Determination of Chlorinated Pesticides in Soil by Solid Phase Extraction Gas Chromatography

Transcription

1 ANALYTICAL SCIENCES MARCH 1999, VOL The Japan Society for Analytical Chemistry 283 Determination of Chlorinated Pesticides in Soil by Solid Phase Extraction Gas Chromatography Mohammad A MOTTALEB and Mohammad Z ABEDIN Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL, UK A multi-residue technique is presented for the extraction and quantitative determination of some widely used organochlorine pesticides such as lindane, heptachlor, aldrin, dieldrin, endrin and p,p -DDT in soil. The chlorinated pesticides were extracted from soil with different solvents, followed by clean-up of the sample extract using C 18 solid phase extraction cartridge; the analytes were eluted with hexane and determined by gas chromatography with electron-capture detection. Average recoveries exceeding 89% were obtained for the pesticides. The solid phase extraction method was applied to extract the lindane from contaminated soil and soil-leachates. Column leaching tests were performed; we found some movement of the lindane between operationally defined soil phases or layers. Keywords Organochlorine pesticides, soil, solid phase extraction, leaching test, gas chromatography, recovery Organochlorine pesticides (OCPs) have become popular topics of research and have attracted the attention of environment scientists because of both their resistance to degradation and their toxicity to animals, including human beings. Pollution due to OCPs has aroused the significant interest in determination of the OCPs species in water, soil, and sediment samples. Analysis of these compounds by liquid-liquid extraction (LLE) and subsequent purification by column chromatography involves tedious preparation and requires fairly large volumes of solvents, which produce huge amounts of waste. The method often requires several hours of extraction times to yield acceptable, although sometimes incomplete recoveries. In recent years a new technique, the solid phase extraction (SPE) with reversed bonded-phase silica sorbent, has been developed for extraction and enrichment of the organic compounds from liquid samples. 1 7 The technique presents a viable alternative to conventional LLE methods for many compounds that are present in trace levels in environmental matrices. Although the application of the SPE method for analysis of OCPs in water samples has been investigated 8 14, there are few examples of applications to soil sample extraction. Balinova and Balinov 15 described a method for the extraction of some chlorinated pesticides using a glass column and analysis by gas chromatography (GC). The pesticides were purified using a mini-column To whom correspondence should be addressed. M. A. M. present address: Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh. M. Z. A. present address: Department of Chemical Technology and Polymer Sciences, Shahjalal University of Science and Technology, Sylhet 3100, Bangladesh. packed with Florisil and Al 2 O 3. The clean-up procedure was laborious and the extract was evaporated, which is again time-consuming. Seidel et al. 16 compared the simultaneous steam distillation and solvent extraction (SDE) with Soxhlet and supercritical fluid extraction (SFE). The SDE method requires a complicated device and Florisil columns were used for cleaning-up the OCPs. Although steam distillation was useful for particular application, it provided the variable results and was not applicable to all compounds in this class. Lopez-Aviala et al. 17 evaluated the Soxhlet extraction procedure for extracting OCPs from soils and sediments. The method involves a 3-stage extraction procedure with a temperature-controlled oil bath. The method is relatively less laborious, but error-prone manual manipulations are difficult to eliminate. The present study demonstrates a rapid and reliable method for analysis of six persistent chlorinated pesticides (Fig. 1) in soil samples, applying the SPE technique and determination by GC with electron capture detection (ECD). Furthermore, leaching tests were performed on lindane-contaminated soils to obtain information about whether lindane becomes mobilized or immobilized in soil, which could be useful in predictions of the fate of this pollutant. Experimental Reagents Methanol (HPLC grade) and hexane (AR grade) were obtained from Rathburn Ltd., UK. A pesticide mixture containing lindane, heptachlor, aldrin, dieldrin, endrin and p,p -DDT was obtained from Sigma Chemical Ltd.,

2 284 ANALYTICAL SCIENCES MARCH 1999, VOL. 15 Soil samples The soil samples were obtained from the ICI, Wilton Center, UK. The non-contaminated soils were sampled from known sites and after transportation to the laboratory were air-dried; all large stones and extraneous materials were removed by hand. The lindane-contaminated samples were collected from three different layers. The physical descriptions of the samples are: (i) Layer 1 (horizon 2 13 cm; depth 0 15 cm); samples were the mixtures of top soil and clinker and very dark brown color, having common stones with high proportion of clinkers; (ii) Layer 2 (horizon cm; depth cm); samples were ground and dominated by clinker and finer ash, having dark gray matrix with yellow color and common clinker fragments; (iii) Layer 3 (horizon cm; depth cm); samples were made-up of ground dominated by clinker and grayish brown color, having larger clinker fragments than Layer 2. All the samples were stored in air-tight containers until required. Fig. 1 Organochlorine pesticides [(i) lindane, (ii) heptachlor, (iii) aldrin, (iv) dieldrin, (v) endrin, (vi) p,p -DDT]. UK at the concentration range from 2000 to 5000 µg ml 1 in methanol. A stock solution of 50 ml was prepared by diluting 10 µl of the mixture with hexane. The internal standard (heptachlor epoxide) was made by diluting the supplied stock (1 mg ml 1 ) to 10 ml with hexane. The stock solution was serially diluted to prepare the final working solutions. For solid phase extraction, all solutions were made in 70% aqueous methanol. Apparatus A Pye Unicam GC equipped with a 63 Ni ECD, connected to a Dionex 4400 integrator or plotter was employed. A 5 µl glass syringe was used to inject 1 µl volume of the sample into a DB-5 fused silica megabore column (30 m 320 µm i.d.; 0.25 µm film thickness) manufactured by J&W Scientific (Folson, CA, USA). 18 The GC operating conditions were: injector temperature 250 C, oven temperature 220 C, detector temperature 300 C, carrier gas (nitrogen) flow rate 1.0 ml mil 1, make-up nitrogen flow rate 30 ml min 1. For SPE, octadecyl (C 18 ) and octyl (C 8 ) cartridge or column (SuplcCo. Ltd., UK) of 3 ml and 6 ml capacity with 500 mg packing material were used. Extraction of pesticides from non-contaminated soil A 5 g soil sample was spiked with a known amount of analytical standards of the pesticides. The soil was thoroughly mixed with pesticide solutions in methanol (10 ml). To ensure homogenization, the spiked samples were sonicated for 30 min. After evaporation of the solvent, the soil was extracted with 50 ml of (i) acetone:water (2:1), (ii) methanol:water (2:1) and (iii) methanol for 2 h. The extract was then filtered (Whatman grade 41) and the filtrate was passed through the solid phase extraction columns. Unspiked soil samples were treated similarly. Extraction of lindane from contaminated soil (a) Extraction with shaking: Each soil sample (5 g) was extracted three times with 35 ml of methanol for 6 h. After extraction, water was added to make 70% aqueous methanol. The extract was filtered through Whatman grade 41 filter paper. For SPE 75 µl of the filtrate and 75 µl (corresponding to 75 ng ml 1 ) of the internal standard were diluted to 2 ml with aqueous methanol. (b) Extraction under sonication: A frequency sweeping ultrasonic bath (Decon FS 200, Sussex, UK) was used for sonication. Each sample (5 g) was extracted with 35 ml methanol for 2 h. To homogenate the mixture, 15 ml of water was added at the end of extraction. The extract was then filtered. For SPE, 75 µl of the filtrate and 75 µl (corresponding to 75 ng ml 1 ) of the internal standard were diluted to 2 ml with aqueous methanol. Column leaching experiment The experimental set-up consisted of two plastic pot columns containing 830 g of each type of soil (column A: soil from 2 13 cm horizon; column B: soil from cm horizon). Silica filter paper was placed on the top of the columns. Artificial rain water was applied to two soil columns A and B. The amount of artificial rain water passed through the column (500 ml, applied five times 100 ml aliquots) was equivalent to one months natural rainfall per week. After percolation through the soil, the leachates were collected, filtered and applied to SPE. Soil samples were taken from columns for lindane analysis. The composition and

3 ANALYTICAL SCIENCES MARCH 1999, VOL Table 1 Characteristics of artificial rain water ph 5.2 Conductivity 55 ms Chloride 10 mg ml 1 Nitrate 0.5 mg ml 1 Sulfate 2.4 mg ml 1 Calcium 0.3 mg ml 1 Magnesium 0.4 mg ml 1 Potassium 0.3 mg ml 1 Sodium 6.0 mg ml 1 Cadmium <0.01 ng ml 1 Copper <0.10 ng ml 1 Manganese <0.10 ng ml 1 Zinc <5.0 ng ml 1 amount of artificial rain water used in the experiment was based on analysis and annual precipitation figures in Glasgow rain water. The artificial rain water was fully characterized before being applied to the soil columns. The parameters are shown in Table 1. Solid phase extraction The solid phase extraction was essentially carried out according to the method described by Deans et al. 2 The SPE cartridges were solvated by passing through them two column volumes of HPLC grade methanol, followed by two columns volumes of 1% aqueous methanol for pre-equilibration. Sample extracts (2 ml) were then passed through the column. The interference elution was carried out by ml of 1% aqueous methanol. The columns were air-dried by drawing air Table 2 Recoveries (%) of selected organochlorine pesticides after SPE with 500 mg C 18 cartridge using different eluents (pesticide concentration: ng ml 1 ) Pesticide Lindane Heptachlor Aldrin Dieldrin Endrin p,p -DDT through them for 10 min. Compounds retained in the cartridge were eluted with 2 ml of Analar grade hexane for GC analysis. Results and Discussion Ethylacetate Recovery, % Hexane GC optimization The OCPs mixture were separated by use of fused silica column. The gas chromatogram of a mixture of six organochlorine pesticide standards is shown in Fig. 2. The pesticides were resolved and eluted in a reasonable time from the column under the GC conditions employed. The GC conditions, such as oven temperature, carrier gas flow rate, injector and detector temperatures, were optimized and this resulted in a good resolution of the separated solutes. The optimum conditions are described in the experimental section. Factors affecting the solid phase extraction Various factors affecting the reliability of SPE procedure have been studied. The sample loading rate and analyte elution rate were determined in order to maximize the recovery of the analytes. Maximum recoveries were obtained when samples were loaded at a rate of 3 4 ml min 1 and analyte elution was carried out at a flow rate of 2 3 ml min 1. Fig. 2 GC-ECD chromatograms of standard six organochlorine pesticides. Conditions: standard pesticide solution containing 20 ng ml 1 each of lindane, heptachlor, aldrin, heptachlor epoxide (internal standard) and 50 ng ml 1 each of dieldrin, endrin and p,p -DDT, injected volume 1 µl, column DB-5 fused silica megabore (30 m 320 µm i.d.; 0.25 µm film thickness), injector temperature 250 C, oven temperature 220 C, detector temperature 300 C, carrier gas (nitrogen) flow rate 1.0 ml mil 1, make-up nitrogen flow rate 30 ml min 1. Peak identitiy: a, lindane; b, heptachlor; c, aldrin; d, heptachlor epoxide (internal standard); e, dieldrin; f, endrin; g, p,p -DDT. Selection of eluting solvent and column The recoveries obtained when 2 ml of standard pesticide samples were passed through a C 18 cartridge and eluted with 2 ml of hexane and ethyl acetate, are given in Table 2. The results show that slightly higher recoveries were obtained with elution when hexane was considered as solvent. When ethylacetate was used as an eluent, the resolution of the peaks was also poorer. However ethylacetate has been used to elute organophosphorous pesticides from SPE columns. 13 The C 18 and C 8 sorbents were compared for the recoveries of pesticides. The recoveries obtained when 2 ml sample were passed through the C 18 and C 8 cartridges and the pesticides eluted with 2 ml of hexane are given in Table 3. For all the pesticides, the recoveries followed the order C 18 >C 8. This is expected 19 as C 18 is more non-polar than C 8 and therefore, most strongly

4 286 ANALYTICAL SCIENCES MARCH 1999, VOL. 15 Table 3 Recoveries of analytes eluted with hexane from C 18 and C 8 columns (pesticide concentration: ng ml 1 ) Analyte C 8 column % Recovery C 18 column Lindane Heptachlor Aldrin Dieldrin Endrin p,p -DDT Mean results of two determinations. Table 4 Impact of column drying time (pesticide concentration: ng ml 1 ) Fig. 3 Effect of load volume on the recoveries of chlorinated pesticides with C 18 sorbent. Analyte % Recovery 0 min 5 min 10 min 30 min Lindane Heptachlor Aldrin Dieldrin Endrin p,p -DDT Elution volume: 2 ml(hexane); mean results of two determinations; 0.5 g sorbent column. retains the essentially non-polar pesticides (Fig. 1). This is reasonable due to the mechanism of solid bonded phase extraction which is based on non-polar interaction occurring between the column-hydrogen bonds of the sorbent and the carbon-hydrogen bonds of the analyte. Removal of all residual water from the column after application of the samples is a necessary step to maximize the analyte recoveries. Impact of the column drying time was studied by passing air through the column up to 30 min. The results are given in Table 4. Although significant losses were not observed for column drying time extended up to 30 min, the optimum time for column drying using vacuum from water suction has been found as 10 min, for a 0.5 g sorbent column. Less than maximum recoveries were obtained by column drying extended up to 30 min. This is expected, as the drying the column for such a long time might cause some evaporative losses. Effect of load volume One of the factors that affect SPE recovery is the volume of the sample passed through the column bed. In order to investigate the effect of the sample volume on the 500 mg SPE column, a number of pesticide solutions were made and up to 200 ml was loaded onto the C 18 column (3 ml capacity). Some constant analyte masses were introduced onto the column. There were no appreciable changes in the recovery rates up to a Fig. 4 Elution characteristics of (A) lindane, heptachlor, aldrin and (B) dieldrin, endrin and p,p -DDT. load volume of 160 ml. The recoveries of 32 µg of each of the lindane, heptachlor, aldrin and 80 µg of each of dieldrin, endrin and DDT as shown in Fig. 3. Lindane and heptachlor suffer some losses with increasing the loading volume up to 200 ml. This might be attributed to poor retention of the analytes such as high volume in 500 mg sorbent with only 3 ml capacity. To investigate the preferential elution of the SPE cartridge, a 2 ml aliquot of the standard pesticide solution was introduced to C 18 column and the analytes were eluted with hexane. The hexane eluate was collected dropwise in pre-weighed glass vials which were

5 ANALYTICAL SCIENCES MARCH 1999, VOL Table 5 Recovery of pesticides from non-contaminated soil (pesticides concentration: 4 10 µg kg 1 ). Analyte Acetone:water (2:1) Methanol:water (2:1) Methanol % Rec SD RSD, % % Rec SD RSD, % % Rec SD RSD, % Lindane Heptachlor Aldrin Dieldrin Endrin p,p -DDT Table 6 Amount of lindane extracted from contaminated soil (µg g 1 )(solvent=methanol) Horizon/ cm 2 13 (Layer 1); depth 0 15 cm (Layer 2); depth cm (Layer 3); depth cm Extraction method Shaking Sonication 31.13± ± ± ± ± ±3.8 then re-weighed and analyzed individually by GC. The results were used to prepare a cumulative elution profile for each of the analytes. Figure 4 represents the results of the elution profile. It can be seen that all of the pesticides were eluted in the first 1.5 ml fraction of the eluent and none in the subsequent fraction. Effect of extracting solvents on recovery Recoveries of the analytes under study, obtained by using different solvent systems are presented in Table 5. It has been found that the recoveries are higher when methanol was used as the extracting solvent. Extraction with acetone:water (2:1) and methanol: water (2:1) provided the relatively lower recovery and poor repeatability of the results. The relatively lower recovery by mixed solvent agrees with reports of other workers. 15 It can be explained by the fact that the presence of water can affect the analyte recovery due to the difference in soil hydration. The recovery results indicate that the choice of solvents and solvent types, elution and loading flow rates, column drying time etc. are important factors to maximize the pesticide recoveries from the soil. Determination of lindane in soil sample The amount of lindane obtained from the soil samples of different horizons was investigated and is presented in Table 6. The results depict that the amount of lindane in horizon cm was found to be higher that of the horizons 2 13 cm and cm. The amount of lindane in horizon cm was significantly lower, corresponding to µg g 1 of soil. The Fig. 5 Amount of lindane removed versus volume of leachate. transport of lindane by rain water during subsoil passage brings about the difference in the amount at different soil layers. Leaching out of lindane from soil sample by rain water The amount of lindane which leached out from the columns A and B after each application of 500 ml of rain water was determined. The results were used to prepare the cumulative removal of profiles for lindane. This is shown graphically in Fig. 5. This illustrates that lindane is continuously being leached out after each application of the rain water through the columns. The soil samples taken from the columns A and B after leaching experiment were subjected to lindane analysis (Table 7). The total amount of lindane in leachate from column A was found to be 4.0 mg in 6.0 l of rain water. The amount of lindane in 830 g soil of column A was mg before leaching and mg after leaching. The recovery of this sample corresponds to 91.95%. Similarly, in the soil horizon cm, total lindane before leaching was found as mg and mg after leaching. An amount of 3.4 mg was found to be present in 3.5 l leachates. The recovery was 89.23%. One of the leachates was spiked with a known amount of lindane. The spiked rain water leachate was extracted with SPE and analyzed and 89.50% of the lindane was recovered. In conclusion, solvent extraction of chlorinated pesti-

6 288 ANALYTICAL SCIENCES MARCH 1999, VOL. 15 Table 7 Recovery (%) of lindane from the polluted soils Soil sample Amount of lindane before leaching Amount of lindane after leaching Difference /mg Amount of lindane found in leachate/mg Recovery,% 2 13 cm (horizon) mg/830 g mg/830 g cm (horizon) mg/830 g mg/830 g cides from soil samples with methanol is a suitable technique for sample preparation. Solid phase extraction can be applied to a clean-up of crude sample extracts from soils in analyzing the selected pesticides. Successful application of the method requires careful consideration of certain factors, such as sorbent types, loading and elution flow rates, and sorbent drying time. The method provides analysis of chlorinated pesticides in soils and water; recoveries of over 90% for most of the compounds could be achieved. Leaching tests on soils exhibited that lindane can be leached out by rain water from soil, thereby causing pollution to natural waters. The authors would like to thank Dr. C. J. Dowle, ICI, UK for supplying the soil samples and supplementary information. The authors would also like to thank Professor D. Littlejohn and Miss A Duncan for giving the laboratory facilities and rain water results. References 1. Y. Odanaka, O. Matano and S. Goto, Fresenius J. Anal. Chem., 339, 368 (1991). 2. I. S. Deans, C. M. Davidson and D. Littlejohn, Analyst[London], 118, 1375 (1993). 3. H. W. Newsome and P. Andrews, J. AOAC Int., 76, 707 (1993). 4. I. J. Barnabas, J. R. Dean, S. M. Hitchen and S. P. Owen, Anal. Chim. Acta, 291, 261 (1994). 5. J. R. Dean, I. J. Barnabas and I. A. Fowlis, Anal. Proc. Including Anal. Commun., 32, 305 (1995). 6. G. Font, J. Manes, J. C. Motto and Y. Pico, J. Chromatogr., 642, 135 (1993). 7. R. J. Ozretich and W. P. Schroeder, Anal. Chem., 58, 2041 (1986). 8. I. Cruz and D. E. Wells, Anal. Chim. Acta, 283, 280 (1993). 9. G. A. Junc and J. J. Richard, Anal. Chem., 60, 451 (1988). 10. W. E. Johnson, N. J. Fendinger and J. R. Plimmer, Anal. Chem., 63, 1510 (1991). 11. J. Sharf, R. Wiesollek and K. Bachmann, Fresenius J. Anal. Chem., 342, 813 (1992). 12. J. Beltran, F. J. Lopez and F. Hernandez, Anal. Chim. Acta, 283, 297 (1993). 13. G. H. Tan, Analyst[London], 117, 1129 (1992). 14. C. L. de la Colina, F. Sachez-Rasero, G. D. Cancela, E. R. Taboada and A. Pefia, Analyst[London], 120, 1723 (1995). 15. A. M. Balinova and I. Balinov, Fresenius J. Anal. Chem., 339, 409 (1991). 16. V. Seidel and W. Linder, Anal. Chem., 65, 3677 (1993). 17. V. Lopez-Avialia, K. Bauer and J. Milanes, J. AOAC Int., 76, 864 (1993). 18. Dionex-Application Note K. C. V. Horne, Sorbent Extraction Technology, Analytichem Internaltional, Harbor City, (Received March 2, 1998) (Accepted November 10, 1998)