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Journal of Chromatography A, 1124 (2006) 205–216

Sol–gel immobilized cyano-polydimethylsiloxane coating for capillarymicroextraction of aqueous trace analytes ranging from

polycyclic aromatic hydrocarbons to free fatty acids

Sameer Kulkarni, Li Fang, Khalid Alhooshani, Abdul Malik ∗Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, CHE 205, Tampa, FL 33620-5250, USA

bstract

Sol–gel coating containing highly polar cyanopropyl and nonpolar poly(dimethylsiloxane) components (sol–gel CN-PDMS coating) was devel-ped for capillary microextraction (CME). The sol–gel chemistry provided an efficient means to immobilize the CN-PDMS coating by establishinghemical anchorage between the coating and the fused silica capillary inner surface. This chemical bond provided excellent thermal and sol-ent stability to the created sol–gel coating. For the extraction of polar and nonpolar analytes, the upper allowable conditioning temperaturesere 330 ◦C and 350 ◦C, respectively. To our knowledge, this is the first time when a CN-PDMS thick coating survived such a high operation

emperature. The prepared sol–gel CN-PDMS coating provided effective extraction of polar and nonpolar analytes simultaneously from aqueousamples. The cyanopropyl moiety in sol–gel CN-PDMS coatings provided effective extraction of highly polar analytes such as free fatty acids,lcohols, and phenols without requiring derivatization, pH adjustment or salting out procedures. The PDMS moiety, on the other hand, providedfficient extraction of nonpolar analytes. The extraction properties of the sol–gel CN-PDMS coatings can be fine tuned via manipulation of relative

roportions of 3-cyanopropyltriethoxysilane and hydroxy-terminated PDMS in the sol solution used to create the coatings. Detection limits ofanogram/liter (ng/L) were achieved for both highly polar and nonpolar analytes directly extracted from aqueous media using sol–gel CN-PDMSoated microextraction capillaries followed by GC analysis.

2006 Elsevier B.V. All rights reserved.

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eywords: Sol–gel; Cyano-PDMS coating; Immobilization; Capillary microextatty acids; Alcohols; Phenols; Aldehydes; Ketones; Amines; Polycyclic aroma

. Introduction

Solid-phase microextraction (SPME) [1] of polar analytesuch as carboxylic acids, alcohols, phenols, etc. from an aqueousedium often poses difficulty because of high affinity towardsater. For efficient extraction of such analytes from aqueous

amples, the SPME coating must have polarity, high enough, toompete with water for the analyte molecules. However, polartationary phases (coatings) are difficult to immobilize on a sil-ca substrates using conventional techniques [2]. The absencef chemical bonds between the SPME coating and fused silicaber is considered to be responsible for low thermal and solvent

tability of conventionally prepared SPME coatings [3]. If suchoatings are used for the extraction of polar analytes from aque-us media, desorption step becomes problematic, often leading

∗ Corresponding author. Tel.: +1 813 974 9688; fax: +1 813 974 3203.E-mail address: malik@cas.usf.edu (A. Malik).

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021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.chroma.2006.06.109

n (CME); In-tube SPME; Trace analysis; Preconcentration; Polar analytes; Freedrocarbons (PAHs)

o undesired effects such as incomplete desorption and samplearryover.

Sol–gel coatings [3,4] were developed to provide an effectiveolution to these problems inherent in conventional SPME coat-ngs. The sol–gel coatings offer several advantages. First, sol–geloatings are chemically anchored to the fused silica substrate.he presence of chemical bonds ensures thermal stability of theoatings, and thereby facilitates application of higher tempera-ures for effective desorption of high-boiling analytes. Thanks tohese chemical bonds, sol–gel coatings also possess high solventtability [4]. Second, the sol–gel coatings usually give a poroustructure enhancing the surface area of the extraction phase.his enhanced surface area allows the use of thinner coatings

o achieve faster extraction and desired level of sample capacity3]. Third, the selectivity of a sol–gel coating can be easily fine

uned by changing the composition of the used sol solution.

Both conventional and sol–gel coatings have been used toxtract polar compounds (e.g., free carboxylic acids, alcohols,tc.) from aqueous media. Pan et al. [5] have demonstrated

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06 S. Kulkarni et al. / J. Chrom

he possibility of achieving improved limits of detection forhort-chain (C2–C10) fatty acids via headspace extraction andn-fiber derivatization on poly(acrylate) (PA) coated SPMEber. Mixed phase coatings such as carbowax/divinylben-ene (CW/DVB) [6–8], polydimethylsiloxane/divinylbenzenePDMS/DVB) [8,9], polydimethylsiloxane/carboxen (PDMS/arboxen) [8,10], polydimethylsiloxane/carboxen/divinyl-enzene (PDMS/CAR/DVB) [8] have been used for extraction ofighly polar compounds. The use of 3-(trimethoxysilyl)propylethacrylate-hydroxyl-terminated silicone oil (TMSPMA-OH-SO) [11] and hydroxy-terminated silicone oil-butyl metha-rylate-divinylbenzene (OH-TSO-BMA-DVB) [12] copolymers sol–gel coatings for the extraction of alcohols and fatty acids,ith adjustments in temperature and pH of sample matrix, has

lso been reported. In all these reports manipulation to theperational conditions (e.g., sample derivatization, temperatureontrol, and pH of sample matrix, etc.) was used to improvextraction efficiency. This increases the number of steps,aking the extraction process time-consuming and error-prone.

t is, therefore, highly desirable to develop coatings capablef providing efficient extraction of polar compounds withoutequiring such adjustments of operating conditions.

Chromatographic stationary phases containing cyano func-ional group [13] are known to be extremely polar, and mayrove to be highly effective in the microextraction of polarnalytes from aqueous samples. Cyanopropylsiloxanes exhibitoth polar and polarizable characteristics and are among theost useful stationary phases with respect to polarity at both

ow and high temperatures. The cyano group, attached tohe siloxane backbone via a three-methylene ( CH2) spacer,s dipolar and strongly electron attracting, hence displayingipole–dipole, dipole-induced dipole, and charge-transfer inter-ctions. The unshared electron pair in the nitrile nitrogen mayorm intermolecular hydrogen-bonds with suitable hydrogen-onor sample molecules such as phenols. These characteristicsf cyano stationary phases are responsible for increased affin-ty of these phases for alcohols, ketones, esters, and analytesearing �-electrons. Cyano stationary phases have been usedn GC [14–18], HPLC [19–21], capillary electrochromatogra-hy (CEC) [22] and as an extraction medium in solid-phasextraction (SPE) [23–25]. Although cyanopropylpolysiloxanesight be useful for extracting highly polar compounds, con-

entionally prepared cyano coatings (having no chemical bondith the substrate) are not stable at elevated temperatures

26,27]. The sol–gel approach solves this problem by chem-cally anchoring such polar coatings onto the fused silicaurface.

Although sol–gel approach has been used to prepare cyanotationary phases for GC [28] and CEC [22], we are not awaref any report on the preparation and use of surface bondedyanopropyl-based thick sol–gel coatings in SPME or capil-ary microextraction (CME also called in-tube SPME). In thisaper, we describe in situ creation of sol–gel coatings containing

yanopropyl and poly(dimethylsiloxane) components (sol–gelN-PDMS coatings), and explain how sol–gel chemistry canrovide an effective means to immobilize such coatings on thenner walls of fused silica capillaries for use in CME. We also

aSTw

r. A 1124 (2006) 205–216

emonstrate the effectiveness of sol–gel CN-PDMS coatings inhe extraction of highly polar analytes from aqueous sample

atrices—an analytical task that is difficult to accomplish usingonventional SPME coatings.

. Experimental

.1. Equipments

All the CME-GC experiments with sol–gel CN-PDMSoated capillaries were performed on a Shimadzu Model 17AC (Shimadzu, Kyoto, Japan) equipped with flame ioniza-

ion detection (FID) system and a split-splitless injector. Aarnstead Model 04741 Nanopure deionized water system

Barnstead/Thermolyne, Dubuque, IA) was used to obtain16.9 M� water. A homebuilt, gas pressure-operated capillarylling/purging device [29] was used to rinse the fused silicaapillary with solvents, to fill the extraction capillary with sololution, to expel the sol solution from the capillary at the end ofol–gel coating process, and to purge the capillary with helium.

vortex shaker model G-560 (Scientific Industries, Bohemia,Y) was used to mix sol–gel ingredients. A Microcentaur modelPO 5760 microcentrifuge (Accurate Chemical and Scientificorp., Westbury, NY) was used to separate the sol solution from

he precipitate (if any) at 14 000 rpm (∼15 × 915 g). A JEOLodel JSM-35 scanning electron microscope (SEM) was used

or the investigation of surface morphology of the sol–gel CN-DMS coated capillaries. An in-house designed liquid sampleispenser [30] was used to perform CME via gravity-fed flowf the aqueous samples through the sol–gel CN-PDMS coatedapillary.

.2. Materials and chemicals

Fused silica capillary (250 �m I.D.) with a protectiveolyimide coating on the external surface was obtained fromolymicro Technologies (Phoenix, AZ). HPLC-grade solventsdichloromethane, methanol, and tetrahydrofuran (THF)),imwipes, polypropylene microcentrifuge tubes (2.0 mL), and.0 mL borosilicate vials (used to store standard solutions)ere purchased from Fisher Scientific (Pittsburgh, PA). Poly-

yclic aromatic hydrocarbons (PAHs) (acenaphthene, fluorene,henanthrene, fluoranthene), aldehydes (nonanal, isopropyl-enzaldehyde, 4-tert-butylbenzaldehyde, dodecanal), ketonesbutyrophenone, valerophenone, hexanophenone, benzophe-one, anthraquinone), aniline derivatives (N,N-dimethylaniline,-butylaniline, acridine, benzanilide), substituted phenols

2,4-dichlorophenol, 2,4,6-trichlorophenol, 4-chloro-3-methyl-henol, pentachlorophenol), fatty acids (caproic acid, nonanoiccid, decanoic acid, undecanoic acid), tetraethoxysilane (TEOS,9%), 1,1,1,3,3,3-hexamethyldisilazane (HMDS, 99.9%), tri-uoroacetic acid (TFA, 99%) were purchased from AldrichMilwaukee, WI). Alcohols (1-heptanol, 1-octanol, 1-nonanol,

nd 1-decanol) were purchased from Acros (Pittsburgh, PA).ilanol terminated PDMS was obtained from United Chemicalechnologies (Bristol, PA) and 3-cyanopropyltriethoxysilaneas purchased from Gelest (Morrisville, PA).

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S. Kulkarni et al. / J. Chrom

.3. Preparation of sol–gel CN-PDMS coatedicroextraction capillaries

A sol–gel coating solution was prepared as follows:rst 50 mg of PDMS (sol–gel active polymer), 50 �L of-cyanopropyltriethoxysilane (sol–gel precursor) and 50 �LEOS (sol–gel co-precursor) were dissolved in 700 �L dichloro-ethane (solvent) contained in a polypropylene microcen-

rifuge vial. Subsequently, 10 �L of HMDS (surface deactivationeagent) and 50 �L of TFA (sol–gel catalyst) containing 5%ater were added to the microcentrifuge vial and mixed formin using a vortex shaker. The resulting solution was cen-

rifuged at 14 000 rpm (∼15 × 915 g) for 5 min and clear super-atant of the sol solution was transferred to another clean vial.his sol solution was used to fill a hydrothermally treated [31]

used silica capillary (2 m) using a helium pressure-operatedlling/purging device [29]. After 10 min of in-capillary resi-ence time, the free unbonded part of the excess sol solutionas expelled from the capillary under helium pressure (50 psi)

eaving behind a surface-bonded sol–gel coating on the capillaryurface. The sol–gel coated capillary was subsequently driedy purging with helium for an hour under the same pressure.his was followed by temperature-programmed conditioningf the coated capillary in a GC oven from 35 ◦C to 150 ◦C at◦C/min with a hold time of 300 min at 150 ◦C. The capil-

ary was further conditioned from 150 ◦C to 300 ◦C at 2 ◦C/min,olding it at 300 ◦C for 60 min under helium flow (1 mL/min).efore using for extraction, the sol–gel CN-PDMS coated cap-

llary was rinsed with 1 mL dichloromethane/methanol (1:1/v) mixture and conditioned again from 35 ◦C to 300 ◦Ct 5 ◦C/min, holding it at 300 ◦C for 30 min under heliumow (1 mL/min). The conditioned capillary was then cut into2 cm long pieces that were further used in capillary micro-xtraction.

.4. Sol–gel CME-GC analysis

Stock solutions (10 mg/mL) of the selected analytes wererepared in methanol or THF and stored in surface-deactivatedmber glass vials. For extraction, fresh aqueous samples wererepared by further diluting these stock solutions to ng/mLevel concentrations. A Chromaflex AQ column (Knotes Glass,ineland, NJ) was modified as described previously [30] andsed for gravity-fed sample delivery in capillary microextrac-ion. A 12 cm long piece of thermally conditioned sol–gelN-PDMS coated microextraction capillary (250 �m I.D.) wasertically connected to the lower end of the gravity-fed sam-le dispenser. The aqueous sample (50 mL) was poured intohe dispenser from its top end and allowed to flow through the

icroextraction capillary under gravity. The extraction was car-ied out for 30–40 min for equilibrium to be established. Theapillary was then detached from the dispenser and the residualample droplets were removed by touching one of the ends of

icroextraction capillary with Kimwipe tissue. After this, the

apillary was installed in the GC injection port, keeping ∼3 cmf its lower end protruding into the GC oven. This end washen interfaced with the inlet of a GC capillary column using a

cso(

r. A 1124 (2006) 205–216 207

eactivated two-way press-fit quartz connector. Under splitlessonditions, the extracted analytes were then thermally desorbedrom the capillary by rapidly raising the temperature of the injec-ion port (from 30 ◦C to 300 ◦C at 60 ◦C/min) while keeping theC oven temperature at 35 ◦C. Such a rapid temperature pro-ram of the injection port facilitated effective desorption of thextracted analytes from the sol–gel CN-PDMS microextractionapillary and their focusing at the GC analysis column inlet.ollowing this, the GC oven was temperature programmed from5 ◦C to 300 ◦C at rate the of 20 ◦C/min to achieve separationf the focused analytes on the GC column. A flame ionizationetector (FID) maintained at 350 ◦C was used for analyte detec-ion.

. Results and discussion

In the sol–gel process, a gel can be formed via hydrolyticolycondensation of a sol–gel precursor followed by aging andrying [32]. Since the introduction of sol–gel SPME coatingsn 1997 [3], various sol–gel organic-inorganic hybrid polymeric

aterials have been prepared to coat SPME fibers [33,34], asell as inner walls of the fused capillaries for use in CME or

n-tube SPME [30,35–37].In this work, sol–gel chemistry was used to chemically

ind highly polar cyanopropyl and nonpolar PDMS moieties ton evolving sol–gel network structure, and use such a hybridrganic-inorganic material as a surface-bonded coating in aused silica capillary to provide efficient extraction of aqueousrace analytes from a wide range of polarity. Table 1 lists thehemical ingredients used to prepare the sol solution for creat-ng the sol–gel CN-PDMS coated microextraction capillaries.ere, the key sol–gel precursor is 3-cyanopropyltriethoxysilanehich provides chemically bonded cyanopropyl moieties in the

reated sol–gel coating via hydrolytic polycondensation reac-ions.

.1. Hydrolytic polycondensation reactions leading to theormation of sol–gel CN-PDMS network

The used sol solution contained 3-cyanopropyltriethoxy-ilane (sol–gel precursor) and TEOS (sol–gel co-precursor). Its well-known from the basic principles of sol–gel chemistry38] that such alkoxysilane compounds are capable of under-oing hydrolytic polycondensation reactions (Figs. 1 and 2)n the presence of a sol–gel catalyst. In this research, tri-uoroacetic acid was used to catalyze the sol–gel reactions.olycondensation of the hydrolyzed precursors and silanol-

erminated PDMS would lead to a three-dimensional sol–geletwork, incorporating highly polar cyanopropyl and nonpolarDMS moieties in the organic-inorganic hybrid structure. Frag-ents of this sol–gel network, especially the ones growing in

hemically anchored to the capillary inner surface via conden-ation with the surface silanol groups, leading to the formationf a surface-bonded sol–gel coating on the capillary inner wallsFig. 3).

208 S. Kulkarni et al. / J. Chromatogr. A 1124 (2006) 205–216

Table 1Names, functions, and chemical structures of sol–gel CN-PDMS coating solution ingredients

Name of chemical Function Structure

3-Cyanopropyltriethoxysilane Sol–gel precursor

Tetraethoxysilane (TEOS) Sol–gel co-precursor

Silanol-terminated poly(dimethylsiloxane) (PDMS) Sol–gel active polymer

Trifluroacetic acid/5% water (v/v) Catalyst and source of water CF3COOHSolvent Dichloromethane CH2Cl2

1,1,1,3,3,3-Hexamethyldisilazane (HMDS) Deactivation reagent

3

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.2. Surface morphology of sol–gel CN-PDMS coating

Surface morphology and thickness of sol–gel CN-PDMSoatings in microextraction capillaries were investigated usingcanning electron microscopy. Fig. 4 represents scanning elec-ron microscopic images of a sol–gel CN-PDMS coated capillaryn two different orientations at magnifications: 10 000× (4a) and

0 000× (4b). From Fig. 4a it is evident that coating thicknesss uniform, and was estimated at 1 �m. Fig. 4b represents theurface morphology of the sol–gel CN-PDMS coating obtainedt 20 000× magnification. As can be seen from this SEM image,

Pt

Fig. 1. Hydrolysis of 3-cyanopropyltriethoxysilane (p

ol–gel CN-PDMS coating possess a roughened, porous struc-ure. The porous structure provides enhanced surface area whichn turn translates into improved sample capacity of sol–gel CN-DMS coatings.

.3. Thermal and solvent stability of sol–gel CN-PDMSoated capillaries

Fig. 5a and b illustrate the thermal stability of sol–gel CN-DMS coatings in microextraction capillaries used for extrac-

ion of polar (Fig. 5a) and nonpolar (Fig. 5b) analytes. The GC

recursor) and tetraethoxysilane (co-precursor).

S. Kulkarni et al. / J. Chromatogr. A 1124 (2006) 205–216 209

Fig. 2. Growth of sol–gel CN-PDMS polymer chains within a fused silicachs

pcstitotcce

Ff

Fig. 4. Scanning electron microscopic images of a 250 �m I.D. sol–gel CN-PDMS coated fused silica capillary used in CME illustrating (a) uniform coat-i

apillary via polycondensation of (A) a hydrolyzed sol–gel precursor, (B) aydrolyzed co-precursor, and (C) a sol–gel active polymer contained in the sololution filling a fused silica capillary.

eak areas of four extracted alcohols did not show any significanthanges after the CN-PDMS coated capillaries were conditionedtepwise for 1 h at 290, 300, 310, 320, and 330 ◦C. The enhancedhermal stability can be attributed to the strong chemical bond-ng between sol–gel CN-PDMS coating and the inner walls ofhe fused silica capillary. It should be noted that the performancef the sol–gel CN-PDMS coated capillary with regard to extrac-

ion of alcohols was not affected even after subjecting it to aonditioning temperature of 330 ◦C. When the microextractionapillary was heated above 330 ◦C, a reduction in peak area ofxtracted alcohols was observed. Since the polar cyanopropyl

ig. 3. Sol–gel CN-PDMS coating chemically anchored to the inner walls ofused silica capillary.

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ng thickness on the inner surface of the fused silica capillary, magnification:0 000×, and (b) porous fine structure of sol–gel CN-PDMS coating, magnifi-ation: 20 000×.

oieties are mainly responsible for the extraction of these polarnalytes, it can be assumed that such a peak area reduction isssociated with degradation of the cyanopropyl moiety in theol–gel coating above 330 ◦C. On the other hand, no signifi-ant change was observed in peak area of nonpolar compoundsxtracted with sol–gel CN-PDMS coated capillaries conditionedp to 350 ◦C. This indicates that the nonpolar PDMS componentf the sol–gel coating (which is responsible for the extractionf nonpolar analytes) was still intact even when the capillaryas conditioned at 350 ◦C. We are not aware of any reports on

yano coatings that can provide stable performance at such a highemperature (330 ◦C). By comparison, even the thin conven-

ionally prepared cyanopropylpolysiloxane based GC coatings∼0.25 �m) that should, in principle, provide better thermal sta-ility (compared to thicker coating like the ones used in theresent work) have upper temperature limit of ∼275 ◦C [39].

210 S. Kulkarni et al. / J. Chromatogr. A 1124 (2006) 205–216

Fig. 5. Effect of conditioning temperature on the performance of sol–gel CN-PDMS microextraction capillary in the extraction of alcohols (a) and PAHs (b)used as test solutes. CME-GC conditions: extraction time, 30 min; 5 m × 250 �mI.D. sol–gel PDMS column; splitless injection; injector temperature: initial3tg

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Fig. 6. Illustration of the extraction kinetics of moderately polar (valerophe-none, 20 �g/L), and polar (2,4,6-trichlorophenol, 200 �g/L; 1-nonanol, 40 �g/L;nonanoic acid, 200 �g/L) analytes extracted on a 12 cm × 250 �m I.D. sol–gelCN-PDMS coated capillary from aqueous samples. Extraction conditions: trip-licate extraction for 10, 20, 30, 40, 50, 60, and 70 min. GC analysis conditions:10 m × 250 �m I.D. sol–gel PDMS column; splitless injection; injector temper-atc

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0 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min; GC oven tempera-ure programmed from 35 ◦C to 300 ◦C at a rate of 20 ◦C/min; helium carrieras; FID temperature 350 ◦C.

Sol–gel CN-PDMS coating showed excellent stabilityoward organic solvents. As it can be seen in Table 2, thextraction performance of the sol–gel CN-PDMS capillaryemained practically unchanged after rinsing it with 50 mL ofichloromethane/methanol mixture (1:1, v/v) over a 24 h period.

.4. Extraction kinetics of moderately polar and highlyolar compounds on sol–gel CN-PDMS coated

icroextraction capillary

Fig. 6 illustrates the extraction kinetics of valerophenone,,4,6-trichlorophenol, 1-nonanol, and nonanoic acid on a sol–gel

able 2eak area repeatability data (n = 3) for free fatty acids before and after rinsing the sichloromethane/methanol (1:1, v/v) for 24 h

ame of the analyte Peak area

Before rinsing A1 (arbitrary units)

ctanoic acid 11.9onanoic acid 9.2ecanoic acid 8.6

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ture: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min; GC ovenemperature programmed from 35 ◦C to 300 ◦C at a rate of 20 ◦C/min; heliumarrier gas; FID temperature 350 ◦C.

oated microextraction capillary. The CME experiments werearried out using aqueous samples of individual test ana-ytes. The extraction equilibria for valerophenone and 2,4,6-richlorophenol were reached faster (∼20 min) than those for-nonanol and nonanoic acid (∼30 min). These results indicatehat, compared with 1-nonanol and nonanoic acid, valerophe-one and 2,4,6-trichlorophenol have lower affinity for the aque-us matrix, resulting in their faster extraction. On the otherand, greater hydrophilic nature of 1-nonanol and nonanoiccid makes the extraction process slower, which is evident fromonger equilibration time.

.5. CME-GC analysis of non-polar and moderately polarompounds using sol–gel CN-PDMS coatedicroextraction capillaries

Polycyclic aromatic hydrocarbons are among the most com-on environmental pollutants found in air, water, and soil in

he USA, and other industrialized countries where petroleumroducts are heavily used. Due to their potential or proven

ol–gel CN-PDMS coated microextraction capillary with a mixture (50 mL) of

Relative change in peak area(A) = |(A2 – A1)/A1| × 100 (%)

After rinsing A2 (arbitrary units)

11.5 3.49.4 2.28.2 4.7

arcinogenic activities, US Environmental Protection AgencyEPA) has placed 16 unsubstituted PAHs in its list of 129 pri-rity pollutants [40]. In our study, a sol–gel CN-PDMS coatedapillary was used to extract four of these PAHs from an aqueous

S. Kulkarni et al. / J. Chromatogr. A 1124 (2006) 205–216 211

Fig. 7. CME-GC analysis of aqueous samples of PAHs. Extraction conditions:sol–gel CN-PDMS coated extraction capillary (12 cm). extraction time, 30 min.,GC analysis conditions: 10 m × 250 �m I.D. sol–gel PDMS column; splitlessinjection; injector temperature: initial 30 ◦C, final 300 ◦C, programmed at araa

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Fig. 8. CME-GC analysis of aldehydes. Extraction conditions: sol–gel CN-PDMS coated extraction capillary (12 cm); extraction time, 30 min; GC analysisconditions: 10 m × 250 �m I.D. sol–gel PDMS column; splitless injection; injec-tor temperature: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min;G ◦ ◦ ◦hb

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ate of 60 ◦C/min; GC oven temperature programmed from 35 ◦C to 300 ◦C atrate of 20 ◦C/min; helium carrier gas; FID temperature 350 ◦C. Peaks: (1)

cenaphthene; (2) fluorene; (3) phenanthrene; and (4) fluoranthene.

ample (12 �g/L concentration of each) (Fig. 7) for CME-GCnalysis. For all the analyte types studied, the peak area relativetandard deviation (RSD) was under 6%. The detection limits ofg/L were obtained for PAHs in the CME-GC-FID experimentssing sol–gel CN-PDMS microextraction capillaries.

Aldehydes and ketones are known to have toxic and carcino-enic properties, and therefore, their presence in the environments of great concern [41]. They are formed as by-products inhe drinking water disinfection processes. Many of these by-roducts have been shown to be carcinogens or carcinogenuspects [42,43]. Due to the polar nature of these compounds,hey are often derivatized [44] for GC analysis to avoid unde-irable adsorption responsible for peak tailing and analyte loss.lthough analytical derivatizations are effective, they involve

dditional steps in the sample preparation scheme. These reac-ions may not be quantitative, especially for samples containingltra trace concentrations of the analytes. They may also produceide products capable of interfering with the analysis. For these

easons, it is not always desirable to use derivatization of the tar-et analyte. The data presented in Table 3 indicate that a sol–gelN-PDMS coated capillary can extract underivatized aldehy-es (Fig. 8) from aqueous media to provide detection limits in

foiw

C oven temperature programmed from 35 C to 300 C at a rate of 20 C/min;elium carrier gas; FID temperature 350 ◦C. Peaks: (1) nonanal, (2) isopropyl-enzaldehyde, (3) 4-tert-butylbenzaldehyde, and (4) dodecanal.

he 12–16 ng/L range. An analogous trend was also observedor CME-GC analysis of ketones as evident from Fig. 9. Theol–gel CN-PDMS microextraction capillaries showed excellentun-to-run and capillary-to-capillary extraction reproducibilityTable 3) for aldehydes, ketones, and PAHs demonstrating theersatility of the sol–gel CN-PDMS coatings and the sol–gelrocedure used to prepare the extraction capillaries.

Aromatic amines are used as intermediates in the pharmaceu-ical, photographic, dye, and pesticide industries [45–47]. Theyave also been employed in rubber industry as antioxidants andntiozonants [48]. Many of these aromatic amines, which areound in air, water and soil [49–52], have been classified asutagenic and carcinogenic [53,54]. This makes quantitative

etection of aromatic amines very important.In our study, different anilines were directly extracted from

queous samples. Fig. 10 represents a gas chromatogram ofmixture of four underivatized aromatic amines extracted

rom an aqueous sample. The detection limits of ng/L werebtained for the aromatic amines in the CME-GC-FID exper-ments using sol–gel CN-PDMS microextraction capillariesith excellent run-to-run and capillary-to-capillary extraction

212 S. Kulkarni et al. / J. Chromatogr. A 1124 (2006) 205–216

Table 3Run-to-run and capillary-to-capillary repeatability (peak area and retention time), and detection limit data for non-polar and moderately polar analytes in threereplicate measurements by CME-GC using sol–gel CN-PDMS coated microextraction capillaries

Chemical classof analyte

Name of analyte Peak area repeatability (n = 3) Retention time (tR)repeatability (n = 5)

Detection limits,S/N = 3 (ng/L)

Capillary-to-capillary Run-to-run Mean tR (min) RSD (%)

Mean peak area(arbitrary unit)

RSD(%)

Mean peak area(arbitrary unit)

RSD(%)

PAHs Acenaphthene 5.9 6.6 6.5 2.4 7.47 0.06 2.9Fluorene 5.3 3.4 5.8 2.3 8.09 0.06 3.0Phenanthrene 6.1 4.5 6.6 4.2 9.13 0.05 3.1Fluoranthene 5.3 1.8 5.7 3.6 10.45 0.05 4.1

Aldehydes Nonanal 6.3 2.8 6.0 2.2 5.30 0.08 16.8Isopropylbenzaldehyde 5.0 4.6 4.5 4.8 6.12 0.07 22.44-Tertbutylbenzaldehyde 9.7 3.3 8.9 3.6 6.67 0.08 13.1Dodecanal 6.6 5.5 6.7 4.2 7.44 0.06 12.0

Ketones Butyrophenone 3.5 4.0 3.4 1.9 6.18 0.09 7.0Valeropheone 5.7 3.8 5.4 4.0 6.90 0.06 2.7Hexanophenone 8.7 4.0 8.8 5.0 7.58 0.07 2.3Benzophenone 6.1 4.2 5.7 3.2 8.45 0.05 3.4Anthraquinone 5.6 4.5 5.6 4.0 10.04 0.04 4.7

Fig. 9. CME-GC analysis of ketones. Extraction conditions: sol–gel CN-PDMScoated extraction capillary (12 cm); extraction time, 30 min, GC analysis con-ditions: 10 m × 250 �m I.D. sol–gel PDMS column; splitless injection; injectortemperature: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min; GCoven temperature programmed from 35 ◦C to 300 ◦C at a rate of 20 ◦C/min;helium carrier gas; FID temperature 350 ◦C. Peaks: (1) butyrophenone, (2)valerophenone, (3) hexanophenone, (4) benzophenone, and (5) anthraquinone.

Fig. 10. CME-GC analysis of aromatic amines. Extraction conditions: sol–gelCN-PDMS coated extraction capillary (12 cm); extraction time, 30 min; GCanalysis conditions: 10 m × 250 �m I.D. sol–gel PDMS column; splitless injec-tion; injector temperature: initial 30 ◦C, final 300 ◦C, programmed at a rateof 60 ◦C/min; GC oven temperature programmed from 35 ◦C to 300 ◦C at arate of 20 ◦C/min; helium carrier gas; FID temperature 350 ◦C. Peaks: (1) N,N-dimethylaniline, (2) N-butylaniline, (3) acridine, and (4) benzanilide.

S. Kulkarni et al. / J. Chromatog 24 (2006) 205–216 213

Table 4Run-to-run and capillary-to-capillary repeatability (peak area and retention time), and tion limit data for moderately polar and polar analytes in three replicatemeasurements by CME-GC using sol–gel CN-PDMS coated microextraction capillar

Chemical class ofanalyte

Name of analyte Peak area repeatability (n = 3) Retention time (tR)repeatability (n = 5)

Detection limits,S/N = 3 (ng/L)

Capillary-to-capillary R un Mean tR (min) RSD (%)

Mean peak area(arbitrary unit)

RSD(%)

M ak area(a y unit)

RSD(%)

Aromatic amines N,N-dimethylaniline 8.7 4.1 2.4 4.88 0.11 114.9N-butylaniline 11.5 3.8 1 1.3 6.64 0.07 11.4Acridine 8.6 5.0 1.6 9.22 0.06 10.4Benzanilide 10.7 3.9 1 3.6 10.03 0.04 35.9

Phenols 2,4-Dichlorophenol 6.0 4.6 4.1 5.37 0.08 136.92,4,6-Trichlorophenol 4.5 5.8 4.0 6.31 0.13 161.54-Chloro-3-methylphenol 6.5 5.8 5.0 6.58 0.13 56.9Pentachlorophenol 6.7 3.7 4.5 8.95 0.08 39.1

Alcohols 1-Heptanol 2.7 4.6 5.2 4.75 0.19 60.31-Octanol 3.4 5.3 4.2 5.61 0.16 4.31-Nonanol 5.3 4.4 3.5 6.39 0.11 1.61-Decanol 3.7 5.9 5.3 7.12 0.10 1.4

Acids Caproic acid 5.0 4.0Nonanoic acid 5.9 4.4Decanoic acid 4.8 4.1Undecanoic acid 5.2 4.2

Fig. 11. CME-GC analysis of chlorophenols. Extraction conditions: sol–gel CN-PDMS coated extraction capillary (12 cm); extraction time, 30 min, GC analysisconditions: 10 m × 250 �m I.D. sol–gel PDMS column; splitless injection; injec-tor temperature: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min;GC oven temperature programmed from 35 ◦C to 300 ◦C at a rate of 20 ◦C/min;helium carrier gas; FID temperature 350 ◦C. Peaks: (1) 2,4-dichlorophenol, (2)2,4,6-trichlorophenol, (3) 4-chloro-3-methylphenol, and (4) pentachlorophenol.

FPctGh(

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5.64.96.66.5

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4.7 4.6 4.47 0.12 197.26.1 3.2 7.29 0.12 38.74.3 5.1 7.96 0.07 13.45.1 5.3 8.54 0.06 9.3

ig. 12. CME-GC analysis of alcohols. Extraction conditions: sol–gel CN-DMS coated extraction capillary (12 cm); extraction time, 30 min, GC analysisonditions: 5 m × 250 �m I.D. sol–gel PDMS column; splitless injection; injec-or temperature: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min;C oven temperature programmed from 35 ◦C to 300 ◦C at a rate of 20 ◦C/min;elium carrier gas; FID temperature 350 ◦C. Peaks: (1) 1-heptanol, (2) 1-octanol,3) 1-nonanol, and (4) 1-decanol.

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14 S. Kulkarni et al. / J. Chrom

epeatability characterized by RSD values of less than 5%Table 4).

.6. CME-GC analysis of highly polar compounds usingol–gel CN-PDMS coated microextraction capillaries

Chlorophenols (CPs) have been widely used as preservatives,esticides, antiseptics, and disinfectants [55]. They are also usedn producing dyes, plastics and pharmaceuticals. In the envi-onment, CPs may also be formed as a result of hydrolysis,xidation, and microbiological degradation of chlorinated pes-icides. As a result, CPs are often found in waters [56], soils,nd sediments [57]. CPs constitute an important group of prior-ty toxic pollutants listed by EPA [40] because of their possiblearcinogenic properties.

In our study, four CPs were extracted using CN-PDMS coated

apillaries. CPs are highly polar compounds with significantffinity toward water. CN-PDMS coated capillaries were effec-ive in extracting four underivatized CPs from an aqueous sampleFig. 11). We were also able to achieve lower detection lim-

ig. 13. CME-GC analysis of free fatty acids. Extraction conditions: sol–gelN-PDMS coated extraction capillary (12 cm); extraction time, 30 min, GCnalysis conditions: 5 m × 250 �m I.D. sol–gel PDMS column; splitless injec-ion; injector temperature: initial 30 ◦C, final 300 ◦C, programmed at a rate of0 ◦C/min; GC oven temperature programmed from 35 ◦C to 300 ◦C at a rate of0 ◦C/min; helium carrier gas; FID temperature 350 ◦C. Peaks: (1) caproic acid,2) nonanoic acid, (3) decanoic acid, and (4) undecanoic acid.

iamf

Fhe5pohd

r. A 1124 (2006) 205–216

ts (e.g., 136 ng/L for 2,4-dichlorophenol, by CME-GC-FID)ompared to other literature reports (150 ng/L for the same com-ound, by SPME-GC-FID) [34].

Fig. 12 represents a gas chromatogram for a mixture of alco-ols. These highly polar analytes were extracted from aqueousamples using sol–gel CN-PDMS capillaries. The presented datahow excellent affinity of the sol–gel CN-PDMS coating towardhese highly polar analytes that are often difficult to extract fromqueous media in underivatized form using commercial coat-ngs. Moreover, the achieved low ng/L level detection limitsTable 4) and small RSD values for run-to-run and capillary-to-apillary peak area repeatability (<6%) also demonstrate out-tanding performance of the sol–gel CN-PDMS coating.

The determination of free fatty acids in various matri-es such as blood plasma and urine is of great importanceecause they are key metabolites and intermediates in biolog-

cal processes [58]. They are widely dispersed in nature andre often produced from humic substances during water treat-ent [59]. Being very hydrophilic, underivatized short-chain

atty acids are usually difficult to extract from aqueous matri-

ig. 14. CME-GC analysis of a mixture of nonpolar, moderately polar andighly polar compounds. Extraction conditions: sol–gel CN-PDMS coatedxtraction capillary (12 cm); extraction time, 30 min, GC analysis conditions:m × 250 �m I.D. sol–gel PDMS column; splitless injection; injector tem-erature: initial 30 ◦C, final 300 ◦C, programmed at a rate of 60 ◦C/min; GCven temperature programmed from 35 ◦C to 300 ◦C at a rate of 15 ◦C/min;elium carrier gas; FID temperature 350 ◦C. Peaks: (1) 1-heptanol, (2) 2,4-ichlorophenol, (3) decanal, (4) nonanoic acid, (5) fluorene, and (6) acridine.

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S. Kulkarni et al. / J. Chrom

es. In the present study, sol–gel CN-PDMS microextractionapillaries provided efficient extraction of fatty acids withoutsing any derivatization, pH adjustment or salting-out proce-ures. Experimental data on detection limits are presented inable 4 and demonstrates remarkable ability of sol–gel CN-DMS capillaries to reproducibly extract short-chain fatty acidsFig. 13) from aqueous medium. Detection limits for underiva-ized caproic acid and decanoic acid were 197 ng/L and 16 ng/L,espectively. These values are better than those reported inhe literature (e.g., 500 ng/L for caproic acid and 20 ng/L forecanoic acid, respectively, by SPME-GC-GID) using in situ

erivatization of fatty acids on polyacrylate coated SPME fiber5].

A mixture containing analytes from different chemicallasses representing a wide polarity range was extracted from

imca

ig. 15. CME-GC analysis of mixture of two alcohols and two free fatty acids on (a)xtraction time, 30 min, GC analysis conditions: 5 m × 250 �m I.D. sol–gel PDMSrogrammed at a rate of 60 ◦C/min; GC oven temperature programmed from 35 ◦Ceaks: (1) 1-heptanol, (2) 1-octanol, (3) octanoic acid, (4) nonanoic acid.

r. A 1124 (2006) 205–216 215

n aqueous sample using a sol–gel CN-PDMS coated capillary.t is evident from the chromatogram (Fig. 14) that a sol–gel CN-DMS coated capillary can simultaneously extract non-polar,oderately polar, and highly polar compounds from an aqueousedium.Finally, the extraction performance of sol–gel CN-PDMS

apillary was compared with the sol–gel PDMS capillary. Fig. 15ompares the extraction of an aqueous sample containing twolcohols and two free fatty acids obtained on two sol–geloated microextraction capillaries: a sol–gel CN-PDMS cap-llary (Fig. 15a) and a sol–gel PDMS capillary (Fig. 15b). It

s evident from these figures that in the absence of highly polar

oieties (such as cyanopropyl), the sol–gel PDMS coating aloneannot compete with water to provide extraction of highly polarnalytes like free fatty acids and alcohols.

sol–gel CN-PDMS capillary (12 cm) and (b) sol–gel PDMS capillary (12 cm);column; splitless injection; injector temperature: initial 30 ◦C, final 300 ◦C,

to 300 ◦C at a rate of 15 ◦C/min; helium carrier gas; FID temperature 350 ◦C.

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. Conclusion

To our knowledge, this is the first report on sol–gel CN-DMS coated microextraction capillaries developed for effec-

ive preconcentration and trace analysis of polar and non-polarompounds. In such coatings, the cyanopropyl moieties arerimarily responsible for the extraction of polar analytes andhe PDMS moieties are mainly responsible for the extractionf nonpolar analytes. In conjunction with GC-FID, the sol–gelN-PDMS coated microextraction capillaries provided lowg/L level detection limits for polar and nonpolar analytesirectly extracted from aqueous media without requiringerivatization, pH adjustment, or salting out procedures. Theol–gel CN-PDMS microextraction coatings were characterizedy remarkable performance repeatability. The run-to-run andapillary-to-capillary peak area RSD values for these coatingsere lower than 5% and 6%, respectively. The sol–gel CN-DMS coatings showed excellent thermal and solvent stability.he integrity of the sol–gel CN-PDMS coatings and hence theirxtraction abilities for polar analytes was fully preserved evenfter conditioning at 330 ◦C.

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