Backpressure rise, Eluent percipitation?

[Petsav]

Hello,

I have been trying to adapt an IC-ICP-MS/MS method for Sb speciation using an Dionex AS22 Column and unsurprisingly I have been having problems with low resolution between SbV and III.

The base eluent i have been using is 1.5 mM Phthalic acid and 10 mM Na2EDTA in 2:98 MeOH:H2O with the pH adjusted to 4 using Ammonia and HCl. I tried to add 4 mM Oxalic acid (from 0.2M solution in H2O, so 2% of the Oxalic acid solution and 98% of the base eluent) with the LC quaternary pump and the chromatograms looked absolutely fantastic, but after an hour or two the backpressure started to rise rapidly leading to the LC shutting down. I was able to slowly flush the system with 2% MeOH in H2O and get the pressure back down. I am using Type 1 water and Analytical grade or better chemicals.

My guess is that some kind of poorly soluble oxalate is forming in the system and clogging it?

The LC system i am using is an Agilent 1260 Infinity II Bio-Inert so my reckoning is that the eluent path materials shouldn't be a problem.

I also tried with the oxalic acid solution also being adjusted to pH 4 but the chromatograms were useless.

So i guess my question is if anybody has any ideas on how to prevent this/stabilize the eluent solution since i would very much like to achieve a reliable way to get the chromatograms that i got with the oxalic acid.

[vmu]

What is the eluent bottle made of? Try to use plastic bottle.

[Petsav]

What is the eluent bottle made of? Try to use plastic bottle.

I have been using bottles made of Duran glass that came with the machine.

I will give plastic a try, PE and PP both should probably be fine as long as the bottles are clean?

Is your thought that the Oxalic acid pulls some metal contamination from the storage bottle that then precipitates out once it is mixed with the base eluent and the pH rises?

[vmu]

I will give plastic a try, PE and PP both should probably be fine as long as the bottles are clean?

Yes, they should be good.

Is your thought that the Oxalic acid pulls some metal contamination from the storage bottle that then precipitates out once it is mixed with the base eluent and the pH rises?

Glass contains calcium. Calcium oxalate is poorly soluble. However, I am not sure that it causes the problem.

Sb(III) oxalate is insoluble too...

[Petsav]

I will give plastic a try, PE and PP both should probably be fine as long as the bottles are clean?

Yes, they should be good.

Is your thought that the Oxalic acid pulls some metal contamination from the storage bottle that then precipitates out once it is mixed with the base eluent and the pH rises?

Glass contains calcium. Calcium oxalate is poorly soluble. However, I am not sure that it causes the problem.

Sb(III) oxalate is insoluble too...

That makes sense, will be a couple days until i can test the plastic bottles but will update then. Yeah i was thinking about the Sb(III) oxalate too but figured that since i only ran a couple injections with Sb(III) and (V) standard in them and a few of just water and the pressure seemed to only start to noticeably climb like 20 min after the Sb ones and they gave 2 nice sized peaks that it probably wasn't it.