Gas Chromatography - Organic Chemistry

In summary, the difference in stationary and mobile phases between GC and TLC contributes to the varying times for molecules to reach the detector in GC, while in TLC the stronger polar-polar interactions on the polar stationary phase result in more consistent migration times for molecules. Additionally, the random adsorption/desorption process in the column further contributes to the variable migration times in GC.
  • #1
singinglupine
15
0
Why do ten molecules of toluene that enter a GC column at exactly the same time not reach the detector at the same time?

Why are London Dispersion forces of the sample molecules strongly influencing GC measurements? And why are they negligible in TLC experiments on silica gel?

I have no idea how to answer the first question. The second question I'm thinking deals with the fact that the stationary phase in TLC, silica gel is polar, while the stationary phase in GC is relatively non polar. So in GC there are more weak interactions, nonpolar to nonpolar and in TLC there are stronger interactions, polar-polar aka dipole-dipole interactions.
 
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  • #2
singinglupine said:
Why do ten molecules of toluene that enter a GC column at exactly the same time not reach the detector at the same time?
At the very least, remember diffusion: if you drop dye into a glass of water, the dye molecules will diffuse to eventually homogeneously fill the glass of water. Same thing with toluene molecules in a GC column. They spread out based on the statistical distribution of their velocities.

singinglupine said:
The second question I'm thinking deals with the fact that the stationary phase in TLC, silica gel is polar, while the stationary phase in GC is relatively non polar. So in GC there are more weak interactions, nonpolar to nonpolar and in TLC there are stronger interactions, polar-polar aka dipole-dipole interactions.
This is a good answer. I might mention also that the mobile phase in GC is generally also nonpolar (helium or argon), whereas the mobile phase in TLC can be either polar or nonpolar.
 
  • #3
What slows down the molecules in the column is the constant adsorption/desorption process. That process is random, so the time a given molecule will take to go down the column is variable.
 

1. What is gas chromatography?

Gas chromatography is a technique used in organic chemistry to separate and analyze the components of a mixture based on their physical and chemical properties. It involves passing a gaseous mixture through a column filled with a stationary phase, where the different components of the mixture will interact differently and be separated as they travel through the column. The separated components can then be detected and identified using a detector.

2. What types of compounds can be analyzed using gas chromatography?

Gas chromatography is mainly used for the analysis of volatile compounds, which are substances that can easily be converted into a gas at room temperature. This includes organic compounds such as hydrocarbons, alcohols, esters, and fatty acids. Inorganic gases such as carbon dioxide and nitrogen can also be analyzed using gas chromatography.

3. How does gas chromatography differ from other chromatography techniques?

Gas chromatography differs from other chromatography techniques, such as liquid chromatography, in that it uses a gaseous mobile phase instead of a liquid. This allows for faster separations and the ability to analyze volatile compounds. Gas chromatography also typically uses a higher temperature and lower pressure, making it a faster and more efficient method.

4. What factors can affect the separation and analysis in gas chromatography?

The separation and analysis in gas chromatography can be affected by several factors, including the type of stationary phase used, the temperature and pressure of the system, and the flow rate of the carrier gas. The chemical properties of the compounds being analyzed, such as their boiling points and polarity, can also impact the separation and analysis.

5. What are some common applications of gas chromatography in organic chemistry?

Gas chromatography is widely used in organic chemistry for various purposes, such as analyzing the purity of a compound, identifying unknown compounds, and quantifying the amount of a compound in a mixture. It is commonly used in environmental, pharmaceutical, and forensic analyses, as well as in the production and quality control of various products, such as food, beverages, and cosmetics.

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