2
$\begingroup$

I'm supposed to analyse the two GC graphs in the attached picture (I quickly drew them instead of taking a picture). The question states ths "A mixture contains 3-Methylhexane. To determine the mass of 3-Methylhexane in the Mixture, a 1:1 - Mixture of pure 3-Methylhexane with the comparative substance Hexane was produced and analysed by GC (first graph). The second graph shows a GC of the same mixture after adding 1g of Hexane. Question: Find the mass of 3-Methylhexane in the mixture."

Now, first thing I'm confused about is the area of the hexane peaks. How could the area be the same if there is more ($\pu{1 g}$ more) hexane in the second graph?

The areas of the peaks are:

|----------+--------+----------------|
| analysis | hexane | 3-methylhexane |
|----------+--------+----------------|
| run 1    |  0.675 |           1.02 |
| run 2    |  0.675 |           0.90 |
|----------+--------+----------------|

Could someone give me a hint of how to proceed?

As depicted in the scheme (not drawn accurately) I calculated an area ratio about the data of run 1 (left hand side) as: 0.675 / 1.02 = 0.66. But how do I determine masses now?enter image description here

Improve this question
$\endgroup$
9
  • $\begingroup$ Don't worry about the absolute areas. Work with the ratios to set up the initial equations. You have two equations in two unknowns: x/y = r1 and (x+1g)/y = r2. Solve for x and y. $\endgroup$
    – Buck Thorn ♦
    Commented Aug 29, 2023 at 6:39
  • 1
    $\begingroup$ Something seems seriously confused in the task. $\endgroup$
    – Poutnik
    Commented Aug 29, 2023 at 7:59
  • $\begingroup$ Poutnik is right. First difficulty : the area of $3$-methylhexane cannot be $1.02$ and $0.9$ as given. Visually, the first area looks about $1.3$ or $1.4$ and not $1.02$. Second difficulty : The area of the two peaks of hexane are exactly the same (to less than $0.1$ %), although the second graph is made with more hexane. This is difficult to understand. The whole problem looks as if both graphs have been interchanged, and some more $3$-methylhexane has been added in the first graph, with the same amount of hexane in both graphs. $\endgroup$
    – Maurice
    Commented Aug 29, 2023 at 8:55
  • $\begingroup$ Did you perform the GC with a FID/TIC? Especially with compounds as volatile as hexane, the absolute numbers and peak shapes can vary depending on temperature gradient, column type, column length, use of an automatic sampler vs. a manual injection, etc. Then, the raw data collected, same protocol on both for background correction and integration criteria? For an analysis of added standards, the ratio of your additional $\pu{1 g}$ to the total ($\pu{10 g}$ vs $\pu{100 g}$, for example) equally is a detail to keep track. (It can be less relevant for the specific question here, though.) $\endgroup$
    – Buttonwood
    Commented Aug 29, 2023 at 10:55
  • $\begingroup$ @Maurice Sorry for confusing you. I just quickly drew these graphs in MS Paint because otherwise I would have had to take a picture. The values are as close to the correct ones as I could calculate from the (low res) image I received. I would totally agree with your conclusion that 3-methylhexane would have to have been added, but the task doesn't say that for some reason. $\endgroup$
    – Ladan
    Commented Aug 29, 2023 at 15:49

0

Reset to default

Browse other questions tagged or ask your own question.