Response factor of FID

[wl_yang]

I am doing a research on FID response factor. However, an important issue is confusing me......

I do need the response factore that could be directly used to correct the concentration measured from the instrument. I call it "absolute RF".

However, all the RFs I've got from the literatures are kind of "relative" RF. For example, Dietz (1967) first tabulated the response factors for various compounds for FID, using n-heptane as a standard with a define RF of 1.00. The popularly-used Dietz RF can be expressed as follows: RF(Dietz)=[(area of compound)/(mass of compound)]/[(area of reference)/(mass of reference)]. Different references were used in different literatures, such as tetradecane and fluoranthene. Although I have a big list of these "relative" RFs, they could not be used directly.

Therefore, I am trying to convert the relative ones to the absolute ones. A technican in our lab gave some "absolute RF" values to me, but they seem too far from the ones that I convert from the literature "relative RF".

I really wish to get some help or discussion on this issue with somebody here. Thanks!!

[skunked_once]

I question whether there can be an "absolute" response factor because of the response variation between detectors and variations in response due to hdrogen and air flow rates. In analytical chemistry practices, all detectors need to be calibrated individually based on standards of known concentration. Aren't response factors relative by definition?

[JI2002]

Spike the target compound standard with internal standard and inject it to GC/FID. You can calculate RRF from the responses and concentrations of the IS and target compounds. Your RRF should be very close to what you find in the books.

[lmb]

Dear wl_yang,

I do not know much about FID, but still a few following comments might be helpful.

First, I think that it is important for answering your questions to recognize that FID is sensitive to amount (not to concentration) of a substance. Once you know the sensitivity of FID to the amount, you can find its sensitivity to the concentration in each particular case (column, carrier gas flow, flow of make-up gas, etc.), but this would be the next layer of calculations.

As far as I understand it, FID generates one ion (one unit of electric charge) per certain amount of carbon atoms in a substance that it burns. In more technical terms, FID has a certain ionization efficiency which (as I remember) is somewhere between one ion per 100000 to 1000000 carbon atoms. Ionization efficiency of a particular FID in a particular GC analysis depends on the FID design and other factors. Because it is sensitive to the number of carbon atoms, FID is more sensitive to substances with higher molecular weight (having more carbon atoms per molecule).

Basically, ionization efficiency of FID is all one needs to know in order to figure out its sensitivity to amount or concentration of any substance in any GC analysis. However, it involves many additional details. I hope that these comments give you a useful starting point.

Good luck,
lmb