Chromatography Forum

A while ago I posted a spectrum of a mystery peak that was showing up in our chromatography.

Well, it looks like it is a contaminant from the pH probe used during pH adjustment of the mobile phase. I am guessing the preservative in the VWR buffers (Dowicide A) is the culprit (not to mention the pthalate in the pH 4 buffer). The main Amax's we were seeing in our mystery peak were at 245 nm and 285 nm which match up nicely with the spectra of the pH 2 and 7 standardization buffers.

We are now looking into preservative free pH solutions as well as different types of probes that we can clean better prior imersion in the liquid.

I just wanted to give you all the heads up if you too were chasing after a mystery peak.

Domino

Or, make it a practice to never, ever immerse a pH electrode directly in the mobile phase. To measure pH, withdraw an aliquot into a test tube or beaker, measure the pH, and then throw away the aliquot

By the way, thanks for the update; it's a learning experience for all of us.

I'm sure that Bill Tindall can say it better than I, but you probably should revise the method for preparing buffers to use precisely weighed ingredients. You can eliminate the pH titration and all the troubles associated with it. When the solution is made, you pour a bit into a vial and check the pH and put it in the logbook.

It is just hard when you are doing development. People want to go fast and removing aliquots is a slow process. But - if they want good chromatography, they will spend the time - LOL!!

We have been trying to go to weighed aliquots too since we have found some differences in pH meters and probes. It is easier if you just take it out of the equation.

Thank you for the feedback!!

Domino.