OrganicChemistry323A's Photobucket 11 GC album media

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OrganicChemistry323A — Thu, 18 Jun 2009 15:26:19 MDT

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Once the sample is injected, push the green-light button (on gas chromatography apparatus) and the machine will do the rest to acquire the data and produce a GC chart. Note that the column here is preset at 101 degree Celsius in the test run. Considerable preliminary investigation is often required by your instructor for any mixture to be separated successfully into its components. Run a reference sample first then a sample of unknown composition side-by-side.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:54 MDT

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The data begins to appear on the screen on the lower left. The following information will be seen from a final GC print-out chart: (1) all experimental conditions (2) The more volatile component spends less amount of time in the liquid phase in the column, evaporate faster, separate from the less volatile component, and arrives at the end of the column first. (3) Besides retention time for the identity of each compound, the peak area for each peak will be analyzed by students to determine the mole% composition in the sample mixture. Read "quantitative analysis" section in the textbook. Q: What is the limitation of GC? Is NaCl capable of producing a peak using GC technique? Why not? Read "Factors Affecting Separation" in the textbook.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:51 MDT

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Click on "Acquisition", scroll, down select "Re-Initialize", then set up experimental conditions to acquire GC results by the computer.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:50 MDT

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Before running a GC analysis, the software program "Peak Sample" is opened.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:49 MDT

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Injection of sample into the column through the port (entrance to the column), all at once. Be careful - do not bend the needle!

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:47 MDT

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Hold the syringe with both hands for injection.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:40 MDT

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A syringe is used to receive a sample. Pre-rinse the syringe a couple times with the liquid sample, usually 1-10 micro liters is sufficient. You must obtain a GC chart for a reference sample first, where the composition of reference sample is already known. The retention time of a reference sample, when obtained under controlled conditions, can identify a compound by a direct comparison of it versus the value of reference peak determined under the same conditions.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:37 MDT

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Gas chromatograph: this apparatus carries out a gas-liquid chromatographic separation, and is equipped with a detector which monitors the quantity of each incoming compound in the gas phase.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:36 MDT

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The entire set-up is for gas chromatography: (1)computer & printer (2)gas chromatograph (3) helium gas tank (carrier gas, all connected together). Only hardware and lab procedures are presented here. Prior to coming to the lab, students should read the textbook and understand how GC works.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:31 MDT

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Compressed helium (carrier gas) connected to the gas chromatograph apparatus. This is preset at a fixed flow rate before running the gas chromatograph. Helium provides a mobile gas phase, which constantly elutes compounds in the gas phase through the column and brings the vapor molecules to the detector. The flow rate is an important parameter to be included when a GC chart is obtained. Q: why is helium chosen as a carrier gas? Is there a replacement for helium? Why not use oxygen?

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:30 MDT

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Column (tubing) inside of the Gas Chromatograph chamber (a controlled oven): this is the heart of the entire instrument providing the stationary phase where separation of compounds takes place. The type of packing material, the choice of liquid phase, the length of the tubing, the start-up temperature, and the heating rate of column, collectively play the key role in the separation of gas-liquid. Together with the flow rate of carrier gas, the retention time (residence time) of any compound can be determined, and it stays constant provided the GC chart is obtained under the same experimental conditions. Read Retention Time and Principles of Separation sections in the textbook.

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OrganicChemistry323A — Thu, 18 Jun 2009 15:15:26 MDT

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The GC syringe needle is pointed up, as to push out of air bubbles (which also produce interfering peaks). A sample amount down to as little as 1-2 micron liter is still suitable to produce good results.