Negative Peaks on UPLC w/PDA and FLR Detector

[Mwaite126]

I have recently been using an FLR detector and for one of my eleven compounds it shows a negative signal and all the others are fine. Now I have a PDA in line and I am able to compare the two FLR and the PDA signals so I know it is one of my peaks of interest, I just cannot figure out why it is upside down. with my timed events, emission, excitation and gain all correct in my method set i do not know what else to do, any ideas?

[Alfred88]

Did you dissolve samples in "compatible solvent" ?

[Uwe Neue]

It is possible to create negative peaks with FL. The background at the monitoring wavelength could be residual signal from the excitation wavelength, and if your analyte absorbs the excitation wavelength but does not fluoresce, you can get a negative signal.

If you can integrate the negative signal, you can check it for linearity. If you get decent and linear response over your range of interest, then there is nothing to worry about. Mixtures of positive and negative signals are common with RI for example, so the software should be able to integrate both signals.