Chromatography Forum

Hello,

my case: the same mobile phase, the same injector (manual), the same column and 2 detectors, lambda 214nm, sample in mobile phase, repeatability ideal.

UV:
http://www.fotosik.pl/zdjecie/fe44c5ad8c730357

DAD:
http://www.fotosik.pl/zdjecie/997411d7451a0fb3

Rt - 6min - the reason for the difference and negative part?

Regards

Without knowing more details about the column dimensions, flow rate, temperature, etc. it's hard to be definite. However:
- The t0 noise looks to be at the same time, so that suggests that the flow rate was the same.
- From the time stamps on the chromatograms, it looks like the runs were made on two different days (i.e., the detectors were *not* in series). Combined with the similarities in t0, that suggests that the retention time difference is related to your system chemistry rather than the detector. The proof, of course, would be to re-run with the DAD and see if you get the original retention times again (I'd be willing to bet the price of a pizza that you don't).

The negative dip between 6 and 7 minutes is actually present on both chromatograms; it is more pronounced with the UV detector. Possibly it's a "system peak" generated by the injection process upsetting the equilibrium of the system. Try using your mobile phase as the sample diluent; if the negative dip goes away, that would confirm the "system peak" hypothesis.

Hello

If you're comparing 2 detectors like DAD and VWD (simple UV) you will always have different chromatograms. It is because the optical units are not the same.
Of course asuuming you have the same parameters (reference wavelength, bandwith, slit).
If you want to compare results on 2 detectors you should take response ratio between 2 peaks (or heights to areas ratio).

An like Tom mentioned you need to provide more details about your method.

Regards

Tomasz Kubowicz