Control of time front tilts in planar multi-reflection time-of-flight mass analyzers by local wedge fields

Highlights

• Weak local wedge fields at small ion energies combined with flat accelerating fields create large time front tilts.

• Time front tilts control decoupled from ion trajectory inclination angle control.

• Compensation of time front tilts induced by deflectors in planar multi-reflection time-of-flight analyzers.

• Combination of an orthogonal accelerator with a local wedge field and a deflector for dense packaging ion trajectories.

• Combination of an ion mirror with a local wedge field and a deflector for doubling the ion flight path.

Abstract

It is shown that an electrostatic field structure including a local wedge field in the area of small ion kinetic energies, followed by a planar accelerating field, allows to efficiently vary and control the inclination angle of time fronts (isochronous surfaces) in essentially parallel ion beams. When combined with deflectors, wedge fields allow independent control of the trajectory inclination angle and of the time front inclination angle. Two particular applications of local wedge fields are shown for improvement of planar multi-reflecting time-of-flight (MR-TOF) analyzers. One is a combination of an orthogonal accelerator and a deflector arranged for dense folding of ion trajectories. The other is a combination of an ion mirror and a deflector for reversing the ion motion and elongation of the flight paths.

Introduction

Since first practical implementations of a folded flight path in multi-pass time-of-flight mass analyzers (MP-TOF) [1,2] this technology attracts more and more attention for being potent of ultra-high resolving powers. Two types of MP-TOF analyzers provide simultaneous recording of mass spectra in the unlimited mass range, thus, being feasible for analytical chemistry. One type is based on a spiral ion motion in electrostatic sector fields [3], while the other type utilizes multiple ion reflections between two planar mirrors [4]. In the latter case, referred as multi-reflecting TOF (MR-TOF) analyzers, ions are periodically reflected in the X-direction between two gridless ion mirrors elongated in the “drift” Z-direction. Ion sources are arranged to produce some initial velocity in the drift direction to form a zig-zag path in the XZ-plane, as shown in Fig. 1.

In one type of MR-TOF instruments [[5], [6], [7]], continuous ion beams are injected into planar multi-reflection time-of-flight (MR-TOF) mass analyzers in the Y-direction normal to the XZ-plane. Ions are accelerated in the XZ-plane under a small drift angle with respect to the X-direction by an orthogonal accelerator (OA). This way, ion bunches are formed whose Z-width (typically below 2 mm) is smaller than their Y-height (typically, 4–8 mm), as shown in Fig. 1,A. Let us call this method as Y-injection. The method allows organizing a very dense folding of ion paths with a small Z-shift per one reflection below 10 mm. Conserving small Z-widths along the ion path length is performed by periodic focusing in arrays of two-dimensional lenses [4,6]. In total, ions experience several tens of reflections, the ion path length reaches tens of meters, and the mass resolving power achieves hundreds of thousand. Planar MR-TOF mass spectrometers experimentally demonstrated the mass resolving power up to 500 000 with practical mass spectra [7]. The theoretical limit of the mass resolving power at reasonable analyzer sizes exceeds 1 000 000.

The ion motion in planar MR-TOF analyzers, however, can be organized in a different way as shown in Fig. 1B and C. A continuous ion beam may be injected into the analyzer in the drift Z-direction. We call this method as Z-injection. While using orthogonal accelerators, the method allows forming longer ion bunches, say, 10–30 mm long in the Z-direction and narrow (1–2 mm) in the Y-direction. Clearly, with the Z-injection the Z-shift of an ion bunch per one reflection must be larger compared to the Y-injection scheme, i.e. the trajectory folding is less dense. Consequently, at a fixed analyzer size, the available number of ion reflections is smaller (typically 10–20). However, also the flight time aberrations are reduced. This happens due to a smaller Y-height of the ion bunch inside the mirrors as well as due to the absence of periodic lenses which are not necessary at a small number of ion reflections. A suitable compromise between trajectory folding, the mass resolving power and the OA duty cycle can notable improve the resolving power over singly reflecting TOF mass spectrometers at comparable duty cycles.

For optimizing MR-TOF analyzers with the Z-injection, it is desirable to vary the inclination angle of ion trajectories, for example at ion injection with orthogonal accelerators. The natural drift angle formed by an OA cannot be made small enough because of the practically existing lower limit of the kinetic energy of the continuous ion beam and because of the necessity to bypass the OA rims after the first ion reflection. The drift angle may be reduced by a deflector D1, as shown in Fig. 1B, in order to fold the ion trajectories denser and so to increase the total flight path length. Similarly, bypassing the rim of a detector may require another deflecting unit D2. Reflecting ions in the drift Z-direction to double the ion path length in the analyzer is another example of a drift angle change. Several technically complicated solutions has been proposed to fulfill the goal [8,9], but the easiest way is bending ion trajectories with the aid of a deflector D3 as shown in Fig. 1C [[10], [11], [12]].

However, every deflector tilts the time front (the isochronous surface at which ions of a fixed kinetic energy have the same flight time) with respect to the Z-direction as shown in Fig. 2. Indeed, if all ions have the same kinetic energy in front of the deflector and after leaving it, inside the deflector the electrostatic potential and thus the ion kinetic energy depends on the Z-position of the ion, since the deflecting field E is directed essentially perpendicular to the beam. Ions with different Z-coordinates pass through the deflector with different velocities and as a result the time front is tilted at the exit from the deflector. At small deflection angles, the time front tilt angle γ equals the deflection angle ψ. As a result, the time front occurs tilted with respect to the detector plane which ruins the resolution of the analyzer. For example, at 5 keV ion energy and 10 mm ion beam width in the Z-direction the time front tilt by 1° corresponds to the time broadening of the signal produced by ions with the mass of 1000 u at the detector surface parallel to the Z-direction by 5.6 ns, which is about 10 times larger than the typically achieved time peak width in a planar MR-TOF mass analyzer. With the typical flight time of 150 μs through a compact MR-TOF analyzer with the Z-injection, this broadening would limit the mass resolving power to less than 15000 vs. 150000 in the “perfect” analyzer. Thus, a careful alignment of the time front to at least 0.1° with the Z-direction is necessary.

It may seem that this problem can be easily solved by placing an additional deflecting unit in front of the detector or just by tilting the detector itself [13]. Unfortunately, this is not the case. The reason is illustrated in Fig. 3. Ion packets always possess an angular spread in the “drift” Z-direction which ruins the method of Ref. [13]. This spread is produced by the energy spread in continuous ion beams in OA, typically being ∼1eV. After one mirror reflection, ions starting from one point at different drift angles form a linear time front parallel to the drift Z-direction, because the mirror makes the flight time independent of the ion energy, and consequently of the starting angle. This time front is not aligned with the tilted time front formed by ions starting from different points. As a result, the tilted time front at presence of an angular spread occurs blurred in the X-direction. At a small angular spread this blurring can be negligible after one reflection, but in MR-TOF analyzers it is accumulated after many reflections and finally spoils the resolving power. So, the general rule to be fulfilled reads: the ion time front at multiple reflections in a planar analyzer should be kept essentially parallel to the direction Z of the ion mirror elongation.

There is one more problem concerning deflecting ions in MR-TOF analyzers. In the presence of an energy spread in the ion beam, using deflectors for changing the ion drift angle leads to an undesirable ion beam chromatic defocusing, because the deflection angle depends on the ion energy. In other words, the Z-component of the ion velocity becomes dependent on the ion energy in the X-direction. Leaving this dependence uncompensated at a long ion path length can reduce the ion transmission or even to cause fake peaks in the mass spectra due to a presence of ions which make different numbers of reflections before hitting the detector. In MR-TOF analyzers with the Y-injection, the Z-width of the beam is controlled by periodic lenses. However, at Z-injection, lenses cannot be used because of large flight time aberrations induced at a large Z-width of ion bunches.

Thus, a method is needed for an independent control of the time front tilt, of the ion inclination angle, and of the chromatic defocusing in planar MR-TOF mass analyzers with the Z-injection. In this paper we show a solution provided by local wedge fields within accelerators and mirrors producing an efficient tilting of the time front at minor changes of the inclination angle. Wedge fields have been used to bend energetic ion beams, but typically they work as simple deflectors in which the deflection angle and the tilt of the ion time front are strictly interrelated. On the contrary, local wedge fields, first considered in Refs. [[10], [11], [12]], are concentrated in a narrow vicinity of the ion turning point in ion mirrors or in a vicinity of the continuous ion beam position in orthogonal accelerators. Using local wedge fields in the region of small ion energy, one can compensate for a time front tilt induced by an electrostatic deflector and, thus, to provide an upright (parallel to the drift Z-direction) time front at multiple ion reflections in MR-TOF analyzers. In addition, the chromatic defocusing effects can be also compensated.

Section 2 of the paper explains the action of local wedge fields and presents analytical expressions for the time front tilt, variation of the ion drift angle and for the energy dependence of the ion velocity in the drift direction. Section 3 describes conditions for independent control of time front and trajectory angles when combining a deflector and a local wedge field. Further two sections present typical examples of improving MR-TOF with local wedge fields, within an accelerator in Section 4 and within an ion mirror in Section 5.