UV and PDA detector not giving same result?

[docf07773]

Hi all,

I'm having trouble interpreting some data I have acquired.

During some method development work I've been running sample injections with UV at 214 and 254 nm, both of these give me essentially the same chromatogram with adequate response - note that these wavelengths were chosen simply because they're what the detector was set to from the last time I ran it. I ran the PDA at the same time, from 190-500 nm - expecting to catch any results if the UV didn't pick it up. The PDA chromatogram (all wavelengths) gives the same overall result as the two UV wavelengths, which is what I was expecting/hoping for. The problem is that I don't know how to read the spectrum/ion map for the data produced. I had hoped that I could use the PDA spectrum to pick a suitable, specific wavelength(s) for my UV/Vis detector. I tried drawing the PDA chromatogram with a range of wavelengths - e.g. I tried 200-275 since that covered my UV ranges, but it gives me no data. I don't get any signal until I set the range to 425-430. My understanding of this is that I should set my UV wavelength to 425 with a bit of wiggle room then, and that should be a good wavelength for my sample. But why does my UV show data at 214 and the PDA not? Have I done something silly, or am I fundamentally not understanding the relationship between the UV and the PDA?

Any help would be much appreciated.

[Steve Reimer]

Just a dumb question, but does your PDA have separate UV and vis lamps?

[James_Ball]

Just a dumb question, but does your PDA have separate UV and vis lamps?

If it is the 1100 PDA it would have separate lamps.

Do you have the spectral module for the software so you can look at the iso plot of the spectrum or the 3D spectrum/chromatogram overlay? Those are what I use to determine max absorbance for each peak when developing a new method, much easier than just redrawing at various wavelengths and you can actually see there the absorbance maximizes.