Cavitand-Based Solid-Phase Microextraction Coating for theSelective Detection of Nitroaromatic Explosives in Air and SoilFederica Bianchi,*,† Alessandro Bedini,† Nicolo Riboni,† Roberta Pinalli,† Adolfo Gregori,‡

Leonard Sidisky,§ Enrico Dalcanale,*,† and Maria Careri†

†Dipartimento di Chimica and INSTM, UdR Parma, Universita di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy‡Reparto Carabinieri Investigazioni Scientifiche di Roma, Viale di Tor di Quinto No. 119, 00191 Roma, Italy§Supelco, 595 North Harrison Road, Bellefonte, Pennsylvania 16823, United States

*S Supporting Information

ABSTRACT: A selective cavitand-based solid-phase microextraction coating was synthesized for the determination ofnitroaromatic explosives and explosive taggants at trace levels in air and soil. A quinoxaline cavitand functionalized with acarboxylic group at the upper rim was used to enhance selectivity toward analytes containing nitro groups. The fibers werecharacterized in terms of film thickness, morphology, thermal stability, and pH resistance. An average coating thickness of 50(±4) μm, a thermal stability until 400 °C, and an excellent fiber-to-fiber and batch to batch repeatability with RSD lower than 4%were obtained. The capabilities of the developed coating for the selective sampling of nitroaromatic explosives were provedachieving LOD values in the low ppbv and ng kg−1 range, respectively, for air and soil samples.

The detection of explosives at trace or ultratrace levelsremains an open and challenging issue, which demands

the development of new analytical techniques, featuring at thesame time enhanced selectivity and sensitivity for targetanalytes. Usually, customs and law enforcement units confrontthe following demanding tasks in their activity: (i) the detectionof explosives in sensitive locations like airports, (ii) themonitoring of soil near explosives depot or near sites wherethe explosives are buried (minefields, for example), (iii) theidentification of explosives in suspicious objects. These taskscall for highly reliable, hand-held analytical instrumentationcapable of identifying explosives at trace level, without incurringin false positive/false negative responses. In such context, thedeployment of trained dogs still offers the most efficient andreliable method to detect most of the illegal substances, such asdrugs or explosives. However, dogs work only with a singletrainer at a time and are able to operate efficiently only a fewhours per day; therefore, their deployment requests consid-erable efforts in terms of personnel, cost, and infrastructures.1

For these reasons, great emphasis has been placed to substitutecanine testing with reliable analytical instruments.

Low vapor pressure explosives are labeled with taggants toenhance detectability according to the Montreal Convention.2

Despite this, problems related to selectivity still represent anissue, particularly when explosives need to be detected at traceor ultratrace levels in complex matrixes.Gas chromatography coupled to mass spectrometry or ion

mobility spectrometry (IMS) are some of the most widely usedinstrumental techniques for explosive detection.3 Additionaltechnologies rely on the use of optical laser-based techniqueslike cavity ring-down spectroscopy4,5 or Raman spectroscopy.3

Some of these techniques have been also proposed for thestandoff detection of explosives, being able to detect hazardousmaterials at safe distances.6−9 Studies based on secondaryelectrospray ionization or atmospheric pressure chemicalionization mass spectrometry10,11 have evidenced the need offurther research to meet real challenges of explosive detection.Very recently, a method based on direct real-time selectivepressure chemical ionization12 showed to be promising for

Received: July 8, 2014Accepted: October 10, 2014Published: October 10, 2014

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explosive detection in terms of selectivity and reduction of falsealarm rates.Optical chemosensors are widely used due to their sensitivity

and simplicity in detection.13,14 Fluorescent conjugatedpolymers have been used as optical probes, being able torapidly detect explosives at low concentration levels.15−20

Immunosensors have been also developed for the detection ofexplosives; however, their applications for the gas phasedetection of these compounds is still limited if comparedwith methods based on solution-phase detection.21−24

Since sampling is the critical step which crucially affects theanalytical result, many studies focused on selective materials forexplosives enrichment. In this context, the potential ofnanotechnology has been successfully exploited to developsubstrates with unique recognition properties toward explosivecompounds. Relevant examples include luminescent metalorganic frameworks (MOFs)25,26 and nanowire-based field-effect-transistor arrays.27

Recent advances in sample preparation rely also on thedevelopment of molecularly imprinted polymers (MIPs). MIPsare proposed as coatings for the gas phase extraction ofexplosives both for mass selective devices and for solid-phasemicroextraction (SPME).28−30 Owing to its simplicity, low cost,and possible automation, SPME is a widely used technique forsample collection.31 In addition to commercially availablecoatings,32 different materials like nanostructured polypyrrole,33

polyphthalazine ether sulfone ketone,34 or metal β-diketonatepolymers35 are available for explosive enrichment in both waterand air samples. Explosive detection via air sampling has beenalso obtained via planar SPME-IMS devices, using homemadeIMS supports coated with different materials.36−38

However, selectivity remains the key challenge for thedevelopment of analytical devices for explosive detection.Supramolecular analytical chemistry represents a fascinatingapproach to tackle it.39,40 By a molecular level understanding ofthe receptor−analyte interactions, selective receptors can berationally designed as a function of the analytes to be detectedand the interfering agents to be excluded. The appropriatechoice of synergic weak host−guest interactions allows theboosting of selectivity, retaining at the same time reversibility ofthe responses.The complexation properties of calixarenes toward nitro-

aromatics have been exploited to impart selectivity to a widevariety of 2,4,6-trinitrotoluene (TNT) chemical sensors,41−43

spanning from colorimetric,44 electrochemical,45,46 piezoelec-tric47 to surface-enhanced Raman spectroscopy48 transductionmodes, as well as to extract explosives from water and soil.49,50

Here we propose an alternative approach, based on the use ofmolecular receptors as selective preconcentrators for nitro-aromatics. The ideal preconcentrator must at the same timeincrease sensitivity, by concentrating volatile compoundspresent in air at very low level, and enhance selectivity bytrapping only the desired compounds out of the many presentin air.To this purpose we report a quinoxaline-bridged cavitand

decorated at the upper rim with a carboxyl group(QxCOOHCav) to be used both as SPME coating anddynamic headspace sorbent for the selective gas-phaseextraction of nitroaromatic compounds from environmentalmatrixes like air and soil. The multiple π−π and CH−πinteractions, responsible for the aromatic inclusion within thecavity,51−54 are strengthened by the presence of an additionalsynergistic H-bonding interaction with one NO2 group of the

guests, thus promoting the confinement of the aromaticexplosive taggants within the cavity.The QxCOOHCav receptor is compared to the commonly

used commercial fibers in terms of thermal and pH stability,film thickness, and selectivity. Finally, the selective enrichmentof nitroaromatic explosives and taggants is demonstrated in realworld air and soil samples.

■ EXPERIMENTAL SECTIONChemicals and Materials. For the synthesis, all solvents

were dried over 3 or 4 Å molecular sieves. All reagents,nitroaromatic explosives (nitrobenzene NB, 2-nitrotoluene 2-NT, 3-nitrotoluene 3-NT, 4-nitrotoluene 4-NT, 2,6-dinitroto-luene 2,6-DNT, 2,4-dinitrotoluene 2,4-DNT 3,4 -dinitrotoluene3,4-DNT), and solvents were purchased by Sigma-Aldrich.TNT was obtained from Reparto Carabinieri InvestigazioniScientifiche of Rome and recrystallized in the laboratory. TenaxTA cartridges (90 mg, 20−35 mesh) were purchased fromSuperchrom (Milan, Italy), whereas SPME bare fused silicafibers with and without assembly and PDMS-DVB 65 μm fiberswere purchased from Supelco (Supelco, Bellefonte, PA).Duralco 4460 epoxy glue was from Cotronics Corp. (Brooklyn,NY).

Cavitands Synthesis. 2,3-Dichloroquinoxaline-6-carboxylicacid,55 tetraquinoxaline cavitand 4QxCav, and triquinoxalinecavitand 3QxCav were prepared according to literatureprocedures.56 The synthesis of the 4QxCOOHCav is shownin Scheme 1. The synthesis of 2,3-dichloroquinoxaline-6-carboxylic acid and the characterization of the 3QxCav and4QxCOOHCav are reported in the Supporting Information(Table S-1).

4QxCOOHCav. To a stirred solution of 3QxCav (0.40 g,0.33 mmol) and K2CO3 (0.16 g, 1.15 mmol) in dry DMF (11mL), 2,3-dichloroquinoxaline-6-carboxylic acid (0.088 g, 0.36mmol) was added. The mixture was heated at 60 °C overnightand then quenched in acidic water (HCl 1 N). The precipitateobtained was filtered and washed to neutrality. The crudeproduct was purified by column chromatography (SiO2, 9/1,CH2Cl2/EtOH, v/v) affording 4QxCOOHCav as a white solid.The recovered 3QxCav is further reacted, bringing the overallyield closed to 100%. (Scheme 1).

Fiber Preparation. The fiber coating was obtained byvertically dipping the silica support of the fiber in the Duralco4460 epoxy glue and after 2 min in the cavitand powder threetimes. Four fibers were prepared for each kind of cavitand.

Fiber Characterization. Thermogravimetric analysis(TGA) was performed on a TGA 7 instrument (PerkinElmer,Walthan, MA) over the temperature range 25−400 °C (heatingrate, 25 °C min−1) in air. Coating thickness and surfacemorphology were investigated by using scanning electron

Scheme 1. Synthesis of 4QxCOOHCav

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microscopy (SEM) with a Leica 430i instrument (Leica, Solms,Germany). Fiber bleeding was investigated by desorbing thefibers in the GC injection port for 2 min at 250 °C, respectively.pH resistance was evaluated by sampling (immersion analysis,room temperature, extraction time, 30 min) 50 ng L−1 ofnitrobenzene, 2-, 3-, and 4-nitrotoluene in water at pH 2, pH 4,and pH 7. Five replicated measurements for each pH valuewere performed. Fiber-to-fiber and batch-to-batch repeatabilitywere evaluated both for headspace and for immersion analysisby using four fibers in each case. Nitrobenzene andtrinitrotoluene were analyzed in the case of headspace andimmersion analysis, respectively. Six replicated measurementsfor each fiber were always performed.SPME Analysis. All the SPME experiments were performed

by using the Combi-PAL GC autosampler (CTC Analytics, AG,Switzerland). Prior to use, all the fibers were conditioned in theGC injection port at 270 °C for 1 h under a helium flow. Airsampling of nitroaromatic explosives was performed byexposing the QxCav-based fibers in an air atmospherecontaining a mixture of the nitroaromatic compounds.Extraction was carried out at room temperature for 30 min.The same procedure was applied using the 65 μm PDMS-DVBfiber (Supelco). Soil samples were analyzed by exposing theSPME fiber in the headspace above the sample by operatingunder the optimized extraction conditions.Adsorbent Cartridges for Dynamic Headspace (DHS)

Sampling. The adsorbent cartridges were obtained bymanually filling glass tubes (i.d.= 3.5 mm, l = 16 cm) with 0.1 g of the cavitand-based materials (∼40 mesh, Figure S-1 inthe Supporting Information). Small silanized glass wool plugswere used to avoid loss of material during desorption.Dynamic Headspace Analysis. Prior to use, all the

cartridges were conditioned at 270 °C for 1 h under a nitrogenflow. Sampling of an air mixture containing mononitroaromaticexplosives was performed by using the 4QxCOOHCav,4QxCav, and 3QxCav adsorbents under the followingconditions: room temperature; extraction time, 30 min;extraction flow, 40 mL min−1. The same procedure was appliedusing the Tenax TA cartridges. Thermal desorption was carriedout by using a TCT thermal desorption cold trap (TD800,Fisons Instruments, Milan, Italy). Desorption was thermallyperformed at 220 °C for 10 min under a helium flow (10 mLmin−1) focusing the analytes in a glass lined tube at −120 °Cwith liquid nitrogen. The nitroaromatic explosives were injectedinto the GC capillary column by heating the cold trap to 220°C. Three independent extractions were always performed.GC/MS Analysis. A HP 6890 series Plus gas chromatograph

(Agilent Technologies, Palo Alto, CA) equipped with a MSD5973 mass spectrometer (Agilent Technologies) was used.Helium was used as the carrier gas at a constant flow rate of 1mL min−1; the gas chromatograph was operated in splitlessmode for 1 min with the programmed temperature vaporization(PTV) injector (Agilent Technologies) maintained at thetemperature of 200 °C and equipped with a 1.5 mm i.d.multibaffled liner (Agilent Technologies). Chromatographicseparation was performed on a 30 m × 0.25 mm, df 0.25 μmHP-5 ms capillary column (Agilent Technologies). The transferline and source were maintained at the temperatures of 220 and150 °C, respectively. Preliminarily, full scan electron impact(EI) data were acquired to determine appropriate masses forselected-ion monitoring mode (Table S-2 in the SupportingInformation) under the following conditions: ionization energy,70 eV; mass range, 35−250 amu; scan time, 3 scan s−1; electron

multiplier voltage, 2200 V. Signal acquisition and data handlingwere performed using HP Chemstation (Agilent Technolo-gies).

Optimization Procedure. The experiments were carriedout on blank soil samples (0.5 g in 2.5 mL of water) spiked withnitroaromatic compounds each at 1 μg kg−1. A 23 two-levels fullfactorial design (FFD) followed by the multicriteria method ofdesirability functions was carried out.57,58 The effects oftemperature of extraction (T), time of extraction (t), andpercentage of sodium chloride (NaCl) added to the sampleswere evaluated: low and high levels were T, 40−70 °C; t, 15−45 min; NaCl, 0−30% (w/v). The best regression models wereobtained by a forward search stepwise variable selectionalgorithm, and the optimal conditions were evaluated by theglobal desirability D.59

Method Validation. Method validation was performedaccording to EURACHEM guidelines60 following the sameprocedure reported in a previous study.61

Samples. Propelling charges and soil samples taken in theproximity of explosion sites were supplied by RepartoCarabinieri Investigazioni Scientifiche (RIS), Roma, Italy.

■ RESULTS AND DISCUSSION4QxCOOHCav Synthesis. The design of an appropriate

receptor for nitroaromatic explosives was based on our previousexperience in selective detection of aromatic compounds likebenzene, toluene, ethylbenzene, and xylene isomers withtetraquinoxaline-bridged cavitands.51,52 The molecular recog-nition properties of 4QxCav toward aromatic hydrocarbons arebased on π−π and CH−π interactions both with thequinoxaline cavity walls and with the resorcinarene scaffold.62

These multiple weak interactions, made possible by thecomplete confinement of the guest within the cavity, render4QxCav the receptor of choice for the selective detection ofaromatic over aliphatic hydrocarbons. To succeed in the aim ofdeveloping novel coatings for the selective sampling ofnitroaromatic explosives and explosive taggants, the quinoxa-line-based cavitands required the insertion of a suitablesubstituent at the upper rim. A carboxylic acid was chosenbecause of its capability of introducing a synergistic H-bondinginteraction with the NO2 group of the guest, thus strengtheningthe interactions among nitro-compounds and receptors.63 Asdepicted in Scheme 1, the carboxylic acid was placed on the topof one of the four tetraquinoxaline walls. Since 4QxCOOHCavcannot be obtained by the direct bridging of resorcinarene, dueto the favorite formation of 4QxCav, an alternative syntheticstrategy was pursued. It is based on the synthesis of 3QxCavfollowed by the bridging of its residual two hydroxyl groupswith 2,3-dichloroquinoxaline-6-carboxylic acid. The last qui-noxaline wall was introduced directly without the need of anyprotecting group for the carboxylic unit: this approach led tothe synthesis of the desired cavitand without additionalprotection/deprotection steps. Despite the low yield of4QxCOOHCav (20%), the unreacted 3QxCav was recoveredquantitatively, thus making possible to bring the overall yield of4QxCOOHCav close to 100% by performing multiplereactions.

Thermal Stability of the Cavitands. The thermal profileof the obtained materials was recorded by means of TGA(Figures S2−S4 in the Supporting Information). A very goodstability from room temperature to 400 °C was obtained for4QxCOOHCav in air, which mirrors that observed for 4QxCav.The presence of the carboxylic group does not reduce the

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thermal stability of the cavitand. The small initial loss of weightis related to the thermal release of residual solvent trapped inthe cavity. By comparison the precursor 3QxCav is less stableand slowly decomposes from 130 °C onward, due to thepresence of the two thermally labile phenolic OHs.The thermal stability of the coatings were also evaluated by

conditioning the fibers in the GC injector port at 250 °C. Bydesorbing the fibers, no significant bleeding was observed forboth 4QxCav and 4QxCOOHCav, thus confirming the highthermal resistance of the coatings.The pH influence was also tested by using the developed

fibers for sampling of nitrobenzene, 2-, 3-, and 4-nitrotoluene(immersion analysis) in aqueous solutions at different pH (inthe 2−7 pH range) for 30 min at room temperature. ANOVAdid not show significant differences (p > 0.005) among themean responses (n = 5 for each pH value), thus assessing thecapabilities of the developed coatings for the sampling ofsolutions under different pH conditions.The morphology of the obtained fibers was investigated by

scanning electron microscopy under different magnifications(Figure 1), revealing a reasonably homogeneous coating with a

large surface area on the entire surface of the fiber in which thecavitand particles of the powder are distributed in a variety oforientations. The average thickness of 4QxCOOHCavdeveloped coating was found to be 50 ± 4 μm (n = 3).Finally, the performance of the developed materials were

evaluated in terms of fiber-to fiber and batch-to-batchrepeatability using SPME both for headspace and immersionanalyses. As shown in Table 1, all the cavitands tested in thestudy allowed one to obtain a RSD always lower than 4.1%,

thus proving the feasibility of the proposed procedure in thedevelopment of stable and repeatable coatings.

Sampling of Nitroaromatic Explosives and ExplosiveTaggants. The host−guest interactions of the quinoxaline-based materials were exploited for the selective sampling ofnitroaromatic explosives and explosive taggants at trace levels inair and soil samples using both dynamic headspace and SPME.Preliminary experiments were carried out by performing DHSanalyses of an air mixture of mononitroaromatic compoundseach at 10 ppbv. The experiments were carried out bycomparing the performances of 3QxCav, 4QxCav, and4QxCOOHCav with those of the commercial adsorbentTenax TA. 4QxCav was used as reference material beingcharacterized only by a quinoxaline-based structure able tointeract with aromatic guest only via π−π and CH−πinteractions.The 4QxCOOHCav showed the highest extraction efficiency

with GC responses from 3 to 20 times higher than thoseachieved using the other adsorbents, thus proving that thesynergistic presence of π−π, CH−π, and H-bondinginteractions is able to strengthen the retention of nitrocompounds (Figure 2). As expected, the performances of

3QxCav were not satisfying. The open cavity of the receptordid not allow the complete engulfment of the analytes, thusreducing the adsorption of the investigated compounds.As for extraction, owing to the predominant presence of the

cavitand powder with respect to the epoxy glue, the involvedmechanism is adsorption. Finally, the cavitands were exposed toaliphatic compounds (from C5 to C10) at concentrations 50times higher than those of the nitroaromatic guests to assess thesuperior selectivity of 4QxCOOHCav during sampling. Theseanalytes are adsorbed through unspecific dispersion interactionswith the aliphatic chains at the lower rim of the cavitand.64 Thecomplete desorption of aliphatic hydrocarbons was obtained atlow temperature (50 °C), whereas stronger conditions (220°C) were required for the release of nitro compounds, thusconfirming their specific inclusion inside the cavity (Figure S-5in the Supporting Information).

Figure 1. Scanning electron micrographies of the 4QxCOOHCav-based coating at two different magnification: (a) 100× and (b) 1000×.

Table 1. Fiber-to Fiber and Batch-to-Batch RepeatabilityUsing the 4QxCOOHCav Fibera

fiber-to-fiber RSD% batch-to-batch RSD%

headspace analysisnitrobenzene

2.8 3.6

immersion analysistrinitrotoluene

2.2 4.1

a4 fibers, 6 replicated measurements for each fiber.

Figure 2. DHS sampling of air containing a mixture ofmononitroaromatic compounds each at 10 ppbv followed by GC-(SIM)-MS analysis. Sampling time, 30 min; room temperature (RT).

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The host−guest interactions of the 4QxCOOH fiber werefurther exploited by performing SPME experiments. Theenrichment capabilities toward nitroaromatic compoundswere evaluated in terms of enhancement factors (EFs).65 EFswere calculated as the ratio of the concentration of the analytein the fiber after extraction to that of the analyte in the gasstandard mixture (i.e., using the ratio of the chromatographicpeak area of the analyte after SPME, extraction time, 30 min atRT, to that before extraction obtained by the direct injection ofthe same gas standard solution, n = 3). The highest EFs wereobserved for NB (25 500 ± 1 800), 4-NT (23 000 ± 1 000),3,4-DNT (22 100 ± 1 700), 3-NT (17 300 ± 1 900), and 2-NT(11 900 ± 1 600), whereas the lowest values were obtained forTNT (2600 ± 200), 2,6-DNT (6800 ± 500), toluene (7400 ±800), and 2,4-DNT (10 000 ± 600). Toluene was chosen as thereference compound to assess the complexation capabilities ofthe cavitand toward analytes bringing NO2 substituents. Theachieved results suggest that steric hindrance affects cavityinclusion. Because of their shape, TNT, 2,6-DNT, and 2,4-DNT were not favorably trapped inside the cavity, whereastoluene complexation was depressed by the absence of nitrosubstituents.Control experiments were performed by using both a plain

epoxy glue coated fiber to rule out glue physisorption effectsand with a fiber coated with 2,3-diphenoxy quinoxaline(diPhQx) as a mimic of the aromatic cavitand walls, to excludeunspecific π−π interactions. The extraction performance of the4QxCOOHCav coating was always higher than that of thecontrol fibers (Figure 3). The absence of the receptor in the

first case and the lack of a preorganized cavity in the secondcase led to low responses. These two control experimentssupport the need of host−guest interactions to boost SPMEselectivity.The SPME method for the determination of nitro explosives

in air was validated obtaining excellent results with LOD valuesin the low ppbv range (Table 2), thus proving the potential ofthe method for the determination of these compounds atultratrace levels.The complexation capabilities of 4QxCOOHCav were also

exploited toward the sampling of nitroaromatic explosives andexplosive taggants present in soil. By using a 23 full factorialdesign and the multicriteria method of desirability functions,the optimal extraction conditions were found in correspond-ence to an extraction temperature of 70 °C, an extraction timeof 45 min, and 30% of NaCl obtaining a global desirability D =0.70. The regression models used to search for the highestglobal SPME-GC-MS recovery within the explored domain and

the values of the single desirability di are reported in Table S-3in the Supporting Information.Taking into account that good signals for all the nitro-

aromatic explosives could be obtained by operating under theseconditions, the results of the optimization procedure weresatisfying. Finally, by operating under the optimized extractionconditions, the performances of the 4QxCOOHCav, 4QxCav,and PDMS/DVB fibers were compared. The 4QxCOOHCavfiber showed the best extraction efficiency obtaining GCresponses 10−50 times higher than those achieved by using the4QxCav and the commercial coating. Toluene gave the lowestextraction efficiency due to the absence of H-bondinginteractions with the −COOH substituents.Method validation proved the suitability of the 4QxCOOH-

Cav coating for the detection of nitroaromatic explosives attrace levels with LOD values in the low ng kg−1 range (Table3). The achieved LOD values were about 2 orders of magnitude

lower than those reported by the USEPA methods 8330 and8330B. The major features of the method were an easyautomation, reduced analysis time and reduced samplehandling. It is to be noted that low LOD values werepreviously obtained also both by Holmgren et al.66 and Felt etal.,67 but in both cases explosive analysis was carried out afterperforming a solid−liquid extraction procedure.Good linearity was proved in the 120−1200 ng kg−1 range

for all the analytes by applying Mandel’s fitting test. Methodprecision was evaluated testing two concentration levels, i.e.,150 and 1500 ng kg−1. Good results were obtained both interms of intraday repeatability and intermediate precision, withRSD values always lower than 10%. In the case of intermediateprecision, ANOVA showed that mean values were notsignificantly different among the 3 days obtaining p values >0.05. Extraction recoveries ranging from 83.4 ± 4.5% to 95.3 ±3.2% (n = 3) were calculated at 200 and 700 ng kg−1, thus

Figure 3. SPME sampling of air containing a mixture of nitroaromaticexplosives each at 10 ppbv followed by GC-(SIM)-MS analysis.Sampling time, 30 min; RT.

Table 2. LOD and LOQ Values in Air Using the4QxCOOHCav

LOD (ppbv) LOQ (ppbv)

toluene 0.100 0.40NB 0.004 0.052-NT 0.014 0.053-NT 0.008 0.034-NT 0.006 0.022,6-DNT 0.020 0.062,4-DNT 0.012 0.043,4-DNT 0.007 0.022,4,6-TNT 0.060 0.30

Table 3. LOD and LOQ Values in Soil Using the4QxCOOHCav

LOD (ng kg−1) LOQ (ng kg−1)

toluene 60 200NB 38.2 1102-NT 36.4 953-NT 32 804-NT 25.8 782,6-DNT 18.4 482,4-DNT 12.6 44.83,4-DNT 12.4 42.62,4,6-TNT 18 60.5

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showing the good efficiency of the developed method in termsof extraction recovery.The developed method was finally applied for the analysis of

two debris samples collected on two different explosion scenes.The analysis of the two blind samples revealed the differentorigin of the explosives: in the first sample, the absence of TNTand the high amount of di- and mononitrotoluenes allowed oneto assess the use of a propelling charge (smokeless powder,Figure S-6 in the Supporting Information); by contrast, in thesecond sample the presence of a high amount of TNT andDNTs coupled to the detection of other compounds likeparaffin proved the use of military explosive coming from themunition case of unexploded ordnance (UXO) to produce animprovised explosive device (IED) (Figure 4).

■ CONCLUSIONSA new SPME coating based on 4QxCOOHCav as a molecularreceptor was developed and proposed for the selectivedetermination of nitroaromatic explosives and explosivetaggants at ultratrace levels in environmental air and soilsamples. A synergistic ensemble of weak interactions wasarranged in a single receptor to boost analytical performancesvia enhanced selectivity. The cavitand was specifically designedto complex nitroaromatics by adding a H-bonding interactionto the already present ability of the cavity to engulf aromaticcompounds. To this purpose the upper rim of the cavitand wasfunctionalized with a carboxylic group, preorganized for H-bonding with the NO2 group of the analytes protruding fromthe cavity rim. The main features of the developed material areexcellent thermal stability, a very good repeatability, and thepossibility of a selective adsorption of nitroaromatics in thepresence of high amounts of aliphatic hydrocarbons. The sameremarkable selectivity and sensitivity were also proved incomplex matrixes like soil, obtaining GC responses 10−50-foldhigher than those obtained using commercial devices.

■ ASSOCIATED CONTENT*S Supporting InformationAnalytical and spectral characterizations of the developedcoating like SEM images, TGA, 1H NMR, and electrospray(ESI) mass spectra to support the characterization of thedeveloped materials and a detailed description of the GC/MSexperimental conditions used in this work. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*Federica Bianchi: e-mail, federica.bianchi@unipr.it; phone,+39 0521 905446; fax, +39 0521 905556.*Enrico Dalcanale: e-mail, enrico.dalcanale@unipr.it; phone,+39 0521 905463; fax, +39 0521 905556.Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the European Union through theDOGGIES project (Grant FP7-SEC-2011-285446). Interdipar-timental Center for Measurements “G. Casnati” of theUniversity of Parma is acknowledged for the use of the NMRand MS facilities.

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Figure 4. GC-SIM-MS chromatogram of a debris sample collected onan explosion scene. The inset shows the GC responses including theminor analytes extracted. Extraction conditions: sample, 0.1 g;extraction temperature, 70 °C; extraction time, 45 min.

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