Chromatography Forum

After many runs the AS4 Dionex column has lost its resolving power.

I though I could dilute the mobile phase to improve resolution.

I followed the clean-up protocols and while they helped they did that marginally.

I understand that when ion exchange sites in the column are unavailable peak resolution decreases. So since the recommended cleanup protocol didn't help too much...
is there a hard-core way to clean up the column? I am willing to take risks and experiment.

lgchrom wrote:
After many runs the AS4 Dionex column has lost its resolving power. I though I could dilute the mobile phase to improve resolution. I followed the clean-up protocols and while they helped they did that marginally. I understand that when ion exchange sites in the column are unavailable peak resolution decreases. So since the recommended cleanup protocol didn't help too much...is there a hard-core way to clean up the column? I am willing to take risks and experiment.

Are you seeing loss in resolution of the early peaks or the late eluting peaks? Is there a dramatic decrease in the retention time of divalent anions, say by 10% of the original. Also examine the inlet and outlet frit for contamination. Usually 0.1 M HCl followed by ACN followed by 0.1 M NaOH can help. If AS4 coating on the polymer has a low cross link, the stationary phase can be permanently damaged by the pure organic solvent. You may skip the ACN wash in that case.

There is mostly loss of resolution in the early eluting peaks. The divalent ones elute more than 15-20% earlier. What do you think?

lgchrom wrote:
There is mostly loss of resolution in the early eluting peaks. The divalent ones elute more than 15-20% earlier. What do you think?

Assuming that the system works perfectly with another IC column, I would open the column inlet to see if there is frit contamination and/or void has developed at the head of the column and the guard column. This can also be tested by reversing the column direction (the frits are identical on both ends so no worries about reversing the directions for a short time).

Even a small void is enough to degrade the resolution of early eluting peaks. The decrease in the retention time of the divalents indicates there is some loss of capacity but it is not that drastic. The early eluting peaks are more affected by the hardware issues (such as contaminated guards/ voids at column heads/ extra long tubings before and after the column/ column overload - the most ignored aspect)

Try a fresh carbonate eluent after 0.1 M HCl and 0.1 M Na2CO3 washes. Hope that works.

What is the plate count?

Hi lgchrom

The AS4 is a very old Ionpac Column and therefore the column is NOT solvent compatible. If you rinse with more than 5% organic solvent you will destroy the resin. If you send me you email adress i will send you the product manual. In section 5.3 "poor peak resolution" are some procedures to solve such issues desribed. However, how old is your column and how many samples did you run in the column?
Regards,

Sorry, it was a typo. The column is an AS14.

The samples that have run are about 600 drinking water samples. I have seen Shodex columns go for more than 1200 runs of similar samples.

In case of a void volume on top of column, can anything be done about that? maybe pressurising the column from the opposite direction?