Resolving power and acquisition time required to observe deuterium-associated isotopic fine structure is charge state dependent. A. In order to resolve deuterium-associated peaks, it is necessary to detect a mass difference m HDX of ~2.922 mDa, where m HDX is the mass difference between 2H and 1H minus the mass difference between 13C and 12C. Increasing charge states require higher resolving power at a given m/z. Plotted are the resolving powers (m/∆m 50% where ∆m50% is the full width at half-maximum peak height) required to observe deuterium-associated isotopic fine structure for z = 1, 2, and 3 (Eq. 2). ∆m50% was calculated assuming two peaks separated by m HDX/z of equal height and Lorentzian peak shape are resolved with a valley between them reaches half-maximum height for either peak (Eq. 3). Also plotted (in black diamonds) are the resolving powers required to observed various peptide ions we previously monitored in a HDX experiment  to help illustrate the range of resolving powers necessary for one representative set of data. B. Theoretical minimum acquisition time required in order to resolve deuterium-associated isotopic fine structure as a function of absolute mass for various charge states. For a 9.4 T FTICR-MS operating in absorption mode, we assumed a conservative ~1.45-fold improvement in resolving power over the magnitude mode low-pressure limit (Eq. 1, effective field B 0 = 13.6 T ). For the 12 T FTICR-MS operating in absorption mode, we assumed the maximum 2-fold improvement in resolving power over the magnitude mode low-pressure limit, which can be valid for acquisition times of less than a few seconds ,. Again plotted (in black diamonds) are peptide ions monitored from an actual HDX experiment  to help illustrate the range of acquisition times that would be necessary without further optimization.