Improved Miniaturized Linear Ion Trap Mass Spectrometer Using Lithographically Patterned Plates and Tapered Ejection Slit

Concept and simulation of a novel dual-layer linear ion trap mass analyzer for micro-electromechanical systems mass spectrometry

This paper proposed a dual-layer linear ion trap mass analyzer (dLIT) based on micro-electromechanical systems (MEMS) technology and stacked-layer structure for the development of MEMS mass spectrometry. Its basic performance and potential capabilities were explored by ion trajectory simulations. The theoretical formulas were modified by implementing multipole expansion. The simulation results were confirmed to be highly consistent with theoretical calculations in multiple aspects, including stability diagram, secular frequencies, and mass linearity, with only a deviation of 1-2%. In the boundary ejection mode, close to 100% ejection was achieved in a single dimension by applying extra quadrupole DC voltage. Preliminary simulation results showed that dLIT can achieve a peak width of ∼2 mass units (full width at half maximum, FWHM) for m/z 60 ions even at pressures as high as 50 Pa. Furthermore, the application of AC frequency scanning mode in dLIT was also evaluated, and preliminary simulation results yield a peak width of 0.3-0.4 mass units (FWHM). The dLIT offered several advantages, including high-precision fabrication at the sub-millimeter scale, excellent high-pressure performance, and a clear physical model. It preliminarily proved to be an ideal mass analyzer for MEMS mass spectrometry.

Analysis of Multipolar Linear Paul Traps for Ion–Atom Ultracold Collision Experiments

We evaluate the performance of multipole, linear Paul traps for the purpose of studying cold ion–atom collisions. A combination of numerical simulations and analysis based on the virial theorem is used to draw conclusions on the differences that result, by considering the trapping details of several multipole trap types. Starting with an analysis of how a low energy collision takes place between a fully compensated, ultracold trapped ion and an stationary atom, we show that a higher order multipole trap is, in principle, advantageous in terms of collisional heating. The virial analysis of multipole traps then follows, along with the computation of trapped ion trajectories in the quadrupole, hexapole, octopole and do-decapole radio frequency traps. A detailed analysis of the motion of trapped ions as a function of the amplitude, phase and stability of the ion’s motion is used to evaluate the experimental prospects for such traps. The present analysis has the virtue of providing definitive answers for the merits of the various configurations, using first principles.

A Microscale Planar Linear Ion Trap Mass Spectrometer

The planar linear ion trap (PLIT) is a version of the two-dimensional linear quadrupole ion trap constructed using two facing dielectric substrates on which electrodes are lithographically patterned. In this article, we present a PLIT that was successfully miniaturized from a radius of 2.5 mm to a microscale radius of 800 μm (a scaling factor of 3.125). The mathematics concerning scaling an ion trap mass spectrometer are demonstrated-including the tradeoff between RF power and pseudopotential well depth. The time average power for the microscale PLIT is, at best, ~ 1/100 that of the PLIT but at a cost of potential well depth of ~ 1/10 the original. Experimental data using toluene/deuterated toluene and isobutylbenze to verify trap performance demonstrated resolutions around 1.5 Da at a pressure of 5.4 × 10^-3 Torr. The microscale PLIT was shown to retain resolutions between 2.3 and 2.7 Da at pressures up to 42 × 10^-3 Torr while consuming a factor of 3.38 less time average power than the unscaled PLIT. Graphical Abstract.

Double resonance ejection using novel radiofrequency phase tracking circuitry in a miniaturized planar linear ion trap mass spectrometer

RATIONALE

Ion trap mass spectrometers are attractive due to their inherent sensitivity and specificity. Miniaturization increases trap portability for in situ mass analysis by relaxing vacuum and voltage requirements but decreases the trapping volume. To overcome signal/resolution loss from miniaturization, double resonance ejection using phase tracking circuitry was investigated.

METHODS

Phase tracking circuitry was developed to induce double resonance ejection in a planar linear ion trap using the β 2/3 hexapole resonance line.

RESULTS

Double resonance was observed using phase tracking circuitry. Resolution of 0.5 m/z units and improved signal-to-noise ratio (SNR) compared with AC resonant ejection were achieved.

CONCLUSIONS

The phase tracking circuitry proved effective despite deviations from a true phase locked condition. Double resonance ejection is a means to increase signal intensity in a miniaturized planar ion trap.

Experimental Observation of the Effects of Translational and Rotational Electrode Misalignment on a Planar Linear Ion Trap Mass Spectrometer

The performance of miniaturized ion trap mass analyzers is limited, in part, by the accuracy with which electrodes can be fabricated and positioned relative to each other. Alignment of plates in a two-plate planar LIT is ideal to characterize misalignment effects, as it represents the simplest possible case, having only six degrees of freedom (DOF) (three translational and three rotational). High-precision motorized actuators were used to vary the alignment between the two ion trap plates in five DOFs-x, y, z, pitch, and yaw. A comparison between the experiment and previous simulations shows reasonable agreement. Pitch, or the degree to which the plates are parallel along the axial direction, has the largest and sharpest impact to resolving power, with resolving power dropping noticeably with pitch misalignment of a fraction of a degree. Lateral displacement (x) and yaw (rotation of one plate, but plates remain parallel) both have a strong impact on ion ejection efficiency, but little effect on resolving power. The effects of plate spacing (y-displacement) on both resolving power and ion ejection efficiency are attributable to higher-order terms in the trapping field. Varying the DC (axial) trapping potential can elucidate the effects where more misalignments in more than one DOF affect performance. Implications of these results for miniaturized ion traps are discussed. Graphical Abstract ᅟ.

Optimal fabrication methods for miniature coplanar ion traps

RATIONALE

Ion trap mass spectrometers are beneficial due to their intrinsic sensitivity and specificity. Therefore, a portable version for in situ analysis of various compounds is very attractive. Miniaturization of ion traps is paramount for the portability of such mass spectrometers.

METHODS

We developed an optimized design for a planar linear ion trap mass spectrometer, consisting of two trapping plates with photolithographically patterned electrodes. Each plate is constructed using a machined glass substrate and standard microfabrication procedures. The plates are attached to a patterned circuit board via wire bonds then positioned approximately 5 mm apart.

RESULTS

Trapped ions are detected by ejecting them through tapered slits, which alleviate charge buildup. Mass analysis can be performed through either boundary or resonant ion ejection. Better than unit mass resolution is demonstrated with resonant ejection.

CONCLUSIONS

The optimized planar linear ion trap provides good resolution and the potential for further miniaturization. This was accomplished by vigorously testing variables associated with ion trap design including electrical connections, substrate materials, and electrode designs.