Sensitive Monitoring of Volatile Chemical Warfare Agents in Air by Atmospheric Pressure Chemical Ionization Mass Spectrometry with Counter-Flow Introduction

An efficient multi-enzyme cascade platform based on mesoporous metal-organic frameworks for the detection of organophosphorus and glucose

An efficient colorimetric detection platform based on multi-enzyme cascade has been developed for detection of organophosphorus. Firstly, the dual-enzyme platform was prepared and applied for sensitive glucose detection (detection limit 0.32 μM). And then three enzymes, including acetylcholinesterase, horseradish peroxidase and choline oxidase were encapsulated in cruciate flower-like zeolitic imidazolate framework-8 (CF-ZIF-8) through one-step co-precipitation to construct detection platform with acetylcholine chloride as substrate. The acephate inhibited the activity of acetylcholinesterase, obstructed the cascade reaction and reduced the production of H₂O₂, resulting in the changes of color intensity for the colorimetric detection. With suitable size and porous structure, CF-ZIF-8 provided a good microenvironment for guaranteeing the activity and spatial proximity of enzymes. The multi-enzyme platform displayed great performances with the detection limit of 0.23 nM for acephate. It was applied to the detection of acephate in Chinese cabbage and romaine, verifying the practicability of this platform.

Double side nanostructuring of microcantilever sensors with TiO2-NTs as a route to enhance their sensitivity

We reported a new strategy to enhance the sensing performances of a commercial microcantilever with optical readout in dynamic mode for the vapor detection of organophosphorus compounds (OPs). In order to increase significantly the surface area accessible to the molecules in the vapor phase, we nanostructured both sides of the microcantilever with ordered, open and vertically oriented amorphous titanium dioxide nanotubes (TiO2-NTs) in one step by an anodization method. However, due to the aggressive conditions of anodization synthesis it remains a real challenge to nanostructure both sides of the microcantilever. Consequently, we developed and optimized a protocol of synthesis to overcome these harsh conditions which can lead to the total destruction of the silicon microcantilever. Moreover, this protocol was also elaborated in order to maintain a good reflection of the laser beam on one side of the microcantilever towards the position sensitive photodiode and limit the light diffusion by the NTs film. The results related to the detection of dimethyl methylphosphonate (DMMP) showed that TiO2 and the nanostructuring on both sides of the microcantilever with NTs indeed improved the response of the sensor to vapors compared to a microcantilever nanostructured on only one side. The dimensions and morphology of NTs guaranteed the access of molecules to the surface of NTs. This approach showed promising prospects to enhance the sensing performances of microcantilevers.

Functionalized TiO2 Nanorods on a Microcantilever for the Detection of Organophosphorus Chemical Agents in Air

We report the fabrication of nanostructured microcantilevers employed as sensors for the detection of organophosphorus (OPs) vapors. These micromechanical sensors are prepared using a two-step procedure first optimized on a silicon wafer. TiO₂ one-dimensional nanostructures are synthesized at a silicon surface by a solvothermal method and then grafted with bifunctional molecules having an oxime group known for its strong affinity with organophosphorus compounds. The loading of oxime molecules grafted on the different nanostructured surfaces was quantified by UV spectroscopy. It has been found that a wafer covered by vertically aligned rutile TiO₂ nanorods (NRs), with an average length and width of 9.5 μm and 14.7 nm, respectively, provides an oxime function density of 360 nmol cm^-2. The optimized TiO₂ nanorod synthesis was successfully reproduced on the cantilevers, leading to a homogeneous and reproducible TiO₂ NR film with the desired morphology. Thereafter, oxime molecules have been successfully grafted on the nanostructured cantilevers. Detection tests were performed in a dynamic mode by exposing the microcantilevers to dimethyl methylphosphonate (a model compound of toxic OPs agents) and following the shift of the resonant frequency. The nanostructure and the presence of the molecules on a TiO₂ NR surface both improve the response of the sensors. A detection limit of 2.25 ppm can be reached with this type of sensor.

Direct Analysis of Carbonyl Compounds by Mass Spectrometry with Double-Region Atmospheric Pressure Chemical Ionization

Direct analysis of highly reactive volatile species such as the aliphatic aldehydes as vital biomarkers remains a great challenge due to difficulties in the sample pretreatment. To address such a challenge, we herein report the development of a novel double-region atmospheric pressure chemical ionization mass spectrometry (DRAPCI-MS) method. The DRAPCI source implements a separated structural design that uses a focus electrode to divide the discharge and ionization region to reduce sample fragmentation in the ionization process. Counterflow introduction (CFI) configuration was adopted in the DRAPCI source to reduce background noise, while ion transmission efficiency was optimized through simulating the voltage of the focus electrode and the ion trajectory of the ion source. The limits of detection (LODs) of four carbonyl compounds cyclohexanone, hexanal, heptanal, and octanal by DRAPCI-MS were between 0.1 and 3 μg·m^-3, approximately two to eight times lower than those by atmospheric pressure chemical ionization mass spectrometry. Additionally, the DRAPCI-MS method carried out effective in situ analyses of the volatile components in expired milk and the exhaled breath of smokers, demonstrating the DRAPCI-MS as a practical tool to analyze complex mixtures. The DRAPCI-MS method provides a rapid, sensitive, and high-throughput technique in the real-time analysis of gaseous small-molecule compounds.

Detection of chemical warfare agent simulants and hydrolysis products in biological samples by paper spray mass spectrometry

Paper spray ionization coupled to a high resolution tandem mass spectrometer (a quadrupole orbitrap) was used to identify and quantitate chemical warfare agent (CWA) simulants and their hydrolysis products in blood and urine. Three CWA simulants, dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and diisopropyl methylphosphonate (DIMP), and their isotopically labeled standards were analyzed in human whole blood and urine. Calibration curves were generated and tested with continuing calibration verification standards. Limits of detection for these three compounds were in the low ng mL^-1 range for the direct analysis of both blood and urine samples. Five CWA hydrolysis products, ethyl methylphosphonic acid (EMPA), isopropyl methylphosphonic acid (IMPA), isobutyl methylphosphonic acid (iBuMPA), cyclohexyl methylphosphonic acid (CHMPA), and pinacolyl methylphosphonic acid (PinMPA), were also analyzed. Calibration curves were generated in both positive and negative ion modes. Limits of detection in the negative ion mode ranged from 0.36 ng mL^-1 to 1.25 ng mL^-1 in both blood and urine for the hydrolysis products. These levels were well below those found in victims of the Tokyo subway attack of 2 to 135 ng mL^-1. Improved stability and robustness of the paper spray technique in the negative ion mode was achieved by the addition of chlorinated solvents. These applications demonstrate that paper spray mass spectrometry (PS-MS) can be used for rapid, sample preparation-free detection of chemical warfare agents and their hydrolysis products at physiologically relevant concentrations in biological samples.

Gas-phase chemiluminescence of reactive negative ions evolved through corona discharge in air and O2 atmospheres

The reactive performance of negative ions evolved in air and O₂ atmospheres through a gas-phase chemiluminescence method was investigated.

In-Line Ozonation for Sensitive Air-Monitoring of a Mustard-Gas Simulant by Atmospheric Pressure Chemical Ionization Mass Spectrometry

A highly sensitive method for real-time air-monitoring of mustard gas (bis(2-chloroethyl) sulfide, HD), which is a lethal blister agent, is proposed. Humidified air containing a HD simulant, 2-chloroethyl ethyl sulfide (2CEES), was mixed with ozone and then analyzed by using an atmospheric pressure chemical ionization ion trap tandem mass spectrometer. Mass-spectral ion peaks attributable to protonated molecules of intact, monooxygenated, and dioxygenated 2CEES (MH(+), MOH(+), and MO(2)H(+), respectively) were observed. As ozone concentration was increased from zero to 30 ppm, the signal intensity of MH(+) sharply decreased, that of MOH(+) increased once and then decreased, and that of MO(2)H(+) sharply increased until reaching a plateau. The signal intensity of MO(2)H(+) at the plateau was 40 times higher than that of MH(+) and 100 times higher than that of MOH(+) in the case without in-line ozonation. Twenty-ppm ozone gas was adequate to give a linear calibration curve for 2CEES obtained by detecting the MO(2)H(+) signal in the concentration range up to 60 μg/m(3), which is high enough for hygiene management. In the low concentration range lower than 3 μg/m(3), which is equal to the short-term exposure limit for HD, calibration plots unexpectedly fell off the linear calibration curve, but 0.6-μg/m(3) vapor was actually detected with the signal-to-noise ratio of nine. Ozone was generated from instrumentation air by using a simple and inexpensive home-made generator. 2CEES was ozonated in 1-m extended sampling tube in only 1 s.

Direct gas-phase detection of nerve and blister warfare agents utilizing active capillary plasma ionization mass spectrometry

Ultrasensitive direct gas-phase detection of chemical warfare agents (CWAs) is demonstrated utilizing active capillary plasma ionization and triple quadrupole mass spectrometry (MS) instrumentation. Four G- agents, two V-agents and various blistering agents [including sulfur mustard (HD)] were detected directly in the gas phase with limits of detection in the low parts per trillion (ng m(-3)) range. The direct detection of HD was shown for dry carrier gas conditions, but signals vanished when humidity was present, indicating a possible direct detection of HD after sufficient gas phase pretreatment. The method provided sufficient sensitivity to monitor directly the investigated volatile CWAs way below their corresponding minimal effect dose, and in most cases even below the eight hours worker exposure concentration. In general, the ionization is very soft, with little to no in-source fragmentation. Especially for the G-agents, some dimer formation occurred at higher concentrations. This adds complexity, but also further selectivity, to the corresponding mass spectra. Our results show that the active capillary plasma ionization is a robust, sensitive, "plug and play" ambient ionization source suited (but not exclusively) to the very sensitive detection of CWAs. It has the potential to be used with portable MS instrumentation.

Direct quantification of chemical warfare agents and related compounds at low ppt levels: comparing active capillary dielectric barrier discharge plasma ionization and secondary electrospray ionization mass spectrometry

A novel active capillary dielectric barrier discharge plasma ionization (DBDI) technique for mass spectrometry is applied to the direct detection of 13 chemical warfare related compounds, including sarin, and compared to secondary electrospray ionization (SESI) in terms of selectivity and sensitivity. The investigated compounds include an intact chemical warfare agent and structurally related molecules, hydrolysis products and/or precursors of highly toxic nerve agents (G-series, V-series, and "new" nerve agents), and blistering and incapacitating warfare agents. Well-defined analyte gas phase concentrations were generated by a pressure-assisted nanospray with consecutive thermal evaporation and dilution. Identification was achieved by selected reaction monitoring (SRM). The most abundant fragment ion intensity of each compound was used for quantification. For DBDI and SESI, absolute gas phase detection limits in the low ppt range (in MS/MS mode) were achieved for all compounds investigated. Although the sensitivity of both methods was comparable, the active capillary DBDI sensitivity was found to be dependent on the applied AC voltage, thus enabling direct tuning of the sensitivity and the in-source fragmentation, which may become a key feature in terms of field applicability. Our findings underline the applicability of DBDI and SESI for the direct, sensitive detection and quantification of several CWA types and their degradation products. Furthermore, they suggest the use of DBDI in combination with hand-held instruments for CWAs on-site monitoring.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of small volatile molecules using a parylene-matrix chip

RATIONALE

In matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), volatile small molecules have been nearly impossible to analyze because (1) such molecules evaporate under drying and vacuum conditions and (2) the organic matrix creates matrix peaks in the low mass-to-charge (m/z) range (m/z <500). In this work, the analysis of volatile small molecules using MALDI-TOFMS was realized using (1) a parylene-matrix chip to eliminate the matrix peaks of the organic matrix and (2) graphene for the effective adsorption of the small volatile molecules.

METHODS

The parylene-matrix chip was produced by deposition of a partially porous parylene-N thin film on a dried organic matrix array. The sample solution of volatile small molecules was mixed with the graphene and then placed on the parylene-matrix chip for MALDI-TOFMS. Analogs of chemical agents called dimethyl methyl phosphonate (DMMP) and 2-chloroethylethylsulfide (CEES) were used as model compounds for the small volatile molecules, and the sensing parameters were estimated, such as the limit of detection (LOD) and the detection range.

RESULTS

MALDI-TOFMS based on the parylene-matrix chip and graphene as the adsorbent could achieve a LOD of approximately 1 ppb in the detection range of 1 ppm-1 ppb for the highly volatile DMMP and CEES.

CONCLUSIONS

The parylene-matrix chip with graphene can be applied for the detection of volatile small molecule analytes in the m/z ratio range of small molecules (m/z <500) using graphene as an effective adsorbent.

Development of portable mass spectrometer with electron cyclotron resonance ion source for detection of chemical warfare agents in air

A portable mass spectrometer with an electron cyclotron resonance ion source (miniECRIS-MS) was developed. It was used for in situ monitoring of trace amounts of chemical warfare agents (CWAs) in atmospheric air. Instrumental construction and parameters were optimized to realize a fast response, high sensitivity, and a small body size. Three types of CWAs, i.e., phosgene, mustard gas, and hydrogen cyanide were examined to check if the mass spectrometer was able to detect characteristic elements and atomic groups. From the results, it was found that CWAs were effectively ionized in the miniECRIS-MS, and their specific signals could be discerned over the background signals of air. In phosgene, the signals of the 35Cl+ and 37Cl+ ions were clearly observed with high dose-response relationships in the parts-per-billion level, which could lead to the quantitative on-site analysis of CWAs. A parts-per-million level of mustard gas, which was far lower than its lethal dosage (LCt50), was successfully detected with a high signal-stability of the plasma ion source. It was also found that the chemical forms of CWAs ionized in the plasma, i.e., monoatomic ions, fragment ions, and molecular ions, could be detected, thereby enabling the effective identification of the target CWAs. Despite the disadvantages associated with miniaturization, the overall performance (sensitivity and response time) of the miniECRIS-MS in detecting CWAs exceeded those of sector-type ECRIS-MS, showing its potential for on-site detection in the future.

An acetylcholinesterase-based chronoamperometric biosensor for fast and reliable assay of nerve agents

The enzyme acetylcholinesterase (AChE) is an important part of cholinergic nervous system, where it stops neurotransmission by hydrolysis of the neurotransmitter acetylcholine. It is sensitive to inhibition by organophosphate and carbamate insecticides, some Alzheimer disease drugs, secondary metabolites such as aflatoxins and nerve agents used in chemical warfare. When immobilized on a sensor (physico-chemical transducer), it can be used for assay of these inhibitors. In the experiments described herein, an AChE- based electrochemical biosensor using screen printed electrode systems was prepared. The biosensor was used for assay of nerve agents such as sarin, soman, tabun and VX. The limits of detection achieved in a measuring protocol lasting ten minutes were 7.41 × 10⁻¹² mol/L for sarin, 6.31 × 10⁻¹² mol/L for soman, 6.17 × 10⁻¹¹ mol/L for tabun, and 2.19 × 10⁻¹¹ mol/L for VX, respectively. The assay was reliable, with minor interferences caused by the organic solvents ethanol, methanol, isopropanol and acetonitrile. Isopropanol was chosen as suitable medium for processing lipophilic samples.