High pH matrix using LC-MS ESI positive mode

[martharay]

Need a bit of help please. If I'm extracting analytes and my matrix is so basic that my pH is ~10, when I run the sample, will all the base overwhelm my acidic eluent such that no protonated ions will be formed upon ESI? I'm not seeing any analytes reported in a sample of mine I submitted. I don't have control over the instrument, and can't directly talk to the analyst (touchy reasons). I'm working on a theory that the extract should have been brought to neutrality before running. Any ideas out there?

Thanks,

MR

[Kostas Petritis]

It depends... on your analytes and the base that was used to have the pH brought at 10.

There are analytes that has been shown to have the same electrospray response at all the pH spectrum (i.e. acidic, neutral and basic).

What is the base used to bring the pH at 10? If it is something like NH3 it's OK. If you use an organic base with high proton/affinity-gas bacisity then you might have significant suppression effects...

As a rule of thump, I always try to bring the pH to acidic -with formic acid/acetic acid- when I am working in positive ion mode (assuming pH stability of the analyte).

Conclusively, yes, I would give it another try. Maybe you even need to do some chromatography to elute your compound in a matrix free environment in order to see it...

Talking to the analyst would also help

[Uwe Neue]

Besides an MS issue, there could be a chromatography issue as well. Assume that your analytes are anions at pH 10, and that these anions are unretained, a high concentration of something like ammonia in your sample could make them elute as unretained peaks. This is roughly the same as injecting a large volume of DMSO.

[james little]

It would depend on the volume of dissolved analyte you are injecting and also the strength of the % aqueous solvent used in the reversed phase separation.

[Kostas Petritis]

Hmmm,

I made the assumption that there is no chromatography involved after the extraction maybe MR can clarify this ...