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Cavitand-based solid-phase microextraction coatings for the selective detection of nitroaromatic explosives in air and soil Federica Bianchi, Alessandro Bedini, Nicolò Riboni, Roberta Pinalli, Adolfo Gregori, Leonard M. Sidisky, Enrico Dalcanale, and Maria Careri Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac5025045 • Publication Date (Web): 10 Oct 2014 Downloaded from http://pubs.acs.org on October 14, 2014
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Analytical Chemistry
Cavitand-based solid-phase microextraction coatings for the selective detection of nitroaromatic explosives in air and soil Federica Bianchia*, Alessandro Bedinia, Nicolò Ribonia, Roberta Pinallia, Adolfo Gregorib, Leonard Sidiskyc, Enrico Dalcanalea*, Maria Careria a
Dipartimento di Chimica and INSTM, UdR Parma, Università di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy Reparto Carabinieri Investigazioni Scientifiche di Roma, Viale di Tor di Quinto n.119, 00191 Roma, Italy c Supelco 595 North Harrison Road, Bellefonte, PA 16823 USA b
ABSTRACT: A selective cavitand-based solid-phase microextraction coating was synthesized for the determination of nitroaromatic explosives and explosive taggants at trace levels in air and soil. A quinoxaline cavitand functionalized with a carboxylic group at the upper rim was used to enhance selectivity towards analytes containing nitro groups. The fibers were characterized 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 proved achieving LOD values in the low ppbv and ng kg-1 range, respectively for air and soil samples.
The detection of explosives at trace or ultra-trace levels remains an open and challenging issue, which demands the development of new analytical techniques, featuring at the same time enhanced selectivity and sensitivity for target analytes. Usually, customs and law enforcement units confront the following demanding tasks in their activity: (i) the detection of explosives in sensitive locations like airports; (ii) the monitoring of soil near explosives depot or near sites where the explosives are buried (minefields for example); (iii) the identification of explosives in suspicious objects. These tasks call for highly reliable, hand-held analytical instrumentation capable of identifying explosives at trace level, without incurring in false positive/false negative responses. In such context, the deployment of trained dogs still offers the most efficient and reliable method to detect most of the illegal substances, such as drugs or explosives. However, dogs work only with a single trainer at a time and are able to operate efficiently only a few hours per day; therefore, their deployment requests considerable efforts in terms of personnel, cost and infrastructures.1 For these reasons, great emphasis has been placed to substitute canine testing with reliable analytical instruments. Low vapor pressure explosives are labelled with taggants to enhance detectability according to the Montreal Convention.2 Despite of this, problems related to selectivity still represent an issue, particularly when explosives need to be detected at trace or ultratrace levels in complex matrices. Gas chromatography coupled to mass spectrometry or ion mobility spectrometry (IMS) are some of the most widely used instrumental techniques for explosive detection.3 Additional technologies rely on the use of optical laser-based techniques like cavity ring-down spectroscopy,4,5 or Raman spectroscopy.3 Some of these techniques have been also proposed for the standoff detection of explosives, being able to detect hazardous materials at safe distances.6-9 Studies based on secondary electrospray ionization or atmospheric pressure chemical ionization mass spectrometry10,11
have evidenced the need of further research to meet real challenges of explosive detection. Very recently, a method based on direct real-time selective pressure chemical ionization12 showed to be promising for explosive detection in terms of selectivity and reduction of false alarm rates. Optical chemosensors are widely used due to their sensitivity and simplicity in detection.13,14 Fluorescent conjugated polymers have been used as optical probes, being able to rapidly detect explosives at low concentration levels.15-20 Immunosensors have been also developed for the detection of explosives, however, their applications for the gas phase detection of these compounds is still limited if compared with methods based on solution-phase detection.21-24 Since sampling is the critical step which crucially affects the analytical result, many studies focused on selective materials for explosives enrichment. In this context, the potential of nanotechnology has been successfully exploited to develop substrates with unique recognition properties toward explosive compounds. Relevant examples include luminescent metal organic frameworks (MOFs)25,26 and nanowire-based field-effect-transistor arrays.27 Recent advances in sample preparation rely also on the development of molecularly imprinted polymers (MIPs). MIPs are proposed as coatings for the gas phase extraction of explosives both for mass selective devices and for solid-phase microextraction (SPME).28-30 Owing to its simplicity, low cost and possible automation, SPME is a widely used technique for sample collection.31 In addition to commercially available coatings,32 different materials like nano-structured polypyrrole,33 polyphthalazine ether sulfone ketone34 or metal β-diketonate polymers35 are available for explosive enrichment in both water and air samples. Explosive detection via air sampling has been also obtained via planar SPME-IMS devices, using home-made IMS supports coated with different materials.36-38
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However, selectivity remains the key challenge for the development of analytical devices for explosive detection. Supramolecular analytical chemistry represents a fascinating approach to tackle it.39,40 By a molecular level understanding of the receptor-analyte interactions, selective receptors can be rationally designed as a function of the analytes to be detected and the interferents to be excluded. The appropriate choice of synergic weak host-guest interactions allows the boosting of selectivity, retaining at the same time reversibility of the responses. The complexation properties of calixarenes toward nitroaromatics have been exploited to impart selectivity to a wide variety of 2,4,6-trinitrotoluene (TNT) chemical sensors,41-43 spanning from colorimetric,44 electrochemical,45,46 piezoelectric47 to surfaceenhanced Raman spectroscopy48 transduction modes, as well as to extract explosives from water and soil.49-50 Here we propose an alternative approach, based on the use of molecular receptors as selective preconcentrators for nitroaromatics. The ideal preconcentrator must at the same time increase sensitivity, by concentrating volatile compounds present in air at very low level, and enhance selectivity by trapping only the desired compounds out of the many present in 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 and dynamic headspace sorbent for the selective gas-phase extraction of nitroaromatic compounds from environmental matrices like air and soil. The multiple π-π and CH-π interactions, responsible for the aromatic inclusion within the cavity,51-54 are strengthened by the presence of an additional synergistic H-bonding interaction with one NO2 group of the guests, thus promoting the confinement of the aromatic explosive 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 enrichment of nitroaromatic explosives and taggants is demonstrated in real world air and soil samples. EXPERIMENTAL SECTION Chemicals 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-dinitrotoluene 2,6-DNT, 2,4dinitrotoluene 2,4-DNT 3,4 -dinitrotoluene 3,4-DNT) and solvents were purchased by Sigma Aldrich. TNT was obtained from Reparto Carabinieri Investigazioni Scientifiche of Rome and recrystallized in laboratory. Tenax TA cartridges (90 mg, 20–35 mesh) were purchased from Superchrom (Milan, Italy), whereas SPME bare fused silica fibers with and without assembly and PDMSDVB 65 µm fibers were purchased from Supelco (Supelco, Bellefonte, PA, USA). Duralco 4460 epoxy glue was from Cotronics Corp. (Brooklyn, NY, USA). Cavitands synthesis. 2,3-dichloroquinoxaline-6-carboxylic acid,55 tetraquinoxaline cavitand 4QxCav and triquinoxaline cavitand 3QxCav were prepared according to literature procedures.56 The synthesis of the 4QxCOOHCav is shown in Scheme 1. The synthesis of 2,3-dichloroquinoxaline-6-carboxylic acid and the characterization of the 3QxCav and 4QxCOOHCav 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 (11 mL), 2,3dichloroquinoxaline-6-carboxylic acid (0.088 g, 0.36 mmol) was added. The mixture was heated at 60°C overnight and then quenched in acidic water (HCl 1N). The precipitate obtained was
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filtered and washed to neutrality. The crude product 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 overall yield closed to 100%. (Scheme 1). Fiber preparation. The fiber coating was obtained by vertically dipping the silica support of the fiber in –the Duralco 4460 epoxy glue and after 2 min in the cavitand powder for three times. Four fibers were prepared for each kind of cavitand.
Scheme 1. Synthesis of 4QxCOOHCav
Fiber characterization. Thermogravimetric analysis (TGA) was performed on a TGA 7 instrument (Perkin-Elmer, Walthan, MA) over the temperature range 25-400°C (heating rate: 25°C min-1) in air. Coating thickness and surface morphology were investigated by using scanning electron microscopy (SEM) with a Leica 430i instrument (Leica, Solms, Germany). Fiber bleeding was investigated by desorbing the fibers 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 of nitrobenzene, 2-, 3- and 4-nitrotoluene in water at pH 2, pH 4 and pH 7. Five replicated measurements for each pH value were performed. Fiber-to-fiber and batch-to-batch repeatability were evaluated both for headspace and for immersion analysis by using four fibers in each case. Nitrobenzene and trinitrotoluene were analyzed in the case of headspace and immersion analysis, respectively. Six replicated measurements for 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 the GC injection port at 270°C for 1 h under a helium flow. Air sampling of nitroaromatic explosives was performed by exposing the QxCav-based fibers in an air atmosphere containing 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-DVB fiber (Supelco). Soil samples were analyzed by exposing the SPME fiber in the headspace above the sample by operating under the optimized extraction conditions. Adsorbent cartridges for dynamic headspace (DHS) sampling. The adsorbent cartridges were obtained by manually 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). Small silanized glass wool plugs were 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 nitrogen flow. Sampling of an air mixture containing mono-nitroaromatic explosives was performed by using the 4QxCOOHCav, 4QxCav and 3QxCav adsorbents under the following conditions: room temperature, extraction time: 30 min, extraction flow: 40 ml min-1. The same procedure was applied using the Tenax TA cartridges. Thermal desorption was carried out by using a TCT thermal desorption cold trap (TD800, Fisons Instruments, Milan, Italy). Desorption was thermally performed at 220°C for 10 min under a helium flow (10 ml
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Analytical Chemistry
min-1) focusing the analytes in a glass lined tube at −120 °C with liquid nitrogen. The nitroaromatic explosives were injected into 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 MSD 5973 mass spectrometer (Agilent Technologies) was used. Helium was used as the carrier gas at a constant flow rate of 1 mL min-1; the gas chromatograph was operated in splitless mode for 1 min with the PTV injector (Agilent Technologies) maintained at the temperature of 200°C and equipped with a 1.5 mm i.d. multibaffled liner (Agilent Technologies). Chromatographic separation was performed on a 30 m x 0.25 mm, df 0.25 µm HP-5 ms capillary column (Agilent Technologies). The transfer line and source were maintained at the temperatures of 220 and 150 °C, respectively. Preliminarily, full scan EI data were acquired to determine appropriate masses for selected-ion monitoring mode (Table S-2 in the Supporting Information) 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 handling were performed using the HP Chemstation (Agilent Technologies). Optimization procedure. The experiments were carried out on blank soil samples (0.5 g in 2.5 mL of water) spiked with nitroaromatic compounds each @1 µg kg-1. A 23 two-levels full factorial design (FFD) followed by the multicriteria method of desirability functions was carried out.57,58 The effects of temperature of extraction (T), time of extraction (t) and percentage of sodium chloride (NaCl) added to the samples were evaluated: low and high levels were T: 40–70 °C, t: 15–45 min, NaCl: 0–30% (w/v). The best regression models were obtained by a forward search stepwise variable selection algorithm and the optimal conditions were evaluated by the global desirability D.59 Method Validation. Method validation was performed according to EURACHEM guidelines60 following the same procedure reported in a previous study.61 Samples. Propelling charges and soil samples taken in the proximity of explosion sites were supplied by Reparto Carabinieri Investigazioni Scientifiche (RIS), Roma, Italy. RESULTS AND DISCUSSION QxCOOHCav synthesis. The design of an appropriate receptor for nitroaromatic explosives was based on our previous experience in selective detection of aromatic compounds like benzene, toluene, ethylbenzene and xylene isomers with tetraquinoxalinebridged cavitands.51,52 The molecular recognition properties of 4QxCav toward aromatic hydrocarbons are based on π-π and CHπ interactions both with the quinoxaline cavity walls and with the resorcinarene scaffold.62 These multiple weak interactions, made possible by the complete confinement of the guest within the cavity, render 4QxCav the receptor of choice for the selective detection of aromatic over aliphatic hydrocarbons. To succeed in the aim of developing novel coatings for the selective sampling of nitroaromatic explosives and explosive taggants, the quinoxalinebased cavitands required the insertion of a suitable substituent at the upper rim. A carboxylic acid was chosen because of its capability of introducing a synergistic H-bonding interaction with the NO2 group of the guest, thus strengthening the interactions among nitro-compounds and receptors.63 As depicted in Scheme 1, the carboxylic acid was placed on the top of one of the four tetraquinoxaline walls. Since 4QxCOOHCav cannot be obtained by the direct bridging of resorcinarene, due to the favorite formation of 4QxCav, an alternative synthetic strategy was pursued. It is based on the synthesis of 3QxCav followed by the bridging of
its residual two hydroxyl groups with 2,3-dichloroquinoxaline-6carboxylic acid. The last quinoxaline wall was introduced directly without the need of any protecting group for the carboxylic unit: this approach led to the synthesis of the desired cavitand without additional protection/de-protection steps. Despite the low yield of 4QxCOOHCav (20%), the unreacted 3QxCav was recovered quantitatively, thus making possible to bring the overall yield of 4QxCOOHCav close to 100% by performing multiple reactions. Thermal stability of the cavitands. The thermal profile of the obtained materials was recorded by means of TGA (Figures S2S4). A very good stability from room temperature to 400°C was obtained for 4QxCOOHCav in air, which mirrors that observed for 4QxCav. The presence of the carboxylic group does not reduce the thermal stability of the cavitand. The small initial loss of weight is related to the thermal release of residual solvent trapped in the cavity. By comparison the precursor 3QxCav is less stable and slowly decomposes from 130°C onward, due to the presence 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: by desorbing the fibers, no significant bleeding was observed for both 4QxCav and 4QxCOOHCav, thus confirming the high thermal 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 (in the 2-7 pH range) for 30 min at room temperature. ANOVA did not show significant differences (p > 0.005) among the mean responses (n=5 for each pH value), thus assessing the capabilities of the developed coatings for the sampling of solutions 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 the cavitand particles of the powder are distributed in a variety of orientations. The average thickness of 4QxCOOHCav developed coating was found to be 50 ± 4 µm (n=3). a
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4QxCav 4QxCOOHCav
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300000000
3QxCav TENAX
200000000
100000000
0
toluene
Table 1. Fiber-to fiber and batch-to-batch repeatability using the 4QxCOOHCav fiber* Batch-tobatch RSD%
Headspace analysis nitrobenzene
2.8
3.6
Immersion analysis trinitrotoluene
2.2
4.1
2-NT
3-NT
4 -NT
Figure 2. DHS sampling of air containing a mixture of mononitroaromatic compounds each @ 10 ppbv followed by GC-(SIM)MS analysis. Sampling time: 30 min, RT. As for extraction, owing to the predominant presence of the cavitand powder with respect to the epoxy glue, the involved mechanism is adsorption. Finally, the cavitands were exposed to aliphatic compounds (from C5 to C10) at concentrations 50 times higher than those of the nitroaromatic guests to assess the superior selectivity of 4QxCOOHCav during sampling. These analytes are adsorbed through unspecific dispersion interactions with the aliphatic chains at the lower rim of the cavitand.64 The complete desorption of aliphatic hydrocarbons was obtained at low temperature (50°C), whereas stronger conditions (220°C) were required for the release of nitro compounds, thus confirming their specific inclusion inside the cavity (Figure S-5). The host-guest interactions of the 4QxCOOH fiber were further exploited by performing SPME experiments. The enrichment capabilities toward nitroaromatic compounds were evaluated in terms of enhancement factors (EFs).65 EFs were calculated as the ratio of the concentration of the analyte in the fiber after extraction to that of the analyte in the gas standard mixture (i.e. using the ratio of the chromatographic peak area of the analyte after SPME –extraction time: 30 min at RT– to that before extraction obtained by the direct injection of the same gas standard solution, n=3). The highest EFs were observed for NB (25500±1800), 4-NT (23000±1000), 3,4-DNT (22100±1700), 3-NT (17300±1900) and 2-NT (11900±1600), whereas the lowest values were obtained for TNT (2600±200), 2,6-DNT (6800±500), toluene (7400±800) and 2,4-DNT (10000±600). Toluene was chosen as reference compound to assess the complexation capabilities of the cavitand towards analytes bringing NO2 substituents. The achieved results suggest that steric hindrance affects cavity inclusion: due to their shape TNT, 2,6-DNT and 2,4-DNT were not favorably trapped inside the cavity, whereas toluene complexation was depressed by the absence of nitro substituents. Control experiments were performed by using both a plain epoxy glue coated fiber to rule out glue physisorption effects and with a fiber coated with 2,3-diphenoxy quinoxaline (diPhQx) as mimic of the aromatic cavitand walls, to exclude unspecific π-π interactions. The extraction performance of the 4QxCOOHCav coating was always higher than that of the control fibers (Figure 3). The absence of the receptor in the first case and the lack of a preorganized cavity in the second case led to low responses. These two control experiments support the need of host-guest interactions to boost SPME selectivity.
Figure 1. Scanning electron micrographies of the 4QxCOOHCavbased coating at two different magnification: a) 100x; b) 1000x. Finally, the performance of the developed materials were evaluated in terms of fiber-to fiber and batch-to-batch repeatability using SPME both for headspace and immersion analyses. As shown in Table 1, all the cavitands tested in the study allowed to obtain RSD always lower than 4%, thus proving feasibility of the proposed procedure in the development of stable and repeatable coatings.
Fiber-to-fiber RSD%
NB
*4 fibers, 6 replicated measurements for each fiber Sampling of nitroaromatic explosives and explosive taggants. The host-guest interactions of the quinoxaline-based materials were exploited for the selective sampling of nitroaromatic explosives and explosive taggants at trace levels in air and soil samples using both dynamic headspace and SPME. Preliminary experiments were carried out by performing DHS analyses of an air mixture of mono-nitroaromatic compounds each @10 ppbv. The experiments were carried out by comparing the performances of 3QxCav, 4QxCav, 4QxCOOHCav with those of the commercial adsorbent Tenax TA. 4QxCav was used as reference material being characterized only by a quinoxaline-based structure able to interact with aromatic guest only via π-π and CH-π interactions. The 4QxCOOHCav showed the highest extraction efficiency with GC responses from 3 to 20 times higher than those achieved using the other adsorbents, thus proving that the synergistic presence of π-π, CH-π and H-bonding interactions is able to strengthen the retention of nitro compounds (Figure 2). As expected, the performances of 3QxCav were not satisfying: the open cavity of the receptor did not allow the complete engulfment of the analytes, thus reducing the adsorption of the investigated compounds.
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Analytical Chemistry analysis time and reduced sample handling. It is to be noted that low LOD values were previously obtained also both by Holmgren et al.66 and Felt et al.,67 but in both cases explosive analysis was carried out after performing a solid-liquid extraction procedure.
Table 3. LOD and LOQ values in soil using 4QxCOOHCav
toluene NB 2-NT 3-NT 4-NT 2,6-DNT 2,4-DNT 3,4-DNT 2,4,6-TNT
Figure 3. SPME sampling of air containing a mixture of nitroaromatic explosives each @ 10 ppbv followed by GC-(SIM)-MS analysis. Sampling time: 30 min, RT The SPME method for the determination of nitro explosives in air was validated obtaining excellent results with LOD values in the low ppbv range (Table 2), thus proving the potential of the method for the determination of these compounds at ultratrace levels. The complexation capabilities of 4QxCOOHCav were also exploited toward the sampling of nitroaromatic explosives and explosive taggants present in soil. By using a 23 full factorial design and the multicriteria method of desirability functions, the optimal extraction conditions were found in correspondence to an extraction temperature of 70°C, an extraction time of 45 min and 30% of NaCl obtaining a global desirability D=0.70. The regression models used to search for the highest global SPME–GC–MS recovery within the explored domain and the values of the single desirability di are reported in Table S-3.
toluene NB 2-NT 3-NT 4-NT 2,6-DNT 2,4-DNT 3,4-DNT 2,4,6-TNT
LOQ (ppbv)
0.100 0.004 0.014 0.008 0.006 0.020 0.012 0.007 0.060
0.40 0.05 0.05 0.03 0.02 0.06 0.04 0.02 0.30
LOQ (ng kg-1)
60 38.2 36.4 32 25.8 18.4 12.6 12.4 18
200 110 95 80 78 48 44.8 42.6 60.5
Good linearity was proved in the 120-1200 ng kg-1 range for all the analytes by applying Mandel’s fitting test. Method precision was evaluated testing two concentration levels, i.e. 150 and 1500 ng kg-1. Good results were obtained both in terms of intraday repeatability and intermediate precision, with RSD values always lower than 10%. In the case of intermediate precision, ANOVA showed that mean values were not significantly 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 showing the good efficiency of the developed method in terms of 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 different origin of the explosives: in the first sample, the absence of TNT and the high amount of di- and mono-nitrotoluenes allowed to assess the use of a propelling charge (smokeless powder-Figure S-6); by contrast, in the second sample the presence of high amount of TNT and DNTs coupled to the detection of other compounds like paraffin proved the use of military explosive coming from munition case of Unexploded Ordnance (UXO) to produce an IED (Improvised Explosive Device) (Figure 4).
Table 2. LOD and LOQ values in air using the 4QxCOOHCav LOD (ppbv)
LOD (ng kg-1)
the
Taking into account that good signals for all the nitroaromatic explosives could be obtained by operating under these conditions, the results of the optimization procedure were satisfying. Finally, by operating under the optimized extraction conditions the performances of the 4QxCOOHCav, 4QxCav and PDMS/DVB fibers were compared. The 4QxCOOHCav fiber showed the best extraction efficiency obtaining GC responses 10-50 times higher than those achieved by using the 4QxCav and the commercial coating. Toluene gave the lowest extraction efficiency due to the absence of H-bonding interactions with the -COOH substituents. Method validation proved the suitability of the 4QxCOOHCav coating for the detection of nitroaromatic explosives at trace levels with LOD values in the low ng kg-1 range (Table 3). The achieved LOD values were about two orders of magnitude lower than those reported by the USEPA methods 8330 and 8330B. The major features of the method were an easy automation, reduced
Figure 4. GC-SIM-MS chromatogram of a debris sample collected on an explosion scene. The inset shows the GC responses including the minor analytes extracted. Extraction conditions: sample: 0.1 g, extraction temperature: 70°C, extraction time: 45 min. CONCLUSIONS A new SPME coating based on 4QxCOOHCav as a molecular receptor was developed and proposed for the selective determina-
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tion of nitroaromatic explosives and explosive taggants at ultratrace levels in environmental air and soil samples. A synergistic ensemble of weak interactions was arranged in a single receptor to boost analytical performances via enhanced selectivity. The cavitand was specifically designed to complex nitroaromatics by adding a H-bonding interaction to the already present ability of the cavity to engulf aromatic compounds. To this purpose the upper rim of the cavitand was functionalized with a carboxylic group, pre-organized for H-bonding with the NO2 group of the analytes protruding from the cavity rim. The main features of the developed material are excellent thermal stability, a very good repeatability, and the possibility of a selective adsorption of nitroaromatics in the presence of high amounts of aliphatic hydrocarbons. The same remarkable selectivity and sensitivity were also proved in complex matrixes like soil, obtaining GC responses 10-50-fold higher than those obtained using commercial devices.
ASSOCIATED CONTENT Supporting Information Analytical and spectral characterizations of the developed coating like SEM images, TGA, 1H NMR and electrospray (ESI) mass spectra to support the characterization of the developed materials and a detailed description of the GC/MS experimental conditions used in this work. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *Corresponding authors: Federica Bianchi: E-mail:
[email protected]. Phone: +39 0521 905446. Fax: +39 0521 905556 Enrico Dalcanale: E-mail:
[email protected]. Phone: +39 0521 905463. Fax: +39 0521 905556
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the European Union through the DOGGIES project (FP7-SEC-2011- 285446). Interdipartimental Center for Measurements “G. Casnati” of the University of Parma is acknowledged for the use of NMR and MS facilities.
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