Gas Chromatography: Example of Molar Proportion of A & B

In summary, the statement "the areas under the two peaks are directly proportional to the molar amounts of A and B in the mixture" means that the area under each peak on a gas chromatogram is directly related to the number of moles of each substance in the mixture. This relationship may not always be 1:1 and can be affected by the type of analyzer used. To accurately quantify the amounts of A and B, a 1:1 mixture of the compounds can be analyzed and the areas compared to determine the proportion between the two peaks. The internal standard technique can also be used for quantification by comparing the peak areas of a known substance to the analyte peaks. It is important to choose an internal standard that is un
  • #1
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what does it mean when it says; "the areas under the two peaks are directly proportional to the molar amounts of A and B in the mixture?


directly proportional? can anyone give an example of it?

many thanks!
 
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  • #2
If the number of moles of substance doubles, the area under the peak will double too. By analyzing the area under the peak, you can quantitate the relative amounts of A and B in the mixture.
 
  • #3
Depending on the type of analyzer you have on your GC instrument, this won't necessarily be a 1:1 relationship between two different compounds. The relationship for each individual compound is proportional to the number of moles of that compound. Typically GCs use a combustion analysis to measure what is coming off of the column. Therefore, the area under the peak will depend on the particular molecule that is being combusted; in general larger molecules will give a greater peak area because more heat is released in their combustion. The way to solve this problem is to make a 1:1 mixture of two compounds and measure the areas you get from the GC trace of the mixture. Then, since you know that there was an equal number of moles of each compound, you can figure out the proportion that relates the area of the two peaks. This can allow you to calculate the yield of your reaction based just on the GC trace!

Note that in most cases you can assume that the relation between the two different compounds is 1:1, but if you want precise results you should figure out the relation I described.

I bet this sounds complicated. Sorry if I didn't explain it very well.
 
  • #4
Movies has explained this greatly, but I wanted to add this one. "Internal standard technique" is frequently used for quantification, in which a known amount of substance is added and whose peak is known to occur far from the analyte peaks. So, comparing the peak areas with analyte peaks, a quantification is very easy to calculate.
 
  • #5
Chem_tr makes a good point. When using internal standards it is also important that the internal standard be unreactive under the conditions of the reaction you are monitoring. High molecular weight n-alkanes are therefore good for this purpose. I frequently use tridecane.
 

1. What is gas chromatography and how does it work?

Gas chromatography is a technique used to separate and analyze the components of a mixture. It involves injecting a sample into a chromatography column filled with a stationary phase, and then passing a gas (the mobile phase) through the column. The different components of the sample will interact differently with the stationary phase, causing them to elute at different times and be detected by a detector at the end of the column.

2. What is the purpose of using molar proportion in gas chromatography?

Molar proportion, also known as mole fraction, is a measure of the relative amount of a specific component in a mixture. In gas chromatography, it is used to determine the relative concentration of each component in the sample, which can provide information about the composition and purity of the mixture.

3. Can gas chromatography be used to separate and analyze all types of mixtures?

Gas chromatography is most commonly used for separating and analyzing volatile organic compounds, but it can also be used for non-volatile substances by first converting them into volatile forms. However, there are some limitations to what can be separated and detected using gas chromatography, so other techniques may be more suitable for certain types of mixtures.

4. How is the molar proportion of A and B calculated in gas chromatography?

The molar proportion of A and B in gas chromatography is calculated by dividing the peak area of each component by the total peak area of all components in the sample. This will give the mole fraction of each component, which can then be converted to molar proportion by multiplying by 100%.

5. What are some factors that can affect the molar proportion of A and B in gas chromatography?

The molar proportion of A and B in gas chromatography can be affected by factors such as the composition and concentration of the sample, the type of stationary phase and mobile phase used, the temperature and pressure conditions, and the sensitivity of the detector. It is important to carefully control these factors in order to obtain accurate and reproducible results.

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