Problems with carry over

[Sarahr]

Hi
I am analysing open ocean seawater samples for D and L amino acids using pre-column derivatisaton with N-Iso-L-butyryl-L-cysteine and O-pthaldialdehyde. We're using an Agilent 1100 Series HPLC system with a Thermo Electron Corporation BDS Hypersil C18 column. The solvent program employed uses acetonitrile, methanol, 23 mM sodium acetate buffer (pH 6) and HPLC grade water. In between running each sample, the system is washed by injecting 3:2 methanol:water. Despite this we are still getting a significant amount of carry over and as the concentrations within my samples are very low this is causing us lot of trouble. Does anyone have any ideas on how to solve this problem?
Thanks

[HW Mueller]

What do you mean with "carryover", dirt emanating from the column? If so, wash more thoroughly. Have you tried some other, more sensitive derivatizations?
How do you detect?
Your buffer is not so hot.

[Rob Burgess]

I presume you are using the needle wash functionality of the Agilent 1100 A/S...

[Klaus]

Do these peaks decrease by a constant factor when you inject blanks after a sample or do they reach a constant level? The first case would indicate a "injector flush problem" (which you'll surely get rid off with the flush option), the second would indicate some "strange" adsorption effects.
Do they still appear, when you do a blank gradient (without any movement of the injector system by using the "blank run" option of the Agilent)? If so, I would assume some sticky dirt in your column.