Yasuaki Arakawa

High-performance liquid chromatographic determination of dialkyltin homologues using fluorescence detection

Abstract A simple, sensitive and reproducible method has been developed for the simultaneous determination of dialkyltin compounds and their possible metabolites. Dialkyltin compounds were separated by high-performance liquid chromatography on a cyanopropyl-bonded column with n -hexane-ethyl acetate (95:5) containing 5% acetic acid and detected by a fluorescence detector with an excitation wavelength at ca . 420 nm and an emission wavelength at ca . 500 nm after post-column derivatization with Morin reagent. Detection limits ranged from 0. 1 to 1 ng depending upon the dialkyltin species. Recoveries of dialkyltins added to various tissues at the 5-nmole level ranged from 91 to 99%.

Simultaneous determination of trialkyltin homologues in biological materials

Taking advantage of the high sensitivity of an electron capture detector to alkyltin halides, an analytical method has been developed for the simultaneous determination of trialkyltin homologues in biological materials. Trialkyltins were purified as chlorides from tissues by simultaneous extraction with hydrochloric acid and ethyl acetate, replacement of the extraction solution with n-hexane and stepwise elution with n-hexane-ethyl acetate on a silica gel column. Alternatively, gas chromatographic analysis was carried out on 20% DEGS-HG at temperatures from 100 to 120 degrees C. Detection limits reached 1 x 10(-12) g for trialkyltin chlorides. The recoveries of trialkyltins added to various tissues at the 50-pmole level ranged from 97 to 106%. By in vivo studies, it was confirmed that this method is rapid, sensitive and applicable to biomaterials containing more than 1 ng trialkyltins per gram of tissue.

Inhibition of neutrophil chemotaxis by organotin compounds

Organotin compounds such as Bu2SnCl2, Bu3SnCl and Ph3SnCl suppressed significantly not only chemotactic response of neutrophils to stimulation by the chemoattractant fMet-Leu-Phe but also phospholipase activity in situ as measured by the release of [1-14C] arachidonic acid previously incorporated into phospholipids. Moreover, these suppressions were dose dependent and a parallelism was found between dose-dependent inhibition of chemotaxis and that of arachidonate release. These results suggest that the chemotactic response is controlled by the activation of phospholipase activity in neutrophils, and that the inhibitory effects of these organotin compounds on chemotactic response reflect the blockage of phospholipase activation system regulated by phosphorylation of lipomodulin.

Dealkylation and distribution of tin compounds

By using the gas chromatographic methods (Y. Arakawa et al., 1981, J. Chromatogr.), the present study was undertaken to determine whether the extent of the conversion of tetraalkyltins into trialkyltins in rabbits might be related to the length of their alkyl chain, and whether the signs of toxicity of tetra- and trialkyltins in rabbits might be associated with their distributions and accumulations in the tissues including the brain. Various tetraalkyltins (8.5 μmol) were administered iv to male rabbits and the quantity of tetra- and trialkyltins in organs was determined at varying intervals up to 3 hr after treatment. Tetraethyltin was quickly distributed to the liver whereas tetrapropyltin and tetrabutyltin were slowly distributed. Tetraethyltin was more readily converted into the corresponding trialkyltin than tetrapropyltin, which, however, was more readily converted into trialkyltin than tetrabutyltin. About 20% of the tetraethyltin, 4% of the tetrapropyltin, and 1% of the tetrabutyltin were converted to the corresponding trialkyltins, respectively, within 3 hr after iv treatment. Thus, the extent of formation of trialkyltins decreased as the size and stability of the ligand increased. The distribution of three tetraalkyltins to the brain relative to the other organs was poor. The amounts of trialkyltin metabolites found in the brain, however, showed a tendency to increased with time. Particularly, the transfer of triethyltin to the brain was significant and this finding was compatible with the appearance of signs of toxicity. These results show that the extent of the dealkylation and the toxicity of alkyltin compounds depend on the length of their alkyl group, which was associated with the rate of absorption and ultimate distribution of alkyltin compounds.

Magnetoresistance oscillations induced by intersubband scattering of two-dimensional electron gas in Al0.22Ga0.78N/GaN heterostructures

Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al0.22Ga0.78N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al0.22Ga0.78N/GaN heterostructures.

Optical spectroscopy of self-assembled type II GaSb/GaAs quantum dot structures grown by molecular beam epitaxy

We report an optical spectroscopic study of GaSb/GaAs quantum dots (QDs) formed by the Stranski–Krastanow growth mode using molecular beam epitaxy. We identify the QD luminescence by photoluminescence obtained at different excitation energies and densities. We show that, for these structures, not only the spectral position of peaks, but also their relative intensities are critically dependent upon the density of photogenerated carriers. Photoluminescence excitation (PLE) measurements confirm our assignment of the QD related peaks and a feature ∼25–27u2009meV higher in energy than the PLE detection energy is discussed in terms of phonon relaxation.

Photoluminescence studies of GaAs quantum wires with quantum confined Stark effect

We investigated the quantum confined Stark effect in GaAs quantum wires formed in a V-groove structure, demonstrating observation of a blueshift of the photoluminescence peak with the increase of electric fields at 50 K. This blueshift is attributed to the fact that the change in enhanced binding energy of excitons due to the electric field is larger than that in quantized energy levels of electrons and holes. Time-resolved photoluminescence was also measured. The photoluminescence decay time is decreased in small quantum wires of 8 nm width with the increase of electric fields, while the decay time is increased in the quantum wires with a size of 35 nm. These results indicate that the escaping of carriers is more dominant in smaller structures than reduction of the oscillator strength due to the electric fields.

Intestinal uptake site, enterohepatic circulation, and excretion of tetra‐and trialkyltin compounds in mammals

The intestinal uptake site, enterohepatic circulation, and excretion into bile, feces, and urine of alkyltins (tetra and trialkyltin) were investigated after oral, sc, or intestinal administration of the compounds to rats and rabbits. Assays of tetra- and trialkyltins in biological materials were carried out by gas chromatography. The main uptake sites in the small intestine were the jejunum and duodenum for tetraalkyltins and the ileum and jejunum for trialkyltins. Tetra- and trialkyltins were detected in the small intestine and contents of the intestinal lumen after sc injection of these compounds in rats. These facts suggest that tetra- and trialkyltins are transported in the body through enterohepatic circulation. The route, rate, and amount of excretion of tetra and trialkyltins seem to depend on the velocity of dealkylation, doses, physical and chemical properties, and route of administration of the compounds.

Two-photon absorption and multiphoton-induced photoluminescence of bulk GaN excited below the middle of the band gap

Optical nonlinearity in the yellow luminescence (YL) band of GaN was investigated using thick bulk samples. Transient pump–probe measurements revealed strong transmission changes due to two-photon absorption (TPA) even at the middle of the YL band. The TPA coefficient evaluated reaches ∼5u2009cm/GW at about 1.3 eV, which was as large as the mid-gap resonance. The TPA spectrum clearly showed that the observed large nonlinearity originated from the YL band. On the basis of efficient TPA in the YL band, relaxation processes in the multiphoton-induced photoluminescence excitation spectrum were also investigated.

Analysis of gain saturation in In0.02Ga0.98N/In0.16Ga0.84N multiple quantum wells

A way of analyzing the data in a variable stripe length method gain experiment is presented. We confirm that the stripe length dependence of the gain in In0.02Ga0.98N/In0.16Ga0.84N multiple quantum wells is caused by the change of the chemical potential along the excited stripe due to the interaction of the carrier and photon densities, and the gain threshold density is estimated. A trial function assuming a Lorentzian line shape for the stripe length dependence of the gain is compared with the edge emission intensity as a function of the stripe length. This is found to fit very well with our data, even beyond the saturation region.