Simultaneous determination of ascorbic acid and dehydroascorbic acid by HPLC with postcolumn derivatisation and fluorometric detection

Summary A reproducible method is described for the separation and simultaneous and specific quantitation of ascorbic acid and dehydroascorbic acid by ion-pairing reversed-phase HPLC with fluorometric detection. Copper sulphate and copper acetate were compared as oxidizing reagents for ascorbic acid and 1,2-diaminobenzene dihydrochloride and 1,2-diamino-3,4-dimethylbenzene dihydrochloride as derivatising reagents. The HPLC-method was applied to human plasma. The detection limit reaches 16 ng for ascorbic acid and 3 ng for dehydroascorbic acid. Sample preparation is carried out by solid phase extraction with a recovery of 98%; it is compared with conventional precipitation of plasma proteins by metaphosphoric acid.     

Local optical emission spectroscopy of excited species effused from an evaporation cell and a sputter source into dense plasmas – Basic studies for the deposition of thin gradient films

Space resolved optical emission spectroscopy has been applied to determine the distribution of excited species in dense plasmas which are used for the deposition of thin coatings. Typical electron densities and electron temperatures in the plasma facility PETRA ( Plasma Engineering and Technology Research Assembly) are in the range of n(e) = 10(12) cm(-3) and T(e) = 10 eV. During the deposition process material (Al) is evaporated from a vapour cell under controlled conditions. The vapour stream is guided into a dense plasma which is composed of inert gas, Ar or He, and hydrocarbon species produced from the dissociation of C(2)H(2). The evaporated Al-stream which travels with thermal velocity into a plasma of high electron density, is nearly completely ionized due to the short mean free path for electron impact ionization in the above mentioned parameter range. Optical emission spectroscopy has been applied to investigate the interaction processes between the vapour stream and the plasma as well as the transport of the ionized Al along the applied magnetic field. For the measurements space resolved optical emission spectroscopy with an in-situ translation mechanism of the optical fibre has been used to measure the local concentrations of excited Al neutrals and ions as well as the concentration of the background plasma species.     

The negative adsorption of anions by clay suspensions

Summary A generalized expression for the negative adsorption of anions by clay suspensions has been derived using an extension of the procedure developed bySchofield. Negative adsorption of chloride by Na-montmorillonite has been measured in NaCl solutions from 0.1 M to 0.001 M. If the clay was purified by ultrafiltration, measured negative adsorption nearly equalled that calculated from diffuse double layer theory over the entire range of concentration. At very low electrolyte concentrations the measured negative adsorption was less than the calculated amount, probably as a result of the presence of positive charges. Polyanions such as those of metaphosphate or acrylic acid increased the negative adsorption in this region to the theoretical amount. At NaCl concentration between 0.05 and 0.005 M NaCl measured negative adsorption was a maximum of 20 per cent lower than that calculated. This probably represented the error of the approximations made in the theoretical calculations by neglecting the effect of activity corrections in the double layer.

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Zusammenfassung Ein verallgemeinerter Ausdruck für die negative Adsorption von Anionen in Tonsuspensionen wird unter Erweiterung des vonSchofield verwendeten Rechnungsverfahrens abgeleitet. Die negative Adsorption von Chlorid durch Natrium-Montmorillonite wird mit NaCl-L?sungen, 0,1 M-0,00 m, gemessen. Wenn der Ton durch Ultrafiltration gereinigt war, kam die gemessene negative Adsorption über den ganzen Konzentrationsbereich nahezu der aus der Theorie der diffusen Doppelschicht berechneten gleich. Bei sehr niedrigen Elektrolytgehalten war die gemessene negative Adsorption geringer als die berechnete, wahrscheinlich als Ergebnis der Anwesenheit von positiven Ladungen. Polyanionen wie Metaphosphat oder Acryls?ure erh?hten die negative Adsorption in diesem Bereich auf den theoretischen Wert. Für NaCl-Konzentrationen zwischen 0,05 und 0,005 m zeigte die gemessene negative Adsorption ein um 20% niedriger liegendes Maximum als das berechnete. Dies repr?sentiert wahrscheinlich den Fehler in den N?herungen, die in den theoretischen Berechnungen durch Vernachl?ssigung der Effekte der Aktivit?tskorrekturen in der Doppelschicht verursacht werden.

     

A direct route for the preparation of Fmoc/OtBu protected iodotyrosine

Herein the synthesis of an Fmoc/O^tBu orthogonally protected iodotyrosine derivative is reported. This has been achieved via a simple two-step process in an overall 58% yield from commercially available Fmoc-Tyr(^tBu)-OH. The Fmoc/O^tBu orthogonally protected iodotyrosine was also shown to be amenable to Suzuki-Miyaura cross-coupling to deliver a novel bi-aryl tyrosine derivative.     

气相色谱—微波等离子体发射光谱法研究农药1605与人唾液的相互作用

用ＧＣ－ＡＥＤ联用技术研究了农药１６０５，得到１６０５所含六种元素的元素色谱图，并用该方法研究了人唾液与１６０５的相互作用。     

Studies on the preparation and characterization of monodisperse polystyrene latices. VI. Preparation of zwitterionic latices

The preparation of zwitterionic latex particles is reported by using a mixed anionic and cationic initiator system without requiring surface-active agents. Isoelectric points were found from microelectrophoresis experiments and were in the pH range of 3.5-5. Close to the isoelectric point, the latices coagulated as expected, and good stability was achieved outside this narrow range. This range of stability was in good agreement with predictions from current theory. Redispersion after coagulation was found to be difficult as was expected for a hydrophobic colloid. The electrokinetic behavior did not result in the maximum in zeta potential at an electrolyte concentration of 1 mM unlike the situation for other hydrophobic polystyrene latex particles, and hence these systems may be even better models for other colloidal studies.     

The pressure filtrate of colloidal suspensions

Summary The effect of the negative adsorption of co-ions by a colloid on the composition of the pressure filtrate of colloidal suspensions is calculated for a number of different systems. Also the influence of partial retention of electrolytes by the filter is considered.

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Zusammenfassung Der Einfluß, den die Kolloide durch die negative Adsorption von Neben-ionen auf die Zusammensetzung des Druckfiltrats von Kolloidsuspensionen ausüben, ist für verschiedene Fälle berechnet worden. Auch die Folgen der partiellen Zurückhaltung der Elektrolyte durch das Filter wurden betrachtet.

     

NEW PROBES FOR SURFACTANT SOLUTIONS: PHOSPHORESCENT LABELLED DETERGENTS

Abstract— A series of alkyltrimethylammonium bromides containing terminal phosphorescent chromo-phores have been prepared and characterized. 10-(4-bromo, l-naphthoyl)decyltrimethylammonium bromide(BND–10) has a critical micelle concentration (CMC) of 14 mM and a micellar mean aggregation number ( ) of 105. Analogous detergents,BND–11, -8, and -5 have also been prepared; in each case, the naphthoyl group is located at the terminal carbon of an alkyl chain of the indicated length. The normal behavior of increasing and decreasing CMC with increasing aliphatic chain length is observed. When compared with unsubstituted alkyltrimethylammonium bromides, the 4-bromo, 1-naphthoyl chromo-phore is roughly equivalent to 6 methylenes with respect to hydrophobic contribution. A broad phosphorescence maximizing at about 590 nm is readily detected upon photoexcitation of the BND molecules in various solvents. The weak fluorescence of the probes is consistent with efficient intersystem crossing of the BND molecules. BND–10 has a phosphorescence yield of 0.023 and a lifetime of 1 ms in water at 25₀C. The probes are useful for the study of micelle-detergent exchange dynamics.     

Synthesis and conformational analysis of 6-[N,N-bis(dihydroxyphosphorylmethyl)amino]methyl-2-hydroxy-2-oxido-1,4,2-oxazaphosphorinane-4-methylphosphonic acid by NMR spectroscopy

The Kabachnik—Fields methylphosphorylation of 1,3-diaminopropan-2-ol affords a mixture of 1,3-diamino-2-hydroxypropane-N,N,N′,N′-tetrakismethylphosphonic acid and its intramolecular cyclic ester. Subsequent heating of this mixture led to the thermal dehydration of the acid with the 1,4,2-oxazaphosphorinane ring closure and the formation of 6-[N,N-bis(dihydroxyphosphorylmethyl)amino]methyl-2-hydroxy-2-oxido-1,4,2-oxazaphosphorinane4-methylphosphonic acid. A predominant chair conformation of the formed six-membered heterocycle was inferred from the data of 2D homonuclear (¹H, ¹H; J-resolved) and heteronuclear (¹H, ¹³C; HSQC, HMBC) NMR correlation spectra.     

Synthesis and characterization of N-heterocyclic carbene complexes of 1,3,2-dioxaborolane-4,5-dione (NHC-boryl oxalates)

NHC-boranes react with oxalic acid in acetonitrile slowly at room temperature and rapidly at 80?°C to form NHC-boryl oxalates. For example, heating of 1,3-dimethylimidazol-2-ylidene borane with oxalic acid in acetonitrile (80?°C, 2?h) provides the cyclic NHC-boryl oxalate derived from two sequential acid/base reactions in 59% isolated yield. The new NHC-boryl oxalates are robust white solids that are formally complexes of 1,3,2-dioxaborolane-4,5-dione, itself an unknown heterocycle.