Jb. Cunniff et P. Vouros, MASS AND CHARGE ASSIGNMENT FOR ELECTROSPRAY IONS BY CROWN-ETHER ADDUCTION, Rapid communications in mass spectrometry, 8(9), 1994, pp. 715-719
The introduction of 18-crown-6 into the liquid sheath during the elect
rospray ionization mass spectrometry (ESI-MS) of peptides leads to the
formation of crown ether/peptide complexes. The nominal mass spacing
of the peaks from these complexes allows an unambiguous determination
of the charge state of the ions and thus of their actual mass. The 264
Da mass shift is much larger than what is achieved by either Na+ or C
u2+ adduction, and is thus potentially more useful for the determinati
on of charge states, especially for multiply charged high-molecular-we
ight species. The addition of an uncharged crown ether does not appear
to interfere with the ESI process. Because the crown ether is added t
o the liquid sheath and not to the original solution, the method shoul
d be particularly amenable to chromatographic or electrohoretic techni
ques because it is essentially a post-column noncovalent derivatizatio
n procedure. Finally, because the complex formation constants are rela
tively large, the peptide/crown ether complex yields high intensity io
ns, normally the base peak in the spectrum when analyzing pure peptide
samples. Finally, as the crown ether is neutral and noncovalently com
plexed, a constant-neutral-loss scan of their collision-induced dissoc
iation spectra yields a simplified spectrum that is free of chemical n
oise.