Re: Methanol compared with acetonitrile

Briefly, MeOH has a lower proton affinity than (say) acetonitrile. This
means that MeOH2+ is a stronger proton donor than CH3CNH+, so you will
generally see more abundant ions using MeOH as HPLC solvent, compared to
CH3CN, for certain types of analyte.

For example, a while back we tried LC/MS of a plant extract containing
polyphenols, isothiocyanates and nitriles, initially using a
water/acid.acetonitrile gradient (from a previously optimised HPLC/UV
method). The response for the nitriles was, unsurprisingly, very poor
because the acetonitrile solvent, present in massive excess compared to the
analyte, was grabbing most of the charge. Switching to MeOH yielded a
100-fold increase in response for the nitriles (but had little effect on the
phenolics) without compromising the separation.

A useful paper to consult is: "Importance of gas-phase proton affinities in
determining the electrospray ionization response for analytes and solvents"
Ma'an H. Amad, Nadja B. Cech, George S. Jackson and Christie G. Enke;
JOURNAL OF MASS SPECTROMETRY, J. Mass Spectrom. 35, 784^Ö789 (2000).

regards,

Fred Mellon

"Jimmy" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
} I read from STMS that methanol usually gives higher sensitivity with
} ESI and APCI. Why is it so? I know that under RP, we would use higher
} conc. methanol to get similar RT. That would give higher ionization
} effeciency under ESI. But in APCI, everything is evaporated. There
} should be no difference except for the needle caburization issue.
}
} Is it always true that methanol gives higher sensitivity, regardless
} positive or negative mode, ESI or APCI?
}
} Thanks in advance for your kind reply!
}
}