ion pairing with ACN and MeOH

[Malarac]

Hi,

this may be a silly question but I have spent a couple of hours searching the forums and haven't got the answer I need yet. I have been assaying for catecholamines for the past few years and have recently tried a new method to increase my selectivity and sensitivity but have a question regarding its improvement. Currently, I run an agilent 1100 system with a coulochem III ECD and a sunfire C18 RP column 4.6x100; 5um packing @ 1.2mL/min flow rate. My mobile phase is as follows:

75mM NaH2PO4
500uM EDTA
5mM Octane sulphonic acid
18% ACN
pH 5.25


The issue is there are unknown peaks that elute around my peak of interest in a real sample. I originally ran lower ACN and lower Octane concentration but have found that increasing both keeps my amines at a similar retention whilst decreasing the retention of the unknowns (hoping to wash them out before my peaks of interest elute). I was pondering whether to replace some ACN (say 5%) with MeOH given that this should bolster the effect of the ion pairing reagent, is this a correct assumption? see: http://www.sepsci.com/chromforum/viewto ... etonitrile

So my questions is, are there any problems using a combination of ACN and MeOH rather than just switching from one to the other?

Thanks for your help and sorry for the long post.
Cheers,
James

[tom jupille]

"If some is good, more is better"

Since more ACN and more octanesulfonate improved things, why not just continue on that track? If you're in the 20% or so range for ACN, you should be able to go up to 25 mM or so on the octane sulfonate without any problems.

There's no major problem with adding some MeOH, but it will very likely change your selectivity, and there's no a priori way to judge whether the result will be an improvement, you just have to try it.

[Malarac]

Thanks Tom,

will try that then.