Chromatography Forum

Dear members of this forum,

I have a strange behaviour with the backpressure when I was cleaning with acetonitrile my ciano column. The values started to be very variable, from 16 to 1 bar very quickly as I had not any phase stationary in the column.
When I change the column to C18 the behaviour in the pressure change with stable values. I do not understand what happened because the analysis I have done is a routine and the mobile phase is not agressive (buffer phosphate -acetonitrile 50:50). I think the problem with the column was the high pressure.
First I think the problem could be air in the pump but after working with high flow of acetonitrile and water - methyl alcohol (50:50) in order to remove the air (the column was disconected) the back pressure seems to go back to normal values but after 15 minutes the behaviour was strange again (the values change to 15 and 1 bar in few seconds).
I will apreciate any help in order to solve this problem,

Diego

Tequila,

You mentioned phosphate buffer but didn't specify the pH. What is your operating pH? Cyano columns are generally not anywhere near as stable as C18 columns so you probably should keep your pH lower than 7.

did you use pure acn or mixture of water and acn?

The washing cycle is: 30 minutes of a mixture of distiled water - methyl alcohol (50:50) at 1 ml per minute and 30 minutes of pure acetonitrile at 1 ml/min. The pH of the phosphate buffer which I work is 7.00.

Diego

The Zorbax SB-CN is extremely stable at low ph (2) up to 7. If it is another type, it could have been destroyed. Though this is probably not the case here, I have witnessed at high pH the entire contents of a column dissolve and leave you with an empty pipe ! It was a VERY high pH solution that was accidentally put on an instrument by a newbie. When no peaks and very low pressure were observed, I took the ends off and saw daylight down the thing. You do this technique long enough and you see just about everything !

If you had a fair amount of phosphate in there and then put 100% ACN (lines reversed?) in, you could precipitate the phosphate out. This would clog things. Make sure you are running a higher % water for your first step in cleaning the column out, then switch to a higher % organic.

I have generally found CN & Phe columns to be the devil.

If I understand your initial post correctly, you are able to prime all your solvents against no backpressure, but after reconnecting your column, you are unable to maintain pressure. If so it sounds like a check valve is faulty. It would help to know what kind of system you have