Geology Reference
In-Depth Information
8.1. Trace Gas Analyses
Timothy R. Blake
Simone Meinardi
Donald R. Blake
Research group member Gloria Liu
trapping a coal gas sample on the five
column/detector trace gas analytical
system.
Photo by Timothy R. Blake (2007).
Introduction
G as Chromatography (GC) is the technique used by the Blake/Rowland research group for analyzing components of
samples taken in the field. We use chemically treated columns to
the sample into groups of its individual
chemical constituents. This is achieved by the column being coated (for capillary) or filled (for packed) with a bonded
phase substrate to which the species in the sample are attracted by varying degrees. As compounds are moved through
the column by a carrier gas such as helium, the attraction of the substrate on the column provides resistance to this
movement based on chemical attraction. Columns containing various substrates are employed for different separation,
based on different chemical affinities. Chemical properties such as molecular weight, size, and polarity of the sample
components to be analyzed are considered in determining which type of column to utilize. This affinity can be
controlled by temperature, and thus columns are housed in temperature-controlled ovens. The oven that contains the
column is heated as the sample passes further through the column providing intraspecies separation but controlling
interspecies separation. At the end of the column the properly separated compounds pass through a detector that
records the data. Each column in the Blake/Rowland lab is coupled with an Electron Capture Detector (ECD),
Quadrupole Mass Spectrometer Detector (MSD), Flame Ionization Detector (FID). These various detectors
specialize in quantifying specific groups of compounds, similarly to how a person
un-mix
s senses respond to different stimuli.
Thus, different detectors are employed for different columns. A column that preferentially separates hydrocarbons is
coupled with an FID, which essentially counts the carbon atoms that pass through it. As can be seen, the pairing of
column and detector is important to data quality and the Blake/Rowland group has five different column detector
combinations that analyze each volatile organic compound (VOC) sample. The Blake/Rowland group has been
analyzing samples and refining system parameters since the 1970s and was recently recognized as the top lab for
nonmethane hydrocarbons (NMHCs) in the National Science Foundation NSF-funded nonmethane hydrocarbon
intercomparison experiment (NOMHICE) study which compared analysis by 30 International groups.
'
Gas Collection
T he Blake/Rowland group has used sampling canisters (currently seventh generation) in the analysis of gases from
various regions around the world since the 1970s. Samples are collected in evacuated two-liter (2L) stainless steel
canisters through a Swagelock valve (Figure 8.1.1). As the valve is opened at the sampling sight, the surrounding
air fills the evacuated canister and the valve is closed, sealing the sample until analysis (Figure 8.1.2). Canisters are
treated and tested continuously to ensure that they are inert to the quantified gases. The thousands of canisters in the
group
'
s inventory are constantly shipped around the world for sampling and back to the lab for analysis.
For this project samples were collected on site using 2L stainless steel canisters fitted with a 6-foot stainless steel
extension tube attached to the Swagelock valve by an ultra-torr union. The extension was inserted into a hole in the
coal area and the valve was opened at which point a sample from the coal bed rushed into the evacuated canister.
Samples were taken on site and subsequently returned to the lab for analysis.
Search WWH ::




Custom Search