Unstable retention time

[Strep79]

Hi,

I have a new sunfire c18 column and it has only been used 5 times and somehow i am experiencing unstable retention times (a drift of sometimes 1min but usually .5min). What could be the cause of this?

The system that I am using is probably 10yrs old. Could it be because of the pump not working properly (e.g. flow not constant or inaccurate mixing). And lastly how to rectify such a problem.

Thank You

[Chrom user]

Check your flowrate accuracy by using a measuring cylinder and stopwatch. Set your flow to 1ml/min and time for 5min. You should get approx 5ml. If not, your pump is malfunctioning.

[Consumer Products Guy]

If isocratic assay, premix mobile phase and put all reservoir inlets into it. Purge all inlet lines of their old mobile phase, letting the mixed mobile phase in. Run assays and measure retention times; this should isolate whether it is a pumping problem. If this is OK, but allowing pump to mix mobile phase is not, then suspect solvent mixer portion of the pump. Also, I'm assuming you have the column thermostatically temperature-controlled, because shifts in lab temperature will also cause shifts of retention time. We usually set temperatures of 5 degrees over ambient to control this.

[sonal]

Check your temperature, whether it is stable or not throughout the run. I was experiencing a similar problem. Due to frequent power failure my temperature difference was as great as 15degrees which resulted in different Rt's.