Chromatography Forum

I am kind of intrigued by this:
Run a gradient or isocratic method using acetone/MeOH- runs fine. I have a waters alliance 2695. Goes through the following sequence at the end of the run:
Purge- venting system
Purge- zero transducers
Purge- Empty syringe
Purge- Purging sample loop
Purge- waste line
Purge- Fill syringe
Purge- empty syringe

The monent I change to 1% Formic acid/Acetonitrile to 100%Acetonitrile. The method runs fine but when the method ends and starts the washing cycle it stops at:
Purge - Empty syringe (after zero transducers)
The system pressure builds up to over 4,800 psi and then just stops without going through the other sequences as in the acetone/MeOH run above.

Any suggestions ?

thanks

Does this happen only with the mentioned solvents or with any solvents?

It happens with other solvents such as 1% FA and MeOH and especially with the use of particle size of 3.5 micron columns with 50 and 100mm length and ID of 2.1 mm.
Looks like some kind of backpressure when the columns are connected. Does not happen when the column is disconnected

I am not familiar with your system or the sequence you are using to clean the system but if you are trying to purge at high flow-rates and at some point you direct the flow towards the column, then it is maybe normal that you the system overpressurises. Can you provide more info of what exactly happens when you say that the system "purges"? What is the flow rates at that point? Does the flow passes through the column?

Another problem could come from the tubing you use for the waste lines; i. e. on our LC-MS system, from the divert valve before the MS source a red peek tube is going to the waste, due to its leght the counterpressure on the waste line is almost the same of the pressure on the source line.
In your system could be the same and if you increase to much the flow rate during the purge steps you could fall in overpressure.

The flow rate when it "purges" is 3.0ml/min. I think the system overpressures because of the smaller 3.5 micron particle and 2.1 ID. This does not happen with a 4.6mm, 5 micron, 250mm column. I guess I might be stuck using a 5 micron column.

Maybe you should just change your post-run sequence!

Thanks for the suggestion. But the stupid program in the waters 2795 is set in such a way that it has to purge at 3.0 ml/min at the end of the run. I wonder if anyone has run a 3.0 micron, 2.1 ID, 50mm column at 3 ml/min on a waters alliance system ?