Chemistry and Biochemistry

Wire Ion Trap

 

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Introduction 

The electric field in a conventional linear ion trap (LIT) is formed by solid metal electrodes that include machined slits for ion ejection. Under some operating conditions these slits can block a portion of the ejection ions, reducing the sensitivity. We report on a linear ion trap in which the electric field is formed by fine wires, held under tension and accurately positioned using laser-drilled holes in two end plastic plates.  This device was first studied in SIMION and then demonstrated in experiments.

 

Methods 

In this work, we designed a linear wire ion trap mass analyzer (WIT), in which the electric field was formed by 24 straight wires. Each of the four electrodes of a conventional LIT are represented using six wires arranged in a pattern that approximates the LIT electric potential.  These wires are held under mild tension between two plastic plates.  The position of each wire is constrained by a laser-drilled hole on each plate. The plates are held apart using four bolts. Two metal rings were set in the center of plastic plates and served as end electrodes, providing axial confinement.

The configuration of a WIT testing system is shown in Figure A, and the cut view was shown in Figure B. Figure C shows the assembled WIT. The insulation plates are made of polyetheretherketone (PEEK) and were drilled with 24 holes on it. The position of holes were located according to the simulated results with scale of 0.05. The accuracy of the position of holes are ±0.05 mm, which has negligible impact on the mass resolution. The tension applied on the wires is provided by the eight nuts on four bolts. The distance between the two plates is 62 mm. A hot filament electron gun was used for ionizing neutral molecules inside the WIT. The ejected electrons were gated by the Egun gate. In addition, a magnetic ring was set outside the electron gun and used to confine the trajectory of electrons.

 

Results

The mass resolution was evaluated by mixture of toluene and deuterated toluene (toluene-D8) with ratio close to 1:1. Figure shows the mass spectrum for the mixture. The FWHM is in the range of 0.37 to 0.38 under scan rate of 3830 Th/s. The RF frequency is 1.53 MHz, with applied supplementary ac at frequency of 525 kHz, amplitude of 1.2 V (0-p), and offset of 1.3 V. Ions are resonantly ejected at q = 0.78. The potential applied on the end ring electrodes have a similar impact on the resolution and peak intensity as described in RIT study.