4 research outputs found
Compound-Specific Hydrogen Isotope Analysis of Heteroatom-Bearing Compounds via Gas ChromatographyâChromium-Based High-Temperature Conversion (Cr/HTC)âIsotope Ratio Mass Spectrometry
The traditional high-temperature
conversion (HTC) approach toward
compound-specific stable isotope analysis (CSIA) of hydrogen for heteroatom-bearing
(i.e., N, Cl, S) compounds has been afflicted by fractionation bias
due to formation of byproducts HCN, HCl, and H<sub>2</sub>S. This
study presents a chromium-based high-temperature conversion (Cr/HTC)
approach for organic compounds containing nitrogen, chlorine, and
sulfur. Following peak separation along a gas chromatographic (GC)
column, the use of thermally stable ceramic Cr/HTC reactors at 1100â1500
°C and chemical sequestration of N, Cl, and S by chromium result
in quantitative conversion of compound-specific organic hydrogen to
H<sub>2</sub> analyte gas. The overall hydrogen isotope analysis via
GCâCr/HTCâisotope ratio mass spectrometry (IRMS) achieved
a precision of better than Âą 5 mUr along the VSMOW-SLAP scale.
The accuracy of GCâCr/HTCâIRMS was validated with organic
reference materials (RM) in comparison with online EAâCr/HTCâIRMS
and offline dual-inlet IRMS. The utility and reliability of the GCâCr/HTCâIRMS
system were documented during the routine measurement of more than
500 heteroatom-bearing organic samples spanning a δ<sup>2</sup>H range of â181 mUr to 629 mUr
Small-Angle and Ultrasmall-Angle Neutron Scattering (SANS/USANS) Study of New Albany Shale: A Treatise on Microporosity
Small-angle
neutron scattering (SANS) and ultrasmall-angle neutron
scattering (USANS) techniques were applied to study the microstructure
of several New Albany shales of different maturity. It has been established
that the total porosity decreases with maturity and increases somewhat
for post-mature samples. A new method of SANS data analysis was developed,
which allows the extraction of information about the size range and
number density of micropores from the relatively flat scattering intensity
observed in the limit of the large scattering vector <i>Q</i>. Macropores and significant number of mesopores are surface fractals,
and their structure can be described in terms of the polydisperse
spheres (PDSP) model. The model-independent Porod invariant method
was employed to estimate total porosity, and the results were compared
with the PDSP model results. It has been demonstrated that independent
evaluation of incoherent background is crucial for accurate interpretation
of the scattering data in the limit of large <i>Q</i>-values.
Pore volumes estimated by the N<sub>2</sub> and CO<sub>2</sub> adsorption,
as well as via the mercury intrusion technique, have been compared
with those measured by SANS/USANS, and possible reasons for the observed
discrepancies are discussed
A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC Ă GC) and Asphaltene Chemistry
The heavy oil rim of a large Saudi
Arabian oilfield has been shown
to be in vertical and lateral equilibrium, matching predictions of
the gravity term from the FloryâHugginsâZuo equation
of state for asphaltenes in the form of 5.2 nm clusters of the YenâMullins
model. The large (10Ă) vertical gradient of asphaltene concentration
over a very large perimeter (âŤ10 km) of the oilfield provided
a stringent test of this equation-of-state fit. Two-dimensional gas
chromatography (GC Ă GC) and stable isotope analysis (δâŻD
and δâŻ<sup>13</sup>C) were used to determine the consistency
of the liquid-phase components with equilibration and the effects
of biodegradation or thermal maturity on the observed asphaltene gradient.
These analyses confirm homogeneity of equilibrated liquid-phase components
of similar chemical character and equilibrated asphaltene isotopes.
Biodegradation is minimal and there is no maturity variation among
the samples. Thus, the large asphaltene gradient did not result from
these secondary processes and is not a remnant from how the reservoir
was charged with crude oil. The results are consistent with original
findings that the oil column is equilibrated. Thermodynamic equilibration
over such large distances (>10 km) requires convective currents
and
provides constraints on fluid dynamic processes in reservoirs. A simple
one-dimensional (1-D) three-component single-phase model is introduced
to account for asphaltene accumulation by way of convective currents
established from a diffusive gas front at the top of the oil column
Organic Reference Materials for Hydrogen, Carbon, and Nitrogen Stable Isotope-Ratio Measurements: Caffeines, <i>n</i>âAlkanes, Fatty Acid Methyl Esters, Glycines, lâValines, Polyethylenes, and Oils
An
international project developed, quality-tested, and determined isotopeâδ
values of 19 new organic reference materials (RMs) for hydrogen, carbon,
and nitrogen stable isotope-ratio measurements, in addition to analyzing
pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and
IAEA-600 (caffeine). These new RMs enable users to normalize measurements
of samples to isotopeâδ scales. The RMs span a range
of δ<sup>2</sup>H<sub>VSMOWâSLAP</sub> values from â210.8
to +397.0 mUr or â°, for δ<sup>13</sup>C<sub>VPDBâLSVEC</sub> from â40.81 to +0.49 mUr and for δ<sup>15</sup>N<sub>Air</sub> from â5.21 to +61.53 mUr. Many of the new RMs are
amenable to gas and liquid chromatography. The RMs include triads
of isotopically contrasting caffeines, C<sub>16</sub> <i>n</i>-alkanes, <i>n</i>-C<sub>20</sub>-fatty acid methyl esters
(FAMEs), glycines, and l-valines, together with polyethylene
powder and string, one <i>n</i>-C<sub>17</sub>-FAME, a vacuum
oil (NBS 22a) to replace NBS 22 oil, and a <sup>2</sup>H-enriched
vacuum oil. A total of 11 laboratories from 7 countries used multiple
analytical approaches and instrumentation for 2-point isotopic normalization
against international primary measurement standards. The use of reference
waters in silver tubes allowed direct normalization of δ<sup>2</sup>H values of organic materials against isotopic reference waters
following the principle of identical treatment. Bayesian statistical
analysis yielded the mean values reported here. New RMs are numbered
from USGS61 through USGS78, in addition to NBS 22a. Because of exchangeable
hydrogen, amino acid RMs currently are recommended only for carbon-
and nitrogen-isotope measurements. Some amino acids contain <sup>13</sup>C and carbon-bound organic <sup>2</sup>H-enrichments at different
molecular sites to provide RMs for potential site-specific isotopic
analysis in future studies