Chromatography Forum

I measure the flow before entering the column and it's 1 ml/min. I connect the column and the flow exiting the column is about 0.6 ml/min. I thought that the column may have become blocked but the back pressure has dropped where I would have expected an increase. Any ideas?

I suspect that you have a check valve problem. There is very little back pressure without the column connected so the check valves don't have to hold much pressure. When you connect the column, the back pressure increases significantly and the check valves may not hold the pressure.
Try replacing them with new check valves.

While you are replacing checkvalves, replace the piston seals and inspect the pistons for damage.

A good argument for monitoring flow, rather than completely relying on the pump setting.

I released that it wasn't a pump problem by switching pumps. However, I discovered a leak in the autosampler injection valve. Problem solved. Thanks for all your sugestions which helped me look in the right direction.