Effective mass of the 2-dimensional electron gas in an Al0.6Ga0.4Sb/InAs single quantum well

The 2-dimensional electron gas (2DEG) in an Al_0.6Ga_0.4Sb/InAs single quantum well is studied using cyclotron resonance and Shubnikov - de Haas (SdH) techniques. The effective mass (m^*) of the 2DEG was obtained from the peak positions of the cyclotron resonance transmission spectra. The results exhibit oscillatory behavior as a function of the magnetic field strength (B). The m^* value extracted from the temperature dependence of the SdH oscillations is in good agreement with the average value of m^* obtained from cyclotron resonance measurements. The effective mass is calculated as a function of B using an electron self-energy model based on the Hartree-Fock approximation. The calculated m^* values also show oscillatory behavior similar to that of the measured cyclotron resonance m^*. Both experiment and theory show that m^* maxima are shifted from the integral values (both odd and even) of the filling factors.     

Immunoaffinity extraction in veterinary residue analysis--a regulatory viewpoint.

The utility of immunoaffinity chromatography (IAC) for the purification of veterinary drug residues from extracts of foods of animal origin is widely acknowledged. However owing to the unique nature of the specific antibody reagent used when synthesising an immunosorbent, certain reassurances are needed before an IAC process can be considered an acceptable stage in a routine analytical method. European Community legislation requires applicants for Maximum Residue Limits or national Marketing Authorization for veterinary medicinal products, used in food producing animals, to provide a routine analytical method in accordance with Volume VI of the Rules Governing Medicinal Products in the European Community. In order to ensure the continued supply of immunosorbent material, applicants wishing to use IAC sample clean-up procedures should consider generating the following data: a standard operating procedure (SOP) for antibody production (or cell-line maintenance in the case of monoclonal antibodies), a specifications document of antibody characterisation, an SOP for immunosorbent synthesis, validation of immunosorbent operating conditions and an assessment of its performance criteria. This paper aims to introduce analysts to the technique of IAC and to suggest how those areas of concern highlighted above may be addressed.     

The microwave spectrum of urea

The microwave spectrum of urea has been recorded between 5 and 50 GHz. The rotational constants obtained from a least-squares fit of 47 b-type transitions are A = 11233.333, B = 10369.369, C = 5416.668 MHz. Centrifugal distortion constants were evaluated subject to the planarity constraints, giving satisfactory agreement up to at least J = 20. They are: τ_aaaa = ?0.04423, τ_bbbb = ?0.04075, τ_aabb = 0.02268, τ_abab = ?0.02806. The diagonal elements of the nuclear quadrupole coupling tensor are χ_aa = 2.16, χ_bb = 1.88, χ_cc = ?4.04. The dipole moment is μ = μ_b = 3.83 D (12.8 × 10^?30 Cm).     

Analysis of azanaphthalenes and their enzyme oxidation products by high-performance liquid chromatography, infrared spectroscopy and mass spectrometry

Adsorption and reversed-phase high-performance liquid chromatography (HPLC) have been successfully used to separate metabolites from the parent heterocycles (isoquinoline, 3-methylisoquinoline, phthalazine, quinazoline, quinoxaline and cinnoline). Retention data are reported. The metabolites, hydroxyazanaphthalenes, which arise as a result of aldehyde oxidase catalysed oxidation, could be extracted in microgram quantities from incubation mixtures by n-butanol. Complete identification of the oxidation products was achieved by collecting fractions corresponding to each compound from the HPLC eluate and subjecting these to infrared and mass-spectral analysis.     

The reactions of alkylamino substituted phosphines with I2 and (Ph2Se2I2)2: structural features of alkylamino phosphonium cations

The reactions of the tris-dialkylamino phosphines (Et2N)3P and (nPr2N)3P, and the pyrrolidinyl substituted phosphines (C4H8N)3P and tBuP(NC4H8)2, with I2 and (Ph2Se2I2)2, have been reported. The reactions with diiodine lead to the formation of [R3PI]I adducts, which are essentially ionic, but show a tendency to display long, soft-soft, II interactions in the solid state. The crystal structures of [(Et2N)3PI]I, (1), [(nPr2N)3PI]I, (2), and [(C4H8N)3PI]I, (3), have all been determined, and display II interactions varying between 3.5170(6) and 3.6389(14) A. The analogous reactions with (Ph2Se2I2)2 lead to the formation of phenylseleno-phosphonium salts, [R3PSePh]I. The structures of [(C4H8N)3PSePh]I, (6) and [(C4H8N)2tBuPSePh] I, (7), have been determined and do not display any soft-soft interactions between the selenium and iodine atoms. All of the phosphonium salts represent examples of structures containing tris-dialkylamino phosphine fragments which show no special nitrogen atom, i.e. all three nitrogen atoms are planar. This type of arrangement is usually observed when a C3 symmetric conformation is observed, (which is the case for 1 and 2), but not for the (C4H8N)3P adducts 3 and 6, where the conformation is closer to Cs, although the nitrogen atoms are still essentially planar. The P-N bonds in all the compounds reported herein are short, ranging between 1.599(12) A and 1.643(12) A, and are consistent with the previously reported short P-N bonds in phosphonium salts featuring tris-dialkylamino substituted phosphines.     

Properties of confined and sheared rhodamine B films studied by SFA-FECO spectroscopy

We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area. We also observed crystallization (solidification) parallel to the shearing direction.     

Visualizing supersonic inlet duct unstart using planar laser Rayleigh scattering

Planar laser Rayleigh scattering (PLRS) from condensed CO₂ particles is used to visualize flow structure in a Mach 5 wind tunnel undergoing unstart. Detailed flow features such as laminar/turbulent boundary layers and shockwaves are readily illustrated by the technique. A downstream transverse air jet, inducing flow unchoking downstream of the jet, is injected into the free stream flow of the tunnel, resulting in tunnel unstart. Time sequential PLRS images reveal that the boundary layer growth/separation on a surface with a thick turbulent boundary layer, initiated by the jet injection, propagates upstream and produces an oblique unstart shock. The tunnel unstarts upon the arrival of the shock at the inlet. In contrast, earlier flow separation on the opposite surface, initially supporting a thin laminar boundary layer, is observed when a jet induced bow shock strikes that surface. The resulting disturbance to this boundary layer also propagates upstream and precedes the formation of an unstart shock.     

The preparation of (-)-grandisine B from (+)-grandisine D; a biomimetic total synthesis or formation of an isolation artefact?

An efficient new alkyne-acetal cyclization procedure has been developed to prepare enantiopure indolizidine building blocks from l-proline and then applied to prepare the Elaeocarpus-derived alkaloids grandisine B and grandisine D in an efficient manner. However, evidence is presented which indicates that grandisine B does not occur naturally but is formed by reaction of grandisine D with ammonia during the extraction/purification process.     

Fourier transform microwave and millimeter wave spectroscopy of quinazoline, quinoxaline, and phthalazine

The pure rotational spectra of the bicyclic aromatic nitrogen heterocycle molecules, quinazoline, quinoxaline, and phthalazine, have been recorded and assigned in the region 13-87 GHz. An analysis, guided by ab initio molecular orbital predictions, of frequency-scanned Stark modulated, jet-cooled millimeter wave absorption spectra (48-87 GHz) yielded a preliminary set of rotational and centrifugal distortion constants. Subsequent spectral analysis at higher resolution was carried out with Fourier transform microwave (FT-MW) spectroscopy (13-18 GHz) of a supersonic rotationally cold molecular beam. The high spectral resolution of the FT-MW instrument provided an improved set of rotational and centrifugal distortion constants together with nitrogen quadrupole coupling constants for all three species. Density functional theory calculations at the B3LYP∕6-311+G?? level of theory closely predict rotational constants and are useful in predicting quadrupole coupling constants and dipole moments for such species.