Chromatography Forum

We have recently completed the analysis of a set of samples using ion-pair chromatography. Although I was quite pleased with the results, the mobile phases we used, (nonafluoro pentanoic acid on one instrument, and pentafluoroheptanoic acid on another instrument - both are Aquity UPLCs), are proving extremely difficult to get rid of; both show quite clear IPR parent ion contamination even after exhaustive flushing sequences. We've tried all the recommended flushing mixes (acid, base, strong solvent, weak solvent) and have also changed a number of different filters (solvent frits, in-line filters, mixer) and tubing (of course there is no column). In 15 years of working with LCs I have really never seen anything quite like this problem; I'm not sure what to do next. I was wondering if anybody had experienced a similar problem and could perhaps advise us on how to proceed.

Kind Regards

You're seeing traces w/o a column in place?

Yes. There is no column fitted; I've narrowed the problem down to the UPLC system (somewhere after the solvent plastic tubing and filters, which have been replaced), the MS is definately clean. Thanks.

Yes, I once used nonafluoropentanoic acid to look at amino acids. It worked beautifully, but it took me a month to get rid of it (from two different LC-MS systems).

I found it lingered not so much in the hplc, but in the waste tubing out of the spray chambers of the MS; replacing the waste tubing was the final thing that sorted it out. I don't know why the waste tubing should matter, but assume there is enough gas exchange back into the spray chamber from these tubes that material can be carried back in. These perfluorinated acids seem to stick strongly to some plastic components, and ionise extremely well.

Having said that, I had, of course, already changed the inlet filters on the solvent lines, rinsed through with water, dilute formic acid, and methanol (for hours, alternating polar and non-polar solvents) and washed the spray chamber copiously, even changing the nebulizer needles.

You will get rid of it eventually.

Generally speaking, the problem comes mainly from on-line degasseurs if you use one...