Salts in ion exclusion chromatography

[Stibnut]

I work at an ethanol plant. We do most of our routine analyses of fermentation samples for sugars, organic acids, and ethanol using an ion exclusion column with an H+ sulfonated PS-DVB resin and an RI detector. I also run non-routine samples from time to time when questions arise.

I've gotten to wondering where the cations go for any salts in our samples. As an experiment, I tried injecting a 2% lithium acetate solution as a sample. Only the acetic acid peak appeared. I was expecting Li+ to come off the column eventually because it's the one cation that PS-DVB has a weaker affinity for than H+, but I saw no sign of it even after 90 minutes of runtime.

Where do cations with weak retention by PS-DVB go when samples containing salts are injected? Do they eventually elute, or do they remain on the column more or less indefinitely? How about salts with stronger affinity for PS-DVB, such as calcium?

If they remain on column, how much salt from samples can be injected before it noticeably changes the chromatography? Is this something I would have to worry about if I inject samples with significant salt concentrations, or are 10 uL injections on a 300 x 7.8 mm column too small to have much impact?

[Stibnut]

I thought I should add a little more context as further explanation.

With our type of column, dextrin molecules (oligomers of four or more glucose units) all coelute as unretained compounds. We call this peak "DP4+" and it gets interpreted as dextrins that were not converted by glucoamylase into fermentable glucose. However, strong anions like chloride and sulfate are also eluted unretained, adding to this peak.

At one of our facilities, the fully fermented beer just before distillation still appears to have roughly 0.5% of "DP4+", making people at this plant think their glucoamylase is behaving inefficiently resulting in lost yield. However, the plant uses sulfuric acid for pH control with a rather low pH target, and based on IC results of a diluted beer sample, I think roughly half of this peak is just sulfate.

I'd like to precipitate out the sulfate by adding a barium salt and then filtering off the barium sulfate. I'd then inject that and get sulfate-free "DP4+" results. I was thinking of making barium propionate from barium hydroxide and propionic acid, since propionic acid does not co-elute with other acids we are monitoring. But barium is strongly retained by PS-DVB, and I'm wondering how careful I'll need to be about getting barium on the column. I might also try something similar by using a silver salt to precipitate silver chloride.

[LuccasLN]

Hi Stibnut,

the short answer is that the cations probably get adsorbed to the matrix. depending on the flow time, ion strength you are using and temperature they might elute though. you don't see those signals because it probably broadens due to adsorption, besides this concentration should still be rather low considering a RID is not the best option for analysis of cations like this.

If you allow me to write my two cents of opinion, i think you are barking at the wrong tree here my friend. you can't understand your signals because analyzing DP4+ oligosaccharides in an ion exclusion column does not work well for quantitative purposes because they elute too close to the V0 with no resolution whatsoever. I suggest you to either go for a AG counterion LEX column or a HILIC column, like a YMC-pack Polyamine II!


hope it helps.

thank you.

best regards,