high variations in retention time

[camino]

Actually I'm validating a method for 16 PAHs in water, the resolution in HPLC an sensibility are good, but the retention times change in the day, from the 12.00 a.m. to 16.00 p.m change about 1.6 minutes (down the tr) for the last compound and the variation is less for the first (about one minute) making about 5 runs. The temperature of the room is the same all the time. The column in this moment is new. The mixture for daily shutdown is water:acetonitrile 50:50 and must be equilibrated for the first analisys of the next day.

Another question, what technice for extraction can I use for the best recovery rate in water, for ng/L concentrations?

[Kostas Petritis]

Camino,

Does your retention times have their initial retention values once you flush your system and re-equilibrate it? If yes, then it means that there is a build up of impurities in your column after each analysis which you do not flush properly which cause this gradual retention time decrease. You must flush your system at the end of each analysis or every x time of analysis with a certain concentration of organic solvent to clean your system.

I do not really get your second question...

[DR]

Assuming ^ isn't the cause, I'd ask: What's in the MP??

If there's any triethylamine (for example), it takes quite a while for the column chemistry to reach equilibrium with some ion pairing reagents. One approach to 'fixing' this problem is to set up a recycling loop w/ a phase in it and just let it pump over night, to 'condition' the column.

[JI2002]

Although the room temp is the same, you may want to put the column in an oven, and set the oven temp slightly higher the room temp.