Determination of polychlorinated biphenyls in ash using dimethylsulfoxide microwave assisted extraction followed by solid-phase microextraction

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1 Talanta 63 (24) Determination of polychlorinated biphenyls in ash using dimethylsulfoxide microwave assisted extraction followed by solid-phase microextraction M. Ramil Criado, I. Rodríguez Pereiro, R. Cela Torrijos Departamento de Química Analítica, Nutrición y Bromatología, Facultad de Química, Universidad de Santiago de Compostela, Santiago de Compostela 15782, Spain Received 12 June 23; received in revised form 18 September 23; accepted 18 November 23 Available online 1 February 24 Abstract A procedure for the determination of several coplanar and non coplanar PCBs in ash samples is described. Analytes were extracted from the samples using dimethylsulfoxide (DMSO) under the action of a microwave field, and then they were concentrated on a PDMS-DVB solid-phase microextraction (SPME) fibre using the headspace mode, after water dilution of the DMSO extract. Determinations were carried out using GC ECD and GC MS detection. Influences of microwave extraction (solvent volume, temperature and time) and SPME conditions (stirring, kind of SPME fibre, salt and water addition, sampling time and temperature) on the performance of the whole analytical procedure were systematically investigated. Working under optimal conditions quantification limits from.2 to 1.5 ng g 1 were obtained for all the compounds, except for PCB 29, which could not be consistently extracted from the sample using the proposed conditions. 23 Elsevier B.V. All rights reserved. Keywords: PCBs; Ash; SPME; DMSO; MAE 1. Introduction Because of their chemical stability and past industrial applications, polychlorinated biphenyls (PCBs) are ubiquitous contaminants in the environment. Nowadays, the production and industrial uses of these compounds have been restricted; however, PCBs can still be introduced in the environment as byproducts of a wide variety of chemical processes, e.g. the non-controlled incineration of different materials such as waste oils, electric equipments and any other samples contaminated with different PCB mixtures [1 3]. From these sources and depending on the vapor pressure of each congener and on the adsorptivity of the ash particles, a fraction of PCBs is released to the atmosphere in the gas phase whilst the rest remain adsorbed on the ash residue. Normally, the determination of PCBs in this solid residue is a multi-step process due to the high surface area of ash particles, the presence of unburnt carbon and the ex- Corresponding author. Tel.: x14387; fax: address: qnisaac@usc.es (I.R. Pereiro). istence of a large number of interfering species, which can be co-extracted with the PCBs. An exhaustive extraction of the sample using an appropriate organic solvent constitutes the first step of most procedures applied to the analysis of PCBs in ash [1]. In this sense, microwave assisted extraction (MAE), sonication, accelerated solvent extraction and obviously the Soxhlet technique have been proposed for the extraction of the PCBs from ash samples [4 9]. As a rule, the more aggressive the conditions the higher the efficiency of the extraction, specially for the coplanar congeners in samples with a high content of carbon [4], but also the amount of co-extracted interferences; therefore, liquid extracts are submitted to one or several clean-up steps, using normal phase sorbents and occasionally carbonaceous sorbents, prior to the determination of the analytes using GC in combination with ECD or MS detection [1,11]. As a consequence, a time consuming and multi-step procedure is obtained. During the last several years, solid-phase microextraction (SPME) has been proposed as low cost, solvent free and selective sample preparation technique for the determination of PCBs. First applications were developed for aqueous matrices and latter, extended to soils and sediments [12 14] /$ see front matter 23 Elsevier B.V. All rights reserved. doi:1.116/j.talanta

2 534 M.R. Criado et al. / Talanta 63 (24) In most cases, solid samples are mixed with water, and PCBs concentrated on a non polar SPME fibre, placed in the headspace over this slurry [14]. The presence of a slurry in the microextraction vial produces a significant decrease in the efficiency of the microextraction in comparison with water samples; however, detection limits at the low ng/g could be obtained for soils and sediments [14]. Another approach considered in the application of SPME to the determination of PCBs in solid samples is based on a first extraction of the analytes from the material followed by filtration and exposition of the fibre to the headspace of a vial containing the liquid extract. As the SPME technique is not compatible with volatile organic solvents, water in sub-critical conditions is normally used to release the compounds from the sample [15,16]. In this case, the efficiency of the microextraction step is very high; however, the yield of the leaching step (for PCBs in soils) is poor. Approximately, only between 1 and 3% of the total PCB content in the sample remains in the aqueous phase when the extraction vessel returns to atmospheric pressure and temperature [16]. The aim of this work has been the optimization of an analytical procedure for the determination of PCBs in ash samples using the SPME technique. As preliminary experiments showed that direct transference of PCBs from the ash to a SPME fibre was not feasible, the sample preparation strategy was divided in two steps: firstly, analytes were extracted from the ash using a water-soluble organic solvent and then, they were selectively pre-concentrated on a non polar SPME fibre. Influence of experimental parameters on the efficiency of the extraction and SPME steps was investigated. 2. Experimental 2.1. Apparatus Determinations of PCBs were performed using two gas chromatographic systems equipped respectively with ECD and MS detection. The GC ECD system was a HP 589 series II gas chromatograph (Hewlett-Packard, Avondale, MA, USA) with a 63 Ni electron capture detector and a split/splitless injection port. Separations were carried out using a BP-5 capillary column (3 m.32 mm i.d., d.f.:.25 m) purchased from Supelco (Bellefonte, PA, USA). Nitrogen was employed as column carrier gas (at a constant flow of 1.5 ml min 1 ) and also as auxiliary gas in the detector. The GC MS system consisted of a Varian CP 38 gas chromatograph (Walnut Creek, CA, USA) equipped with a split splitless injector and connected to an ion-trap mass spectrometer (Varian Saturn 2). Separations were carried out using a BP-5 capillary column (3 m.25 mm i.d., d.f.:.17 m) obtained from Varian. Helium (99.999%) was used as carrier gas at a constant column flow of 1.4 ml min 1. Mass spectra were obtained in the electron ionization mode (7 ev) in the range from 1 to 55 m/z. In both columns PCBs were separated using the following oven program: 3 min at 9 C, first ramp at 2 C min 1 to 17 C (held for 7.5 min), second ramp at 3 C min 1 to 25 C (held for 5 min). Injections of PCB standards in isooctane and thermal desorption of SPME fibres were carried out in the splitless mode for 1 and 3 min, respectively. In both chromatographic systems a conventional liner (4 mm i.d.) was used. A manual SPME fibre holder and 1 m PDMS, 65 m PDMS-DVB and 75 m CAR-PDMS microextraction fibres were obtained from Supelco. Fibres were thermally desorbed for 3 min at 26 C (PDMS and PDMS-DVB) and 3 C (CAR-PDMS). Microwave extraction of PCBs from ash samples was performed using a MES-1 microwave extraction system (CEM, Matthews, NC, USA) equipped with Teflon-lined 1 ml extraction vessels. Numerical analysis of data resulting from the experimental design was carried out by means of the statistical package Statgraphics Plus for Windows, version 3.3 (Manugistics, Rockville, MD, USA) Reagents and materials Dimethylsulfoxide (DMSO), n-hexane, toluene and isooctane for trace analysis were purchased from Aldrich (Milwaukee, WI, USA) and Merck (Darmstadt, Germany). A mixture containing several PCB congeners in isooctane (2,4,4 -trichlorobiphenyl PCB 28; 2,2,5,5 -tetrachlorobiphenyl PCB 52; 2,2,3,4,4,5 -hexachlorobiphenyl PCB 138; 2,2,4,4,5,5 -hexachlorobiphenyl PCB 153; and 2,2,3,4,4,5,5 -heptachlorobiphenyl PCB 18, 1 gml 1 of each one) was obtained from Supelco. In addition, individual standards of 2,3,4,4,5-pentachlorobiphenyl PCB 118, 3,3,4,4 -tetrachlorobiphenyl PCB 77, 3,3,4,4,5-pentachlorobiphenyl PCB 126, 3,3,4,4,5,5 -hexachlorobiphenyl PCB 169 and decachlorobiphenyl PCB 29, were purchased from Dr. Ehrensdorfer (Augsburg, Germany). A standard solution of these ten congeners was prepared in isooctane and further dilutions were made in n-hexane. Mixtures of PCBs in DMSO, used for the optimization of the microextraction step, were prepared by evaporation to dryness of a standard in n-hexane and dilution of the residue in DMSO. As most volatile analytes can be partially lost during solvent evaporation, their real concentration in the DMSO solution is not exactly known; thus, aliquots from the same mixture were used during each set of experiments. A sample of ash (fraction below 6 m), containing an 8.7% of carbon, was obtained from the incineration of wood and wastes from a paper production plant. After checking that PCBs were not present in the sample [4], it was spiked with the congeners considered in this study at different concentration levels between 1 and 1 ng g 1. In order to achieve an approximate simulation of the interaction between the PCBs and the matrix that occurs in real polluted samples, a slurry was prepared by mixing a known amount of the ash with a solution of the PCBs in n-hexane. The mixture was left in the dark, stirred periodically and allowed

3 M.R. Criado et al. / Talanta 63 (24) to air-dry for 2 weeks until constant weight was achieved. Then it was stored at 4 C for 1 month before being used for the optimization of the proposed procedure Extraction of PCBs and SPME concentration Microwave assisted extractions of PCB compounds from spiked ash samples under optimized conditions were carried out using 3 ml of DMSO at 12 C for 1 min. The mixture was centrifuged and the liquid extract separated. Then, a fraction of the DMSO extract (between 5 and 15 ml) was placed in a 11 ml headspace vial, diluted four times with water and 3 mg of sodium chloride per ml of water were added. SPME extractions were performed at 1 C for 5 min using a PDMS-DVB fibre placed in the headspace over the DMSO water solution. 3. Results and discussion 3.1. Preliminary experiments Direct exposition of a PDMS fibre to the headspace of a vessel containing several mg of a spot spiked ash sample (added concentration 1 ng g 1 ) showed that analytes were not transferred in an appreciable amount to the fibre. A previous digestion of the material with hydrochloric acid and/or the preparation of a slurry in the microextraction vessel led to similar negative results. Conversely to these findings, when the microextraction was carried out in the same conditions (HS mode for 6 min at 1 C using a PDMS fibre), over the DMSO extract obtained from spot and long term spiked samples PCBs were concentrated on the SPME fibre and detected in the GC ECD chromatograms. Fig. 1, overlays the GC ECD chromatograms corresponding to long term spiked samples (1 ng g 1 per congener). In both cases 1.5 g of ash were extracted during 3 min with 2 ml of DMSO. One of the samples was sonicated and the other one microwave assisted extracted at 15 C(warning: DMSO solutions have a risk of explosion when they are heated in closed vessels at temperatures over 22 C, therefore extraction devices without accurate temperature control should not be used). Then, 5 ml aliquots of the liquid extract were placed in 11 ml vessels and a PDMS fibre exposed to the headspace of the vials in the conditions given in the above paragraph. As shown in Fig. 1, microwave assisted extraction resulted more effective to leach the coplanar congeners (PCBs 77, 126 and 169) from the ash than sonication. On the other hand, the congener 29 was better extracted with sonication than using microwave assisted extraction. Because of the acute toxicity of coplanar congeners MAE was selected as the extraction technique to pass PCBs from the sample to DMSO, previously to their concentration on a SPME fibre. Moreover, as SPME and microwave extraction conditions were completely independent, both steps involved in the analytical procedure were optimized separately Optimization of SPME conditions Studies dealing with the solid-phase microextraction of organic compounds from DMSO solutions are scarce [17,18] and, to our knowledge, no results have been published for Counts min Fig. 1. Overlay of GC ECD chromatograms corresponding to a long term spiked ash sample. Compounds were concentrated on a SPME fibre after extraction in DMSO using sonication (solid line) or MAE (dotted line) (1) PCB 28, (2) PCB 52, (3) PCB 77, (4) PCB 118, (5) PCB 153, (6) PCB 138, (7) PCB 126, (8) PCB 18, (9) PCB 169, (1) PCB 29.

4 536 M.R. Criado et al. / Talanta 63 (24) PCB compounds. Thus, the effect of different experimental parameters in the yield of the microextraction was studied in detail. Measurements were carried out using the GC ECD system. The equation below represents the predicted mass of analyte concentrated on the fibre working in the headspace mode: K fs V f C V s n = K fs V f + K hs V h + V s where n is the mass of analyte absorbed by the coating, V f, V s, and V h are the volumes of the fibre, sample and headspace respectively, K fs and K hs are de fibre/sample and the headspace/sample distribution constants respectively and C is the initial concentration of analyte in the sample Stirring, sodium chloride and water dilution of DMSO solutions The effect of these factors on the yield of the microextraction was evaluated using a factorial experimental design at two levels with a total of 1 experiments. In all cases, 5 ml aliquots of the same PCB solution in DMSO were placed in 11 ml vials and the SPME was carried out at 1 C, using a PDMS fibre placed in the headspace over the sample. The sampling time was fixed in 45 min. Low and high levels for each factor in the factorial design experiments, together with the standardized value of the main effect for each one, obtained using the software package Statgraphics Plus, are given in Table 1. The addition of sodium chloride showed a positive and significant influence (at the confidence level of 95%) on the peak areas of most congeners, except PCB 29. Stirring affected positively to the yield of the microextraction for seven PCB congeners; however, it never achieved the statistical significance bound. For this reason, and in order to avoid possible cross contamination problems, due to the sorption of PCBs on magnetic bars [19], samples were not stirred in subsequent experiments. The volume of water added to the DMSO aliquot showed either a positive or negative influence depending on each congener; however, it was not statistically significant for most PCBs. First order interactions showed a minor effect on the yield of the microextraction (results not shown) except for the case of salt-water volume, which was statistically significant for all congeners with more than four chlorine atoms. For these analytes, as shown in the surface response plot for PCB 153 Fig. 2, when salt was added to the sampling vial the highest efficiency of the microextraction corresponded to the lowest Peak area (x 1 4 ) NaCl (mg) Water volume (ml) Fig. 2. Surface response plot for PCB 153 in the experimental factorial design. volume of water considered in the factorial design (3 ml), whilst the opposite behavior was obtained in absence of salt. In order to confirm this pattern, and also to obtain the optimum dilution factor of the DMSO aliquot, the influence of water was reevaluated using an univariant approach. Two sets of experiments were carried out, in the first one salt was not used, whilst in the second one 3 mg of sodium chloride per ml of water were added to the SPME vessel. In both cases, the volume of DMSO was fixed in 5 ml and water aliquots from 1 to 7 ml were added, thus the total amount of each congener in the SPME vial remained constant but their concentration (C ) decreased with the increase of the water volume; moreover, the values of K fs, K hs, V h and V s also changed in function of the added water volume. Obtained results showed that in absence of salt, the peak area of each PCB became higher with the addition of water until a volume of 5 ml and then it decreased slightly (figure not shown). In this case, it was assumed that the diminution of the concentration of PCBs with the addition of water was less significant than the change in the distribution constant between the fibre and the sample (K fs was displaced to higher values when DMSO is diluted with water). On the other hand, when sodium chloride was added, the highest responses were obtained for a water volume of 2 ml. A possible explanation to this behavior was that the affinity of PCBs with the fibre was higher in presence of salt than in a non-saline medium, therefore, the diminution in the concentration of the compounds when water was added to the SPME vessel was not compensated by the additional improvement in affinity of the compounds with the fibre, causing a decrease in the peak areas. Table 1 Experimental domain and standardized values, with their signs, for the main effects associated to each factor in the SPME of PCBs from DMSO solutions Factor Low level High level Standardized main effects for each congener; PCB NaCl (mg/ml water) a 1.4 a 3.2 a 7.4 a 5.7 a 8.7 a 8.9 a a 5.5 a Water volume (ml) a a Stirring No Yes a Statistically significant factors at the 95% confidence level.

5 M.R. Criado et al. / Talanta 63 (24) Table 2 Influence of temperature and fibre coating on the yield of the headspace microextraction of PCBs from DMSO:water solutions Compound Fibre coating Microextraction temperature 2 ( C) 1 ( C) PCB 28 PDMS 4 37 PDMS-DVB 49 1 PCB 52 PDMS 3 52 PDMS-DVB 33 1 PCB 77 PDMS 6 51 PDMS-DVB 6 1 PCB 118 PDMS 5 64 PDMS-DVB 5 1 PCB 153 PDMS 5 89 PDMS-DVB 5 1 PCB 138 PDMS 3 82 PDMS-DVB 3 1 PCB 126 PDMS 3 79 PDMS-DVB 2 1 PCB 18 PDMS 1 99 PDMS-DVB 1 1 PCB 169 PDMS 2 1 PDMS-DVB 1 88 PCB 29 PDMS 2 1 PDMS-DVB 4 77 Normalized peak areas for each congener. From these results it was decided to carry out the microextraction using DMSO solutions diluted four times with water (5:2 ml), in presence of 6 g of sodium chloride Fibre coating, temperature and time The influence of the fibre coating in the yield of the microextraction was evaluated at room temperature and 1 C using PDMS, PDMS-DVB and CAR-PDMS fibres. SPME was always performed in the headspace mode for 45 min. At both temperatures, the extraction efficiency of the CAR- PDMS fibre was very low in comparison with the other two (results not shown). PDMS and PDMS-DVB fibres gave similar results at room temperature; however, at 1 C the latter produced higher responses for the most volatile congeners, Table 2. In general, for both fibre coatings better signals were obtained at 1 C than at room temperature. The microextraction efficiency using direct sampling, instead of headspace sampling, was not evaluated, since the high percentage of DMSO could have damaged either the stationary phase, or the bound between the silica core and the polymeric sorbent. Fig. 3 overlays the extraction time profiles, at 1 C, for PCB 28 (trichlorobiphenyl), PCB 118 (pentachlorobiphenyl) and PCB 18 (heptachlorobiphenyl) obtained using PDMS and PDMS-DVB fibres. For the PDMS one, all the congeners (excepting PCBs 18, 169 and 29) achieved the equilibrium within the period of time considered (figure not rea Peak a Peak area Peak area PCB Time (min) PCB Time (min) PCB Time (min) Fig. 3. Kinetics of the microextraction for PCB 28, 118 and 18 using PDMS (solid line) and PDMS-DVB (dotted line) fibres. Microextraction was carried out at 1 C in the headspace mode. shown), whilst with the PDMS-DVB the kinetic of the extraction was much slower, and only PCB 28 achieved the equilibrium between the sample and the fibre after a period of 2 h. Anyhow, the pre-concentration capacity of the latter was higher than that of the former for the most volatile congeners and similar for the rest of PCBs, considering a sampling time around 1 h. Therefore, the PDMS-DVB was chosen for further experiments. The PDMS fibre, a more extended sorption material, could be also used as a good alternative to carry out this method in routine analysis. In further experiments, the microextraction time was limited to 5 min, a similar value to the duration of the chromatographic separations. Obviously, a better sensitivity could be achieved for most congeners, with the PDMS-DVB fibre, using longer extraction times Vial size Influence of the microextraction sampling vessel on the amount of each PCB extracted from the sample to the PDMS-DVB fibre was compared using 22 and 11 ml glass vials. A 2 ml aliquot of a PCB solution in DMSO plus 8 ml of water were placed in the small vials versus 5 ml of the same

6 538 M.R. Criado et al. / Talanta 63 (24) DMSO mixture plus 2 ml of water in the large ones; therefore the concentration of each PCB and the composition of the matrix was the same in both vials (3 mg of NaCl per ml of water was added in both vessels). Microextraction was carried out at 1 C using a PDMS-DVB fibre for 5 min. Slightly higher peak areas were obtained using the 11 ml vials, probably because of their wide cross section (5 cm i.d. versus 2 cm in the 22 ml vials) which facilitates the transference of the analytes to the headspace of the vessel and thus to the SPME fibre. In addition, the increase in the sample volume (V s ) could also contribute to this behavior Performance of the SPME over DMSO water solutions The linearity of the response for the SPME procedure was tested using solutions containing absolute amounts of each congener between.1 and 5 ng, at five different levels. Correlation coefficients of.998 (PCBs 28 and 52) and.999 (for the rest of congeners) were obtained. The repeatability of the microextraction using solutions containing 2.5 ng of each analyte ranged from 4 to 8%, Table 3. The efficiency of the SPME step (5 min at 1 C) was evaluated by comparing the amount of each compound concentrated on the PDMS-DVB fibre (measured against a calibration curve obtained for PCB standards in isooctane injected in the splitless mode) with its initial amount in the sampling vessel. SPME sampling efficiencies from 16% (PCB 29) up to 31% (PCB 153) were achieved, Table 3. Comparison of the microextraction efficiency in DMSO:water solutions and ultrapure water was done using the sum of peak areas for the considered PCB congeners extracted from both solutions under same SPME conditions. The presence of DMSO in the extraction vessel (5 ml of DMSO + 2 ml of water) produced a diminution in the yield of the SPME of only 21% respect the microextraction over ultrapure water (data not shown) Optimization the microwave assisted extraction Influence of temperature, time and DMSO volume in the MAE step were optimized to maximize the sensitivity in Table 3 Performance of the SPME over DMSO:water (5:2 ml) solutions Compound Correlation coefficient (R 2 ) (.1 5 ng) R.S.D., n = 5 replicates (%) Microextraction efficiency (%) PCB ± 4 PCB ± 3 PCB ± 5 PCB ± 2 PCB ± 2 PCB ± 3 PCB ± 4 PCB ± 2 PCB ± 3 PCB ± 4 Headspace sampling at 1 C, 5 min, 65 m PDMS-DVB fibre. the determination of PCBs in ash. The effect of the microwave extraction temperature was studied at 2 C intervals from 6 to 16 C (figure not shown). Initially, signals rose with the increase in the extraction temperature up to C; however, a significant decrease was observed at 16 C. Therefore, the extraction temperature was fixed at 12 C. The effect of solvent volume on the peak areas of each compound was investigated between 2 and 5 ml. Obviously, the larger the solvent volume the higher the total amount of each PCB in this extract. However, as only 5 ml were used in the SPME step, the amount of each congener in the microextraction vial depended on the efficiency of the MAE extraction and the concentration of PCBs in the DMSO extract. Experimentally, a decrease in the peak areas was obtained for volumes larger than 3 ml for most congeners. Thus, the DMSO extraction volume was fixed at 3 ml. As this value was higher than the 5 ml considered during the optimization of the microextraction conditions, the effect of decreasing the headspace and increasing the sample volume in the microextraction vessels, without modifying the DMSO:water ratio and the amount of salt per ml of water, was reevaluated. Up scaling the solvent volumes three times (15 ml of DMSO plus 6 ml of water) improved the peak areas of all compounds in a 2 3%. The effect of time on the efficiency of the extraction, working at 12 C with 3 ml of DMSO, was investigated at 1, 2, 3 and 4 min. A significant effect on the yield of the process was not obtained (results not given), and therefore microwave extraction time was limited to 1 min Quantification and recovery evaluation It is well known that SPME is an extraction technique affected by matrix effects, therefore quantitative measurements in real samples normally require applying the standard addition technique. In order to evaluate the figures of merit of the extraction process a series of analyses of subsamples from the long term spiked material were carried out. In this study, GC MS was employed to avoid the interferences appearing in the early retention part of the GC ECD chromatograms, that affected the measurement of PCB congeners 28 and 52. Two different materials containing ca. 5 and 1 ng g 1 of each congener were analysed. Four replicates of the raw material and standard additions at three levels in duplicate were carried out. The obtained concentrations for each congener were compared with the theoretical values to calculate recoveries. Table 4 summarises the results in this experiment DMSO microwave assisted extraction followed by SPME versus Soxhlet extraction Fig. 4 overlays the chromatograms obtained for two subsamples of ash (1.5 g each) spiked with the selected compounds. One of them was processed following the procedure

7 M.R. Criado et al. / Talanta 63 (24) counts min Fig. 4. GC ECD chromatograms for long term spiked ash samples (1.5 g of ash containing 5 ng g 1 of each congener). Soxhlet extraction with toluene (solid line) and DMSO leaching followed by SPME concentration (dotted line). (1) PCB 28, (2) PCB 52, (3) PCB 77, (4) PCB 118, (5) PCB 153, (6) PCB 138, (7) PCB 126, (8) PCB 18, (9) PCB 169, (1) PCB 29. described in this paper while the other one was submitted to Soxhlet extraction for 24 h with 9 ml of toluene. This extract was reduced to ca. 2 ml under nitrogen stream, passed through a column containing anhydrous sodium sulphate, alumina and florisil, and finally, adjusted to 1 ml with toluene. A 1. l aliquot of the extract was injected in the GC ECD system in the splitless mode. It is evident that the proposed procedure led to higher peaks than the Soxhlet technique. Moreover, it should be noticed the enormous differences in time required for the total analytical process and the number of critical manipulations involved in each procedure. DMSO extraction plus SPME showed a repeatability for the whole procedure using electron capture detection (ECD) of 4 9% for all considered congeners except for PCBs 28 and 52 which are affected by interferences using ECD (4 12% for GC MS, Table 4). Using MS detection quantification limits in the range ng g 1 were obtained for all PCBs except for the congener 29, Table 5. Table 4 Recoveries of the whole sample preparation procedure (microwave extraction plus SPME) for long term spiked ash samples containing different concentrations of PCBs Compound Quant. ions (m/z) Recovery (%) ± R.S.D. Spiked concentration (5 ng g 1 ) Spiked concentration (1 ng g 1 ) PCB ± 7 99 ± 12 PCB ± 9 17 ± 9 PCB ± 8 83 ± 1 PCB ± 6 96 ± 5 PCB ± 5 11 ± 7 PCB ± 7 99 ± 6 PCB ± ± 8 PCB ± 9 99 ± 6 PCB ± 1 17 ± 1 Detection was performed using GC MS. Table 5 Comparison of quantification limits (S/N = 1) for the proposed method with those obtained using microwave extraction with toluene, solvent evaporation, clean up and concentration of the extract to 1 ml Compound QL (S/N = 1) (ng g 1 ) DMSO extraction plus SPME PCB PCB PCB PCB PCB PCB PCB PCB PCB MAE with toluene In both cases 1.5 g of sample were taken and GC MS was used as detection technique. These results compares favourably with those attainable for the same sample using microwave assisted extraction with toluene, followed by solvent evaporation, a sample clean-up step similar to that described for Soxhlet extractions, and further concentration of the final extract to 1 ml [4]. 4. Conclusions The combination of a microwave assisted extraction step using DMSO, a water-soluble and non-volatile organic solvent, followed by headspace SPME using a PDMS-DVB fibre, allowed the determination of several PCBs in ash samples. When compared with classical extraction approaches, the proposed method avoids some time consuming operations such as solvent evaporation and clean up of the extracts without apparent losses of selectivity and with a considerable improvement in the achieved detection limits. On the other hand, the above described approach expanded the application field of the SPME technique to the determination

8 54 M.R. Criado et al. / Talanta 63 (24) of PCBs in high adsorptive solid matrices, for which direct transference of the analytes from the sample to the SPME fibre, fails or presents an extremely low efficiency. Acknowledgements This work has been financially supported by the Spanish DGICT (project BQU23-29). M. Ramil acknowledges a FPU grant from the Spanish Ministry of Education. References [1] D. Voutsa, H. Terzi, C. Samara, Th. Kouimtzis, L. Muller, Organohalogen Compd. 58 (22) 449. [2] N. Takeda, M. Takaoka, T. Fujiwara, H. Takeyama, S. Eguchi, Chemosphere 43 (21) 763. [3] S.I. Sakai, K. Hayakawa, H. Takatsuki, I. Kawakami, Environ. Sci. Technol. 35 (21) 361. [4] M. Ramil Criado, I. Rodríguez Pereiro, R. Cela Torrijos, J. Chromatogr. A 985 (23) 137. [5] M. Martínez, J. Díaz-Ferrero, R. Martí, F. Broto-Puig, L. Comellas, M.C. Rodríguez-Larena, Chemosphere 41 (2) [6] J.S. Yang, J.H. Jeong, D.W. Lee, Y.S. Chang, J. Liquid Chromatogr. Relat. Technol. 24 (21) [7] J.S. Yang, D.W. Lee, S. Lee, J. Liquid Chromatogr. Relat. Technol. 25 (22) 899. [8] B.E. Richter, J.L. Ezzell, D.E. Knowles, F. Hoefler, A.K.R. Mattulat, M. Scheutwinkel, D.S. Waddell, T. Gobran, V. Khurana, Chemosphere 34 (1997) 975. [9] L.J. Fitzpatrik, O. Zuloaga, N. Etxebarria, J.R. Dean, Rev. Anal. Chem. 19 (2) 75. [1] A.K. Djien Liem, Trends Anal. Chem. 18 (1999) 429. [11] O. Zuloaga, N. Etxebarria, L.A. Fernández, J.M. Madariaga, Trends Anal. Chem. 17 (1998) 642. [12] P. Landin, M. Llompart, M. Lourido, R. Cela, J. Microcolumn Sep. 13 (21) 275. [13] J.L. Zhou, K. Maskaoui, Y.W. Qiu, H.S. Hong, Z.D. Wang, Environ. Pollut. 113 (21) 373. [14] M. Llompart, K. Li, M. Fingas, J. Microcolumn Sep. 11 (1999) 397. [15] K.J. Hageman, L. Mazeas, C.B. Grabanski, D.J. Miller, S.B. Hawthorne, Anal. Chem. 68 (1996) [16] S.B. Hawthorne, C. Grabanski, B. Carol, K.J. Hageman, D.J. Miller, J. Chromatogr. A 814 (1998) 151. [17] C.C. Camarasu, J. Pharm. Biomed. Anal. 23 (2) 197. [18] A.R. Raghani, J. Pharm. Biomed. Anal. 29 (22) 57. [19] E. Baltussen, P. Sandra, F. David, H.G. Janssen, C. Cramers, Anal. Chem. 71 (1999) 5213.