A rapid method for the sampling of atmospheric water vapour for isotopic analysis

Analysis of the stable isotopic composition of atmospheric moisture is widely applied in the environmental sciences. Traditional methods for obtaining isotopic compositional data from ambient moisture have required complicated sampling procedures, expensive and sophisticated distillation lines, hazardous consumables, and lengthy treatments prior to analysis. Newer laser‐based techniques are expensive and usually not suitable for large‐scale field campaigns, especially in cases where access to mains power is not feasible or high spatial coverage is required. Here we outline the construction and usage of a novel vapour‐sampling system based on a battery‐operated Stirling cycle cooler, which is simple to operate, does not require any consumables, or post‐collection distillation, and is light‐weight and highly portable. We demonstrate the ability of this system to reproduce δ¹⁸O isotopic compositions of ambient water vapour, with samples taken simultaneously by a traditional cryogenic collection technique. Samples were collected over 1 h directly into autosampler vials and were analysed by mass spectrometry after pyrolysis of 1 µL aliquots to CO. This yielded an average error of < ±0.5‰, approximately equal to the signal‐to‐noise ratio of traditional approaches. This new system provides a rapid and reliable alternative to conventional cryogenic techniques, particularly in cases requiring high sample throughput or where access to distillation lines, slurry maintenance or mains power is not feasible. Copyright © 2009 John Wiley & Sons, Ltd.     

The nature of marbled Terra Sigillata slips: a combined μXRF and μXRD investigation

In addition to the red terra sigillata production, the largest Gallic workshop (La Graufesenque) made a special type of terra sigillata, called “marbled” by the archaeologists. Produced exclusively at this site, this pottery is characterized by a surface finish made of a mixture of yellow and red slips. Because the two slips are intimately mixed, it is difficult to obtain the precise composition of one of the two constituents without contamination from the other. In order to obtain very precise correlation at the appropriate scale between the color aspect and the elemental and mineralogical phase distributions in the slip, combined electron microprobe, X-ray micro spectroscopies and micro diffraction on cross-sectional samples were performed. The aim of this study is to discover how potters were able to produce this unique type of terra sigillata and especially this particular slip of an intense yellow color. Results show that the yellow component of marbled sigillata was made from a titanium-rich clay preparation. The color is due to the formation of a pseudobrookite (TiFe₂O₅) phase in the yellow part of the slip, the main characteristics of that structure being considered nowadays as essential for the fabrication of stable yellow ceramic pigments. Its physical properties such as high refractive indices and a melting point higher than that of most silicates widely used as ceramic colorants are indeed determinant for this kind of application. Finally, the red parts have a similar composition (elementary and mineralogical) to the one of standard red slip.     

FTIR analysis of the hydrolysis rate in the sol–gel formation of gadolinia-doped ceria with acetylacetonate precursors

This study reveals that Fourier transform infrared is an effective analytical tool in probing the extent of hydrolysis of cerium and gadolinium acetylacetonates dissolved in methanol. It is found that these acetylacetonates have relatively fast hydrolysis rates at the early stage of hydrolysis (<6 h). However, their hydrolysis rates become very slow beyond 6-h of hydrolysis and decrease to near zero after 55-h of hydrolysis. As a result, hydrolysis cannot be completed even after 170 h at room temperature. These phenomena suggest that the first and second acetylacetone ligands chelated to Ce³⁺ and Gd³⁺ ions can actually be hydrolyzed with relative ease at room temperature, similar to alkoxide ligands. It is the last acetylacetone ligand that is very difficult to be hydrolyzed.     

Photoredox-enabled 1,2-dialkylation of α-substituted acrylates via Ireland–Claisen rearrangement

Herein, we report the 1,2-dialkylation of simple feedstock acrylates for the synthesis of valuable tertiary carboxylic acids by merging Giese-type radical addition with an Ireland–Claisen rearrangement. Key to success is the utilization of the reductive radical-polar crossover concept under photocatalytic reaction conditions to force the [3,3]-sigmatropic rearrangement after alkyl radical addition to allyl acrylates. Using readily available alkyl boronic acids as radical progenitors, this redox-neutral, transition-metal-free protocol allows the mild formation of two C(sp³)–C(sp³) bonds, thus providing rapid access to complex tertiary carboxylic acids in a single step. Moreover, this strategy enables the efficient synthesis of highly attractive α,α-dialkylated γ-amino butyric acids (GABAs) when α-silyl amines are used as radical precursors – a structural motif that was still inaccessible in related transformations. Depending on the nature of the radical precursors and their inherent oxidation potentials, either a photoredox-induced radical chain or a solely photoredox mechanism is proposed to be operative.

A photocatalytic 1,2-dialkylation of α-substituted acrylates is enabled by a reaction cascade combining reductive radical-polar crossover with the established Ireland–Claisen rearrangement for the synthesis of valuable tertiary carboxylic acids.     

An efficient approach towards the convergent synthesis of "fully-carbohydrate" mannodendrimers

Glycosylation of sugar trityl ethers with sugar 1,2-O-(1-cyano)ethylidene derivatives (the trityl-cyanoethylidene condensation) has been applied to the synthesis of highly branched (dendritic) mannooligosaccharides incorporating a Manalpha1-->3(Manalpha1-->6)Man structural motif. The convergent synthetic strategy used to assemble these oligosaccharides was based on the use of glycosyl acceptors and/or a glycosyl donor already bearing this structural motif. The former were represented by mono- and ditrityl ethers of ManalphaOMe, Manalpha1-->3ManalphaOMe, and Manalpha1-->3(Manalpha1-->6)ManalphaX, where X=OMe or SEt. The pivotal glycosyl donor was the peracetylated 1,2-O-(1-cyano)ethylidene-3,6-di-O-(alpha-D-mannopyranosyl)-beta-D-mannopyranose (1), prepared by orthogonal Helferich glycosylation of the known 1,2-O-(1-cyano)ethylidene-beta-D-mannopyranose with tetra-O-acetyl-alpha-D-mannopyranosyl bromide followed by O-acetylation. Glycosylation of acetates of methyl 6-O-trityl-alpha-D-mannopyranoside and methyl 3,6-di-O-trityl-alpha-D-mannopyranoside with one equivalent of the donor 1 gave rise to the isomeric tetrasaccharide derivatives, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6ManalphaOMe and Manalpha1-->3(Manalpha1-->6)Manalpha1-->3ManalphaOMe, respectively. The latter derivative was further mannosylated at the remaining 6-O-trityl acceptor site to give the protected pentasaccharide Manalpha1-->3(Manalpha1-->6)Manalpha1-->3(Manalpha1-->6)ManalphaOMe. The isomeric pentasaccharide, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6(Manalpha1-->3)ManalphaOMe, was prepared by reaction of 1 with the 6-O-trityl derivative of (Manalpha1-->3)ManalphaOMe. In a similar fashion, 6'- and 6"-O-trityl derivatives of the branched trisaccharide Manalpha1-->3(Manalpha1-->6)ManalphaOMe served as precursors for two isomeric mannohexaosides. The 3,6-di-O-trityl ether of ManalphaOMe and the 6',6"-di-O-trityl ether of Manalpha1-->3(Manalpha1-->6)ManalphaX (X=OMe or SEt) were efficiently bis-glycosylated with the donor 1 to give the corresponding protected mannoheptaoside and mannononaoside. The yields of these glycosylations with the donor 1 ranged from 50 to 66 %. Final deprotection of all the oligosaccharides was straightforward and afforded the target products in high yields. Both the acylated and deprotected products were characterized, and the intersaccharide connectivities were elucidated by extensive one- and two-dimensional NMR spectroscopy. The described blockwise convergent approach allows assembly of a variety of 3,6-branched mannooligosaccharides.     

Undesired Bulk Oxidation of LiMn2O4 Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes

Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink-casted LiMn₂O₄ electrodes by a rotating ring-disk electrode (RRDE) setup and X-ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn−K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH-dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn₂O₄ in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an E^RHE-pH diagram that illustrates robustness of LiMn₂O₄ against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.     

Labelling of peptides with <Superscript>99m</Superscript>Tc complexes through the modified C-terminal group

Easy and efficient way to modify of the C-terminus of the peptide using amino acid lysine has been proposed and experimentally verified. The synthesis yield was high (≥85%) and it was found that reactions can be carried out even with microgram amounts of the peptide. Using Fmoc-alanine (as the peptide model) with modified C-terminus the procedure of peptide labelling with monovalent and trivalent ^99mTc complexes has been elaborated.