mysteries of ion chromatography

[lfleck]

I am looking for advice with a suppressed ion chromatography method. I am using a Dionex CS14 column, eluent is 10 mM MSA with 2.5% acetonitrile. The regenerant is 100 mM TBAOH.

My problems are two fold. (at least)
1st, The retention time of my analyte (quaternary amine) is drifting inward. It appears to be related to the column lot. I have contacted Dionex but have not gotten a response. The two new columns that I have tried start out with about 15 minute retention time and then drift in about 0.1 min per injection. This continues until all retention is lost. I have a second method for the same compound. It uses an eluent of 15 mM MSA, 1% acetonitrile and 10% methanol. This produces stable retention for the same compound. I have increased the acid concentation and observed improved peak shape but still have the retention time variation issue.

My second problem comes from trying to find a solution to the first.
I experience a large increase in noise (up to 20 time larger) if I change the flow rate of the eluent. The method flow rate is 1.5 mL/minute. I have tried flow rates from 1.2 mL/minute to 1.75 mL/minute. The noise does not appear to be influenced by the flow rate of my regenerant.

Thanks in advance for all responses.

[KC]

I have done quite a bit of work with quarternary amines and might suggest the following:
Channel A mobile phase: 0.01M hexanesulfonate w/0.1% H3PO4
Channel B 100% methanol
flow 1.00 mL/min
Column: YMC Phenyl 250mm
Gradient
time %A %B
0.0 90 10
1.0 90 10
6.0 75 25
7.0 75 25
7.01 90 10
12.0 90 10

I validated this method and ran literally 100s of samples and it worked like a champ. I know some may say the ion pair reagent concentration changes during the run because of the gradient but as long as the gradient is shallow enough it should not make a difference. If the quarternary amine happens to be diquat dibromide your retention will be about 6.5 min with this method and it will remain remarkable consistent. The first few injections of the day may shift around slightly but once equilibrated to these conditions your retention will remain constant. Would also bet heptanesulfonic or octanesulfonic would work just as well as the hexanesulfonate.

[Einar Ponten]

It seems to me that something else in the sample injected is completely retained and thereby blocks the functional groups leading to loss of retention for less charged analytes (your target).

If the matrix is complicated you could try to make a pre-treatment by passing the sample through a SPE with a cation exchanger and removing poly-cations.

An alternative approach, if you have another detector than the conductivity detector (=MS), is to use HILIC as separation mode. It works nicely for quaternary amines and the problem (if any) with poly-cations will also be solved.

[Mark Tracy]

I was hoping that one of my colleagues would answer this one... I also think that you have a contaminant building up on the column. The second method with 10% methanol in it apparently does not permit accumulation. You can try using an OnGuard RP cleanup cartridge and see if that selectively removes the contaminant.

As for the noise, I have seen where a bubble stubbornly won't exit the detector. Momentarily block the outlet with your finger, then release it; the pressure surge might dislodge a bubble. Be careful not to overpressure the suppressor. Also, if the pump is contaminated with a hydrophobic ion, it will have a pulsing baseline for days. (In your case it could also poison the column.)

[lfleck]

Thanks for the replies.

I am experiencing problems when I inject the neat material dissolved in water. Both of the new columns have displayed the issue before any samples were injected. I have tried flushing the column with 100 mM methanesulfonic acid and also with acetonitrile and mixtures of the two. This did not improve retention on the column or stop the retention time shift. Injections of solutions containing acetonitrile (up to 50%) increase the retention time drift.

[Chris Pohl]

lfleck,

Well, your description leaves quite a puzzle since everything I'm familiar with would suggest you should observe the inverse of what you described. It's not a great idea to use methanol in the eluent with carboxylic acid-base stationary phases since this results in slow conversion of the acid sites to methyl esters, resulting in a progressive loss of capacity (this is reversible, by simply omitting the methanol and allowing hydrolysis to occur but at room temperature this can take up to a week). If you had told me that you saw the drifting retention time problem with the methanol-acetonitrile eluent system, this certainly wouldn't surprise me given that we have observed such drifting retention time with methanol containing eluent systems. But curiously, in your case this is the only eluent system that doesn't exhibit the drift! Is there any chance your proportioning valve is leaking and you are inadvertently adding methanol with your acetonitrile only eluent system? Is it possible you have reversed the eluent description and the symptom?