who investigated the magnitude of the activity coefficient of alcohol in various inert solvent solutions and developed equations that described the entropy and enthalpy of associations within the solution. Hoy6 determined the individual components of solubility parameter using the following methodology: 1. The total solubility parameter, δt, is determined using the Clausius-Clapeyron equation, which is a measure of the change in vapor pressure of a substance at various temperatures. 2. Regression analysis of molar volume is determined as a function of temperature, molecular weight, and density, allowing the calculation of an aggregation number that is an estimate of δh. 3. δp is then calculated by a group molar attraction method, using eq 3, where Fp is the individual group contribution. (9) Fedors, R. F. Polym. Eng. Sci. Technol. 1974, 14, 147-154. (10) Hansen, C. M. J. Paint. Technol. 1967, 39, 104-117. (11) Null, H. R.; Palmer, D. A. Chem. Eng. Prog. 1969, 65, 47-61. (12) Weihe, I. A.; Bagley, E. B. Am. Inst. Chem. Eng. J. 1967, 13, 836-844.
∑F )/V
δp ) (
p
(3)
4. Determination of δh is then achieved by rearranging eq 1, and solving for δh. van Krevelen and Hoftzyer13 also determined the individual components using a group contribution additive method, not unlike that of Fedors.9 This paper outlines a procedure for predicting the optimum extraction solvent for analytes of environmental interest. Matrix-analyte interactions are not taken into account, although they will influence the extraction procedure. EXPERIMENTAL SECTION Extractions were performed using an ASE 200 accelerated solvent extractor (Dionex (UK) Ltd., Camberley, Surrey, U.K.) with 11-mL extraction cells. The operating conditions for the extraction of 4,4′-DDT, 4,4′-DDE, and 4,4′-DDD were as follows: temperature, 100 °C; pressure, 2000 psi; static extraction time, 10 min; number of static flush cycles, one. For the extraction of PCP, the conditions were as follows: temperature, 100 °C; pressure, 2000 psi; static extraction time, 5 min; number of static flush cycles, two. The number of repeat extractions was fixed at six for each analyte. Analysis of the extracts was achieved on a GC-MSD (HP G1800A GCD system, Hewlett-Packard, Palo Alto, CA) in selected ion monitoring (SIM) mode. A DB-5ms column (J&W Scientific, Folsom, CA), of 30 m × 0.25 mm i.d. × 0.25 µm film thickness, was used throughout for separation. The temperature program used for the analysis of DDT and its metabolites was as follows: 120 °C, held for 2 min and then to 290 °C at a rate of 5 °C/minute, with a final hold of 2 min. The ions monitored were m/z 235 and 237 for 4,4′-DDT and 4,4′-DDD and m/z 246 and 318 for 4,4′-DDE. The splitless injection volume was 0.5 µL. The temperature program used for the analysis of PCP was as follows: 90 °C, held for 2 min and then to 250 °C at a rate of 5 °C/min, with a final hold of 2 min. The ions monitored were m/z 321 and 323. The splitless injection volume was 1.0 µL. Calibration plots ranging from 0 to 5 µg mL-1 were constructed for each analyte (four) using at least five data points. The resultant linear calibration plots exhibited correlation coefficients exceeding 0.995 in each case. Soil. Zeneca Environmental Laboratories Brixham, U.K.) provided a soil contaminated with DDT and its metabolites (DDE and DDD). The certified reference material (CRM524), an industrial site soil, contaminated with PCP was supplied from the Laboratory of the Government Chemist (LGC, Teddington, London). Chemicals. Solvents were obtained from Fisher Scientific (Loughborough, Leicestershire, U.K.) and were certified analytical grade. The headspace of the extraction cells were filled with Hydromatrix (Varian Ltd., Surrey, U.K.). A pesticide standard was purchased from Supelco (Walton-on-Thames, U.K.) and comprised 20 organochlorine pesticides. PCP was purchased from Aldrich Chemical Co. (Gillingham, U.K.). Derivatizing agent, N,O-bis(trimethylsilyl)acetamide (BSA), was purchased from Aldrich Chemical Co. Hexachlorobenzene (Aldrich Chemical Co.) was used as the internal standard. The structures of the analytes used in this study are shown in Figure 1. (13) van Krevelen, D. W.; Hoftzyer, P. J. Properties of Polymers; Their Estimation and Correlation with Chemical Structure; Elsevier: Amsterdam, 1976.
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Table 1. Calculation of Individual Group Contributions for Methanol group contribution
group
to dispersion (Fd), J1/2 cm3/2 mol-1
to polarity (Fp), J1/2 cm2 mol-1
to hydrogen bonding (Uh), J mol-1
molar volume (V), cm3 mol-1
CH3 OH
420 210
0 500
0 20000
33.5 10.0
total
630
500
20000
43.5
Table 2. Calculation of Individual Group Contributions for the Analyte, 4,4′-DDT group contribution
group
to dispersion (Fd), J1/2 cm3/2 mol-1
to polarity(Fp), J1/2 cm2 mol-1
to hydrogen bonding (Uh), J mol-1
molar volume (V), cm3 mol-1
2 × sPhs 2 x ClsCHd 3 × Cl 1 × CH >C
