FID and concentrations of analytes

[tolisfoto]

Are there upper limits in the concentration of a compound for FID detector? What must be a typical concentration of a standard for FID detector?

thanks in advance

[MikeD]

Must depend a little on design and gas flows, but I would guess above 10 micrograms per second the FID is starting to go non-linear. From your peak widths and split ratio (if any) you will have to convert 10 ug/s to mass or concentration injected in your own application.

[oscarBAL]

Sorry, MikeD, but I really dont understand that, probably is usual but for me is the first time that read about sencibility in termes of ug/sec, have sence, but could you explaime why?

[MikeD]

Well, first of all you are unlikely to reach the upper limit of linearity for the FID unless you are using a packed column with no split. A capillary column would overload long before the FID reaches its limit.

The linear range of the FID is specified in mass/sec because manufacturers measure it that way, perhaps with propane mixtures fed continously to the detector.

The following relation between mass/sec and the peak mass is not mathematical - it is a crude approximation good enough for the purpose. Say the peak width at base is 5 seconds and the peak contains 1 microgram. That is an "average" of 1/5 = 0.2 ug/sec through the FID and a peak of about twice that at 0.4 ug/sec. Therefore in this example the detector is well inside the linear limit. It is worth noting , however, that 1 microgram on a typical capillary will cause peak distortion or fronting.

[oscarBAL]

[Thank you Mike for your explanation, now is clear for me.