Geology Reference
In-Depth Information
UCM. The semivolatile hydrocarbons in the highest rank coals, like anthracite from Pennsylvania (Figure 11.3.1E),
are largely purged out of or condensed into the coal matrix by more extreme geochemical forces. For example, this
anthracite contains only low concentrations of alkylated benzenes likely representing pyrolytic residues of the
geochemical devolatilization process. Collectively, the hydrocarbon scans of these coal samples demonstrate
molecular changes with increasing rank; that is, the early loss of heteroatomic moieties at low rank, the formation
of saturated hydrocarbons (e.g., normal alkanes) and PAHs at intermediate rank, and the devolatilization/con-
densation of semivolatile hydrocarbons at high rank (Van Krevelen, 1993). These general features provide a
context for interpreting features observed in other coals and coal tars encountered in the environment.
Polycyclic Aromatic Hydrocarbons
V irtually all fossil fuels and fossil fuel by-products contain PAH patterns with characteristics of hydrocarbon types
or specific sources. The high-resolution hydrocarbon scan sometimes depicts the presence of PAHs; however, this
method is incapable of comprehensively measuring the majority of PAHs due to the widely varying PAH
concentrations and obscuring effects of coelution with non-PAH compounds. Mass spectrometry solves this
problem because PAHs ionize with characteristic fragmentation patterns that are accurately measured and differ-
entiated from coeluting compounds. The GC/MS instrument provides the technological means for resolving the
concentration of a full range of PAHs and revealing source specific patterns.
Coal predominantly contains variable proportions diagenetic and petrogenic PAHs while coal tar contains high
proportions of pyrogenic PAHs. Diagenetic PAHs are specific PAH isomers (e.g., retene and perylene) produced
by the natural oxidative degradation of vegetative matter. The presence of these compounds in environmental
samples indicates the presence of modern or diagenetic residues. Petrogenic PAHs contain higher proportions of
alkylated PAHs relative to parent PAHs. This pattern indicates the presence of organic matter that minimally
experienced diagenesis and catagenesis (coalification or petroleum formation). The presence of petrogenic PAHs
signifies ancient organic matter that became fossil fuel. Pyrogenic PAHs contain lower proportions of alkylated
PAHs relative to parent PAHs. Thermal exposure preferentially breaks the more reactive bonds of the alkylated side
chain before disrupting the more stable PAH ring structure and results in a shift from alkyl-PAH enrichment in
fossil fuels to a parent (nonalkyl) PAH enrichment in coal tars and coke.
The reference method for the PAH analysis is EPA Method 8270D (USEPA, 2008). This analysis employs the
semivolatile extract described previously. The TEM measurement is used to generate a 1 mL extract with no
more than 10mg/mL of extractable material to maintain consistent instrument performance over time, as
discussed previously. 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 mass spectrometer detector operated in the selected ion monitoring mode (GC/MS SIM). Based on personal
experience, one of the better GC systems for the characterization of PAHs 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 helium with an
isobaric flow rate of 1mL/minute.
The instrument is initially calibrated with selected PAH isomers containing two to six rings at multiple concentra-
tion levels between 1 and 200 µg/mL (Table 11.3.2). The resolution of instrument is assured by demonstrating that
height of the valley between benzo[
b
]fluoranthene and benzo[
k
]fluoranthene is less than 40% of the height of
benzo[
]fluoranthene 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 benzo[
b
] perylene relative to phenanthrene 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.
g,h,i
Once the instrument accuracy, precision, and sensitivity are assured, field sample extracts can be analyzed.
A comparison of the PAH fingerprints demonstrate a variety hydrocarbon patterns in coals of different rank
(Figure 11.3.2), recently reviewed by Stout and Emsbo-Mattingly (2008). For example, low rank coal, like the
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