Bad sensitivity in low concentrations (HPLC FLD)

[wistar]

First of all, sorry for my english, my native language is Spanish.
Im working with a HP 1046A Detector.
when Injecting Doramectin previusly derivatized i cant get a good peak of 100ng Standard sample, but when i inject a 400ng standard i get a great peak.
Someone told me that my flow cell may be too big, and the smallest samples are diluting inside it..
Somebody have an idea of what can be happening?
I need to read 10ng, that shouldnt be a problem with this detector.. but i cant find a way to solve this.

Regards.

[Peter Apps]

What do you get at 200 ng, is the response half of what you get at 400 ng ? In other words, is the response at different concentrations linear over any range of concentrations ?

Dilution will affect all peaks similarly.

Are you derivatizing different concentrations of Doramectin, or diluting derivatized Doramectin ?

Peter

[wistar]

200 ng can be seen as a good peak but much smaller than half of 400 ng.
So the areas are not linear.
i evaporate each sample and derivatize separataley.

[Peter Apps]

To test whether the problem is with the derivatization or the instrument; derivatize a double batch of 400 ng; inject half of it at full concentration, dilute the other half x2 and inject half of that, then dilute the remainder x 2 and inject that. You will then have a 400 - 200 - 100 ng series, all derivatized under the same conditions.

Peter