Chromatography is a term that refers to the separation of chemicals in a mixture so that they may be used or detected separately. Gas chromatography involves separation of compounds with comparatively low boiling points for easy detection in complex mixtures. The separation compound is based on their variations in vapor pressures and level of affinity to solid material at the test instrument. Since the vapor pressure of each compound depends on the intermolecular forces pulling molecules together, gas chromatography utilizes any of the properties of matter. In this technique, a sample of test chemical is injected through a syringe into a specialized instrument. A carrier gas, which is normally helium, is used to sweep the injected sample into the instrument where it is separated into analytes, the individual chemical components. Both vapor pressure differences and affinity to stationary phase facilitate the separation. Since vapor pressure is directly related to ambient temperatures, the temperatures of the instrument are adjusted to achieve the desired pressure during the run. At the exit of analyte molecules in gas chromatograph, a detector senses these molecules.
For this experiment, thermal conductivity detector TCD was used to measure the variations in carrier gas properties. The alterations result from the existence of the separated molecules of analyte inside the carrier gas flow. The analyte molecules attach separately to the stationary phase, hence they travel at varying rates via the GC column. This implies that their column retention times are different. A peak signals the presence of an analyte once it appears within the detector. Therefore, a gas chromatogram contains a sequence of peaks, each sample component for one. A computer screen or chart recorder is used to display the chromatogram.
Figure 1 Gas Chromatography System Schematic
For this experiment, the following equipment and material were used:
- Gas chromatograph
- Samples with equal volumes of pure butanol, hexanol, octanol, and decanol were prepared
- Chart paper was run at a rate of 1 cm per minute
- 5 μL of every sample were injected into GC and chromatographs collected on chart paper after the run
- The collected chromatograph was labeled as per sample name and necessary critical parameters collected
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For subsequent tests, the temperature settings were changed to maximize separation of peaks.
In conclusion, the experiment was able to collect the relevant data required for the separation of alcohols using gas chromatography. Since the injected test samples were pure in composition, the required parameters were simply read and recorded from chromatographs generated in the GC instrument.
Crouch, Stanley R., R. Stanley, and Douglas A. Skoog. “Principles of instrumental
analysis.” Brooks/Cole Cengage Learning: Belmont, CA 6 (2007): 498-542.