Results and Discussion
Focusing of Electrosprayed Ions
A 2D intensity map of ions impinging on the deposition surface was constructed from intensity profiles (dN) at each position along the x-axis, shown in Fig. 2.2. Potentials applied to the sprayer and electrode were 5 and 4 kV, respectively. The resulting intensity plot indicates that the highest ion intensity occurs in the region nearest the spray tip.
Cross-sections (through center of ellipse opening plane, parallel to the y-axis of Fig. 2.2) of spatially resolved ion intensity were recorded using the IonCCD while the elliptical electrode was held at a constant potential of 4 kV and the sprayer was positioned 25 mm from the opening plane of the ellipse (z = -25 mm). The spray potential was varied from 4.6 to 6 kV, corresponding to offset potentials of 0.6-2 kV. While an increase in the offset potential increased the overall ion intensity, this also had the effect of broadening the profile (i.e. widening the deposition area). The broadening was likely due to the electrical fields created by the potential difference of the spray tip from the ellipse. The asymmetric nature of the ion beam was most likely the result of the orthogonal position of the sprayer relative to the center axis of the ellipse. The spray was positioned in this manner so that neutral droplets expelled from the spray tip would impinge on the electrode wall rather than be accepted into the MS. The asymmetry is not observed when the spray direction is in-line with the center axis. The effect the offset voltage (spray potential-ellipse potential) has on both the intensity and spatial distribution of ions exiting the ellipse is shown in Fig. 2.3a.
If one considers the relationship between the maximum recorded intensity as a fraction of the total intensity (Imax/Itot) as a function of the potential offset between the spray tip and the focusing potential it is possible to develop a method of quantifying the focal abilities of the electrode under different conditions. A plot of this is shown in Fig. 2.3b. From the plot it is clearly evident that lowering the potential offset of the spray tip from the focusing potential results in greatly increased focal abilities. Additionally, the asymmetric nature of the ion distribution is almost entirely eliminated at lower potentials compared to the distributions obtained at higher offset potentials. While a smaller number of ions are created, the
Fig. 2.3 a Ion intensity profile at different offset voltages and b maximum IonCCD signal (Imax) as a fraction of total signal (Itot) for different offset potentials. Potential applied to ellipse was 4 kV and sprayer was 25 mm from the ellipse opening plane (z = -25 mm) for values shown. Spray was in the direction of decreasing values on the IonCCD™ pixel axis. The electrode arrangement corresponds to that shown in Fig. 2.1
better focusing will allow a larger percentage of ions to be sampled by a mass spectrometer, improving overall efficiency and reducing the sample volume needed for analysis. This effect is likely due to the repulsive nature of ESI plume, which is magnified at higher potentials as the density of charge increases at the spray tip.
The effect of a gas flow on the focal and transport properties of the device was examined by subjecting the ions to a nitrogen flow from a compressed gas cylinder. The pressure of the nitrogen line, as determined by the regulator, was approximately 5 psi and the interior of the device was assumed to be at atmospheric pressure. Spatial ion distributions were recorded by the IonCCD at different offset potentials while the ellipse potential was held constant in the same manner as described earlier. Results of these experiments show an increased intensity and an increased symmetry in the ion beam cross-section; however, the better focus at low offset potentials was not observed. This effect is likely due to the transport of ions and charged microdroplets from areas of low electric field strengths to those of higher strength where they are more highly influenced by electrostatics. Additionally, the removal of solvent vapor decreases its partial pressure within the spray plume to afford more effective solvent evaporation according to Henry’s Law.