Hamish Stewarta, Christian Hocka, Anastassios Giannakopulosa, Dmitry Grinfelda, Richard Hemingb, Alexander MakarovaaThermo Fisher Scientific, Bremen, Germany, bUniversity of Münster, Münster, Germany.

A Rectilinear Pulsed-Extraction Ion Trap with Auxiliary Axial DC Trapping Electrodes

ABSTRACTPurpose: Characterisation of a novel RF ion trap incorporating auxiliary DC-only axial trapping electrodes.

Methods: The ion trap was characterised with regard to trapping and ion extraction properties, mass range and space charge effects via analysis of the extracted ion clouds within a custom-made linear time-of-flight analyser. Extraction of ions was performed by rapidly quenching the applied RF phases at the optimum times, followed by a pulsed electric field gradient across the trap. Measurements of the spatial distributions of the extracted ion packets were carried out using an IonCCD imaging detector. Results were compared to ion optical simulations and RF trapping theory.

Results: Ions were axially constrained by the auxiliary trapping electrodes even when the trap body was floated to +4KV relative to the axial DC apertures, and good time-of-flight properties were retained.

The efficacy of trapping higher m/z ions was reduced by greater auxiliary electrode voltages, and this is shown to be due to DC counteracting the RF pseudopotential, which known to be weak for high m/z ions with low parameter q. On the other hand, low auxiliary electrode voltages limit the space charge capacity for low m/z (high q) ions, which become axially unstable due to their greater density in RF trapping devices.

INTRODUCTIONRadiofrequency ion traps are used as ion preparation devices for Thermo Scientific™ Orbitrap™ based mass spectrometers and time-of-flight mass analysers, where they serve to accumulate, thermalize and shape ion populations prior to pulsed extraction into the analyser. Ions are radially confined within a trap by a quadrupole RF pseudopotential.

Typically ions are axially confined by DC electrodes at the termini of the trap. Here a rectilinear ion trap is presented and characterised whereby axial trapping is accomplished via a pair of negatively biased DC-only rectangular electrodes, positioned in gaps between split RF electrodes in a manner shown in Figure 1. These electrodes create an electric potential that penetrates to some extent to the center of the trap, creating an axial DC trapping well of a length controlled by the length of the electrodes.

Extraction of ions is accomplished by quenching the RF and applying an extraction potential, as push and pull voltages shown in Figure 1. Applying the opposite extraction voltages across the split Y electrodes increases the field gradient on the axis. Optionally the ion trap potential may be lifted prior to extraction to increase the energy of the extracted ions.

Auxiliary DC Electrode -5V

Split Y Electrode (Top)RF1, Pull = -600V

Split Y Electrode (Bottom)RF1, Push = +600V

X Electrode RF2, Pull = -600V

To Analyser

X ElectrodeRF2, Push = +600V

To Analyser

DC Aperture

DC Aperture

X Electrode

Auxiliary DC Electrode Split

Push Electrode

Split Pull Electrode

Figure 1. Diagrams of the rectilinear ion trap showing applied RF phases and voltages; and photographs of the device with and without front DC aperture

Preferentially immediately prior to extraction RF2 is quenched as it ascends to ground, a ½ cycle before RF1 is quenched, and then relevant extraction voltages may be applied. This optimizes the trapped ions’ phase space distribution, minimising the velocity spread in the direction of extraction (scenario A in Figure 2 below), resulting in shorter ion turnaround time and narrower time focus.

Figure 2. i) Illustration of ejection schemes A and B and their influence on the ion energy and velocity distributions along the axis of extraction. ii) Oscilloscope trace of opposing RF phases at the point of extraction. iii) Trace showing 4KV lift of ion trap whilst RF applied.

RF1

RF2

Trap Lift+4 KV

ii) iii)

i)

METHODExperimental Setup

A small time-of-flight analyser was assembled in front of the ion trap, incorporating a stack of accelerating electrodes with progressive voltages U1 and U6 and an ion detector as shown in Figure 3. Ions were injected axially into the ion trap where they were accumulated and cooled before ejection into the acceleration stack. By adjusting the acceleration voltages U1 and U6 across the stack, the position of the time focus was brought to the detector.

Time resolved and quantitative ion data was gathered with time-of-flight detectors (ETP, Photonis). Spatial data was generated with an MCP-CCD based imaging detector (IonCCD, OI Analytical).

Values such as ion trap lift voltage, RF amplitude, auxiliary DC electrode voltage and ion number were scanned and and experimental results were compared to simulations performed with the MASIM 3D software package.

Figure 3. Sectional view of the experimental setup

Figure 4. Representation of experimental apparatus in MASIM 3D simulation

Ion Trap

Acceleration Stack

Detector Plane

RESULTSTime-of-Flight Focal Quality

The time spread of ions recorded at the time focus is dependent on the spatial-velocity distribution of the trapped ions, as well as the extraction field gradient. The result is also affected by the scatter against background gas and the detector time response.

Figure 5 shows measured time responses and their reduction with increasing RF amplitude. Simulated data points are included based on the model of thermalized ions with initial distributions according to RF trapping theory. If estimated detector time response is subtracted, then experiment and simulation are in reasonable agreement for the three masses. The agreement for m/z 1522 is slightly worse, which may be attributed to scattering with background ions or stronger perturbation of the RF by the auxiliary DC electrodes.

Figure 5. Ion arrival time spread (full width half maximum) versus RF amplitude @3MHz, raw and shifted by estimated 1.25 ns detector response. Several simulated points are included.

IonCCD Imaging Measurement

For 2.5 mm long auxiliary DC trapping electrodes, the imaging detector recorded a total axial width of 2.4 mm, consistent across m/z 195, 524 and 1522 as would be expected with DC axial trapping, and just slightly smaller than the length of the electrodes themselves.

Figure 6. Ion axial distribution recorded at IonCCD imaging detector

Aux Electrode Length = 2.5mmσ = 0.4 mm6σ = 2.4 mm

High m/z Limit

It was observed that higher auxiliary DC voltage generally improves the trapping efficiency, but eventually higher m/z ions start to be lost. It can be seen that this threshold increases with stronger RF amplitude, indicating that there is an interplay between the radial DC field and the RF pseudopotential.

Figure 7. Relative signal areas for ions when auxiliary DC electrode voltage scanned for top: Differing m/z ions, and below: m/z 1522 at differing RF amplitudes

Field simulations show that the DC electrodes indeed counteract the radial trapping pseudopotential. Higher m/z ions, for which the RF trapping well is insufficiently deep, are attracted to the auxiliary electrodes and may be sucked out. Figure 8 shows how the overall radial trapping well is reduced and eliminated by progressively stronger auxiliary electrode DC potentials.

An ion trap with four auxiliary electrodes is also shown. These are less harmful to the RF pseudopotential as the radial DC field component is octupolar rather than quadrupolar. On the other hand, the four electrodes are no longer conveniently aligned at the zero level of the extraction potential and would need extraction voltages to be applied for optimised ion extraction.

Figure 8. Simulated radial pseudopotential well (1000V RF @ 4.5 MHz, m/z 1522) plus perturbation from auxiliary DC electrodes at 5-30V, for traps with 2 and 4 auxiliary electrodes

PO65225-EN 0518S

CONCLUSIONSA linear RF extraction ion trap incorporating auxiliary DC axial trapping electrodes has been characterised:

Trapped ion spatial and energy distributions and time-of-flight properties align well with theory.

At low auxiliary DC voltages, ion confinement is limited by the axial potential barrier, and the m/z ions escape the ion trap first under space charge. This effect imposes a lower restriction on this voltage.

The higher auxiliary DC voltages restrict the mass range by disruption of the RF trapping well for high m/z ions. This limitation may be mitigated by adopting a four-electrode auxiliary DC scheme.

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Figure 10. Simulated ion axial and radial distributions for an equal mixture of m/z 195, 524 and 1522 under space charge, within an ion trap with 4mm auxiliary DC electrodes at -5V

Axial Dist.

Radial Dist.# Ions

Figure 9. Scan of fill time and detected ion number for three m/z ions within Calmix sample

Space Charge Effects

When an RF trap is overfilled high m/z ions are the first to be lost, as they are pushed out of their relatively shallow radial trapping well. However the opposite trend was observed when filling this trap at various fill times with ions from calibration solution. Low mass ions were the first to be lost, as shown in Figure 9 below.

This counter-intuitive space charge trend can be rationalised by understanding that the DC potential well is relatively shallow (~400 mV when 5V DC applied) and the ion leakage is also axial. Lower m/z ions are better confined radially and thus have higher charge density, so these ions are the first to pass over the axial potential barrier. MASIM 3D simulations of radial and axial ion distributions are drawn in Figure 10 and demonstrate the axial expansion of lower m/z ions under space charge.

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