MASS AND CHARGE ASSIGNMENT FOR ELECTROSPRAY IONS BY CROWN-ETHER ADDUCTION

The introduction of 18-crown-6 into the liquid sheath during the electrospray 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 Cu2+ adduction, and is thus potentially more useful for the determination of charge states, especially for multiply charged high-molecular-weight species. The addition of an uncharged crown ether does not appear to interfere with the ESI process. Because the crown ether is added to the liquid sheath and not to the original solution, the method should be particularly amenable to chromatographic or electrohoretic techniques because it is essentially a post-column noncovalent derivatization procedure. Finally, because the complex formation constants are relatively large, the peptide/crown ether complex yields high intensity ions, normally the base peak in the spectrum when analyzing pure peptide samples. Finally, as the crown ether is neutral and noncovalently complexed, a constant-neutral-loss scan of their collision-induced dissociation spectra yields a simplified spectrum that is free of chemical noise.