simultaneous derivatization and extraction of nitrophenols in soil and rain samples using modified...

Simultaneous derivatization and extraction of nitrophenolsin soil and rain samples using modified hollow-fiberliquid-phase microextraction followed by gaschromatography–mass spectrometry

Hamid Reza Sobhi & Ali Esrafili & Hadi Farahani &Mitra Gholami & Mohammad Mehdi Baneshi

Received: 5 December 2012 /Accepted: 25 April 2013 /Published online: 7 May 2013# Springer Science+Business Media Dordrecht 2013

Abstract A simple and sensitive method based on amodified hollow-fiber liquid-phase microextractionfollowed by gas chromatography–mass spectrometryhas been successfully developed for the extraction andsimultaneous derivatization of some nitrophenols(NPs) in soil and rain samples. Microwave-assistedsolvent extraction was used for the extraction of NPsfrom the soil, while the rain sample was directlyapplied to the previously mentioned method. Briefly,in this method, the analytes were extracted from aqueoussamples into a thin layer of organic solvent (dodecane+10 % tri-n-octylphosphine oxide) sustained in the poresof a porous hollow fiber. Then, they were back-extractedusing a small volume of organic acceptor solution (25μl;

10 mg/L N-methyl-N-(trimethylsilyl)trifluoroacetamide,as derivatization reagent, in acetonitrile) that was locatedinside the lumen of the hollow fiber. Under theoptimized extraction conditions, enrichment factorsof 255 to 280 and limits of detection of 0.1 to0.2 μg/L (S/N=3) with dynamic linear ranges of1–100 μg/L were obtained for the analytes. Theaccuracy of the approach was tested by the relativerecovery experiments on spiked samples, with re-sults ranging from 93 to 113 %. The method wasshown to be rapid, cost-effective, and potentiallyinteresting for screening purposes.

Keywords Nitrophenols .Modified hollow-fiber liquid-phasemicroextraction . Gas chromatography–massspectrometry . Soilandrainsamples .Microwave-assistedsolvent extraction

AbbreviationsHF-LPME

Hollow-fiber liquid-phasemicroextraction

GC-MS Gas chromatography–mass spectrometryTOPO Tri-n-octylphosphine oxideACN AcetonitrileLPME Liquid-phase microextractionSLM Supported liquid membraneNPs NitrophenolsSIM Selected ion monitoringLODs Limits of detectionRSDs Relative standard deviations

Environ Monit Assess (2013) 185:9055–9065DOI 10.1007/s10661-013-3235-y

H. R. SobhiDepartment of Chemistry, Payame Noor University,Tehran, Iran

A. Esrafili (*) :M. GholamiDepartment of Environmental Health Engineering, Schoolof Public Health, Tehran University of Medical Sciences,Tehran, Irane-mail: [email protected]

H. FarahaniResearch Institute of Petroleum Industry (RIPI),P.O. Box 14665-1137, Tehran, Iran

M. M. BaneshiSocial Determinant of Health Research Center, YasujUniversity of Medical Science,Yasuj, Iran

Introduction

Despite recent technological advances, most analyticalinstrumentation cannot handle sample matrices direct-ly and, as a result, a sample preparation step is com-monly introduced (Xu and Lee 2008; Sarafraz-Yazdiand Amiri 2010; Ulrich 2000; Melwanki and Fuh2008; Ghambarian et al. 2011). For organic trace anal-ysis, this step mainly comprises of extractions, whichserve to isolate compounds of interest from a samplematrix. Ultimately, the concentration of target com-pounds is enhanced (enrichment) and the presence ofmatrix components is reduced (sample cleanup)(Moradi et al. 2011; Pawliszyn 2003; Smith 2003;Raynie 2004; Raynie 2006).

Liquid-phase microextraction (LPME) techniqueshave been used in recent years as an eco-friendly andvery efficient alternative in the domain of sample prep-aration, especially for chromatography and electropho-resis (Lambropoulou and Albanis 2007; Demeestere etal. 2007). LPME has attracted increasing attention be-cause it requires very little solvents and minimal expo-sure to toxic organic solvents, which make it a simple,quick, inexpensive, and virtually solvent-free sam-ple preparation method (Asensio-Ramos et al.2011; Hai-Yang and He 2010; Jeannot et al.2010). Nowadays, LPME is widely used for theanalysis of organic compounds (Xiao et al. 2006a,b; Peng et al. 2007; Wu and Lin 2006; Zhu et al.2002; Deng et al. 2005) and inorganic trace ele-ments (Xia et al. 2004, 2005, 2006) in environ-mental, biological, and food sample analysis.

LPME has three main operating modes: headspaceLPME, direct-immersed LPME, and hollow-fiberLPME (HF-LPME) (Jun Xiong and Hu 2008). HF-LPME has become very popular because it provideshigh analyte enrichments, low consumption of hazard-ous organic solvents, low cost, and clean extracts(Pedersen-Bjergaard and Rasmussen 2005).

HF-LPME can be carried out in either a two-phaseor three-phase mode. In the two-phase mode, theanalytes are extracted from an aqueous sample matrixand into a thin layer of organic phase inside the wallpores of a hollow fiber and further into the samesolvent placed inside the hollow fiber (acceptor solu-tion) (Pezo et al. 2007). In the three-phase mode, theanalytes are first extracted from an aqueous samplematrix and into a thin layer of organic phase insidethe wall pores of a hollow fiber and then extracted

once more into an aqueous acceptor phase locatedinside the hollow fiber (Rodriguez et al. 2008).

Recently, Ghambarian et al. (2010) have suc-cessfully developed a simple, modified HF-LPME,based on using two immiscible organic solvents,initially applied for the determination of somechlorophenols in solid and aqueous samples. Inthe method, an organic solvent (dodecane) isimmobilized in the pores of the hollow fiber, pro-viding a supported liquid membrane (SLM), and anadditional organic solvent (acetonitrile [ACN] ormethanol) is placed within the lumen. This quanti-tative HF-LPME is an efficient and satisfactoryanalytical procedure, with excellent accuracy andprecision reported.

Nitrophenolic compounds are common pollutantsin aquatic environments. These compounds are typi-cally toxic and degrade extremely slow. They mainlyoriginate from chemical, agricultural, medical, andother industries (Chung et al. 2012). They haveobtained considerable attention in wastewater and en-vironmental analysis programs due to the humanhealth hazards they pose, even at micrograms per literlevels (Tremp et al. 1993), so their determination attrace level is of great importance (Karim Asadpour-Zeynali 2012; Toral et al. 1999, 2002).

The aim of the present study was to assess thesuitability of the HF-LPME technique for the de-termination of selected nitrophenols (NPs) in soiland rain samples. The factors affecting themicroextraction efficiency were studied in detailand the optimal conditions were established. Theresulting method was validated for quantitative pur-poses in combination with gas chromatography–mass spectrometry (GC-MS).

Experimental

Reagents and real samples

All NPs (1,2-dinitrophenol, 1,3-dinitrophenol, 1,4-dini-trophenol, 1,2,3-trinitrophenol, and 1,2,4-trinitrophenol)were purchased fromMerck (Darmstadt, Germany). An-alytical grade n-octanol, n-dodecane, n-docanol, dihexylether, tri-n-octylphosphine oxide (TOPO), and ACNwere supplied by Fluka (Buchs, Switzerland). N-meth-yl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) as de-rivatization reagent, sodium hydroxide, and sodium

9056 Environ Monit Assess (2013) 185:9055–9065

chloride were of analytical grade from Sigma (St. Louis,MO, USA). The stock standard solutions of each analytewere prepared separately in methanol at 1,000 mg/L andstored at 4 °C. Mixtures of standard working solutionsfor extraction at different concentration levels were pre-pared by dilution with water purified by a Milli-Q waterpurification system from Millipore Company (Bedford,MA, USA). The Accurel Q3/2 polypropylene hollowfiber membrane (600 μm i.d., 200 μm wall thickness,and 0.2 μm pore size) were supplied by Membrana(Wuppertal, Germany).

The soil and rain samples were taken from threedifferent parts of Tehran (Tehran, Iran) and they wereselected for validation of the proposed method. Allpreviously mentioned samples were collected in am-ber glass bottles, stored at 4 °C, and analyzed within4 days after collection.

Instrumentation

The gas chromatographic system consisted of anAgilent (Centerville Road, Wilmington, USA) series7890A GC coupled to an Agilent MSD 5975C quad-rupole mass spectrometer. The GC was fitted with HP-5 MS capillary column (30 m×0.25 mm i.d., 0.25 μmfilm thickness) from Agilent J&W Scientific (Folsom,CA, USA). Helium (99.999 %) was used as the carriergas at the flow rate of 1.0 ml/min. The followingtemperature program was employed for the separation:70 °C for 1 min, increased to 200 °C at 10 °C/min andheld for 1 min, and finally increased to 300 °C at50 °C/min and held for 3 min. The MS quadrupoleand the MS source temperatures were set at 150and 230 °C, respectively. Data acquisition wasperformed in the full scan mode (m/z in the rangeof 50–700) to confirm the retention times ofanalytes and in selected ion monitoring (SIM)mode (m/z=184 for dinitrophenols and m/z=229for trinitrophenols) for quantitative determinationof NPs. A dwell time of 100 ms was used foreach mass operated in SIM mode with high reso-lution. The filament delay time was set at 3 min.

A magnetic stirrer from Heidolph (Kelheim, Ger-many) was employed for stirring the aqueous solu-tions. A 25-μl (model 702 N) syringe was purchasedfrom Hamilton (Bonaduz, Switzerland) for extractionpurposes. Also, a 1.00-μl microsyringe (zero deadvolume, conical tip needle; SGE, Ringwood, Austra-lia) was used for injection into the GC.

The modified HF-LPME procedure

The modified HF-LPME apparatus has been intro-duced first in detail by Ghambarian et al. (2010).The extraction is carried out as follows: (1) 20 ml ofpure water spiked with the analytes (each at the levelof 50 μg/L) was placed in a 22-ml sample vial. (2) A25-μl HPLC syringe and a conventional medical sy-ringe needle were inserted through the silicon septum,the former served to introduce the acceptor solution(10 mg/L MSTFA in ACN) into the hollow fiber priorto extraction and collect this solution after extraction,while the latter needle was utilized for supporting thehollow fiber. (3) An 8.8-cm length of hollow fiber wasplaced between the two needle ends and subsequentlyimmersed in an extracting organic solvent for severalseconds to ensure that the pores of the hollow fibermembrane were filled with the extracting solvent. (4)The diffused solvent in the lumen of the fiber wasremoved by blowing air with a 5-ml syringe, and25 μl of the organic acceptor solution was carefullyinserted into the lumen of the fiber using themicrosyringe. (5) The fiber was placed in the aqueoussample solution stirred at 750 rpm. (6) Once the ex-traction time was reached, the acceptor solution wasflushed into a microtube, of which 1.0 μl was injecteddirectly into the GC-MS.

Results and discussion

In three-phase LPME, the amount of analyte extractedat a certain time depends on the mass transfer of theanalyte from the aqueous sample to the organic sol-vent in the hollow fiber and the analyte’s partitioncoefficient between the aqueous sample and the or-ganic phase. There are several parameters, such astype of organic solvent, sample pH, extraction time,addition of NaCl, and stirring rate, that have an impacton the extraction efficiency.

Firstly, the method was optimized for the extractionof selected NPs in pure water. A univariate approachwas employed to optimize the influential factors in thisstudy. Quantifications were based on the peak area ofthe analytes from the average of three replicate mea-surements. A fixed concentration of each analyte(50.0 μg/L) was used in the optimization process.The optimized method was then used for the analysisof the real samples.

Environ Monit Assess (2013) 185:9055–9065 9057

Effect of organic solvent

The selection of the most appropriate organic solventis of great importance in LPME, in order to achievesatisfactory analyte pre-concentration (Pedersen-Bjergaard and Rasmussen 2008; Shen and Lee2003). Specifically, the criteria for the selection of asuitable organic solvent in HF-LPME are as follows:The solubility of the analytes in the organic sol-vent should be higher than in the donor phase butlower than in the acceptor phase so that theanalytes could be transferred from the donor phaseinto the acceptor phase with high extraction effi-ciency. Secondly, it should be of low volatility toprevent solvent loss and be immiscible with waterto avoid dissolution during extraction and serve asa barrier between the donor and acceptor phases.Lastly, the solvent should be compatible with hol-low fiber and have no or less toxicity (Lee et al.2008; Rasmussen and Pedersen-Biergaard 2004).Regarding the previously mentioned facts, five or-ganic solvents, namely, n-octane, n-dodecane, n-tetradecane, n-hexadecane, and n-dodecane wereevaluated as the extraction solvents. It is worthnoting that TOPO was added to all previously men-tioned solvents (at 10 % w/w level) in order to increasethe polarity of the organic membrane (SLM). Figure 1depicts that (n-dodecane+TOPO) presented the maxi-mum extraction efficiency and thus was selected as themost suitable solvent.

Effect of the pH of the donor phase

Adjustment of the pH can enhance extraction, asdissociation equilibrium is affected together withthe solubility of the acidic/basic target analytes. Intwo-phase or three-phase LPME, there are many re-ports where pH changes in the donor aqueous solu-tion resulted in higher analyte pre-concentrations(Pedersen-Bjergaard and Rasmussen 1999, 2000;Pedersen-Bjergaard et al. 2002; Zhao et al. 2002;Zhu et al. 2001; Andersen et al. 2002).

In order to extract the NPs into the organic phase, thepH of the donor phase was set to be acidic so that theanalytes could be kept in their undissociated forms. Theeffect of pH in the range of 1–6 was investigated.The variations of the extraction efficiency of NPsvs. pH are shown in Fig. 2. Therefore, pH 1 waschosen as the optimum value for the subsequentextractions.

Effect of stirring rate

Agitation of the sample is routinely applied to accel-erate the extraction kinetics (Zhang et al. 1994). In-creasing the agitation rate of the donor solutionenhances the extraction, as the diffusion of analytesthrough the interfacial layer of the hollow fiber isfacilitated, and improves the repeatability of the extrac-tion method (Pedersen-Bjergaard and Rasmussen2000). Moreover, in HF-LPME, the organic solvent is

0

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25

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50

1 2 3 4 5 6

Pea

k A

rea

pH

1,2 Dinitrophenol 1,3 Dinitrophenol 1,4 Dinitro phenol 1,2,3 trinitrophenol 1,2,4 trinitrophenol


Fig. 1 Effect of organic solvent on the extraction efficiency of the method (conditions: pH of donor phase, 1.0; stirring rate, 1,000 rpm;extraction time, 40 min; without addition of salt)

sealed and protected by the hydrophobic hollow fibermembrane, so it is easier to handle and can tolerate ahigher stirring rate.

In our experiments, partitioning of the analytes intothe organic solvent was enhanced with increase of thestirring rate from 200 to 1,000 rpm (Fig. 3). However,higher stirring rates were not evaluated since it wouldcause excessive air bubbles on the surface of thehollow fiber, which could lead to poorer precision

and possible experimental failure. Therefore, we chose1,000 rpm as a suitable value for further extractions.

Effect of salt addition

The addition of salt often increases ionic strength andthus increases the extraction efficiency due to the “salt-ing-out effect” (Wu et al. 2008). In HF-LPME methods,depending on the target analytes, an increase in the ionic

0

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30

35

40

45

50

1 2 3 4 5 6

Pea

k A

rea

pH


Fig. 2 Effect of donor phase pH on the extraction efficiency of the method (conditions: organic solvent (dodecane+10 % TOPO);stirring rate, 1,000 rpm; extraction time, 40 min; without addition of salt)

0

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40

50

60

Pea

k A

rea

1000 800 600 400 200

Fig. 3 Effect of stirring rate on the extraction efficiency of the method (conditions: pH of donor phase, 1.0; organic solvent (dodecane+10 % TOPO); extraction time, 40 min; without addition of salt)


strength of the aqueous solution may have various ef-fects upon extraction: it may enhance (Varanusupakul etal. 2007), not influence (Xia et al. 2007), or even limitthe extraction (Ebrahimzadeh et al. 2010).

In order to investigate the effect of salt addition,spiked samples with various concentrations of sodiumchloride in the range of 0–4 M were studied. Theresults (Fig. 4) revealed that salt addition restrictedthe extraction of the target analytes. A possible expla-nation for this observation may be due to an increase

in viscosity, which in turn decreases the mass transferof the analyte into the organic membrane solvent. As aresult, we decided not to add salt to the sample solu-tions in the following experiments.

Effect of extraction time

There are two liquid–liquid interfaces in HF-LPME (i.e.,source phase–organic phase, organic phase–receivingphase). Thus, it is supposed that the solute molecules

0 10 20 30 40 50 60

1,2 Dinitrophenol

1,3 Dinitrophenol

1,4 Dinitro phenol

1,2,3 trinitrophenol

1,2,4 trinitrophenol

Peak Area

4

3

2

1

0

Fig. 4 Effect of salt addition on the extraction efficiency of the method (conditions: pH of donor phase, 1.0; organic solvent(dodecane+10 % TOPO); extraction time, 40 min; stirring rate, 1,000 rpm)

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Fig. 5 Effect of extraction time on the extraction efficiency of the method (conditions: pH of donor phase, 1.0; organic solvent(dodecane+10 % TOPO); stirring rate, 1,000 rpm, without addition of salt)


Table 1 Figures of merit

Analyte LDR (μg/L) LOD (μg/L) RSD (n=5) EF Extraction percent

1,2-Dinitrophenol 1–100 0.1 5.6 260 52

1,3-Dinitrophenol 1–100 0.1 6.1 275 55

1,4-Dinitrophenol 1–100 0.1 4.7 280 56

1,2,3-Trinitrophenol 1–100 0.2 7.3 255 51

1,2,4-Trinitrophenol 1–100 0.2 6.4 270 54

Table 2 Analysis of real samples

Sample 1,2-Dinitrophenol

1,3-Dinitrophenol

1,4-Dinitrophenol

1,2,3-Trinitrophenol

1,2,4-Trinitrophenol

Soil 1 Concentration beforespiking

– – 3.7 – –

Concentration afterspiking

8.7 9.4 13.1 9.5 9.3

Recovery % 87 94 94 95 93

RSD % 7.3 6.1 5.7 4.8 6.2


– – 2.7 3.1 –


9.1 9.7 12.5 12.8 12.1

Recovery % 91 97 98 97 95

RSD % 7.1 5.3 5.3 5.7 7.4


– – – – –


9.3 9.6 9.5 9.7 9.8

Recovery % 93 96 95 97 98

RSD % 8.3 7.9 6.3 7.4 8.2

Rain water1

Concentration beforespiking

– – – – –


9.5 9.7 9.3 9.8 9.7

Recovery % 95 97 93 98 97

RSD % 4.2 3.5 3.7 4.1 4.6

Rain water2


7.8 – – – –


17.1 9.4 9.5 9.7 9.6

Recovery % 93 94 95 97 96

RSD % 4.8 4.2 5.5 6.3 6.4

Rain water3


4.8 – 3.8 – –


14.3 9.7 13.5 9.5 9.4

Recovery % 95 97 97 95 94

RSD % 3.5 3.7 4.4 4.5 4.2

All concentrations are in micrograms per liter


require a relatively long time to pass through theseinterfaces. LPME is not an exhaustive extraction tech-nique, so maximum sensitivity is attained at equilibriumconditions. On the other hand, there is no need to reach acomplete equilibrium for accurate and precise analysis(Hoffmann et al. 1996; Palmarsdottir et al. 1997). How-ever, to increase the precision and sensitivity of theLPMEmethod, it is necessary to select an exposure timethat guarantees the equilibrium between the phases.

To study the effect of the extraction time, the ex-periments were performed at various time values in therange of 15–90 min. As shown in Fig. 5, the extractionefficiencies increased rapidly by increasing the extrac-tion time up to 40 min and then after remaining rela-tively constant. Consequently, the exposure time of40 min was selected for subsequent extractions.

Evaluation of the method performance

To evaluate the practical applicability of the proposedmethod, the calibration curves were plotted using 10spiked levels including 0.1, 0.2, 0.5, 1.0, 2.5, 5.0, 10,25, 50, and 100 μg/L. Each standard sample wasextracted by the proposed method at the optimum con-ditions. For each level, three replicate extractions wereconducted. The limits of detection (LODs) based on thesignal-to-noise ratio of 3 in the SIM mode, the

correlation coefficients (R2), the linear ranges, the rela-tive standard deviations (RSDs), and the enrichmentfactors (EFs) were calculated and are summarized inTable 1. In order to examine the EF of each analyte, aseries of standard solutions (at concentrations of 0.1,0.5, 1.0, 2.5, 5.0, 10, 25, 50, 75, and 100 mg/L) in theextracting solvent were prepared and were directlyinjected into the GC-MS. Then, peak area was plottedagainst analyte concentration (for all analytes). The EFwas calculated as the slope ratio of the LPME calibra-tion curve to that of the non-extraction (directly injected)curve.

Real sample analysis

Under the optimum conditions, the performance of themethod was tested by analyzing the analytes in sixdifferent real samples. The results listed in Table 2indicate that the number of analytes were detected insome mentioned real samples. All rain samples wereput under the extraction procedure described in the“The modified HF-LPME procedure” section, whilein the case of soil samples, the following pretreatmentwas made: microwave-assisted solvent extraction wasused for the extraction of NPs from 1 g of soil using10 ml of pure water at 600 W for 5 min at 90 °C. Thesoil was further rinsed with an additional 10 ml pure

1,2-

Din

itrop

heno

l

1,3-

Din

itrop

heno

l

1,4-

Din

itrop

heno

l

1,2,

3 -T

r ini

trop

heno

l

1,2,

4-T

rini

trop

heno

l

(B)

(A)

Fig. 6 The GC-MS chromatograms of the soil sample without spike (A) and spiked (B) by the target compounds at a concentrationlevel of 10.0 μg/L after HF-LPME


water to attain a 20-ml aqueous sample. Finally, theresulting sample underwent the modified HF-LPMEprocedure.

All real samples were spiked with the analyte stan-dards at different concentration levels to assess matrixeffects. The relative recoveries of the analytes aregiven in Table 2; these varied between 93 and113 %, which indicated that the matrices in our presentcontext had almost little effect on the method. TheGC-MS chromatogram of the rain sample spiked withthe target compounds at a concentration level of10.0 μg/L for each analyte after HF-LPME is shownin Fig. 6.

Furthermore, the proposed procedure is organicsolvent-minimized and, thus, an environmentallyfriendly approach to the determination of the selectedpollutants in the real samples. So, it follows the path ofan efficient “green analytical chemistry” methodologyto develop a cleaner technique with reduced toxic re-agents consumption and waste generation.

Conclusion

The results from this work show that the modified HF-LPME technique in combination with GC-MS is avalid means of enrichment and quantification of se-lected NPs at trace level in soil and rain samples.Simplicity, low cost, ease of operation, no possibilityof sample carryover, and high EFs are among theadvantages of the proposed method. The establishedapproach demonstrates good sample cleanup withhigh sensitivity and reproducibility. Also, excellentextraction efficiency is achieved almost independentof the matrix in real sample analysis. Subsequently, itcan be extended to other applications.

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