Space focus time of flight mass spectrometer

Abstract

A Time of Flight mass spectrometer is disclosed wherein a fifth order spatial focusing device is provided. The device which may comprise an additional stage in the source region of the Time of Flight mass analyser is arranged to introduce a non-zero fifth order spatial focusing term so that the combined effect of first, third and fifth order spatial focusing terms results in a reduction in the spread of ion arrival times T of ions arriving at the ion detector.

Claims

1. A mass spectrometer comprising: an ion source for generating ions; and an orthogonal acceleration Time of Flight mass analyser located downstream of the ion source for analyzing ions generated thereby, the Time of Flight mass analyser comprising a source region, an ion detector, and a drift region disposed between said source region and said ion detector wherein ions separate according to their time of flight as they travel through said drift region; the Time of Flight mass analyser source region comprising an extraction stage, a first acceleration stage, and a further stage, wherein said further stage comprises a field free region in said Time of Flight mass analyser source region and the extraction stage performs orthogonal acceleration of ions into the filed free region.

2. The mass spectrometer of claim 1, wherein said Time of Flight mass analyser comprises a multi-pass Time of Flight mass analyser.

3. The mass spectrometer of claim 1, further comprising one or more collision, fragmentation or reaction cells disposed between the ion source and the Time of Flight mass analyser source region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the present invention will now be described, by way of example only, together with other arrangements given for illustrative purposes only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a conventional Wiley & McLaren two stage source Time of Flight geometry;

(3) FIG. 2 illustrates the concept of turnaround time;

(4) FIG. 3 shows how high initial extraction fields in a two stage source of a Time of Flight mass analyser lead to shorter analysers which are impracticable;

(5) FIG. 4 shows how the addition of a one stage reflectron in an orthogonal acceleration Time of Flight mass analyser allows the combination of high extraction fields and longer flight times;

(6) FIG. 5 illustrates Liouvilles's theorem and shows an optical system comprising N optical elements with each element changing the shape of the phase space but not its area;

(7) FIG. 6A shows a conventional Time of Flight mass analyser having a two stage source geometry and a two stage reflectron and FIG. 6B shows an embodiment of the present invention comprising a Time of Flight mass analyser comprising a three-stage source;

(8) FIG. 7A shows the space focusing characteristics of a conventional Time of Flight mass analyser having a two stage source and two stage reflectron and FIG. 7B shows the corresponding residuals;

(9) FIG. 8A shows the odd terms of space focusing characteristics of a Time of Flight mass analyser according to a preferred embodiment comprising a three stage source and a two stage reflectron, FIG. 8B shows the even terms of the space focusing characteristics of a Time of Flight mass analyser according to the preferred embodiment and FIG. 8C shows the corresponding residuals;

(10) FIG. 9 shows the space focusing residual aberrations for a larger beam according to an embodiment of the present invention comprising a three stage source and a two stage reflectron;

(11) FIG. 10 illustrates the resolution enhancement which may be achieved according to the preferred embodiment; and

(12) FIG. 11 illustrates higher order correlation for pre-extraction velocity-position (phase space).

(13) FIG. 12 illustrates a mass spectrometer according to some embodiments.

(14) FIG. 13 depicts an operating environment according to some embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(15) A preferred embodiment of the present invention will now be described.

(16) If Eqn. 1 is rewritten in terms of velocity vo then this leads to a relationship for the turnaround time t such that:

(17) $\begin{array}{cc}{t}^{}=\frac{\mathrm{Lp}.Math.\mathrm{mv}}{q\mathrm{Vp}}& \left(4\right)\end{array}$

(18) The term my is the momentum of the ion beam and the region length Lp is inherently related linearly to the extent of the beam in the pusher.

(19) A fundamental theorem in ion optics is Liouville's theorem which states that: For a cloud of moving particles, the particle density p(x, p.sub.x, y, p.sub.y, z, p.sub.z) in phase space is invariable (Geometrical Charged-Particle Optics, Harald H. Rose, Springer Series in Optical Sciences 142) where p.sub.x, p.sub.y and p.sub.z are the momenta of the three Cartesian coordinate directions.

(20) According to Liouville's theorem a cloud of particles at a time t.sub.1 that fills a certain volume in phase space may change its shape at a later time t.sub.n but not the magnitude of its volume. Attempts to reduce this volume by the use of electromagnetic fields will be futile although it is possible to sample desired regions of phase space by aperturing the beam (rejecting unfocusable ions) before subsequent manipulation. A first order approximation splits Liouville's theorem into the three independent space coordinates x, y and z. The ion beam can now be described in terms of three independent phase space areas the shape of which change as the ion beam progresses through an ion optical system but not the total area itself.

(21) This concept is illustrated in FIG. 5 which shows an optical system comprising N optical elements with each element changing the shape of the phase space but not its area. Conservation of phase space means that the x p.sub.x term will be constant and so expanding the beam will lead to lower velocity spreads. This is because the x p.sub.x is proportional to the Lp*mv term in Eqn. 4. These lower velocity spreads can ultimately lead to a proportionally lower turnaround times for a fixed extraction field.

(22) Accordingly, an orthogonal acceleration Time of Flight mass spectrometer with the ability to spatially focus larger positional spreads x will result in a reduced turnaround time and hence higher resolution if the beam is further expanded prior to the extraction region and the field in the extraction region remains constant. Alternatively, if the beam is clipped by an aperture prior to the extraction region then the aperture size can be increased resulting in improved transmission and sensitivity for the same resolution if the beam undergoes no further expansion.

(23) FIG. 6A shows a conventional Time of Flight geometry comprising a two stage Wiley/McLaren source, an intermediate field free region and a two stage reflectron.

(24) A typical space focusing approach for conventional Time of Flight mass analyser as shown in FIG. 6A is illustrated in FIGS. 7A and 7B. The geometry is configured to provide second order focusing together with an opposing first order term as illustrated in FIG. 7A. The resulting residuals have a lower absolute time spread than either the third order or first order terms individually (FIG. 7B).

(25) FIG. 6B shows a preferred embodiment of the present invention wherein the known two stage Wiley/McLaren source has been replaced by a three stage source. The first stage of the source has the same extraction field as the extraction region of the known two stage Wiley/McLaren source as shown in FIG. 6A. According to the preferred embodiment the geometry is preferably configured to introduce higher order space focusing terms. These higher order space focusing terms are preferably arranged such that the odd powers (see FIG. 8A) combine to minimise the overall residuals and also so that even powers (see FIG. 8B) will also combine to minimise the overall residuals. The combined residuals are plotted in FIG. 8C on the same scale as FIG. 7B and illustrate how according to the preferred embodiment substantially improved space focusing may be obtained.

(26) The improved space focus according to the preferred embodiment and as illustrated by FIG. 8C allows expansion of the beam as shown in FIG. 9. In FIG. 9 the ion beam width is scaled by a factor of 1.5 when compared with FIG. 7B yet the absolute time spreads are comparable. According to an embodiment the ions in the wider beam have a reduced spread of velocities which enables the spread in ion arrival times at the ion detector to be reduced thereby improving resolution.

(27) A simulation was performed which compared the two different geometries shown in FIGS. 6A and FIG. 6B. The improvement in resolution according to the preferred embodiment is illustrated in FIG. 10.

(28) The dashed line peak shown in FIG. 10 shows the enhanced resolution obtained according to the preferred embodiment and corresponds to the preferred three stage source which receives a 1.5 wider ion beam having a proportionally lower velocity spread. The resolution enhancement is compared with that obtained conventional as represented by the solid line peak. The vertical scale is normalised for comparison purposes but in reality the area of the two peaks is the same.

(29) The initial conditions of an ion beam in the simulation were defined by a stacked ring RF ion guide (SRIG) in the presence of a buffer gas. The ions typically adopt a Maxwellian distribution of velocities on exit from the RF element due to the thermal motion of gas molecules with a beam cross section of 1-2 mm.

(30) Simulations of the velocity spreads were performed using SIMION and a hard sphere model. The hard sphere model simulated collisions with residual gas molecules in the stacked ring RF ion guide. These ion conditions were then used as the input beam parameters for the different geometry types.

(31) Using a similar principle to that used for the correction of linear (first order) velocity-position correlations, it is also possible to arrange the pre-extraction phase space so as to include non linear (>1.sup.st order) odd power terms as shown in FIG. 11. These higher order terms can be used to compensate for the higher order odd powered space focus terms further reducing the absolute time spread.

(32) FIG. 12 illustrates a mass spectrometer according to some embodiments. As shown in FIG. 12, a mass spectrometer 1205 may include an ion source 1210 operative to provide ions to a mass analyser 1220, such as a Time of Flight mass analyser. A source region 1230 may include one or more stages 1232a-n, such as an extraction stage and a first acceleration stage. In some embodiments, a beam expander 1225 may be arranged upstream from the source region 1230. A drift region 1240 may be arranged between the source region 1230 and an ion detector 1260. In some embodiments, the mass spectrometer 1205 may include a reflectron 1250 having one or more stages 1252a-n, such as a first deceleration or acceleration stage and a second deceleration or acceleration stage.

(33) Although the preferred embodiment relates to providing a third or further stage in the source region of the Time of Flight mass analyser, other embodiments are also contemplated wherein an additional acceleration or deceleration region may be provided within the intermediate field free region between the source and the reflectron. Other embodiments are also contemplated wherein an additional acceleration, deceleration or field free region may be provided with the reflectron. Embodiments are contemplated wherein one or more additional regions are provided within the source and/or field free region and/or reflectron.

(34) Although the preferred embodiment is primarily concerned with a device arranged and adapted to introduce a fourth and/or fifth order spatial focusing term, further embodiments are contemplated wherein a sixth and/or seventh and/or eighth and/or ninth and/or higher order spatial focusing term may be introduced.

(35) FIG. 13 depicts an operating environment according to some embodiments. As shown in FIG. 13, operating environment may include a mass spectrometer 1305 having an ion source 1310, an optional beam expander 1315, an extraction stage 1330. Mass spectrometer 1305 may be operative to facilitate the passage of ions orthogonally into a field free region 1332 and then into a first acceleration stage 1334 where the ions are accelerated through drift region 1340 towards a reflectron 1350 which may cause the ions to turn around so that they then reach detector 1360.

(36) Although the present invention has been described with reference to preferred embodiments it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the accompanying claims.