Geology Reference
In-Depth Information
The procedure involves the transfer of a small aliquot of this extract (nominally 50 µL) into a preweighed
aluminum pan resting on a hot plate at about 35°C (5°C below the solvent boiling point). After the solvent
evaporates, the aluminum pan containing the extractable material is reweighed and the mass determined by
difference. The TEM is calculated as the mass of the residue divided by the 50 µL of solvent. Based on this
concentration, 10mg of the final extract can be diluted in 1mL DCM for the high-resolution hydrocarbon scan and
the alkylated PAH analysis. Similarly, 25mg can be solvent exchanged to hexane before it is fractionated for the
measurement of saturated hydrocarbons and geochemical biomarkers.
High-Resolution Hydrocarbon Scan
T he high-resolution hydrocarbon scan depicts the dominant semivolatile hydrocarbons extracted from the field
sample. Often this scan yields recognizable hydrocarbon patterns, but it frequently yields novel features that
necessitate supplemental testing. Therefore, the high-resolution scan is instrumental in determining the degree to
which the sample resembles known hydrocarbon patterns, interesting variations of known patterns, or new patterns
for geochemical inquiry.
The reference method for the high-resolution hydrocarbon scan is EPA Method 8015C (USEPA, 2008). This
analysis employs the semivolatile TEH extract described previously. The TEM measurement is used to generate a
1mL extract with no more than 10mg/mL of extractable material. Based on personal experience, the injection of
more than 10 mg/kg into a GC injection port degrades the instrument performance over time. Internal standards
are added to the 1 mL extract to minimize the effects of evaporative loss during the sample analysis. The extract is
injected into a GC instrument equipped with capillary column and a flame ionization detector (GC/FID). Based
on personal experience, one of the better GC systems for TEH analysis is the Agilent 6890 using a 95% dimethyl-
5% diphenyl polysiloxane, fused silica capillary column with 0.32 mm inner diameter, 30m length, and 0.25 µm
film thickness. The instrument run program begins with the oven temperature set to 60°C for 2 minutes, then
increase the temperature by 10°C/minute for 10 minutes, then increase the temperature by 25°C/minute for 2
minutes, then hold the oven temperature at 310°C for 3 minutes. The carrier gas is hydrogen with an isobaric flow
rate of 3°mL/minute.
The initial calibration involves the analysis of normal alkanes and selected isoprenoid hydrocarbons eluting
between
-C 40 run at multiple concentrations between 1 and 200°µg/mL (Table 11.3.1). The resolution
of the instrument is assured by demonstrating that height of the valley between phytane and
n
-C 9 and
n
-C 18 is less than
40% of the height of phytane using a common baseline in a standard with equal concentrations of both
compounds. In addition to developing relative response factors, the initial calibration standards are used to
demonstrate the absence of mass discrimination by assuring the ratio of
n
-C 20 is greater than
0.85. A crude oil reference sample is run with every initial calibration to verify comparable pattern resolution
and quantitative precision over time. A continuing calibration standard is run every day to demonstrate
quantitative precision over time.
n
-C 36 relative to
n
Once the instrument accuracy, precision, and sensitivity are assured, field sample extracts can be analyzed. The
high-resolution hydrocarbon scans demonstrate a variety of hydrocarbon patterns in coals of different rank (Figure
11.3.1). This figure is not intended to be comprehensive of all coals from all ranks; rather, it demonstrates
distinctive hydrocarbons patterns from different US formations and geochemical histories. For example, the
hydrocarbon pattern in low ranked coal, like the North Dakota lignite (Figure 11.3.1A), contains complex and
undulating unresolved complex mixture (UCM) topped by clusters of resolved terpenoid peaks. Moving up the
rank scale, the Wyoming subbituminous coal (Figure 11.3.1B) contains long chain
-C 36 ) with
a strong odd-carbon preference attributed to plant waxes from tree leaves. It also shares many lignite features, like
the complex clusters of terpenoid peaks and an undulating UCM. More significant compositional changes occur in
the high-volatile bituminous coals. For example, alkylated naphthalenes dominate the sample of high-volatile
bituminous coal from Colorado (Figure 11.3.1C). In addition to PAHs, the contour of the UCM becomes more
broad and homogenous. This loss of features from the native biomass coincides with the onset of more extreme
geochemical alterations. However, these alterations fail to completely change the signatures of the original biomass
as evident in the continued presence of long chain, odd-carbon dominated
n
-alkanes (
n
-C 25 to
n
-alkanes. The high-resolution hydro-
carbon scan of medium-volatile bituminous coal from Colorado (Figure 11.3.1D) demonstrates the dominant
appearance of a homologous series of normal alkanes eluting between
n
-C 44 + with no odd-carbon
preference. The sample also demonstrates the presence of alkylated naphthalenes and more uniform and unimodal
n
-C 8 to
n
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