Enhancing the Water Stability of Al‐MIL‐101‐NH2 via Postsynthetic Modification

The resistance of metal–organic frameworks towards water is a very critical issue concerning their practical use. Recently, it was shown for microporous MOFs that the water stability could be increased by introducing hydrophobic pendant groups. Here, we demonstrate a remarkable stabilisation of the mesoporous MOF Al‐MIL‐101‐NH₂ by postsynthetic modification with phenyl isocyanate. In this process 86 % of the amino groups were converted into phenylurea units. As a consequence, the long‐term stability of Al‐MIL‐101‐URPh in liquid water could be extended beyond a week. In water saturated atmospheres Al‐MIL‐101‐URPh decomposed at least 12‐times slower than the unfunctionalised analogue. To study the underlying processes both materials were characterised by Ar, N₂ and H₂O sorption measurements, powder X‐ray diffraction, thermogravimetric and chemical analysis as well as solid‐state NMR and IR spectroscopy. Postsynthetic modification decreased the BET equivalent surface area from 3363 to 1555 m² g^?1 for Al‐MIL‐101‐URPh and reduced the mean diameters of the mesopores by 0.6 nm without degrading the structure significantly and reducing thermal stability. In spite of similar water uptake capacities, the relative humidity‐dependent uptake of Al‐MIL‐101‐URPh is slowed and occurs at higher relative humidity values. In combination with ¹H‐²⁷Al D ‐HMQC NMR spectroscopy experiments this favours a shielding mechanism of the Al clusters by the pendant phenyl groups and rules out pore blocking.     

Inductively-coupled plasma/atomic emission spectrometry of sulfur with a vacuum scanning spectrometer and its application to the analysis of coal liquefaction catalysts

Inductively-coupled plasma/atomic emission spectrometry with a high-resolution vacuum scanning monochromator is described for the determination of sulfur at 180.734 nm. The behavior of the signal-to-background ratio is investigated as functions of RF power, argon gas flow rate and observation height above the load coil. Under the operating conditions selected, the detection limit is 3 μg l^?1. The Se I 196.090-nm line is chosen as internal standard, because the S/Se line pair exhibited the least change with carrier gas flow rate and acid concentration of solution. Sulfur in NiMo and CoMo/ Al₂O₃ catalysts used for coal liquefaction is determined as S(II) and S(VI) species. The total amount of the species agreed well with the sulfur value obtained by the conventinal combustion method.     

Separation strategy for acidic chiral pharmaceuticals with capillary electrochromatography on polysaccharide stationary phases

The effect of five factors on the capillary electrochromatographic enantioseparation of acidic compounds was studied using an experimental design. The studied factors were pH, acetonitrile content in the mobile phase, temperature, buffer concentration, and applied voltage. These experiments allowed defining a generic separation strategy applicable on acidic compounds with chemical and structural diversity. The starting screening conditions consist of a 45 mM ammonium formate electrolyte at pH 2.9 mixed with 65% acetonitrile, an applied voltage of 15 kV, and a temperature of 25 degrees C. The screening phase occasionally can be followed by an optimization procedure. Evaluation of the proposed strategy pointed out that it allows achieving baseline resolution within a relatively short time when a beginning of separation is obtained at the starting conditions. This strategy revealed enantioselectivity for 11 compounds out of 15, of which 10 could be baseline-separated after the proposed optimization steps.     

Simplex optimisation of conditions for the determination of antimony in environmental samples by using electrothermal atomic absorption spectrometry

Analysis of the total antimony in plant material was unsuccessful using the electrothermal atomic absorption spectrometry (ETAAS) conditions recommended by the instrument manufacturer. For this reason, an optimisation procedure utilising the Plackett-Burman method, simplex optimisation and visualisation of the generated response surface via principal components analysis, was carried out. The Plackett-Burman method was used to eliminate four of the initial variables chosen. Four variables (atomisation temperature, atomisation time, ash temperature and modifier concentration) were subsequently optimised using the composite modified simplex method and the results were visualised as a contour diagram, after reduction to two principal components. The optimised conditions were used for the analysis of both an acid digested pine needle standard reference material (NIST 1575) and a pond weed sample, collected from a contaminated site at Yellowknife Bay, Yellowknife, NWT, Canada. The total concentration of antimony present in the pine needles was statistically indistinguishable from the non-certified value, as was the value for the pond weed sample, compared with a value determined by neutron activation analysis (NAA). The results for the analysis of the pond weed sample by ETAAS agreed with those obtained from a subsequent analysis by inductively coupled plasma-mass spectrometry.     

Synthesis and characterization of mixed-morphology CePO4 nanoparticles

Cerium phosphate nanoparticles with diameters of 10-180 nm were synthesized by a variety of solution techniques. X-ray diffraction (XRD) determined the crystalline phase(s) present in each sample. Population, shift, and spin-lattice relaxation ³¹P solid-state nuclear magnetic resonance (NMR) measurements accounted for all the ³¹P nuclei expected in each sample, and were able to distinguish between phosphorous nuclei in different environments and phases. Transmission electron microscopy (TEM) characterized the morphology and crystallinity of the powder samples as well as of the sintered compacts of the powders. In conjunction with TEM, energy-dispersive spectroscopy (EDS) provided a measure of the composition of the bulk intergranular regions within each CePO₄ sample. The presence of an amorphous, phosphate-rich intergranular phase was found in those samples prepared by dissolution of ceria in H₃PO₄ under various conditions.     

In situ extraction/preconcentration of PCBs and PAHs from aqueous samples using polytetrafluoroethylene tubing

In situ extraction/preconcentration of organics from water samples was accomplished using polytetrafluoroethylene (PTFE) tubing. Polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were concentrated inside the tubing by flowing aqueous samples through it. The adsorbed PCBs and PAHs were then recovered by solvent desorption. The eluent was subsequently analyzed using gas chromatography with electron capture detection (GC-ECD) or gas chromatography with flame ionization detection (GC-FID). Multiple samples were simultaneously processed to concentrate organics onto several PTFE tubings. Analyses of seawater and surface microlayers using this technique demonstrated that organics in the surface microlayers were elevated with respect to those in the water column, consistent with previous findings.     

Photoelectric evidence for a neutral excited state in fully polymerized polydiacetylene-p-(toluenesulfonate) single crystals

The intensity dependence of photocurrent transients excited by the 1.06 μ line of a Nd laser has been measured. Observation of an I² and an I³ branch indicate that charge carriers can be generated via two-quantum absorption and that at high intensities photoionization of a neutral excited state with a lifetime of the order 10^?10 s dominates.     

Determination of urinary arsenic and impact of dietary arsenic intake

An analytical method based on microwave decomposition and flow injection analysis (FIA) coupled to hydride generation atomic absorption spectrometry (HGAAS) is described. This is used to differentiate arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) from organoarsenic compounds usually present in seafood. Without microwave digestion, direct analysis of urine by HGAAS gives the total concentration of As(III), As(V), MMA and DMA because organoarsenic compounds such as arsenobetaine, usually found in most seafood, are not reducible upon treatment with borohydride and therefore cannot be determined by using the hydride generation technique. The microwave oven digestion procedure with potassium persulfate and sodium hydroxide as decomposition reagents completely decomposes all arsenicals to arsenate and this can be measured by HGASS. Microwave decomposition parameters were studied to achieve efficient decomposition and quantitative recovery of arsenobetaine spiked into urine samples. The method is applied to the determination of urinary arsenic and is useful for the assessment of occupational exposure to arsenic without intereference from excess organoarsenicals due to the consumption of seafood. Analysis of urine samples collected from an individual who ingested some seafood revealed that organoarsenicals were rapidly excreted in urine. After the ingestion of a 500-g crab, a 10-fold increase of total urinary arsenic was observed, due to the excretion of organoarsenicals. The maximum arsenic concentration was found in the urine samples collected approximately between 4 to 17 hr after eating seafood. However, the ingestion of organoarsenic-containing seafoods such as crab, shrimp and salmon showed no effect on the urinary excretion of inorganic arsenic, MMA and DMA.     

Charge carrier governed temperature dependence of the electric field gradient for111Cd in tellurium

Using the TDPAC-method with the proble nucleus¹¹¹Cd the electric quadrupole interaction (QI) in the trigonal semiconductor Te has been investigated in the temperature range 77 KT655 K. In contrast to most metals the quadrupole frequencyv _Q increases with increasing temperature. This is a consequence of the fact that in metals the charge carrier density is temperature independent, whereas in semiconductors it strongly increases with temperature. A comparison between the temperature variation of the quadrupole coupling constantv _Q and that of the charge carrier density leads to the conclusion that the QI in Te is governed by changes in the free electron density.