Increase in retention time

[veronika]

I use Waters Spherisorb ODS-2 column (4.6 x 250 mm; 5 um). Column is 4 months old. Mobile phase is 10% methanol:90% water. Column shipping solvent was acetonitrile. At the beginning of my work, my supervisor told me I could wash column in methanol at the end of day (and leave it in methanol). 3 weeks ago I start washing and keeping my column in acetonitrile. I wash column for 30-45 minutes (flow = 1 ml/min) at the end of day, with acetonitrile. 2 weeks ago I noticed increase in retention time of my analyte - each day I had changes in retention time. Now my analyte has retention time 11 minutes, and 3 weeks before, retention time was 5 minutes. I have cca 30 injections per week. What can I do to decrease RT ?
Thanks for advice.
Veronika[/img][size=9][/size][size=12][/size]

[Uwe Neue]

What is the nature of your analyte? Is it a basic compound?

[veronika]

The analyte is 5-hydroxymethylfurfural (HMF; CAS.no: 67-47-0).

[Uwe Neue]

In order to bring the retention back to where it was, increase the methanol concentration in the mobile phase.

[veronika]

Thank you very much for the answer.

OK, I'll increase methanol in mobile phase.
What do you think, should I wash the column in methanol instead of acetonitrile ?

[Uwe Neue]

I would continue to store the column in acetonitrile.

[veronika]

Thank you very much for the answer.
Best regards,
Veronika