Geology Reference
In-Depth Information
CO/CO
2
system
Methane system
VOC system
Figure 8.1.3. Gas-handling manifolds for the three analytical systems.
Methane concentrations are high enough (parts per million level) to measure with a smaller sample size than the
other systems require. The FID at the end of the column responds to carbon and outputs the electrical response to
Chromeleon Chromatography software which generates a chromatogram. Because the concentrations of
methane are high compared to other hydrocarbon species in the sample, these other compounds are separated
on the column but do not show up on the chromatogram [since they are about 1000 times less concentrated (parts
per billion)].
CO/CO
2
System
T
he second system used in the group
s chromatographic analysis is the CO/CO
2
system. This system is really two
systems with samples loaded through a common port. The CO
2
portion of this system works the exact same way as
the methane system but with a larger sample size of about 10 cm
3
being loaded into a 30 cm
3
sample loop. This
sample is injected into the column and output to a TCD and the response is sent to Chromeleon software which
outputs a chromatogram.
'
The CO unit of this system uses a 10 cm
3
sample size on a 20 cm
3
sample loop which is injected into a 5 meter
¼
molecular sieve column. This system also incorporates a catalyst (which O
2
destroys) to change the CO
to methane because the FID at the end of the column does not count carbon atoms in the CO form. By
converting the carbon from CO to CH
4
, the quantity of carbon is maintained. Therefore, the column outlets to a
four-port switching valve and after 3.5 minutes of venting to the room (sufficient time to allow O
2
to
completely elute from the column) the switching valve redirects the flow to a nickel oxide catalyst maintained
at 285°C bathed in ultra high purity (UHP) hydrogen. Upon passing through the catalyst CO is converted to
methane which is then directed to the FID. Detector output is sent to Chromeleon software and converted to
chromatogram form.
″
VOC System
T
he third analytical system is set up in much the same way as the other two. However, because more volatile
compounds such as N
2
,O
2
,Ar,CH
4
, and CO are orders of magnitude more concentrated than the compounds that
the VOC system is designed to measure, these compounds are removed from the sample by implementing
preconcentration inside the sample loop (Colman et al., 2001)
A 120 cm
3
sample is drawn through the sample
loop that is submerged in a liquid nitrogen dewar (Figure 8.1.4). The less-volatile compounds adhere to the cold
glass beads (which provide surface area for the compounds to adsorb onto), while the volatile compounds do not
adhere to the beads (due to their low boiling points) and are subsequently pumped out by a vacuum pump. Once a
known volume is trapped, the external liquid N
2
dewar is replaced with hot water and the compounds are
volitalized and ready for injection into the columns Figure 8.1.5.
.
