Question

how is it theoretically possible to determine the diastereomeric ratio of isomers using gas chromatography ?

Answer 1

Diastereomers are the compounds having at least 2 stereo centres. These isomers can be theoretically separated by chromatographic techniques.

Considering the GC, in case of a mixture of 2 diastereomers, 2 peaks will be obtained. As the molecular formula for the diastereomers is same, in high likelihood, both isomers will give same GC response. This means, if you inject the same amount of each isomer in GC, you will get the same peak area.

By dividing the area of the peaks for each isomer you will theoretically get the diastereomeric ratio.

Let us say the isomers are x and y.

1 mg of x and 1mg of y will give same area in gc.

Now, if your mixture shows gc peak area of 1000 for x and 100 for y, then you can say that you have the dr of 1000/100 ie 10/1.

Based on GC area it shows that x is 10 times more than y in your mixture.

Alternatively,

You can generate concentration vs area plot for x and y separately. This is done by injecting different concentrations of x and y and measuring the peak area.

Now when you inject the mixture, you can compare the peak area obtained for unknown sample against the plot and determine the concentration of x and y from the plot. Concentrations can then be extrapolated to exact quantity of x and y. Dividing these quantities will give the dr