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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: socomplicated on December 14, 2004, 10:09:59 AM

Title: Gas chromatography
Post by: socomplicated on December 14, 2004, 10:09:59 AM

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!

Title: Re:Gas chromatography
Post by: Demotivator on December 14, 2004, 10:17:00 AM

k is a proportionality constant.
Area1 = k x moles1
Area2 = k x moles2
therefore
Area1/Area2 = k(moles1)/k(moles2)
the k cancels out:
Area1/Area2 = moles1/moles2

Title: Re:Gas chromatography
Post by: socomplicated on December 14, 2004, 10:28:21 AM

Quote from: Demotivator on December 14, 2004, 10:17:00 AM

k is a proportionality constant.
Area1 = k x moles1
Area2 = k x moles2
therefore
Area1/Area2 = k(moles1)/k(moles2)
the k cancels out:
Area1/Area2 = moles1/moles2

Demotivator,

do you mean moles or molar?  ::)

Title: Re:Gas chromatography
Post by: Demotivator on December 14, 2004, 10:37:21 AM

I don't remember, it depends on how k is defined. It doesn't matter when dealing with a ratio, though, because all components come from the same volume of solution, so the volume cancels out. ie:
(moles1/liter)/(moles2/liter) = moles1/moles2

Title: Re:Gas chromatography
Post by: socomplicated on December 14, 2004, 10:43:16 AM

okay, thanks!   ;)

Title: Re:Gas chromatography
Post by: socomplicated on December 14, 2004, 10:49:33 AM

by the way, Demo; according to this saying; "the areas under the two peaks are directly proportional to the molar amounts of A and B in the mixture" what do you think of the molars of A and B if the area of A is 600 and B is 200 ? do you have any ideas about it?  ???

thanks again!

Title: Re:Gas chromatography
Post by: Mr Amino on December 14, 2004, 11:41:15 AM

Basically it means that if one peak is twice as big as the other, then it is present in twice the molar concentration.

So if one is 600 and the other is 200, the substance with the 600 peak has 3X the concentration of the other one with the 200 peak.

Title: Re:Gas chromatography
Post by: Demotivator on December 14, 2004, 11:48:11 AM

Right.
Also, typically a standard of known concentration is run in analysis, so if one determines the area for the known concentration of the standard, one can calculate the concentrations of the others.

Title: Re:Gas chromatography
Post by: jdurg on December 14, 2004, 03:22:40 PM

Really, for any quantitative analysis on a GC or GC/MS you MUST run a calibrated standard.  Otherwise your results are meaningless.

Title: Re:Gas chromatography
Post by: socomplicated on December 14, 2004, 03:38:48 PM

thanks!  :D

Title: Re:Gas chromatography
Post by: movies on December 14, 2004, 03:58:07 PM

If the molecules are different then the value of k can be different too.  If the k is different then it won't cancel out in the equation Demotivator showed.  You can account for it by solving for that ratio by running a GC with a solution of known concentration.

I think that is what jdurg means by a "calibrated standard."