Chromatography Forum

Hi folks..

Quick question for RI users out there..

I'm running a methd that has a # of peaks before 5 min and noting for 20 min followed by a couple peaks at 30 min. Would I be able to shrink the analysis time be increasing the flow rate between 5 and 30 min? and would this have any negative effects on my analysis? I'm assuming, since the composition of the solvent is not changing (RI should have no issue with this), and there are no components coming out at those times, everything should be OK.. any comments??

Thanks
- hubert

Sounds OK, as long as the increased flow rate doesn't overpressure.

With any type of concentration-sensitive detector, changes in flow rate will effect peak area, but not peak height (at least in theory).

So if you are quantitating by area, it would be important to allow enough time for the flow rate to come back to its appropiate value before the peaks of interest pass through the detector. In other words, if there is a peak at 20 minutes, don't reset the flow at 19.9 minutes. Allow some time - maybe a minute - for the flow rate to reestablish itself.

If you are quanitating by height then (theoretically) you wouldn't have to worry about this.

I'll agree with Adam -- in theory. In practice, my experience has been that RI detector response is somewhat flow sensitive, so it is doubly important to let flow stabilize.

That said, for state-of-the-art columns (certainly 3-micron, and to a certain extent 5-micron), the van Deemter plots for small molecules are fairly flat, which means that you shouldn't lose too much efficiency by simply running at the highest flow rate your system will tolerate through the whole run. This assumes you can collect data fast enough to characterize the early peaks.

To add a few points to Tom's post: this might not be trivial at all with an RI detector. Flow rate changes create changes in mobile phase temperature, especially if you are using a column with a smaller particle size. I have not played with an RI in probably over 20 years, but if my memory serves me correctly, you change ANYTHING you get a drift in the baseline.

Our friend Uwe is correct!

In fact, RI detectors are quite sensitive to ANY change. The only thing it seems to be less sensitive is to peaks!

Therefore, you should expect baseline drift with flow changes.

Thanks everyone...

That's what I thought.. I might try it and see what happens.. I will post the results if I do..

Thanks again everyone..

- Hubert