Anal. Chem. 1997, 69, 3304-3313
Quantitative Analysis of Pesticides in Postconsumer Recycled Plastics Using Off-Line Supercritical Fluid Extraction/GC-ECD C. Nerı´n,* R. Batlle, and J. Cacho
Department of Analytical Chemistry, Centro Polite´ cnico Superior, Universidad de Zaragoza, C/Marı´a de Luna, 3, E-50015, Zaragoza, Spain
The supercritical fluid extraction (SFE) of several organochlorine and organophosphorus pesticides and two metabolites, o,p-DDE and 4,4′-dichlorobenzophenone, from recycled plastics was studied. The recycled plastics, used as agricultural soil covers, were a mixture of lowdensity polyethylene (∼90% w/w) and ethylene-vinyl acetate copolymer (∼10% w/w). The SFE was optimized and compared to the classical sonication and total dissolution extraction methods. The plastic was extracted with supercritical CO2 at different extraction conditions, depending on the physical matrix characteristics. Average recoveries of the 11 chemicals under study ranged from values greater than 90% with an overall relative standard deviation of 4.3% for plastic film to values lower than 40% with an overall standard deviation of 7.2% for plastic pellets. The variables affecting the whole extraction process, and the results obtained in the comparison between the three extraction methods were discussed. The determination of chemicals present in a solid or liquid matrix usually involves at least one extraction step. Soxhlet, microwave, ultrasonic, and total dissolution extractions are the most common ones. In these cases, further evaporation of the solvent is needed to get a concentrated extract for chromatographic analysis. However, these methods are time consuming and involve the use of large volumes of solvent that are often toxic and carcinogenic. Supercritical fluid extraction (SFE) technology provides a useful sample preparation method that reduces or eliminates the use of halogenated solvents, extracts samples more quickly and efficiently, and greatly simplifies concentration and cleaning of the extracted analytes. For these reasons, SFE has shown many applications for the extraction of a wide range of analytes from various matrixes.1-11 Working with SFE-CO2 and modifying the (1) Hedrick, J. L.; Mulcahey, L. J.; Taylor, L. T. Mikrochim. Acta 1992, 108, 115-132. (2) Barnabas, I. J.; Dean, J. R.; Owen, S. P. Analyst 1994, 119, 2381-2394. (3) Bowadt, S.; Hawthorne, S. B. J. Chromatogr. 1995, 703, 549-571. (4) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1996, 68, 487R514R. (5) Stuart, I. A.; MacLachan, J.; McNaughtan, A. Analyst 1996, 121, R11R28. (6) Snyder, J. L.; Grob, R. L.; McNally, M. E.; Oostdyk, T. S. Anal. Chem. 1992, 64, 1940-1946. (7) Schafer, K.; Baumann, W. Fresenius Z. Anal. Chem. 1989, 332, 884-889. (8) EPA Method 3560. Supercritical fluid extraction of total recoverable petroleum hydrocarbons. January 1995. (9) EPA Method 3561. Supercritical fluid extraction of polynuclear aromatic hydrocarbons. January 1995.
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experimental variables such as pressure, temperature, polarity of CO2, and matrix characteristics by using a modifier12,13 and collection method,14 the extraction of different analytes has been studied.12,15-18 Results from these investigations have shown that each of these SFE parameters could play a significant role in the extraction efficiency of the analytes. The predominance of one SFE parameter over another is dependent upon the sample matrix and the target analytes. One of the matrixes in which it is important to know the content of minor components is recycled plastic. Among the new recycled polymers, postconsumer films from agricultural uses can be mentioned. These films are mainly low-density polyethylene (LDPE, ∼90% w/w) and ethylene-vinyl acetate copolymer (EVA, ∼10% w/w) which are mixed in the recycling plant. As more and more plastic films are used in agricultural uses, especially in mulching, the amounts of this postconsumer residue has grown considerably. Mulching is a technique in which, in order to obtain higher yields at an earlier stage in the season, agricultural fields are covered to prevent loss of moisture.19 Plastic films have been used in Europe for this application for the last twenty-five years.20 In the Seville area, in the south of Spain, more than 5000 metric tons of plastic film, mainly based on LDPE, are annually used for the mulching of cotton. Even greater quantities are used in strawberry cultivation as well as other harvests. This film is normally laid in March or April and stays on the ground for 1-1.5 months. Once the film has served its role in helping the plants in the early stage of their development, it is removed and transported to a recycling plant. (10) EPA Method 3562. Supercritical fluid extraction of polychlorinated biphenyls (PCBs) and organochlorine pesticides. July 1995. (11) Hawthorne, S. B. Anal. Chem. 1990, 62, 633A-642A. (12) Levy, J. M.; Storozynski, E.; Ashraf-Khorassani, M. ACS Symp. Ser. 1992, No. 48, 336-361. (13) Levy, J. M.; Dolata, L.; Ravey, R. M.; Storozynski, E.; Holowczak, K. A. J. High Resolut. Chromatogr. 1993, 16, 368-371. (14) Ashraf-Khorassani, M.; Houck, R. K.; Levy, J. M. J. Chromatogr. Sci. 1992, 30, 361-366. (15) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987, 59, 1705-1708. (16) Ashraf-Khorassani, M.; Kumar, M. L.; Koebler, D. J.; Williams, G. P. J. Chromatogr. Sci. 1990, 28, 599-604. (17) Lopez-Avila, V.; Dodhiwala, N. F.; Beckert, W. F. J. Chromatogr. Sci. 1990, 28, 468-476. (18) Levy, J. M.; Ravey, R. M.; Houck, R. K.; Ashraf-Khorassani, M. Fresenius J. Anal. Chem. 1992, 344, 517-520. (19) Llop, C.; Pe´rez, A. Makromol. Chem., Makromol. Symp. 1992, 57, 115121. (20) Robledo de Pedro, F.; Martı´n Vicente, L. ANAIP, Madrid, Spain, 1981. S0003-2700(97)00219-9 CCC: $14.00
© 1997 American Chemical Society
One of the better alternatives is to recycle the plastic again for the same use.19 Unfortunately, as was previously demonstrated,21,22 LDPE and EVA used as agricultural soil covers can absorb pesticides and other chemicals applied to the crop. When these plastics are recycled, the pesticides remain in the plastic even after five recycling steps.21 Moreover, polymer recycling, which includes heating of the polymer above its melting point, could increase the degree of crystallinity, which, in turn, could more effectively retain the pesticides, making them much more difficult to extract. For these reasons, it is important to know both the concentrations and identity of pesticides remaining in the recycled plastic that is to be used again. The quantitative determination of chemicals in plastics, especially in LDPE, is a difficult task due to the special characteristics of the polymeric matrixes. The analytical process usually involves an extraction step in which the polymer can either swell the solvent in contact or be changed in its structure, but in any case, the quantitation of additives or contaminants in them is not guaranteed. When the polymer sample is thick or has a high degree of cristallinity, such as, for instance, in pellets, total dissolution is often necessary in order to achieve quantitative results. However, some polymers are very difficult to dissolve even using large volumes of organic solvents, and alternative extraction procedures such as SFE can be very appropriate.16,23-28 The present paper shows SFE applied to postconsumer recycled plastics used as agricultural soil covers, either in films or as pellets. The results obtained are compared to those obtained by both ultrasonic and total dissolution of the plastic film methods. The pesticides recoveries as well as the precisions obtained from the extraction methods have been compared. The behavior of modifiers in SFE and the interaction between supercritical CO2 and the polymeric matrix are discussed. Finally, recycled plastics from five sequential recycling cycles were analyzed by both optimized SFE and sonication extraction. The results obtained are discussed. Safety Considerations. All handling of pesticide standards should be performed while wearing disposable, impervious laboratory gloves. Skin contact with highly concentrated pesticide compounds can be extremely harmful. Many of these highly toxic materials, such as, for example, organophosphorus pesticides, are toxic by the dermal route. All handling of neat standards should be performed within a chemical fume hood or a glovebox to minimize inhalation of vapor and potential skin contact. If primary standard material should come in contact with the skin, thorough washing with copious amounts of soap and water should be performed immediately. Such an exposure should always be reported to supervisory personnel and a physician (21) Nerı´n, C.; Batlle, R.; Cacho, J. Proceedings of the VIII Jornadas de Ana´ lisis Instrumental, Barcelona, Spain, 1996. (22) Nerı´n, C.; Torne´s, A. R.; Domen ˜o, C.; Cacho, J. J. Agric. Food Chem. 1996, 44, 4009-4014. (23) Ashraf-Khorassani, M.; Levy, J. M. J. High Resolut. Chromatogr. 1990, 13, 742-747. (24) Ashraf-Khorassani, M.; Boyer, D. S.; Levy, J. M. J. Chromatogr. Sci. 1991, 29, 517-521. (25) Levy, J. M.; Cavalier, R. A.; Bosch, T. N.; Rynaski, A. F.; Huhak, W. E. J. Chromatogr. Sci. 1989, 27, 341-346. (26) Engelhardt, H.; Zapp, J.; Kolla, P. Chromatographia 1991, 32, 521-537. (27) Lou, X.; Janssen, H. G.; Cramers, C. A. J. Chromatogr. Sci. 1996, 34, 282290. (28) Cotton, N. J.; Bartle, K. D.; Clifford, A. A.; Dowle, C. J. J. Appl. Polym. Sci. 1993, 48, 1607-1619.
consulted at once. Delayed effects are characteristics of pesticides and prompt reporting of exposure is essential. EXPERIMENTAL SECTION Chlorpyrifos, 99.7%; procymidone, 99.9%; malathion, 98.5%; endosulfan-β, 98.8%; vinclozolin, 99.8%; tolclofos-methyl, 99.3%; 4,4′dichlorobenzophenone, 99.3%; bromopropylate, 99.6% and tetradifon, 99.9% were from Dr. S. Ehrenstorfer, GmbH (Augsburg, Germany). o,p-DDE was Certified Reference Material (Terdington, UK). Chlorobenzilate, 99.0%, was from Riedel de Ha¨en (Seelze, Germany). 2,4,5,2′,3′,4′-Hexachlorobiphenyl, PCB 138, used as internal standard, was from Chem Service (West Chester, PA). All the solvents used were from Merck Suprapur quality for gas chromatography (Darmstad, Germany). Carbon dioxide, SFE-60 grade, was supplied from Carburos Meta´licos (Barcelona, Spain). A Varian (Harbor City, CA) Star 3400 CX gas chromatograph, equipped with a 60 m × 0.25 mm i.d. SGL-5 (Sugelabor, Madrid, Spain) fused-silica column (0.25 µm film thickness) was used. An empty precolumn of 2 m × 0.32 mm i.d. (Supelco, Bellefonte, CA) was connected to the column after the injection port. The detector used was an ECD (63Ni) which was maintained at 300 °C. Hydrogen carrier gas flow was ∼1.5 mL/min. Injections (1 µL) were made in the splitless mode (injector split vent opened after 0.60 min) by a Varian 8200 autosampler. The injector was held at 210 °C. The initial column temperature was 50 °C, held for 1.00 min; ramped at 25 °C/min up to 215 °C, and held for 2.00 min; a second rate at 2 °C/min up to 250 °C and held for 1.50 min; a third rate at 25 °C/min up to 290 °C and held for 5.00 min. Quantitation was accomplished by relative heights vs PCB 138 used as internal standard, which was added just before the chromatographic injection. Standards were injected daily to verify the GC system response. A Prepmaster (Suprex Corp., Pittsburgh, PA)14 stand-alone supercritical fluid extraction system was used for all the extraction experiments. Extractions were accomplished with a 5.00 mL extraction vessel (Suprex) using operation conditions that are listed in Table 6. The filling of the extraction vessel was made in “sandwich” mode,29 i.e., silanized glass wool, anhydrous sodium sulfate as drying agent, sample, and, finally, more silanized glass wool. An off-line collection module, the Accutrap (Suprex),14,30 was used to perform the cryogenic adsorbent trap collection on a silanized glass bead cartridge (80/100 mesh, Suprex). This collection module also includes a liquid pump for delivering an appropriate liquid solvent for analyte desorption from the trap. The material under study is a postconsumer mixture of 90% (w/w) LDPE and 10% (w/w) EVA supplied as a film of 90 µm thickness and as pellets from EGMASA (Sevilla, Spain). Pellets were sequentially recycled five times in an injection machine.31 The following conditions of the injection machine were used: injection temperature, 30 ( 5 °C; average injection speed, 110 ( 30 mm/s; melting temperature, 195 °C; holding injection time, 20 s; cooling time, 40 s; maximum temperature of injector mouth, 210 °C. (29) Reimer, G. J.; Noot, D.; Sua´rez, A. Int. J. Environ. Anal. Chem. 1995, 59, 91-96. (30) Levy, J. M.; Houck, R. K. Am. Lab. 1993, 25, 36R-36Y. (31) Nerı´n, C.; Salafranca, J.; Cacho, J. Food Addit. Contam. 1996, 13, 243250.
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RESULTS AND DISCUSSION Experimental Design. The supercritical fluid extraction process involves two different and independent steps. The first one is the extraction of the interesting analytes from the matrix, and the second one is the collection of these analytes extracted from the supercritical fluid stream. Both steps have a close relationship, but they are controlled by independent variables. For this reason, the joint optimization of the two steps is not possible, and they have to be independently studied. The main variables affecting the first step, the extraction itself, are the temperature (Te) and pressure (P) of the extraction cell, which determine the most important extraction parameter, the density of supercritical fluid. This parameter has a great influence on the solubility ability of the supercritical fluid, through the wellknown equation
δ ) 1.25Pc0.5[F/F1]
(1)
where δ is the Hildebrand solubility parameter, Pc is the critical pressure of the fluid, F is the density of the supercritical fluid, and F1 is the density of the liquid gas under standard conditions.32,33 The flow (F) of supercritical fluid used in the dynamic step of the process is important since it determines, together with the dynamic extraction time, the total amount of supercritical fluid in contact with the sample. Very high values of CO2 flow are expensive and can produce a high dilution of the analytes, which can escape the trap used for collection. On the other hand, too low values provide a nonefficient extraction. From a general point of view, CO2 should be a reasonably good extracting agent for organochlorine pesticides having a typical δ value of 8-11 (cal/cm3)1/2, since CO2 at its critical pressure and temperature values (73 atm and 31 °C, respectively) has a Hildebrand solubility parameter of 10.7 (cal/cm3)1/2.34 However, these solubility considerations are only a part of the extraction problem. The extraction of a given analyte depends, among other features, on the ability of supercritical fluid to diffuse within the matrix, and consequently, it depends on the physical characteristics of the matrix. As a result of this behavior, the extraction conditions of the same group of pesticides are not the same in different matrixes as could be inferred based only on solubility criteria, and even more, such conditions also vary with the physical state of the same matrix. On the other hand, the variables involved in the second step, the collection process, are the adsorption and desorption temperatures from the cryogenic trap (Ta and Td, respectively) and the volume (V) and rate of solvent necessary to quantitatively recover the analytes from the trap. The optimization scheme used in this work is a slightly modification of the method proposed by Li and co-workers.35 The first step of the scheme is to define the criteria for optimization. A maximum of 15 min (static and dynamic) was established for the extraction time to enable an extraction to be carried out within a reasonably short period. This way, the amount of supercritical (32) Giddings, C. J.; Myers, M. N.; King, J. W. J. Chromatogr. Sci. 1969, 7, 276283. (33) Kane, M.; Dean, J. R.; Hitchen, S. M.; Tomlinson, W. R.; Tranter, R. L.; Dowle, C. J. Analyst 1993, 118, 1261-1264. (34) Monin, J. C.; Barth, D.; Perrut, M.; Espitalie, M.; Durand, B. Adv. Org. Geochem. 1988, 13, 1079-1094. (35) Li, K.; Ong, C. P.; Li, S. F. Y. J. Chromatogr. Sci. 1994, 32, 53-56.
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fluid in contact with the sample was only controlled by the flow used in the dynamic extraction step. The extraction efficiency appeared to be the highest achievable within the experimental range compared to both sonication and total dissolution methods. First, the optimization of the collection process was carried out. For this purpose, the conditions proposed by AshrafKhorassani and Levy23 for the extraction of additives from polyethylene films were selected. These conditions are Te 100 °C, P 450 atm, and F 2.0 mL. The next step of the scheme is to perform a set of pre-fixed experiments. For this work, 11 experiments were selected to optimize the parameters Ta, Td, and V. According to previous experience, the selected ranges of cited parameters were the following: Ta ranged from -15 to -50 °C; Td varied from 10 to 45 °C; and volume (V) of the solvent, n-hexane, in this case necessary to quantitatively wash the analytes from the trap, ranged from 0.7 to 1.7 mL. Table 1 shows the conditions tested for the collection of pesticides from recycled plastic film of 90 µm thickness as well as the concentration found for pesticides. As can be seen, the highest efficiency is achieved when Ta is -15 °C, Td is 10 °C, and V is 1.7 mL. The results obtained by using a Ta of -50 °C while the other variables remain constant were very similar. However, -15 °C was selected since the consumption of CO2 as cooling fluid was considerably lower. As the collection method is independent from the physical state of the matrix, the optimization of this step was carried out only with the postconsumer recycled film. The following step in the analysis is to optimize the extraction conditions for the extraction of these pesticides in recycled pellets (90% LDPE + 10% EVA, postconsumer mixture). The values tested in this case were as follows: Te varied from 25 to 75 °C; P ranged from 200 to 400 atm, and F ranged from 0.5 to 2.0 mL/ min. It can be observed from Table 2 that the highest efficiency of the extraction is obtained when the temperature is 75 °C at 400 atm of pressure and with a dynamic CO2 flow of 2.0 mL/min. Slightly higher values were obtained for some pesticides in different conditions, but the general behavior was the best under those mentioned above. Figure 1a shows a typical chromatogram. As the optimum values are the maximum selected, higher values of these variables were also tested and are shown in Table 2. As can be seen, the results obtained are very similar or, in most of cases, lower than those obtained by using the proposed conditions. As was mentioned above, from the literature and according to our previous experience, it seems clear that the optimum SFE conditions change if the matrix characteristics change. So, another optimization procedure must be applied to the analysis of recycled film. Table 3 shows the conditions tested and the concentrations found for the pesticides from this kind of matrix. For this matrix, the optimum SFE conditions found are Te 75 °C, P 200 atm, and F 2.0 mL/min. Figure 1b shows a typical chromatogram. The extraction values obtained from pellets were very much lower (50-95%) than those obtained by using total dissolution method. This difference could be attributed to the difficulties in extracting the compounds from the pellets. As it is well known, the sequential steps of melting and cooling the polymer during the serial recycling process increase the
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0.263 0.226 0.125 0.334 0.101 0.296 0.242
