Gas Chromatography

Gas Chromatography (GC) is used to separate volatile components of a mixture.  A small amount of the sample to be analyzed is drawn up into a syringe.  The syringe needle is placed into a hot injector port of the gas chromatograph, and the sample is injected.  The injector is set to a temperature higher than the components’ boiling points.  So, components of the mixture evaporate into the gas phase inside the injector.  A carrier gas, such as helium, flows through the injector and pushes the gaseous components of the sample onto the GC column.  It is within the column that separation of the components takes place.  Molecules partition between the carrier gas (the mobile phase) and the high boiling liquid (the stationary phase) within the GC column.

Top View of Oven and Columns

 

Two columns will fit inside the oven of our GCs.  A heating element is used to raise the oven temperature, when desired, and thus raise the column temperature.  GC columns typically have a metal identification tag clipped onto the column that lists column length and diameter, what material is inside, and the maximum operating temperature.

After components of the mixture move through the GC column, they reach a detector.  Ideally, components of the mixture will reach the detector at varying times due to differences in the partitioning between mobile and stationary phases.  The detector sends a signal to the chart recorder which results in a peak on the chart paper.  The area of the peak is proportional to the number of molecules generating the signal.

To use the GC, follow these simple steps:

1.      Wash a syringe with acetone by filling the syringe completely and ejecting the waste acetone onto a paper towel.  Wash 2-3 times.

2.      Pull some of your sample into the syringe.  You will most likely need to remove air bubbles in the syringe by rapidly moving the plunger up and down while the needle is in the sample.  Usually 1-2 mL of sample is injected into the GC.  It is okay to have small air bubbles in the syringe.  However, you do not want to inject mostly air or your peaks will be too small on the chart recorder.

3.      Make sure the chart recorder is on and set to the appropriate chart speed (Arrow A). Set the baseline using the zero on the chart recorder (Arrow B). With the pen in place, turn on the chart (Arrow D), make sure the pen is down (marking the paper) and the paper is moving.

Arrow A Set chart speed in cm/min
Arrow B Set zero so that the baseline is ~ 1 cm from bottom (right edge) of chart paper
Arrow C  Record (but do not adjust) full scale setting
Arrow D Switch to turn movement of chart paper on and off.

4.      Inject your sample onto either column A or column B as instructed.  Hold the syringe level and push the needle completely into the injector.  Once you can no longer see the needle, quickly push the plunger and then pull the syringe out of the injection port.

Injection Notes:
A)   The injectors are very hot, so be careful not to touch the silver disk.
B) The needle will pass through a rubber septum, so you should feel some resistance.  For some of our GC’s, the column does not align properly in the injector, so the needle hits the front of the metal column.  If you feel that you are pushing against metal, pull the needle out of the injector and try again, perhaps at a slightly different angle.  The needle should completely disappear into the injector for proper injection of the sample onto the GC column.
C) Inject quickly for best results.  Do not hesitate to inject once the needle is properly positioned in the injection port.
D) Remove the syringe immediately after injection.  (Carrying out notes C and D helps to insure that all of the sample enters the GC column at approximately the same time.)

5.      Mark your injection time on the chart recorder.  This can be done by adjusting the zero just after the sample is injected.  It is often convenient for one person to inject the sample while a lab partner marks the injection time at the chart recorder.

6.      Clean your syringe immediately after injection.  Syringe needles often clog quickly and must be replaced if they are not cleaned after each use.

7.      Record the settings of your chart recorder during a run.  You need to know the chart speed and the full-scale setting.

8.      Record the settings of your GC during a run.  A knob on the bottom center of the GC can be turned to read column (or oven) temperature, detector temperature, and injector port temperature in °C.  The bridge current is displayed in mA.  Note that there are two scales on the display.  Be careful to read the appropriate scale!

Arrow A Top scale is reported in milliamps and is used to read the bridge current.
Arrow B Bottom scale is reported in degrees Celsius and is used to read all temperatures.
Arrow C Typically the ONLY knob to be adjusted by students.  Knob is turned for corresponding reading on the scale above the knob.
Arrow D Increasing the Attenuator setting decreases the area of a peak on the chart recorder.  This knob should only be adjusted with permission of instructor.  Always return the knob to its original setting if you are given permission to change it.

Analysis of the Gas Chromatograph

Report the retention time of each peak (in minutes), the identity of each component in the mixture, and the percent composition of the mixture.  To determine the percent composition, you will first need to find the area under each curve. 

Area = (height) x (width at ½ height)

Mark retention time, height, half-height, and width at ½ height on your GC trace.  Show your calculations either in your final report or directly on the chromatograph.

You may assume that each component of the mixture causes the same response in the detector.  Therefore, the areas under the curves can be used to calculate percent composition of the mixture of alkenes. (This is a reasonable assumption when the components of the mixture are very similar in structure, as are 2-methyl-1-butene and 2-methyl-2-butene.) 

% Component 1 = [(area under peak 1)/(total area)] x 100%

The sample used to obtain the GC trace that is shown above did not have any solvent in it. Student samples will have at least one solvent present, so you will see another peak in your GC traces that typically appears very soon (usually within a minute) after injection. It is normal for the solvent peak to go off scale.

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