By Anonymous on Thursday, June 10, 2004 - 01:46 am:

Dear all,

Can you please recommend any acceptable ion pairing agents for LCMS? We are using an API2000 Tandem MS. I have been trying to separate two very hydrophilic compounds and am having difficulties getting good rentention and peak shape. Will appreciate any help that you can give.

Thanks!

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By AllsepTech on Thursday, June 10, 2004 - 05:05 am:

I would recommend you to check the following link. It describes the retention of polar compounds without ion-pairing reagent. Mobile phases are LC/MS friendly ACN/water with modifiers(formic acid, ammonium acetate and formate, TFA). If you compound is basic or acidic then this is the way to go. You can achieve practically any retention by controlling dual interactions on the column (reverse phase and ion exchange on one column)

http://www.lcgcmag.com/lcgc/data/articl ... rticle.pdf

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By MG on Thursday, June 10, 2004 - 06:36 am:

Another option is hydrophilic interaction chromatography (HILIC). You use a polar stationary phase, with acetonitrile being the weak solvent, and aqueous buffer being the strong solvent. It is like normal phase, but with MS friendly mobile phases. There are columns sold for this purpose, or some people use ordinary normal-phase columns. Search this forum and you'll find several posts on the subject.

Yet another option is to use trifluoroacetic acid (TFA) or heptafluorobutyric acid (HFBA) with a reverse phase column, but only if you are working in positive ion mode. Some have argued on this forum that TFA is not an ion pair reagent, but it will improve retention and peak shape for polar basic compounds. I would recommend avoiding this second option if possible.

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By Anonymous on Thursday, June 10, 2004 - 09:58 am:

You could try using a polar endcapped C18 column
like YMC-Pack ODS AQ, Phenomenex Aqua,
Macherey-Nagel Pyramid, Waters Atlantis, Aquasil
C18.

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By Anonymous on Monday, June 14, 2004 - 07:16 pm:

Dear Anonymous, MG and AllsepTech,

Thank you all f or your responses. I tried using several buffers like TFA, Ammonium Acetate and Ammonium Trifluoroacetate. The only columns that we have available at the moment are C18 from Hypersil, Zorbax, and Xterra. It would take a while for us to order a new one from those that you just recommended. Using the Hypersil C18 column, I was able to get good separation for my peaks within 2 minutes. However, the peaks were not sharp and symmetrical. With regular HPLC-UV, EC, or FLD runs, we normally add a small amount of TEA to improve the peak shape but I do not know if this is possible if I am using LCMS with the API2000 Tandem MS. Can I do so without harming my MS system/results? Do you have any recommendations?

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By MG on Tuesday, June 15, 2004 - 02:34 pm:

If you are using positive ionization, do not use TEA. It will suppress your signal quite badly. I've been able to use it with negative ion APCI. I can't recall if I've used it with negative ESI. I don't think you will harm your MS (TEA is volatile), but you might have to do quite alot of flushing to get the TEA out of your LC, once you are finished with the project.

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By Anonymous on Friday, June 18, 2004 - 07:39 pm:

It could be related to your sample injection solvent. If the compounds are very polar, you should inject them in water, or water with TFA, NH4Ac or NH4TFA.