ELECTRODE SYSTEM OF A LINEAR ION TRAP

Technical Field

Invention relates to the field of mass spectrometry, in particular it relates to design of a linear ion trap and its electrode system which forms a trapping field. Ion trap can be used directly for mass analysis and also for trapping ion cloud for some time and preparation of ion population for further analysis in downstream mass analyzers.

Background Art

Linear ion traps with trapping field formed by four elongated electrodes (rods) arranged around common axis (trap axis) are known in the art. Shortest distance from the axis to the electrode surface - r ₀ is called 'field radius' or the inscribed radius of the trap. This is a major geometrical parameter of the trap. Main difference in the design of linear ion traps lies in the shape of working surfaces of electrodes, i.e. the inner shape of electrodes which define field shape in radial direction. Trapping field in such traps is created by application of radiofrequency potentials RF+ and RF- (further in the text - RF supply) , positive phase on one pare of oppositely placed electrodes and negative phase on the other pair correspondingly. Amplitude and frequency Ω of the RF supply are also main parameters of the ion trap because they define a mass range of trapped ions. Fields created by variable AC . potentials are used for manipulations with ion cloud. Positive and negative potentials (AC+ and AC-) are applied to the one pair of oppositely placed electrodes. Falling into resonance with excitation field ions increases amplitude of their vibration and can appear at the electrodes. In an ion trap with radial ejection ions are ejected to detector through the slits in electrodes, slits are cut parallel to the axis of trap. It is possible to use ejection slits in all four electrodes.

Methods of ion manipulations in ion traps are based on resonance excitation of ion vibrations. That is why main (secular) frequency of ion vibrations should be well defined and should depend on ion mass only. In order to achieve this, a returning force of the effective potential of the trap should be linearly proportional to the distance of ion from the trap axis. Only quadrupole fields have such property. In order to create quadrupole fields electrodes of the trap should have hyperbolic shape, because hyperbolas are equipotential surfaces of the quadrupole fields.

Patent US 6,797,950 describes a linear ion trap with four extended electrodes arranged symmetrically around longitudinal axis of the trap, each electrode has a hyperbolic shape of working surface. Manufacturing and accurate assembling of hyperbolic electrodes is complicated and expensive process. These problems become even more difficult with miniaturization of ion traps. Presence of ejection slits introduces imperfections to the shape of trapping field, resulting in reduction of electrical field in the vicinity of the slit. Due to this ion traps with hyperbolic electrodes are designed with rather narrow slits - not more than 10% of the inscribed radius.

Ion trap described in US 6,838,666 better satisfies requirements of miniaturization and manufacturing. Electrodes of this trap are extended flat plates. At the same time such simplification of design results in significant degradation of the trapping field shape due to significant deviations of the trapping field from quadrupole. It is known that field strength is reduced near flat electrode surfaces, thus use of flat plate electrodes only increases effect of field reduction in the vicinity of ejection slits. Secular frequency of ions becomes dependent not only on ion mass, but also on the amplitude of ion vibrations. While amplitude of ion vibration is increased and ions approach ejection slits, ions fall out of resonance with excitation field. As the result, ions either not ejected through the slit to detector, or ejected after significant time delay, which significantly reduces resolving power of mass analysis.

Field shape can be improved to some extend by variation of potential along the surface of flat electrode. Patent application WO 2005/119737 describes a linear ion trap in which flat electrodes are separated into a number of longitudinal strips. RF potential is applied to the strips in certain proportion. Advantage of this trap is that electrodes can be manufactured with the use of printed circuit board technology. With the use of several strips per electrode the shape of trapping field can be rather close to quadrupole. At the same time such solution for the problem results in significant complication of power supply.

International patent application WO 2007/025475 describes a number of designs for linear electrodes for mass analyzers with different shapes of working surfaces. Those designs have one common feature - working surface has a shape with two or more steps. Part of described designs, in particular those electrodes, which have curved step surfaces, have the same advantages and problems, as above described electrodes of hyperbolic surfaces. Mentioned patent application also presents electrodes with flat planar steps. Such electrodes are capable to form field close to quadrupole, but manufacturing of those electrodes is much simpler than of hyperbolic electrodes. Particularly those electrodes, which are presented in fig.l of application description WO 2007/025475, are selected as prototype. Nevertheless even those electrodes are not free of disadvantages. Ejection slit is located at the upper flat step of such electrodes. Presence of flat surface in the region of slit results in reduction of resolving power of mass analysis.

Disclosure of Invention

A problem which is solved by present invention is improvement of resolving power of mass analyzer with simultaneous simplification of electrode design. Technical result is a compensation of field reduction in the region of ejection slit. Target is achieved by modification of electrode design.

Claimed electrode system of a linear ion trap has four electrodes, each pair oppositely located. Plains of symmetry of electrode pairs are perpendicular to each other. Difference from prototype is that each electrode of at least one pair has in a cross section substantially a shape of isosceles triangle. Top of the triangle is directed towards longitudinal axis of the trap. The best result is achieved when angle between shoulders of the triangle is from 130° to 152°. In other words angle between working surfaces of electrodes is 130-152°. The width of slit for ejecting ions in such electrode is less than 24% of the inscribed radius of the trap .

Brief Description of the Drawings

Further the efficiency of claimed electrode shape is explained and examples of implementation of the ion trap with electrodes, having cross-section in the shape of isosceles triangle (for simplicity such electrode is called further triangular electrode) . Invention is illustrated by figures, where the following is presented:

Fig. 1 - drawing of the ion trap electrode system in 3d projection,

Fig. 2 - cross-sectional view of an ion trap with identical electrodes, schematically, Fig. 3 - graphs of ion vibration amplitude in X direction (direction of excitation) as a function of time t in ion traps with triangular electrodes at several values of angles a between shoulders of the triangle: A - for a=140°, B - for a=142°, C - for =134°,

Fig. 4 - graph of number of ions (as function of time t) of mass 1891 Da ejected per unit time from an ion trap with triangular electrodes of angle o=140° at several values of excitation pulses amplitude: a - for U _A c =0.4 V (resolving power 4571); b - for U _AC =0.5 V (resolving power 6603); c - for U _AC = 0.6 V (resolving power 2971),

Fig. 5 - resolving power dependence from angle a for several values of slit width for a trap with inscribed radius 5 mm,

Fig. 6 - dependence of optimum angle a at which maximum resolving power is achieved against ejection slit width.

Fig. 7 - cross-sectional view of an ion trap with two triangular and two flat electrodes, schematically.

Best Mode for Carrying Out the Invention

Authors of present invention have realized that it is possible to use for a linear ion trap electrodes which working surface consists of flat parts while keeping high resolving power when ion trap is used as a mass analyzer. Flat parts of electrode are adjacent to each other creating a prism with cross-section in the shape of isosceles triangle. Electrode system with four such electrodes is presented in Fig. 1. For trapping ions in longitudinal direction (along the trap axis) it is possible to use diaphragms from both sides of the trap or segmentation of electrodes as in prototype (not shown in Figures) .

Claimed system contains two pairs of electrodes 1. In each pair electrodes are oppositely located. Plains of symmetry of pairs are perpendicular to each other. Each of two electrodes of at least one pair has a cross-section substantially a shape of isosceles triangle with a top directed towards longitudinal axis of the trap. Parameter r ₀ in Fig. 1 is a radius of the circle inscribed between electrodes, a - is an angle between working planes 2 of electrode 1. Angles at the bottom of triangle can be cut, as shown in Fig. 1, in the rest part working surface is made flat. In the context of present invention the shape of electrode cross section 'isosceles triangle' should be understood as a shape of main external contour of the cross section. Inside this contour, i.e. in the interior of electrode behind the working surfaces 2, cavities 3 of arbitrary shape can be present. In proposed ion trap design each of two opposed electrodes of pair have longitudinal slit (slit width designated as d) for ejecting ions towards detector, slit is placed at the top of triangle, i.e. in the plane of electrode symmetry. Above mentioned excitation potentials AC+ and AC- are applied between those electrodes.

Most important parameter of an ion trap as a mass analyzer is the resolving power, which equals to the mass of ions to the peak width of the ion current expressed in mass units. In order to define the resolving power for proposed ion traps with triangular electrodes a simulation of mass selective resonance ejection of ions was undertaken.

An ion trap with electrodes as shown in Fig. 2 with inscribed radius r ₀ =5mm and slit width d = 0.8mm was used for modelling. Periodic trapping RF supply was a bipolar square wave signal with duty cycle 0.5 and amplitude ^=500^. Such shape of RF supply is most convenient for implementation of frequency scan by gradual increasing of the signal period. In simulations increase of the square wave period was 5Ops after every 20 complete RF cycles.

Modelling was performed for singly charged ions of mass 1891Da. For better statistics the ion group consisted of 1000 identical particles. Random distribution of initial locations for ions was in accordance with normal distribution with standard deviation 0.05 mm in both radial directions X and Y, which corresponds to symmetrical ion cloud in the trap centre. Initial period of the square wave RF supply was selected near 2.5yus so that resonance ejection of ions happened approximately after 20-30 ms . For modelling of ion collisions with buffer gas a model of hard sphere collisions was used. Helium at pressure of 0.2mTorr was used as a buffer gas. Modelling assumed that fields are independent of axial location along the trap. Such assumption is valid at least for the central part of the trap.

For ejection of ions an additional small excitation signal is applied between opposite electrodes of the trap. In modelling such excitation voltage (AC) was applied between electrodes in X direction as shown in Fig. 2. Additional excitation was implemented as pulses of positive polarity with duration equal to 1.5 of the period of main RF supply and repetition rate every 3 RF cycles. Such excitation leads to resonance with ion vibrations when secular frequency of ions approaches 1/3 of the frequency of main trapping supply. As a result of gradual increase of the RF period, secular frequency of ions increase comes into resonance with additional excitation. Amplitude of ion vibrations grows and reaching electrodes of the trap ions penetrate through the slit and directed towards detector. In modelling ions that reach detector in fixed time intervals (typically 20/y ) are counted and a histogram of ion current is plotted. This histogram reflects the peak shape from ions of the same mass.

For a linear ion trap with ejection slit width of 0.8 mm it was found that electrode angle of 140° is optimal for ion ejection. Fig. 3A shows a time domain of ion vibration amplitude in the direction of excitation (X) for an ion trap with electrode angles 140°. Approximately at 20 ms ion falls into resonance with excitation field and amplitude of its vibrations starts to grow. Increase of the vibration amplitude is uniform and after another 1.5 ms ion is ejected through the slit in positive X direction, because coordinate of ion becomes bigger than the inscribed radius of the trap (5 mm) . Although due to random initial conditions and stochastic collisions with buffer gas molecules ions will reach detector at different times, the spread of ejection times for ions of similar mass will be small, if vibration amplitude increases as shown in Fig. 3A. Hence one can expect high resolving power for this case .

In case when electrode angle a is slightly bigger than optimum, increase of vibration amplitude is as shown in Fig. 3B. In this case amplitude grows slowly and stays long at the level of 4 mm, i.e. ions undergo long vibrations in the vicinity of the slit. Arriving into the slit region at some favourable RF phase, ion though penetrates through the slit to detector, , but time of ejection is hardly predictable. As a result the time spread of ion ejection times appears very big and resolving power is small. In the other case, when electrode angle o is smaller than optimum (Fig. 3C) initial growth of vibration amplitude is interrupted at approximately 4mm and ion falls out of resonance with excitation field. Amplitude is suddenly reduced and ion comes into resonance with excitation field ones again. This process repeats several times and ion motion has a beat character as shown in Fig. 3C. While vibrating ion never reach ejection slits and does not appear at the detector. Ejection of ions in this case can be ensured by increase of excitation amplitude, but this also results in bigger spread of ejection times and lower resolving power as compared to the optimum case.

In order to define resolving power of the trap one needs to do simulations with big amount of ions and find time of ejection for each ion. From results of such simulation one can plot a histogram of number of ions ejected at different time, which will give a peak shape from which resolving power can be found. Examples of such histograms for an ion trap with electrode angle 140° are presented in Fig. 4. Curves «a», «b», «c» correspond to different amplitude of excitation pulses (a - for U _ex =0.4 V (resolution 4571); b - for U _ex =0.5 V (resolution 6603); c - for U _ex = 0.6 V (resolution 2971)). The best peak shape is in a case «b» . Peak width at half maximum is 0.18 ms . This width can be expressed in mass units with account for a scan speed. In conditions of present modelling scan speed is 1591Da/s, consequently, the peak width corresponds to AM = 0. 18 10 ^"3 * 1591 = 0.29 Da . Resolving power is defined as ratio of ion mass to the peak width R - M IAM and equals 1891/0.29 = 6603 .

Similar modelling with identification of the optimum amplitude of excitation pulses and definition of maximum resolving power has been done for ion traps with triangular electrodes at several values of slit width against electrode angle. Results are presented in Fig. 5.

Maximum resolving power 6600 is achieved with a slit width of 0.8 mm (or 16% of the inscribed radius) at electrode angle of 140°. It should be mentioned that such resolving power at similar conditions can be achieved in ion traps with hyperbolic electrodes only. Graph of resolving power against electrode angle shows that at higher angles (over 140°) resolving power sharply reduced down to several hundred, while at smaller angles the resolution is gradually reduced down to 2000 at angle 130°. Although resolving power 2000 not so high, it is still two times higher than maximum resolution which can be achieved in ion traps with flat electrodes. Consequently the range of angles from 140° to 130° is of practical interest. At smaller slit width of 0.4 mm (or 8% of the inscribed radius) the optimum angle is shifted towards higher value - 148°. Maximum resolving power is slightly smaller - 6000, although still high. In other respects behaviour of the curve is similar to the case of slit width 0.8 mm. At slit width of 1.2 mm (or 24% of the inscribed radius) maximum resolving power is only 2000 and optimum angle is 130°.

Similar modelling was done for ion traps with triangular electrodes with zero slit width, i.e. without a slit. In this case ions can not be ejected to detector and disappear on electrodes. Although such trap can't operate as a mass spectrometer, such device can be used for preparation of the ion cloud for further stages of analysis by downstream mass analyzers. By means of resonance excitation unwanted ion can be removed on electrodes and ions of desired mass left in the ion trap volume. So measurement of resolving power in an ion trap without slits has not only theoretical interest. According to the curve at Fig. 5 optimum angle in this case is 152° and maximum resolving power is over 4000.

Results of defining of optimum angles are summarized in Fig. 6. Solid curve in this figure corresponds to angle values at which maximum resolving power is obtained* for corresponding slit width. Shaded area corresponds to a range of angles where resolving power is not less that 80% of maximum. One can see from the graph that the region of angles from 130° to 152° depending on the slit width is of practical interest for ion traps with triangular electrodes.

As described above, the resolving power of a trap is defined by configuration of the electrical fields created by trap electrodes . Later is not changed when dimensions of the trap are proportionally reduced or increased. That is why, although modelling has been done for an ion trap with the inscribed radius of 5 mm, the quality of ion trap operation will not degrade if ion trap of different inscribed radius is used, suggesting that all other dimensions are proportionally changed. That is why we may state that region of angles shown in Fig.6 will be identical for ion traps of geometry- described in this invention if slit width equals corresponding part of the inscribed radius. So upper range of slit width 1.2 mm in Fig. 6 corresponds to 24% of the inscribed radius .

Fig. 7 shows cross sectional view of the central part of the trap with triangular electrodes in X direction and simple flat electrodes in Y direction. Optimum electrode angles for this trap can be defined by methods described above. Thus this geometry falls into a 'family' of traps described in present invention.

As follows from above description suggested electrode system for a linear ion trap allows achieving high resolving power which is comparable with resolution of ion traps of hyperbolic geometry, i.e. significantly higher than can be achieved by prototype ion traps . At the same time the working surface of electrodes in proposed system is composed of flat surfaces, which are placed at certain angle to each other, with top of angle directed towards ion trap axis. Manufacturing of such electrodes is much simpler. Angle in the region of ejection slit compensates for local reduction of the field strength.