I have swithched from a 50x4.6 mm column to 50x2.1 mm one. Can anyone tell me if the 6mm flow cell is compartible to the 2.1 mm ID colum? I did slow down the flow 4 times but I saw a shoulder on the peaks. The peaks were also broader. Do I have to change connecting lines? And which dimensions are most suitable? I'm using LC/UV HP 1100. Thanks a lot in advance.

Yes Try first the red capillary tubes (0.12 mm). It could be due to extracolumns effects. I´ve used the standard cell of HP1100 DAD with very short columns (30 x 2.1 mm) at high flow-rates (0.5 - 0.8 ml/min) for high speed separations (<1 min)without problems. But I know that it would be better to change the cell.
Good luck

Did you decrease the sample injection volume? The shoulder and peak broadening might be due to that. Another thing to try is a weaker injection solvent (although that may increase your run time).

I agree. We injected tipically 2 ul of sample using 30 x 2.1 mm columns (I´m the 2nd anonymous). With 20 ul of vol inj. in mobile phase we saw band broadening, but it was OK with analites dissolved in water. But it worked better with 0.12 mm capillary tubes instead of 0.2 mm