Preparative gas chromatography with glass capillary columns

The present paper describes an automated system for preparative gas chromatography with glass capillary columns, controlled by a microprocessor. The effluent from the capillary column is divided by a pneumatically controlled splitter and any desired split ratio between traps and detector can be obtained. Moreover, a second pneumatic control allows instantaneous change-over to a different split ratio, thus minimizing loss of material during collection. The effluent containing the compounds of interest is passed through a multiple manifold and collected in coiled glass capillary traps. To ensure maximum trapping efficiency even for very small amounts of material, the inner walls of the capillary traps are wetted with a suitable solvent, which gives a quantitative recovery of micro- and nanogram amounts of material. After repetitive sampling, sufficient amounts of material can be obtained for NMR spectroscopy and possibilities exist to enrich trace components with the aid of a double column system. Two examples of such applications are given, employing mixtures of both synthetic and natural origin.     

Comparison of the hole mobility in undoped and boron-doped polycrystalline CVD diamond films

The drift mobility of nonequilibrium holes injected in undoped polycrystalline diamond films was determined, by a transit-time technique, as ca. 10⁻³ cm²/(V s). This hole mobility is three orders of magnitude lower than the “equilibrium” mobility in boron-doped diamond films [0.1–1 cm²/(V s)], determined from the films' dc conductivity. This difference is explained by the effect of a nonequilibrium charge carrier trapping during the carrier transport in polycrystalline diamond. Received: 3 December 1997 / Accepted: 9 April 1998     

The composition and architecture of hydrated hydroxy anions as observed in the crystalline state – A review

Search of the Cambridge Structural Database reveals that hydrated hydroxide anions can be quite complex in their stereochemistry. As in the case of hydronium cations [I. Bernal. C.R. Chim. 9 (2006) 1454], they can be acyclic as well as cyclic. Moreover, it is documented here that hydrated hydroxide anions exist in the form of geometrical isomers or conformers. A simple example occurs in species of composition H₅O₃⁻ which exist in isomeric forms in which their stereochemistry depends on whether the OH⁻ is the hydrogen donor to waters or vice versa. All illustrations in this survey were generated with DIAMOND.     

Reaction,trapping, and multifractality in one-dimensional systems

In the first part of this paper, we present two variants of the A+AA and A+AP reaction in one dimension that can be investigated analytically. In the first model, pairs of neighboring particles disappear reactively at a rate which is independent of their relative distance. It is shown that the probability density (x) for a nearest neighbor distance equal tox approaches the scaling form(x) c exp(–cx/2)/(cx)^1/2 in the long-time limit, withc being the concentration of particles. The second model is a ballistic analogue of the coagulation reaction A+A A. The model is solved by reducing it to a first-passagetime problem. The anomalous relaxation dynamics can be linked in a direct way to the fractal time properties of random walks. In the second part of this paper, we discuss the complications that arise in systems with disorder. We present a new approach that relates first-passage-time characteristics in a one-dimensional random walk to properties of random maps. In particular, we show that Sinai disorder is a borderline case for the appearance of multifractal properties. Finally, we apply a previously introduced renormalization technique to calculate the survival probability of particles moving on the line in the presence of a background of imperfect traps.     

Effect of γ irradiation on optical properties of Ce-doped phosphate and silicate scintillating glasses

A set of Ce³⁺ activated silicate- and phosphate-based scintillating glasses were submitted to γ irradiation in the ⁶⁰Co radioisotope source “Calliope” (ENEA-Casaccia in Rome, Italy) in the dose range between 1 and 250 Gy (3.7 Gy/h). The effect of ionising radiation was probed by transmission measurements performed before and after each irradiation on all analysed samples. From these data, the radiation-induced absorption coefficient was calculated, proportional to the density of colour centres induced by irradiation in the solid matrix. Results are discussed by taking into account the possible dependence of radiation hardness on the composition of glass matrix.     

The synthetic versatility of alkoxycarbonyl- and hydroxymethyl-piperazine-2,5-diones

Alkoxycarbonylpiperazine-2,5-diones are versatile precursors for the α-functionalisation of piperazine-2,5-diones. The alkoxycarbonyl group activates the α-carbon position to alkylation reactions and this provides a mild and selective method for the extension of the carbon framework of piperazine-2,5-diones. In addition, the alkoxycarbonyl group can be converted to the carboxy group, which in turn can be ‘deleted’ or manipulated for the installation of carbon and/or heteroatom substituents where desired, the latter via N-acyliminium chemistry. We also demonstrate that hydroxymethylpiperazine-2,5-diones complement carboxypiperazinediones as precursors for the generation of N-acyliminium ions.     

Investigations into the chromatographic behaviour of silica gels produced from ester silicates

Summary Silica gels produced from ester silicates (ES-gels) are excellent chromatographic supports. In comparison with other silica gels the RP-materials obtained from them show little peak tailing even with polar, and, in particular, with basic compounds. Gels produced by various manufacturing processes have been classified by adsorption with methyl pyridinium chloride. ES-gels yield very low methyl pyridinium chloride values and small asymmetry parameters. The results indicate that there are strongly acidic, structurally-related surface centres which cause peak tailing on most commercial gels. It was shown that surface silanols on silica gels do not, in themselves, lead to peak tailing.     

A trapping column for the coupling of reversed-phase liquid chromatography and capillary gas chromatography

A new technique for coupling reversed-phase liquid chromatography (RPLC) with gas chromatography is described. A fraction eluting from an RPLC column is trapped on a short column packed with polymeric adsorbent. After the mobile phase has been displaced with water, the analytes are desorbed with ethyl acetate. Following a delay time to enable the water to be flushed to waste, the ethyl acetate containing the analytes is introduced into the gas chromatograph under conditions suitable for partially concurrent solvent evaporation, i.e. below the solvent boiling point and at a rate just exceeding the evaporation rate. Post-column addition of water to the RPLC eluent helps to prevent breakthrough of compounds which are only modestly retained on the trapping column. The relationship between the capacity factors of the analytes on the trapping column and the required dilution factor is discussed. Polycyclic aromatic hydrocarbons are used as test compounds to study the system.     

Electrodeless dielectrophoresis: Impact of geometry and material on obstacle polarization

Insulator‐based (electrodeless) dielectrophoresis (iDEP) is a promising particle manipulation technique, based on movement of matter in inhomogeneous fields. The inhomogeneity of the field arises because the excitatory field distorts at obstacles (posts). This effect is caused by accumulation of polarization charges at material interfaces. In this study, we utilize a multipole expansion method to investigate the influence of geometry and material on field distortion of posts with arbitrary cross‐sections in homogeneous electric fields applied perpendicular to the longitudinal axis of the post. The post then develops a multipole parallel or anti parallel to the excitatory field. The multipoles intensity is defined by the post's structure and material properties and directly influences the DEP particle trapping potential. We analyzed posts with circular and rhombus‐shaped cross‐sections with different cross‐sectional width‐to‐height ratios and permittivities for their polarization intensity, multipole position, and their particle trapping behavior. A trade‐off between high maximum field gradient and high coverage range of the gradient is presented, which is determined by the sharpness of the post's edges. We contribute to the overall understanding of the post polarization mechanism and expect that the results presented will help optimizing the structure of microchannels with arrays of posts for electrodeless DEP application.     

Diffusion measurement of implanted Sb into Si,using SiO2 encapsulation

Large, asymmetric atomic relaxations have been shown to play a crucial role in the structure and properties of several point defects in oxide materials. Examples include trapped hole centers in alkaline-earth oxides and E1′ and E4′ oxygen-vacancy centers and peroxy-radical defects in silicon dioxide. Schirmer's “bound small polaron” model, applied in particular to the alkaline-earth oxide defects, and model treatments of the E1′ center in SiO2 by Yip, Griscom and Fowler clearly illustrate the important spectroscopic consequences of such atomic relaxations. In fact, such effects had been incorporated in Lüty's classic model of the Type II FA center in alkali halides. Edwards and Fowler have recently applied MNDO and MINDO/3 quantum-chemistry approaches to the E1′, E4′, and peroxy radical defects in SiO2. These calculations generally corroborate suggested models and bear as well on possible creation mechanisms. Large relaxation effects are likely to be important in many other defects in oxide materials.