Retention Time and Buffer Concentration

[syx]

Dear Members,
We run a method using CN-column (Waters Spherisorb S5 CN RP and Luna CN). The mobile phase contains 0.01M KH2PO4 buffer pH 5.5 - MeOH - ACN (30:35:35). We found that the retention time was unstable (approx 17 minutes and 19 minutes). We thought that it was caused by insufficient buffer concentration. Then, we increased the concentration up to 0.025M and … the retention was changed to approx 9 minutes (Spherisorb) and 11 minutes (Luna). How could buffer concentration change the retention time?

Best regards,
Siswanto Tanuatmojo

[HW Mueller]

Rather looks as if you had a pH problem, phosphate isn´t much of a buffer at 5.5 anyway. If the change in the ionic strength is really behind this you would indeed have a very interesting/unusual example. What are the analytes?

[syx]

The substance is desloratadine.

I try the method based on the abstract of J. Pharm. Biomed. Anal. 2004 Mar 10; 34(5) pp 1013-9. I do not know the exact retention time in the full text journal. Hypersil CN column is used in the journal method. I do not have that type of column. I have tred the method on LiChrospher 100 CN, Phenomenex Luna 5u CN, and Waters Spherisorb S5 CN-RP columns. LiChrospher 100 CN gave high pressure but less retention, so we do not use it any longer in this research.