Chromatography Forum

Hey everybody, I am familiarizing with GC-detectors and I was wondering about linear range.

In FID e.g, the detection limit is 2pg/s and the linear range is >10^7. I understand what they mean but what I don't get is about the realtionship between them. I know that with higher linear ranges, you can solve more different range of solutes, but why?

Thanks in advanced

There is no connection between the two claims, per se.

One can have a low detection limit but no linearity. This means that at the lowest limit of detection, if you were to double the concentration over a range of concentrations, say 2, 4, 8 16, 32, 64, 128, ........1024 picograms, a graph of the peak areas would not be consistent or linear.

ECDs are like this. Which is why they are great for methods that want to give results in not more than xxx picograms. But if you wanted to determine the exact amount you would have to prepare a std at the amount you think is present to compare to your sample.

Now a FID is linear over a large range. That means if you made a std at say:

64 picograms (having an area of 1000) and when you did a test you found a peak of 10,000, or 100,000 you could infer than your sample contained 10 X the amount of std or 640 picograms or 100 X the amount of std, or 6400 picrograms, because the detector is linear over this range.

You could also assume that while performing a purity test for a solvent (assuming an equi-molar response) that by measuring all the peaks, from a 0.001% to 99%) a true percent level purity measurement could be made.

I hope I was clear.

best wishes,

Rod

Thank you so much Rod, your answer was really clear. It took to read it twice because I am still learning CG but it has been really useful.

Best wishes,

Tere.