Does anyone have any comments/concerns about recycling mobile phase? Has anyone encountered any problems in an analysis caused by recycling their mobile phase?

Thanks

Unless you want to perform a tedious cleanup you might as well forget recycling if your column has even a small bleed or if you analyze samples in a difficult and strong matrix (serum/plasma, etc.). The latter case causes "bleeding" of matrix components after a few injections. You can check on this by running a spectrum on baseline, but this may not be sensitive enough. If you have strong tailing you may even have a problem with recycling during analysis of ideal samples.

A number of years ago, I recycled mobile phase for about a day in a "single-column" IC system. The mobile phase was an aqueous buffer and we were using direct conductivity detection. We could actually see the baseline ramp up a little bit after every injection. Granted, we were making large injections, but that experience taught me that recycling mobile phase has a lot of pitfalls!

If you want to pursue it, you might look at a mobile phase recycling valve. I know Alltech has one in their catalog (other vendors probably do as well). This ties into the detector and diverts mobile phase to waste when the signal is above a certain threshold level (i.e., when analytes are eluting). I don't have any "hands-on" experience with it, but it seems like it might remove some of the problems.

Has anyone out there tried this?

We tried the recycler (Alltech) for pharmaceutical samples with UV detection and we could save almost 70% of mobile phase. It worked very well. But I agree with H.W. Mueller, I wouldn't recomended for dirty samples like biological, plants, etc, or trace analysis. However, we recycled mobile phase in catecholamines assay with EC detection and it worked very well.

I have in years past recycled mobile phases quite a lot. Typically this was for assays of relatively "clean" materials -- purity assay of a technical material, for example. Even so, there is a slow accumulation of the sample material in the mobile phase.
This prompted me (and probably lots of other folks) to suggest to Alltech that they come out with a "dump" valve tied in to the detector. Eventually they did come out with the device mentioned above ("Solvent Recycler 3000"). I have not used it myself, but if set up correctly it should greatly reduce the interferences accumulating in the mobile phase.
Remember that "invisible" components will accumulate in the mobile phase and a "recycler" set to the wavelength of the chromatogram will not eliminate these. Some diode-array detectors give you the option of outputting two fixed wavelengths. If one of these were set to a very low wavelength, and this were used to control the recycler, then a greater portion of the sample components would be kept out of the mobile phase reservoir. Alternatively, the same effect could be got from the use of a second, spare, detector downstream of the usual one.
I do not recommend recycling for certain sensitive mobile phases, such as weakly buffered mobile phases where the pH matters to the separation.
Surprisingly, though, the accumulation of small amounts of sample in the mobile phase is not necessarily a problem, if you observe a few precautions:
1) Always use a large quantity of mobile phase. I recommend 4-5 liters.
2) Make sure the recycle line drips the returning mobile phase into the reservoir. Since the intake to the pump is at the bottom, this maximizes the distance between the two lines.
3) Use a magnetic stirrer, and take care that it works well. It's more important that it stir reliably (and that the stir bar never bounces to the side of the container and vibrates) than that it stir fast.
4) I don't recommend degassing by sparging with helium when recycling mobile phase, due to the likelihood of changing the %organic. I recommend using one of the modern membrane degassers instead. However, sparging can be an acceptable means of degassing (it's all I ever used) if you sparge rapidly at first, then turn the sparging rate way down to maintain a head of helium.
5) Be sure to wash your system with clean solvent after your run, or you may accumulate analyte and excipients through the membrane degasser, and the autosampler. I note that Alltech has an attachement ("Autoclean Accessory") for their recycler that will do this function for you automatically. (Note: I have no connection to Alltech Associates.)

I agree. But it would be better to change old columns (4-4.6 mm ID) to microbore 2.1 mm ID columns and 0.15-0.3 ml/min, saving 60-80% of mobile phase, than recycling. At least with new methods.