A Novel Technique for Rapid Extraction of Volatile Organohalogen

Environ. Sci. Technol. 2003, 37, 3978-3984

A Novel Technique for Rapid Extraction of Volatile Organohalogen Compounds from Low Permeability Media† IULIANA DINCUTOIU,‡ TADEUSZ GO Ä R E C K I , * ,‡ A N D BETH L. PARKER§ Departments of Chemistry and Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada

A new method for rapid extraction of Volatile Organic Compounds (VOC) from low permeability, unconsolidated geologic media such as clayey soil has been developed and tested using trichloroethylene (TCE) as the model compound. The technique is based on a combination of sonication and mechanical agitation of the samples. The sample vials, mounted in a special holder attached to an orbital shaker, were immersed in an ultrasonic bath during extraction. The extracts were analyzed by gas chromatography with electron capture detection (GC-ECD). The method was validated using clay samples from a TCEcontaminated industrial site by comparing TCE recoveries to those obtained by the standard methanol extraction. In all cases, the recoveries obtained with the new method were the same or better than the recoveries obtained with the reference method. The extraction time was shortened from 5 days with the standard method to less than 2 h with the new method. The new technique makes it possible to analyze a large number of samples in a short time, without the need for sample preservation and prolonged storage. It has good potential for on-site analysis to facilitate decisions while field investigations are in progress.

Introduction Pollution of water, air, and soil with Volatile Organic Compounds (VOCs) is considered a severe problem due to the impact these compounds have on living organisms. Many contaminated industrial sites exist across North America and Europe where VOCs such as chlorinated solvents occur in fine-grained geologic deposits. The need to conduct risk assessments and develop remediation plans prompts investigations to determine the subsurface VOC distribution in these clayey or silty soils. However, soil is one of the most difficult matrices to analyze. The extent to which analytes are attached to the matrix determines their recovery during extraction. Among various types of soil, low permeability clayey media are one of the most difficult to analyze because of the matrix characteristics. * Corresponding author phone: (519)888-4567 x 5374; fax: (519)746-0435; e-mail: [email protected]. † Presented in parts at Pittcon 2002, New Orleans, LA, March 1722, 2002 and Enviroanalysis 2002, Toronto, ON (Canada) May 27-30, 2002. ‡ Department of Chemistry. § Department of Earth Sciences. 3978

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The mineralogical composition, particle size, density, and porosity are some of the important parameters to be considered when VOCs are extracted from soil. The adsorption capacity of VOCs in soils is determined to a large extent by the available surface area.1 It is also strongly related to the diffusivity of a contaminant in soil.1,2 The adsorption capacity determines the matrix capability to release the compound during extraction. It varies with the water content of the soil and aging as well as chemical and physical properties of the soil.3,4 In a study of VOC sorption onto clay, sand, and limestone, it was found that clay had the highest sorption capacity.1 In the investigations reported in this paper, the soil samples were obtained from a site where the VOC contamination has been in the ground for a few decades. This is common for most VOC contaminated field sites. In permeable deposits such as sand or gravel, emphasis in contaminated site investigations is placed on sampling the groundwater using wells or other devices. In recent years the use of mobile, on-site laboratories that conduct field VOC analyses in groundwater has become common. Although there is a similar need for field VOC analysis of clayey soil, no suitable methods exist. The long time required to extract VOCs from clay by the currently used methods is very inconvenient (especially when a large number of analyses must be performed) and renders field analysis for the evaluation of the level of contamination practically impossible. Field analysis minimizes the time between sample collection and analysis. This eliminates to a large extent the problems related to contamination, losses, and/or degradation of the sample during transportation and storage. Onsite investigations produce immediate results, supporting real-time decisions, interactive sampling, and cost-effective solutions, especially when corrective measures are immediately required. The goal of this study was to develop a method that would produce results equivalent to those obtained by the method currently in use but in a much shorter time. Because no standards or reference materials were available for the experiments, the only way to assess the performance of the new method was to compare the results obtained by this method for each individual sample to the results obtained for the same samples after they were allowed to equilibrate with the extracting solvent (methanol), as in the standard procedure.

Background Extraction of organic analytes from soils is still conducted mostly by Soxhlet extraction and flask extraction. Both these methods are time-consuming and require large amounts of solvents.5-7 Soxhlet extraction is generally not suitable for VOC extraction from soils, due to large VOC losses when refluxing the solvent for the typical long extraction times of this technique. Besides, further losses occur if concentration of the extract is required. A number of new techniques for the determination of organic compounds in solid matrices have been developed recently; however, little attention has been paid to extraction from low permeability media, such as clay. Modern extraction techniques that can be useful in the extraction of organics from soils include Pressurized Liquid Extraction (PLE) (also called Accelerated Solvent Extraction (ASE)), Supercritical Fluid Extraction (SFE), Microwave Assisted Extraction (MAE), headspace analysis (HS), purge-and-trap (PT), ultrasonic extraction, and Solid-Phase Microextraction (SPME). Many of these techniques proved to be superior to Soxhlet extraction considering factors such as solvent consumption, extraction time, and analyte recovery 10.1021/es0340275 CCC: $25.00

 2003 American Chemical Society Published on Web 08/06/2003

for soils with high to medium permeabilities.3,6,8-10 Both ASE and SFE yield good recoveries in a short time for some semi- and nonvolatile organic compounds in soil samples,7 but they are generally not well suited for VOC determinations. In both techniques, losses of volatile analytes might occur at the extract collection stage (hot solvent discharge to a vented flask in ASE; supercritical fluid decompression in SFE). The instrumentation is expensive, and successful analyte recovery depends among others on the type of soil.7-9 MAE is also used primarily for semi- and nonvolatile analytes. It is less expensive than many of the new methods. Nevertheless, the technique is not very popular. One reason may be the fact that the effectiveness of MAE is highly dependent on matrix characteristics.11 In all three techniques, further VOC losses might occur if concentration of the extract is required. The most popular techniques for VOC analysis in soil samples are static headspace and purge-and-trap.3 The latter is recommended by U.S. Environmental Protection Agency (U.S.-EPA) for volatile contaminants that have diffused onto external surfaces but not into internal micropores of soil.9,12 Consequently, this method is not suitable for VOC analysis in clay samples. Many previous studies have shown that hot methanol extraction is more efficient than the approved purge-and-trap and headspace techniques in terms of accuracy regardless of the type and age of the contaminated soil.12,13 Static headspace analysis requires a thermodynamic equilibrium between the solid sample and the gas phase. This equilibrium is usually difficult to achieve. High temperatures are required to increase the diffusivity and the gas/solid distribution ratio of the volatile analytes, and that may lead to analyte losses. In addition, quantitative calibration is very difficult in this method.14 Static headspace can be used for matrices that behave as though the VOCs are “dissolved” in the matrix, while soils represent an adsorption system where the VOC distribution is determined by adsorption coefficients.14 Consequently, this method in its typical implementation is not suitable for clay samples. SPME is useful mainly for gaseous and liquid samples. It relies on analyte equilibration between the sample and the extracting device (SPME fiber). Thus, all factors affecting equilibration must be controlled for accurate quantitative analysis.15 Despite all the advantages of regular SPME in the analysis of air and water samples, it has severe limitations when it comes to quantitative analysis of soil samples,9 including the analysis of volatile compounds.14 Zhang and Pawliszyn16 reported good recoveries of BTEX from spiked clay samples with the use of an internally cooled SPME fiber, but the device is not available commercially. Solvent extraction is a very reliable method for VOC evaluation in soil.13,17 Hewitt concluded that methanol extraction is a more reliable method of recovering VOCs from soil compared to methods based on vapor partitioning.13 Sonication extraction using focused high-intensity sound waves is an enhanced solvent extraction technique. Vibrations in the ultrasonic range create cavitation in the solvent and the porous medium, resulting in high differential fluid velocities. These velocity perturbations are capable of overcoming the forces binding soil particles to each other and clay microparticles to the soil matrix.18 In addition, ultrasonic radiation generates heat, which further aids the extraction process. Sonication improves mass transfer conditions and helps disintegrate larger particles. U.S.-EPA recommends the use of sonication for the extraction of nonvolatile and semivolatile organic compounds from soils.19 The instrumentation is simple and the cost is low. To the best of our knowledge, few if any articles were published with regard to VOC analysis in low permeability media. Difficulties in measuring VOCs in soil are under

investigation by the U.S.-EPA. This paper presents the results of our research on the development of a sonication extraction method for the analysis of trichloroethylene (TCE) in natural clayey samples. The analyte is a widely spread environmental pollutant, found at many contaminated sites.

Experimental Section Solvents and Standards. HPLC grade methanol (Fischer Scientific) was used for TCE extraction from natural clayey samples. HPLC grade hexane, 99+% purity (Sigma-Aldrich) was used as exchange solvent for GC analysis. Sigma-Aldrich 1, 2-dibromoethane (DBE) of 99.5% purity, and BDH-assured grade TCE of 99.5% purity were used for standards and solvent preparation. Initially, DBE was used as internal standard for GC injection. In further experiments, DBE was used as a surrogate standard in methanol extraction to compensate for possible losses during sample preparation and the entire process of extraction and assessment of TCE recovery. DBE concentration in the injection solvent, hexane, ranged from 500 to 550 ppb. DBE concentration in methanol was around 38.8 mg/L, so that in the injection solvent its concentration was 500-550 ppb. Standards of TCE with DBE as internal standard were prepared for eight levels of concentration, starting with 1 ng/mL to 3 µg/mL (final concentrations of TCE in hexane). Samples. Soil samples were collected from an inactive industrial site in Kitchener, Ontario, where TCE has existed in the subsurface for at least 20 years.20 The TCE occurs in various geologic layers, from which a silty clay layer was selected to obtain the core samples for this study. Care was taken to collect the samples from the same location, at a depth of about 5 m. The clayey layer is classified geologically as a glacial till, which is a common type of deposit in the glaciated interior region of North America. This layer is comprised mineralogically of quartz, feldspars, carbonate minerals, and nonswelling clay minerals, primarily illite. The layer occurs below the water table and, therefore, under field conditions, is water saturated. The wet and dry bulk densities are 2.24 and 1.96 g/cm3, respectively, solid density of 2.88 g/cm3, porosity of 0.3, and fractional organic carbon of 0.24% dry weight basis.20 Standard Procedure. The standard method used in our laboratory for the determination of VOC concentrations in low permeability soils was described by Parker.21 In this method, concentrations are determined on small subsamples from cores using methanolic extraction. Only a brief description is provided here. The core segments were collected using equipment described by Zapico et al.22 A core barrel (1.52 m length) was driven into the ground using a pneumatic hammer. While the barrel was driven, the soil core entered an aluminum tube (5.1 cm outside diameter) held inside the barrel. Inside the tube, a piston connected by wire-line to surface maintained suction to prevent soil from falling out, ensuring complete recovery and retention of pore water. Immediately after raising the coring system from the ground, the aluminum tube containing the core was removed from the barrel and Teflon caps were placed on the ends, followed by preservation at 4 °C. At the time of subsampling for VOCs, the core tube was split longitudinally and stainless steel subcoring devices were used to collect samples of around 6-10 g, which were placed in 25 mL vials with screw caps and Tegrabond septa. The subsampling procedure was done fast and care was taken to wrap the two pieces of the core in aluminum foil in order to avoid significant losses of the analyte(s). Each vial was weighed before and after collection of the clay sample. Twenty milliliters of HPLC grade methanol (MeOH) were added to each vial, which was then agitated vigorously for 1 min using a standard Mini-vortex stirrer (VWR) and for another 30 min on an orbital shaker model 3520 (Lab-Line Instruments, Inc., Melrose Park, IL) at 300 VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. TCE Concentration in Diluted Clay Extracts Obtained by the Standard Method Currently in Use in the Laboratory TCE concn TCE concn after shaking after 5 days % increase average sample (ng/mL) (ng/mL) after 5 days increase% RSD%

FIGURE 1. Schematic diagram of the device for simultaneous shaking/sonication. rpm. Samples prepared in this way were allowed to equilibrate for 5 days, following which they were centrifuged for 30 min with an IEC CRU-5000 centrifuge (Damon/IEC Division) at 1700-1800 rpm. Samples for final analysis by gas chromatography were prepared by adding 20 µL of the MeOH extract to 1.5 mL of hexane containing DBE as the internal standard. Methanol was the extraction solvent of choice because it efficiently penetrates the intraparticle regions, which results in an increase in solute diffusivity and allows a reasonable detection limit to be achieved. Since methanol is very polar, it is incompatible with the nonpolar GC column stationary phase used in the final determination of VOC. Thus, solvent exchange was required before the extract was injected into the GC. Analyses were performed on an HP6890 GC equipped with a µ-ECD detector and a capillary fused silica column, 30 m × 530 µm × 3.0 µm DB-624 (J&W). The carrier gas, helium (Ultrahigh Purity, Praxair), was set to a constant flow of 8.1 mL/min. The injection was performed in pulsed splitless mode. An aliquot of 1 µL of the liquid sample was injected using the autoinjector. The injector temperature was set to 225 °C, the column was kept for 3 min at 55°C, followed by an increase to 105 °C at a rate of 30 °C/min. At this temperature the column was held for 2 more min. The detector was operated at 300 °C using nitrogen (ECD grade, Praxair) as the makeup gas at a flow rate of 60 mL/min. The limit of detection for the method is ∼0.15 mg/kg soil (w.w.) for a 10 g soil sample. The linear working range of the method is 0.45-450 mg/kg soil. It can be extended to higher concentrations by spiking the hexane with a smaller volume of the methanolic extract or by diluting the extract. On the other hand, the limit of detection of the method can be improved by a factor of 76 (down to ∼2 µg/kg) by elimination of the solvent exchange step. This can be achieved by using direct cool on-column injection of the methanolic extract to the GC column.23 Sonication. A CREST TRU-SWEEP sonicator model 275D was used, featuring digital control of heat, time, and power. The TRU-SWEEP technology ensures uniform distribution of the ultrasound energy in the sonicating bath by eliminating standing waves and the resultant “hot spots”. The power level was set at maximum (corresponding to 90W sonic power), and the frequency of the ultrasound radiation was 38.5 kHz. Since the bath tended to overheat slightly during prolonged sonication, water circulation was introduced by slowly adding cold running water to the bottom of the bath and draining warm water from the top. For the experiments involving sonication followed by mechanical agitation, the procedure consisted of initial mechanical agitation of the samples (as in the standard procedure), followed by sonication at 45 °C for 0.5 h and mechanical agitation for an additional 0.5 h. A special device was developed to allow simultaneous sonication and mechanical agitation of the samples. The design of the device is illustrated in Figure 1. An aluminum plate equipped with three vertical aluminum rods, each provided with two clamps to hold the vials, was connected 3980

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

584 1142 536 986 27 56 19 43 27 56 39 55 111 111 112 77 114 53 274

676 1497 702 1217 47 80 30 56 47 80 60 71 138 168 173 106 161 92 372

15.8 31.1 31.0 23.4 74.1 42.9 57.9 30.2 74.1 42.9 53.8 29.1 24.3 51.4 54.5 37.7 41.2 73.6 35.8

43.4

17.6

to an orbital shaker. The entire assembly was positioned next to the ultrasonic bath, so that the vials could be immersed in the bath while being shaken at the same time. In the two-step procedure, the initial mechanical agitation of the samples (as in the standard procedure) was followed by simultaneous shaking (330 rpm) and sonication at 45 °C. Three different sonication extraction times were examined: 0.5 h, 1 h, and 1.5 h. The one-step procedure was a shortened version of the two-step procedure, in which the initial mechanical agitation step was eliminated. Extraction times of 1 and 1.5 h were examined. In all cases, the sample size, solvent volume, and the postextraction procedure were identical to the ones used in the standard method. Consequently, the limits of detection and the working range were also the same.

Results and Discussion Organic contaminants in aged clay samples can be found throughout the bulk of the material, which indicates that they enter the micropores in clay. Since transport in the micropores is mostly via molecular diffusion, a very slow process, it is highly desirable to make the diffusion distances during extraction as short as possible. This is typically accomplished by vigorous agitation of the samples, which leads to disintegration of the clay subcores into smaller aggregates. However, the time required to accomplish complete disintegration of all clay particle aggregates can be very long, which creates a bottleneck in the sample preparation process. We propose to achieve the same goal in a much shorter time by combining mechanical agitation with sonication. The latter delivers additional energy to the sample, promoting disintegration of small aggregates as a result of cavitation and enhancing solvent penetration of the micropores. Development of a Spiking Procedure. Spiked samples are usually used in the development of new extraction techniques. To be useful in the study, spiked clay samples would have to mimic natural contaminated clay as much as possible. Apart from having known and controlled total analyte concentration in the sample, analyte distribution throughout the spiked subcore would have to be uniform. Besides, it would have to be demonstrated that spiked samples behave in the same way as the naturally aged native samples. Several different approaches to spiking, usually incorporating an aging period, were tried. While it was

TABLE 2. TCE Concentration in Diluted Clay Extracts Obtained by Sonication Extraction Followed by Shaking and 5 Days of Equilibration

sample 1 2 3 4 5 6 7 8

extraction time, h 0.5

TCE concn after shaking (ng/mL)

TCE concn after 5 equilibration days (ng/mL)

concn difference (ng/mL)

relative difference %

220 161 52 196 146 153 113 231

223 161 51 190 137 143 100 215

3 0 -1 -6 -9 -10 -13 -16

1.35 0.00 -1.96 -3.16 -6.57 -6.99 -13.00 -7.44

possible to produce spiked samples with uniform analyte distribution that produced extraction time profiles similar to those of the native samples, analyte recoveries from the spiked samples were never quantitative and varied widely due to losses incurred at the different stages of spike preparation. Consequently, this approach was abandoned in favor of using real samples from a contaminated site. Solvent Extraction using the Standard Method. Initial experiments were performed using the standard solvent extraction procedure (see Experimental Section) to establish a reference for further experiments. The main goal was to find the average increase in TCE concentration during the equilibration period of 5 days. A series of experiments was conducted on 19 clay samples that were analyzed according to the standard procedure. The extracts were analyzed immediately following the 30 min agitation on the orbital shaker and after 5 days of equilibration with methanol. The results are presented in Table 1. The average increase in TCE concentration in the extract over the period of 5 days of equilibration was 43.4%, with a standard deviation of 17.6%. Sonication Extraction. Early experimental procedure was based on the standard method of VOC extraction from clay for the primary stage of core and sample collection and placement into vials containing methanol for preservation and extraction. The difference was that after shaking on the orbital shaker, the vials with clay samples were sonicated at a constant temperature of 45 °C. Different sonication times were investigated, from 0.5 h to 8 h. At every 0.5 h interval, aliquots of the extract were collected and analyzed in order to determine the extraction time profile of TCE from clay. The results demonstrated no need for sonication for more than 1.5 h, as no further increase in TCE concentration in the extracts was observed after this time. Moreover, during the 8 h experiment, a decrease in analyte concentration ranging from 1.8 to 7.5% was observed. Consequently, the remaining experiments were conducted only for three time intervals: 0.5 h, 1 h, and 1.5 h. TCE loss during prolonged sonication could have been caused by analyte decomposition; however, it seemed more plausible that the loss was caused by sorption into the septum material (PTFE-lined poly(dimethylsiloxane)), especially following collection of the aliquots of the extract when the PTFE lining was pierced. The latter possibility was evaluated using standard solutions of about 23 µg/mL TCE in methanol. The solutions were stored for 2.5 h at 45 °C, and the observed analyte losses ranged from 1.1% to 4.3%, close to the values observed for the sonicated samples. Thus, it was assumed that analyte decomposition did not occur during prolonged sonication and that the losses were due to sorption. Compared to the TCE concentration in MeOH immediately after agitation of the sample vials on the orbital shaker, an increase in concentration ranging from 2.0 to 13.1% was observed after the sonication of the clay samples. However, an additional increase in TCE concentration by

average relative difference % -4.72

SD

calculated Student’s t (absolute value)

critical Student’s t

4.69

2.851

2.306

8.6-53.8% was observed after the 5 days of equilibration following the sonication. The increase was statistically significant at 95% probability level. Thus, it was clear that while sonication may have increased the rate of extraction somewhat, it did not yield complete analyte recovery in a short time, the stated goal of this study. Visual examination of the samples revealed that the clay particles settled very rapidly at the bottom of the vials during sonication. Apparently, the additional energy introduced to the system helped overcome repulsive forces between clay particles, promoting their settling. The layer of clay found at the bottom of the vials immediately after sonication, typically stratified according to the particle size, was usually much more compacted than the layers formed in vials that were only mechanically shaken and allowed to settle over 5 days. In fact, very vigorous shaking was required to resuspend the particles that settled at the bottom of the sonicated vials. Thus, while sonication accelerated the disintegration of clay particle aggregates, it also promoted the settling of the particles, leading to the formation of a very compact layer of clay at the bottom of the vials. Transport of the analyte molecules by molecular diffusion was most likely very slow in these layers. Formation of low permeability layers in clayey soils subjected to sonication was also observed in the study of Reddi et al.18 Sonication Followed by Mechanical Agitation. Keeping the disintegrated clay particles in suspension seemed vital in order to achieve complete analyte recovery in a short time. Reddi et al.18 observed that subsonic vibrations following ultrasonic excitation mobilized additional clay fines. It was decided therefore to resuspend the tight clay layer formed at the bottom of the vials after sonication by vigorous shaking of the vials for 30 min using the orbital shaker (see Experimental Section). Once the particles were dispersed again in the solvent, the diffusion distances were reduced, and the extraction process was accelerated as a result. Because no standards or reference materials were available for the experiments, the only way to assess the performance of the new method was to compare the results obtained by this method for each individual sample to the results obtained for the same samples after they were allowed to equilibrate for 5 additional days with the extracting solvent (MeOH), as in the standard procedure. The two sets of results were compared using Student’s t-test for the individual differences.24 The zero hypothesis was that the two methods produced identical results, and the differences were caused by random factors only, while the alternative hypothesis stated that the differences were caused by systematic factors. Because analyte levels in the natural samples varied widely, we chose to compare percent relative differences rather than absolute differences. The results for this version of the technique are presented in Table 2. Since the average difference had a negative sign, we compared the absolute value of tcalc with the value of tc VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. TCE Concentration in Diluted Clay Extracts Obtained for the Two-Step Simultaneous Sonication and Shaking Procedure, for Three Time Intervals and after Five Equilibration days

sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

extraction time, h 0.5

1

1.5

TCE concn after sonication (ng/mL)




1485 700 1189 1316 1414 1669 1584 1338 1615 497 956 1286 1325 1585 1069 595 1018 763 1064 1655 1177 773 465 91 73 39 111 75 457 745 554 1133 380 787 1071 807 1709 1542 608 729 1639 1598 633 1564 25 34 92 49 38 65 43 38 25 53

1478 695 1178 1301 1415 1676 1603 1345 1642 503 957 1261 1306 1561 1041 605 1074 811 1118 1752 1289 792 478 89 72 37 128 78 463 746 552 1131 383 788 1063 805 1742 1589 633 756 1603 1613 656 1602 25 34 90 48 41 64 43 39 24 53

-7 -5 -11 -15 1 7 19 7 27 6 1 -25 -19 -24 -28 10 56 48 54 97 112 19 13 -2 -1 -2 17 3 6 1 -2 -2 3 1 -8 -2 33 47 25 27 -36 15 23 38 0 0 -2 -1 3 -1 0 1 -1 0

-0.5 -0.7 -0.9 -1.2 0.1 0.4 1.2 0.5 1.6 1.2 0.1 -2.0 -1.5 -1.5 -2.7 1.7 5.2 5.9 4.8 5.5 8.7 2.4 2.7 -2.2 -1.4 -5.4 13.3 3.8 1.3 0.1 -0.4 -0.2 0.8 0.1 -0.8 -0.2 1.9 3.0 3.9 3.6 -2.2 0.9 3.5 2.4 0.0 0.0 -2.2 -2.1 7.3 -1.6 0.0 2.6 -4.2 0.0

at the 95% probability level. We found that |tcalc| > tc(95%); therefore, we had to conclude that the two sets of results differed statistically significantly at the chosen probability level. The decrease in concentration observed for nearly all samples after they were equilibrated with methanol for the additional 5 days, albeit small, could not be explained by random factors only. This does not mean, however, that the method developed did not yield good results. On the contrary, it seems that complete or nearly complete analyte recovery was achieved in the short time required for the new procedure, following which analyte losses (e.g. through sorption) could only occur during further equilibration. 3982

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average relative difference % 1.40

SD

calculated Student’s t


3.87

1.916

2.049

0.67

1.89

1.589

2.086

0.69

3.93

0.432

2.447

The method proposed involved a three-step procedure (shaking-sonication-shaking), which we considered cumbersome. In addition, it would be advantageous to further shorten the sample preparation time. Consequently, we decided to combine shaking and sonication into a single step. Simultaneous Shaking and Sonication - Two-Step Procedure. Initial processing of the samples was kept unchanged. The samples were shaken for 30 min on the orbital shaker, following which they were placed in the device allowing simultaneous sonication and agitation (see Figure 1). Initially, the vials were mounted vertically during pro-

TABLE 4. TCE Concentration in Clay Extracts Obtained for the One-Step Simultaneous Shaking and Sonication Extraction

sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

extraction time, h 1

1.5

TCE concn after sonication (ng/mL)




845 842 817 953 802 282 687 757 147 490 921 923 801 375 595

847 837 812 948 797 283 683 757 146 487 949 918 798 376 596

2 -5 -5 -5 -5 1 -4 0 -1 -3 28 -5 -3 1 1

0.2 -0.6 -0.6 -0.5 -0.6 0.4 -0.6 0.0 -0.7 -0.6 3.0 -0.5 -0.4 0.3 0.2

cessing. The observed increase in TCE concentration varied from 26.4 to 45.1% during sonication, and further 3.8-20.9% after the 5 additional days of equilibration following sonication. The average initial increase in TCE concentration was higher than that observed in the experiments involving sonication only (see above), but complete analyte recovery was not achieved during sonication. Visual examination of the vials indicated that clay particles still tended to settle at the bottom (although to a lesser extent), despite the mechanical agitation. Apparently, the circular motion of the orbital shaker combined with the circular cross-section of the vials did not provide enough agitation. To alleviate this problem, we changed the position of the vials immersed in the sonication bath from vertical to horizontal (the position in which the vials are normally agitated on the orbital shaker). The results of these experiments, summarized in Table 3, proved that this approach achieved the stated goal. The change in TCE concentration after 5 days of equilibration following the combined agitation/sonication was negligibly small, with both positive and negative signs. A total of 54 clay samples were investigated for three different extraction times. The samples were analyzed immediately after sonication and after 5 additional days of equilibration. The tcalc values calculated for the differences between the resultant pairs of results were lower than the tc at the 95.0% probability level for all three extraction times. For the 28 samples investigated at 0.5 h extraction time, the interpolated tc value was 2.049, while tcalc was 1.916. For 1 h extraction, tcalc ) 1.589 (n ) 24) compared to tc ) 2.086. Similarly, for the six samples sonicated for 1.5h, tcalc ) 0.432, while the critical value was 2.447. Since in all cases tcalc values were lower than the critical values, it can be concluded that the two methods did not differ significantly at the 95% probability level, and the differences between the results obtained using the two methods could be explained by random factors. This approach to extraction of TCE from clay samples yielded the same results as the standard methanol extraction method, but in a much shorter time (less than 2 h compared to 5 days). This indicates that the method has a very good potential in the analysis of VOCs in clay, especially when the results have to be produced fast (e.g. in field analysis). Simultaneous Shaking and Sonication - One-Step Procedure. The one-step procedure was a shortened version of the two-step procedure described above, in which initial processing of the samples (30 min agitation using an orbital shaker) was eliminated. Table 4 summarizes the results of the experiments. Changes in TCE concentration in the extracts after 5 days were negligibly small, with both negative

average relative difference %

SD

calculated student’s t


-0.34

0.435

2.056

2.365

0.15

1.191

0.345

2.306

and positive signs. Seven clay samples were investigated for the 1 h extraction time and eight clay samples for the 1.5 h extraction time. Analyses were performed immediately after the extraction and after 5 equilibration days. From Table 4, the tcalc values for the two extraction periods were lower than the tc values at the 95% probability level. For the seven samples investigated at 1 h extraction time, tcalc was 2.056, while the critical value tc was 2.365. For the eight sample series investigated at 1.5 h extraction time, the tcalc was 0.345, while tc was 2.306. It can be concluded, therefore, that the two methods did not differ significantly at the 95% confidence level, and random factors could explain the differences between the results. Similarly to the two-step procedure, the one-step procedure yielded excellent results in a very short time, with minimum sample manipulation. The clear advantages of performing TCE extraction by combining shaking with sonication without initial sample processing make this technique very attractive for VOC analysis in low permeability media including clay, especially in the field when results are required in near real-time. The number of samples that can be processed in a single batch by this method can be easily increased by using a larger ultrasonic bath.

Acknowledgments We express our gratitude to CRESTech, NSERC and the University of Waterloo Consortium Solvents-in-Groundwater Research Program for the financial support of this study.

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Received for review January 9, 2003. Revised manuscript received April 9, 2003. Accepted April 24, 2003. ES0340275

A Novel Technique for Rapid Extraction of Volatile Organohalogen

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