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Long‐term stable water isotope and runoff data for the investigation of deforestation effects on the hydrological system of the Wüstebach catchment, Germany

Long‐term stable water isotope and runoff data for the investigation of deforestation effects on the hydrological system of the Wüstebach catchment, Germany

The described long-term stable water isotope and runoff data set is directly accessible via a digital object identifier (DOI) and is freely available at: https://doi.org/10.34731/y6tj-3t38. Please note, the download of the data is possible under the category “Transfer options ” of the landing page. The data is stored in an Excel spread- sheet with 4 worksheets. No gaps were filled in the data sets. Data will receive new DOIs with each update; preceding versions will not F I G U R E 2 Runoff data of the two runoff gauging stations WU14

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Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale

Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale

Studies on the micro-scale (Boy et al., 2008; Goller et al., 2005), supported by solute data and end member mixing analysis at the meso-scale (Bücker et al., 2011; Crespo et al., 2012), showed that fast “organic horizon flow” in forested catchments dominates during discharge events if the min- eral soils are water saturated prior to the rainfall. Due to an abrupt change in saturated hydraulic conductivity (K sat ) between the organic (38.9 m d −1 ) and the near-surface min- eral layer (0.15 m d −1 ), this organic horizon flow can con- tribute up to 78 % of the total discharge during storm events (Fleischbein et al., 2006; Goller et al., 2005). However, the overall importance of this organic horizon flow is still dis- putable because the rainfall intensity rarely gets close to such a high saturated hydraulic conductivity. In 95 % of the mea- sured rainfall events between June 2010 and October 2012 the intensity was below 0.1 m d −1 (≈ 4.1 mm h −1 ) and was therefore 15 times lower than the saturated hydraulic con- ductivity of the mineral soil layer below the organic layer under forest vegetation and around 30 times lower than the saturated hydraulic conductivity of the top soil under pas- ture vegetation (Zimmermann and Elsenbeer, 2008; Crespo et al., 2012). The same conclusion holds true for the occur- rence of surface runoff due to infiltration access on pasture (lacking a significant organic layer). Solely based on rainfall intensities, surface runoff is therefore relatively unlikely to
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Construction and specification of a cryogenic vacuum extraction device for application in stable water isotope analysis

Construction and specification of a cryogenic vacuum extraction device for application in stable water isotope analysis

Stable isotopes of water - particularly 1 H and 2 H as well as 16 O and 18 O - can assist in the so- lution of hydrogeochemical and biological problems due to their natural and overall existence in water cycles (Clark and Fritz, .1997). They improve the understanding of the origin, forma- tion, and flow path of water and, therefore, provide insights across a range of spatial scales from the cell to the plant community, ecosystem, region or global and over temporal scales (Dawson et al., 2002). Thereby, physicochemical differences, determined by the dissimilar numbers of neutrons and, therefore, by different masses of the stable water isotopes, lead to different chemical behaviours of isotopes in the environment. This phenomenon is called iso- tope fractionation (Unkovich et al., 2001). This different behaviour in physical and chemical reactions is the major reason that in nature stable water isotopes exist in different ratios and can consequently be used as natural tracers. The differences in isotope ratios are expressed relatively to an international standard in per mil [‰] (Aggarwal et al., 2007).
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Exploring water cycle dynamics by sampling multiple stable water isotope pools in a developed landscape in Germany

Exploring water cycle dynamics by sampling multiple stable water isotope pools in a developed landscape in Germany

standing results from studies in forested catchments. Spatio- temporal studies of stream water in developed, agricultur- ally dominated, and managed catchments are less abundant. This is partly caused by damped stream water isotopic sig- natures excluding traditional hydrograph separations in low- relief catchments (Klaus et al., 2015). Unlike the distinct watershed components found in steeper headwater counter- parts, lowland areas often exhibit a complex groundwater– surface-water interaction (Klaus et al., 2015). Sklash and Far- volden (1979) showed that groundwater plays an important role as a generating factor for storm and snowmelt runoff pro- cesses. In many catchments, streamflow responds promptly to rainfall inputs but variations in passive tracers such as water isotopes are often strongly damped (Kirchner, 2003). This indicates that storm runoff in these catchments is domi- nated mostly by “old water” (Buttle, 1994; Neal and Rosier, 1990; Sklash, 1990). However, not all old water is the same (Kirchner, 2003). This catchment behaviour was described by Kirchner (2003) as the old-water paradox. Thus, there is evidence of complex age dynamics within catchments and much of the runoff is stored in the catchment for much longer than event water (Rinaldo et al., 2015). Still, some of the physical processes controlling the release of old water from catchments are poorly understood and roughly modelled, and the observed data do not suggest a common catchment be- haviour (Botter et al., 2010). However, old-water paradox behaviour was observed in many catchments worldwide, but it may have the strongest effect in agriculturally managed catchments, where surprisingly only small changes in stream chemistry have been observed (Hrachowitz et al., 2016).
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An Investigation of the Last Interglacial's Climate Characteristics: Insights from a Stable Water Isotope Equipped Climate Model

An Investigation of the Last Interglacial's Climate Characteristics: Insights from a Stable Water Isotope Equipped Climate Model

red sediment layer. This is thought to suggest either the addition of fresh water to the surface water, or the mixing of isotopically ligher brines resulting from sea ice spreading linked to a freshening event. Interestingly enough, Nicholl et al. [2012] also find that there is no coarse Ice Rafted Debris (IRD) – an indicator of iceberg rafting – present at the site, suggesting that it was not deposited by iceberg discharge from a desintegrating ice sheet. Deposition from surface plumes of suspended sediment is also unlikely, due to the distance of both sites from the mouth of the Hudson Strait. A similar layer of carbonate-rich, red colored sediment was depsoited during the final outburst of glacial Lake Agassiz 8.4 ka B.P. [Kerwin, 1997, St-Onge and Lajeunesse, 2007, Lajeunesse and St-Onge, 2008]. The final discharge of Lake Agassiz has been associtaed with a cooling event 8.2 ka B.P., which has been shown to be associated with a perturbation of the AMOC [Hillaire-Marcel et al., 2007]. Nicholl et al. [2012] suggest that analogues of this event should also be found in other high-resolution climate records.
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Improved interpretation of stable water isotope signals in snow and ice from northern Greenland

Improved interpretation of stable water isotope signals in snow and ice from northern Greenland

To date, most ice core studies on the Greenland ice sheet have been carried out point-wise (e.g., Dye 3, GRIP, GISP2, NGRIP), which begs the question of how representative one single long ice core record is for deriving a comprehensive record of past climate. A study of ice cores from south- ern Greenland revealed that winter season stable water iso- topes are largely influenced by the North Atlantic Oscilla- tion (NAO) and are strongly related to southwestern Green- land air temperatures. On the other hand, summer season sta- ble water isotope ratios show higher correlations with North Atlantic sea surface temperature conditions (Vinther et al., 2010). In particular, northern Greenland has been little inves- tigated so far. The summit in Greenland’s center is the high- est site and separates Greenland into a northern and southern part. Northern Greenland differs significantly from the south in terms of lower air temperatures and lower snow accumula- tion rates (Fischer et al., 1998c). Thus, the results from south- ern Greenland are not directly transferable to the northern part.
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Diffusive gradients in thin films measurement of sulfur stable isotope variations in labile soil sulfate

Diffusive gradients in thin films measurement of sulfur stable isotope variations in labile soil sulfate

Abstract A diffusive gradient in thin films (DGT) technique, based on a strongly basic anion exchange resin (Amberlite IRA-400), was successfully tested for 34 S/ 32 S analysis in la- bile soil sulfate. Separation of matrix elements (Na, K, and Ca) that potentially cause non-spectral interferences in 34 S/ 32 S analysis by MC ICP-MS (multi-collector inductively coupled plasma–mass spectrometry) during sampling of sulfate was demonstrated. No isotopic fractionation caused by diffusion or elution of sulfate was observed below a resin gel disc loading of ≤79 μg S. Above this threshold, fractionation to- wards 34 S was observed. The method was applied to 11 dif- ferent topsoils and one mineral soil profile (0–100 cm depth) and compared with soil sulfate extraction by water. The S amount and isotopic ratio in DGT-S and water-extractable sulfate correlated significantly (r 2 = 0.89 and r 2 = 0.74 for the 11 topsoils, respectively). The systematically lower 34 S/ 32 S isotope ratios of the DGT-S were ascribed to mineralization of organic S.
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Characterisation of microbial transformation of halogenated organic contaminants using compound-specific stable isotope analysis

Characterisation of microbial transformation of halogenated organic contaminants using compound-specific stable isotope analysis

The advance in society as result of the industrial revolution has led to an increasing anthropogenic impact on the environment. As a result of the increasing number of applications and chemical production, as well as initial lack of awareness of the potential negative impacts on the environment and human health, the environment has been subject to exploitation and contamination. After awareness arose, efforts were made to understand the fate of contaminants in the environment. Soon the importance of microorganisms as key factors influencing the removal of, e.g. groundwater contaminants, was recognised. A reliable assessment of the contribution of biodegradation to the overall natural attenuation, however, is a challenge and methods and approaches were developed allowing addressing the in situ activity of microorganisms. These approaches require an understanding of biotransformation of contaminants, and the relevant environmental conditions supporting these activities in situ. In this work, concepts based on the application of stable isotopes were developed and investigated. These are an alternative and can serve as complementary tools to common geochemical analysis, concentration analysis, microbiology and molecular biology and are discussed in relation to the biotransformation of common halogenated groundwater contaminants. The approaches developed allow the investigation of the in situ activity of microorganisms, both qualitatively and quantitatively, and were used to characterize the dehalogenation reaction in organohalide-respiring bacteria. This work focuses on the enrichment of microorganisms capable of organohalide respiration, determination of their substrate spectrum, the investigation of the relevance of these organisms in situ and the development of stable isotope approaches to detect in situ biotransformation of chlorinated ethenes and monochlorobenzene. The Habilitation thesis is based on 27 publications (see chapter 7 for the complete list) and the discussion includes further recent unpublished data.
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Determination of Thallium in Brain Tissue by Stable Isotope Dilution and Field Desorption Mass Spectrometry 

Determination of Thallium in Brain Tissue by Stable Isotope Dilution and Field Desorption Mass Spectrometry 

Since thallium naturally occurs as a m ixture o f two stable isotopes and since isotopically enriched thallium is com m ercially available, stable isotope dilution appears to be the method o f choice fo r a mass spectrometric quantitation. The utility o f this technique o f internal standardization fo r quantita­ tive studies by field desorption mass spectrometry has been demonstrated fo r a number o f organic compounds [1 3 ] as well as fo r organic [1 4 ] and inorganic cations [1 5 ]. Coupling o f the mass spec­ trometer to a multi-channel analyzer is especially advantageous fo r these investigations because it allows integrating electrical recording o f the FD ions in the mode o f repetitive scanning. Thus the fluctuations o f the F D ion currents can be com ­ pensated effectively and quantitative data o f good precision are obtained. F ig. 1 shows the isotopic abundances of naturally occuring thallium, o f the isotopically enriched thallium standard, and o f a model mixture measured by this procedure.
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Stable hydrogen isotope ratios in crystal water of clay minerals

Stable hydrogen isotope ratios in crystal water of clay minerals

Hydrogen is the most abundant element in the Universe. But the utilization of the H isotopic composition (?H-2 value) of soil to elucidate biogeochemical processes or to serve as a palaeo climate proxy is still in its infancy. In our research, we will focus on the ?H-2 value of nonexchangeable H in the clay fraction of soils. The ?H-2 value of structural H in clay minerals – mainly from C-poor subsoils - has been studied since the 1970s. The ?H-2 value of clay minerals mainly depends on (a) the average ?H-2 value of ambient water at the site and time of formation, and on (b) the size of the equilibrium isotopic fractionation factor between water and clay mineral at the temperature of formation. In our research, we will focus on the ?H-2 value of nonexchangeable H in the clay fraction of soils. Only nonexchangeable H in in structural water of minerals preserves its inherited ?H-2 value and does not exchange with water at temperatures usually occurring in soil environments at the Earth’s surface. Nonexchangeable H is bound in crystal water, which integrates the ?H-2 value of soil water over several millennia. This is in turn determined by palaeoclimatic variations of the precipitation’s ?H-2 signal with distinguishable shifts e.g., from Pleistocene to Holocene. For a global data set, Ruppenthal (2014) reported a close correlation of bulk soil ?H-2 values with those of the mean local precipitation and confirmed this for organic matter, while the clay fraction of soils was up to now not studied. We will adapt a steam equilibration method with water vapor of known H isotopic composition – formerly applied by Ruppenthal (2014) on SOM and bulk soil – to clay fractions and compare our results to the hitherto used heating treatments (200-250°C) under vacuum. We expect that the ?H-2 signal of the clay fraction of Bt horizons will serve to differentiate soils developed in different climatic epochs (e.g., Holocene, last interstadial, last interglacial) by analyzing dated palaeo soil samples. To test the hypothesis that there is a similar global regression line of the ?H-2 values in structural water of clay as up to now reported for bulk soils and soil organic matter, we will analyze the clay fraction in a global set of soil samples.
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Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution

Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution

On glacial-interglacial time scales oceanic carbonate chemistry determines atmospheric pCO2. However, the underlying mechanisms and possible feedbacks with climate change are still not well understood. Knowledge of the nature and amplitude of natural fluctuations in the past can be used to assess the stability of modern subsystems and their potential range of variations in the future. Understanding the climate system therefore requires the reconstruction of physical, chemical and biological parameters that characterize the ocean carbonate system over glacial and interglacial time scales as well as the transitions between them. Over the past decade a number of proxy relationships based predominantly on foraminifera have been established on the basis of laboratory and field experiments. Among others, the stable boron isotopic composition in foraminiferal shells was found to record marine pH (Spivack et al., 1993), Ba/Ca was used to infer alkalinity (Lea and Boyle, 1989) and differences between the influence of [CO3 2- ] on the stable oxygen and carbon isotopic composition of Globigerinoides sacculifer and G. ruber was found to be useful for past carbonate ion concentration estimates (Bijma et al., 1999; Spero et al., 1999). More recently, some new proxies such as foraminiferal U/Ca (Russell, 2001), S/Ca (Erez et al., 2001) and the CaCO3 size fraction index (Broecker and Clark, 1999) have been found to record [CO3 2- ].
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The pivotal role of sulfite species in shaping the oxygen isotope composition of sulfate: new insights from a stable isotope perspective

The pivotal role of sulfite species in shaping the oxygen isotope composition of sulfate: new insights from a stable isotope perspective

During DSR, where sulfate is reduced over intermediates such as adenosine 5´- phosphosulfate (APS) and sulfite to sulfide, the oxygen isotope composition in the residual sulfate is dependent on the oxygen isotope composition of the surrounding water (Mizutani and Rafter, 1973) despite the fact that sulfate does not exchange directly its oxygen with the water (Chiba and Sakai, 1985; Lloyd, 1968). This observation is generally explained as a result of DSR-mediated equilibrium oxygen isotope fractionation between residual sulfate and the water driving the oxygen isotope composition of sulfate towards a constant value of approximately 23‰ (Zeebe, 2010) to 26‰ (Fritz et al., 1989) at 22°C. This mechanism was intensively investigated both by experimental studies as well as modeling approaches (Blake et al., 2006; Brunner and Bernasconi, 2005; Brunner et al., 2005; Brunner et al., 2012; Fritz et al., 1989; Turchyn et al., 2010; Wortmann et al., 2007). It is assumed that the oxygen isotope fractionation observed during DSR results from reversible enzymatic steps during sulfate reduction, enabling the oxidation of sulfoxy intermediates that rapidly exchange oxygen isotopes with ambient water back to sulfate. Sulfite is considered to be an important intermediate in DSR and is known to rapidly exchange oxygen isotopes with water, unlike the more oxidized sulfoxy anions in DSR, such as APS or cell-internal sulfate (Brunner et al. 2012, Kohl et al., 2012).
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The Nitrogen Cycle of the Eastern Mediterranean Sea from a Stable Isotope Perspective

The Nitrogen Cycle of the Eastern Mediterranean Sea from a Stable Isotope Perspective

The cause of the oligotrophy of the EMS is attributed to the general water mass circulation observed in the Mediterranean Sea. The circulation of the Mediterranean Sea is driven by an excess of evaporation over precipitation. The general circulation is an anti-estuarine thermohaline cycle open to the Atlantic, consisting of two closed sub-cycles in the western and eastern basin, respectively (Lascaratos et al., 1999). The communication between the two basins is constricted by the shallow trench between Sicily and Africa. The net water transport across this trench into the EMS is approximately 1 to 1.5 Sv (Moretti et al., 1993). The circulation starts at the Straits of Gibraltar when North Atlantic surface waters flow into the Western Mediterranean basin and move towards the Eastern basin. At this point it is important to note that the circulation in the upper 100–300 m of the Western Mediterranean Sea is dominated by Atlantic water that separates Mediterranean and frontal waters (Prieur and Sournia, 1994). Persistent density fronts resulting from the interaction of saline Mediterranean and fresher Atlantic waters are associated with higher primary production rates than in surrounding waters (Lohrenz et al., 1988) and in the eastern basin (Azov, 1991), explaining the recorded longitudinal gradient of oligotrophy (Ignatiades et al., 2005).
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Reassessing Hydrological Processes That Control Stable Isotope Tracers in Groundwater of the Atacama Desert (Northern Chile)

Reassessing Hydrological Processes That Control Stable Isotope Tracers in Groundwater of the Atacama Desert (Northern Chile)

Abstract: A collection of 514 stable isotope water samples from the Atacama Desert is being reassessed geostatistically. The evaluation reveals that adjacent Andean catchments can exhibit distinct δ 18 O and δ 2 H value ranges in meteoric waters, despite similar sample altitudes of up to 4000 m above sea level (a.s.l.). It is proposed that the individual topographic features of each catchment at the western Andean Precordillera either inhibit or facilitate vapor mixing processes of easterly and westerly air masses with different isotopic compositions. This process likely causes catchment-specific isotope value ranges in precipitations (between − 7‰ and − 19‰ δ 18 O) that are being consistently reflected in the isotope values of groundwater and surface waters of these catchments. Further, due to evaporation-driven isotopic fractionation and subsurface water mixing, isotope samples of the regional Pampa del Tamarugal Aquifer plot collectively parallel to the local meteoric water line. Besides, there is no evidence for hydrothermal isotopic water-rock interactions. Overall, the observed catchment-dependent isotope characteristics allow for using δ 18 O and δ 2 H as tracers to delineate regionally distinct groundwater compartments and associated recharge areas. In this context, δ 18 O, δ 2 H and 3 H data of shallow groundwater at three alluvial fans challenge the established idea of recharge from alluvial fans after flash floods.
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Electrodeposition of aluminium in different air and water stable ionic liquids

Electrodeposition of aluminium in different air and water stable ionic liquids

Moreover, alloys such as Co-Al [227] and Al-Cr [228] alloys can be electrodeposited on different substrates in the acidic melt of this system. 1.6 Scanning tunneling microscopy, STM The scanning tunneling microscopy was first introduced in 1982 by Gerd Binnig and Heinrich Rohrer [229]. They were awarded the Nobel Prize in Physics in 1986. Nowadays, scanning tunneling microscopy (STM) is employed in many fields in academics and industry. The application of the scanning tunneling microscopy (STM) to probe the structure at the electrode/electrolyte interface is one of the most important advances in electrochemistry over the past 2 decades [230]. Sonnenfeld and Hansma [231] were the first ones to study a surface immersed in a liquid. To achieve this purpose, they combined the normal STM with a potentiostat to be able to perform electrochemical STM measurements. They succeeded to get images of highly oriented pyrolytic graphite (HOPG) in aqueous media. Following these initial efforts in 1986, a rapid series of advances was done to apply in a variety of electrochemical studies in both aqueous [232-234] and ionic electrolytes [70, 72, 235-238]. STM performed under electrochemical conditions (EC-STM technique) gave an insight into various processes, such as for example the initial stages of electrodeposition. Despite the extensive studies in aqueous media [232-234], in situ STM studies on electrode / electrolyte interface behaviour in chloroaluminate ionic liquids are difficult, as chloroaluminates are strongly hygroscopic and liberate highly aggressive HCl upon reaction with ambient water. Therefore, Endres et al. [239] developed a new design suited for working under inert-gas atmosphere for several days. Recently, STM was used successfully to investigate the electrodeposition of metals, alloys [235, 102, 237,
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Tracing carbon fluxes within two distinct microbial communities in anaerobically methane oxidising mats by stable isotope probing

Tracing carbon fluxes within two distinct microbial communities in anaerobically methane oxidising mats by stable isotope probing

Figure 3.15: Total 13 C-assimilation rates for bacterial lipid biomarker compounds of ANME-1 (left) and -2 (right) dominated communities incubated with different 13 C-labelled subst[r]

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Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH

Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH

In ECHAM5-JSBACH the same land hydrology model is used as in ECHAM5. The model comprises three surface wa- ter reservoirs: a snow layer (sn), water at the skin layer of the canopy or bare soil (wl), and a soil water layer (ws). These three types are each represented by a single layer bucket model, and each of them has a prescribed maximum field ca- pacity. The snow reservoir is filled by snowfall and depleted by snowmelt or sublimation. The skin layer and the soil layer are filled by rainfall and snowmelt in the following order: first the skin layer is filled until its water holding capacity is ex- ceeded, and secondly the non-intercepted water fills the soil reservoir. The modeled depletion of the skin layer can only occur by evaporation; the depletion of the soil water reservoir occurs by evapotranspiration. There is no exchange between these two reservoirs. If the soil water reservoir is saturated, surface runoff occurs. Drainage occurs independent of the new precipitation, and it is calculated if the amount of soil water reaches 5 % or more of the maximal soil water capac- ity. The runoff and drainage scheme is based on examination of variations of the field capacity for soil water over the land surface (D¨umenil and Tondini, 1992). Furthermore, lakes are prescribed by a lake mask; to calculate the evaporation over larger lakes (i.e., grid cells with a lake fraction greater than 50 %) the same scheme as for the ocean is used. A more de- tailed description of the land hydrology model can be found in Roeckner et al. (2003).
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Lithium Isotope Effects in Cation Exchange Chromatography of Lithium Lactate in Water-Dimethyl Sulfoxide and Water-Acetone Mixed Solvent Media 

Lithium Isotope Effects in Cation Exchange Chromatography of Lithium Lactate in Water-Dimethyl Sulfoxide and Water-Acetone Mixed Solvent Media 

Strong correlations of S with solvent partitions between the solution and the exchanger phases were found in both systems, which was qualitatively explainable by considering the lithium[r]

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Water stable, antimicrobial active nanofibres generated by electrospinning from aqueous spinning solutions

Water stable, antimicrobial active nanofibres generated by electrospinning from aqueous spinning solutions

In comparison to nanofibres from the corresponding pure materials the inorganic-organic composite benefits by combining the material properties of the both materials such as flexibility, low density and toughness of PVA and thermal / water stability of the silica component [38-44]. Combination of poly(vinyl alcohol) with an inorganic sol consisting of silica results in an interlinked hybrid system due to strong interactions between the hydroxyl groups of the PVA and the residual silanol groups of the inorganic phase in form of hydrogen bonds [39]. The network between PVA and silica suppresses the swelling of PVA in water and enhances its thermal stability [39]. As a consequence of the strong interaction between the inorganic and the organic phase, the spinning behaviour and the morphology of the hybrid fibres differ significantly from that of pure PVA and silica, respectively. The PVA/silica hybrid generally tends towards the formation of beads within the fibres [36; 37]. In this work, we firstly optimised fibre morphology and electrospinning of the PVA/silica hybrid material with regard to reduce bead formation within the fibre structure and to obtain homogeneous fibre webs by adjusting the PVA content and the reaction time between the organic and the inorganic component. In order to provide the hybrid nanofibres with antimicrobial activity, silver ions were used as add-on to the spinning solution. The silver nanoparticles were generated within the fibre matrix by UV irradiation of the hybrid fibres. The resulting hybrid fibres have excellent antimicrobial activity and water stability. In addition it was examined whether intensive heat conditions influence morphology and antimicrobial activity of the Ag-nanoparticles within the nanofibre matrix.
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Monitoring the Sphingolipid de novo Synthesis by Stable-Isotope Labeling and Liquid Chromatography-Mass Spectrometry

Monitoring the Sphingolipid de novo Synthesis by Stable-Isotope Labeling and Liquid Chromatography-Mass Spectrometry

a substrate. It would be straightforward to add the preferred fatty acids of other CerS isoforms, with or without labels, to increase the complexity of the assay and to assure even more detailed insights into sphingolipid de novo synthesis. In the present study, however, we focused on labeled physiological SPT substrates. In the future, follow-up studies with atypical SPT substrates, a broader spectrum of CerS substrates (all preferably stable- isotopically labeled), and combinations of these are envisaged. From an analytical point of view, the labeling of both the fatty acid and amino acid precursor molecules with stable-isotopes is advantageous. Most published MS-based SPT assays used labeled serine only ( Ren et al., 2018 ; Harrison et al., 2019 ). Since serine is a small molecule (chemical formula: C 3 H 7 NO 3 ), the
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