Bart,
I can think of two possibilities for decreasing the amount of
the sodium adduct in your sample. The simplest is to rinse the probe
tip (or sample spot) with distilled water after the mixture of sample
and matrix dries. You can either dip the probe into a solution of
distilled water or put a droplet of water on the dried sample and
wick it away with a Kimwipe.
The other, and probably more effective, possibility is to
purify the sample using reversed phase HPLC. A simplified way of
doing this is to use a Zip tip ( made by Millipore) or a SuproTip
(made by The Nest Group). Both have reversed phase chromatographic
material in a pipet tip that binds proteins from an aqueous solution.
The bound protein can be rinsed with water(or water containing 5%
acetonitrile) to remove hydrophilic contaminants, such as salts and
glucose, followed by eluting the bound protein onto the mass spec
sample holder with a couple of microliters of alpha cyano matrix in
about 50% acetonitrile with the usual 0.1% TFA.
Best wishes,
Dan Brune
Dept of Chem. & Biochem.
Arizona State Univ.
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