Triazine-Based Covalent Organic Framework: A Promising Sorbent for

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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 Kanchan Sinha Roy, Raghavender Goud D, Avik Mazumder, Buddhadeb Chandra, Ajay Kumar Purohit, Meehir Palit, and Devendra K. Dubey ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02354 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

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 Kanchan Sinha Roy, Raghavender Goud D, Avik Mazumder, Buddhadeb Chandra, Ajay Kumar Purohit, Meehir Palit, Devendra Kumar Dubey* Vertox Laboratory, Defence Research and Development Establishment, Jhansi Road, Gwalior-474002, Madhya Pradesh, India Fax: (+) 0751-2341148, E-mail: [email protected]

KEYWORDS: Triazine based covalent organic framework, Solid-phase extraction, Chemical warfare agents, Hydrocarbon backgrounds, Organic samples, GC-MS analysis & 1NMR analysis.

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ABSTRACT: 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 background using covalent triazine-based frameworks (CTFs). This nitrogen rich CTFs sorbent has shown an excellent SPE performance towards sample clean-up 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 n-hexane and the extraction solvent 1 mL dichloromethane (DCM). Under the optimized conditions, the SPE method had good linearity in 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 0.9867–0.9998 for all analytes. Limits of detection (LOD) (S:N=3:1) in SIM mode was 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. Limit of quantification (LOQ) (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 SIM mode. The recoveries of all analytes ranged from 87−100 % with the relative standard deviations (RSDs) ranging 1− 8 %. This method was also successfully applied for the sample preparation of 1H-NMR analysis of sulfur and nitrogen mustards in presence of hydrocarbon backgrounds. Therefore, this SPE method provides single sample preparation for both NMR and GC-MS analysis.


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1.

Introduction:

Chemical Warfare Agents (CWAs) 1-2, especially nerve and blister agents 3-4 are extremely toxic and deadliest chemicals. These chemicals were used in many conflicts like World War I & II, Iran-Iraq War, Tokyo subway etc. to gain political, territorial, religious, psychological and military superiority5-7. The use of CWAs had been restricted globally after the implementation of the Chemical Weapons Convention (CWC) in 19978. The CWC is implemented through strict verification regime by The Organization for Prohibition of Chemical Weapons (OPCW) located at The Hauge, The Netherlands9-10. In spite of the implementation of CWC, CWAs have still been stockpiled and used by the extremists in several events for mass destruction or targeted operation1114.

Recently in chemical attacks in Syria, CWAs were used for mass destruction, whereas in

Malaysia and North Korea, and London UK, CWAs have been used for target operations11-14. These episodes are raising the questions to the world about homeland security. Even in long-term exposure at low concentration, these agents will cause damage to human and animal bodies leading to eventual death. Therefore, strict monitoring and precise measurement of these harmful agents in environmental and other complex samples with high accuracy have great practical significance to minimize their exposure for both human and environment. This emphasized an urgent mandate to the growing field of analytical chemistry towards the development of identification and quantification methodologies for these toxic agents in complex samples, especially with improved sensitivity, accuracy, simplicity, reliability and multi-detection ability. Use of CWAs can be verified by the identification of convention related compounds (CRCs) in environmental as well as synthetic matrices15-16. In this context, off-site analysis related to CRCs, plays an essential role to validate the offence of CWC. During on-site inspections, OPCW


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inspectors collect samples from the production, storage, and suspected sites of chemical weapon activity15-16. For the unambiguous identification, these complex samples are sent to the “designated laboratories” for off-site analysis15-17. Among these collected samples, organic liquids are very common as these can be used as medium for spread of CWAs or be used as solvent during synthesis and storage. Analysis of organic samples from different sources like organic waste, incineration waste, engine or machine fuel, silicon oil or vacuum pump oil provide the information regarding the suspected area or production plant prohibited by CWC. Therefore, organic samples have great significance in CWC verification15-17. Several chromatographic and spectroscopic techniques are employed in conjunction to identify the CRCs15-20. Among them, gas chromatography coupled with mass spectrometer (GC-MS) is most reliable and primary analytical technique21-22. In addition to GC-MS, Nuclear Magnetic Resonance (NMR) spectroscopy has also been used as an important analytical technique23-25. But most of the sources of organic samples are usually adulterated with hydrocarbon backgrounds or other interfering chemicals which create difficulties in chromatographic and spectroscopic analysis15-16, 19-20. In presence of hydrocarbon backgrounds or interfering chemicals, both GC-MS and NMR analysis may give spurious information because analytes signals may eclipsed under background signals26-30. Therefore, sample preparation is of utmost value for clean-up of the interfering backgrounds and successful identification of the desired analytes. Sample preparation is an inevitable part for extraction and enrichment of target analytes from complex matrix sample prior to instruments for the trace level analysis31. Extraction techniques are promising tool in sample preparation, used for separation and enrichment. There are various extraction techniques like solvent exchange (SE), liquid–liquid extraction (LLE), solid phase extraction (SPE), solid phase micro extraction (SPME), single drop micro extraction


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(SDME), dispersive liquid-liquid extraction (DLLE) and electro-membrane extraction (EME) etc. reported in literature32. The above mentioned sample clean-up methods have their own limitations, and application of these methods depends on the nature of matrix background and the type of analytes. Single sample preparation cannot be universalized for all kinds of samples or analytical techniques. Generally, LLE with acetonitrile and silica based SPE methods are most common for the analysis of CRCs in hydrocarbon containing organic matrices19-20. These conventional methods are mostly preferred over the current state-of-the-art like SPME, SDME, DLLE, EME, etc. in sample preparation because of their easy usage. These conventional techniques are also recommended by the OPCW in the form ‘Blue Book’ named as “Recommended Operating Procedures for Analysis in the Verification of Chemical Disarmament”, known as ROP19-20. The acetonitrile (ACN) extraction shows limited efficacy for relatively nonpolar analytes due to low partitioning of nonpolar analytes into polar ACN19-20. As a result, recovery of nonpolar sulfur mustard is relatively low27. It does require back extraction using n-hexane for further removal of the hydrocarbon background. On other hand, silica based conventional SPE methods also have some limitations. According to ROP, polar analytes are not efficiently extracted from silica (silanol) based SPE cartridges19-20. It is also clearly mentioned in ROP that sulfur mustard behaves like a hydrocarbon and not efficiently retained on a silica cartridge19-20. Again, it is also expected that silica based SPE method would have restricted the extraction of nitrogen-containing analytes (like basic nitrogen mustards) due to their strong adsorption on the acidic silica surface33-34. In ROP, methanol (MeOH) and ethyl acetate (EtOAc) were recommended as extracting solvent for polar and nonpolar analytes respectively from silica SPE cartridges19-20. The presence of labile moieties like P=O, P−CN, P=S, P−F and C−Cl makes nerve agents and mustards more reactive towards the hydroxyl (−OH) functionalities present on the sorbent as well as extractant. Therefore,


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use of methanol as an extracting solvent may cause hydrolysis of the real agents in silica cartridge and results reduction in their recoveries. Hence, there is a dire need for the development of new analytical methodology that can overcome the difficulties associated with the conventional methods for extraction of analytes of varying polarities. In general, SPE is preferred over the LLE or other sample preparation techniques due to its robustness, chromatographic superiority and good reproducibility. SPE delivers a straightforward way of sample preparation and plays an important role in eliminating background chemicals. In SPE method, elimination of backgrounds can be achieved by partitioning of the desire analytes from the complex sample to the surface of SPE sorbent. This partition either kinetically or thermodynamically is driven by the physiochemical properties of the SPE sorbent. Therefore, adsorbent material packed in the SPE cartridge plays an important role during extraction because it determines the selectivity and sensitivity of the method. There are many sorbents categorized into silica-based, carbon-based and polymeric sorbents, commercially available or prepared inhouse for SPE35-37. Among them, recently, polymeric sorbents have gained considerable interest towards the SPE due to their stability and ability to tune with the variety of functionalities for broad range of analyte38-39. But, these sorbents either favor the irreversible sorption or there may be a lack of effective transportation of the intended analytes to the bed of the sorbent39. This suggests the use of compound and / or class-specific sorbents that can impart some selectivity; especially in a complex sample containing high interfering species. Previously, our group had reported polymeric materials as SPE sorbent for the extraction of nerve agents and mustards from organic matrices29-30. In both the cases, retention mechanism of analytes is mainly based on surface phenomena and they also bring hydrocarbon backgrounds down maintaining analytes concentration. Therefore, the obtained chromatogram in both cases were not enough clean for trace


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level detection. In such cases, we cannot analyze the prepared sample aliquot by 1H NMR spectroscopy which can also be used as primary technique for the identification of CWAs like sulfur mustard (SM) and nitrogen mustards (HNs). In addition, polymeric sorbents can also generate high backpressure in the SPE column due to swelling and heterogeneous structure which could possibly lead to slow adsorption / desorption kinetics and impaired size-selective adsorption40-42. Further leaching of polymer impurities sometime affects the chromatographic profiles. These considerations suggest that there exist scope for the development of SPE sorbent to further improve the matrix effects in sample preparation. Especially, considering increasing complexity in sample matrices, the search of new materials for extraction of organic pollutants has resulted in the development of new and versatile sorbents43-44. Recently, covalent organic frameworks (COFs), have emerged as a new class of crystalline porous material45-47. COFs have several advantageous features like high crystallinity, manupulable surface property, low density, easy synthesis, up-scalability and ability to interact with guest species through-out the bulk because of their porous structure46-48. Covalent organic frameworks (COFs) are synthesized by integrating the organic building block (monomers) units via strong covalent bond to form an ordered network structure with atomic precision49-50. They are mainly composed of light weight elements (N, O, B, C, H and Si) and prepared by using the same reticular chemistry as used for metal organic frameworks (MOFs)51. COFs hold the advantages of unique ordered network structure, inherent porosity, well-defined pore aperture, large specific surface area, low crystal density, excellent thermal and chemical stability, and facilely tailored functionality. Compared with their analogues like metal organic frameworks (MOFs)52, conjugated microporous polymers (CMPs)53 and hyper crosslinked polymers (HCPs)54, COFs have been proved to be more stable in water and acidic media and also


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overcome the difficulty to regulate the structures and functionalities or the lack of sustainable synthesis of other porous materials. Metal-free structure of COFs also has great potential as SPE sorbent for sample preparation of NMR spectroscopy. These fascinating features of COFs have been already demonstrated in the wider range applications including gas adsorption47, storage47,

56,

55,

gas

catalysis47, sensing57, photo-electricity47, optoelectronics58 and drug delivery59.

However, in case of CWAs and their related compounds the applicability of COFs materials has not been studied in details. Considering their fascinating features, structural diversity, and facilely tailored functionality, COFs have prompted us to investigate their usefulness as extraction materials against CWAs and their related compounds. The unique properties of COFs make them to be promising candidates as sorbents in the various extraction fields60-62. In this study, we have devoted an attempt towards the covalent triazine-based frameworks (CTFs)47, 63-64, a sub class of COFs, due to its excellent physicochemical stability65 and easy synthetic process63-64. CTFs have shown excellent adsorption reversibility with a complete and fast adsorption / desorption kinetics as they exhibit several specific and nonhydrophobic mechanisms including hydrogen bonding, electrostatic attraction, and electrondonor-acceptor interaction towards the guest molecules65. Their nitrogen enriched backbone also impart balance in their surface polarity which also favors the interactions of varying polarities. Therefore, we have synthesized CTFs via condensation of piperazine and cyanuric chloride and used as SPE sorbent against extraction of CWAs and their related compounds present in an organic matrix.


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2.

Experimental Section:

2.1.

Chemicals: 2,4,6-Trichloro-1,3,5-triazine

(cyanuric

chloride,

CC),

1,4-Diazacyclohexane

(Piperazine), N,N-Diisopropylethylamine (DIPEA), tri-n-butyl phosphate (TBP) (Internal Standard), deuterated chloroform-d1 and solvent 1,4-Dioxane, Ethanol, Dodecane were obtained from Sigma-Aldrich (New Delhi, India). HPLC grade solvents like acetonitrile, acetone, ethyl acetate, chloroform, dichloromethane, n-hexane and methanol were procured from E. Merck (Mumbai, India). Silicon oil was also procured from E. Merck (Mumbai, India). The diesel was procured from nearest petrol pump (Bharat petrolium) in Gwalior, India. The SPE silica cartridges (normal phase, 200 mg, 3 mL) of Agilent Technologies (USA) were used. The polypropylene SPE tubes (empty, 3 mL) with frits were obtained from Sigma-Aldrich (New Delhi, India). CWAs simulants like O,O'-dibutyl methylphosphonate (DBMP), O,O'-dicyclohexyl methylphosphonate (DHMP), O,O'-diethyl N, N-diethylphosphoramidate (DEDEPA), O,O'-diethyl ethylphosphonate (DEEP), O,O'-diethyl methylphosphonate (DEMP), O,O'-diethyl propylphosphonate (DEPP), O,O'-diisopropylmethylphosphonate (DIMP), O,O'-dimethyl N,N-dipropylphosphoramidate (DMDPPA), O,O'-dimethyl methylphosphonate (DMMP), O,O'-dipropyl methylphosphonate (DPMP) were synthesized (in micro gram quantities) and purified according to the literature66. The nerve agents like Tabun (O-ethyl-N,N-dimethylphosphoramidocyanidate), Sarin (O-isopropyl methylphosphonofluoridate), Soman (O-pinacolyl methylphosphonofluoridate), Cyclosarin (O-cyclohexyl

methylphosphonofluoridate),

VX

(O-ethyl

S-2-

diisopropylaminoethyl

methylphosphonothiolate), DFP (diisopropyl fluorophosphates), and the blister agents like Sulfur mustard [bis(2-chloroethyl)sulphide], nitrogen mustards like HN1 [bis(2-chloroethyl)ethylamine], HN2 [bis(2-chloroethyl)methylamine] and HN3 [tris(2-chloroethyl)amine] were synthesized


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(microgram quantities) and purified according to the reported procedures in OPCW scheduled facility of DRDE, Gwalior. Caution: The nerve and blister agents used in this study are highly toxic. Therefore proper precautions should be taken during the handling of these agents. All the experiments should be carried out in an efficient fume hood equipped with alkali scrubber by wearing appropriate protective gears. After use, all the solvent waste, contaminated glassware and pippet tips should be decontaminated using decontamination solution (approximately 15% w/v solution of bleach and alkali). 2.2.

Synthesis of covalent triazine–based framework: In 150 mL of 1,4-dioxane, Piperazine (3.73 g, 43.3 mmol) was dissolved at 15 °C and to

this solution DIPEA (18.9 mL, 108.4 mmol) was added. To this reaction mixture a solution of Cyanuric chloride (5.00 g, 27.1 mmol) in 50 mL of 1,4-dioxane, was added dropwise with continuous stirring at 15 °C under nitrogen environment. With this addition a white precipitate was observed and stirred at 15 °C for 1 hr. After that, the mixture was continued for stirring at room temperature (25 °C) for 2 hr and then refluxed at 85 °C for 21 hr. The off-white precipitate was filtered and the resultant solid was washed with 1,4-dioxane. Afterwards, the crude product was suspended in ethanol (EtOH) for three times over the period of 12 h. Finally, the resultant offwhite solid was dried at room temperature under vacuum for overnight. This material was designated as CTFs and used for solid phase extraction. 2.3.

Solid phase extraction procedure: SPE cartridges were prepared in-house by packing of 50 mg of CTFs materials in the 3 mL

polypropylene SPE tube. The SPE cartridges were pre-conditioned using 1 mL acetone followed


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by 2 mL n-hexane. Pre-conditioned cartridges were loaded with 1 mL of organic sample containing selected analytes and allowed to pass through the cartridges under the gravity / gentle flow of nitrogen. Then, in order to eliminate the backgrounds or interferences from all the cartridges, washing was performed with n-hexane. Finally the elution was carried out with optimized solvents. Final volume of the eluate was maintained to 500 μL by concentrating under mild flow of nitrogen. To the concentrated eluate 25 μL of internal standard [tri-n-butyl phosphate (TBP)] solution (100 μg mL−1) was added and 1 μL of that sample was subjected to GC-MS analysis in EI mode (SIM & full scan).Recoveries of all analytes were calculated by comparing the relative response of analytes of treated samples to that of the control samples (6 μg mL−1). Matrix matched control samples were prepared by spiking the analytes in to the similarly treated blank samples (i.e. preconditioning, loading, washing and eluting with extracting solvent) which did not contain any analytes like blank n-hexane and / or n-hexane spiked with 5000 μg mL−1 diesel background. All the experiments were carried out in triplicate runs and the values presented in figures represent the average of triplicate runs. 2.4.

GC-MS analysis: An Agilent 6890N gas chromatograph equipped with Agilent 5973N mass-selective

detector was used for the GC-MS analysis of all the samples in the electron ionization (EI) mode. A capillary column (30 m × 0.25 mm I.D., 0.25 µm film thicknesses) DB-5MS (Agilent technologies) was used as stationary phase. The following GC method was used for the analysis. GC oven temperature from 50 °C (held for 2 min) to 300 °C @ 20 °C min−1 (held for 2 min). Carrier gas was helium at a constant flow rate of 1.0 mL min−1. Injection temperature was 250 °C and all analyses were carried out in splitless mode. The samples were injected manually using a 10 μL micro syringe (hamilton) and the injection volume was 1 μL. The temperature of EI source


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was 230 °C, quadrupole temperature was 150 °C and ionization energy was 70 eV. For quantitation studies, selected ion monitoring (SIM) mode was used (dwell time was 100 ms). For qualitative studies fullscan mode was used, and scan range was from m/z 35 to 450 (3.47 scans per second). 2.5.

NMR analysis: All NMR experiments were performed on a 5 mm broad band observe (BBFO) smart probe,

installed on a Bruker AV III 600 MHz NMR spectrometer (Bruker Biospin, Fallenden, Switzerland). The instrument software Topspin 3.5pl7 had been used for control, data acquisition and processing. Before carrying out experiments the NMR probe was maintained at 25 °C (under 400 L min−1 air flow) and 5 minutes equilibration time was given to each sample. The frequencyfield locked in deuterated chloroform-d1 (CDCl3) for all the samples. TopShim© gradient shimming tool of the instrument software had been used to establish the homogeneous magnetic field. Throughout the complete period of NMR experimentation auto shim mode was maintained for all the samples to maintain the stability of the established magnetic field homogeneity. Each 1H-NMR

spectrum was acquired by using 24 transients (rf pulse of 12.58 µs, spectral width 8000

Hz and central frequency of 2500 Hz) using the Bruker library pulse program zg. 3.

Results and Discussion

3.1.

Synthesis of covalent triazine–based framework: 2, 4, 6-trichloro-1, 3, 5-triazine is a common industrial chemical, also known as cyanuric

chloride (CC). CC is extensively used in the synthesis of triazine based hyper-branched polymers and dentrimers67-68. These structures possess a high degree of regularity due to its symmetric (D3h) and planar structure67-68. Sequential selective nucleophilic displacement of ‘Cl’ atoms in


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appropriate conditions imparts regularity in their structure64. Therefore, CC was taken as planar trigonal building unit along with mono-protic divalent nucleophile piperazine as a linear linker for the synthesis of COFs. According to reported procedure64, COF was prepared by the copolymerization of CC and piperazine in the presence of N, N-diisopropylethylamine (DIEPA, act as an acid scavenger) (Scheme S1). With temperature, piperazine displaced ‘Cl’ atoms of CC (Scheme S1) via nucleophilic substitution and builds a 2D framework structure of CTFs as shown in (Scheme S1). The tertiary ‘N’ atoms present in the CTFs are proficient for reversible binding of guest molecules. 3.2.

Characterization of synthesized CTFs: The chemical structure of synthesized CTFs was confirmed by FTIR analysis. The FT-IR

spectrum in Figure S1 shows the typical bands at 1490 and 1299 cm−1 due to the stretching and breathing modes of the aromatic C–N in the triazine and band at 2922 cm−1 due to the absorption of alkane C–H in the piperazine. Distinctly, the absence of the characteristic peak at 850 cm−1 in the FT-IR spectrum of CTFs, which is responsible for the stretching vibration of C-Cl in the cyanuric chloride, confirmed that all of the ‘Cl’ atoms were substituted completely. The thermal stability of the material was checked by thermogravimetric analysis (TGA) with the heating rate of 10 °C min−1 up to 800 °C separately under the environment of air and gaseous nitrogen (N2). Thermograms depicted in Figure S2 indicate that the synthesized CTFs material was stable both in inert condition as well as normal atmosphere and start to decompose near 290 °C in air and 370 °C in nitrogen atmosphere. Information about the particle size and shape was collected from Scanning Electron Microscope (SEM) analysis. SEM images in Figure 1 showed that CTFs adopts a uniform sheet-shaped morphology with average particle diameter of 400–450 nm which indicated that poly-condensation of CC and piperazine impart to a certain degree of structural


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regularity. The surface area, pore size and pore volume of the material were measured by N2 adsorption isotherm and the observed elemental composition and details of surface properties are shown in Table 1. The measured BET surface area of material was 166 m2g−1 which is close to the reported value64. The observed (%) elemental composition for C15H24N9 was found ‘C’ 53.73, ‘H’ 5.94, and ‘N’ 40.33 by the CHNS analysis. These values are close to calculated values (Table 1).

Figure 1: SEM images of CTFs particles at (a) 10000X and (b) 20000X magnifications

Table 1: Elemental composition, BET surface area, pore size and pore volume observed for the synthesized CTFs. Elemental composition C%

CTFs

3.3.

H%

N%

Theoretical

54.53

7.32

38.15

Experimental

53.73

5.94

40.33

Surface properties BET surface area (m2g−1)

Pore size (nm)

Pore volume (cm3g−1)

166

7.3

0.26

Optimization of solid phase extraction (SPE) conditions: The applicability of synthesized CTFs as SPE sorbent was assessed via extraction of the

CWAs and related compounds from organic matrix. To achieve the best extraction and enrichment


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performance, several affecting factors like washing solvent and amount of washing solvent, elution solvent and amount of elution solvent were investigated. 3.3.1. Optimization of elution solvent and its volume: The extraction performance of CTFs as SPE sorbent was first tested against polar and nonpolar CWC related organophosphorus esters shown in Figure 2 (spiked concentration of each analytes was 3 μg mL−1). To find the best recoveries of selected organophosphorus esters, various solvents like DCM, CHCl3, acetone and EtOAc were employed as eluting solvent considering their chromatographic profile in nonpolar GC-column. The recoveries of selected analytes with 1 mL of different eluting solvents were depicted in Figure 3. These results showed that all extracting solvents were able to recover both polar as well as nonpolar analytes efficiently (≈ 80-100%); particularly with DCM, CHCl3 and EtOAc. This indicates that the CTFs would have ideal hydrophobic-hydrophilic balance throughout its frameworks due to high nitrogen content. The piperazine rings adjacent to the nitrogen rich triazine moiety would also maintain the microenvironment polarity of the cavity of CTFs. This microenvironment of the cavity provides a thermodynamically favourable condition for the retention of the analytes by shattering the hydrophobic solvation between the analytes and nonpolar n-hexane. Therefore, both polar as well as nonpolar analytes are adsorbed by the framework of CTFs, dictated by reversible non-covalent interactions. However, DCM was selected as elution solvent for further optimization, as it yielded comparatively higher recoveries of the analytes, further DCM also have better chromatographic response in gas chromatography (GC). To perform NMR analysis, elution can be performed using deuterated chloroform (CDCl3) as good recoveries of analytes with CHCl3 were also observed. DCM and CHCl3 also display good recoveries of nerve and blister agents, as shown in Figure 4. These results indicate the good performance of the CTFs as SPE sorbent.


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To get a high enrichment factor and ensure complete desorption of target analytes, different amounts of DCM (1 mL, 0.5 mL + 0.5 mL and 1.5 mL) were used for elution. Results depicted in Figure 5 show that highest recoveries (87-100%) were achieved even with 1 mL of DCM. No further increase in recovery was observed with higher amount of DCM. Therefore, 1 mL DCM was fixed as elution. Nerve Agents: O

O

O

P

P

NC

O

O P

P

P

N

S

Tabun

Cyclosarin

Soman

O

O

F

F

F Sarin

O

O

O

N

VX

Blister Agents:

Cl

S

Cl

Cl

N

Cl

N

Cl

Cl

Nitogen Mustard (HN1)


Sulfur Mustard (SM)

Cl

N

Cl

Cl


CRC related organophosphorus ester and markers of nerve agents: O O

O

O

P O

O

O

O

O,O'-Diethyl methylphosphonate

O,O'-Dimethyl methylphosphonate

O

P

P

O P

O

O

O,O'-Diisopropyl methylphosphonate

O,O'-Diethyl ethylphosphonate

O O

O O

O

O

O,O'- Diethyl-N,Ndiethylphosphoramidate

O P O

O,O'-Dicyclohexyl methylphosphonate

O

O

O,O'-Dibutyl methylphosphonate

N

P

O

O O,O'- Dipropyl-N,Ndimethylphosphoramidate

O

O

P O

N

O

O

O,O'-Dipropyl methylphosphonate

O,O'-Diethyl propylphosphonate

P

O

P

P

O

O

P F

O

Diisopropyl fluorophosphate (DFP)

Figure 2: Structures of nerve agents, blister agents and CWC-related organophosphorus esters.


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DCM

120

CHCl3

EtOAc

Acetone

Recovery (%)

100 80 60 40 20 0

Figure 3: Recovery % of selected organophosphorus esters in different eluting solvents using CTFs sorbent.

Recovery (%) nerve agents

(a)

DCM

Recovery (%)

120

CHCl3

100 80 60 40 20 0 Sarin

(b)

DFP

Soman

Tabun

Cy. Sarin

Vx

Recovery (%) of sulfur and nitrogen mustards DCM

120

Recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CHCl3

100 80 60 40 20 0 HN2

HN1

SM

Figure 4: Recovery % of (a) nerve agents and (b) blister agents in DCM and CHCl3.


17


120

1mL DCM

(500 + 500)uL DCM

1mL + 500uL DCM

100

Recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

80 60 40 20 0

Figure 5: Recovery (%) of selected organophosphorus esters with different amounts of DCM.

3.3.2. Background Elimination: Organic matrices were commonly encountered with high hydrocarbon background such as diesel fuel, which interferes with analyte signals in chromatographic and spectroscopic analysis. This causes difficulties in GC-MS and NMR based identification process of analytes. In GC-MS analysis, hydrocarbon backgrounds can mask the analyte peaks in the chromatogram. Therefore, elimination of background chemicals is essential before instrumental analysis and it is affected by the washing. In this regard, the selection of washing solvent and optimization of its volume are very crucial in order to effectively eliminate the background and prevent the loss of desired analytes. In solid phase extraction, the desired analytes get absorbed on the CTFs framework along with some diesel background. This is because of hydrophobic interaction between diesel background and the hydrophobic part of CTFs framework. Therefore, to eliminate the hydrocarbon backgrounds, n-hexane was selected as washing solvent. Results of washings with 1 mL of n-hexane are depicted in Figure 6. It is quite evident from these results that 1 mL of n-hexane eliminated most of the hydrocarbon background without losing the analytes. Further increase in


18

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the washing amount of n-hexane to 2 and 3 mL, caused analytes loss with no further appreciable removal of backgrounds. Hence, 1 mL n-hexane was preferred for washing. (a)

(b)

(c)

(d)

Figure 6: Total ion chromatogram obtained from GC-EI-MS analyses of the (a) selected analytes spiked in the organic matrix with a diesel background, (b) after washing the diesel background for the organophosphorus esters, (c) after washing the diesel background for the nerve agents and (d) after washing the diesel background for the blister agents with 1 mL n-hexane.


19


3.4.

Page 20 of 35

Analytical performance of the optimized SPE method: After optimization of the extraction & enrichment parameters with synthesized CTFs, an

analytical method was developed using CTFs as SPE sorbent. In order to investigate the practical applicability of the developed SPE method, the linearity range, limits of detection (LODs) and limits of quantiﬁcation (LOQs) were determined under the optimized SPE conditions. These quantitative parameters for the selected analytes are presented in Table 2, Table 3 and Table 4. From these results it could be seen that, the method had good linearity in 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 good correlation coefﬁcients (r2) between 0.9867–0.9998 for all analytes. The LODs and LOQs for selected analytes were determined based on signal-tonoise (S:N) ratio of 3:1 and 10:1 respectively and illustrated in Table 2, Table 3 and Table 4. Observed LODs of each analytes were re-evaluated by analysis of the identical spiked concentration in n-hexane. It was also observed that the S:N ratio for LOD of each analyte was greater than 3. The intra-day / inter-day precision and accuracy were determined by relative standard deviation (RSD) for selected analytes at three quantification levels in triplicate for three consecutive days for 3 weeks. The intra-day precision for organophosphorus esters, nerve agents and mustards were found in the range of 2.08–3.80 %, 1.00–3.13 % and 1.65–5.39 % respectively. The corresponding inter-day mean precision for organophosphorus esters, nerve agents and mustards lies in the range of 1.49–7.54 %, 2.72–6.28 % and 2.53–7.51 % respectively. These parameters demonstrate the applicability and reliability of the developed method.


20

Page 21 of 35

Table 2: Analytical figures of merit for organophosphorus esters under the optimized SPE conditions Linearity Range

Coefficient of determination

Intra-day repeatability

Inter-day reproducibility

LOD

LOQ

in SIM

in SIM

(µg mL−1)

(r2)

(RSD %)

(RSD %)

(µg mL−1)

(µg mL−1)

DMMP

0.18-10

0.9941

2.65

3.29

0.050

0.200

DEMP

0.18-10

0.9984

2.92

4.88

0.050

0.180

DIMP

0.15-10

0.9996

2.97

2.39

0.040

0.150

DEEP

0.15-10

0.9867

3.04

4.23

0.040

0.150

DEPP

0.12-5

0.9967

3.24

3.59

0.035

0.120

DPMP

0.10-5

0.9961

3.76

1.49

0.030

0.100

DEDEPA

0.10-5

0.9979

3.61

2.88

0.025

0.100

DMDPPA

0.075-5

0.9978

3.65

3.93

0.020

0.075

DBMP

0.050-5

0.9912

3.80

7.54

0.015

0.050

DHMP

0.075-5

0.9898

2.08

5.68

0.020

0.075

CWC related phosphonates

Analytes

Sample loading volume was 1 mL n-hexane and elution was performed with 1 mL DCM.

Table 3: Analytical figures of merits for nerve agents under the optimized SPE conditions.

Nerve agents

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Analytes

Linearity Range (µg mL−1)

Coefficient of determination (r2)

Intra-day repeatability (RSD %)

Inter-day reproducibility (RSD %)

LOD in SIM (µg mL−1)

LOQ in SIM (µg mL−1)

Sarin

0.050-20

0.9976

2.97

2.72

0.015

0.050

DFP

0.040-20

0.9991

1.00

4.91

0.020

0.070

Soman

0.070-20

0.9996

1.50

2.75

0.025

0.080

Tabun

0.100-20

0.9936

3.13

6.28

0.030

0.100

Cy. Sarin

0.040-20

0.9981

2.46

3.87

0.010

0.040



21


Table 4: Analytical figures of merits for blister agents under the optimized SPE conditions.

Blister agents

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

Analytes

Linearity Range (µg mL−1)

Coefficient of determination (r2)

Intra-day repeatability (RSD %)

Inter-day reproducibility (RSD %)

LOD in SIM (µg mL−1)

LOQ in SIM (µg mL−1)

HN1

0.300-20

0.9918

2.35

4.05

0.090

0.300

HN2

0.250-20

0.9978

2.85

2.53

0.075

0.250

HN3

0.200-20

0.9985

1.65

7.51

0.050

0.180

SM

0.350-20

0.9998

5.39

4.91

0.100

0.350


2.5.

Evaluation of the developed SPE method: To study the matrix effect on the developed SPE method, the extraction of the CWAs and

its related compounds, at three different concentrations, was determined taking silicon oil and vacuum pump oil as challenge matrix. Results presented in Table S1 & Table S2 show that recoveries of all analytes ranged from 75−91 % with relative standard deviation (RSD) of 2−7 %. This method could successfully be employed for organophosphorus esters also. Results of extraction are shown in Figure S3 for n-hexane: dodecane (50:50 v/v) matrix. In order to check sample clean-up ability of the optimized SPE method, a comparison of chromatographic profile of organophosphorus compounds is presented with those of other methods26-27, 30 (Figure 7). The clean chromatogram (Figure 7d) obtained with CTFs sorbent suggested that this method can effectively eliminate most of the hydrocarbon backgrounds; thus signifies its superiority over other methods. Better performance of CTFs further support the fact ascribed to the polarity of the ‘N’ rich framework which restricts the nonpolar hydrocarbon to adhere on its bulk. Hence, hydrocarbon easily passes through the cavities. Therefore, CTFs materials have an advantage as a SPE sorbent in sample preparation, particularly for the elimination of hydrocarbon backgrounds from organic matrices.


22

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(a)

(b)

(c)

(d)

Figure 7: Total ion chromatogram obtained from GC-EI-MS analyses after the sample clean-up of n-hexane matrix spiked with the selected CWC related organophosphorus esters (3 μg mL−1) and diesel background (5000 μg mL−1). The sample clean-up was performed with (a) acetonitrile extraction, (b) silica based SPE cartridges, (c) previously developed polymeric SPE sorbent30 and (d) present optimized SPE method based on CTFs sorbent.


23


3.6.

Page 24 of 35

Application of the developed method: Generally, X{1H} NMR experiments (where X=31P or

19F)

can be performed for the

detection and identification of CWC related organophosphorus and / or fluorine containing compounds19-20. Additionally Heteronuclear single quantum coherence (HSQC) type of experiments can aid in the detection of the P-C connection with enhanced sensitivity23-24. In such experiment, hydrocarbon background does not alter the analysis since the nerve agents or other organophosphorus and / or fluorine compounds can easily be identified selectively from diverse matrices. But analysis and identification of analytes devoid of ‘P’ and / or ‘F’ like sulfur and nitrogen mustards, simple 1H NMR experiments can only be used25. In this job, the hydrocarbon backgrounds interfere with the 1H NMR signals of these agents due to spectral overlaps and dynamic range problems. Therefore in order to impart selectivity and remove/reduce spectral inference, tagging of the analytes with ‘P’ / ‘F’ containing moieties is a viable option69. But these methods require clean chemical reactions and they may not occur universally in all types of matrices or proceed to completion. The use of LC-NMR spectroscopy for sample clean-up28 also has its own limitations70. Therefore, the hydrocarbon backgrounds elimination ability of CTFs sorbent based SPE method was explored for the 1H NMR analysis of these analytes in presence of hydrocarbon backgrounds containing organic matrices. Each analyte (SM and HN3) was separately spiked at concentration of 10 μg mL−1 in n-hexane containing diesel (5000 μg mL−1) background and also in dodecane (50:50 v/v) matrix to prepare two sets of samples (two samples for each). For the clean-up of these samples, the optimized sample preparation protocol was followed using CDCl3 as eluting solvent (instead of DCM). This was done on the basis of elution solvent optimization experiments wherein chloroform had shown its compatibility with the synthesized CTFs sorbent and could easily elute sulfur and nitrogen mustards from the CTFs


24

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material with good recoveries (80-84%). Control experiments were performed wherein the spiked samples were subjected to 1H NMR analysis without any sample preparation. In this case the 1H NMR analyses were found to be unsuitable for the detection of the target analytes (Figure-8). When the same experiments were performed on these samples subjected to sample clean-up using the optimized conditions described herein, discernable signals of these analytes (separately spiked in sample) could easily be observed and identified in 1H NMR spectra (shown in Figure-8). In this way the developed SPE method overcame the difficulties associated with the 1H NMR analysis of sulfur and nitrogen mustards in presence of highly proton rich hydrocarbon backgrounds.

Before clean-up the diesel backgrounds

(a)

Before clean-up the dodecane backgrounds

After clean-up the dodecane backgrounds

After clean-up the diesel backgrounds

SM

SM

Before clean-up the diesel backgrounds

After clean-up the diesel backgrounds

(b)

(c)

Before clean-up the dodecane backgrounds

(d)

After clean-up the dodecane backgrounds

HN3

HN3

Figure 8: Comparision in 1H NMR Spectra obtained from 1H NMR analyses of (a) SM spiked at 10 μg mL−1 in diesel backgrounds (5000 μg mL−1) containing n-hexane matrix, (b) SM spiked at 10 μg mL−1 in dodecane (50:50 v/v) matrix, (c) HN3 spiked at 10 μg mL−1 in diesel backgrounds (5000 μg mL−1) containing n-hexane matrix (d) HN3 spiked at 10 μg mL−1 in dodecane (50:50 v/v) matrix before and after the sample clean-up.


25


Page 26 of 35

The optimized SPE method can also be applied for the extraction of sulfur mustard spiked at 5 μg mL−1 in n-hexane: dodecane (50:50 v/v) matrix. Generally, the chromatographic response of sulfur mustard in GC-MS becomes supress under the huge hump of dodecane matrix as shown in Figure 9a. The developed SPE method successfully cleaned this sample by trim down the hump of dodecane and sulfur mustard was easily identified as illustrated in Figure 9b. Therefore, the developed SPE method may also provide single sample preparation protocol for both NMR and GC-MS instruments when CDCl3 using as elution solvent. (a)

(b)

Figure 9: Total ion chromatogram obtained from GC-EI-MS analysis of sulfur mustard spiked at 5 μg mL−1 in n-hexane: dodecane (50:50 v/v) matrix (a) before and (b) after the sample clean-up.


26

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4.

Conclusion: In this work, the nitrogen rich covalent triazine based framework (CTFs) was synthesized

and successfully applied for the first time as a new SPE sorbent for the extraction and enrichment of CWAs and their related compound from hydrocarbon backgrounds. Various extraction parameters e.g. eluting solvent, its volume, washing solvent were optimized and rationale of its performance as a SPE sorbent is presented. Comparative evaluation with convention methods and recent developed methods, the optimized SPE methods shows excellent clean-up and enrichment ability towards the analytes of varying polarity. The high nitrogen content on its framework facilitates the selective removal of hydrocarbon backgrounds by maintaining the polarity balanced in the framework. Encouraged by the effective clean-up ability of this material, this method was also successfully applied for 1H NMR analysis of sulfur and nitrogen mustards in hydrocarbon backgrounds. The developed SPE method can also provide single shot sample preparation for both NMR and GC-MS instruments. The advantageous features of the framework structure of CTFs materials make it a superior SPE sorbent for the extraction and enrichment of CWAs and their related compounds from hydrocarbon backgrounds. The outcomes of the current study signify that this SPE method can be effectively employed during the off-site analysis of CWAs for their extractions and also expect that it will open up a new way in the sample preparation field for wider applications. ASSOCIATED CONTENT Supporting Information: Synthesis, characterization of CTFs and validation data of developed SPE method have been provided as the Supporting Information.


27


Page 28 of 35

AUTHOR INFORMATION Corresponding Author *Email Id: [email protected] Fax: (+) 0751-2341148 ORCID ID: Devendra Kumar Dubey: 0000-0003-3820-2233 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Kanchan Sinha Roy is thankful to DRDE, Gwalior and DRDO, New Delhi for providing fellowship. We appreciate Mr. Manoj K. Sahoo for providing chemicals. We are thankful to Dr. B. N Acharya, Dr. G.K Prasad and Dr. P.K Gutch for their support for characterization of materials. We are also thankful to Dr. Joyti Ranjan Acharya and the OPCW scheduled facility in our establishment for providing the real agents. This manuscript is assigned DRDE accession no. DRDE/VTX/004/2019.

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Triazine-Based Covalent Organic Framework: A Promising Sorbent for

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