Ion Transfer Efficiency

The fraction of ions which traverse the entire device varies greatly, with a large dependence on the potentials applied to the electrodes in addition to the spray emitter. A summary of the results of transmission efficiency experiments is shown in Table 3.1.

Under most the most common operating conditions, such as those give in Fig. 3.5a, c, 1-10 % of the total spray current is transmitted to the detection surface. It is possible to increase transmission efficiency by iteratively tuning the potentials on each electrode as well as the spray tip. With the tip position maintained in the

Table 3.1 Efficiencies of ion transfer from spray tip to deposition surface under typical operating conditions. Potentials applied to Esource, E1, E2, and E3 are identical to Fig. 3.5a-c. Spray voltage is given as the difference of potential applied to the Esource and nanoESI spray tip

Spray voltage (kV)

Isource (nA)

Isurface (nA)

Transmission efficiency (%)

0.750

3.52

0.28

7.8

1.00

19.65

0.50

2.5

1.25

37.99

0.67

1.8

1.50

56.56

0.75

1.3

1.65

67.70

0.87

1.3

same position as the data collected in Table 3.1 (inserted 1 cm into Esource) it was possible to transmit 11 % of an 11.10 nA current through the device. To achieve this, potentials of 3.89, 3.00, 2.64, 1.89, and 1.37 kV were applied to the spray tip, Esource, Ej, E2, and E3, respectively. While the ion distribution at the surface was not mapped under these conditions, simulations show a significant increase in spot size at the detection surface. By further adjustment of potentials and inserting the nanoESI emitter further into Esource (approximately 10 mm distant from the opening of Ej) a 55 % transmission of a 4.01 nA spray current was achieved. Potentials applied to the spray tip, Esource, Eb E2, and E3 were 3.65, 2.50, 1.76, and 1.27 kV, respectively.

Simulations were used as the primary tool in determining where ion losses might occur within the devices. Under typical operating conditions as shown in Fig. 3.5a-c, a simulation was performed in which the ions were originated uniformly within a cylindrical volume extending from the tip of a nanoESI emitter (inserted 1 cm into Esource as is visible in Fig. 3.1c) to the exit of Esource. From this simulation an “acceptance volume” (defined as the volume within the source capable of transmitting ions to the deposition surface) was determined. The transmission efficiency was also calculated from the simulated trajectories of 4 x 104 ions (104 each of tetrapropyl-, tetrabutyl-, tetrahexyl-, and tetradodecylammonium cations) distributed uniformly within the cylindrical volume. The resulting acceptance volume is shown in Fig. 3.6a along with a plot of origin locations of all ions within acceptance volume in Fig. 3.6b. Electrode points and resulting deposition spot are shown as well.

From the calculated volumes of the ion origins and resulting acceptance volume a maximum transfer efficiency from ions originating only within Esource can be calculated as 40.5 %. This number assumes that ion generation occurs only within Esource which is not representative of the dynamic nature of spray ionization in which gas phase ions may be released from droplets projected beyond the confines of Esource. A qualitative examination of simulated ion trajectories suggests that the majority of losses occurs to the walls of Esource with minor losses to the walls of E1. Ion losses within electrodes Ej-E3 are assumed to be minimal and the transmission efficiency of more than 50 % achieved by positioning the spray tip within 10 mm of the entrance of E1 supports this reasoning. It is expected that with further optimization of the size and shape of electrodes, transmission efficiency may be increased significantly.

encompassing ion origin locations represented by green mesh, and acceptance

Fig. 3.6 encompassing ion origin locations represented by green mesh, and acceptance

volume represented by red mesh (a); 3D scatter plot showing origin location of ions within acceptance volume (red points), electrode points (blue), and resulting deposition spot (black) (b). Potentials applied to emitter and electrodes are identical to Fig. 3.5a-c

 
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