Baseline Resolution of Isobaric Phosphorylated and Sulfated Peptides and Nucleotides by Electrospray Ionization FTICR MS:  Another Step toward Mass Spectrometry-Based Proteomics

Electrospray ionization broadband FTICR mass spectrometry at a mass resolving power, m/Δm 50% ≥ 400 000 has achieved the first direct mass spectral resolution of phosphorylated and sulfated peptides (or nucleotides) of the same nominal mass. The elemental composition difference in each case is PH versus S (9.5 mDa), requiring a minimum mass resolving power ((m 2 − m 1)/m 1) of 118 000 (C terminal amidated cholecystekinin fragment 26−33 (CCK-8), DY(PO3H2)MGWMDF-NH2 versus DY(SO3H)MGWMDF-NH2) or 65 400 (adenosine triphosphate vs 3-phosphoadenosine 5‘-phosphosulfate). The isobaric mass doublets were detected in broadband mode (400 < m/z