MALDI-TOF mass spectrum of insulin and β-lactoglobulin. A mixture of 5 pmol each of insulin (I) and β-lactoglobulin (L) was ionized by MALDI, which produces predominantly singly charged molecular ions from peptides and proteins—the insulin ion (I+H)+ and the lactoglobulin ion (L+H)+. Molecules with multiple charges, such as those for β-lactoglobulin indicated by the blue arrows, as well as small quantities of a singly charged dimer of insulin (2 I+H)+ also are produced.
[Data from J. T. Watson, Introduction to Mass Spectrometry, 3d ed. (Lippincott-Raven, 1997), p. 282.]