Anal. Chem. 2003, 75, 6109-6118
Optimization of Accelerated Solvent Extraction for Polyhalogenated Dibenzo-p-Dioxins and Benzo-p-furans in Mineral and Environmental Matrixes Using Experimental Designs Hugues Preud’homme*,†,‡ and Martine Potin-Gautier†
LCABIE-UMR CNRS 5034, Laboratoire de Chimie Analytique, Universite´ de Pau et des Pays de l’Adour, B.P. 155 64013 Pau Cedex, France, and Laboratoires De´ partementaux des Pyre´ ne´ es Atlantiques, 5 rue des Ecoles, 64150 Lagor, France
This paper deals with the optimization of accelerated solvent extraction (ASE) for the analysis of the polychlorinated dibenzo-p-dioxins, benzo-p-furans (PCDD/Fs), mixed bromine/chlorine-dibenzo-p-dioxins, and benzop-furans (so-called MXDD/Fs) in solid samples. Previous theoretical studies have shown that these compounds exhibit similar electronic properties. It is reasonable to assume that there is little difference in the behavior, formation, and toxicity of PCDD/Fs and MXDD/Fs. Indeed, for most of the cases, the affinity is defined by these weak interactions. Only eight native standards are available for the MXDD/Fs; hence, the use of similar compounds (native and 13C12-labeled), such as PCDD/ Fs, is required to optimize and to validate experimental methods. This would allow conclusions to be applied for the MXDD/Fs without extended studies involving complex synthesis methods. Experimental design methodology was used to evaluate the influence of five parameters (temperature, pressure, static time, number of cycles, and solvent nature) on the polyhalogenated dibenzodioxin and -furan (PXDD/Fs) extractions in different materials. The extraction profiles and the optimal operating conditions were determined for each matrix from the modeling of extraction performance. The two following effects, the relative peak area and the co-extracted matrix (CEM), were screened in this study. The temperature of extraction was found to be the most important parameter. ASE offers automation and appears to be as efficient as Soxhlet or Soxtet; however, a major benefit was that a 4-fold decrease in extraction time was obtained. Results suggest that extraction efficiency was quantitative with extraction times as low as 15 min for all congeners at 130 °C with a mixed solvent (n-hexane/acetone (1/1)). Under these operating conditions, the CEM and the degradation of the highly brominated compounds were minimized. The analysis of some real life samples from municipal solid waste incinerators showed significant amounts of PXDD/Fs.
* Corresponding author. E-mail:
[email protected]. † Universite ´ de Pau et des Pays de l’Adour. ‡ Laboratoires De ´ partementaux des Pyre´ne´es Atlantiques. 10.1021/ac0341665 CCC: $25.00 Published on Web 10/16/2003
© 2003 American Chemical Society
The formation and environmental fate of polychlorinated dibenzo-p-dioxins and benzo-p-furans (PCDD/Fs) are of great interest because of their toxicity and persistence.1,2 From the literature,3 it is known that fly ash and exhaust gases of municipal solid waste incinerators (MSWI) can contain a few hundred nanograms of international toxic equivalent quantities (I-TEQ) per gram or per cubic meter under normal conditions (Nm3) of PCDD/Fs. It is well-known that the incineration of chlorinated compounds and brominated compounds (such as flame retardants) generates highly toxic compounds, for example, poly(chlorobiphenyl)s (PCBs), PCDD/Fs, and poly(bromodibenzo)dioxins and -furans (PBrDD/Fs) and mixed brominated/chlorinated dioxins and furans (MXDD/Fs). The theoretical chemistry of these compounds has been investigated, and their potential toxicological behavior has been established.4 These theoretical considerations suggest that environmental and health assessments for the MXDD/Fs should be based on the molecular electronic properties without discriminating over the nature of the halogen.5 The internationally accepted and most accurate technique to analyze PCDD/Fs is using the isotopic dilution method combined with an analytical method involving a hyphenated technique, such as high-resolution gas chromatography-high-resolution mass spectrometry (HRGC-HRMS).6 However the analysis and the extraction of such compounds is time-consuming. The extraction takes from 24 to 48 h, according to classical techniques, such as the Soxhlet. A further disadvantage of this method is the need for large solvent volumes (350 mL). Automation of a large number of samples is also difficult because of the unknown characteristics of each sample and the risk of cross contamination. An alternative solution is the use of Soxtet (Tecator system), which allows extraction of 3 or 6 samples in the same run on the same process but requires a smaller solvent volume (100 mL) and reduced time (45 min to 2 h). Microwave-assisted extraction (MAE) represents (1) Safe, S. CRC Crit. Rev., Toxicol. 1990, 21, 51-88. (2) Rappe, C. Pure Appl. Chem. 1996, 68, 1781-1789. (3) Horstmann, M.; McLachan, M. Organohalogen Compd. 1995, 22, 309-313. (4) Preud’homme, H.; Potin-Gautier, M. Organohalogen Compd. 2002, 55, 4549. (5) Gallo, M. A.; Scheuplein, R. J.; Van Der Heijden, K. A. Biological Basis for Risk Assessment of Dioxins and Related Compounds; Barbury Report 35; Cold Spring Harbor Laboratory Press: Plainview NY, 1991. (6) Ballschmiter, K.; Bacher, R.; Mennel, A.; Fisher, R.; Riehle, U.; Swerev, M. J. High Res. Chromatogr. 1992, 15, 260-270.
Analytical Chemistry, Vol. 75, No. 22, November 15, 2003 6109
one of the most recent extraction methods. It involves the rapid heating of an organic sample with a permanent dipole moment by alignment of molecules, followed by their rapid return to disorder under nonionizing radiation. As the system expands, the cell walls rupture, allowing essential oils to flow toward the organic solvent. This process is different from classical solvent extraction, in which the solvent diffuses into the matrix and extracts the components by solubilization.7 However, even if the PXDD/Fs are nonthermally labile, the microwave radiation is known to generate, with a low energy cost, some nonconventional syntheses, principally in the presence of aromatic and unsaturated compounds (such as Diels-Alder reactions, cycloaddition, and FriedelCraft acylation).8 For this reason, and despite its potential advantages, we have not studied MAE in this work. The reduction of costs, mainly by the reduction of sample preparation time, for routine environmental analysis has been investigated worldwide in the past few years.9-14 No applications of ASE extraction of MXDD/Fs has be done, and considerable work is still required to optimize its efficiency. We have investigated the ASE 200-Dionex with a multisolvent system to see if sample preparation time can be reduced in the extraction of polyhalogenated dibenzodioxins and -furans (PXDD/Fs) from solid samples. The method of optimization chosen was a chemometric approach using a central composite design for obtaining the optimal conditions. The main advantage of this technique was to describe in details, the many parameters, and the two antagonist responses in a large yield. The influence of each of the parameters and their interactions could be well-identified for different responses screened. The use of an experimental design enabled a subsequent benefit in terms of labor time and number of experiences to optimize the conditions. In this technique, conventionally heated (50-200 °C) liquid solvents are used under elevated pressures (50-200 bar) to extract solid samples quickly (
