Anion Exchange Retention Shift

[dsduncan]

We have a method that quantifies sulfamate and sulfate at 15µg/mL. The problem we are having is a loss of retention of the peaks over time. The retention of sulfate has decreased from ~34min to ~22min, and sulfamate from ~7min to ~5min, when running on consecutive days with the same instrument, MP etc. We have seen this same thing using similar columns from other manufacturers.

Column: Silica-SAX, 5µm, 4.6x250mm
MP: 30mM sodium salicylate, pH ~6 (no adjustment)
Temp: 30°C
Flow: 1.0mL/min
Detector: RI

Any suggestion on how to approach this problem are appreciated. My background is mostly in RP method development and my ion chromatography knowledge is a bit lacking. I have some resin based columns I could try but not sure where to start with the mobile phase. I am limited to running isocratic due to the RI detector.

[tom jupille]

What you're seeing is consistent with contamination of the column by something more strongly bound than sulfate. What else is in your sample matrix?

[dsduncan]

Hi Tom, thanks for your reply.
My sample also contains 5mg/mL of Topiramate. There may also be quantities of sodium starch glycolate present (I'm not sure of the extent of it's solubility in the mobile phase).

[tom jupille]

Hmmm, tough call.

I'd be tempted to suspect the sodium starch glycolate (because it's polyanionic), but no guarantees on how to get it off.

First thing to try would be a wash with a low-pH eluant (glycolic acid has a pKa in the high 3's, if memory serves. The idea here is to suppress the ionization of the sodium starch glycolate so that it will let go of the SAX column (might be a good idea to have some ACN in there as well to get and keep the stuff in solution). You may then be faced with a fairly long re-equilibration with the salicylate.

Longer-term, this seems like a good candidate for a guard cartridge (frequently changed!). That's a stopgap; a better long-term solution would be to improve the sample workup (SPE with a reversed-phase packing, maybe?)

[Uwe Neue]

It is also possible that the underlying problem is the stability of this type of packing.

[dsduncan]

Tom: I will try to wash the column as you suggested to see if I can restore the retention. Glycolic acid has a pKa around 3.8, if I use a phosphoric acid elutent around pH 2.5 with 10% ACN would that be suitable? If I can restore the column to it's original state I could introduce a restoration process that the analysts could follow at the end of the analysis.

Uwe: I have some IC-Pak anion columns on hand from a previous project, could you suggest an elutant as a starting point for method development? I looked on waters website but can't seem to find any application notes for sulfamate on this column. It would be ideal if I could develop new chromatography that would eliminate this problem.

[Uwe Neue]

I would start with the same mobile phase that you have. Chances are that you will get less retention on the IC-Pak Anion column. If this is the case, dilute the mobile phase with water to reduce the ionic strength. If you have very little retention, you need to reduce the ionic strength drastically.

[tom jupille]

Glycolic acid has a pKa around 3.8, if I use a phosphoric acid elutent around pH 2.5 with 10% ACN would that be suitable

At this point, your guess is as good as mine!

Starch, per se, is not very soluble in ACN, and I'm assuming that the glycolic acid ester of reasonably high MW starch isn't very soluble in water, so yes, something on the order of 10 - 20% ACN seems reasonable (but I'd listen to any suggestions from people with more direct experience!).

[dsduncan]

Hi Guys, thanks again for your help. I just wanted to provide an update on the work so far.

Tom:
Flushing the column with phosphoric acid and 20% ACN restored the retention on the SAX column back to it's original state. So it seems that there is something being more strongly bound to the column as you suggested.

Uwe:
I have found the IC-pak to be more reproducible. I am not seeing the same decreasing retention as I saw with the silica-SAX column. I reduced the concentration of sodium salicylate from 30mM down to 3mM to increase the retention of sulfamate and sulfate, and added 10% ACN to suppress the retention of topiramate.

The problem now is sulfamate is coming just after the topiramate peak. If I reduce the ionic strength even more the sulfate peak is being retained much too long. I was thinking to try and change the eluent to give less selectivity for sulfamate and sulfate, that way I could lower the ionic strength to pull sulfamate away from the topiramate interference, and still have sulfate at a reasonable retention time. Any suggestions on how I might approach this? I was thinking my next step is to change to an eluent with a different pH to see the effect.