Chromatography Forum

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?

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.