Chromatography Forum

Hi All,

Could someone please be kind helping me understand the definition of detection limit of the FID, which by vendors is given as e.g. 2 pg C/s. What I do not understand is the time part of it.

Does this mean that if we have 2 pg of methane which is passed through the fid in 0,5 s, giving 1,5/0,5 pg C/s = 3 pg C/s the peak is detected BUT if the methane is passed in 2 s, giving 1,5/2 pg C/s = 0,75 pg C/s, the peak is not detected? Is the detection limit flow dependent?

Thanks in advance,
Chromos

FID is mass dependent. So the quoted limits are defined with a time element as the on column amount will be spread over the peak width. You are correct in your calculation but it is not flow rate dependent other than in how this affects your peak width.

Hi All,

Could someone please be kind helping me understand the definition of detection limit of the FID, which by vendors is given as e.g. 2 pg C/s. What I do not understand is the time part of it.

Does this mean that if we have 2 pg of methane which is passed through the fid in 0,5 s, giving 1,5/0,5 pg C/s = 3 pg C/s the peak is detected BUT if the methane is passed in 2 s, giving 1,5/2 pg C/s = 0,75 pg C/s, the peak is not detected? Is the detection limit flow dependent?

Thanks in advance,
Chromos

Although the FID is mass sensitive, the detection limit still depends on the peak height. So I think it depends on flow rate.

The specification for detection limit of mass per unit time is indpendent of carrier flow rate, which is why the manufacturers of the detectors specify in those terms - it makes the detector performance independent of the column and its operating conditions.

Of course, a chromatographic peak is not a continuous introduction of a fixed amount of carbon per unit time, so the detection limit in terms of mass per unit time does not translate straightforwardly to a detection limit in terms of mass in a peak - mainly because peak height and width for a given amount of carbon do depend on flow rate, temperature and column dimensions.

Peter

Thanks for the replies! So how do I translate e.g. 2 pg C/s into something useful like how many ppm propane I can detect in some inert gas?

You have to make so many assumptions, and measure so many things that are not straightforward to measure, that it is better to just inject some standards of known concentration and work from there.

Peter