Gas Chromatography

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Purpose:      The purpose of the gas chromatography lab is to find out how different substances interact with the surface of a solid. Chromatography is a separation technique that depends on the relative distribution of the components of a mixture between a mobile phase and a solid stationary phase. Chromatography measures the tendency of a substance to interact with the surface of a solid or to remain in a mobile phase. When doing a chromatography lab the mobile phase has to be a substance that is either in a liquid or a gas state. In this lab the mobile phase was a gas, which is why this is called a gas chromatography lab. The different gases tested in this lab were CHCl3 and CH2Cl2. It is determined to what extent a gas interacts with the solid by injecting a known amount of the mobile gas into the carrier gas and then measuring the concentration that comes out at the end of the column. From this there was a detector that transferred the information to a computer were it was graphed. The tendency of the gas to interact with the solid is determined by the number of theoretical plates. A substance that interacts more strongly with the surface of the solid will take more time to be carried across the stationary phase.

Procedure:     The pieces of a Gas Chromatograph are the gas supply, injector, column and the detector. The gas supply, or carrier gas, is the gas from the valves at the lab tables. First a coil had to be made out of copper, which would serve as the burner for the detection system. A pipet was used as the column to put the solid stationary substance into. The solid phase in this experiment was Tide. The pipet was filled with Tide detergent and cotton was inserted in both end of the pipet. The column was then secured horizontally to a ring stand using clamps. The tip of the column should be in a vertical position. The copper coil is then placed in the vertical part of the column with the coil about 1/8” above the end of the column. It is important that the copper coil be placed at the right height because if it is too low the flame will not get enough air and if the copper is to high the flame will burn below the coil.

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"Gas Chromatography." 26 Apr 2017
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On the other open end of the column the latex coupling/buret valve assembly is connected to the gas supply and to the column. It is attached to the gas supply by using a Bunsen burner hose. The buret valve is used as the adjuster for controlling the gas flow through the column. Another important part of the gas chromatography setup is the flame shield and detector. The flame shield is an open-ended cylinder made of black construction paper. It is used to prevent drafts and room light from causing errors in the signal from the detector. The coil should be centered in the middle of the flame shield. The detector/stopper assembly should be secured to the ring stand using a clamp and the detector should be facing directly at the flame through a cut out portion of the cylinder. After this is done a wire gauze pad should be placed over the cylinder to reduce the light that shines in through the top of the cylinder. The last part of the setup is the computer setup. The alligator clips should be connected to the wires of the sensor. Polarity is not important. The face of the sensor should be about 1/8” back from the end of the straw. Once on the computer the icon labeled GC startup should be double clicked to launch the data collection program. The y-axis should go from 1 to 5 volts with the units decreasing upwards. The x-axis should go from 0 to 400 seconds.
Once setup of the gas chromatography is complete, the system is ready to be tested. With the gas turned on and a flame with a height of about 3/8” begin the data collection system and adjust the flame until it is reading between 4 and 5 volts. After the flame is correct the actual experiment is ready to be started. First a septum vial of CH2Cl2 should be retrieved from the fume hood. Using the syringe collect 0.30 ml of CH2Cl2 of vapor, not liquid. Then inject the sample into the latex coupling as close as possible to the column. Quickly release all the vapors into the coupling. The data collection program should be started when the gas is being injected into the gas flow. The flame should turn to a green color. After that sample is done, follow the same guidelines using 0.60 ml of CHCl3 vapor. After the two separate samples are done, a mixture of the two will be used. Use 0.20 ml of CH2Cl2 and 0.30 ml of CHCl3 vapors.

A drawing of the apparatus of the Gas Chromatography can be found on the next page.

          Theoretical Plates:
                         N = (tr/wb)2      tr = retention time
                                    wb= baseline peak width

               CHCl3      tr = 175 s          wb = 350 – 90
                                        wb = 260

                         N = 16(175/260)2
                         N = 5.38

               CH2Cl2 tr = 80 s          wb = 160 – 45
                                    = 115

                         N = 16(80/115)2
                         N = 7.74
               Mixture      CH2Cl2               CHCl3
                         Tr = 75               tr = 175
                         Wb=115               wb=290
                         N=16(75/115)2          N= 16(175/290)2
                         N= 6.81               N= 5.83

                         Total N for mixture = 12.64

          Area of Peaks for Mixture:
                         Area = .5(wb x ht)

                         CH2Cl2               CHCl3
                    A = .5(30mm)(110mm)     A = .5(74mm)(82mm)
                    A = 1680 mm2               A = 3034 mm2
          Composition of Mixture:
                    Area of CH2Cl2 = 1680 mm2
                    Area of CHCl3 = 3034 mm2

                    1680/4714 * 100 = 35.6%
                    3034/4714 * 100 = 64.4%

                    Mixture is:      35.6% CH2Cl2
                              64.4% CHCl3

Conclusion:     The purpose of this lab was to find out how the different gases tested interacted with the surface of solid. The lab worked well for testing this. To even further test the interaction we sent of a mixture of the two gases through the column at the same time. By doing that we can compare the graphs of the single gases and of the mixture. Below is some of the information from that.

     Table 1
     CH2Cl2     CHCl3     Mixture CH2Cl2     Mixture CHCl3
TR     80     175     75     175
WB     115     260     115     290
N     7.74     5.38     6.81     5.83

The elution profiles of the single gases compared to that of the mixture actually match up really well. The biggest difference is the number of plates between that of the CH2Cl2 by itself and the CHCl3 of that in the mixture. The number of plates for the CHCl3 is pretty close for both the mixture and the single gas. The values of the retention times and base width are actually really close. The retention time for both the single gas of CHCl3 and the mixture are exactly the same as are the base width of the CH2Cl2. I would expect the measurements to be the same or close for all of these because if though one is a mixture the gases should separate and move through the column at the same rate they did the first time when they were the only mobile phase.

As with any experiment, there were more than likely some systematic and random errors that occurred. Some possible systematic errors that could have occurred would be that the flame shield and wire gauze pad did not completely block out all exterior light from entering. Therefore, the sensor could have been picking up on small amounts of extraneous light. Another systematic error could be that the coil was not wound into a tight enough coil, which would make the flame sputter and not burn as true. Some random errors, which could have taken place would be that the computer might have collected the information correctly if the sensor was malfunctioning any or if the computer was lagging. The biggest problem noticed in our lab was that it was hard to get the flame the right height because the valve on the buret would either turn to far, or not far enough. We did end up getting the flame to the right height though. Overall this experiment was a success and achieved what it was set out to do, explain how the different gases used interacted with the surface of the powder tide.

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