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 in tense [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 pro minent fragments were observed to contain two lithium ions, and precur sor ion scans (constant B2/E) confirmed that these were derived from a metastable [M + 2Li-H]+ precursor. Due to poor precursor-ion resoluti on in fragment-ion scans at constant B/E, mono- and dilithiated precur sors separated by only 6 u may produce fragments observed in each othe r's spectra. We find that use of labeled (LiCl)-Li-6 quickly and simpl y alleviates interpretation problems associated with this effect.