Figure 3.

Mass scale-expanded spectra of one isotopic peak for ¹³C,¹⁵N doubly depleted p16 tumor suppressor protein, from a single ESI FT-ICR mass spectrum. The theoretical isotopic distribution is shown at upper left, in which ★ denotes the isotopic peak ≈2 Da above the monoisotopic mass. The theoretical isotopic fine structure for the starred peak is shown at upper right. Double-depletion improves the SNR of the monoisotopic, ³⁴S, and ¹⁸O species, by reducing the abundances of species containing ¹³C and/or ¹⁵N.