Atomic Force Microscopy of Novel Zeolitic Materials Prepared by Top‐Down Synthesis and ADOR Mechanism

Top‐down synthesis of 2D materials from a parent 3D zeolite with subsequent post‐synthetic modification is an interesting method for synthesis of new materials. Assembly, disassembly, organisation, reassembly (ADOR) processes towards novel materials based on the zeolite UTL are now established. Herein, we present the first study of these materials by atomic force microscopy (AFM). AFM was used to monitor the ADOR process through observation of the changes in crystal surface and step height of the products. UTL surfaces were generally complex and contained grain boundaries and low‐angle intergrowths, in addition to regular terraces. Hydrolysis of UTL to IPC‐1P did not have adverse effects on the surfaces as compared to UTL. The layers remained intact after intercalation and calcination forming novel materials IPC‐2 and IPC‐4. Measured step heights gave good correlation with the X‐ray diffraction determined d₂₀₀‐spacing in these materials. However, swelling gave rise to significant changes to the surface topography, with significantly less regular terrace shapes. The pillared material yielded the roughest surface with ill‐defined surface features. The results support a mechanism for the majority of these materials in which the UTL layers remain intact during the ADOR process as opposed to dissolving and recrystallising during each step.     

Determination of N-nitroso derivatives of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in soils by pressurized liquid extraction and liquid chromatography-electrospray ionization mass spectrometry

To aid in the evaluation of the potential toxicity of N-nitroso derivatives of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), we describe a pressurized liquid extraction (PLE) followed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) method for determination of RDX and its N-nitroso derivatives: hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) in soils. Sandy loam soil was spiked with RDX and its N-nitroso derivatives (MNX, DNX, and TNX). Acetonitrile was used as the PLE extraction solvent at 100 degrees C and 1500 psi for 15 min. Florisil was used to cleanup extracts following PLE. Instrumental analysis employed LC-ESI-MS, in which 1mM acetic acid was added to the mobile phase to facilitate formation of acetate adduct ions [M+CH(3)COO](-). The method detection limits (MDLs) for RDX, MNX, DNX, and TNX were 1.46, 1.46, 1.69, and 1.93 ng/g, respectively. High recovery (91.1-108.3%), good precision (RSD: 3.2-12.4%), and reproducibility were achieved. This method proved effective and was applied to monitor the reductive biotransformation of MNX in soils with the presence of earthworms (Eisenia fetida).     

Direct observation of defect levels in InN by soft X-ray absorption spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.     

Separation and preconcentration procedures for the determination of lead using spectrometric techniques: a review

Lead is recognized worldwide as a poisonous metal. Thus, the determination of this element is often required in environmental, biological, food and geological samples. However, these analyses are difficult because such samples contain relatively low concentrations of lead, which fall below the detection limit of conventional analytical techniques such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry. Several preconcentration procedures to determine lead have therefore been devised, involving separation techniques such as liquid-liquid extraction, solid phase extraction, coprecipitation and cloud point extraction. Citing 160 references, this paper offers a critical review of preconcentration procedures for determining lead using spectroanalytical techniques.     

Determination of As, Ge, Hg, Pb, Sb, Se and Sn in coal slurries by CVG-ETV-ICP-MS using external or isotopic dilution calibration

A method for the multi-elemental determination of As, Ge, Hg, Pb, Sb, Se and Sn in coal reference materials by slurry sampling chemical vapor generation (CVG) using external calibration and isotopic dilution (ID) calibration and detection by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) is proposed. As, Ge, Sb, Se and Sn were determined using the external calibration, while, Hg, Pb, Se and Sn were determined by isotopic dilution. About 50–250 mg of sample was mixed with an acid solution, containing aqua regia and HCl, in an ultrasonic bath. For the isotopic dilution calibration, the enriched isotopes ²⁰¹Hg, ²⁰⁶Pb, ⁷⁷Se and ¹¹⁹Sn were added to the slurry in an adequate amount in order to produce an altered isotopic ratio close to 1. The vapor produced by the reaction of the sample slurry with the reducing agent was transported to the vaporizer and trapped in a Ir-treated graphite tube at 200 °C, before vaporization at 2100 °C and transportation of the vapor to the plasma. The accuracy of the method was assured by the analysis of four certified reference coal samples, using external calibration with aqueous solutions, prepared in the same medium and subjected to the same CVG and trapping procedure as the slurries and also by isotopic dilution calibration. The obtained concentrations were in agreement with the certified values, using the t-Student test for a confidence level of 95%. The detection limits (3 s; n = 5) of isotopic dilution, in ng g^− 1, were: 0.4 for Hg, 900 for Pb, 0.3 for Se and 0.2 for Sn. For external calibration, the detection limits, in ng g^− 1, were: 1.6 for As, 0.1 for Ge, 0.3 for Sb, 0.9 for Se and 7.5 for Sn. The relative standard deviations generally were lower than 14%, adequate for slurry analysis.     

Challenges in determining perchlorate in biological tissues and fluids: implications for characterizing perchlorate exposure

The ability to measure environmental contaminants in biological tissues and fluids is important in the characterization of exposure. However, the analysis of certain contaminants in these matrices presents significant challenges. Perchlorate (ClO₄⁻) has emerged as a potential contaminant of concern primarily in drinking water and also in contaminated food. Significant advances have been made in the analysis of perchlorate in environmental matrices (water, soil) by ion chromatography (IC). In contrast, the analysis of perchlorate in extracts of biological tissues and fluids (vegetation, organs, milk, blood, urine, etc.) presents several challenges including small sample sizes, extracts with high matrix conductivity, and co-elution of other ions during IC analysis. To be able to detect low concentrations of perchlorate in biological samples, interferences must be removed or minimized, such as through the use of preparative chromatography cleanup techniques and/or alternative analytical methods less susceptible to common interferences (preconcentration or mass spectrometric detection). We present discussion and examples of the challenges encountered in the analysis of tissue extracts and fluids for perchlorate by IC and how some of those analytical challenges have been overcome.     

Antimicrobial peptide-based array for Escherichia coli and Salmonella screening

A method based on solid-phase microextraction (SPME) followed by gas chromatography with microwave-induced plasma atomic emission detection for determining 16 pesticides of different chemical families (organochlorines, organophosphorus compounds and pyrethrins) in honey is proposed. Parameters affecting the sample enrichment step, such as sample mass, ionic strength, absorption and desorption times and temperatures, were carefully optimized in the direct immersion mode. Element-specific detection and quantification was carried out by monitoring the chlorine (479 nm), bromine (478 nm) and sulphur (181 nm) emission lines, which provided nearly specific chromatograms. The matrix effect was evaluated for samples of different floral origin, it being concluded that standard addition calibration was required for quantification purposes. The detection limits ranged from 0.02 to 10 ng g⁻¹, depending on the compound and the honey sample under analysis. The method is reliable and can be considered useful for routine monitoring. None of the honey samples analyzed contained the studied compounds at concentrations above the corresponding detection limits.     

Separation of racemic sulfoxides and sulfinate esters on four derivatized cyclodextrin chiral stationary phases using capillary gas chromatography

The separation of 17 chiral sulfoxides and eight chiral sulfinate esters by gas chromatography (GC) on four derivatized cyclodextrin chiral stationary phases (CSPs) (Chiraldex G-TA, G-BP, G-PN, B-DM) is presented. Many of these compounds are structural isomers or part of a homologous series. Differences in enantioselectivity of the methyl phenyl sulfoxide isomers on the derivatized gamma cyclodextrin and the heptakis 2,6-di-O-methyl-beta-cyclodextrin (i.e. B-DM) CSPs are discussed. Under the conditions of this study, the molecular mass cut-off for the GC separation of these compounds was approximately 230. Compounds of higher molecular mass were not eluted from the CSPs at reasonable times and temperatures, but these higher molecular mass enantiomers can be separated by liquid chromatography and capillary electrophoresis. The enantiomeric separation and elution order of a sulfinate ester containing two stereogenic centers as well as 15 chiral sulfoxides is presented. The G-TA and B-DM CSPs generally gave opposite elution orders for most of the compounds studied.     

Formation of isoprostane bicyclic endoperoxides from the autoxidation of cholesteryl arachidonate

Autoxidation of polyunsaturated fatty acids and esters leads to a complex mixture containing hydroperoxides and cyclic peroxides. Prostaglandin bicyclic endoperoxides have been detected from the autoxidation of cholesteryl arachidonate by LC-MS and GC-MS techniques. All four possible types (I-IV) of bicyclic endoperoxides have been found starting from different regioisomeric hydroperoxides of cholesteryl arachidonate. Furthermore, the stereochemistry of Type IV bicyclic endoperoxides has been determined by conversion to pentafluorobenzyl (PFB) ester, trimethylsilyl (TMS) derivatives, and comparison with synthetic standards by the use of GC-MS. All eight possible diastereomers of the derivatized isoprostanes were observed and were separated by gas chromatography. The bicyclic endoperoxides with the two alkyl chains syn on the cyclopentane ring were formed preferentially to those with anti configuration, a result anticipated from earlier work. Substantial amounts of the anti-substituted isoprostanes, including PGF(2)(alpha), were, however, observed in the product mixture.