Structural Characterization of Chemical Warfare Agent Degradation Products in Decontamination Solutions with Proton Band-Selective 1H−31P NMR Spectroscopy

Triazine-Based Covalent Organic Framework: A Promising Sorbent for Efficient Elimination of the Hydrocarbon Backgrounds of Organic Sample for GC-MS and 1H NMR Analysis of Chemical Weapons Convention Related Compounds

The strict monitoring and precise measurements of chemical warfare agents (CWAs) in environmental and other complex samples with high accuracy have great practical significance from the forensic and Chemical Weapons Convention (CWC) verification point of view. Therefore, this study was aimed to develop an efficient extraction and enrichment method for identification and quantification of toxic agents, especially with high sensitivity and multidetection ability in complex samples. It is the first study on solid-phase extraction (SPE) of CWAs and their related compounds from hydrocarbon backgrounds using covalent triazine-based frameworks (CTFs). This nitrogen-rich CTF sorbent has shown an excellent SPE performance toward sample cleanup by selective elimination of hydrocarbon backgrounds and enrich the CWC related analytes in comparison with the conventional and other reported methods. The best enrichment of the analytes was found with the washing solvent (1 mL of n-hexane) and the extraction solvent (1 mL of dichloromethane). Under the optimized conditions, the SPE method had good linearity in the concentration range of 0.050-10.0 μg mL^-1 for organophosphorus esters, 0.040-20.0 μg mL^-1 for nerve agents, and 0.200-20.0 μg mL^-1 for mustards with correlation coefficients ( r²) between 0.9867 and 0.9998 for all analytes. Limits of detection ( S/ N = 3:1) in the SIM mode were found to be in the range of 0.015-0.050 μg mL^-1 for organophosphorus esters, 0.010-0.030 μg mL^-1 for nerve agents, and 0.050-0.100 μg mL^-1 for blister agents. Limits of quantification ( S/ N = 10:1) were found in the range of 0.050-0.200 μg mL^-1 for organophosphorus esters, 0.040-0.100 μg mL^-1 for nerve agents, and 0.180-0.350 μg mL^-1 for blister agents in the SIM mode. The recoveries of all analytes ranged from 87 to 100% with the relative standard deviations ranging from 1 to 8%. This method was also successfully applied for the sample preparation of ¹H NMR analysis of sulfur and nitrogen mustards in the presence of hydrocarbon backgrounds. Therefore, this SPE method provides the single sample preparation for both NMR and GC-MS analyses.

pH-Dependent Piecewise Linear Correlation of 1H,31P Chemical Shifts: Application in NMR Identification of Nerve Agent Metabolites in Urine Samples

The NMR-observable nuclei of the acidic and basic compounds experience pH dependence in chemical shift. This phenomenon can be exploited in NMR titrations to determine p K_a values of compounds, or in pH measurement of solutions using dedicated pH reference compounds. On the other hand, this sensitivity can also cause problems in, for example, metabolomics, where slight changes in pH result in significant difficulties for peak alignment between spectra of set of samples for comparative analysis. In worst case, the pH sensitivity of chemical shifts can prevent unambiguous identification of compounds. Here, we propose an alternative approach for NMR identification of pH-sensitive analytes. The ¹H and X (¹³C, ¹⁵N, ³¹P, ...) chemical shifts in close proximity to the acidic or basic functional group should, when presented as ordered pairs, express piecewise linear correlation with distinct slope, intercept, and range. We have studied the pH dependence of ¹H and ³¹P chemical shifts of the CH₃-P moiety in urinary metabolites of nerve agents sarin, soman and VX using 2D ¹H-³¹P fast-HMQC spectroscopy. The ¹H and ³¹P chemical shifts of these chemicals appear in very narrow range, and due to subtle changes in sample pH the identification on either ¹H or ³¹P chemical shift alone is uncertain. However, if the observed ¹H and ³¹P chemical shifts of the CH₃-P moiety of individual compounds are presented as ordered pairs, they fall into distinct linear spaces, thus, facilitating identification with high confidence.

Oxidative decontamination of chemical warfare agent VX and its simulant using N,N-dichlorovaleramide

The efficient, operationally simple and safe oxidative decontamination of chemical warfare agent O-ethyl-S-2-(N,N-diisopropylaminoethyl)methylphosphonothioate (VX) and its non-toxic simulant O,S-diethyl methylphosphonothioate (OSDEMP) has been reported.

Selective excitation with asymmetric adiabatic pulses for NMR spectroscopy

Existing selective pulses are mainly constructed in the forms of classically shaped pulses, such as the Gaussian pulses, or generated by using numerical optimization methods. However, all of these pulses are highly sensitive to radiofrequency (RF) intensity variation, which means their performance is highly dependent on the accuracy and stability of the RF intensity. Even a slight RF intensity deviation can cause severe degradation in the excitation profile. To solve this problem, we propose a method for narrow selective excitation by sequential application of a pair of phase-opposite asymmetric adiabatic pulses, all within two scans. By retaining the adiabatic character, the new method is highly robust to RF intensity variation. Moreover, it has flexible excitation bandwidth, ranging from line-selective to narrow-band-selective pulses. The method is tested both in numerical simulations and solution-state NMR experiments.

Chemical analysis of bleach and hydroxide-based solutions after decontamination of the chemical warfare agent O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate (VX)

Detailed chemical analysis of solutions used to decontaminate chemical warfare agents can be used to support verification and forensic attribution. Decontamination solutions are amongst the most difficult matrices for chemical analysis because of their corrosive and potentially emulsion-based nature. Consequently, there are relatively few publications that report their detailed chemical analysis. This paper describes the application of modern analytical techniques to the analysis of decontamination solutions following decontamination of the chemical warfare agent O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate (VX). We confirm the formation of N,N-diisopropylformamide and N,N-diisopropylamine following decontamination of VX with hypochlorite-based solution, whereas they were not detected in extracts of hydroxide-based decontamination solutions by nuclear magnetic resonance (NMR) spectroscopy or gas chromatography-mass spectrometry. We report the electron ionisation and chemical ionisation mass spectroscopic details, retention indices, and NMR spectra of N,N-diisopropylformamide and N,N-diisopropylamine, as well as analytical methods suitable for their analysis and identification in solvent extracts and decontamination residues.

Identification tree based on fragmentation rules for structure elucidation of organophosphorus esters by electrospray mass spectrometry

Organophosphorus compounds have played important roles as pesticides, chemical warfare agents and extractors of radioactive material. Structural elucidation of phosphonates poses a particular challenge because their initial forms can be hydrolyzed, thus, degradation products may predominate in samples acquired in the field. The analysis of non-volatile organophosphorus compounds and their degradation products is possible using electrospray tandem mass spectrometry ESI-MS/MS. Here, we present a generic strategy that allows the unambiguous identification of substituents for two families of organophosphorus compounds: the phosphonates and phosphates. General fragmentation rules were deduced based on the study of decomposition pathways of 55 organophosphorus esters, including examples found in the literature. Multistage MS (MS(n)) experiments at high resolution in a hybrid mass spectrometer provide accurate mass measurements, whereas collision-induced dissociation experiments in a triple quadrupole give access to small fragment ions. The creation of a specific nomenclature for each possible structure of organophosphorus compound, depending on the alkyl side chain linked to the oxygen, was achieved by applying these fragmentation rules. This led to the creation of an 'identification tree' based upon the unique consecutive decomposition pathways uncovered for each individual compound. Hence, seven structural motifs were created that orient an unequivocal identification using the 'identification tree'. Despite the similar structures of the ensemble of phosphate and phosphonate esters, distinct identifications based upon characteristic neutral losses and diagnostic fragment ions were possible in all cases.

31P-edited diffusion-ordered 1H NMR spectroscopy for the spectral isolation and identification of organophosphorus compounds related to chemical weapons agents and their degradation products

Organophosphorus compounds represent a large class of molecules that include pesticides, flame-retardants, biologically relevant molecules, and chemical weapons agents (CWAs). The detection and identification of organophosphorus molecules, particularly in the cases of pesticides and CWAs, are paramount to the verification of international treaties by various organizations. To that end, novel analytical methodologies that can provide additional support to traditional analyses are important for unambiguous identification of these compounds. We have developed an NMR method that selectively edits for organophosphorus compounds via (31)P-(1)H heteronuclear single quantum correlation (HSQC) and provides an additional chromatographic-like separation based on self-diffusivities of the individual species via (1)H diffusion-ordered spectroscopy (DOSY): (1)H-(31)P HSQC-DOSY. The technique is first validated using the CWA VX (O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate) by traditional two-dimensional DOSY spectra. We then extend this technique to a complex mixture of VX degradation products and identify all the main phosphorus-containing byproducts generated after exposure to a zinc-cyclen organometallic homogeneous catalyst.

A set of triple-resonance nuclear magnetic resonance experiments for structural characterization of organophosphorus compounds in mixture samples

The (1)H, (13)C correlation NMR spectroscopy utilizes J(CH) couplings in molecules, and provides important structural information from small organic molecules in the form of carbon chemical shifts and carbon-proton connectivities. The full potential of the (1)H, (13)C correlation NMR spectroscopy has not been realized in the Chemical Weapons Convention (CWC) related verification analyses due to the sample matrix, which usually contains a high amount of non-related compounds obscuring the correlations of the relevant compounds. Here, the results of the application of (1)H, (13)C, (31)P triple-resonance NMR spectroscopy in characterization of OP compounds related to the CWC are presented. With a set of two-dimensional triple-resonance experiments the J(HP), J(CH) and J(PC) couplings are utilized to map the connectivities of the atoms in OP compounds and to extract the carbon chemical shift information. With the use of the proposed pulse sequences the correlations from the OP compounds can be recorded without significant artifacts from the non-OP compound impurities in the sample. Further selectivity of the observed correlations is achieved with the application of phosphorus band-selective pulse in the pulse sequences to assist the analysis of multiple OP compounds in mixture samples. The use of the triple-resonance experiments in the analysis of a complex sample is shown with a test mixture containing typical scheduled OP compounds, including the characteristic degradation products of nerve agents sarin, soman, and VX. The viability of the approach in verification analysis is demonstrated in the analysis of the 30th OPCW Proficiency Test sample.

Enhanced enzymatic thermal stability and activity in functionalized mesoporous silica monitored by (31) p NMR

Organophosphorus hydrolase (OPH) is immobilized on ammonium-modified mesoporous silica particles. Thermal stability and activity are measured with a (31) P NMR assay of the conversion of paraoxon (toxic) to its non-toxic hydrolysis product. After immobilization, OPH is significantly more active at room temperature and retained activity even after being heated to 45 °C for 1 month.