The Experts below are selected from a list of 9240 Experts worldwide ranked by ideXlab platform
Alexis T Bell - One of the best experts on this subject based on the ideXlab platform.
-
Isotopic Tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts
Journal of Physical Chemistry B, 2001Co-Authors: Kaidong Chen, Enrique Iglesia, Alexis T BellAbstract:Kinetic analysis and Isotopic Tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic Isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...
-
Isotopic Tracer and kinetic studies of oxidative dehydrogenation pathways on vanadium oxide catalysts
Journal of Catalysis, 1999Co-Authors: Kaidong Chen, Andrei Y Khodakov, Jun Yang, Alexis T Bell, Enrique IglesiaAbstract:Kinetic analysis and Isotopic Tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic Isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These Isotopic Tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.
-
Isotopic Tracer studies of chain propagation and termination during fischer tropsch synthesis over ru tio2
Studies in Surface Science and Catalysis, 1993Co-Authors: Kamala R Krishna, Alexis T BellAbstract:Abstract Transient response Isotopic Tracer experiments have been used to study chain growth during Fischer-Tropsch synthesis over a Ru/TiO 2 catalyst. This involves observation of the incorporation of 13 C into reaction products after an abrupt switch from 12 CO/D 2 to 13 CO/D 2 in the feed. Values for the rate constants for initiation, propagation and termination are determined by fitting theoretically generated curves to the observed transient response. The rate coefficient for chain initiation is found to be independent of temperature, whereas the rate coefficients for propagation and termination increase with temperature. The activation energy for chain termination is significantly higher than that for chain propagation, explaining the observed decrease in chain growth probability, α, with increasing temperature. Coverages by reaction intermediates are also estimated. The dominant species are monomeric building units which occupy 0.2–0.6 ML. Alkyl species that are the direct hydrocarbon product precursors occupy
-
estimates of the rate coefficients for chain initiation propagation and termination during fischer tropsch synthesis over ru tio2
Journal of Catalysis, 1993Co-Authors: Kamala R Krishna, Alexis T BellAbstract:Transient response Isotopic Tracer experiments have been used to study chain growth during Fischer-Tropsch synthesis over an Ru/TiO2 catalyst. This involves observation of the incorporation of 13C into reaction products after an abrupt switch from 12CO/D2 to 13CO/D2 in the feed. Values for the rate coefficients for initiation, propagation, and termination are determined by fitting theoretically generated model curves to the observed transient responses. The rate coefficient for chain initiation is independent of temperature and D2/CO ratio. The rate coefficients for propagation and termination increase with temperature. The rate constant for propagation is not affected by the D2/CO ratio. The rate coefficient for termination increases linearly with increasing D2/CO ratio. The activation energy for chain propagation is 8 kcal/mol, whereas the activation energy for chain termination is 20 kcal/mol. The relative values of these two activation energies explains the observed decrease in chain growth probability, α, with increasing temperature. Coverages by reaction intermediates are also estimated. The dominant species are monomeric building units which occupy 0.3-0.6 ML. Alkyl species that are the direct hydrocarbon product precursors occupy < 0.2 ML and adsorbed CO covers 0.7 ML.
-
an Isotopic Tracer study of the deactivation of ru ti02 catalysts during fischer tropsch synthesis
Journal of Catalysis, 1991Co-Authors: Kamala R Krishna, Alexis T BellAbstract:An investigation of the causes of catalyst deactivation during Fischer-Tropsch synthesis over Ru/Ti02 catalysts has been carried out. The effects of Ru dispersion, TiO2 phase, and TiO2 surface area on catalyst activity and selectivity were examined. CO chemisorption capacity and carbon species accumulation were determined as a function of reaction time using Isotopic Tracer techniques in conjunction with temperature-programmed surface reaction (TPSR). The results of this investigation show that all of the catalysts undergo deactivation with time, with no change in product selectivity. Initially, deactivation is very rapid followed by a slower activity loss. The long-term rate of deactivation is proportional to the initial CO turnover frequency, obtained by extrapolation of the long-term activity data. Deactivation is accompanied by a progressive loss in CO chemisorption capacity as well as an accumulation of various types of carbon species. Carbidic carbon, Cα, and alkyl carbon chains, Cβ, were observed to accumulate as reaction proceeds. Cβ consists of two species, C′β which is the precursor to C2+ hydrocarbon products and C″β which consists of longer alkyl chains and does not participate in the production of gas phase products. The rapid initial loss in activity in the first 10 min correlates with the Cα + Cβ accumulation; the long-term loss in CO uptake and catalyst activity are probably due to C″β. The alkyl chains comprising C″β do not undergo hydrogenolysis under reaction conditions, probably due to inaccessibility to hydrogen.
Philip G Robertson - One of the best experts on this subject based on the ideXlab platform.
-
Isotopic evidence for episodic nitrogen fixation in switchgrass panicum virgatum l
Soil Biology & Biochemistry, 2019Co-Authors: Sarah S Roley, Chao Xue, Stephen K Hamilton, James M Tiedje, Philip G RobertsonAbstract:Abstract Perennial grasses can assimilate nitrogen (N) fixed by non-nodulating bacteria living in the rhizosphere and the plant's own tissues, but many details of associative N fixation (ANF) remain unknown, including ANF's contribution to grass N nutrition, the exact location of fixation, and composition of the associated microbial community. We examined ANF in switchgrass (Panicum virgatum L.), a North American perennial grass, using 15N-enriched N2 Isotopic Tracer additions in a combination of in vitro, greenhouse, and field experiments to estimate how much N is assimilated, where fixation takes place, and the likely N-fixing taxa present. Using in vitro incubations, we documented fixation in root-free rhizosphere soil and on root surfaces, with average rates of 3.8 μg N g root−1 d−1 on roots and 0.81 μg N g soil−1 d−1 in soil. In greenhouse transplants, N fixation occurred only in the early growing season, but in the field, fixation was irregularly detectable throughout the 3-month growing season. Soil, leaves, stems, and roots all contained diazotrophs and incorporated fixed N2. Metagenomic analysis suggested that microbial communities were distinct among tissue types and influenced by N fertilizer application. A diverse array of microbes inhabiting the rhizosphere, and possibly aboveground tissues, appear to be episodically contributing fixed N to switchgrass.
Enrique Iglesia - One of the best experts on this subject based on the ideXlab platform.
-
mechanism and site requirements for activation and chemical conversion of methane on supported pt clusters and turnover rate comparisons among noble metals
Journal of Physical Chemistry B, 2004Co-Authors: Enrique IglesiaAbstract:Isotopic Tracer and kinetic studies are used to probe the identity and reversibility of elementary steps required for H2O and CO2 reforming of CH4 on supported Pt clusters and to demonstrate a rigorous kinetic and mechanistic equivalence for CO2 and H2O reforming, CH4 decomposition, and water-gas shift reactions. Reforming rates are exclusively limited by C−H bond activation on essentially uncovered Pt crystallite surfaces and unaffected by the concentration or reactivity of co-reactants (H2O, CO2). Kinetic Isotopic effects are consistent with the sole kinetic relevance of C−H bond activation (kH/kD = 1.58−1.77 at 873 K); these isotope effects and measured activation energies are similar for H2O reforming, CO2 reforming, and CH4 decomposition reactions. CH4/CD4 cross exchange rates are much smaller than the rate of methane chemical conversion in CO2 and H2O reforming reactions; thus, C−H bond activation steps are irreversible, except as required by the approach to equilibrium for the overall reforming rea...
-
deuterium Isotopic Tracer studies of thiophene desulfurization pathways using propane or dihydrogen as co reactants
Journal of Catalysis, 2002Co-Authors: Enrique IglesiaAbstract:Isotopically labeled reactants (D2 and C3D8) were used in order to probe H2S formation pathways during thiophene desulfurization on Co/H-ZSM5 at 773 K using H2 or propane as co-reactants. With D2/C4H4S or C3D8/C4H4S reactants, both D and H were present in the hydrogen sulfide formed, suggesting that desulfurization can occur via both direct thiophene decomposition with intramolecular hydrogen transfer and deuterium addition from D2 or C3D8. Thiophene, however, becomes readily deuterated via rapid exchange with D-containing intermediates formed from D2 or C3D8, suggesting that thiophene protonation-deprotonation steps are fast. These Isotopic equilibration processes prevent a definitive assessment of the contributions of these two pathways to hydrogen sulfide formation. The D-content in hydrogen sulfide and in hydrocarbons are higher with C3D8/C4H4S than with D2/C4H4S reactants, indicating that propane is more effective than dihydrogen as a source of hydrogen for desulfurization reaction. C3H8/C4H4S and C3D8/C4H4S reactants gave similar reactions rates for hydrocarbon formation, suggesting that C–H bond activation steps do not limit hydrocarbon formation rates. A normal kinetic isotope effect was observed for thiophene desulfurization, indicating that the abstraction of fragments from propane-derived intermediates for reactions with thiophene is more difficult for deuterated intermediates. A comparison of thiophene desulfurization rates with C3H8/D2/C4H4S, C3H8/C4H4S, and D2/C4H4S mixtures confirmed that propane is a more effective source of hydrogen than H2. The deuterium content in all products formed from C3H8/D2/C4H4S mixtures was lower than in products formed from C3H8/D2 reactants. These findings suggest that thiophene inhibits the hydrogen adsorption–desorption steps responsible for the incorporation of D-atoms into adsorbed intermediates from D2 and for the desorption of dihydrogen during propane reactions.
-
Isotopic Tracer studies of thiophene desulfurization reactions using hydrogen from alkanes on h zsm5 and co h zsm5
Journal of Catalysis, 2001Co-Authors: Enrique IglesiaAbstract:Abstract Reaction pathways for the desulfurization of thiophene using alkanes as hydrogen sources were probed by measuring the chemical and Isotopic composition of products formed from 13 C-labeled C 3 H 8 and unlabeled C 4 H 4 S mixtures on H-ZSM5 and Co/H-ZSM5. Aliphatic hydrocarbons formed only from propane carbon atoms while aromatic molecules contained carbon atoms from both propane and thiophene. Hydrogen-deficient thiophene and thiophene-derived species react with surface hydrogen and alkenes formed from propane and desorb as unreactive aromatics, in steps that lead to the irreversible exit of alkenes from oligomerization-cracking cycles. These steps remove kinetic bottlenecks in thiophene desulfurization resulting from the irreversible formation of strongly adsorbed unsaturated species, which cannot desorb without reactions with hydrogen or hydrogen-rich surface species. This kinetic coupling between alkane and thiophene reactions leads to the observed concurrent increase in the rates of both propane aromatization and thiophene desulfurization compared with those achieved with each pure reactant. The scavenging of unsaturated intermediates formed via thiophene decomposition using hydrogen or hydrogen-rich intermediates formed from propane decreases the rate of bimolecular Diels–Alder reactions, which lead to larger organosulfur compounds and to low H 2 S selectivity. As a result, H 2 S selectivities are higher and deactivation rates are lower when propane is present as a co-reactant during reactions of thiophene.
-
Isotopic Tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts
Journal of Physical Chemistry B, 2001Co-Authors: Kaidong Chen, Enrique Iglesia, Alexis T BellAbstract:Kinetic analysis and Isotopic Tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic Isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...
-
Isotopic Tracer and kinetic studies of oxidative dehydrogenation pathways on vanadium oxide catalysts
Journal of Catalysis, 1999Co-Authors: Kaidong Chen, Andrei Y Khodakov, Jun Yang, Alexis T Bell, Enrique IglesiaAbstract:Kinetic analysis and Isotopic Tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic Isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These Isotopic Tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.
Zhifang Chai - One of the best experts on this subject based on the ideXlab platform.
-
study of the effect of a chinese medicine on calcium absorption with Isotopic Tracer technique
Journal of Radioanalytical and Nuclear Chemistry, 2007Co-Authors: Xueying Mao, Zhifang Chai, Weiyue Feng, Min Wang, Fengbao ZhangAbstract:The effects of two Chinese prescriptions, Zhugu and Shengu capsules were studied on calcium absorption. The absorption ratios of Zhugu and Shengu was 30.4% and 21.5%, respectively.
-
investigation of mercury containing proteins by enriched stable Isotopic Tracer and size exclusion chromatography hyphenated to inductively coupled plasma isotope dilution mass spectrometry
Analytica Chimica Acta, 2007Co-Authors: Junwen Shi, Zhifang Chai, Weiyue Feng, Meng Wang, Fang Zhang, Bing Wang, Motao ZhuAbstract:In order to investigate trace mercury-containing proteins in maternal rat and their offspring, a method of enriched stable Isotopic Tracer (Hg-196 and Hg-198) combined with size-exclusion chromatography (SEC) coupled to inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS) was developed. Prior to the analysis, Hg-196- and Hg-198-enriched methylmercury was administrated to the pregnant rats. Then the mercury-containing proteins in serum and brain cytosol of the dam and pup rats were separated by size-exclusion columns and the mercury was detected by ICP-MS. The ICP-MS spectrogram of the tracing samples showed significantly elevated Hg-196 and Hg-198 Isotopic signals compared with the natural ones, indicating that the detection sensitivity could be increased by the Tracer method. The contents of mercury in chromatographic fractions of the dam and pup rat brain cytosol were quantitatively estimated by post-column reverse ID-ICP-MS. The quantitative speciation differences of mercury in brain cytosol between the dam and pup rats were observed, indicating that such studies could be useful for toxicological estimation. Additionally, the Isotopic ratio measurement of Hg-198/Hg-202 in the tracing samples could be used to identify the artifact mercury species caused in the analytical procedure. The study demonstrates that the Tracer method combined with high-performance liquid chromatography (HPLC)-ICP-IDMS could provide reliably qualitative and quantitative information on mercury-containing proteins in organisms. (c) 2006 Elsevier B.V. All rights reserved.
-
Isotopic Tracer studies on the metabolism and functional roles of mineral elements in institute of high energy physics china
Journal of Nuclear Science and Technology, 2006Co-Authors: Zhiyong Zhang, Chunying Chen, Weiyue Feng, Zhifang ChaiAbstract:Human and animal bodies contain practically all naturally occurring mineral elements and only 14 of them have been shown to be essential to health. Imbalance in mineral elements (on the cellular level) is one of the basic causes of diseases. Within the past decade, there has been a growing interest in the role of mineral elements in human health and disease. Isotopic Tracer method is especially suited to such studies and has played an important role. In this paper, some recent progress in the study of metabolism and functional roles of mineral elements made by the Isotopic Tracer method in key laboratory of nuclear analytical techniques, institute of high energy physics is outlined, including rare earth elements, selenium, calcium, chromium and iron.
-
Isotopic Tracer studies of chemical behavior of rare earth elements in environmental and biological sciences
Radiochimica Acta, 2004Co-Authors: Z Y Zhang, Zhifang ChaiAbstract:Rare earth elements (REE), a group of elements with atomic numbers from 57 to 71, have been widely applied in recent years not only in industry but also in agriculture, forestry, animal husbandry and medicine. Numerous anthropogenic activities make REE to easily enter the environment and finally the human body via the food chain. Therefore, detailed studies on chemical behavior of these metals in environmental and biological systems are imperative. Isotopic Tracer method is especially suited to such studies and has played an important role in assessing the environmental effects of REE. In this paper, some recent progress in the study of chemical behavior of REE in environmental and biological sciences made by the Isotopic Tracer method is outlined.
-
study of chromium containing proteins in subcellular fractions of rat liver by enriched stable Isotopic Tracer technique and gel filtration chromatography
Analytical and Bioanalytical Chemistry, 2003Co-Authors: Weiyue Feng, Zhifang Chai, Jing Liu, Peiqun Zhang, Yuxi Gao, Jiujiang ZhaoAbstract:Ten male Wistar rats were intravenously injected with a single approximately physiological dose of enriched stable Isotopic Cr-50 Tracer solution (200 ng 50Cr3+/100 g body wt). The fundamental distribution patterns of the chromium-containing proteins in the nucleic, mitochondrial, lysosomal, microsomal, and cytosolic subcellular fractions of the rat liver were investigated by means of Sephadex G-100 gel chromatography combined with neutron activation analysis via 50Cr (n, γ) 51Cr reaction. In total, nine kinds of Cr-containing proteins were found in the five subcellular fractions, whose relative molecular masses were 96.6±6.2, 68.2±1.4, 57.9±4.7, 36.6±1.2, 24.2±1.8, 14.0±1.5, 8.8±0.6, 6.9±0.4, and 4.2±0.4 kDa. Approximately 64.5% of Cr proteins accumulated in the cytosolic fraction. The second enriched part was the nucleic fraction; about 12.2% Cr proteins were stored in this section. The 4.2-kDa molecular mass might contain the so-called low molecular weight chromium-containing substance; however, in this research, it was only observed in the mitochondria, lysosome, and microsome. In the mitochondrial fraction, most of the Cr proteins were present as relatively low molecular weight substances: about 56% of chromium-containing proteins had molecular masses ≤6.9 kDa. Nevertheless, more than 69% of Cr-containing proteins were observed with molecular masses ≥57.9 kDa in the liver cytosolic fraction.
Pablo G Toral - One of the best experts on this subject based on the ideXlab platform.
-
endogenous synthesis of milk cis 9 trans 11 conjugated linoleic acid in dairy ewes quantification using 13c labeled vaccenic acid and comparison with estimates based on cobalt administration
Journal of Dairy Science, 2020Co-Authors: Gonzalo Hervas, Pilar Frutos, Pablo G ToralAbstract:ABSTRACT Isotopic Tracers are used to directly quantify the effect of mammary Δ9-desaturation on milk fatty acid (FA) composition, but very few studies have applied this method to measuring the endogenous synthesis of rumenic acid (RA; cis-9,trans-11 conjugated linoleic acid) in cows and goats, and no publications exist in ewes. In sheep, knowledge about the contribution of stearoyl-coenzyme A desaturase (SCD) to milk FA secretion is derived mostly from indirect estimates based on inhibition of the enzyme by oral administration of cobalt, a cost-effective method that has not been validated to date. To fill both gaps, we conducted an Isotopic Tracer assay in sheep to quantify the proportion of endogenous RA in milk for the first time in this species. We then compared the results with estimates derived from a Co administration assay performed on the same animals. First, 5 lactating ewes received an intravenous injection of 200 mg of [1-13C]trans-11 18:1 (vaccenic acid, VA), the precursor for RA production by SCD activity. At −24, −15, 0, 9, 24, 33, 48, 57, 72, 81, and 96 h post-injection, we recorded milk yield and collected milk samples to examine fat concentration and FA profile. We conducted compound-specific isotope analysis of VA and RA by gas chromatography–combustion isotope ratio mass spectrometry. Afterward, in the Co administration assay, ewes received a daily dose of 7 mg of Co/kg of body weight for 5 d. We analyzed milk samples for composition before and on the last days of cobalt dosing. On average, 17% of the injected amount of [1-13C]VA was transferred to milk within 96 h post-injection, and up to 29% of the VA taken up by the mammary gland was desaturated to milk RA. Under our conditions, the mean proportion of this conjugated linoleic acid isomer deriving from Δ9-desaturation represented 82 to 90% of the amount secreted in milk. However, the proportion estimated in the Co assay with calculations based on individual FA concentrations was lower (on average, 46%). When we calculated the same estimates based on changes in Δ9-desaturation ratios after Co dosing, the higher values of endogenous RA (75%) did not differ from the results of the Isotopic Tracer assay. Nevertheless, correlation analysis between the results obtained through [1-13C]VA or Co administration revealed no significant relationship, which would prevent acceptance of the latter as a reliable alternative to Isotopic labeling to examine mammary Δ9-desaturation in dairy ewes.