Ion Transport Efficiency
The overall efficiency of ionization by nanoESI and ion/charged droplet transport to a collector surface was measured with and without using the elliptical focusing electrode through the deposition of the dye rhodamine B. The sprayer was 17 mm from the grounded aluminum collection plate (15 mm from the opening plane of the ellipse) in both experiments while voltages of 4 and 5.5 kV were applied to the ellipse and sprayer (1.5 kV difference) in the experiment with the focusing electrode and a spray potential of 1.5 kV was used without the ellipsoidal lens. Rhodamine B was diluted in 4:1 methanol:water (60 p.L, 1 mM) and sprayed at a grounded aluminum plate. Following the deposition, the resulting material on a 1 cm2 area on the grounded aluminum collection surface, (centered on the deposition area), was redissolved in 1:1 methanol:water (v:v), diluted to a known volume and analyzed for concentration by UV-Vis spectrophotometry. Around 15 % of the rhodamine B in the spray solution was deposited in the 1 cm2 collection area using nanoESI without the elliptical electrode. With the elliptical electrode and using the same sprayer to counter electrode distance, the entire visible spot was confined within the 1 cm2 collection area. Measurement of the concentration of the
Scheme 2.1 Rhodamine B species in solution at low pH 1, moderate pH 2, and high pH 3
dye taken up in a fixed volume of solvent showed that the contents of this spot corresponded to 70 % of the theoretical yield. These results show that the elliptical electrode is able to concentrate ions/charged droplets into the deposition area to produce a four-fold increase in ion intensity when compared to nanoESI without the use of a focusing electrode. The 1 cm2 collection area was chosen to represent applications in ambient surface preparation techniques. It is not representative of applications in which ion transfer into a mass spectrometer from an atmospheric pressure ion source is of interest; in such cases the sprayer would be placed closer to a much smaller entrance capillary.
It should be noted that the 70 % efficiency value measured includes both ionization and transport to the collector surface and both processes individually must therefore be highly efficient. Attempts to draw distinctions between dry ion, solvated ion, and charged droplet focusing were not made, as these are all instances of charged particle focusing which was the objective of the experiment; however, it does seem to be important to determine the significance of the transport of analyte in the form of charged droplets. To make this distinction, the above deposition experiments were repeated using a rhodamine B solution doped with ammonium hydroxide, to give a pH of approximately 9 (0.148 M NH4OH). Rhodamine B exists in three different forms which depend on the solution pH, as shown in Scheme 2.1 .
At low pH (-1 to 0) form 1 is favored, but in a solution of moderate pH (1-3), as is found in the spray solution absent a pH modifier, the cation 2 can be assumed. In a solution of higher pH the zwitterion (3) will be the favored form. Although droplet desolvation during drying in air will result in a lowered pH, this is assumed to contribute minor changes to the form of rhodamine B as the change is rarely more than 2 pH units . Because of this it is assumed the deposition at low to moderate pH involves ionic species while at high pH the zwitterion should dominate. The deposition of the basic ammonium solution showed a deposition efficiency of 10 % when compared to theoretical yield (with the use of the elliptical electrode). These results are indicative of the large role played by solution phase ions in the conveyance of chemical species by electrospray, as the zwitterion (3) is effectively neutral, limiting transport to a mechanism associated with charged droplets and incidental neutral collisions.