UV MS

[Mouleyre]

Is it possible when analysing materials on LC-MS and LC-UV, and you see an impurity on the MS but not sure where it would elute on the UV, is it possible to figure out where it would elute if you had rrt of other known impurities on both the UV and MS chromatography.I think it could be possible to see the fragment on the UV currently but not sure

[Multidimensional]

Normally, a good quality MS system includes a scanning diode-array detector (DAD) INLINE. Running them separately on different systems makes the data NOT comparable. "Retention" is not changed by the detector used. Retention times are a function of the HPLC METHOD used (a:"method" includes all parameters, settings, the column, detector(s), mobile phase etc).

Running on the same system with the same HPLC method having both detectors inline allows you to easily collect two dimensions of data (UV/VIS plus some type of mass detection) for the peaks which are observed. The retention times (for the same Peak) will be nearly identical this way, factoring in the small difference in delay volume in tubing from the DAD flow cell to the MS source. Because of differences in physical/chemical properties (and mobile phase, instrument settings etc), no single detector can "see" all compounds. The MS detector will not detect many types of compounds. The DAD will also not "see" many types of compounds too. No ":universal" detector exists for HPLC, BUT having the two detectors inline will allow you to collect more data so you can later compare the results obtained to real standards.

[lmh]

Yes, it's best to run on one system that's equipped with UV and MS detection, because you can find the delay time between the two detectors by using any large peak in a simple sample, where you have no doubt that you're looking at the same chemical on both. The delay time will be the same for all chemicals, because it's just a matter of volume flowing down an inert bit of tubing. So you can use it to find the time that any UV peak would have arrived at the MS, or the time that any MS peak should have been passing through the PDA.

If you are obliged to work on two separate systems because that's what you have, and it can't be changed, then of course you can try something similar. The best you can do is make your chromatography as identical as possible: run the same gradient with the same column and the same solvents. The retention times will be different, but they will (hopefully) be consistently different. If there were no such consistency between different systems, there would be no point in literature methods or publishing typical chromatograms. You will need to check several peaks at different retention times to plot yourself a sort of "calibration curve" of retention time on the UV system versus retention time on the MS system. Simplistic explanation: if one system pumps a bit faster, then from an isocratic point of view, everything comes out at a fraction of the other one's retention times: things might come out at 90% of the retention time. But if one system has a greater delay volume post-column, everything will come out a fixed amount of time later (not a percentage). So in this simple situation, the conversion chart would be some sort of linear thing that probably doesn't pass through the origin, nor have zero slope. In a real comparison, it might be a curvy thing. The process isn't perfect, it's not guaranteed to work, but in the right circumstances it might get you out of a hole.

This is the sort of thing that's probably totally inappropriate in a regulatory environment, but necessary and informative in a university with limited funding.