Journal of Chromatography A, 1192 (2008) 203–207

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Journal of Chromatography A

journa l homepage: www.e lsev ier .com/ locate /chroma

Novel approach to microwave-assisted extraction and micro-solid-phase

extraction from soil using graphite fibers as sorbent

Li Xu, Hian Kee Lee ∗

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leanu-SPE). �-Spoly

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tractisorpton dess sp

and 5d carMAE

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Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapor

a r t i c l e i n f o

Article history:Received 16 January 2008Received in revised form 19 March 2008Accepted 19 March 2008Available online 25 March 2008

Keywords:Microwave-assisted extractionMicro-solid-phase extractionGraphite fiberGraphenePolycyclic aromatic hydrocarbons

a b s t r a c t

A single-step extraction–csolid-phase extraction (�(PAHs) from soil samplesenclosed within a sealedgraphite fiber was used asamples and to extract andtion of water. The best ex20 min, and an elution (deraphy (GC)–flame ionizatiand 3.6 ng/g. With GC–maranges were between 0.1analysis. Granular activatein the PAH extraction. Theand marine sediments, de

1. Introduction

Sample preparation is a tedious and yet unavoidable procedurein analytical chemistry. Microwave-assisted extraction (MAE) hasattracted strong interest in the sample preparation field, especiallyfor environmental matrixes, owing to its efficiency, and time-savingand solvent-saving advantages. Camel wrote a detailed reviewabout its theory and applications in environmental samples in 2000[1]. More recently, Srogi also presented a comprehensive reviewabout its applications in environmental analytical chemistry [2].

Generally, another concentration and/or cleanup procedureis required after MAE, particularly of “dirty” samples, to max-imize analyte recoveries and minimize interferences. Differentapproaches have been employed to cleanup and further con-centrate the extracts, such as solid-phase extraction (SPE) orsolid-phase microextraction (SPME) [3–7], saponification [8–10],gel permeation chromatography [8,11,12], liquid–liquid extraction(LLE) or liquid-phase microextraction [13,14]. These follow theMAE process as a separate step. There are few reports about thecombination of MAE with the subsequent cleanup procedure inone step. Hitherto, only such a combination, that of involving

∗ Corresponding author. Tel.: +65 6516 2995; fax: +65 6779 1691.E-mail address: chmleehk@nus.edu.sg (H.K. Lee).

0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.chroma.2008.03.060

43, Singapore

p procedure involving microwave-assisted extraction (MAE) and micro-has been developed for the analysis of polycyclic aromatic hydrocarbons

PE is a relatively new extraction procedure that makes use of a sorbentpropylene membrane envelope. In the present work, for the first time,rbent material for extraction. MAE–�-SPE was used to cleanup sedimentoncentrate five PAHs in sediment samples prepared as slurries with addi-on conditions comprised of microwave heating at 50 ◦C for a duration ofion) time of 5 min using acetonitrile with sonication. Using gas chromatog-tection (FID), the limits of detection (LODs) of the PAHs ranged between 2.2ectrometry (MS), LODs were between 0.0017 and 0.0057 ng/g. The linear0 or 100 �g/g for GC–FID analysis, and 1 and 500 or 1000 ng/g for GC–MSbon was also used for the �-SPE device but was found to be not as efficient–�-SPE method was successfully used for the extraction of PAHs in rivertrating its applicability to real environmental solid matrixes.

© 2008 Elsevier B.V. All rights reserved.

MAE-headspace (HS)-SPME, has been realized by modifying themicrowave oven device [15–17]. In this configuration, the sampleextracted by microwave is placed in the microwave oven, while theHS-SPME takes place in a condenser outside the oven through a

connection between the sample and the condenser.

SPME has been a popular extraction technique since its introduc-tion in the 1990s by Pawliszyn’s group [18]. SPME is the method ofchoice to cleanup and concentrate the extract by microwave irradi-ation because it is a completely solvent-free procedure. In addition,various sorbents are available commercially or may be prepared inhouse, affording a reasonable range of high selectivity and sensi-tivity.

The introduction and creation of novel phases for SPME isan essential part of the development of this technology. Car-bon materials have recently been attracting growing interestdue to their excellent adsorption properties. For example, acti-vated carbon fibers have been investigated for SPME applicationsinstead of the conventional coated silica ones, for the enrich-ment of organochlorine pesticides [19–21], benzyl chloride [22] andchlorohydrocarbons [23]. Another kind of activated carbon fiber,Toyobo-KF, has also been evaluated for the extraction of DDT in ani-mal fat [24]. All the above reports demonstrated that carbon fiberis more effective than the conventional granular activated carbon,due to the special surface structure and excellent adsorption prop-erties of the former. A carbon-coated fiber was also described for

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204 L. Xu, H.K. Lee / J. Chrom

the SPME of BTEX (benzene, toluene, ethylbenzene, p-xylene ando-xylene) [25].

It is known that the characteristic structures and electronicproperties of carbon nanotubes allow them to interact effectivelywith organic molecules. The surface, made up of hexagonal arraysof carbon atoms in graphene sheets, interacts strongly with thebenzene ring of aromatic compounds [26,27]. Hence, they haveshown to be an effective sorbent phase in extraction [28]. Basheer etal. [29] reported the first instance of micro-solid-phase extraction(�-SPE) that used muti-walled carbon nanotubes (MWCNTs) as sor-bent, applied for the enrichment of organophosphorus pesticides.This �-SPE method was based on the packing of the MWCNTs in asealed porous polypropylene membrane envelope (2 cm × 1.5 cm).Since the porous membrane afforded protection of the MWCNTs, nofurther cleanup of the extract was required. The consumption of sol-vent in the extraction was much less compared to conventional SPE.

SPME also has some disadvantages, including fiber fragility, costand problems with analyte carryover, etc. �-SPE was demonstratedto be able to address these disadvantages [29]. Very recently, thesame authors developed this �-SPE device containing C18 sorbentto extract acidic drugs from wastewater [30].

In the present work, graphite fiber was for the first time usedas the sorbent in �-SPE. The procedure involved the simultaneousMAE of polycyclic aromatic hydrocarbons (PAHs) as model com-pounds, from solid samples. To our knowledge, so far, MAE hasnot been reported for the extraction of analytes from solid sam-ples with simultaneous cleanup and concentration by �-SPE. Noadditional cleanup process is needed, because of the protection,as mentioned above, offered by the porous membrane of the �-SPE device. After optimization of the microextraction conditions,the established method was applied to real sediment samples. Theresults were also compared to sonication-assisted extraction (SAE)and conventional extraction using a stirring bar in conjunction with�-SPE using the same sorbent. Moreover, normal granular activatedcarbon material was also used in the �-SPE device for comparisonwith the performance of graphite fibers.

2. Experimental

2.1. Chemicals and reagents

Accurel polypropylene sheet membrane (0.2-�m pore size)was bought from Membrana (Wuppertal, Germany). HPLC-

grade methanol, hexane and acetonitrile were obtained fromFisher (Loughborough, UK). The PAHs, fluorene, phenanthrene,anthracene, fluoranthene and pyrene, were purchased fromSupelco (Bellefonte, PA, USA). Graphite fibers were obtained fromJohnson-Matthey Materials Technology (Billingham, UK). Ultrapurewater was produced on a Nanopure (Barnstead, Dubuque, IA, USA)water purification system.

2.2. Gas chromatography (GC)–flame ionization detection (FID)and GC–mass spectrometry (MS) analysis

GC–FID analysis was performed on a Hewlett-Packard (Palo Alto,CA, USA) series 6890 instrument. A ZB-1 column (30 m × 0.25 mmI.D., 0.25 �m film thickness) from Phenomenex (Torrance, CA,USA) was used for separations. Helium was used as carrier gasat a flow rate of 1.8 mL/min. The GC conditions were as follows:initial oven temperature 80 ◦C for 2 min, increased to 300 ◦C atthe rate of 10 ◦C/min, then held at 300 ◦C for 2 min. The injectortemperature was 280 ◦C. All injections were in splitless mode andmade in triplicate.

A 1192 (2008) 203–207

GC–MS analysis was carried out using a Shimadzu (Tokyo, Japan)QP2010 GC–MS system equipped with a Shimadzu AOC-20i autosampler and a DB-5 fused silica capillary column (30 m × 0.25 mmI.D., film thickness 0.25 �m) (J&W Scientific, Folsom, CA, USA). Allthe injection and temperature program conditions were the sameas for the GC–FID analysis. All standards and samples were analyzedin selective ion monitoring (SIM) mode at least in triplicate.

2.3. Scanning electron microscopy (SEM)

A JSM-5200 scanning electron microscope (JEOL, Tokyo, Japan)was used for SEM observation of the graphite fiber. To preparesamples for SEM, the graphite fiber was fixed on the stub by adouble-sided sticky tape and then coated with platinum by a JFC-1600 (JEOL) Auto fine coater for 50 s.

2.4. Sample preparation

Stock solutions (0.2 mg/mL of each analyte) were preparedseparately in methanol. Soil samples (previously checked to bePAH-free) were mixed with acetone until the soil was covered bythe solvent. Appropriate volumes of the stock solutions were addedto the above slurry to give the desired analyte concentration. Inthis experiment, 5 �g/g was used to study extraction performanceunder different conditions followed by GC–FID, unless stated oth-erwise. The prepared soil samples were dried at room temperatureand then stored in the refrigerator until analysis. Soil samples werecollected from several locations in Singapore.

2.5. Microwave-assisted extraction (MAE)

Briefly, 10 mg of graphite fibers was weighed and put into thepolypropylene membrane envelope (1 cm length × 0.5 cm width),whose edges were heat-sealed as previously described [29]. Thesegraphite fiber �-SPE devices were preconditioned and cleaned bymethanol for 10 min before use.

MAE was carried out using a CEM (Matthews, NC, USA) MES-1000 microwave extraction system equipped with a solvent detec-tor. Approximately 1.0 g of the soil sample was accurately weighedand quantitatively transferred to a Teflon-lined extraction vessel.Ten milliliters of water and the �-SPE device were then added tothe sample. Closed-vessel MAE was performed under controlledtemperature for a specified time. Two minutes were allowed for thetemperature to be ramped up from room temperature to the desired

level. When the irradiation period was completed, 10 min wasallowed for the extraction vessel to cool down to room temperature.

In comparative experiments, �-SPE devices with granular acti-vated carbon were also prepared in the same way and used in thesame extraction process.

2.6. Sonication-assisted desorption

After extraction, the �-SPE device was removed, rinsed in ultra-pure water, dried with lint-free tissue, and placed in a 200-�L vial.Analyte desorption or elution was carried out by sonication for aprescribed time using 150 �L of acetonitrile. This final extract wasdirectly used for analysis (GC injection volume: 1 �L).

2.7. Sonication-assisted extraction (SAE) and agitation-assistedextraction (AAE)

SAE was carried out using a sonicator (Midmark, Versailles, OH,USA) for 20 min followed by the sonication-assisted elution for5 min in 150 �L of acetonitrile. AAE was performed by a magneticstirrer with a stirring bar at 1000 rpm for 20 min.

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L. Xu, H.K. Lee / J. Chrom

3. Results and discussion

3.1. Properties of a graphite fiber

Carbonaceous materials, especially activated carbon, have beenwidely employed as sorbents because of their excellent adsorptioncapacity. It is therefore important and interesting to develop similarmaterials with suitable properties to fully exploit their adsorp-tion capability. The proportion of graphite in a carbon fiber canrange from 0 to 100%. Carbon fibers refer to those fibers that are atleast 92% by weight of carbon in composition. The fiber is called agraphite fiber due to the presence of high proportion of graphite.The carbon fibers employed here contain 99.5% by weight of carbon.The cylindrical shape of the fiber allows analytes to be adsorbedfaster, thus accelerating the adsorption (and desorption) processes[31] because of the high surface area. Besides, these fibers areeconomical to procure, and, based on our experience, were morefacilely handled compared to CNTs.

Fig. 1 shows the SEM image of a graphite fiber at 1000× magni-fication. The smooth surface is clearly observed. The dimension ofthe fiber is 6.4 mm (length) × 8 �m (diameter).

3.2. Optimization of MAE–�-SPE

MAE is an extraction technique which uses polar solvents toextract target compounds primarily from solid matrixes. Althoughorganic solvents are normally used, water is regarded as a more sat-isfactory “solvent” because of its high polarity and has been appliedto MAE in some cases [3,4,15–17]. Another benefit is that water isnon-toxic and more environmentally acceptable than organic sol-vents, thus eliminating undesirable exposure to operators. In thisstudy, water was used to extract the PAHs from soil samples, whilstat the same time, acted as the sample media for �-SPE. The min-imum water volume required by the microwave instrument was10 mL and using this volume of water also ensured that soil sampleswere completely immersed.

Hexane, methanol and acetonitrile were investigated as elu-tion solvents for the �-SPE device after extraction. Under the sameextraction and elution conditions, acetonitrile showed the highestchromatographic signals, followed by methanol and hexane.

Temperature is of prime importance in ensuring efficient MAE,as elevated values usually enhance the extraction, as a result of anincreased diffusivity of the solvent into the interior of the matrixunder high temperatures, as well as an enhanced desorption of the

Fig. 1. SEM of a single graphite fiber.

Fig. 2. Influence of holding (heating) temperature on MAE efficiency.

components from the active sites of the matrix [1]. Fig. 2 shows theinfluence of temperature on the extraction efficiency. For all the tar-

get analytes, analytical signals increased from 35 to 50 ◦C and thendecreased with further increase of the temperature. The reason forthis observation may be ascribed to the evaporation of solutes as thetemperature was raised beyond 50 ◦C. Ideally, mass transfer associ-ated with this process is from the soil to the aqueous phase, then tothe graphite fibers. However, because of their semi-volatile proper-ties, the analytes are in some degree distributed into the headspaceof the extraction vessels under high temperature, leading to the lossof availability of these compounds to the graphite fibers. Anotherfactor may lie in the fact that the adsorption process is exothermic.With the increasing temperature, an enhanced desorption of thecomponents from the sorbent may also occur. Consequently, theextraction efficiency decreased with the increase of the tempera-ture above 50 ◦C. On the other hand, analyte diffusivity and releasefrom the matrix materials was inefficient below a threshold tem-perature [32] (that is why heating is necessary for extraction in thefirst place). Thus, a temperature of 50 ◦C, based on our observation(Fig. 2), appeared to be the most satisfactory compromise amongstthe conflicting effects.

Fig. 3. Time profile of MAE of PAHs.

206 L. Xu, H.K. Lee / J. Chromatogr. A 1192 (2008) 203–207

Fig. 4. Time profile of elution of PAHs.

Fig. 3 describes the effect of holding (heating) time durationon the extraction efficiency. All the model analytes exhibited thehighest analytical signals at 20-min heating time. This result canbe explained by the same reason offered above for the influence oftemperature. Due to the increase of the heating time duration, theprobability of the semi-volatile compounds being released to theheadspace is enhanced, thus making them unavailable for extrac-tion. As a result, there is reduced extraction by the graphite fibers.Thus, it is reasonable to surmise that longer heating times lead to adecrease in extraction efficiency as far as MAE–�-SPE is concerned.In subsequent experiments, 20 min was used as heating time.

The effect of the duration of elution time with sonication wasalso investigated from 5 to 30 min, as shown in Fig. 4. It can be

observed that almost all the analytes showed similar intensitiesof chromatographic signals under different elution time durationfrom 5 to 20 min. An elution time of 30 min leads to only slightlydecreased chromatographic signals. These results demonstratedthat elution time duration has little effect on the elution efficiency.On the basis of the foregoing, 5 min was adequate and acceptableto elute all the adsorbed analytes.

Based on the above discussion, optimum heating time was20 min at 50 ◦C, followed by sonication-assisted analyte elution in150 �L of acetonitrile for 5 min. All the following experiments werecarried out under these optimized conditions.

3.3. Comparison

MAE was compared with both SAE and AAE. From Fig. 5, itcan be clearly seen that for all the tested analytes, MAE showedhigher chromatographic signals among three methods with thesame graphite fibers as sorbent. A comparison in the use of differentcarbon materials, graphite fiber and granular activated carbon, wasalso made. From Fig. 5, it is seen that carbon fibers exhibited higherextraction efficiency than granular activated carbon. Even under

Table 1Regression data and LODs of analytes

Analytes GC–FID

Linear range (�g/g) r2 LOD (ng/g) RSD (n = 5

Fluorene 0.1–50 0.9995 3.4 4.1Phenanthrene 0.1–50 0.9999 2.2 5.5Anthracene 0.1–50 0.9973 2.3 10.1Fluoranthene 0.1–100 0.9999 3.6 4.2Pyrene 0.1–100 0.9996 2.9 8.1

Fig. 5. Comparison of different methods and materials. GF = graphite fiber,AC = activated carbon.

MAE, activated carbon exhibited lower extraction efficiency thancarbon fibers by SAE and AAE. This indicated that graphite fiber is avery good material for extraction of the PAHs, with high efficiency.

Many extraction methods have been developed for PAHs incontaminated soils. For example, the United States Environmen-tal Protection Agency (EPA) has a method making use of microwaveextraction from sediment, Method 3546 [33]. For aqueous samples,EPA Method 3535A for SPE is available [34]. The present methodproposed here combines MAE and �-SPE in which cleanup andconcentration can be achieved in a single step, avoiding tediousadditional sample purification/cleanup procedures. It can be con-sidered to be a green method, because of the use of water as the MAEextractant “solvent”. Additionally, the operation is simple, time-

saving and the required equipment is generally affordable. Seeingthat the �-SPE itself is involved in extraction from a liquid matrix,it is reasonable to believe that it can be effectively used directly foraqueous samples, apart from soil samples.

MAE has been widely used for the extraction of PAHs, from soilsamples, including those that are of higher molecular weights thanthe ones considered in the present work. It is therefore reasonableto assume that when the analytes are released into the slurryby MAE, extraction by �-SPE should be expected to proceed asefficiently as has been demonstrated here for the lower molecularweight compounds, given the affinity of the graphite fibers foraromatic compounds. Future work will be directed towards thisdirection.

3.4. Method evaluation

All the tested PAHs exhibited good linearity with good squaredregression coefficients (r2), as illustrated in Table 1. As far as GC–FIDwas concerned, the linearity range was wide, from 0.1 to 50 �g/g forfluorene, phenanthrene and anthracene, and from 0.1 to 100 �g/gfor fluoranthene and pyrene with regression coefficients greater

GC–MS

) (%) Linear range (ng/g) r2 LOD (ng/g) RSD (n = 5) (%)

1–500 0.9962 0.003 7.71–500 0.9897 0.0017 5.21–500 0.9910 0.0024 4.91–1000 0.9912 0.0057 9.71–1000 0.9932 0.0045 10.2

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[22] T.H. Sun, L.K. Cao, J.P. Jia, Chromatographia 61 (2005) 173.[23] T. Sun, N. Fang, Y. Wang, J. Jia, J. Yu, Anal. Lett. 37 (2004) 1411.

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Table 2Level of PAHs (�g/g) in Singapore coastal sediment samples extracted with MAE–�-SPE using graphite fibres as sorbent with analysis by GC–FID or GC–MS

PAH Location

1a 2 3 4a

Fluorene 7.39 0.24 0.15 3.65Phenanthrene 8.97 0.20 0.10 3.66Anthracene 14.6 0.30 0.15 3.51Fluoranthene 12.6 0.42 0.19 1.77Pyrene 15.0 0.63 0.13 1.53

a Determination from the calibration plots of GC–FID. All the others were deter-mined from the calibration plots of GC–MS.

than 0.9900. Reproducibility was assessed by a series of five inde-pendent experiments carried out on different days using the spikedsoil samples. The relative standard deviations (RSDs) for the fivetested PAHs were lower than 10.1%. Limits of detection (LODs) (cal-culated based on peak height and at a signal-to-noise ratio of 3)were between 2.2 and 3.6 ng/g. These results are comparable toprevious reported values obtained by GC–MS [32,35], in whichPAHs were extracted by dynamic LPME and microwave-assistedsolvent extraction. When GC–MS by SIM mode, instead of GC–FID,was used, LODs were 0.0017, 0.0024, 0.003, 0.0045 and 0.0057 ng/gfor phenanthrene, anthracene, fluorene, pyrene and fluoranthene,respectively. These values are three orders of magnitude better thanthose obtained by GC–FID. The linearity ranged from 1 to 500 ng/g(fluorene, phenanthrene and anthracene) and 1 to 1000 ng/g (fluo-ranthene and pyrene).

It is stated in EPA Method 3546 [33] that this procedure has beenvalidated for solid matrices containing 50–10 000 ng/g of semi-volatile organic compounds (to which PAHs belong), indicating thatthe present MAE–�-SPE method has much lower LODs.

There have been some other recent studies on the extraction anddetermination of PAHs in soil samples. In an MAE method combinedwith microextraction (headspace SPME) [15] for landfill leachates,the LODs obtained for those PAHs that were common with thepresent work ranged from 0.25 to 1.5 ng/g. The other studies men-tioned here are not based on microextraction. Villar et al. extractedPAHs from sewage sludge using MAE followed by purification by asilica column, and reported the LODs to be between 5 and 12 ng/gby HPLC-fluorescence detection [36]. Wang et al. made a system-atic comparison among Soxhlet extraction, MAE and acceleratedsolvent extraction of PAHs from soils with GC–MS detection, andthese three methods gave LODs in the range of 0.29 and 0.67 ng/g

[37]. A study on the MAE of PAHs in marine sediments followedby cleanup by an alumina column provided LODs of between 0.03and 0.45 ng/g [38]. In comparison, the present MAE–�-SPE methodgave much lower LODs (0.0017–0.0057 ng/g).

3.5. Applications

Four different soil samples (river and marine sediments),collected from local sites, were analyzed using the developedextraction method with GC–FID or GC–MS analysis. The results areshown in Table 2. As expected, since PAHs are ubiquitous, all thesediments were contaminated with PAHs. The individual concen-trations of PAHs ranged from 0.10 to 15.0 �g/g, similar to the levelsof contamination of soil by PAHs elsewhere [36,39,40]. In addition,the results demonstrated that the MAE-�-SPE approach is feasiblefor application to genuine environmental solid sample analysis.

4. Conclusion

In this study, graphite fiber was investigated as a novel sorbentfor �-SPE. When combined with MAE, this new material exhibited

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A 1192 (2008) 203–207 207

excellent extraction capability for PAH compounds directly fromsoil samples. Water was shown to be a good extractive mediumfor MAE. Cleanup and concentration could be accomplished in asingle step, avoiding extra tedious procedures normally associatedwith post-treatment processing after MAE (as required by most ofthe EPA methods). Satisfactorily low LODs from 2.2 to 3.6 ng/g andgood reproducibility (RSD < 10.1%) could be obtained when MAE-�-SPE was coupled with GC–FID. When GC–MS was used, lowerLODs, ranging from 0.0017 to 0.0057 ng/g, were achieved. MAE-�-SPE was demonstrated to be a fast and robust approach and can bedirectly applied to detect PAHs in real environmental solid samples.

Acknowledgement

The authors gratefully acknowledge the financial support of thisresearch by the National University of Singapore (grant number R-143-000-303-112). L.X. also wishes to thank the University for anaward of a research scholarship. The technical assistance of FrancesLim is acknowledged.

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