Bob:
I have used sodium phytate from Sigma (inositol hexaphospate, or IP6) and
cyclic GMP (Fluka #51055), which have good solubility in electrospray
solvent solutions, and good stability when stored as stock solutions at
-20C. Both need to be eluted through a cation exchange resin (H+ form)
to remove sodium ions. Weighing .05 to .1 mmol of the standard and about
.5 to 1 gram of cation xchg resin gives enough std solution to last years
-> 5 to 10 ml of 10mM stock. The free acid is less soluble in aqueous
solution than the salt, but it gets diluted in washing it from the resin
with H2O. cGMP gives a negative ion with 344.2 m/z, and I get a signal
with fair SNR with as little as 10 to 20 uM injected in my single quad HP
5989. Likely your sensitivity would be better.
With IP6, I have used the procedure of Phillippy et al. "Preparation of
inositol phospate from sodium phytate by enzymatic and non-enzymatic
hydrolysis" Anal. Biochem. 1987,162, 115-121, to prepare a std solution
containing a series of IPx's (x=3 to 6) with the singly-charged negative
ions at 80 m/z intervals.
Of course many compounds are available as free acids, which would save the
de-salting step. Just the same, doing a cation exchange step also on my
unknowns increases their negative ion sensitivity many-fold. (if the sample
volume is not too limited) I have also used cluster complexes of Na and
TFA to calibrate for negative ion mode, per a JASMS article of a couple
years ago. For routine check of calibration I like the cGMP best, since
its signal disappears quickly.
Larry
>Hi,
>
>Need to find a good and cheap compound (including the solvent system) >for
>optimizing negative ion mode of LCQ. Thanks.
>
>
>Bob
Larry Gross lgross@ucsd.edu
Howard Hughes Medical Institute
University of California, San Diego
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