GC-FID Help

[jocelyn04]

Hi

I wonder if anyone could point me in the right direction. I realize that most might find this to be basic knowledge.

I’ve just started an MSc Analytical Chemistry course and completed my first lab session on GC-FID (FID response to compounds containing atoms other than C and H)

GC is all very new to me and so far we haven’t had any lectures on GC as yet, but I’ve been consulting books, however I couldn’t find any calculations for determining concentration from peak height/area and retention time.

Can anyone tell me which calculations I should be using, or how to find one? So far I have calculated the concentration of each component in my standard mixture.

I have three chromatographs for my standard sample and three for my unknown. Is it better to take an average of the three?

Any help that could be given would be very much appreciated

Thanks!!

[PJ8]

I've been taught to use peak area for concentration calculations.

For a single level calculation (ie. only one concentration of known standard)

concentration (known standard)/ peak area (known standard) = concentration (sample) / peak area (sample)

Therefore concentration (sample) = peak area (sample) * [concentration (known standard) / peak area (known Standard)]

If you have multiple standards of the same concentration then averaging them should improve the accuracy of your answer.

If you have a multi-level standard set-up (eg. standards differing significantly in concentration) then you can plot a graph of concentration against peak area, find the peak area of your sample on that and extrapolate it's concentration (preferably you need standards with lower and higher concentration than the sample to do this.)
Again if you have standards of the same concentration you can average the peak area to improve accuracy.

I would calculate a result for each unknown seperately and report the average or the highest depending on the nature of the testing. This all assumes your unknown is the same analyte as your standard.

[jocelyn04]

My standard solution is a mixture of Benzene, Chlorobenzene, Dichlorobenzene and Trichlorobenzene, where the weights of each component are known. My unknown is a mixture of the same components but with unknown weights of each

Thanks, that is really helpful

[jocelyn04]

I had to normalise the peak areas of each component of the sample to 100%, then compare the results to the % composition from my accurately weighed sample to obtain the correction factor. Does anyone know how i would do that?

[PJ8]

I think for % area normalisation sum total areas, then divide each individual area by this total and multiply by 100 to get an area % for each peak. Total for all peaks should equal 100%.

[DR]

I suspect that, in addition to getting you to properly calculate the amount of each analyte in the unknowns, your instructor wants you to be able to discuss the relative responses of each.

To do this, compare the area counts per unit of concentration for each analyte. You should notice that (if memory serves), per mg of analyte, the area goes down a lot for each halogen substitution.