Ion Transport and Beam Profiling
The assembly used to demonstrate the transmission and focusing of ions beams at atmospheric pressure in the experiments discussed within this chapter is shown in Fig. 2.1 as a cutaway view with relevant axes identified. It was manufactured from a solid stainless steel block which was milled out to form a polished, half-ellipsoidal cavity (4.3 and 1.8 cm major and minor radii, respectively). Holes were drilled on the top portion of the block, to allow for the insertion of nanoESI tips for the generation of ions, as well as a single inlet on the bottom portion through which a supplementary gas flow could be introduced. Dry nitrogen was delivered from a compressed cylinder for experiments in which a gas flow was employed. A polyether ether ketone spacer (PEEK) provided electrical isolation of the ellipsoidal electrode from grounded aluminum plate covering the opening of the cavity. For all experiments the spacing between the aluminum plate and the opening of the stainless steel electrode (z = 0) was 2 mm.
Generation, transport, and subsequent focusing of ions was accomplished by first inserting a nanoESI spray tip through one of several holes in the top of the steel electrode. Once the spray tip was inserted, the potential on the spray tip and ellipsoidal electrode were adjusted such that the difference between the two was the effective spray voltage, and the difference between the electrode and aluminum plate was the focusing potential. For example, a potential of 3 kV applied to the electrode, 4.0 kV to the sprayer, with the aluminum plate held at ground results in a
Fig. 2.1 Two dimensional representation of focusing assembly showing the stainless steel electrode (black), spray tip (blue), PEEK insulator (gray), and the grounded aluminum plate (green). Direction of axes are shown in the lower left with the x-axis orthogonal to the paper. The dashed line represents the center (z) axis of the electrode and the orange line is used to show the opening plane of the ellipse and is used to define z = 0. Red traces are plots of ion trajectories
positive-mode electrospray at a spray voltage of 1.0 kV with a focusing potential of 3.0 kV. Focusing potentials ranging from 1 to 6 kV with spray voltages in the range of 0.6-3 kV were used for all experiments described.
The full profile of the ion beam exiting the ellipse was determined by mounting the IonCCD with the pixel axis parallel to the y-axis and sprayer. Ion intensity profiles were taken in 0.5 mm increments across the x-axis as shown in Fig. 2.2. This experiment, including the ionization, ion transport, and detection steps was done in air without supplementary gas flow. The elliptical electrode and sprayer were held at potentials of 4 and 5 kV, respectively, while the sprayer was 25 mm from the opening plane of the ellipse (z = -25 mm). The instrument calibration mixture was used as the spray solution in all cases.