Headspace single-drop microextration for the detection of organotin compounds

The performance of single-drop microextraction (SDME), coupled with gas chromatography/mass spectrometry, was assessed for the determination of tributyltin compounds in water and solid samples. Experimental parameters impacting the performance of SDME, such as microextraction solvent and sampling and stirring time, were investigated. Analytical results obtained by SDME were compared with those generated by conventional solid phase microextraction (SPME) and liquid-liquid extraction (LLE) for the determination of TBT in PACS-2 sediment certified reference material (CRM).     

3-Deaza-2′-deoxyadenosine: Synthesis via 4-(methylthio)-1H-imidazo[4,5-c]pyridine 2′-deoxyribonucleosides and properties of oligonucleotides

The synthesis of 4-(methylthio)-1H-imidazo[4,5-c]pyridine 2′-deoxy-β-D -ribonucleosides 2 and 9 and the conversion of the N¹-isomer 2 into the 2′,3′-didehydro-2′,3′-dideoxyribonucleoside 3a or (via 7 ) 3-deaza-2′-deoxyadenosine ( 1 ) is described. Phosphonate building blocks of 1 were employed in solid-phase synthesis of self-complementary base-modified oligonucleotides. Their properties were studied with regard to duplex stability and hydrolysis by the restriction enzyme Eco RI.     

Electron spin resonance g tensors for complexes of Ne and Ar with AlO: theoretical studies related to the large matrix effect observed for AlO

For Ne(n)-AlO (n=2, 4, 6, 8, 10) and Ar(n)-AlO clusters (n=2, 4, 6, 8), the perpendicular (relative to AlO) component of the g tensor was calculated by second-order perturbation theory, using multireference configuration-interaction wave functions. The rare-gas (Rg) atoms were placed axially and/or off axially (one or two rings of four Rg atoms each), and the distance of the Rg atoms from the Al and O atoms, or from the AlO axis, was varied from 4 to 12 bohrs. Rg atoms placed axially mostly increase g(perpendicular), whereas off-axially placed ones lower it below the gas-phase value of AlO. The largest deviations from g(perpendicular) of isolated AlO occur at Ne-Al,O distances of 5-6 bohrs, and Ar-Al,O distances of 6-9 bohrs, with maximal lowerings of about 1600 ppm for Ne and about 2200 ppm (estimated) for Ar in the case of two axial and eight off-axial Rg atoms. Electron spin resonance studies by Knight and Weltner found large matrix effects for AlO, with downshifts of g(perpendicular) observed to be about 450 and 1150 ppm in Ne and Ar matrices, respectively.     

Ab initio calculations of electron-spin magnetic moments for Li,Be and B hydrides inX 2∑+ states

Electron-spin magnetic moments, in the form ofg-shifts, have been computed at the ROHF level for theX ²∑⁺ states of LiH⁺, BeH²⁺, LiH^?, BeH and BH⁺. A perturbative approach, complete to second-order in appropriate Breit-Pauli operators, has been used. Retention of two-centre integrals has proven vital. First-order terms are important, especially in describing the negativeg _∥ shifts observed experimentally in²∑⁺ molecules. The relativistic mass correction dominates in first-order, except for LiH^? where the two-electron spin-Zeeman gauge correction supersedes. Second-order terms contribute negatively, and only to the Δg _⊥ component. Along the isoelectronic series LiH^? → BeH → BH⁺, the magnitude of Δg _⊥ increases due to the dependence of spin-orbit coupling on nuclear charge. The relation ofg-shifts to electronic structure and bonding is explored.     

Quantum-limited noise performance of a mode-locked laser diode

Low-noise operation of a 9-GHz hybridly mode-locked laser diode is demonstrated. The integrated timing jitter was 47 fs (10 Hz to 10 MHz) and 86 fs (10 Hz to 4.5 GHz), with a pulse width of 6.7 ps. The noise performance as a function of filter bandwidth and oscillator noise is also addressed.