Ion Deposition Image Collection
An IonCCD™ detector (OI Analytical, AL, USA) was used to examine the profiles of ion intensity on a deposition surface. In these cases, the IonCCD™ was mounted onto an automated XY moving stage (Dover Motion, MA, USA). The stage was then scanned at a fixed rate across the ion stream as it exited the electrodes. The intensities from IonCCD™ line scans were then reconstructed into two dimensional images of the ion plume at the deposition surface using a custom Matlab script. The script considers both the timestamps from the individual detection periods and the velocity of the moving stage when reconstructing an image. This process is illustrated in Fig. 3.2.
For experiments discussed in this chapter, the integration time of the IonCCD™ was set to 100 ms and the detector was scanned across the ion plume at a rate of 0.5 mm/s. Examination of the annular focusing effects within the source region uses neat acetonitrile (ACN) as the spray solution. In other cases, a 10 ^M equimolar mixture of, tetrabutyl- (TBAB), tetrahexyl- (THAB), tetraoctyl- (TOAB), and tetradodecylammonium bromide (TDDAB) in ACN was used as the spray solution. For IMS separation, an 2 mM equimolar mixture of TBAB, THAB, TOAB, and TDDAB in ACN was used as the spray solvent.
Ion Transmission Efficiency
An evaluation of the efficiency of ion transfer from the nanoESI spray tip to the deposition surface is performed in a manner similar to that discussed in Sect. 3.2.4. Briefly, a 4.7 MX resistor is placed in series with the nanoESI spray tip and the voltage drop across this resistor is used to calculate the total spray current using Ohm’s Law. This is compared to readings taken simultaneously at the deposition surface with a Keithley 6487 picoammeter (Keithley Instruments, OH, USA).