Deprotonation of trinitrotoluene by dichloromethane in atmospheric pressure chemical ionization mass spectrometry.

RATIONALE: Trinitrotoluene (TNT) and its derivatives are found in the environment. Depending on the analysis conditions, TNT may be detected as a deprotonated molecule or a molecular ion through atmospheric pressure chemical ionization (APCI). Dichloromethane (CH2 Cl2 ) is used as an extraction solvent for TNT from environmental samples and as an ionizing agent to generate chlorinated species. APCI mass spectra of TNT dissolved in CH2 Cl2 revealed the presence of only the [TNT - H]- ion. This phenomenon was investigated by reactions with the CH2 Cl2 -related reactant ions. METHODS: TNT in CH2 Cl2 was ionized through APCI, and CH2 Cl2 and acetone were used as the eluent. The vaporizing temperature was varied from 200 to 350°C. TNT-related ions and reactant ions were analyzed. Detection limits for TNT were determined under different conditions. Energy-minimized structures of the product ions were used to interpret the analysis results. RESULTS: Significant [TNT - H]- ion was observed, but the [TNT + Cl]- and TNT•- ions were not detected. The [HCl + Cl]- and [3acetone + Cl]- ions were detected as reactant ions when using CH2 Cl2 and acetone as eluent, respectively. Other potential reactant ions such as CH2 Cl2 •- and [CH2 Cl2  + Cl]- were not observed. The detection limit improved as the vaporizing temperature increased. The favorability of [TNT - H]- formation was explained on the basis of the heats of reactions. CONCLUSIONS: The [TNT - H]- ion was primarily produced by the deprotonation reaction between CH2 Cl2 •- and neutral TNT. This reaction was determined to be significantly exothermic. CH2 Cl2 can be helpful for the analysis of TNT in environmental samples through APCI mass spectrometry.

Test method for vapor collection and ion mobility detection of explosives with low vapor pressure.

RATIONALE: Ion mobility spectrometry (IMS) has been widely used for on-site detection of explosives. Air sampling method is applicable only when the concentration of explosive vapor is considerably high in the air, but vapor pressures of common explosives such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), and pentaerythritol tetranitrate (PETN) are very low. A test method for analyzing the vapor detection efficiency of explosives with low vapor pressure via IMS was developed using artificial vapor and collection matrices. METHODS: Artificial explosive vapor was produced by spraying an explosive solution in acetone. Fifteen collection matrices of various materials with woven or nonwoven structures were tested. Two arrangements of horizontal and vertical positions of the collection matrices were employed. Explosive vapor collected in the matrix was analyzed using IMS. RESULTS: Only three collection matrices of stainless steel mesh (SSM), polytetrafluoroethylene sheet (PFS), and lens cleansing paper (LCP) showed the TNT and/or RDX ion peaks at an explosive vapor concentration of 49 ng/L. There was no collection matrix to detect PETN vapor at or lower than 49 ng/L. For the PFS, TNT and RDX were detected at a vapor concentration of 49 ng/L. For the LCP, TNT and RDX were detected at vapor concentrations of 14 and 49 ng/L, irrespectively. CONCLUSIONS: The difference in the explosive vapor detection efficiencies could be explained by the adsorption and desorption capabilities of the collection matrices. The proposed method can be used for evaluating the vapor detection efficiency of hazardous materials with low vapor pressure.

Direct detection of diphenylamino radical formed by oxidation of diphenylamine using atmospheric pressure chemical ionization mass spectrometry.

RATIONALE: Diphenylamine (DPAH) and its derivatives have been widely used as antioxidants. Its antioxidation behaviors are started with the formation of diphenylamino radical (DPA• ) by loss of hydrogen atom through reaction with oxygen or ozone. DPAH was oxidized by ozonation to produce DPA• and DPA• was directly detected using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The influence of oxidation time on generation of DPA• and consumption of DPAH was also investigated. METHODS: Experiments were performed using a home-made ozone bubbling apparatus for oxidation of DPAH. DPAH solution (1000 ppm) in acetone was stirred during the ozonation. The oxidized sample was diluted 100-fold and analyzed using APCI-MS. Structures and energies of DPAH, DPA• and detected ions were obtained using electronic structure calculations with Spartan 10 and Gaussian 09. RESULTS: DPA• was detected in positive APCI-MS in the form of DPAH•+ by proton transfer reaction with protonated acetone. Formation of DPAH•+ from DPA• was more favorable than that of [DPAH + H]+ from DPAH and it was also much more favorable than that of DPA+ from DPA• . By increasing the oxidation time, the relative abundance of DPAH•+ was increased whereas that of [DPAH + H]+ was notably decreased. CONCLUSIONS: Existence of DPA• was directly analyzed using APCI-MS. DPAH and DPA• were differently detected as [DPAH + H]+ and DPAH•+ , respectively. Direct APCI-MS analysis is a suitable technique for identification and quantitation of DPA• .

The influence of different types of reactant ions on the ionization behavior of polycyclic aromatic hydrocarbons in corona discharge ion mobility spectrometry.

RATIONALE: Some polycyclic aromatic hydrocarbons (PAHs) are considered to be cancer-causing chemicals, and ion mobility spectrometry (IMS) is used for on-site detection of such hazardous chemicals. In IMS, the ionization behavior of analytes is affected by the types of reactant ions (RIs). In the present work, the influence of different types of RIs on the ionization behaviors of PAHs in an ion mobility spectrometer equipped with a corona discharge ionization source was investigated using various RIs. METHODS: Selected PAHs were dissolved in anisole, fluorobenzene, chlorobenzene, or bromobenzene. The IMS analysis procedure was performed as follows: (a) the PAH solution was dropped onto the smear matrix; (b) the smear matrix was immediately inserted into the sample inlet to minimize evaporation of the solvent; and (c) the IMS analysis was performed. The lowest amount studied was 10 ng. Variations in the IMS spectra with time were investigated. RESULTS: PAHs were not ionized by RIs of protonated molecules ([M + H]+ ) such as air/moisture and acetone, but they were ionized by charge transfer reactions with RIs of molecular ions (M•+ ) of solvents such as anisole, fluorobenzene, chlorobenzene, and bromobenzene. The PAH ions were detected following a time delay of ~1-5 s after the sample introduction, and the times at which the maximum intensities for the PAHs were observed were different. The detection limits of PAHs in chlorobenzene were on the whole better than those in other solvents, whereas those in fluorobenzene were worse. The detection limits of pyrene and benzo[a]anthracene were better than those of the other PAHs irrespective of the solvent used. CONCLUSIONS: PAH molecules were ionized by charge transfer reactions with RIs of the solvents, and their ions were detected ~1-5 s after sample introduction. The order of the ionization efficiency was chlorobenzene > anisole > bromobenzene > fluorobenzene.

Complex formation of lactic acid by atmospheric pressure chemical ionization.

Oligomers and polymers of lactic acid are generally synthesized through condensation reactions by dehydration at high emperature under catalysis. In the present work, ionization behaviors of lactic acid produced by atmospheric pressure chemical ionization were investigated. Influence of the sample concentration, the heating zone temperature, and the source fragmentor voltage on kinds and abundances of the product ions was examined. Complex formation of the product ions with neutral species was also investigated. Not only lactate, [M-H]- and its complexes but also ions of condended species, [nM-(n-1)H2 O-H]- with n = 2-5, and their complexes were observed. The condensation reactions occurred in an aerosol state generated in the heating zone for evaporation. By increasing the concentration of lactic acid, abundances of the product ions were increased and the increase of larger ones was noticeable. By increasing the heating zone temperature, abundances of the product ions were decreased and the decrease of larger ones was remarkable. By increasing the source fragmentor voltage, abundances of small product ions were increased and those of the complexes, [nM-(n-2)H2 O-H]- with n = 2-5, were significantly decreased. Complex formation of lactate with the neutral condensed species was more favorable than that of the condensed oligomer ions with a neutral lactic acid. The experimental results were explained by energies and structures of the product ions and neutral species obtained by theoretical calculations.

Preparation and Characterization of Model Tire-Road Wear Particles.

Tire tread wear particles (TWPs) are one of major sources of microplastics in the environment. Tire-road wear particles (TRWPs) are mainly composed of TWPs and mineral particles (MPs), and many have long shapes. In the present work, a preparation method of model TRWPs similar to those found in the environment was developed. The model TRWPs were made of TWPs of 212-500 µm and MPs of 20-38 µm. Model TWPs were prepared using a model tire tread compound and indoor abrasion tester while model MPs were prepared by crushing granite rock. The TWPs and MPs were mixed and compressed using a stainless steel roller. The TWPs were treated with chloroform to make them stickier. Many MPs in the model TRWP were deeply stuck into the TWPs. The proper weight ratio of MP and TWP was MP:TWP = 10:1, and the double step pressing procedure was good for the preparation of model TRWPs. The model TRWPs were characterized using scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The model TRWPs had long shapes and the MP content was about 10%. The model TRWPs made of TWPs and asphalt pavement wear particles showed plate-type particles deeply stuck into the TWP. Characteristics of model TRWPs can be controlled by employing various kinds and sizes of TWPs and MPs. The well-defined model TRWPs can be used as the reference TRWPs for tracing the pollutants.

Influence of smear matrix types on detection behaviors and efficiencies of polycyclic aromatic hydrocarbons using ion mobility spectrometry.

Influence of smear matrix types on detection behaviors and efficiencies of polycyclic aromatic hydrocarbons (PAHs) with different molecular weights in ion mobility spectrometry (IMS) were investigated. Various smear matrices of stainless steel mesh (SM), cellulose paper (CP), and cotton fabric (CF) were employed. Anisole was used as the solvent and IMS analysis was performed without evaporation step of the solvent to apply charge transfer reactions between PAH molecules and the molecular ions of solvent. Shapes of reactant ion peaks (RIPs) were varied according to the smear matrix types. At the beginning of the sample inlet, intensity of RIPs of air and moisture notably decreased due to the lots of solvent vapor. The SM with good gas permeability showed relatively strong RIPs of air and moisture, whereas the CP with no gas permeability showed weak ones. Detection times and efficiencies of PAH ions were varied according to the smear matrix types as well as the kinds of PAHs. PAHs were on the whole detected well in 1-3 s after the sample inlet. Detection limits of PAHs measured using the SM were slightly better than those measured using the CP, while those measured using the CP were much better than those measured using the CF. The experimental results could be explained by structures of the smear matrices and evaporation behaviors of the PAH solutions.