Geology Reference
In-Depth Information
11.3. Sample Analysis
Gas chromatograph/mass spectrometer,
Agilent Model 6890.
Photo Courtesy of Alpha Analytical Laboratory, Mansfield, Massachusetts, 2009.
Introduction
C
omplex hydrocarbon patterns, like those comprising the TEH of coal and coal tars, consist of thousands of
compounds in widely varying concentrations. The exploration of these hydrocarbon assemblages benefits greatly
by the use of multiple analytical techniques.
Some methods describe the dominant features while others specialize in low concentration constituents with potent
source specific information. For example, bulk measurements, like TOC, help relate modern analyses with older
datasets whereas highly detailed measurements, like high-resolution gas chromatography in combination with mass
spectrometry (GC/MS), help identify dominant semivolatile hydrocarbons with common (e.g., normal alkanes) and
highly variable patterns (e.g., terpanes). Diagnostic measurements derived from GC/MS, like the concentrations
and distributions of alkylated PAHs and geochemical biomarkers, often reveal source-specific patterns employed
by modern geochemists to define formation-specific source signatures of coal and coal by-products. Collectively,
multiple methods are used to develop synoptic source signatures across the vast molecular landscape occupied by
coal and its by-products.
Total Organic Carbon
T
he field samples are often tested for TOC for the purpose of normalizing the hydrocarbon concentrations;
minimizing the influence of mineral and moisture content; and comparing results with literature values. TOC is
commonly measured by EPA Method 9060A (USEPA, 2008). The samples are thoroughly homogenized, and
water is removed by evaporation for at least 4 hours in an oven heated to 105°C. The dried sample is ground up
with mortar and pestle. Phosphoric acid is added to remove inorganic carbon (e.g., carbonates). Once homogenized
and purged of water and carbonates, an aliquot of sample (2
10mg) is loaded into a tin cup and catalytically
combusted at ~1000°C in a stream of oxygen to CO
2
. The gas passed through scrubber columns that remove sulfur
and halogens. The CO
2
can be measured by several types of detectors including infrared and thermal conductance.
The TOC concentration is calculated in mg/kg based on a calibration factor based on multiple calibration standards
formulated with carbon source, such as potassium hydrogen phthalate. The initial calibration curve should be
verified by accurately measuring the concentration of an independent reference sample with known TOC content.
The nominal detection range is 0.01
-
-
70% TOC. All samples should be analyzed in duplicate and reported as the
average of the replicate runs.
Total Extractable Material
T
he semivolatile sample extracts are measured for TEM to assure that the proper mass of organic material is
injected onto the GC capillary column or silica gel cleanup column. The TEM is measured from the 10mL sample
extract. All gravimetric measurements are performed on an electronic balance capable of measuring
0.0001 g.
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