Chromatography Forum

I'm facing troubles with retention times, for example: when I start to run a sequence of samples, my first samples are read at 174-172 bar and sulfate elutes at 15.77min, but, when the reading advances, the sixth sample is read at 166-164 bar and sulfate elutes at 16.27min. It also variates among days, I mean by this that today my retention time for sulfate might be 15.3min and tomorrow 16.2mins even 17.1mins. How can I get more constant retention times?
I'm using a Dionex IonPac As22 4x250mm, with a 2.0mL/min flow and a 1.7mM NaHCo3/1.8mM Na2CO3 eluent with a SAMS-CARS supressor system.

Thanks for your help.-

Hi Iperdomo,

Is the column thermostatted--this is key for IC work, I think? Next, is the LC system free of leaks?

In between runs, how is the column stored?

Are the samples pre-treated--or what is the nature of the samples you are running?

Hi!
Is the column thermostatted--this is key for IC work, I think? Next, is the LC system free of leaks?
No, it's not. Do I need a Thermostatted chamber? My supplier suggested something like an column oven (I'm not sure if it's the right name in english), are these the same?

Respecting the leaks, no, there are no leaks.

In between runs, how is the column stored?
I do not store it, I use it continuously, almost daily.

Are the samples pre-treated--or what is the nature of the samples you are running?
Yes, I filter the samples throug a 0.2um filter.

Thank you.

Hi Iperdomo,

Yes, a thermostatted chamber is a column heater (or column oven). Consider purchasing one and using it, I think.

No leaks and filtered samples are both good.

While I am thinking of it, how is the mobile phase prepared?

I weight 0.2856g NaHCO3 and 0.3816g Ca2CO3, and dissolve it in 2L with ultrapure water. Then I filter it throught a 0.45um filter, and use an ultrasonic cleaner to degassify it.

Hello again, Iperdomo,

The eluent preparation seems reasonable as long as the water you use has little carbon dioxide within it to start with. In case you do not have this document:

https://www.nestgrp.com/pdf/Zp1/Sp1/ION_Manual.pdf

Thanks a lot, mattmullaney!