Search results for " fractionation"
showing 10 items of 212 documents
Reconstruction of drip-water δ<sup>18</sup>O based on calcite oxygen and clumped isotopes of speleothems from Bunker Cave…
2013
Abstract. The geochemical signature of many speleothems used for reconstruction of past continental climates is affected by kinetic isotope fractionation. This limits quantitative paleoclimate reconstruction and, in cases where the kinetic fractionation varies with time, also affects relative paleoclimate interpretations. In carbonate archive research, clumped isotope thermometry is typically used as proxy for absolute temperatures. In the case of speleothems, however, clumped isotopes provide a sensitive indicator for disequilibrium effects. The extent of kinetic fractionation co-varies in Δ47 and δ18O so that it can be used to account for disequilibrium in δ18O and to extract the past dri…
Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite
2011
Abstract Understanding the relationship between stable isotope signals recorded in speleothems (δ 13 C and δ 18 O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO 2 . The water entering the cave flows as a thin film towards the drip site. CO 2 degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scal…
Modelling the δ18O value of cave drip water and speleothem calcite
2010
Abstract Stable isotope signals recorded in speleothems have provided important insights about past climate variability in recent years. Quantitative reconstruction of mean annual temperature and the amount of precipitation, however, remains difficult because the stable isotope signals are influenced by various processes. Here we present a drip water model, which shows how these climate parameters affect the oxygen isotope signal of cave drip water. In the model the dependence of the δ 18 O value of drip water on mean annual temperature is established by correlation to the amount of winter precipitation and winter temperature. Application of the model to two caves in western Germany reveals…
Lithium isotopes and partition coefficients in inorganic carbonates: Proxy calibration for weathering reconstruction
2021
We report Li measurements from cave-analogue carbonate-precipitation experiments in order to: i) assess the expected isotope fractionation factors applicable to speleothem growth, and ii) contribute to the wider understanding of lithium incorporation in carbonates. The experimental setup closely mimics natural processes (e.g. precipitation driven by CO2 degassing, low ionic strength solution, thin solution film) but within a laboratory setting that allows for controlled growth conditions (temperature, pCO2, drip rate, carbonate saturation index and the composition of the initial solution). For the main batch of calcite growth experiments our average 1000lnαcalcite-solution is -8.5&plu…
Constraining speleothem oxygen isotope disequilibrium driven by rapid CO2 degassing and calcite precipitation: Insights from monitoring and modeling
2020
Abstract Oxygen isotopes are the most commonly applied speleothem proxy for reconstructing Quaternary changes in precipitation and/or temperature. These interpretations are either limited to qualitative wetting and drying trends or rely on theoretical, experimental and/or empirical equilibrium isotope fractionation factors for more quantitative constraints. These various fractionation factors have similar temperature sensitivities, but their absolute values differ, and cave calcite does not appear to generally precipitate in isotopic equilibrium with its drip water. Rapid CO2 degassing paired with calcite precipitation, both occurring under disequilibrium conditions, are a set of mechanisms…
Improving Fractionation of Human Milk Proteins through Calcium Phosphate Coprecipitation and Their Rapid Characterization by Capillary Electrophoresis
2018
This work describes a simple sample pretreatment method for the fractionation of human milk proteins (into their two main groups, whey and caseins) prior to their analysis. The protein-extraction protocol is based on the addition of calcium phosphate to nonadjusted pH human milk. The combination of calcium ions with phosphate results in an effective coprecipitation of caseins. To assess the suitability of this fractionation protocol, the protein extracts were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), LC-MS/MS, and capillary electrophoresis (CE) analysis. The results evidence a significant decrease in contamination of casein fraction with whey proteins…
Erythropoietin restores the anemia-induced reduction in radiosensitivity of experimental human tumors in nude mice
2003
Abstract Purpose The effect of recombinant human erythropoietin (rhEPO) on the radiosensitivity of human tumor xenografts growing in anemic and nonanemic nude mice was studied. Methods and materials Anemia was induced by total body irradiation ([TBI], 2 × 4 Gy) of mice before tumor implantation into the subcutis of the hind leg. The development of anemia was prevented by rhEPO (750 U/kg s.c.) given 3 times weekly starting 2 weeks before TBI. Fourteen days after fractionated TBI (tumor volume of approx. 40 mm 3 ), single-dose irradiation of the tumor with varying doses was performed so that in full dose–response relationship for the probability of tumor cure was obtained. Results Radiation-i…
Fractionated External Beam Radiotherapy as a Suitable Preparative Regimen for Hepatocyte Transplantation After Partial Hepatectomy
2010
Purpose Hepatocyte transplantation is strongly considered to be a promising option to correct chronic liver failure through repopulation of the diseased organ. We already reported on extensive liver repopulation by hepatocytes transplanted into rats preconditioned with 25-Gy single dose selective external beam irradiation (IR). Herein, we tested lower radiation doses and fractionated protocols, which would be applicable in clinical use. Methods and Material Livers of dipeptidylpeptidase IV (DPPIV)-deficient rats were preconditioned with partial liver external beam single dose IR at 25 Gy, 8 Gy, or 5 Gy, or fractionated IR at 5 × 5 Gy or 5 × 2 Gy. Four days after completion of IR, a partial …
Primary optic nerve sheath meningioma
2007
BACKGROUND. Radiotherapy (RT) has occasionally been practiced in the treatment of optic nerve sheath meningioma (ONSM). Recently, stereotactic fractionated RT (SFRT) has been introduced as a tool with better precision for RT delivery. A comprehensive review was undertaken to provide more insight into this matter. METHODS. A literature search was performed to identify reports dealing with both clinical aspects (diagnosis) and treatment in ONSM, focusing on RT in primary (p)ONSM. In particular, major emphasis was placed on the role of SFRT in pONSM. RESULTS. SFRT was capable of achieving excellent local tumor control, with improved/stable functional capacity in ≥80%, accompanied with very low…
Development and validation of a pressurized liquid extraction liquid chromatography–tandem mass spectrometry method for perfluorinated compounds dete…
2009
Abstract This paper describes the development and validation of an analytical methodology to determine eight perfluorinated compounds (PFCs) in edible fish using pressurized liquid extraction (PLE) with water and solid-phase extraction (SPE) with an ion-exchanger as extraction and pre-concentration procedures, followed by liquid chromatography–quadrupole-linear ion trap mass spectrometry (LC–QqLIT–MS). The rapidity and effectiveness of the proposed extraction procedure were compared with those most commonly used to isolate PFCs from fish (ion-pairing and alkaline digestion). The average recoveries of the different fish samples, spiked with the eight PFCs at three levels (the LOQ, 10 and 100…