Ion Trajectory Simulation
Several simulations were performed using the SDS algorithm to determine the trajectories of ions within the electrode and analyze the effect of varying potentials applied to each component as well as the positioning of the nanoESI tip within the electrode. The files used to generate the geometry for machining the electrode were converted to potential array (PA) files using the SL Toolkit included with SIMION 8.0. These arrays were refined using the skipped point refining method to solve for the electric field within the electrode. In each simulation the reduced mobility and diameter of each ion was estimated via the SDS algorithm. 2D images of simulated ion intensity at the deposition surface were constructed from simulation data using custom Matlab programs developed for SIMION data analysis.
Mass Spectrometer Interface
The electrodes were coupled to the atmospheric pressure inlet (API) of the mass spectrometer by drilling a 2.36 mm hole through the aluminum plate and inserting the 3.15 mm protrusion of the 2.28 mm outer diameter API capillary. The voltage on the capillary, as set in the LTQ software, was 15 V and electrical contact caused the potential on the aluminum plate to match this. Ions were sampled into the vacuum system of the MS through combined effects of suction at the opening of the API capillary as well as electrostatic focusing as a result of the potential applied to the focusing electrode. It is important to note that because of the relatively large area (1 cm2) to which focused ions are directed, only a fraction of the ions impinging on the aluminum plate are sampled by the MS. LTQ calibration mix containing caffeine, MRFA peptide, and Ultramark 1621 was used as the spray solution in all experiments. The dependence of mass spectral intensity on the voltage applied to the different components was tested by scanning the potential of the ellipse from 1 to 6 kV while the sprayer was held at a constant offset of +1 kV in relation to the ellipse potential. The intensities of the protonated caffeine ion (m/z 195), MRFA peptide (m/z 524), and a peak from the Ultramark 1621 (m/z 1322) were recorded as a function of the potential applied to the ellipse.