Chromatography Forum

Hi,

I have two lines of experimentation going. All of my work uses aqueous mobile phases. I would optimally like to have two different LC systems - one system dedicated to each project, but alas I only have one system at this time. I would like to keep these projects separate because the buffering compound for the mobile phase (phosphate, 0.005 M) of the first project is a very serious contaminant for the other type of experiment. I switched the system over to the phosphate buffered experiments for about a week, but when I tried to switch back to get back to the previous experiments I found it took about a week to flush the phosphate out of the system so I could get meaningful results again from my first project.

Does anyone generally have any input about why it took so long for the residual phosphate that was trapped somewhere in the LC to be flushed out? Is it always this way when you switch an LC over to a different mobile phase?

Also, I have a lot of stainless tubing in the system. Would switching as much tubing as I can over to PEEK help with this ?

Thanks,

Dave

Hi Dave,
I don't think that Peek tubing will solve the problem.It will help a little bit. I guess the problem is in your pump head. When switching to another mobile phase please clean that first. I would recommend to purge the pump head when disconnected to the injection valve, with water! Which pH is your buffer and is it possible to switch to a volatile buffer? What kind of injection valve do you use, or Autosampler? What is the % of ACN in your mobile phase? If possible shorten the tubings where ever possible before changing to Peek. Is it possible to rinse the pistons in your pump head from the backside? BR Gerhard

I would use PEEK tubing regardless of application.
If cleaning the pumpheads do not solve your problem it could be filter related.
Do you have inline filters in your HPLC, if they are metal they would have a much larger surface area than your tubing.
Do you have an inlet filter on the inlet tubing to the pump (the part that goes in your mobile phase bottle), that should be dedicated to one analysis type.
I am not sure but I seem to recall sometime running dilute nitric acid to get rid of phosphate contamination in my system.

Dave, did you prime the system with the new eluent/s (with no phosphate in it/them) or did you just switch to them running e.g. 1 ml/min or whatever flow rate you apply for the analysis?
Especially if there is a degasser in the system it could take awhile to flush a previous eluent at a “traditionalâ€