help please. Sugar chromatography

[kapangaluc]

We have Dionex system with a carboPac PA 10 column, an eletrochemical detector (ED40). We elute sucrose, raffinose and stachyose using 90mM NaOH at 1ml/min under isocratic conditions.
The problem we have is that always right after sucrose elutes we see a negative drop in the baseline, which flattens again right before raffinose and stachyose elute, but it never goes back up to the initial point. This is a repeatable pattern, we have observed this in every standard and sample that we run.
The has brought us problems to quantify the sugars, as the software tend to overstimate the amount of sucrose in the sample.
We had never seen this before. We have been using this system for years and we have always had nice chromatograms. All the parts in the system have been replaced and are brand new (column, guard column, working eletrode, reference electrode etc). The mobile phase has not changed and it has always been properly degased.

I would really appreciate if anyone could give me some pinter on what the problam maybe. I REALLY need this or I will never finish my dissertation research

[panagiotc]

Hi -

This problem of negative "peaks" is due to the dissolved O2 of your sample. Sparging your sample with an inert gaz (He or N2) is not doing the job. We had also experienced this problem and the best way to eliminate is to degass online. I strongly recommend you to contact DIONEX and ask for an inline degasser (ISSY tube etc, I don't exactly remember the references).

Here is a paper that discuss this issue: Cheng and Kaplan 2001 Anal. chem. 73:458-461.

If you need more info you can e-mail me at christos.panagiotopoulos@univmed.fr

good luck

Christos

[kapangaluc]

Christos,
Thank you for your reply. I will check that paper out and contact the company about this. If not luck I'll contact you later.
Kapangaluc

[John Guajardo]

Hi kapangaluc,

Is this happening in your blank runs as well? If you have a GP40 or GP50 pump, I would recommend checking the degas options and adjusting them as follows:

- Increase the cycle duration (e.g., 30 seconds to 120 seconds)
- Decrease the time between cycles. (e.g., 4 minutes to 2 minutes)

The degas options are found through the "menu of screens" on the front panel of your pump (remember to disconnect it from Chromeleon first).

You should also consider blanketing your eluent with helium at all times if you're not already doing so.