Help with peak integration

[Ralph]

Good day,

I am using a dionex AS18 column set with a hydroxide eluent generator.

I am attempting to check for Cl in an ethylene glycol matrix.

The EG matrix has a consistent 'Lump' on the chromatogram from 3 minutes to 6.5 minutes, and peaking at 4.25 minutes at 1.20uS. The Cl peak comes off at 4.43 minutes, so its almost directly on top of the glycol lump.

What I need to figure out, should I force the Cl peak baseline to zero, which will give me 78.7 ppm; Or should I run the Cl peak base line across the top of the glycol lump, which will give me 5.8 ppm.

As you can see, the difference is rather large.

Any ideas?

Thanks for your help.
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[Mark Tracy]

Try preparing a chloride-free glycol solution and inject that. You can use that for baseline subtraction. I bet that you will get closer to 5.8 ppm than 78.

Are you preparing your calibration standards in the glycol matrix? You probably should at least check that against standards in water.

[DR]

^ what he said.

My addition - about 100 years ago, the last time I touched an IC, I had problems with filters. Basically, If I used syringe filters, they added a significant amount of chloride, so I didn't use them.

[HW Mueller]

I am stumped, having used IX for separating ions from nonionic stuff..... Is the glycol ionized under those conditions? One can not change the conditions so no ionization takes place? Or is the column just overwhelmed via van der Waals forces .... (reverse phase of glycol...)?

[Chris Pohl]

Ralph,

The disturbance in the baseline is caused by oxidation of ethylene glycol as it passes through the regenerant chambers of the suppressor (I assume you are using the recycle mode). This oxidation process produces glycolic acid and a fraction of this passes back across the membrane (since it is a relatively weak acid and the anode chamber where oxidation occurs is an acidic environment) into the eluent chamber where glycolic acid becomes ionized (since it is now in deionized water). The whole process can actually cause glycolic acid to recirculate for a few minutes before the concentration becomes low enough for it to dissipate from the regenerant chambers. This is the reason that you see a relatively broad disturbance in the baseline.

There are two workarounds: one is to use a separate water stream to supply the regenerant chambers. This keeps ethylene glycol from ever entering the regenerant chambers and thus prevents the oxidation reaction. The other option, which may or may not be viable depending upon your analytical requirements, is to put a relatively large delay volume between the cell exit line and the inlet to the regenerant chambers of the suppressor. This will cause significant dilution of the ethylene glycol and depending upon your analytical requirements may be sufficient to circumvent the problem you describe. If you really want to get fancy, you can run the cell effluent through a capped, stirred bottle which can dramatically dilute the ethylene glycol depending upon the volume of the bottle and render it virtually invisible.