Dilithiated Fragments Of A Monolithiated Precursor Ion - Unraveling Linked Scan Spectra At Constant-B/E Using Labeled Lithium Ions

Liquid secondary-ion mass spectra of the macrocyclic immunosuppressant rapamycin, ionized from a matrix containing lithium chloride, show intense [M + Li]+ peaks and little or no [M + 2Li - H]+. Linked scanning, at constant B/E, of the [M + Li]+ ion proved difficult to interpret, so a labeling experiment using (LiCl)-Li-6 was performed. Several prominent fragments were observed to contain two lithium ions, and precursor ion scans (constant B2/E) confirmed that these were derived from a metastable [M + 2Li-H]+ precursor. Due to poor precursor-ion resolution in fragment-ion scans at constant B/E, mono- and dilithiated precursors separated by only 6 u may produce fragments observed in each other's spectra. We find that use of labeled (LiCl)-Li-6 quickly and simply alleviates interpretation problems associated with this effect.