摘要:

Site-specific Natural Isotopic Fractionation of hydrogen and carbon in nicotine, glycerol and vanillin was studied by NMR spectroscopy (SNIF-NMR) in order to point out specific mechanistic and environmental effects. Several products (cellulose, sugars, acids, nicotine) isolated from tobacco leaves present related carbon and nitrogen fractionation profiles. In nicotine, it is possible to measure eleven site-specific isotope ratios (D/H)iwhich give useful information about the climate of the region of production. The origin of glycerol in fermented beverages can be determined by2H and13C-NMR, and the botanical origin of triglycerides can also be checked in the same way.

ISSN:1025-6016    

DOI:10.1080/10256019808234056

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Isotope ratios of technically and biologically produced halogenated hydrocarbons (HHC) are presented. Compared to the parent compounds, there is a significant enrichment of deuterium depending on the manner of production, whereas the carbon isotope ratio was only slightly influenced. Furthermore, there is nearly no alteration of the carbon isotope ratio due to biological degradation, so that the carbon fingerprint of the parent compounds is maintained in the metabolites. In contrast to this, the hydrogen isotope ratio of the metabolites reflects the ratio of the surrounding water. These results show that the isotopic fingerprint is a useful tool to distinguish between age, different charges of chemical dumping, and in addition to that, whether there is a contamination with technical products or residual metabolites.

ISSN:1025-6016    

DOI:10.1080/10256019808234057

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The potential of carbon and sulphur isotope ratios to group crude oils with respect to their origin was investigated. Sample selection was based on the actual crude oil imports to Germany. Analysed curde oils from Algeria, the Community of Independent States (CIS), Middle East, Nigeria, the North Sea and Venezuela make up over 86% of the German crude oil imports. The oil as received was deasphalted and the maltene fraction was separated by MPLC into saturated, aromatic and polar fractions. Due to overlapping areas, it is not possible to group the crude oils by their δ13C values alone. A complete grouping of the crude oils with respect to their origin can only be achieved by the combined use of δ13C and δ14S of crude oils, and isotope type-curves. In some cases isotope type-curves enable differentiation between different oil fields of the same geographical origin.

ISSN:1025-6016    

DOI:10.1080/10256019808234058

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The error budget of137Cs measurements by γ-ray spectrometry with a large Ge-detector is considered. Sampling, sample preparation and counting contributions are formulated and estimated for the three most important counting geometries: Cylindrical aliquots on top of the detector, the Marinelli beaker and small preconcentrated amounts in an anticoincidence array.

ISSN:1025-6016    

DOI:10.1080/10256019808234059

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Determining the concentration and15N abundance of the trace gases NO and N2O in air is difficult owing to their very low concentration in the atmosphere (NO < 1 ppb(v); N2O ≈ 0.32 ppm(v)). Although15N analysis of N2O in ambient concentrations can be carried out using a Gas Chromatograph Quadruople Mass Spectrometer (GC-QMS) and a dosage of 2 mL air by means of a sample loop, this system is not sensitive enough to measure the ambient concentration of NO and its15N abundance. Therefore the concentration of NO must be enriched by cryotrapping. The improved15N analytical apparatus described here facilitates the sensitive and sufficiently accurate measurement of the ambient NO with a15N abundance > 10at.% and thus represents an advance for15N2O analysis (better sensitivity).

ISSN:1025-6016    

DOI:10.1080/10256019808234060

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The kinetic isotope effect (KIE) in the decarbonylation of formic acid(FA) assisted with acetic anhydride (AA) has been studied in the temperature interval 18–100°C. The13C KIE lying in the range from 1.0541 (at 18.4°C) to 1.0463(at 100.1°C) indicates that the rupture of the carbon—oxygen single bond is the reaction step which determines the13C fractionation in the observed production of carbon monoxide from formic acid assisted with Ac2O similarly to what has been observed in the decarbonylation of pure liquid formic acid. The magnitude of the13C KIE (smaller than the value expected for “full C—O bond rupture”) indicates again that there are intrinsic side processes lowering the13C fractionation in the course of decarbonylations of formic acid. The mechanism of decarbonylation of FA in the liquid medium consisting initially of “HCOOH–Ac2O” has been discussed and the most probable main and side minor reaction channels have been selected. The mechanistic conclusions have been corroborated by presenting the supplementary13C KIE data for CO formation in a “HCOOH–Ac2O–EDS” system, where the “overall13C KIE” is observed (EDS = etidronate disodium). The presented isotope data and their interpretation contribute to the full understanding of the decarbonylation of formic acid.

ISSN:1025-6016    

DOI:10.1080/10256019808234061

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The metabolism and biokinetics of trace metals in humans can be successfully studied employing stable isotopes of the investigated elements as tracers. For the estimation of the bioavailability and the intestinal absorption from solid food, materials are required which have been intrinsically labelled with the chosen stable tracer, since the use of an extrinsic label may lead to erroneous results.

ISSN:1025-6016    

DOI:10.1080/10256019808234062

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Molybdenum (Mo) has been identified as an essential trace metal for humans. The present study was aimed at the assessment of data on intestinal Mo absorption from aqueous solutions and from foodstuffs in humans applying the methodology for instrinsic labelling described in Part 1. The intestinal absorption of Mo was investigated by means of a double tracer method in 3 healthy volunteers on a total of 15 occasions. When administered as aqueous solution, almost complete uptake of Mo was observed up to doses of 1 mg and only a slight decrease for higher doses. But addition of black tea reduces the absorbed fraction by about a factor of ten. Studying Mo absorption from food, intrinsically labelled cress showed a reduced uptake as compared to extrinsically labelled cress and aqueous solutions. Even less Mo was absorbed from an extrinsically labelled composite meal. The data obtained demonstrate a pattern of intestinal Mo absorption which is different from that of other essential trace metals,e.g., Fe or Co.

ISSN:1025-6016    

DOI:10.1080/10256019808234063

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A membrane inlet system to a mass spectrometer (MIMS) allows us to monitor the abundance of18O in CO2during the exchange of18O between HCO3−, dissolved CO2and H2O that occurs after dissolving18O-labelled NaHCO3in aqueous solution. Using a mathematical model we quantitate intracellular carbonic anhydrase activity and membrane permeabilities for HCO3−, CO2and H2O of isolated cells or intact epithelia added to the solution. For these calculations it was necessary to determine (a) the relation between MS signal and [C18O16O], (b) MIMS response kinetics and (c) CO2leakage during the experiment. The three parameters were determined by stepwise addition of HCl to HCO3−solution: (a) was found to be linear, (b) response times were 7.5±2s (10°C), 4.5±1s (20°C), 3.5±0.6s (37°C), and (c) CO2leakage was < 1‰ s−1.

ISSN:1025-6016    

DOI:10.1080/10256019808234064

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A methylchloroformate derivative was used for the simultaneous determination of plasma enrichments of 1-13C-phenylalanine, 1-13C-tyrosine,15N-phenylalanine and15N-tyrosine by gas chromatography/mass spectrometry. All four tracer enrichments could be measured in a single GC run. A specific ion fragment was obtained for each tracer. This approach allowed an easy determination of the “tracer to tracee ratios”. Each ion fragment could be measured with an appropriate single-to-noise ratio and precision in samples obtained from 100 μl plasma. The derivatization consists of a fast one-step reaction. Therefore it is well suited for studies involving a large number of samples, such as non-steady state bolus studies.

ISSN:1025-6016    

DOI:10.1080/10256019808234065

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor