What does it mean when two compounds from the GC-FID analysis share a single area %?

[n8cole]

Here is a fictitious example to clarify my question.

Regarding the % composition of the mixture, what does it mean that propane and butane share the same area %?

My guess is that a certain percentage of propane and a certain percentage of butane make up the 15%. But, if that is the case, how is the GC-FID able to identify the two distinct compounds, propane and butane, but not determine their individual area %s?

 

[TiCl4]

This is likely due to how the GC program is set up. Elution times from the GC must be quite similar at the given setup, causing a unimodal or broad pead. Thus, the program was told that "x" to "y" time is considered a mixture of propane and butane since it does not have the resolution to identify quantities of individual components. GC does not ever actually "identify" compounds. It just checks peaks vs time, compares that to the database its given for the certain method, and spits out what corresponds to that time.

There could also be to distinct peaks for each, but with elution times close enough that the right tail of the first overlaps the left tail of the later chemical. A common practice if the overlap is small is to take the area % halfway between the two distinct peaks, but it may be for this GC that the peaks are too close for that approach to work.

It would be helpful if you could post a picture of the curve.