Accelerated Solvent Extraction: AnEvaluation for Screening of Soils forSelected U.S. EPA SemivolatileOrganic Priority PollutantsJ O H N A . F I S H E R

Environment Protection Authority of Victoria, Melbourne,Victoria 3001, Australia

M A R G A R E T J . S C A R L E T T * A N DA A R O N D . S T O T T

School of Chemical Sciences, Swinburne University ofTechnology, P.O. Box 218, Hawthorn, Victoria 3122, Australia

Accelerated solvent extraction (ASE) was evaluated as apossible rapid, low solvent replacement for Soxhlet andbath sonication/shaking extraction in established soil screeningmethods. A spiked uncontaminated clay loam and soilscontaminated with organochlorine pesticides (OCPs) andpolycyclic aromatic hydrocarbons (PAHs) were extractedwith 1:1 dichloromethane/acetone. Extracts were analyzedby gas chromatography with mass spectrometric detectionin selective ion monitoring mode. For the spiked soil, ASErecoveries were higher than those from Soxhlet extractionfor most analytes and generally close to 100% at the 4 and20 mg/kg levels. At the 0.4 mg/kg level, background peakscaused significant interference for both techniques. Thiswas a greater problem for ASE as the method blank gavea more crowded, complex chromatogram. For the OCP soil,ASE gave higher results than Soxhlet but lower resultsthan bath sonication/shaking, for which a more effectivesolvent for OCPs was used. For the PAH soils, ASErecoveries were equivalent or superior to bath sonication/shaking, with ASE giving approximately double the totalPAH content for matrices containing small stones and/orcoal. ASE would be a suitable replacement for existingextraction methods; however, more work is required to reducebackground interference.

IntroductionThe Environment Protection Authority of Victoria (EPAV),like all regulatory agencies, is examining its analytical methodsto find ways of reducing solvent use, particularly the use ofrelatively toxic chlorinated solvents such as dichloromethane.Aims are to protect the health of laboratory staff, to minimizethe production of waste, and to reduce the emission of ozone-depleting and photochemically-active chemicals. Cost sav-ings on solvent purchase and disposal are a significant sidebenefit.

Large volumes of organic solvents are required to extractsemi- and nonvolatile organic analytes from a diverse rangeof sample matrices. Little of the solvents used are currentlyrecovered. The majority are either released to the atmosphereduring extract concentration or collected for subsequentdisposal as waste. Established extraction procedures also

involve several manipulative steps making them time-consuming and labor-intensive.

Method 3540 (1) for screening soils and solid wastes forU.S. EPA semivolatile organic priority pollutants (2) is a typicalexample, involving Soxhlet extraction with approximately 300mL of 1:1 dichloromethane/acetone for 16-24 h. Thealternative probe ultrasonication extraction Method 3550 (1)is much quicker but less efficient (3). However it also requiresapproximately 300 mL of 1:1 dichloromethane/acetone. Bothof these methods have already been modified by the EPAVto reduce solvent use and extraction time.

The newly developed accelerated solvent extraction (ASE)technique (4) offers an order of magnitude additionalreduction in solvent use coupled with faster sample processingtimes. ASE achieves rapid extraction with small volumes ofconventional organic solvents by using high temperatures(up to 200 °C) and high pressures (up to 20 MPa) to maintainthe solvent in the liquid state.

The objective of this work was to determine whether theASE Method 3545 (1) could replace established EPAV extrac-tion techniques for screening soils. We used both spikeduncontaminated soil and real samples to evaluate theperformance of ASE.

Samples of an uncontaminated clay loam were spiked atthree levels with a representative mixture of U.S. EPAsemivolatile organic priority pollutants that covered a rangeof analyte classes. ASE recoveries were compared with thosefrom Soxhlet extraction.

Although recovery data are already available for Method3545 from spiked soils (1), this may not be directly applicableto Australian soils. In comparison with the correspondingsoils from North America and Europe, Australian soils differin organic matter content and are typically lower in nutrients,especially phosphate (5). We were also interested to deter-mine whether or not the higher temperature and pressureconditions of ASE would lead to larger or more complex blanksthan Soxhlet extraction. In addition, because the ASE methodrecommends that soil samples be finely ground beforeextraction, we used the spiked soil to carry out a preliminaryinvestigation of the effect of sample grinding.

We also evaluated ASE using real soil samples from severaldifferent local sites that were known to be contaminated witheither polycyclic aromatic hydrocarbons (PAHs) or orga-nochlorine pesticides (OCPs). ASE recoveries were comparedwith those from Soxhlet extraction and/or pre-existing datafrom bath sonication/shaking.

Since we were interested in a general screening procedurerather than a method for analyzing a particular class ofcompounds, we treated all samples as if they containedunknown semivolatile organic pollutants. Hence 1:1 dichlo-romethane/acetone was used as the solvent for all ASE andSoxhlet extractions, and extracts were not cleaned up. Noattempt was made to optimize the solvent or extractionconditions on the basis of prior knowledge about the sample.

Experimental SectionChemical Reagents and Standard Solutions. All solventsused were distilled-in-glass (Omnisolve grade, EM Science)and were checked for interferences prior to use. CelluloseSoxhlet thimbles were from Microfiltration Systems. Granularanhydrous sodium sulfate (AR grade, Mallinkrodt) was heatedto 600 °C for 2 h prior to use.

The composition of the mixture used for spiking theuncontaminated soil is given in Table 1. An approximately2500 mg/L concentrated stock solution in acetone wasprepared for each component in the mixture; all reagentswere at least 97% pure. Separate spiking and quantitation

* Corresponding author telephone: +61-3-9214-8387; fax: +61-3-9819-0834; e-mail: mscarlett@swin.edu.au.

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standards, containing on average 175 and 25 mg/L percomponent respectively, were prepared by mixing aliquotsof the concentrated stock solutions and diluting with acetone.

Approximately 500 mg/L concentrated stock solutions ofindividual OCPs in acetone were prepared from the purereagents (pesticide grade, Chemservice). Aliquots were mixedand diluted to give a 1 mg/L quantitation standard in hexane.A 1 mg/L PAH quantitation standard was prepared by dilutinga 20 mg/L commercial standard (Ultrascience) with dichlo-romethane and wrapping it in foil to protect against pho-todegradation. The semivolatile spiking standard was madefreshly before each use; the three quantitation standards wereprepared freshly every 2 weeks and stored at -18 °C.

Instrumental Conditions. Dionex ASE 200 Instrument.The conditions of Method 3545 were used. Hence 10 g of soilwas extracted for 10 min with 1:1 dichloromethane/acetoneat 100 °C and 10 MPa pressure in an 11 mL cell.

GC/MS Instrument. All extracts were analyzed on aHewlett-Packard (HP) 5971 mass selective detector interfacedto an HP5890 Series II gas chromatograph. Splitless 1 µLinjections of sample extracts, blank extracts, and quantitationstandards were made with an HP7673 autosampler on to aBPX-5 column (25 m × 0.22 mm i.d. × 25 µm film thickness;SGE Ringwood, Victoria, Australia). Helium was used as thecarrier gas. The injector port and detector interface tem-perature were maintained at 250 and 280 °C, respectively.The spiked soil extracts were analyzed using the followingcolumn temperature program: 45 °C held for 4 min, increasedat 10 °C/min to 125 °C, then at 30 °C/min to 205 °C, and at10 °C/min to 300 °C and held for 5 min, giving a total run timeof 29 min. For the OCP extracts the column temperatureprogram was as follows: 60 °C held for 2 min, increased at15 °C/min to 240 °C, and held for 13 min, giving a total runtime of 25 min. For the PAH extracts the program was asfollows: 40 °C held for 4 min, increased at 10 °C/min to 310°C, and held for 8 min, giving a total run time of 35 min.

Data on most sample extracts were aquired using real-time SIM mode, whereas extracts of instrument and methodblanks were run in full-scan mode. In all cases, quantitationwas based on intensities of selected ions.

The retention times for the components of the semivolatilespiking standard are shown in Table 1. The recommendedcharacteristic ions (6) were used to confirm the identity andto quantitate each analyte in the spiked soil extracts and inthe extracts of the OCP- and PAH-contaminated soils. Exceptwhere noted, 1-bromonaphthalene was added as internalstandard to sample extracts, and quantitation standards andconcentrations were obtained from ratios of peak areas. Forthe spiked soils, method blanks were not subtracted becausethey vary from one subsample to the next.

Soxhlet Extraction. Before use, the glassware were rinsedwith 1:1 hexane/acetone and allowed to air dry. Thimbleswere washed by extraction in the Soxhlet apparatus with 1:1hexane/acetone for 1 h. For each soil, 10-15 g subsampleswere extracted with 150 mL of 1:1 dichloromethane/acetoneat approximately 5-6 cycles/h. The contaminated soils wereextracted for 7 h, allowing each sample to be completed in

1 day. For the spiked soils, the extraction time was reducedto 4 h; this was considered to be sufficient as the pollutantshad not been in contact with the soil for very long.

Sample Information and Treatment. Spiked Soil Samples.The uncontaminated soil used for spiking was an acidic redclay loam, high in hydrated iron oxide, with a moisture contentof 2.5%. The soil was free flowing and was supplied alreadysieved to a particle size of less than 2 mm. It contained bothdecomposed plant material and very small root fibers. Thetotal organic carbon content was determined to be 1.1% (dryweight basis) from the Walkley-Black method (7), in closeagreement with the results obtained by a commerciallaboratory using a total organic carbon analyzer.

Spiked soil samples containing approximately 0.4, 4, or 20mg/kg per analyte were prepared as follows. The soil wasfirst ground in a solvent-washed mortar and pestle until itpassed through a 1 mm sieve as specified in Method 3545.Approximately 100 g of the ground soil was weighed accuratelyinto an amber glass bottle; sealed with a PTFE-lined, screw-top lid; cooled to -18 °C; and then spiked while cold tominimize loss of volatiles. The spiking standard was addedat various sites throughout the soil from a glass syringe; thesample was mixed by tumbling for 2 h and stored at -18 °Cuntil use.

For each spiked sample, duplicate 10 g subsamples wereremoved and subjected to ASE and Soxhlet extraction. Theresulting extracts were reduced by Kuderna-Danish con-centration, followed where necessary by nitrogen blowdownat room temperature, to give analyte concentrations between1 and 30 mg/L.

To assess the effect of grinding on the ASE results, twoadditional sets of spiked samples were prepared. One setreceived no grinding and so contained a range of particlesizes up to a maximum of 2 mm; for the other set, grindingwas done after spiking to simulate the preparation of real soilsamples. An additional duplicate run, where the test mixwas spiked directly into ground soil in the extraction cell, wasperformed at the 4 mg/kg level to test if any loss of volatilesoccurred during the ASE procedure.

OCP-Contaminated Soil. A 1500 g blended sample wasprepared from eight separate soil samples known to becontaminated with organochlorine pesticides. The mixture,which was a dry and sandy soil (moisture content 0.2%), wastumbled for 2 h, ground in a mortar and pestle until it passedthrough a 2 mm sieve, and tumbled for a further 2 h.

Triplicate 15 g subsamples were removed for extractionby ASE and Soxhlet. The ASE extract was analyzed asobtained, whereas the Soxhlet extract was first concentratedby a factor of 20 by nitrogen blowdown at room temperature.Dieldrin and aldrin were determined in both sets of extractsby GC/MS with SIM. Quantitation was by peak area usinga single point external standard. Results were compared witha weighted average based on pre-existing data on the eightsoils from bath sonication/shaking with 4:1 hexane/acetoneand analysis by GC with electron capture detection (ECD)(8).

PAH-Contaminated Soils. Four different soils known tobe contaminated with polycyclic aromatic hydrocarbons wereextracted by ASE. They were a sandy loam containing smallstones (soil SS), a clay (soil C), a sandy loam containing smallstones and pieces of coal (soil SSC), and a dark loam (soilDL). Soils SS, C, and SSC with a moisture content of 5-10%were not free flowing, whereas the dark loam soil, DL, wasfree-flowing in spite of its high moisture content of 16%.

For ASE, 20 g of each soil was placed in a mortar andpestle, the larger stones were removed, and the soil was groundwith an equal volume of anhydrous sodium sulfate until itpassed through a 2 mm sieve. Subsamples of 10 g wereweighed out immediately into extraction cells.

For the soils SS, C, and SSC, we compared our ASE resultswith pre-existing data from bath sonication/shaking (9). For

TABLE 1. Components of Semivolatile Spiking Standarda

compound tR (min) compound tR (min)

o-xylene 6.6 diethyl phthalate 16.1phenol 9.3 heptadecane 16.62,4-dichlorophenol 12.8 pyrene 20.6naphthalene 13.1 endrin 21.41-bromonaphthaleneb 15.6 DDT 22.1

a For the spiking standard, the concentration is approximately 175mg/L per component in acetone; for the quantitation standard, theconcentration of each component is approximately 25 mg/L. b Internalstandard.

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the soil DL, which had not been previously analyzed, wecarried out Soxhlet extraction for comparison with ASE, as itis expected to be more efficient than sonication extraction(3). In keeping with established EPAV practice of minimizingsample handling for PAH screening, soils were neither sievednor ground. Soils SS, C, and SSC were simply stirred withsufficient anhydrous sodium sulfate to produce a free-flowingpowder prior to bath sonication. Mixing with sodium sulfatewas omitted for the Soxhlet extraction of soil DL as it wasalready free-flowing.

The solvent for all three extraction methods was 1:1dichloromethane/acetone, and extracts were analyzed by GC/MS with SIM. The Soxhlet extract for soil DL was relativelyclear and colourless and hence was injected directly. How-ever, all soils gave very dark dirty ASE extracts, and thesewere diluted before injection to give approximately 1 mg/Lfor each analyte based on the Soxhlet or bath sonication data.

Blank Samples. (a) Instrument Blanks. Potential con-tamination from the sample cells, cellulose discs, solvent,and instrument components for the ASE instrument or fromthe glassware, thimble, and solvent in the case of the Soxhletapparatus was assessed by doing an extraction with no soilpresent.

(b) Method Blanks. The uncontaminated clay loam wasused to assess whether peaks due to naturally occurringorganic compounds extracted from the soil matrix wouldinterfere with the analyte peaks in the ASE and Soxhletchromatograms. Compounds in the ASE and Soxhlet blankextracts were identified by matching the mass spectra of thepeaks in the total ion chromatograms in full-scan mode withthe Wiley library containing 130 000 mass spectra.

Results and DiscussionASE and Soxhlet Method Blanks. Figures 1 and 2 show typicaltotal ion chromatograms of extracts of the ASE and Soxhletmethod blanks, both of which vary considerably from onerun to the next. Although the total organic matter contentof the uncontaminated red clay loam was relatively low (5),these chromatograms are complex and contain many closelyspaced peaks, including a large number that are more intensethan the analyte peaks at the 0.4 mg/kg level. This is clearlyseen in the total ion chromatogram of the 0.4 mg/kg ASEextract given in Figure 3.

Both ASE and Soxhlet method blanks contain interferingpeaks that co-elute with certain analytes in the spikingmixture. Hence extracts must be analyzed in SIM mode.However, even with SIM, it is not possible to eliminateinterference when the mass spectra of the analyte and theco-eluting interfering compound have major fragment ionsin common. Co-elution is a greater problem for the ASEprocedure as the ASE method blank is considerably morecrowded.

Furthermore, the analytes phenol, naphthalene, andheptadecane were found in both ASE and Soxhlet methodblanks at a concentration equivalent to at least 0.1 mg/kg inthe soil. Hence, background peaks from the red clay loamsoil clearly cause significant interference in the determinationof phenol, naphthalene, and heptadecane by both the ASEand Soxhlet methods at the 0.4 mg/kg level.

ASE Recoveries for the Spiked Soil. Figure 4 showsrecoveries for ASE extraction of soil spiked at 0.4, 4, and 20mg/kg. At the 4 and 20 mg/kg levels, recoveries are close to100% for most analytes. The lower values for xylene can beattributed to losses during sample handling, as direct spikingof soil in the extraction cell gave a recovery for xylene of 87%.This indicates minimal loss of volatiles during the extractionprocess, as for example when air in the collection vial isdisplaced by hot solvent through the vent for the needleassembly. It is also worth noting that the ASE techniqueseems to perform well at the 20 mg/kg level, which is outsideits validated range (1).

At the 0.4 mg/kg level, background peaks interferesignificantly in the determination of several analytes, includingphenol, naphthalene, and heptadecane as discussed previ-ously. Thus, interference from the background is responsiblefor the heptadecane recovery above 100%. The effect of thelow level of heptadecane in the blank is compounded by thedifficulty in selecting a characteristic ion for heptadecanesaliphatic side chains are ubiquitous in natural organic matter.

Background interference is also responsible for theunrealistically high recoveries for endrin. Endrin is aparticularly difficult analyte to quantitate as its mass spectrumis highly fragmented, containing a large number of lowintensity peaks between 40 and 270 m/z. This makes itdifficult to select a characteristic ion that will not be producedby natural organic compounds in the soil and also leads topoor sensitivity. Chance variation in the size of the contri-bution from the interfering background peaks is presumablyresponsible for the higher recovery observed at the 4 mg/kglevel as compared with the 0.4 mg/kg level.

Background interference can cause an apparent increasein recovery that may disguise the effect of losses, particularlyfor analytes present at low concentration. Although back-ground interference may contribute to recoveries e100%, itis usually not suspected unless recoveries above 100% areobtained.

Comparison of ASE and Soxhlet Recoveries for the SpikedSoil. ASE recoveries are higher for most analytes than thosefrom Soxhlet extraction as seen in Figure 5, which comparesthe two techniques for soil spiked at the 4 mg/kg level.(Soxhlet recoveries at both the 0.4 and 20 mg/kg levels weregenerally lower than those shown.) Evaporation losses fromthe condenser over the 4 h period of the extraction process

FIGURE 1:. Typical total ion chromatogram of ASE method blank(uncontaminated red clay loam). Tentative peak identification: 1,4-hydroxy-4-methyl-2-pentanonesan impurity in acetone; 2, 1,4-dimethoxybenzene; 3, 1,2,3- or 1,3,5-trimethylbenzene and 2,2,4,4-tetramethyl-3-pentanone; 4, phenol and 4-methyl-2-hexanone; 5, 2,6-dimethyl-2,5-heptadien-4-one; 6, 1-bromonaphthalene internal standard;7, bis(2-ethylhexyl) phthalate plasticizer impurityspresent in theinstrument blank.

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may contribute to reduced Soxhlet recoveries for xylene,naphthalene, and, to a lesser extent, phenol.

Again interferences may disguise losses. Soxhlet back-ground peaks interfere significantly in the determination ofphenol, naphthalene, and heptadecane especially at lowerconcentrations. Background interference is responsible forthe heptadecane recovery of 221% at the 0.4 mg/kg level.

Effect of Grinding on ASE Recoveries from the SpikedSoil. The major effect of grinding on these samples was tocause a significant reduction in the recovery of xylene. Forexample, at the 4 mg/kg level, the recovery of xylene whenthe sample was ground and then spiked was 54%, which isvery little different from the value of 48% obtained whengrinding was omitted. However, when the sample was firstspiked and then ground, the recovery of xylene fell to 13%.There was little other effect, and hence a range of particlesizes to 2 mm in the sample does not appear to significantlyaffect ASE extraction efficiency or sample homogeneity. Basedon these results and given the possibility of loss of volatilesand reactive analytes on grinding, real samples were onlyground to a particle size of up to 2 mm for ASE extraction.

Origin of the Method Blanks. We have assigned a numberof the blank peaks, and several of these are indicated in Figures1 and 2.

A small number of the interfering peaks in the ASE andSoxhlet method blanks also occur in the instrument blanks.These include the intense peak 1, due to 4-hydroxy-4-methyl-2-propanone, an impurity in the acetone extraction solvent,and the small peak 7, due to bis(2-ethylhexyl) phthalate, acommon plasticizer and widespread contaminant. Both

compounds are also present at low concentration in thequantitation standard. The intense symmetrical group ofpeaks located between 14 and 20 min in the Soxhlet methodblank is due to residual hydrocarbon impurities from thethimble as solvents and other reagents were checked andfound to be free of these artifacts. The peaks were reducedby a factor of approximately 2 after 1 h of Soxhlet extractionof the thimble but not eliminated. We have been unable tofind any mention of this interference in the literature, norhave we been able to isolate any source of hydrocarboncontamination in our laboratory. The internal standard1-bromonapthalene is responsible for the intense peak 6 at15.6 min present in all chromatograms.

Both ASE and Soxhlet method blanks contain manyadditional interfering peaks that are absent from the instru-ment blanks. A number of these must arise from naturallyoccurring organic matter in the red clay loam. Sources ofsoil organic matter include compounds secreted by root fibers,the cellulose, lignin, essential oils and waxes from plant cellsand their partial microbial oxidation products, and humicand fulvic acids from decomposed plant matter. A wide rangeof compound classes would be expected to extract into asolvent like 1:1 dichloromethane/acetone. On heating, manyof these compounds may break down, oxidize, or undergoother chemical reactions, including reactions with acetonein the solvent mixture.

Compounds present in both ASE and Soxhlet blanks thatare likely to arise from the soil organic matter include theanalytes phenol, naphthalene, and heptadecane. Phenol,which is one of two compounds responsible for peak 4, hasbeen reported at low levels in uncontaminated soil (9).Naphthalene and heptadecane are found among the digestionproducts of humic substances under a variety of conditions(10). (Although heptadecane in the Soxhlet blank comes

FIGURE 2:. Typical total ion chromatogram of Soxhlet extractionmethod blank (uncontaminated red clay loam). Tentative peakidentification: 1, 4-hydroxy-4-methyl-2-pentanonesan impurity inacetone; 3, 1,2,3- or 1,3,5-trimethylbenzene and 2,2,4,4-tetramethyl-3-pentanone; 4, phenol and 4-methyl-2-hexanone; 6, 1-bromonaph-thalene internal standard; thimble, group of peaks due to residualhydrocarbons from the thimble; 7, bis(2-ethylhexyl) phthalateplasticizer impurityspresent in the instrument blank. Note thatpeaks 2 and 5 corresponding to 1,4-dimethoxybenzene and 2,6-dimethyl-2,5-heptadien-4-one, respectively, in Figure 1 are absent.

FIGURE 3:. Typical total ion chromatogram of ASE extract of redclay loam spiked at 0.4 mg/kg. Analyte peaks: x ) o-xylene; ph) phenol; cp ) 2,4-dichlorophenol; n ) naphthalene; ep ) diethylphthalate; hd ) heptadecane; py ) pyrene; en ) endrin; DDT. Peaks1-7 are present in the ASE method blank (see Figure 1).

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mainly from the thimble, the soil may provide an additionalsmall contribution.) It is also likely that the soil organic matteris the origin of the compounds tentatively identified astrimethylbenzene isomers and 2,2,4,4-tetramethyl-3-pen-tanone responsible for peak 3, which is particularly intensein the Soxhlet blank, and 4-methyl-2-hexanone, whichcontributes to peak 4, as aromatic hydrocarbons and aliphaticketones with similar structures are produced by digestion ofhumic substances (10).

However the ASE method blank contains a number ofmajor peaks that are absent from the Soxhlet method blank,including peak 2 assigned to 1,4-dimethoxybenzene, apossible breakdown product of humic substances. Thissuggests that reactions involving the soil organic matter aremore important under the high temperature and pressureconditions of ASE. The fact that a second extraction on thesame soil sample caused some peaks such as peak 2 to greatly

increase in intensity, whereas other peaks shrank or evendisappeared, supports this view.

Not all additional ASE peaks necessarily arise from thesoil organic matter, however. For example, peak 5 has beenassigned to 2,6-dimethyl-2,5-heptadien-4-one, which is formedunder extreme conditions by aldol condensation of acetone.Since this peak is not present when clean sand is extractedin place of the red clay loam, it may be that the hydrated ironoxide in the red mountain soil is acting as a basic catalyst forthis reaction.

Since this work was completed, it has been suggested (12)that dissolved oxygen in the system may be responsible forsome of the reactions observed during the ASE extractionand that the ASE blank may be reduced if the extraction solventwere degassed just prior to use.

Blended OCP-Contaminated Soil. The ASE and Soxhletresults for the dieldrin and aldrin content of the blended

FIGURE 4:. ASE recoveries for spiked red clay loam. Duplicate analyses.

FIGURE 5:. Comparison of ASE and Soxhlet recoveries for red clay loam spiked at 4 mg/kg. Duplicate analyses.

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sample are compared in Table 2 with the values calculatedfrom bath sonication/shaking data on the component soils.Whereas the ASE and Soxhlet methods are general screening

procedures using the semivolatile screening solvent, 1:1dichloromethane/acetone, the sonication method has beenoptimized for OCPs. It uses 4:1 hexane/acetone, in whichthe nonpolar OCPs would be expected to be more soluble,as extraction solvent.

The ASE results are higher than the Soxhlet results,indicating more efficient extraction, although the Soxhletvalues may have been improved if the extraction time wasincreased beyond 7 h. Both methods give very variable resultsdue to sample inhomogeneitysthe blended OCP sample wasobserved to be stratified. Grinding to less than 1 mm mayhave reduced this problem and also increased the ASE (andSoxhlet) recoveries. The ASE concentrations are somewhat

FIGURE 6. Comparison of PAH analysis by ASE and bath sonication/shaking for contaminated soil SSC. Soil SSC is a sandy loam containingsmall stones and lumps of coal. Extracted with 1:1 dichloromethane/acetone. For ASE, soil sieved to 2 mm and ground with anhydroussodium sulfate. For bath sonication/shaking, soil not sieved or ground but mixed with anhydrous sodium sulfate. Average of duplicatedeterminations shown.

FIGURE 7. Comparison of PAH analysis by ASE and Soxhlet extraction for contaminated soil DL. Soil DL is a dark loam. For ASE, soilsieved to 2 mm and ground with anhydrous sodium sulfate. For Soxhlet extraction, sieving, grinding, and anhydrous sodium sulfate omitted.Error bars for n ) 3.

TABLE 2. Analysis of OCP-Contaminated Blended Soil

concn (mg/kg) ASEa Soxhleta sonicationb

dieldrin 1.35 ( 0.38 1.11 ( 0.34 1.61aldrin 0.10 ( 0.04 0.11 ( 0.03 0.20

a n ) 3; extraction with 1:1 dichloromethane/acetone. b Concentrationvalues are a weighted average over the individual soils in the blendedsample. Data obtained by bath sonication/shaking with 4:1 hexane/acetone and analysis by GC with ECD.

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lower than the bath sonication/shaking values but aresatisfactory given that a general screening method is beingcompared with a method targeted to a particular class ofanalyte.

PAH-Contaminated Soils. Both ASE and bath sonication/shaking methods give a similar PAH concentration profile forthe soils SS, C, and SSC as shown in Figure 6 for the sampleSSC. The total PAH content is given in Table 3.

For the clay sample (C), very similar concentrations of theindividual analytes were obtained by both ASE and sonication.Both extraction techniques appear to have been equallyeffective in disrupting the interactions between the analytesand the matrix. In addition, since clay particles are small,the grinding used in the ASE sample preparation has notimproved recoveries. Grinding has also not caused significantlosses of volatiles, presumably because the analytes are heldstrongly to the surface of the small clay particles.

However for sample SS, which contains small stones, andsample SSC, which contains small stones and lumps of coal,the ASE method gives much greater concentrations of mostanalytes than those obtained by the sonication extraction,resulting in a total PAH content about twice as large. ASErecoveries of 76% and 84% were obtained for naphthaleneand pyrene when the difficult SSC matrix was spiked at themilligrams per kilogram level. Hence the much higher largerPAH content obtained by ASE for these samples is most likelydue the higher extraction efficiency of the ASE method,coupled with the effect of the smaller particle size rangeproduced by grinding the ASE samples.

Figure 7 compares ASE and Soxhlet results for the darkloam soil, sample DL. Again both methods gave very similarconcentration profiles, but the ASE values are uniformly lower,giving a total PAH content of 105 ( 12 mg/kg in comparisonwith 147 ( 9 mg/kg for Soxhlet extraction. The ASE resultsare also more variable.

Since sample DL was ground with anhydrous sodiumsulfate for ASE whereas for Soxhlet it was not, the reducedASE recoveries may be due to poorer permeation of the solventthrough the sample in the presence of sodium sulfate (12) asthe sample has a high moisture content. Alternatively, PAHsmay be lost by adsorption on the ceramic surfaces of themortar and pestle; however, it is unlikely that all analyteswould be equally affected. Similarly, evaporation and/ordegradation on grinding is not consistent with the nearlyidentical concentration profiles obtained from the twotechniques. It is possible that ASE recoveries would beincreased with respect to Soxhlet if 1:1 hexane/acetone, amore efficient solvent for ASE extraction of PAHs (13), wereused rather than the general screening solvent.

ConclusionsThe performance of ASE for the general screening ofcontaminated soils for semivolatile organic pollutants is atleast equivalent to Soxhlet extraction and bath sonication/shaking.

For the spiked uncontaminated soil, ASE recoveries weremostly higher than those from Soxhlet extraction. Both

techniques gave high blanks that interfered in the determi-nation of certain analytes at low levels, even though the redclay loam soil used had a low natural organic content.Background interference was a greater problem for ASE asthe blank chromatogram was much more crowded. In orderto achieve the low detection limits for certain analytes, e.g.,endrin, required in assessing contaminated sites, an ap-propriate extract cleanup procedure may be necessary.Background interference is likely to be more important forextraction of soils containing more natural organic matterthan the red clay loam used in this work, resulting in higherdetection limits. We are currently investigating this problemfurther.

ASE recoveries were superior to Soxhlet for the blendedOCP-contaminated soil and were equivalent to or better thanthose from bath sonication/shaking for the PAH-contami-nated soils, especially for difficult matrices. ASE recoverieswere lower than those from Soxhlet extraction for the darkloam PAH soil (sample DL), but this is acceptable in ascreening method given the practical advantages of muchfaster sample processing time and reduced solvent use. Asecond extraction on the same sample may have improvedrecoveries without adding greatly to the overall analysis time.The effect on ASE recoveries of different grinding regimes,and particularly the use of drying agents such as sodiumsulfate, needs to be systematically investigated using certifiedreference materials.

The ASE method is very easy and quick to use. It wouldbe a suitable low solvent alternative to the EPAV’s currentextraction procedures for screening contaminated soils forsemivolatile organics.

AcknowledgmentsWe would like to thank to the Linbrook Company (MtWaverley, Victoria, Australia) and the Dionex Corporation(Salt Lake City, UT) for making a Dionex ASE 200 instrumentavailable, Mr. Ivo Tence (EPAV) for supplying the bathsonication data on the PAH-contaminated soils, and Mr. JohnGood (EPAV) for supplying the bath sonication data on theindividual OCP-contaminated soils and for providing the redclay loam soil. A.D.S. would like to thank the VictorianEducation Foundation (Victoria, Australia) for the provisionof a postgraduate scholarship. Disclaimer: The use of aproprietary accelerated solvent extraction Instrument in thiswork should not be taken as endorsement of any product bythe Environment Protection Authority of Victoria or theSwinburne University of Technology.

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USEPA SW-846; U.S. GPO: Washington, DC, July 1995.(2) Keith, L. H.; Teillard, W. A. Environ. Sci. Technol. 1979, 13, 416-

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N.; Pohl, C. Anal. Chem. 1996, 68, 1033-1039.(5) (a) Rayment, G. E.; Higgenson, F. R. Australian Laboratory

Handbook of Soil and Water Chemical Methods, Australian Soiland Land Survey Handbook; Inkata Press: Melbourne, 1992.(b) Leeper, G. W.; Uren, N. C. Soil Science: An Introduction, 5thed.; Melbourne University Press: Melbourne, 1993.

(6) Standard Methods for the Examination of Water and Wastewater,19th ed.; American Public Health Association: Washington, DC,1995; pp 6-28, 6-85, 6-86.

(7) Page, A. L.; Miller, R. H.; Keeney, D. R. Methods of Soil Analysis,Part 2, Chemical and Microbiological Properties, 2nd ed.; SoilScience Society of America: Madison, WI, 1989.

(8) Determination of Organochlorine Insecticides and PolychlorinatedBiphenyls (PCBs) in Soil, EPAV Method 6012; Draft; EPAV:Victoria, June 1995.

(9) Semivolatile Organics in Soil and Solid Wastes, EPAV Method6000; EPAV: Victoria, May 1995.

TABLE 3. Comparison of Total PAH Content from ASE andBath Sonication/Shakinga

total PAH (mg/kg)b sample SS sample C sample SSC

ASEc 5.3 43 1900sonicate & shaked 2.2 48 1000

a SS, a sandy loam containing small stones; C, a clay; SSC, a sandyloam containing small stones and pieces of coal. b Average of duplicatedeterminations. c Sieved to 2 mm, ground with anhydrous sodiumsulfate, and extracted with 1:1 dichloromethane/acetone. d Not sievedor ground but mixed with anhydrous sodium sulfate and extractedwith 1:1 dichloromethane/acetone.

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(10) Brady, N. C. The Nature and Properties of Soils, 10th ed.;Macmillan: New York, 1990.

(11) Greenland, D. J.; Hayes, M. H. B. The Chemistry of SoilConstituents; Wiley: Chichester, 1979.

(12) Knowles, D. Dionex Corporation, Salt Lake City Technical Centre,personal communication, 1995.

(13) Hofler, F.; Jensen, D.; Ezzel, J. L.; Richter, B. E. Chromatographie1995, 15, 68-71.

Received for review July 17, 1996. Revised manuscript re-ceived November 5, 1996. Accepted November 9, 1996.X

ES9606283

X Abstract published in Advance ACS Abstracts, February 1, 1997.

VOL. 31, NO. 4, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1127