Fluorescence Detector x Negative Signal

[Carlos Teixeira]

My Dears,

Why a Fluorescence Dectector can produce negative signal?

Kinds Regards,

Carlos Teixeira

[Uwe Neue]

the simplest explanation is probably straylight (residual light from the excitation wavelength) that is absorbed.

[HW Mueller]

A bit more detail: There is always some scattering of light in the cell, so there is always a bit of light hitting the detector (the amount depends on the solvent passing through). If you now pass an absorber through the cell which does not fluoresce (or not fluoresce at your emmision wavelength), one has less light to scatter, thus a negative peak.

[Rafael Chust]

In my opinion, basically the same way there is negative peaks in UV detectors...

You must find a solution looking at the stuff you are trying to analyse. Fluorescence has to do with excite a molecule under a given wavelenght and, when this molecule comes back to its original state, emits a radiation under another wavelenght.

Therefore, what might be happening as there must be some molecules - or functional groups - that are absorbing the wavelenght emitted by other molecules - or functional groups.

This might looks confusing, but imagine a mechanism like a chain reaction and you will be able to understand what I am saying!

[HW Mueller]

If you absorb part of a fluorescence you get a dent in or a lowering of a peak, an actual real negative peak (one that goes below the baseline) would be hard to get that way, you can only absorb light if there is any. A possibility would be if you used a mobile phase which fluoresces at the settings you use.... a fairly bad procedure.