TO DOWNLOAD A COPY OF THIS POSTER, VISIT WWW.WATERS.COM/POSTERS ©2017 Waters Corporation

OVERVIEW

Annular geometry ion mobility separation device gives

increased space charge capacity (~50x over a SRIG)

Trap filling simulations via solution of the Poisson

equation show the requirement for an interface that

evenly fills the trapping volume

Novel “pancake” ion guides allow even filling of, and

extraction from, the annular device

INTRODUCTION

In conventional ion mobility mass spectrometry (IM-MS) ions are

released into the ion mobility separator (IMS) in discrete packets. One

method to improve the duty cycle of the device is trapping of the ions

prior to the IMS. As ion sources get brighter, and IMS drift times get

longer, the space charge capacity of the trap increasingly limits the

utilisation of the ion beam. Furthermore, space charge effects within the

IMS can have deleterious effects on resolution and transmission.

One approach to alleviating space charge effects is to increase the

trapping volume, thus reducing the charge density. In standard T-Wave

IM the ions are confined in a 5mm diameter stacked ring ion guide

(SRIG, Figure 1a). Alternate phases of RF are applied to the rings to

give RF confinement. Recently an extended geometry has been

implemented [1] in which ions are confined in a 50mmx5mm slot, greatly

increasing the trapping volume (Figure 1b). In this system we apply

alternate phases of RF to a series of top/bottom plate electrodes to give

confinement in the y-axis, while a DC voltage on the edge electrodes

gives confinement in the x-axis.

If we take an extended slot geometry, remove the DC edge electrodes

and fold it around on itself we arrive at an annular geometry which gives

us a large trapping volume in a compact physical space (Figure 1c).

While the full annular system makes best use of the available space a

half annular system is also possible, again with DC edge electrodes for

confinement at the open edges. This geometry offers about half the

capacity of the full annular system, however it is more straightforward to

manufacture (Figure 1d).

SIMULATION OF A NOVEL ANNULAR GEOMETRY ION MOBILITY TRAP AND SEPARATION DEVICE WITH HIGH SPACE CHARGE CAPACITY

David Langridge, Martin Green, Kevin Giles Waters Corporation, Wilmslow, UK

METHODS

SIMION 8.1 [2] was used to solve the Poisson equation in 2D. The

Poisson equation is periodically re-solved (refined) to generate a new

space charge field. For accurate results we require that the distance

ions move between successive refines is less than 1 grid unit of the

charge density grid.

Between refines, the ions move under the influence of the total electric

field in the system, given by the sum of the space charge field, an RF

effective potential field and a static DC field. Collisions with the

background gas were modelled using the SDS method [3]. The fields for

the RF effective potential and the DC electrodes need only be

calculated once for a given geometry, then scaled by the applied

voltage (and RF frequency / ion m/z for the RF effective potential).

As we solve the Poisson equation in 2D we obtain a linear charge

density (LCD), this can be equated to a number of charges if we define

a z-axis extent for the trapping volume. For the trap filling calculations

we periodically spawn new ions into the system with each ion

representing a portion of the total linear charge density.

Unless otherwise indicated calculations presented here were performed

with: 125V0-p 2.5MHz RF, +50V on the DC edge electrodes, 2.5 torr N2

buffer gas, mass 556 z=+1 ions. The RF electrodes are segmented in

the z-axis with an electrode width of 0.5mm and a gap of 1mm.

RESULTS

Trap capacity

Figure 2 shows several equilibrium charge density distributions for

mass 556 z=+1 ions. Note that for all systems we simulate only a

quarter of the system due to the x-y symmetry, the total LCD is therefore

scaled up by a factor of 4.

Figure 2a shows the charge density in a SRIG (Figure 1a) for a total

linear charge density of 1.44e-15 C/mm, or 9e3 charges/mm. At this low

level of charge the ion distribution is not significantly different from the

thermal distribution in the absence of space charge.

Figure 2b shows the charge density in a SRIG with LCD 1.44e-12 C/

mm, or 9e6 charges/mm, 1000 times more charge than Figure 2a. Due

to the mutual charge repulsion the ions are distributed in a narrow ring

with a maximum at radius ~2.1mm. This is likely to be close to the

space charge limit for this system. We would expect some heating

effects as the ions are forced up close to the steep-sided RF walls.

Figure 2c shows the charge density for an extended slot system

(Figure 1b) with LCD 1e-11 C/mm, or 6.25e7 charges/mm. The ions are

compressed into a narrow band between the mutual charge repulsion

and the steep sided RF barrier. The peak charge density and y-axis

maximum of this band matches the SRIG result from Figure 2b, hence

we estimate these systems as being equally filled. The space charge

capacity of the extended plates system is therefore around 7x greater

than the SRIG.

For the annular system (Figure 1c) we scale up from 44mm to 324mm,

therefore we would expect an increase in the space charge capacity of

~7x over the extended slot, or ~50x over a SRIG. The space charge

capacity of the half annular system (Figure 1d) is therefore about 25x

greater than a SRIG.

Trap filling

Conventionally the IMS trap is filled from an incoming axial ion beam,

this is effectively a point fill. If we take the same approach to filling an

extended geometry trap we are not able to fully utilise the increased

charge capacity due to the relatively high pressure in the IMS (2.5 torr

N2). Diffusion and space charge effects cause the ions to spread across

References

1. Kevin Giles et. al., 63rd ASMS conference, St. Louis, 2015.

2. SIMION 3D v8.1, Scientific Instrument Services Ltd.

3. Appelhans, A.D., Dahl, D.A., Int. J. Mass Spectrom. 244, 1–14 (2005).

the trapping volume, however, due to the low mobility of the ions, the

trapping volume is not evenly filled.

Figure 3 shows the charge density distribution for a point filled extended

slot system (in this case the system is 100mm wide in x). The total

charge injected is 8e-13 C/mm over 10 ms, we plot the charge density

at 2.5, 5, 7.5 and 10ms. This linear charge density equals 1e7 charges

in a 2mm axial trapping volume. The fill rate of 1e9 charges/second is a

typical source current.

After 10ms fill time the charge density is at a maximum near x=0 and

declines linearly out to around x=25mm. Clearly we are not evenly filling

the trapping volume and there would be no advantage from having a

trap with a total x-axis dimension greater than 50mm.

Ion mobility separation

For an IMS scan we accumulate ions at the entrance end of the annular

IMS, this can be done with a DC barrier voltage applied to selected ring

electrodes. At the start of the IMS cycle the DC barrier voltage is

dropped to allow the ion packet to pass into the T-Wave drive region

where they undergo ion mobility separation. Alternatively, the device

can be operated in a pass-through mode. In this case ions are not

trapped at the entrance end of the annular IMS hence we transmit a

continuous beam of ions.

Figure 8 shows simulated drift time peaks for two ion species defined to

match the isomeric reverse peptides GRGDS and SDGRG (mass 492,

z=+2, ccs= 211.7 Å2 and 222.7 Å

2 respectively) . The drift length in the

annular IMS was 250mm, with an 800 m/s 30V T-Wave applied. The

drift time is measured from the trap release time until the ions have

passed from the exit pancake into the exit SRIG. We obtain a mobility

resolution of ~40 compared to 45 from the annular IMS alone, a loss of

~12%. We expect to be able to reduce this with further optimisation of

the exit pancake geometry and voltage settings.

Calculations of transmission through the system show losses of less

than 0.1% over a wide m/z range of ions. Losses generally increase at

high m/z, however various parameters can be tuned to reduce these

losses if desired, e.g. DC voltage offsets and RF frequency / voltage.

CONCLUSION

An annular geometry ion mobility separation device with an

increased space charge capacity, ~50x over a SRIG.

Simulation of space charge via solution of the Poisson equation demonstrates the need for an interface to evenly fill the annular

volume.

Novel “Pancake” ion guides allow even filling of, and extraction

from, the annular ion guide.

A split deflection lens allows control of the angular distribution

of the ion beam.

Simulated drift peaks show that the pancake ion guides exhibit good transmission characteristics and lead to only a small

reduction in ion mobility resolution.

Figure 4 illustrates the pressure dependence of this effect, showing the

same point fill trap simulation except at 0.1 torr N2. After 10ms the trap

is near evenly filled out to ~42mm where the DC edge plate voltage

contains the ions. The peak charge density is about 5x higher in the

high pressure vs the low pressure case.

Pancake ion guides

To make full use of the annular trap capacity we therefore require an

interface that takes an incident axial ion beam and distributes it evenly

around the annular trapping volume. We also need an exit interface to

bring the annular beam back down to an axial beam to be passed on to

the rest of the analyser.

This is achieved via novel “pancake” ion guides at the entrance / exit of

the annular IMS. These ion guides are formed from concentric ring

electrodes with alternate phases of RF applied to give RF confinement.

We apply a DC gradient to move ions radially outward on the entrance

pancake, and radially inward on the exit pancake. DC offsets are used

to transmit ions into and out of the pancake ion guides.

Figure 5 shows a slice through the cylindrically symmetric system, with

the motion of a trace ion taken from a SIMION simulation. Figure 6

shows DC potential energy surfaces for: a) the entrance region, b) the

exit region and c) the entrance pancake. The corrugations in the

entrance SRIG and the annular IMS are a static time slice of the T-

Wave voltages. While in this case we have simulated a T-wave drive in

the annular IMS a DC gradient is equally possible.

Due to the cylindrical symmetry ions are equally likely to take any

angular path across the entrance pancake ion guide, hence we evenly

fill the annular guide. For the half annular system this would result in a

loss of about 50% of the ion beam. To overcome this issue a deflection

lens is used to guide the ions. The first ring electrode of the entrance

pancake is split into four quadrants, application of small DC bias

voltages to these quadrants allows steering of the incoming ion beam

onto the entrance pancake and thus control of the angular distribution of

ions. Figure 7 shows a SIMION simulation where the deflection lens

voltages are varied to send three trace ions in different directions.

Sweeping or stepping the deflection lens voltages allows us to evenly fill

the majority of the half annular trapping volume without loss of ions.

Figure 2. Equilibrium charge density distributions for a) SRIG with 1.44e

-15 C/mm LCD, b) SRIG with 1.44e-12 C/mm LCD, c) extended slot

system with 1e-11 C/mm LCD.

Figure 6. DC potential energy surfaces for a) entrance region, b) exit re-

gion, c) entrance pancake. The corrugations in the entrance / exit regions

are static time slices of T-Wave voltages.

a)

b)

c)

Figure 3. Charge density distribution for a point fill extended slot system

at 2.5 torr N2, plotted at 2.5, 5, 7.5 and 10ms. Total charge is 8e-13 C/

mm after 10ms.

Figure 4. Charge density distribution for a point fill extended slot system

at 0.1 torr N2, plotted at 2.5, 5, 7.5 and 10ms. Total charge is 8e-13 C/

mm after 10ms.

Figure 1. Plate geometries for a) SRIG, b) linear extended slot ion

guide, c) annular ion guide, d) half annular ion guide.

Figure 7. SIMION simulation showing the operation of the deflection

lens. For each trace ion the deflection lens voltages were adjusted to

direct the ions in the chosen direction.

Figure 8. Drift time plots for the full system for the doubly charged re-

verse peptide ions.

Figure 5. x/y section from a cylindrically symmetric SIMION model of

the full system with the various regions labelled. An example ion trace is

shown travelling through the system.

Entrance SRIG

Exit SRIG

Entrance pancake

Exit pancake

Annular IMS