Enhanced detection of nitroaromatic explosive vapors combining solid-phase extraction-air sampling, supercritical fluid extraction, and large-volume injection-GC

Visual colorimetric sensor for nitroguanidine detection based on hydrogen bonding-induced aggregation of uric acid-functionalized gold nanoparticles

A colorimetric assay is proposed for the quantification of nitroguanidine (NQ), based on triggering the aggregation of uric acid-modified gold nanoparticles (AuNPs@UA) by intermolecular hydrogen bonding interaction between uric acid (UA) and NQ. The red-to-purplish blue (lavender) color change of AuNPs@UA with increasing NQ concentrations could be perceived with the naked eye or detected by UV-vis spectrophotometry. The absorbance versus concentration correlation gave a linear calibration curve in the range of 0.6-3.2 mg L^-1 NQ, with a correlation coefficient of 0.9995. The detection limit of the developed method was 0.063 mg L^-1, lower than those of noble metal aggregation methods in the literature. The synthesized and modified AuNPs were characterized using UV-vis spectrophotometry, scanning transmission electron microscopy (STEM), dynamic light scattering (DLS), and Fourier transform infrared spectroscopy (FTIR). Some critical parameters such as modification conditions of AuNPs, UA concentration, solvent environment, pH, and reaction time were optimized for the proposed method. The non-interference of common explosives (i.e., nitroaromatic, nitramine, nitrate ester, insensitive and inorganic explosives), common soil and groundwater ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Cl^-, NO₃^-, SO₄^2-, CO₃^2-, PO₄^3-) and possible interfering compounds (used as camouflage agents for explosives; D-(+)-glucose, sweeteners, acetylsalicylic acid (aspirin), household powder detergents, and paracetamol) on the proposed method was demonstrated, proving that the procedure was fairly selective for NQ, due to special hydrogen bonding interactions between UA-functionalized AuNPs and NQ. Finally, the proposed spectrophotometric method was applied to NQ-contaminated soil, and the obtained results were statistically compared with those of the liquid chromatography-tandem mass spectrometric (LC-MS/MS) method in the literature.

A novel electrochemical sensor for nitroguanidine determination using a glassy carbon electrode modified with multi-walled carbon nanotubes and polyvinylpyrrolidone

The GC/PVP/MWCNTs electrode is the first electrode for the electrochemical determination of insensitive explosive nitroguanidine using intermolecular hydrogen bonding interactions.

Chemometrics in forensic science: approaches and applications

Forensic investigations are often reliant on physical evidence to reconstruct events surrounding a crime. However, there remains a need for more objective approaches to evidential interpretation, along with rigorously validated procedures for handling, storage and analysis. Chemometrics has been recognised as a powerful tool within forensic science for interpretation and optimisation of analytical procedures. However, careful consideration must be given to factors such as sampling, validation and underpinning study design. This tutorial review aims to provide an accessible overview of chemometric methods within the context of forensic science. The review begins with an overview of selected chemometric techniques, followed by a broad review of studies demonstrating the utility of chemometrics across various forensic disciplines. The tutorial review ends with the discussion of the challenges and emerging trends in this rapidly growing field.

Upconversion luminescence nanosensor for TNT selective and label-free quantification in the mixture of nitroaromatic explosives

This paper reports a rapid, sensitive, and selective nanosensor for the detection of 2,4,6-trinitrotoluene (TNT) in the mixture aqueous solution of nitroaromatics independent of immunoassay or molecularly imprinted technology and complicated instruments. Despite many strategies including immunoassay and molecularly imprinted technologies been successfully developed for the detection of TNT, it is not easy to differentiate TNT from 2,4,6-trinitrophenol (TNP) due to their very similar chemical structures and properties. In this work, the amine functionalized NaYF4:Yb(3+)/Er(3+) upconversion luminescence nanoparticles (UCNPs) whose excitation (980 nm) and emission (543 nm) wavelength were far from the absorbance bands of other usual interference nitroaromatics including 2,4-dinitrotoluene (DNT), nitrobenzene (NB), and especially TNP, were utilized as the luminescent nanosensors for TNT luminescence detection. To make these UCNPs highly water stable and render the charge transfer from UCNPs to TNT easier, amino groups were introduced onto the surface of the UCNPs by coating a polymer layer of ethylene glycol dimethacrylate (EGDMA) hybridized with 3-aminopropyltriethoxysilane (APTS). After binding with TNT through amino groups on the UCNPs, the naked eye visible green upconversion luminescence of the UCNPs was dramatically quenched and thus a sensitive UC luminescence nanosensor was developed for TNT detection. However, other nitroaromatics including TNP, DNT, and NB have no influence on the green UC luminescence and thus no influence on the TNT detection. The luminescence intensity is negatively proportional to the concentration of TNT in the range of 0.01-9.0 µg/mL with the 3σ limit of detection (LOD) of 9.7 ng/mL. The present studies provide a novel and facile strategy to fabricate the upconversion luminescence sensors with highly selective recognition ability in aqueous media and are desirable for label free analysis of TNT in mixed solution independent of immunoassay and molecularly imprinted technology and complicated instruments.

Detection of Nitroaromatic and Peroxide Explosives in Air Using Infrared Spectroscopy: QCL and FTIR

A methodology for processing spectroscopic information using a chemometrics-based analysis was designed and implemented in the detection of highly energetic materials (HEMs) in the gas phase at trace levels. The presence of the nitroaromatic HEM 2,4-dinitrotoluene (2,4-DNT) and the cyclic organic peroxide triacetone triperoxide (TATP) in air was detected by chemometrics-enhanced vibrational spectroscopy. Several infrared experimental setups were tested using traditional heated sources (globar), modulated and nonmodulated FT-IR, and quantum cascade laser- (QCL-) based dispersive IR spectroscopy. The data obtained from the gas phase absorption experiments in the midinfrared (MIR) region were used for building the chemometrics models. Partial least-squares discriminant analysis (PLS-DA) was used to generate pattern recognition schemes for trace amounts of explosives in air. The QCL-based methodology exhibited a better capacity of discrimination for the detected presence of HEM in air compared to other methodologies.

SERS substrate for detection of explosives

A novel gold coated femtosecond laser nanostructured sapphire surface - an "optical nose" - based on surface-enhanced Raman spectroscopy (SERS) for detecting vapours of explosive substances was investigated. Four different nitroaromatic vapours at room temperature were tested. Sensor responses were unambiguous and showed response in the range of 0.05-15 μM at 25 °C. The laser fabricated substrate nanostructures produced up to an eight-fold increase in Raman signal over that observed on the unstructured portions of the substrate. This work demonstrates a simple sensing system that is compatible with commercial manufacturing practices to detect taggants in explosives which can undertake as part of an integrated security or investigative mission.

Ion spectrometric detection technologies for ultra-traces of explosives: a review

In recent years, explosive materials have been widely employed for various military applications and civilian conflicts; their use for hostile purposes has increased considerably. The detection of different kind of explosive agents has become crucially important for protection of human lives, infrastructures, and properties. Moreover, both the environmental aspects such as the risk of soil and water contamination and health risks related to the release of explosive particles need to be taken into account. For these reasons, there is a growing need to develop analyzing methods which are faster and more sensitive for detecting explosives. The detection techniques of the explosive materials should ideally serve fast real-time analysis in high accuracy and resolution from a minimal quantity of explosive without involving complicated sample preparation. The performance of the in-field analysis of extremely hazardous material has to be user-friendly and safe for operators. The two closely related ion spectrometric methods used in explosive analyses include mass spectrometry (MS) and ion mobility spectrometry (IMS). The four requirements-speed, selectivity, sensitivity, and sampling-are fulfilled with both of these methods.

Efficiency of whole and skimmed powdered milk for trapping volatile compounds released from plastic containers in high-temperature applications

Plastic food containers used for high-temperature applications are not completely inert, and potentially harmful chemicals may be transferred to foodstuffs when such containers are heated. The aim of this work was to investigate the role of food fat content on the efficiency of trapping volatile organic compounds from heated plastic packaging. Relatively simple food matrices such as powdered skimmed and whole milk were evaluated with respect to their retention of several selected migrants: toluene, 1-octene, ethylbenzene, o-, m-, and p-xylene, styrene, and 1,4-dichlorobenzene released from containers made of polypropylene (random and copolymer), polycarbonate, and styrene-acrylonitrile copolymer, which are all commonly used in high-temperature applications. The analytical method (purge and trap gas chromatography and mass spectrometry) was optimized for each matrix. The developed procedure had detection limits of 0.01 to 1.2 ng, depending on the analyte and sample matrix, and both reproducibility and repeatability (expressed as relative standard deviation) were below 15%. This method was applied to the different plastic materials. The concentrations of the volatile compounds in both matrices were well below the established specific migration limits. Temperature and fat content of powdered milk were the most influential variables in mass transfer processes. These values were compared with those obtained with either Tenax TA (alternative test medium for fatty food simulants) or Porapak Q (another widely used sorbent). Similar results were found in skimmed powdered milk and Tenax TA, but significant differences were observed for whole powdered milk.

Optimized microwave extraction for trace detection of 2,4,6-trinitrotoluene in soil samples

An optimized microwave assisted extraction method for determination of trinitrotoluene (TNT) and related compounds in soil is presented. The new enhanced method exhibits improved extraction recovery and precision as well as sample handling time. For the separation and detection gas chromatography coupled to a thermoionic probe was used achieving TNT and dinitrotoluene detection limits per injection at the femtogram level. The generated extraction recovery and precision data are given for spiked and certified soil. Determined TNT and related compounds residues in soil collected from different parts of the world are presented.

Use of porous graphitic carbon for the analysis of nitrate ester, nitramine and nitroaromatic explosives and by-products by liquid chromatography–atmospheric pressure chemical ionisation-mass spectrometry

A new LC/MS method was developed for the analysis of sixteen different analytes including the most common organic explosives encountered in forensic investigations. The separation was achieved using a porous graphitic carbon (PGC) column with a binary gradient elution. Molecular modeling suggested a possible interpretation for the elution order of explosive compounds on PGC. The introduction of ammonium formate in the mobile phase resulted in the formation of characteristic adduct ions thus enhancing the mass spectrometric detection of nitrate ester and nitramine compounds. Atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) were compared in terms of sensitivity. The final LC/APCI-MS method allowed easy identification of investigated compounds with limits of detection ranging from 0.04 to 1.06 ng/microl. The analysis of simulated forensic samples confirmed the performance of the method.

Detection of organics using porphyrin embedded nanoporous organosilicas

Molecularly imprinted polymers and silica have been studied as receptor binding site mimics for use in a wide range of separation, catalysis, and detection applications employing transduction mechanisms including conductometric, amperometric, and capacitance. Porphyrins are also well known as sensor components due to the extreme sensitivity of their spectrophotometric characteristics to changes in their immediate environment. We have developed periodic mesoporous organosilicas (PMO) which incorporate a porphyrin into the material for use as an optical indicator of target binding. This material combines the stability, selectivity, and high density of binding sites characteristic of the molecularly imprinted PMO with the sensitivity and selectivity of the porphyrin. We demonstrate binding of p-nitrophenol, p-cresol, 2,4,6-trinitrotoluene, and RDX by the porphyrin-embedded PMOs with selective adsorption of TNT over the other analytes. In addition, the binding of each of the organics by the PMO results in unique changes in the spectrophotometric characteristics of the incorporated porphyrin. These changes can be observed by visual inspection or through the use of fluorescence spectra collected in 96-well format.

Air sampling with Empore solid phase extraction membranes and online single-channel desorption/liquid chromatography/mass spectrometry analysis: Determination of volatile and semi-volatile organophosphate esters

A method for determining organophosphate esters in air samples using C8 Empore solid phase extraction (SPE) membranes has been developed. After the sampling the analytes trapped in the membrane are completely desorbed with methanol, using an extraction cell connected online to the organic modifier channel of a HPLC gradient pump. The addition of water to the mobile phase prior to analytical chromatography ensures that the analytes are refocused and efficiently separated. Sampling with Empore SPE membranes enables the collection of analytes in both the vapour phase and particulate matter. During the air sampling procedure no losses were observed after 24 h of sampling, yielding a total volume of 14.4 m3, even for the most volatile compound used in this investigation (trimethylphosphate). Complete desorption was observed for all the organophosphate esters and recoveries were greater than 95%, with a relative standard deviation of less than 8%. The limits of detection ranged between 0.4 and 19 pg/m3. The effect of particulate matter on the extraction efficiency was investigated in detail by spiking the membranes with reference standard material. It was also found that the SPE membranes could be stored for at least 5 days at room temperature without any evidence of loss. The efficacy of the method was verified using real samples from different common indoor environments. Interestingly, significant quantities of several phosphate esters were found in a NIST standard reference material (urban dust, SRM 1649a).

Determination and characterization of organic explosives using porous graphitic carbon and liquid chromatography–atmospheric pressure chemical ionization mass spectrometry

A new LC-MS method for the determination and characterization of three groups of commonly used organic explosives (nitroaromatic compounds, cyclic nitroamines and nitrate esters) was developed using a porous graphitic carbon (PGC) (Hypercarb) column. Twenty-one different explosive-related compounds--including 2,4,6-trinitrotoluene, its by-products and its degradation products--were chromatographically separated in a single analysis. This efficient separation facilitates the identification of the manufacturer of the explosive using the identified analytes as a fingerprint. A final, conclusive identification of the analytes can be obtained using LC-MS equipped with an atmospheric pressure chemical ionization (APCI) interface. Solvent effects on chromatographic behaviour were investigated, as were the effects of solvent mixtures and mobile phase additives. The number and the relative positions of the nitro groups within analyte molecules influence their order of elution; these effects were investigated. The data thus generated can be interpreted to support a hypothesis concerning the retention mechanism of nitro-containing compounds when using PGC. Limits of detection ranged from 0.5 to 41.2 ng. The new methodology described herein improves the sensitivity and selectivity of explosive detection. The effectiveness of the method is demonstrated by the analysis of soil samples containing explosives residue from test fields in Sweden and Afghanistan.

Surface plasmon resonance immunosensor for highly sensitive detection of 2,4,6-trinitrotoluene

We have examined the sensing characteristics of a surface plasmon resonance (SPR) immunoassay for the detection of 2,4,6-trinitrotoluene (TNT) using an immunoreaction between 2,4,6-trinitrophenol-ovalbumin (TNP-OVA) conjugate and anti-2,4,6-trinitrophenol antibody (anti-TNP antibody). TNP-OVA conjugate was attached to a SPR-gold sensing surface by means of physical immobilization, which undergoes binding interaction with anti-TNP antibody. Both the immobilization and binding processes were studied from a change in the SPR-resonance angle. The quantification of TNT is based on the principle of indirect competitive immunoassay, in which the immunoreaction between the TNP-OVA conjugate and anti-TNP antibody was inhibited in the presence of free TNT in solution. The decrease in the resonance angle shift is proportional to an increase in concentration of TNT used for incubation. The immunoassay exhibited excellent sensitivity for the detection of TNT in the concentration range from 0.09 to 1000 ng/ml with good stability and reproducibility. The immunosensor developed could detect TNT as low as 0.09 ng/ml, within a response time of approximately 22 min. The sensor surface was regenerated by a brief flow of pepsin solution, which disrupts the antigen-antibody complex without destroying the conjugate biofilm. Cross-reactivity of the SPR sensor to some structurally related nitroaromatic derivative and the detection of TNT in the presence of these nitroaromatic compounds were investigated. The cross-reactivity of the SPR sensor to 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB), 2-amino-4,6-dinitrotoluene (2A-4,6-DNT) and 4-amino-2,6-dinitrotoluene (4A-2,6-DNT) were very low (< or =1.1%). The analytical characteristics of the proposed immunosensor are highly promising for the development of new field-portable sensors for on-site detection of landmines.

Injectors for capillary gas chromatography and their application to environmental analysis

The application of different injectors in capillary gas chromatography (GC) analysis of semi-volatile organic contaminants in environmental samples prepared in organic solvents is reviewed. The injectors examined include a split/splitless injector in splitless mode (SS), cold on-column (COC), and programmable temperature vaporizer (PTV) and adaptations of these injector designs. Key issues when selecting an injector include properties of the analyte, such as potential for thermal degradation or discrimination of high boiling point compounds within the injector, and the ability of the GC systems to handle large volume injections (LVI) primarily to lower detection limits and allow direct coupling with sample preparation techniques such as at-line or on-line solid phase extraction (SPE). LVI also require consideration of matrix interference issues. This review examines only injector chamber modifications that are feasible with a standard GC configuration, however some modifications to the chromatographic system to extend the range of applicability of gas chromatography analysis for environmental samples are also noted.

Meisenheimer complexes bonded at carbon and at oxygen

The carbon-bonded gas-phase Meisenheimer complex of 2,4,6-trinitrotoluene (TNT) and the nitromethyl carbanion CH(2)NO(2)(-) (m/z 60) is generated for the first time by chemical ionization using nitromethane as the reagent gas. Collision-induced dissociation (CID) of the Meisenheimer complex furnishes deprotonated TNT, a result of the higher gas-phase acidity of TNT than nitromethane. The formation of Meisenheimer complexes with CH(2)NO(2)(-) in the gas phase is selective to highly electron-deficient compounds such as dinitrobenzene and trinitrobenzene and does not occur with organic molecules with lower electron-affinity such as methanol, methylamine, propionaldehyde, acetone, ethyl acetate, chloroform, toluene, m-methoxytoluene, and even nitrobenzene and p-fluoronitrobenzene. As such, the reaction allows selective detection of TNT in mixtures. Meisenheimer complexes between CH(2)NO(2)(-) and the three dinitrobenzene isomers display distinctive fragmentations. The oxygen-bonded sigma-complex of TNT with the deprotonated hemiacetal anion CH(3)OCH(2)O(-) (m/z 61), represents a different type of Meisenheimer complex. It displays characteristic fragmentation involving loss of HNO(2) upon CID. The combination of a selective ion/molecule reaction (Meisenheimer complex formation) followed by a characteristic CID process provides a second novel and highly selective approach to the detection of TNT and closely related compounds in mixtures. The assay is readily implemented using neutral loss scans in a triple quadrupole mass spectrometer. Gas-phase reactions of denitrosylated TNT with benzaldehyde produce the corresponding dihydrofuran in an aldol condensation, a result that parallels the corresponding condensed-phase reaction.