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Journal of Chromatography A, 1216 (2009) 6549–6553

Contents lists available at ScienceDirect

Journal of Chromatography A

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

ulfonated polyvinyl chloride fibers for cation-exchange microextraction

i Xu a,b, Hian Kee Lee b,∗

College of Pharmacy, Tongji Medical Center, Huazhong University of Science and Technology, Wuhan 430030, ChinaDepartment of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

r t i c l e i n f o

rticle history:eceived 6 May 2009eceived in revised form 22 July 2009ccepted 29 July 2009

a b s t r a c t

Polyvinyl chloride (PVC) fiber was derivatized by concentrated sulfuric acid to yield sulfonated PVC(PVC-SO3H). The PVC-SO3H fiber had dual properties as a sorbent, based on cation-exchange andhydrophobicity. In the present study, the novel fiber was used directly as an individual device for extractionpurposes in the cation-exchange microextraction of anaesthetics, followed by high-performance liquid

vailable online 3 August 2009

eywords:ulfonatedolyvinyl chlorideiber

chromatography-UV analysis. The results demonstrated that this PVC-SO3H fiber-based microextractionafforded convenient operation and cost-effective application to basic analytes. The limits of detection forfour anaesthetics ranged from 1.2 to 6.0 ng/mL. No carryover (because of its disposable usage), and no lossof sorbent phase (which normally occurs in stir-bar sorptive extraction) during extraction were observed.

© 2009 Elsevier B.V. All rights reserved.

icroextraction

ation-exchange

. Introduction

Most analytical methods involve five stages: sampling, samplereparation, separation, detection and data analysis. The samplereparation step is generally regarded as a bottleneck and can takep to 70% or more of the time needed for the whole analytical pro-edure. In many cases, sample preparation is unavoidable becauseost samples are complex and analytical instruments cannot han-

le them directly. The main aim of sample preparation is to cleanp, isolate and concentrate the analytes of interest, while render-

ng them in a form that is compatible with the analytical system1]. Classical sample pretreatment methods include liquid–liquidxtraction (LLE) and solid-phase extraction (SPE), whilst liquid-hase microextraction (LPME) and solid-phase microextractionSPME) represent the most typical miniaturized sample prepara-ion approaches that are solvent-minimized or solventless. LPMEnd SPME have superior advantages to LLE and SPE in terms ofabor-saving, low consumption of solvent and reagents, efficiencynhancement, and environmental friendliness [1–6].

SPME was initially introduced by Pawliszyn’s group [7]. Its solvent-free, portable, simple to use, relatively fast, and cane automated and coupled online to analytical instrumentation,

articularly gas chromatography (GC). Research has progressedxtensively since its introduction. It has developed into a highlyuccessful commercial product and remains a popular choice.

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

021-9673/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.chroma.2009.07.056

The original SPME format involved a polymeric phase coatedonto a fused silica fiber. Extraction is based on the establishmentof equilibrium between the analyte and the coating. The analyte isthen desorbed from the fiber into a suitable separation and detec-tion system. One disadvantage of SPME is that the fiber is fragileand needs to be protected by a metal sleeve that also serves asan injection needle. The fiber has a limited lifetime and can beexpensive. Configurations designed as alternatives to the conven-tional fiber-based SPME have been and continue to be investigated,such as in-tube SPME [8], stir-bar sorptive extraction (SBSE) [9],microextraction in a packed syringe [10], micro-solid-phase extrac-tion [11], thin-film microextraction [12] and polymer-coated hollowfiber membrane microextraction (PC-HFME) [13–17]. These mod-ified configurations have been designed to overcome some of theweaknesses and disadvantages of fused silica fiber-based SPME, andto afford more convenient, cost-effective and robust operation.

PC-HFME was developed in our laboratory very recently[13–17]. In this microextraction approach, a short length ofpolypropylene hollow fiber was coated with a functional polymer,such as polyhydroxylated polyparaphenylene [13], hydroxylatedpolymethacrylate [14] or dihydroxylated polymethalmethacrylate[15,16]. These polymeric coatings served as sorbent phases duringextraction. The coated fiber tumbled freely in the stirred samplesolution and this facilitated extraction. This process combined theadvantages of SPME and SBSE, ensuring enhanced extraction effi-

ciency. Such an extraction device is also disposable, thus avoidingthe occurrence of carryover problems or memory effects.

For use in a similar manner, an inorganic oxide-based fiber, zir-conia hollow fiber, was synthesized via the template and sol–gelapproach and investigated for the enrichment of the degradation

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roducts of a nerve agent [18]. The zirconia fiber represented anndividual device and was directly usable for extraction. Handling

as more convenient than, for example, a sorbent that needs to beoated on a rod or fiber. In addition, it was more easily preparedhan a monolithic material.

In the present study, a hollow fiber membrane, polyvinyl chlo-ide (PVC), normally used in wastewater treatment processes, wasnvestigated as a new sorbent phase after functionalization, for a

icroextraction procedure similar to PC-HFME. To generate the sor-ent, a simple and convenient one-step reaction by concentratedulfuric acid to yield sulfonic acid groups, termed here as PVC-O3H, was carried out. Thus, the PVC-SO3H fiber possessed strongation-exchange characteristic afforded by the sulfuric acid groups.oderately polar anaesthetics were used as model compounds for

nvestigations on the cation-exchange microextraction capability ofVC-SO3H, followed by high-performance liquid chromatographyHPLC)-UV analysis.

. Experimental

.1. Chemicals and reagents

HPLC-grade methanol, 1,2-dichloroethane, tetrahydrofuranTHF) and acetonitrile were obtained from Fisher (Loughbor-ugh, UK). The anaesthetics, procaine, lidocaine, tetracaine andupivacaine were purchased from Aldrich (Milwaukee, WI, USA).mmonia, sodium hydroxide (NaOH), hydrochloric acid (HCl) andulfuric acid (98%) were purchased from Merck (Darmstadt, Ger-any). PVC fibers (I.D. 1.0 mm; O.D. 1.8 mm) were a gift from

inomem Technology (Singapore); they were originally manufac-ured by Microdyne (Wiesbaden, Germany). Ultrapure water wasroduced on a Nanopure (Barnstead, Dubuque, IA, USA) waterurification system.

.2. Apparatus

The determination of the anaesthetics was performed on aaters (Milford, MA, USA) HPLC system. The chromatographic

ystem consisted of a Rheodyne (Cotati, CA, USA) 7725i injectorquipped with a 20-�L sample loop, a Waters 1525EF binary pump,nd a Waters 2487 UV–vis spectrophotometric detector. Data wereollected and processed by Empower version 5.0 (Waters) datanalysis software.

A C18 column (4.6 mm × 150 mm, 5 �m) from Metaphase Tech-ologies (Bellefonte, PA, USA) was used for separations. Separationsere carried out at a column temperature of 22 ◦C with a flow

ate of 2 mL/min. A mobile phase consisting of acetonitrile: 0.3%mmonia solution (60:40, v/v) was used. The detection wave-ength was 220 nm. All the experiments were performed inriplicate.

.3. Sulfonation of PVC fibers [19,20]

The sulfonation of PVC fibers was carried out by first soakingn 1,2-dichloroethane for 4 h to swell the fibers. The pre-swollenbers were soaked in concentrated sulfuric acid (98%) for anotherh at 55 ◦C. During this step, the fibers turned from ivory colour

nto light brown, and then gradually to dark brown. Sulfuriccid is corrosive, but at the same time could react with PVCsee below). After reaction, the fibers were washed with copiousmounts of deionized water to remove excess sulfuric acid, and thenith acetonitrile to remove any possible impurities. The cleaned

VC-SO3H fibers were dried at room temperature and ready forse.

1216 (2009) 6549–6553

2.4. Characterization

A JEOL (Tokyo, Japan) JSM-5200 scanning electron microscope(SEM) was used for observation of the PVC and PVC-SO3H fibers.To prepare samples for SEM, the 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.

Elemental analysis was carried out on a PerkinElmer (Shelton,CT, USA) 2400 analyzer with a CHN combustion tube. Fourier-transform infrared (FTIR) spectroscopy was performed on a VarianFTIR 3100 system (Palo Alto, CA, USA). The fiber was first dissolvedin THF by sonication and a KBr disk was immersed in the THF. TheKBr disk was then dried under vacuum to remove the water beforebeing placed in the FTIR spectrometer for measurement.

The cation-exchange capacity was determined as follows: DryPVC-SO3H fibers (0.1 g) were suspended in 12 mL of standard aque-ous NaOH solution (0.1 M) in a flask and the mixture was shakenfor 12 h. 10 mL of the supernatant was taken from the flask andwas titrated with standard aqueous HCl solution (0.1 M) withmethyl orange as the indicator. The cation-exchange capacity (Qiein mM/g) of the PVC-SO3H fiber was calculated by the followingequation:

Qie = 1.2(10NNaOH − VHCl × NHCl)W

where NNaOH and NHCl are the concentrations of the standard aque-ous NaOH and HCl solution, respectively. VHCl is the volume (mL) ofthe standard aqueous HCl used in the titration, and W is the sampleweight (g).

2.5. Sample preparation

Stock solutions of the four anaesthetics (1.0 mg/mL of each ana-lyte) were prepared separately in methanol. Water samples wereprepared by spiking ultrapure water with the analytes at a knownconcentration (500 ng/mL) to study extraction performance underdifferent conditions. A urine sample was collected from a volun-teer. It was first filtered through a membrane of 0.25-�m poresize, before being diluted with ultrapure water (v:v, 1:9). Subse-quently, the filtrate was directly spiked with the four anaesthetics.The solution pH was adjusted to 5.0 with HCl.

2.6. Extraction procedure

For extraction, a PVC-SO3H fiber of 1.0 cm length was placedin the sample solution, which was agitated for a prescribed time.The volume of the aqueous solution was 10 mL in a 12-mL vial.After extraction, the fiber was taken out and placed in ultrapurewater to remove possible surface contamination (without soni-cation) and then dried by lint-free tissue. The wash solution wasinjected into the HPLC system to check for possible loss of analytes:no peaks were observed at the retention time of target analytes.Finally, it was placed in 100-�L acetonitrile containing 2.0% ammo-nia; the solution was ultrasonicated for a prescribed time to desorbthe analytes. An aliquot of 20 �L of the final elution solution wasdirectly injected into the HPLC-UV system for analysis. However,for the urine sample, considering the complexity of the matrix, thefiber after extraction was subjected to two-step washing, watercleanup followed by acidified acetonitrile wash (wash solutionswere checked by HPLC, to be free of analytes). This acidified acetoni-

trile wash step was to remove any possible interfering substancesthat might have been adsorbed onto the fiber based on hydropho-bic interaction alone, while the anaesthetics were still retained onthe fiber (based on cation-exchange interaction), because the ionicinteraction is much stronger than the hydrophobic one [21]. After

L. Xu, H.K. Lee / J. Chromatogr. A 1216 (2009) 6549–6553 6551

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Fig. 1. SEM images of PV

his step, the fiber was sonicated in alkaline acetonitrile for desorp-ion of the analytes.

. Results and discussion

.1. Characterization of PVC-SO3H fiber

Fig. 1 compares the SEM images of the PVC and PVC-SO3H fibers.he PVC fiber is a highly porous material, which is characterizedy the presence of macro channels of 30–50 �m on the fiber wallnd large mesopores of 15 nm on the wall skeleton. Obviously, afterreatment with concentrated sulfuric acid, there was a differencen the surface morphology of the fiber. The surface roughness isbserved to have increased. This may be explained by the effect oforrosion by, and reaction of PVC with, the concentrated sulfuriccid. It is well-known that most materials that come into contactith sulfuric acid will undergo at least some degree of corrosion.

lemental analysis of PVC-SO3H fiber revealed that the sulfur con-ent was up to 2.66%; there was, as expected, no detectable sulfurn the case of PVC, indicating that the latter had undergone sulfona-ion. Additionally, according to the calculation described in Section.4, the cation-exchange capacity of the PVC-SO3H fiber was esti-

ated to be 1.71 mM/g. The IR spectrum of the PVC-SO3H fiber is

resented in Fig. 2. The peaks around 1030 and 1100 cm−1 weressigned to symmetric and asymmetric stretching of the sulfoniccid groups.

Fig. 2. FTIR spectrum of PVC-SO3H fiber.

nd PVC-SO3H (b) fibers.

3.2. Cation-exchange microextraction of anaesthetics

Sulfonic acid-containing materials have been extensively usedfor cation-exchange purification, concentration and separation pur-poses. They are especially efficient for the extraction of basicanalytes containing nitrogen that is difficult to be extracted bytraditional SPE sorbents [21,22]. In the present study, the cation-exchange performance of the PVC-SO3H material was evaluated byconsidering the factors thought to be influential to the extractionefficiency, such as the pH of the sample solution, the sample agi-tation speed, extraction time, desorption solution and desorptiontime.

3.2.1. Optimization of extraction conditions for anaestheticsFig. 3 depicts the influence of the pH of the sample solution on

the extraction efficiency. The analytical signals increased when thepH increased from 3.0 to 5.0, and then decreased when the pH wasfurther increased to 6.5. Sulfonic acid is a strong acid with a pKa of0.699, which means that the sulfonic acid groups can be ionized ina wide pH range (>2) [23]. The pKas of the studied analytes are allabout 8.0, which implies that low pH should facilitate the protona-tion of the compounds. The strongest analytical signal was found

a compromise between the ionization of the sulfonic acid groupsand the protonation of the analytes, in a way that the cations canbe adsorbed on the ionized fiber based on the ion-exchange mech-anism. When the pH was further increased to 8.0 (not shown in

Fig. 3. Influence of pH of sample solution on extraction efficiency.

6552 L. Xu, H.K. Lee / J. Chromatogr. A 1216 (2009) 6549–6553

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3.2.2. Method evaluationThe reproducibilities of the HPLC peak areas were studied for

four replicate experiments based on different sulfonated PVC fibersfor an aqueous sample containing 100 ng/mL of the four anaesthet-

Fig. 4. Influence of agitation speed on extraction efficiency.

ig. 3), the analytical signals were drastically reduced. This coulde due to two possible reasons. First, the protonation of targetnalytes is largely inhibited at high pH, leading to a weak inter-ction between the sulfonic acid groups and the analytes. Second,o adjust the pH to 8.0, an extra amount of NaOH was introducedo the sample solution. The presence of sodium ions could have aegative impact on the ion-exchange process, due to competitionith the target analytes for sorption sites on the fiber [24]. As a

esult, the interaction between the target analytes and the sulfoniccid groups on the fiber could be seriously weakened. To obviatehe problem observed when a pH of 8 was applied, the followingxperiments were carried out with the sample solution at a pHf 5.

In our experiments, the sample agitation speed was investigatedrom 500 to 1250 rpm for a 20-min extraction. The relationshipetween peak areas and agitation speeds is shown in Fig. 4. Clearly,s the agitation speed was increased, the peak areas of all the testednalytes were enhanced correspondingly. Agitation of the samplenhances convection and facilitates mass transfer between the fibernd the analytes, thus promoting extraction. The maximum settingf the stirrer was 1250 rpm. At this setting, the highest peak areaas observed, as shown in Fig. 4. Therefore, 1250 rpm was selected

s the optimum rate of agitation.Fig. 5 depicts the HPLC peak responses for 10–60 min extraction.

ith increasing extraction time, peak areas increased gradually.his is as expected since mass transfer is a time-dependent process.owever, equilibrium was not attained in this case. The diffusion

n the ion-exchange process is slow, and thus requires time forxchange to take place [25]. Hence, higher signal intensities coulde expected for extraction of >60 min. However, to permit practical

mplementation of the technique, an extraction time 60 min waselected.

The sample salinity influence was investigated in the presence of0–200 mg/mL of NaCl. With the increasing NaCl content, HPLC sig-als were observed to decrease. It is well-known that Ca2+ and Mg2+

re more tightly held than Na+ in cation-exchange chromatographynd the interaction power of cations in SPE should correlate withheir strength as predicted by classical cation-exchange chromatog-aphy [24,26]. Hence, it is expected that they will compete with thenalytes for ionic sites much more strongly than Na+. Speculatively,

he presence of Ca2+ and Mg2+ could have negative influence on thextraction efficiency, even more seriously than Na+. Therefore, lowalinity of the sample solution for cation-exchange extraction wasreferred.

Fig. 5. Extraction time profile.

Analyte desorption conditions were also investigated. It is well-known that basic compounds can be easily eluted from the sulfonicacid surface under alkaline conditions. Based on these previousstudies [22,27], 2% ammonia–acetonitrile solution was used to elutethe adsorbed analytes. A more concentrated ammonia–acetonitrilesolution (>2%) could conceivably be better for desorption. However,considering the incompatibility of a highly alkaline solution withthe HPLC system, 2% ammonia–acetonitrile solution was selected.In addition, desorption time was also studied. Fig. 6 depicts the rela-tionship between chromatographic signals and desorption time. Itcan be clearly seen that the signals increased quickly up to a 10-min desorption. After 10 min, the plot generally flattened out. Itwas deemed that 10 min was sufficient for satisfactory desorption.

Based on the above studies, efficient extraction of the anaesthet-ics could be achieved by extracting for 60 min at a sample pH of 5.0,and with sample agitation at 1250 rpm. After extraction, analyteswere desorbed using a 2% ammonia–acetonitrile solution with a10-min sonication.

Fig. 6. Desorption time profile.

L. Xu, H.K. Lee / J. Chromatogr. A

Table 1Regression data and LODs of anaesthetics.

Analyte Linearity range(ng/mL)

r2 RSD(n = 4) (%)

LOD(ng/mL)

Enrichmentfactor

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rocaine 2–500 0.9958 3.5 1.7 78idocaine 10–500 0.9938 5.3 4.0 62etracaine 2–500 0.9994 6.8 1.2 83upivacaine 10–500 0.9999 3.8 6.0 49

cs. The relative standard deviations (RSDs) were 3.5, 5.3, 6.8 and.8% for procaine, lidocaine, tetracaine and bupivacaine, respec-ively. The linearity was investigated over a concentration range of–500 ng/mL. The four analytes exhibited good linearity with goodquared regression coefficients, ranging from 0.9938 to 0.9999.imits of detection (LODs), calculated at S/N = 3, were as low as.67, 4.0, 1.2 and 6.0 ng/mL for procaine, lidocaine, tetracaine andupivacaine, respectively, under the aforementioned extraction andlution conditions. Satisfactory enrichment factors were obtained>40-fold). These results are presented in Table 1.

.2.3. Application to urine sampleThe developed method was applied to determine its suitability

or a real matrix by considering spiked human urine sample, whichas first checked to be free of the target analytes. The relative recov-

ry (defined as the ratios of HPLC peak areas of the respective spikedrine sample extracts to spiked ultrapure water extracts) were 89.2,6.1, 95.9 and 91.4% for procaine, lidocaine, tetracaine and bupiva-aine, respectively. This means that the cleanup of the fiber usingcidified acetonitrile can eliminate interferences effectively. Therine matrix did not unduly affect the extraction.

. Conclusions

In the present work, polyvinyl chloride (PVC) fiber was suc-essfully modified via a one-step sulfonation procedure usingoncentrated sulfuric acid, yielding sulfonated PVC (PVC-SO3H).

he resulting PVC-SO3H was demonstrated to have cation-xchange property, and was used for the extraction of anaestheticsn urine samples. This represents a novel, versatile and convenientonfiguration of a practical and directly usable microextractionevice. The other key features are the low cost and disposability

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1216 (2009) 6549–6553 6553

of the fibers to remove sample carryover problems without com-promising the extraction efficiency.

Acknowledgements

The authors gratefully acknowledge the financial support of thisresearch by the National University of Singapore/Ministry of Edu-cation (grant number R-143-000-303-112). The assistance of Prof.G.-K. Chuah and Dr. Y.-T. Nie on BET measurements is acknowl-edged.

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