Effects of electron kinetic energy and ion-electron inelastic collisions in electron capture dissociation measured using ion nanocalorimetry

Abstract

Ion nanocalorimetry is used to measure the effects of electron kinetic energy in electron capture dissociation (ECD). With ion nanocalorimetry, the internal energy deposited into a hydrated cluster upon activation can be determined from the number of water molecules that evaporate. Varying the heated cathode potential from -1.3 to -2.0 V during ECD has no effect on the average number of water molecules lost from the reduced clusters of either [Ca(H2O)15]2+ or [Ca(H2O)32]2+, even when these data are extrapolated to a cathode potential of zero volts. These results indicate that the initial electron kinetic energy does not go into internal energy in these ions upon ECD. No effects of ion heating from inelastic ion-electron collisions are observed for electron irradiation times up to 200 ms, although some heating occurs for [Ca(H2O)17]2+ at longer irradiation times. In contrast, this effect is negligible for [Ca(H2O)32]2+, a cluster size typically used in nanocalorimetry experiments, indicating that energy transfer from inelastic ion-electron collisions is negligible compared with effects of radiative absorption and emission for these larger clusters. These results have significance toward establishing the accuracy with which electrochemical redox potentials, measured on an absolute basis in the gas phase using ion nanocalorimetry, can be related to relative potentials measured in solution.

Figure 1

Schematic of the 2.75 T FT/ICR mass spectrometer used in these experiments. The inset shows details of the ion cell and heated metal cathode with copper grid. The heated metal cathode is mounted on the central axis of the vacuum chamber and is positioned 20 cm from the center of the ion cell. MP, CP and TP indicate mechanical pump, cryopump and turbopump, respectively.

Figure 2

[Ca(H₂O)₁₅]²⁺ precursor abundance after 120 ms of electron irradiation as a function of heated cathode potential, measured with two different trapping plate potential conditions: a symmetric 2.0 V trap potential during electron irradiation and ion detection (solid squares); and asymmetrical 8.3/9.2 V source-side/far-side trap potentials during electron irradiation, 2.0 V during ion detection (open circles).

Figure 3

Average number of water molecules lost upon reduction of [Ca(H₂O)₁₅]²⁺ and [Ca(H₂O)₃₂]²⁺ as a function of heated cathode potential. Dashed lines indicate extrapolated least squares fits of the data, and the error bars at 0.0 V indicate the propagated error in the intercept.

Figure 4

Average number of water molecules lost from reduced [Ca(H₂O)₁₅]²⁺ due to EC as a function of electron irradiation time. The dashed line indicates the extrapolated least squares fit to the data, and the error bars at 0.0 s indicate the propagated error in the intercept. The dotted lines represent one standard deviation above and below the average value of the data at 40, 80, and 120 ms.

Figure 5

Dissociation kinetics of (a) [Ca(H₂O)₁₇]²⁺ and (b) [Ca(H₂O)₃₂]²⁺ with (squares, cathode potential −1.5 V) and without (circles, cathode potential +10.0 V) electron irradiation as a function of electron irradiation time. The dashed lines are least squares fits to the data obtained with the heated cathode potential at +10.0 V, and the solid lines are a parabolic ([Ca(H₂O)₁₇]²⁺) and least squares fit ([Ca(H₂O)₃₂]²⁺) to the data with a heated cathode potential of −1.5 V.