GAS CHROMATOGRAPHY OVERVIEW
Gas Chromatography (GC) measures compounds within a mixture that can be vaporized. GC is
primarily used to test the purity of substance. Nearly all industrial quality control labs have one
or several GCs as their primary measurement tool to determine the quality of finished products,
in-process samples, and raw materials. For this lab, we will be using GC to separate components
of a mixture and determine the concentration of each component within the mixture. A simple
block diagram of the GC components is shown below:
(diagram courtesy of http://students.ycp.edu/~chertz/Instr_Portfolio_GC.htm)
A sample is introduced at the sample injection port via a syringe. The syringe breaks the
septum and enters the inlet. The appropriate amount of sample is introduced, where it
promptly converts from a liquid to a gas. The volatile sample then leaves the inlet and begins to
travel through the column; excess vapor is bled through the split vent (not shown). The sample
proceeds through the column with the assistance of a carrier gas. The gas does not chemically
interact with any of the sample components; it merely helps with traveling through the column.
Upon reaching the end of the column, the sample begins to pass through the detector. Two
common detectors (both of which are used in this lab) are the flame ionization detector (FID)
and the thermal conductivity detector (TCD). While both detectors can identify a large range of
components, the FID is primarily used to detect hydrocarbons; the TCD can detect nearly all
components except the carrier gas. Specifically, the FID measures the amount of carbons
passing through. The carbon units are converted to current units; the changes in current are
what “draw” the peaks on either the computer program or the integrator. Because of this
specific measurement of hydrocarbons, the FID cannot measure the amount of water in a
sample, whereas the TCD can determine the concentration of water in a sample mixture. Many
other detectors are available for use, some with more specific functions and detecting
capabilities. The type of detector being used determines what carrier gas is to be used.
This lab uses GCs for the Liquid-Liquid Extraction (LLE), the Tray Distillation Unit (TDU), and the
Packed Distillation Unit (PDU) experiments. Be sure to pay attention to which GC is for which
experiment as well as the sample injection volume for each. By following the techniques
described on the next pages, one can ensure consistency when operating the GC and ultimately
produce accurate chromatograms.
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REFERENCES/ADDITIONAL INFORMATION
Gas Chromatography –Wikipedia (general overview):
http://en.wikipedia.org/wiki/Gas_chromatography
A more detailed explanation of GC operation and components, as well as some history:
http://www.gas-chromatography.net/gas-chromatography.php
More of a chemistry-based explanation of GC aspects:
http://www.chem.ucla.edu/~bacher/General/30BL/gc/theory.html
A detailed troubleshooting guide:
http://www.msp.ch/cs/Reference_Guide.pdf
GC TECHNIQUES
I.
Logging Into ChemStation (for TDU and PDU)
a. In the opening screen, enter the appropriate virtual machine
i. “che-uolab-gc1.lsu.edu” = PDU
ii. “che-uolab-gc2.lsu.edu” = TDU
b. Log in with your username and password
i. Make sure the domain underneath name/password is LSU, not the name
of the virtual machine.
c. When the desktop appears, click on the ChemStation – Online icon
d. The Remote light on the GC should light when the program is loaded.
e. Unit should be in Standby mode, with the “Run and Method Control” tab
highlighted.
f. When you are about to begin a series of run, change the method from Standby
to Start.
i. Note: If you are going to be away from the GC for a while, change the
method back to Standby.
II.
Sample Collection
a. Ensure that you use a clean pipette and GC vial for each sample collection.
Although you may be collecting a mixture of the same three components, there
could potentially be residue of the mixture with a different proportion of the
same components, which could provide false data on the chromatogram.
b. Rinse the inside of the pipette with sample before transferring to a GC vial. This
minimizes the potential of contamination by remnants of previous samples.
c. If you are collecting a sample from an in-process sample point, let some of the
sample flow through the sample point and into a waste container before
collecting sample with a GC vial. This prevents potential contaminants (including
older material) within the inside of the spout or inside the sample line from
being read on the GC.
III.
Pre-Injection
a. Draw up some sample from the vial (draw up more than the recommended
amount); however, inject the sample into a Kim-Wipe instead of the injection
port. This cleans the inside syringe of contaminants from the previous run.
Perform this task a minimum of 3 times.
b. Draw up some sample from the vial (draw up more than the recommended
amount); however, inject the contents back into the vial. Ensure that the plunger
depresses the entire way. This prevents air bubbles from being trapped inside
the syringe. Perform this task a minimum of 3 times.
c. Draw up the appropriate volume of sample (0.1 µL for all three GCs) and feed the
syringe needle into the injection port. Be careful not to bend the needle. Ensure
that the GC is ready to run a new sample prior to injection.
d. When the needle is completely inside the injection port, depress the plunger,
ensuring that all syringe contents are emptied, then immediately hit START on
the GC. Either the integrator or ChemStation (online version) will then begin
plotting the chromatogram.
IV.
Finding Results – ChemStation
a. When you begin a run, the status should change from green to blue, indicating
that a run is in progress. When the run is finished, a popup window will appear
with the report for that run. The results will be towards the bottom of the
report. If you need to make corrections, you can do so in the “Data Analysis” tab
(explained in the next section of this guide).
b. Record and save your results into an Excel file.
V.
Correcting Peaks – ChemStation
a. When running samples from the PDU or the TDU, you should have three distinct
peaks on the chromatogram. However, if recent adjustments have been made to
the GC (especially pressure and flow rate), the program may not recognize the
three pre-assigned peaks, even though there are three distinct peaks present.
The program may also have integrated more or less area than actually shown (ie.
The baseline for one peak integrates below the next peak). The chromatogram
needs to be corrected to display the correct %area for all three peaks.
b. If you have just completed a run with ChemStation, you should be in the setting
of “Run and Method Control” (located in the lower left hand corner of the
screen). Switch the setting to “Data Analysis.”
c. To re-draw baseline/re-integrate peaks:
i. Ensure you are in “Integration” mode (located just above the middle of
the screen). It can take an additional few seconds after clicking the
“Integration” button for the full Integration menu to display.
ii. Delete peaks: Click on the Delete Peak button (button showing a peak
with a red “X” above it). Click on the peaks to be deleted. This will
remove the baseline and eliminate the peak from the overall integration
results.
iii. Draw new baselines: Click on the Draw Baseline button (button showing
one peak with a red line underneath). Click and hold on the left-corner of
the peak, then, while continuing to hold down the mouse button, trace
the new baseline to the right corner of the peak. Upon releasing the
mouse button, the new baseline will be drawn.
iv. Splitting two peaks: Oftentimes during both the PDU and TDU sample
runs, the water and methanol peaks will need to be split since the divide
between the two peaks often doesn’t reach the baseline. Click on the
Split Peak button (button showing two peaks with a red vertical line
dividing the two). Click on the natural divide point between the two
peaks. NOTE: You should have a baseline present before you are able to
split peaks.
d. When all of your baseline/integration corrections have been made, switch from
“Integration” mode to “Calibration” mode (button located to the right of the
“Integration” button). If you did not have to make any integration corrections,
you can proceed directly to “Calibration” mode.
e. To recalibrate peaks:
i. In “Calibration” mode you should have an Excel-like chart with the three
components (water, methanol, isopropanol) and their previously saved
retention times.
ii. Click on each saved retention time and enter the actual retention times
located above each peak.
iii. When all of the retention times have been corrected, click “OK” (located
just above the Excel-like chart). This saves the newly entered retention
times for the next analysis.
iv. In “Calibration” mode, you can also record the response factors. These
are determined through periodic calibrations.
f. While still in “Calibration” mode, click Print Preview (middle of screen). This will
create a new report with the adjustments made (a new .txt file for the PDU-GC
and a .pdf file for the TDU-GC). You should now have values for all three mixture
components as well as more accurate integration values. Record these values.
g. Before logging off from ChemStation, change the selected method from
“START.M” to “STANDBY.M”. This conserves the carrier gas and reduces the inlet
pressure.
INTEGRATION MODE DISPLAY:
CALIBRATION MODE DISPLAY: