Fundamentals and Applications of Ion Mobility Using 3D Printed Devices
Advancements in 3D printing technology have provided (1) easy access to low-cost, open- source robotics, and (2) a fast fabrication technique for analytical devices among others. Using the robotics of a 3D printer, a mass spectrometry-based reaction screening device was built as a low- cost, modest throughput alternative to expensive, very fast systems. Using the 3D printer for fabrication, ion mobility devices were fabricated. Fundamental studies of the motion of ions in these devices were performed in addition to applications of ion mobility-mass spectrometry using a 3D printed drift tube ion mobility spectrometer.
With only simple modification, 3D printer kits provide nearly all the necessary parts for a functional reaction screening device. Replacing the hotend assembly with custom parts to hold a syringe, precise volumes of reaction mixtures can be dispensed, and high voltage applied to the needle for direct analysis of solutions by mass spectrometry. Direct analysis of reaction mixtures in a 96-well microtiter plates was completed in approximately 105 minutes (~65 seconds per reaction mixture, including washing of syringe). Following analysis, product distributions derived from the electrospray mass spectra were represented as heatmaps and optimum reaction conditions were determined. Using low-cost, open-source hardware, a modest throughput for reaction screening could be achieved using electrospray ionization mass spectrometry.
The manipulation of ions at reduced pressures is very well understood, whereas the efficient manipulation of ions at atmospheric pressure is far less understood. Using 3D printing, multiple iterations of atmospheric pressure drift tube ion mobility spectrometers were fabricated with one and two turns in the drift path. Optimum electrode geometries for ion transmission and resolution were determined by both simulation and experiment. Racetrack effects, where ions on the inside of turns have a shorter path than ions on the outside, were determined to be highly detrimental to resolving power. Drift tubes with two turns in opposite directions (a chicane) corrected for racetrack effects and had only marginally poorer resolving power than a straight drift tube. Additionally, ion intensities were nearly identical between optimized straight and turned ion paths, showing that these manipulations can be done with high efficiency. The focusing of ions at reduced pressure using RF ion funnels at reduced pressure can have nearly 100 percent transmission. At atmospheric pressure, RF fields are not nearly as efficient at focusing ions. By using non-uniform DC fields at atmospheric pressure, ions can be focused, but not nearly to the extent as at reduced pressure.
The coupling of atmospheric pressure drift tube ion mobility with ion trap mass spectrometry is inefficient due to the mismatch in duty cycle between the two instruments. For this reason, increasing the amount of data collected from a single experiment is of high importance. Fourier transform ion mobility increases the duty cycle from less than 1% to 25%. When ions are fragmented in the mass spectrometer, they maintain the frequency characteristic of the precursor. Therefore, ions can be fragmented without isolation in the ion trap (reducing duty cycle further) and related precursors and product ions identified through their drift time. Two-dimensional tandem mass spectrometry is a method to collect all tandem mass spectrometry information in a single scan. When coupled with ion mobility, this data can be used to generate functional group- specific ion mobility spectra where ion intensity is measured along a precursor or neutral loss scan line. This was demonstrated for a lipid sample in which head-group specific ion mobility spectra were obtained using head-group specific precursor and neutral loss scan lines.