EP3455873A1

EP3455873A1 - Apparatus and method for detecting ions

Info

Publication number: EP3455873A1
Application number: EP17719848.8A
Authority: EP
Inventors: Michel Aliman; Stefan Butzmann; Albrecht BROCKHAUS; Michael Schmidt; Hin Yiu Anthony Chung; Alexander Laue
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Leybold GmbH
Priority date: 2016-05-10
Filing date: 2017-04-28
Publication date: 2019-03-20
Anticipated expiration: 2037-04-28
Also published as: EP3455873B1; WO2017194333A1; TW201805996A; DE102016208009A1; TWI821157B

Abstract

The invention relates to an apparatus (1) for detecting ions (4a, 4b), comprising: an ion trap (2) having a first electrode, preferably a ring electrode (3) and also having at least one second electrode, preferably a cap electrode (7a, 7b), a storage signal generator (5) for generating an RF storage signal (U_RF), which can be coupled into the first electrode (3) in order to generate an electric storage field (E) in the ion trap (2), and also an excitation device (6a, 6b) for generating an excitation signal (U_stim1, U_stim2) for exciting ions (4a, 4b) stored in the ion trap (2). The storage signal generator (5) is designed to set an amplitude (A_RF) and/or a frequency (f_RF) of the RF storage signal (U_RF). The invention also relates to an associated method for mass-selective detection of ions (4a, 4b). Furthermore, a suitable ionization method with constant or targeted ionization energies is presented.

Description

Apparatus and method for detecting ions

Cross-Reference to Related Application

This application claims priority to German Patent Application 10 2016 208 009.1 , filed May 10, 2016, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

Background of the invention

The invention relates to an apparatus for detecting ions, in particular for examining ions by mass spectrometry, comprising: an ion trap having at least one first electrode, preferably at least one ring electrode, and also having at least one second electrode, preferably a cap electrode, which typically serves for acquiring an ion signal from ions excited in the ion trap, a storage signal generator for generating an RF storage signal, which can be coupled into the first electrode in order to generate an electric storage field in the ion trap, an excitation device for generating an excitation signal for exciting ions stored in the ion trap, and also a detector for detecting an ion signal generated by the excited ions. The invention also relates to an associated method for detecting ions.

An ion trap in the form of a so-called electric ion resonance cell typically has a ring electrode and also two cap electrodes, which each have a hyperbolic geometry in the case of a conventional quadrupole trap in the form of a hyperbolic Paul trap. The two cap electrodes are generally at earth potential, while an RF storage signal in the form of a radio-frequency AC voltage is applied to the ring electrode. By virtue of the RF storage signal, an electric field (quadrupole field) is generated in the ion trap, said electric field also being referred to as an electric storage field, since ions or charged particles in such a field can be stored stably in the ion trap.

Mass spectrometers on the basis of the electric ion resonance cell are usually operated in the so-called "instability mode", in which stored ions are removed from the ion trap in a targeted manner and registered by a particle detector. However, this method has some disadvantages with regard to mass resolution and sensitivity. Over the course of the last few years, therefore, a method has been developed in which the ions can be detected in a broadband manner (one spectrum per measurement) and also non-destructively, in order in this way to improve the properties of the ion resonance cell.

In the case of non-destructive detection, the ions are detected by the

measurement of induced charges on the cap electrode or cap electrodes of the ion trap. In order to generate the induced charges, the ions are excited by an excitation signal to effect oscillations, the frequency of which is dependent on the ion mass or dependent on the mass-to-charge ratio of the excited ions, such that the latter can be detected with the aid of the ion current or ion signal generated at the cap electrodes.

In existing apparatuses of the type described above, the frequency of the electric storage field or of the RF storage signal is constant and can be for example of the order of magnitude of 1 MHz. Reasons for the use of a storage field having a constant frequency include the low-loss generation of the RF storage signal or of the storage field voltage (transformer at resonance) and a compensation of interference effects in the form of crosstalk currents that is significantly simplified by the narrowband nature of the storage signal. Such crosstalk or interference currents arise at the second electrode, at which the ion signal is measured, on account of a capacitive coupling to the first electrode that is caused by the radio-frequency electric storage field. The crosstalk or interference current typically has a constant frequency and amplitude

dependent on the frequency and amplitude of the RF storage signal.

The article "A Novel Electric Ion Resonance Cell Design with High Signal-to- Noise Ratio and Low Distortion for Fourier Transform Mass Spectrometry" by M. Aliman and A. Glasmachers, Journal of the American Society for Mass

Spectrometry, Vol. 10, No. 10, 1999, proposes using an ion trap with a novel design with electrodes having a geometry deviating from a hyperbolic shape instead of a conventional electronic ion trap, e.g. in the form of a Paul trap, in order to reduce a crosstalk current. In the case of the proposed design, the hyperbolic ring electrode is replaced by a series of ring electrodes having a parabolic potential distribution, wherein the hyperbolic geometry of the cap electrodes remains unchanged. The crosstalk current that remains in the case of the novel design can be compensated for by means of an electronic compensation technique.

US 9,035,245 B2 proposes, for the compensation of a possible crosstalk current, carrying out the compensation with the aid of simple electronic components instead of software compensation. For this purpose, the RF storage signal coupled into the first electrode is coupled into the second electrode in antiphase, for example with a phase rotated by 180°, by means of a transformer. The compensation signal coupled into the second electrode via a transformer typically has an amplitude proportional to the amplitude of the RF storage signal, such that given suitable scaling the compensation signal can substantially completely compensate for the crosstalk current from the outset.

However, compensating for the interference or crosstalk current with the aid of the transformer described in US 9,035,245 B2 presupposes that the frequency of the RF storage signal is substantially constant. In order to be able to measure analytes with a large mass-to-charge ratio (m/z), a high amplitude of the storage field is additionally required, wherein an RF storage signal having an amplitude of a few kV is required, for example for the examination of analytes with mass- to-charge ratios of greater than 1000 m/z or equivalent to 1000 amu (given a customary radius of the ring electrode - in the region of a few millimetres - and a storage frequency of the order of magnitude of 1 MHz).

Object of the invention

It is an object of the invention to provide an apparatus of the type mentioned in the introduction and also a method which make it possible to detect ions over a large bandwidth of mass-to-charge ratios.

Subject matter of the invention

In accordance with one aspect, this object is achieved by means of an apparatus of the type mentioned in the introduction in which the storage signal generator is designed to set or to vary an amplitude and/or a frequency of the RF storage signal, to be precise preferably on the basis of at least one predefineable control signal.

The invention proposes operating the apparatus with the ion trap in the form of the ion resonance cell for non-destructive ion detection with an electric storage field, wherein ideally the amplitude and/or the frequency of the RF storage signal and thus also the waveform of the electric storage field can be chosen freely, i.e. virtually arbitrarily. In this way, the mass range in which ions can be stored in the ion trap can be drastically increased since the frequency†RF of the electric storage field influences the mappable mass range with 1/fRF². In this regard, it is possible for example to store and detect or demonstrate the same ion with the mass-to-charge ratio of e.g. 1000 amu at a frequency of 1 MHz and an amplitude of approximately 1 kV of the electric storage field or at a frequency of 500 kHz and an amplitude of thus only 250 V of the storage field in the ion trap. The RF storage signal that is coupled into the first electrode is typically an AC voltage that is applied between the first electrode and a constant potential, generally earth potential.

In order to generate an RF storage signal by means of a storage signal generator for the generation of a radio-frequency storage field whose amplitude and/or frequency can be set, a number of possibilities exist:

In one embodiment, the storage signal generator for setting the frequency and/or the amplitude of the RF storage signal has at least one full-bridge module having at least four electronic components which are driveable by means of a respective control signal, e.g. in the form of transistors, in particular in the form of field-effect transistors (e.g. power MOSFETs "Metal-Oxide- Semiconductor Field-Effect Transistor"). Of the four driveable electronic components of the full-bridge module, in each case two, i.e. a pair, are connected in series in a half-bridge. The two half-bridges each comprising a pair of driveable electronic components are connected in a parallel circuit. Of the two taps between the first and second components of a respective pair, typically a first is connected to earth and a second is connected to the first electrode, such that a voltage U_AB is generated between the two taps, which voltage forms the RF storage signal generated by the signal generator.

In order to drive the controllable electronic components with the aid of control signals, a number of possibilities exist: By way of example, the control signals can be digital signals that make it possible to generate an RF storage signal in the form of a square-wave signal and couple it into the electrode. However, the possibility also exists of driving the controllable electronic components of the full-bridge module with analogue control signals in order in this way to generate a fundamentally arbitrary signal shape or waveform, for example a sinusoidal or triangular RF storage signal. The first electrode, for example in the form of a ring electrode, forms a capacitance CR_ing with respect to earth, such that principally reactive power or power loss P_B with the frequency f_RF of the storage field is supplied:

Ρβ ⁼ CRjng UAB² fRF■

In one development, the full-bridge module has at least one series resistor for reducing the power loss of the driveable electronic components. This is advantageous particularly with the use of driveable electronic components in the form of MOSFETs since in the latter, besides the reactive power loss, additional switching and conduction losses occur as well, which can be reduced by means of a series resistor, the total power loss in the apparatus or in the storage signal generator remaining constant. The power loss generated in the series resistor can be dissipated more easily, however, since the latter can be cooled more easily than the driveable electronic components.

In a further embodiment, the storage signal generator is designed to generate an RF storage signal which has a constant amplitude during at least one partial interval of its period duration. If a plurality of partial intervals of the period duration of the RF storage signal have a constant amplitude, the amplitudes in the different partial intervals can in particular be chosen differently. The duration of a respective partial interval in which the RF storage signal has a constant amplitude is typically at least 1 % or 2% of the period duration and typically a maximum of approximately 10% of the period duration of the RF storage signal.

With the use of a full-bridge module for generating the RF storage signal, shunt currents can also occur besides switching losses. In order to avoid said shunt currents, during the generation of the storage signal it is possible to use a partial interval of the period duration having a constant amplitude, preferably having a zero amplitude (also called "zero phase"). In order to generate the constant amplitude, the driveable electronic components of the full-bridge module are driven suitably. The use of an RF storage signal having one or possibly having a plurality of partial intervals having a constant amplitude can also be used for more precise ionization, as is described in greater detail further below.

With the use of the full-bridge module, the frequency of the RF storage signal can be set in a wide frequency range of between approximately 0 Hz and approximately 1 MHz, if the driveable electronic components are driven with the aid of suitable control signals. In order to increase the (maximum) amplitude of the RF storage signal, with the use of the full-bridge concept a number of possibilities exist: In order to increase the (maximum) amplitude, the operating voltage of the or of an individual full-bridge module can be increased or a plurality of full-bridge modules are connected in series, which each have a dedicated voltage source that provides the supply voltage for a respective full- bridge module. The advantage of such a modular concept is that the power loss is divided among the full-bridge modules connected in series. In addition, more cost-effective electronic components having a lower dielectric strength can be used in the individual full-bridge modules. With the use of a plurality of full- bridge modules, however, the complexity in the driving of the electronic components possibly increases.

In a further embodiment, the storage signal generator for generating the RF storage signal has at least one power analogue amplifier for amplifying an analogue control signal, which preferably comprises a half-bridge module having at least two electronic components which are driveable by means of the analogue control signal. The driveable electronic components can be for example field-effect transistors, in particular MOSFETs. The driveable electronic components are typically interconnected in a half-bridge and are operated in linear amplification operation. With the aid of the power analogue amplifier, given a suitably predefined control signal, practically arbitrary waveforms of the RF storage signal can be generated, for example sinusoidal or triangular RF storage signals.

In a further embodiment, the storage signal generator has at least one isolated DC/DC converter in order to generate a high DC voltage. The DC/DC converter is isolated, that is to say that a converter having galvanic isolation is involved. The isolated DC/DC converter can serve, in particular, to provide a floating supply voltage that can be used for generating an RF storage signal in the form of a bipolar AC voltage, for example a floating supply voltage for the full-bridge module. The isolated DC/DC converter can serve, in the signal generator with the power analogue amplifier, if appropriate, for the voltage supply of gate drivers which are present there and which are used for controlling the current through the MOSFETs. In order to generate a floating supply voltage, a battery can also be used, however, instead of an isolated DC/DC converter.

The storage signal generator described further above can be arranged in the vicinity of the ion trap in order to keep the line capacitance and thus the reactive power small, but it is also alternatively possible to mount or arrange the storage signal generator at a greater distance from the ion trap and to accept the occurrence of additional reactive power.

In a further embodiment, the apparatus comprises a compensation device for generating a compensation signal for compensating for an interference current that is generated by the electric storage field at the second electrode, which serves for measuring an ion signal of the excited ions in the ion trap. As was described further above, the ions in the apparatus, to put it more precisely in the ion trap, are analysed with the aid of a non-destructive ion detection. The ion analysation or the detection of the ions is effected by the measurement of an ion signal in the form of induced charges that are induced by the excited ions at the second electrode or electrodes, typically the cap electrodes. The measurement of the induced charges or of the ion current signal is typically effected by the latter being converted into an ion signal in the form of a voltage proportional to the ion current by means of a measuring amplifier, e.g. by means of a charge amplifier. The measurement transient of the ion current signal or of the ion signal proportional thereto is recorded and typically converted into a frequency spectrum or a mass spectrum by means of a Fourier transformation in a spectrometer. On account of this conversion, the apparatus is also referred to as an (electric) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.

On account of the spatial proximity of the second electrode(s) to the first electrode, into which the RF storage signal is coupled, a capacitive coupling exists between the at least one first electrode and the at least one second electrode, which capacitive coupling has the effect that an interference or crosstalk current is injected into the second electrode and thus into the charge amplifier. The interference current may exceed the ion signal or the ion current possibly by more than eleven orders of magnitude. Without compensation, such an interference current would lead to an overdriving of the charge amplifier, such that the interference current should be compensated for with the aid of suitable means.

Since the electric storage field or the RF storage signal can be varied in terms of frequency and has a fundamentally arbitrary waveform, it is necessary in the present case to provide a compensation device that is suitable for generating a compensation signal that enables such a broadband compensation. The compensation device for generating the compensation signal typically has one or more electronic components, that is to say that the compensation device for this purpose typically does not have recourse to a (pure) software solution. The compensation with the aid of the compensation device can be effected in various ways, as explained below: In one embodiment, the compensation device is designed to generate the compensation signal on the basis of the RF storage signal that is coupled into the first electrode by the storage signal generator. In this embodiment, the RF storage signal, which is typically present in the form of a voltage signal, is used for generating the compensation signal, that is to say that the compensation signal is derived in a suitable form from the RF storage signal. The

compensation signal is typically likewise a voltage signal that can subsequently be converted into a current signal. For this purpose, the compensation signal present in the form of a voltage signal can be fed via a capacitance to the charge amplifier generally used for the measurement of the ion current, such that a current signal is present at the input thereof.

In one development, the compensation device for generating the compensation signal has a voltage source. The storage signal generator, too, typically has a voltage source in order to generate the RF storage signal. The voltage source of the RF storage signal and the voltage source of the compensation device are synchronized with one another, to be precise typically in such a way that the voltage source of the compensation device generates an inverted voltage with respect to that generated by the voltage source of the storage signal generator. The voltage source of the compensation device therefore generates a voltage signal that is phase-shifted by 180° with respect to the RF storage signal generated by the voltage source of the storage signal generator. The compensation signal can be fed - if appropriate in a suitably scaled fashion - to the charge amplifier via a capacitor (see above).

In a further embodiment, the compensation device is designed to generate the compensation signal on the basis of at least one analogue control signal that serves for driving at least one driveable electronic component of the storage signal generator. The analogue control signal can be, for example, the control signal which is fed to the power analogue amplifier of the storage signal generator for the amplification thereof. The analogue control signal can likewise be fed in an inverted and suitably scaled fashion as compensation signal to the charge amplifier via a capacitance, without a dedicated voltage source being required for generating the compensation signal.

If appropriate, the compensation device can be designed not to measure the RF storage signal coupled into the first electrode in the storage signal generator in which said signal is generated, but rather to measure for this purpose directly the voltage dropped across the first electrode.

In an alternative embodiment, the compensation device is designed for generating the compensation signal with the aid of a current flowing through the first electrode or with the aid of a crosstalk current portion between the first electrode and a further cap electrode part. In this case, the current through the first electrode or the crosstalk current portion is measured in a suitable manner or by means of a suitable device. The measured current or the crosstalk current portion is inverted, suitably (i.e. appropriately with respect to the interference current) scaled and subsequently fed as compensation signal to the power amplifier generally present in the apparatus. The compensation signal can be a current signal, but also a voltage signal that is typically converted into a current signal at a capacitance.

In one development, the compensation device is designed to measure the current through an electrical lead for coupling the RF storage signal into the first electrode. Typically, the electrical lead on which the current is measured is arranged between the storage signal generator and the first electrode or the ion trap, that is to say that the current in this case is typically measured outside the ion trap and generally also outside the vacuum environment in which the ion trap is typically arranged. The measurement of the current used for generating the compensation signal can be effected in various ways: In one development, the compensation device comprises a (broadband) transformer or a device for contactless measurement of the current in the electrical lead, for example in the form of one or more coils. A high

measurement accuracy is required for the measurement of the current, said high measurement accuracy typically being ensured during the measurement of the current within the lead.

In the first case, a primary winding of the (broadband) transformer is part of the electrical lead and the voltage dropped across the secondary winding can be converted, by means of a (small) capacitance into a compensation signal in the form of a compensation current which can be fed in an inverted fashion (with an opposite winding sense of the secondary coil in comparison with the primary coil without the provision of further measures) to the charge amplifier to which the ion current is also fed. If appropriate, it may be necessary here additionally to adapt, i.e. to scale, the voltage at the secondary winding by means of an active circuit.

In the second case, the current in the electrical lead is measured contactlessly, typically by means of one or more coils. By way of example, a Rogowski coil in combination with a signal amplifier can be used for this purpose. Rogowski coils can be used for frequencies in the MHz range and thus satisfy the requirements for the measurement of the current in the apparatus.

In a further development, the compensation device has an auxiliary capacitance fitted between the first electrode and the second electrode and the

compensation device is designed for measuring the current through the auxiliary capacitance. Since the current in the first electrode exceeds the current through the coupling capacitance between the first electrode and the second electrode by a plurality of orders of magnitude, it has proved to be advantageous not to measure directly the current through the ring electrode but rather the current through an auxiliary capacitance, typically in the form of a capacitor, which is arranged in parallel with the coupling capacitance. The auxiliary capacitance typically has a value that is of a similar order of magnitude to the coupling capacitance between the first electrode and the second electrode, for example approximately 700 fF. The smaller current simplifies the measurement and reduces the requirements on the current source used for the compensation.

The auxiliary capacitance can be fitted or mounted in the vicinity of the first electrode, for example on the outer side of the ion trap. However, it is also possible to form the auxiliary capacitance as a discrete capacitor, e.g. as a film capacitor, outside the vacuum chamber or the vacuum environment in which the ion trap is typically arranged. This has the advantage that the connecting lines between the auxiliary capacitance and the charge amplifier can be kept short and possible coupled-in interference can be minimized in this way.

In one development, the compensation device for measuring the current through the auxiliary capacitance has an operational amplifier circuit, for example a current feedback operational amplifier, ("current feedback amplifier") or some other suitable operational amplifier circuit, for example a voltage feedback operational amplifier ("voltage feedback amplifier"). On account of the significantly reduced current through the auxiliary capacitance in comparison with the first electrode, it is possible to use an operational amplifier circuit for current measurement instead of a discrete circuit. The current through the auxiliary capacitance can be converted into a proportional output voltage with the aid of the current feedback operational amplifier, which output voltage is converted, by means of a small capacitance, into a compensation signal in the form of a compensation current that can be fed for compensation to an input of a power amplifier.

In a further embodiment, the compensation device for measuring the crosstalk current portion or the current through the auxiliary capacitance has a transconductance amplifier. The transconductance amplifier likewise enables a current measurement, wherein the transconductance amplifier can be realized in particular as a discrete circuit, for example in the form of a transistor operated in common-base connection, but can also be designed as an operational amplifier. The transconductance amplifier in the form of a transistor has a base- emitter voltage of 0 V, such that during the measurement of the current through the first electrode the latter can be kept at earth potential via the

transconductance amplifier or the base-emitter junction thereof, the electric storage field is not influenced and no energy loss occurs. During the

measurement of the current through the first electrode by means of the transconductance amplifier, it is necessary to interrupt the earth connection within the ion trap or the measuring cell. This requirement is obviated during the measurement of the current through the auxiliary capacitance.

In a further embodiment, the apparatus additionally comprises an electron beam generating device for generating an electron beam that can be fed to the ion trap in order to generate ions in the interior of the ion trap. The electron beam generating device makes it possible to generate ions in situ, i.e. in the interior of the ion trap, by impact ionization or possibly by charge exchange ionization. The electron beam generating device can be designed for example in the form of an electron gun using a filament or an incandescent wire. The electron beam generating device is typically fixed to the outer side of the first electrode

(generally the ring electrode). Via a hole in the ring electrode, which typically runs at the level of the plane of symmetry of the ring electrode, the electron beam can pass into the interior of the ion trap.

The optimum ionization energy for the electron impact ionization is generally approximately 70 eV. As soon as the electron beam enters the interior of the ion trap (given an electron energy of, for example, 70 eV for an RF storage signal or for an RF storage voltage of the first electrode (ring electrode) relative to an exit stop of the electron beam generating device that is greater than -70 V), said electron beam is modulated with the electric storage field, as a result of which the electrons acquire different energies, which makes it significantly more difficult to detect or to determine the quantity of ions present in the ion trap. In addition, the electrons are distributed in an undesired manner in the interior of the ion trap and thus generate ions at different locations. Ideally, however, the ions should be ionized only in the central plane or plane of symmetry or at the centre of the (hyperbolic) ring electrode, which is typically rotationally symmetrical and designed to be mirror-symmetrical with respect to the central plane. An interior of the ion trap that is "filled" with ions has a worsened sensitivity and resolution during the detection of the ions.

In one development, the electron beam generating device is designed for feeding the electron beam into the ion trap in a pulsed manner and is preferably synchronized with the storage signal generator in order to feed the electron beam to the ion trap only during at least one partial interval of the period duration of the RF storage signal in which the latter has a constant amplitude, in particular a zero amplitude. In order to reduce the above problems, in this development the electron beam is fed to the interior of the ion trap only during the at least one partial interval - described further above - of the storage field cycle. During said partial interval, the absolute value of the electric storage field in the interior of the ion trap is also constant or completely vanishes (zero amplitude). Consequently, all the electrons have a defined energy, move exclusively in the central plane of the ion trap and generate ions only there, for which reason the number of ions generated in the ion trap can be determined with higher accuracy.

As an alternative to feeding the electron beam into the interior of the ion trap during a partial interval with a zero amplitude, it is also possible to feed the electron beam to the ion trap while the electric storage field has a constant amplitude different from zero. By way of example, an ionization can be carried out over a plurality of period durations of the storage field and in this case the electron energy can be modulated in a targeted manner with the aid of a plurality of partial intervals with a constant, in particular different, amplitude of the electric storage field. By means of in each case different, constant amplitudes of the RF storage field, it is possible to generate different ion subsets with different, defined ionization energies and/or with different ionization rates.

The invention also relates to an apparatus as described further above, in particular an apparatus as is described in the introductory part of the description of the present application, wherein the apparatus is designed simultaneously or at the same time to generate an excitation signal for exciting ions present in the ion trap and to detect an ion signal of the ions excited in the ion trap. In the case of this apparatus, the ions present in the storage field of the ion trap can be excited selectively or in a broadband fashion with the aid of the excitation signal, wherein the ion signal or ion current generated during the excitation is detected at the same time. In particular, in this case, a permanent, if appropriate broadband, excitation of the ions can be effected which is not interrupted by the measurement of the ion signal or the ion current.

In this embodiment, it is possible to compensate for the damping of the oscillations of the ions on account of collisions with other ions. As a result, the requirements made of the vacuum installation in which the ion trap is typically operated also decrease, that is to say that they are less stringent than in the case of conventional ion traps (smaller pumps, etc.). In particular, the

advantage is afforded here that the ion trap can be operated in a targeted manner at a higher operating pressure, e.g. at pressures of more than 1 x 10^~5 mbar, e.g. up to 1 x 10^"3 mbar, or with shorter average path lengths than is the case with conventional ion traps. The vacuum installation can therefore be made significantly more compact, which enables overall a very compact construction of the apparatus or of the mass analyzer. The feeding of the excitation signal and the detection of the ion signal can be effected if appropriate at one and the same electrode.

Preferably, the detector for feeding the ion signal is connected to the at least one second electrode, typically a cap electrode, and the excitation device is designed to feed the excitation signal for exciting the ions stored in the ion trap to the first electrode, typically a ring electrode, or to a further electrode of the ion trap.

In this embodiment, different electrodes of the ion trap are used for the feeding of the excitation signal and the detection of the ion signal. The detection takes place at the cap electrode(s) as in conventional ion traps; the excitation signal is fed either to the first electrode, typically the ring electrode, or to a further electrode. In this way, the measurement channel for the ion signal at the second electrode or cap electrode is always free and is available exclusively for the detection of the ion current or of the induced mirror charges.

In this case, the cap electrode can have two or more parts, wherein a first cap electrode part, which generally lies on the inside in the radial direction and which forms the actual cap electrode, is used for the detection of the ion current, while a cap electrode part typically lying radially on the outside is used for feeding the excitation signal. As in a conventional Paul trap, the cap electrode can have a hyperbolic shape even when divided into two or more cap electrode parts. Although (slight) deviations from the ideal hyperbolic shape lead to higher-order field components, this is generally non-critical for the applications under consideration here. It is therefore also possible for the cap electrode and, if appropriate, the ring electrode of the ion trap to deviate from a hyperbolic shape and to have an (approximately) arbitrary geometry.

In one development, the first electrode, typically the ring electrode, is split into at least two electrode parts, to each of which the RF storage signal can be fed, wherein the excitation signal can preferably be fed to at least one of the electrode parts. In this case, the first electrode is typically divided into two or more electrode parts along the axis of symmetry of the ion trap (corresponding to the excitation direction of the ions). The two electrode parts can be, in particular, two halves of the first electrode, to each of which the RF storage signal is fed, wherein the two halves can be arranged in particular mirror- symmetrically with respect to a central plane of the ion trap.

The apparatus can be designed to feed to a first electrode part the RF storage signal and to a second electrode part the RF storage signal and an excitation signal added to the latter or subtracted from the latter, such that the excitation signal drops between the two electrode parts. Alternatively, it is possible to feed to the first electrode part of the first electrode a signal in which the excitation signal is subtracted from the RF storage signal, while a signal formed from the sum of the excitation signal and the RF storage signal is fed to the second electrode part of the first electrode. The excitation signal applied to the respective electrode part or between the two electrode parts can have a fundamentally arbitrary shape, amplitude and excitation frequency if a compensation device is present in the apparatus. It goes without saying that the first electrode or the ring electrode may possibly also have three or more electrode parts.

In the case of the embodiment described here, it is advantageous, but not absolutely necessary, for the amplitude and/or the frequency of the RF storage signal to be settable. What is essential is that the compensation device enables as broadband compensation as possible of the interference current generated as a result of the excitation signal being fed to the first electrode or to a respective electrode part of the first electrode, that is to say that there is no need for a signal generator that generates an RF storage signal having a settable amplitude and/or frequency. In a further development, the excitation device is designed to generate an excitation signal having at least one excitation frequency for selectively exciting ions in the ion trap, wherein the excitation frequency preferably corresponds to an oscillation frequency of the ions to be excited in the ion trap. In this development, in a targeted manner, individual ions or ion types (having a predefined mass-to-charge ratio) are excited and detected in the ion trap. In order to excite a desired ion species or ions having a desired mass-to-charge ratio, it is possible to use the fundamental frequency of the ion oscillation, but it is also possible to use an excitation frequency that corresponds to the sum of the fundamental frequency and the frequency of the RF storage signal, or an excitation frequency that corresponds to a typically odd-numbered harmonic, i.e. to three times, five times, ... the fundamental oscillation frequency of the ions having the desired mass-to-charge ratio.

In order to obtain a complete frequency spectrum, in this embodiment the excitation frequency of the excitation signal can be varied or run through. In this case, for the generation of a broadband mass spectrum, the RF storage signal can be kept constant, such that a complex, broadband compensation of the interference currents can be dispensed with. In addition, in this case it is not necessary to record the detected ion signals by means of suitable transient recorders and to convert them into a frequency spectrum or mass spectrum by means of a Fourier transformation. If the excitation frequency of the excitation signal is tuned without altering the RF storage signal, a simple, compact and hence cost-effective measurement set-up can thus be realized.

It is also possible, in order to generate a complete frequency spectrum, to keep the excitation frequency of the excitation signal constant and to alter the frequency and/or the amplitude of the RF storage signal. A combination of a variation of the excitation frequency of the excitation signal with a variation of the frequency and/or the amplitude of the RF storage signal is also possible in order to obtain an as complete broadband frequency spectrum as possible. In a further embodiment, the apparatus comprises an amplifier connected to the at least one second electrode for feeding an amplified ion signal to the detector, preferably an in particular frequency-dependent charge amplifier. As was described further above, during the excitation of ions stored in the ion trap, an ion signal is generated in the form of an induced charge or an ion current, which, for detecting ions, is typically led away from the second electrode and fed to an amplifier, typically a charge amplifier. The charge amplifier converts the ion current into an amplified ion signal in the form of a voltage that can be converted into a frequency spectrum or into a mass spectrum by the detector for example by means of an FFT transformation. The use of a frequency- dependent charge amplifier, i.e. of a charge amplifier having frequency- dependent amplification, is advantageous since the interference currents are typically up to twelve orders of magnitude greater than the ion currents to be measured. The interference currents can have in particular frequencies that are in the vicinity of the frequencies of the ion signal to be measured. In order to enable a measurement of the ion signal in the case of a possibly incomplete compensation of the crosstalk currents by the compensation signal, a frequency discrimination is advantageous in which the frequency components of the interference currents or of the crosstalk currents are suppressed by virtue of these frequency components experiencing a lower amplification by the charge amplifier.

In the apparatus, in particular two ion currents can be detected, which are generated at a respective cap electrode and which are respectively fed to an amplifier, in particular a charge amplifier. In order to increase the sensitivity during the detection, the two ion signals amplified by the amplifiers can be subtracted from one another, such that only the difference signal between the two ion currents is detected in the detector. The two amplifiers for amplifying a respective ion current together with the amplifier that brings about the subtraction can form an instrumentation amplifier having three (operational) amplifiers.

In one development, the apparatus additionally comprises a comparison device for comparing the at least one excitation frequency of the excitation signal with the amplified ion signal. The comparison device for comparing the amplified ion signal with the excitation frequency of the excitation signal can serve for selectively detecting ions with specific oscillation frequencies and/or mass-to- charge ratios and may be part of the detector, for example. Alternatively or additionally, the comparison device can also form a part of a control device (see below) in order permanently to store excited ions in the ion trap or to remove them in a targeted manner from the ion trap.

Preferably, the comparison device is designed for mixing a reference signal containing the at least one excitation frequency with the amplified ion signal. In particular, the comparison device can consist of a frequency mixer. The frequency mixer typically generates as output signals a sum signal and a difference signal from the reference frequency and the (ion) frequency of the amplified ion signal, which can be processed further, for example in order to simplify the detection of the excited ions or in order to suitably modify the amplitude and/or the pulse duration of the excitation signal in a control loop. The comparison device can also be designed to perform more complex calculations than the addition and subtraction of frequencies, particularly if said comparison device is used in a control device or in a control loop.

In a further development, the apparatus comprises a filter device for filtering the reference signal prior to feeding it to the comparison device, in particular in the form of a phase-locked loop for generating a reference signal that is phase- synchronous with the amplified ion signal. The phase-locked loop, also referred to as a PLL, makes it possible to feed to the frequency mixer a reference signal adapted to the ion signal in terms of phase which is advantageous for the detection (see below). Instead of a phase-locked loop, some other type of filtering, for example an FFT-based filtering, can be effected in the filter device connected upstream of the frequency mixer.

In a further embodiment, the detector has a low-pass filter for filtering an output signal supplied by the comparison device. The frequency mixer typically generates as output signal two frequencies, namely a sum signal and a difference signal from the reference frequency f_ref and the oscillation frequency fion of the ion signal of the excited ions. If the oscillation frequency fi_0n is in the capture range of the PLL, then after filtering with a low-pass filter that suppresses the sum frequency, a DC voltage results since the following holds true for the filtered output signal: f_out = | fion - fref I- Said DC voltage can be converted into a digital signal by an analogue-to-digital converter. Since a voltage is measured only for the case at the output of the analogue-to-digital converter where the oscillation frequency f_i0n corresponds to the reference frequency f_ref or to the excitation frequency, the ion mass M or the mass-to- charge ratio M / Z of the ions can be deduced with the aid of the digital signal.

For this purpose it is possible to make use of the fact that after the ions have been accelerated out of the centre of the ion trap, the ions carry out an oscillation with a characteristic oscillation frequency for which the following holds true in the case of an electrical ion trap e.g. of the type of the Paul trap: _f.on , JWyZe wherein URF denotes the amplitude of the RF storage signal, r₀ denotes the radius of the ion trap, f_RF denotes the frequency of the RF storage signal, Z denotes the charge number of the ion, M denotes the mass of the ion, m_H denotes the proton mass and e denotes the elementary charge. The oscillation frequency f_ion thus depends on the mass-to-charge ratio M / Z of the ions, on the geometrical parameters (r₀ ²) of the ion trap and also on the storage field or on the RF storage signal. If these parameters are known, the ion mass or the mass-to-charge ratio of the excited ions can be deduced with the aid of equation (1).

In a further embodiment, the apparatus has a control device for selectively storing or removing the ions excited by means of the excitation signal in the ion trap, wherein the control device preferably comprises the amplifier, in particular the frequency-dependent charge amplifier, and also the comparison device and is designed for controlling an amplitude and/or a pulse duration of the excitation signal. The control device can serve to control ion populations having mass-to- charge ratios or having oscillation frequencies that typically correspond to the excitation frequency or to an integral multiple of the excitation frequency on paths having substantially constant oscillation amplitudes, such that they can be stored permanently or over a comparatively long period of time in the ion trap. Alternatively, the control device can be used to remove individual ion

populations from the ion trap in a targeted manner, for example by virtue of their oscillation amplitude being chosen, by means of the excitation signal, to have a magnitude such that they collide with an electrode, typically with a cap electrode, of the ion trap.

The invention also relates to a method for mass-selective detection of ions in an ion trap, in particular an ion trap in an apparatus as described further above, comprising: generating an excitation signal having at least one excitation frequency for exciting ions in the ion trap, and also simultaneously detecting an ion signal generated upon the excitation of the ions in the ion trap, wherein detecting preferably comprises comparing the in particular amplified ion signal with a reference signal containing at least one excitation frequency of the excitation signal.

Preferably, the excitation signal is fed to an electrode or to an electrode part of the ion trap that differs from the electrode or the electrode part at which the ion signal is detected. As a result of the simultaneous or parallel detection and excitation of the ions in the ion trap, it is possible in particular to carry out a comparison between the ion signal generated by the excited ions and a reference signal, which for example may have the excitation frequency of the excitation signal and, if appropriate, also corresponds to the excitation signal in terms of phase, which simplifies the detection of the ions.

In one development, a control of an amplitude and/or of a pulse duration of the excitation signal is effected for storing the excited ions in the ion trap and/or for removing the excited ions from the ion trap, wherein the control comprises a preferably frequency-dependent amplification of the ion signal in order to form an amplified ion signal, and also a comparison of the amplified ion signal with a reference signal containing at least one excitation frequency of the excitation signal.

As was described further above in association with the apparatus, during the excitation of ions by means of an excitation signal and the simultaneous detection of the ion signal generated in the process, it is possible to carry out a control of the excitation signal, typically of the amplitude and/or pulse duration thereof, in order to keep the excited ions in a targeted manner on paths having a constant oscillation amplitude. Alternatively, the amplitude and/or the pulse duration of the excitation signal can be chosen with a magnitude such that the ions collide with the electrodes and are removed from the ion trap in this way.

Further features and advantages of the invention emerge from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be realized in each case individually by themselves or as a plurality in any desired combination in a variant of the invention. Drawing

Exemplary embodiments are depicted in the schematic drawing and are explained in the following description. In the figures:

Figure 1 shows a schematic illustration of an apparatus for detecting ions which comprises an ion trap having a ring electrode, having two cap electrodes, and also a storage signal generator,

Figures 2a-c show three schematic illustrations of the storage signal generator from Figure 1 in each case with a full-bridge module and with an RF storage signal generated thereby,

Figure 3 shows a schematic illustration of a storage signal generator with a plurality of full-bridge modules connected in series,

Figure 4 shows a schematic illustration of a storage signal generator with a power analogue amplifier,

Figure 5 shows a schematic illustration of an apparatus analogous to

Figure 1 with an interference current generated by the RF storage signal and with a compensation signal for the compensation of the interference current,

Figure 6 shows a schematic illustration of a compensation device for

generating a compensation signal on the basis of the RF storage signal generated by the storage signal generator,

Figure 7 shows a schematic illustration of a compensation device for

generating a compensation signal with the aid of a current through a first electrode of the ion trap, Figures 8a-c show schematic illustrations of three compensation devices which determine the current through the first electrode in different ways,

Figures 9a, show schematic illustrations of a compensation device with an auxiliary capacitance, which is designed for measuring the current flowing through the auxiliary capacitance,

Figures 10a-c show a schematic illustration of an electron beam

generating device for feeding an electron beam into the ion trap and also two RF storage signals for generating ions having different ionization energies,

Figure 11 shows a schematic illustration of an ion trap with a potential profile which is generated upon the excitation of ions with an excitation signal fed to the cap electrodes,

Figure 12 shows a schematic illustration of an ion trap with a potential profile which is generated upon the excitation of ions with an excitation signal that is fed to two electrode parts of the ring electrode,

Figure 13 shows a schematic illustration analogous to Figure 12 with a

control device for selectively storing or for removing ions from the ion trap, which comprises an amplifier and a frequency mixer,

Figure 14 shows a schematic illustration of an apparatus for detecting ions, which comprises a frequency mixer and a phase-locked loop, and

Figure 15 shows a schematic illustration of a stability diagram of the Mathieu differential equation. In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

Figure 1 schematically shows an apparatus 1 for detecting ions 4a, 4b which are stored in an ion trap 2. The apparatus 1 serves for examining the ions 4a, 4b by mass spectrometry and is therefore also referred to below as a mass spectrometer. In the example shown, the ion trap 2 is designed as an electrical ion trap (Paul trap) and has a first electrode 3 in the form of a ring electrode. An RF storage signal in the form of an AC voltage U F is applied to the ring electrode 3, which signal generates in the ion trap 2 an electric storage field E in the form of a radio-frequency alternating field, in which ions 4a, 4b of a gas 4 to be examined are dynamically stored. In order to generate the RF storage signal URF, the apparatus 1 has a storage signal generator 5. The gas 4 is fed to the interior of the ion trap 2 by means of a feed device (not illustrated in specific detail).

From the electric storage field E there results an average restoring force that acts on the ions 4a, 4b to a greater extent, the further away the ions 4a, 4b are from the middle or centre of the ion trap 2. In order to measure the mass-to- charge ratio (m/z) of the ions 4a, 4b, the latter are excited by an excitation signal U_stim,i , U_stim,2 (stimulus) to carry out oscillations, the frequency of which is dependent on the ion mass and the ion charge and is typically in the frequency range of kHz to MHz orders of magnitude, e.g. from approximately 1 kHz to 200 kHz. The respective excitation signal U_stim,i , U_stim,2 is generated by a first and a second excitation unit 6a, 6b, downstream of which a respective amplifier is connected.

For perturbation-free, non-destructive detection (i.e. the ions 4a, 4b are still present in the ion trap 2 after the detection), the oscillation signals of the excited ions 4a, 4b are tapped off in the form of induced mirror charges at two second electrodes 7a, 7b (measurement electrodes), which form the cap electrodes of the ion trap 2. The two cap electrodes 7a, 7b are connected to a respective low- noise charge amplifier 8a, 8b via a respective filter.

The charge amplifiers 8a, 8b acquire and amplify, on the one hand, in each case one of the two ion currents lj₀n-i , on2 that are generated at the cap electrodes 7a, 7b on account of the excitation, and on the other hand keep them at virtual earth potential. From the ion currents lj_0ni , lion2 converted into voltage signals by the charge amplifiers 8a, 8b, an ion signal u_i0n(t) is generated by means of difference formation, the temporal profile of said ion signal being illustrated at the bottom right in Figure 1.

The ion signal u_i0n(t) is fed to a detector 9, which, in the example shown, has an analogue-to-digital converter 9a and a spectrometer 9b for fast Fourier analysis (FFT) in order to generate a mass spectrum, which is illustrated at the top right in Figure 1. In this case, the detector 9 or the spectrometer 9b firstly generates a frequency spectrum of the characteristic ion resonant frequencies f_i0n of the ions 4a, 4b stored in the ion trap 2, which frequency spectrum is converted into a mass spectrum on the basis of the dependence of the ion resonant frequencies fi_0n on the mass and charge of the respective ions 4a, 4b. The mass spectrum represents the number of detected particles or charges as a function of the mass-to-charge ratio m/z.

In a conventional apparatus 1 for examining ions 4a, 4b, use is made of an RF storage signal URF which can have a constant frequency f_RF of the kHz to MHz order of magnitude, e.g. 1 MHz, and a constant (maximum) amplitude A_RF of hundreds of volts.

The apparatus 1 shown in Figure 1 , to put it more precisely its storage signal generator 5, is designed to set the amplitude A_RF and the frequency fRF of the RF storage signal U_RF over a comparatively large range of values, for example in a range of values of approximately 0.1 Hz to approximately 1 MHz in the case of the frequency fR_F of the RF storage signal U F. The amplitude A_RF can likewise be varied in order to generate a desired waveform, for example a sinusoidal, triangular or rectangular shape of the RF storage signal URF.

Figures 2a-c show exemplary embodiments of the signal generator 5, in which the setting of the waveform and the frequency f_RF of the RF storage signal URF is effected with the aid of a full-bridge module 10. The full-bridge module 10 has four transistors T1 to T4 in the form of power MOSFETs. A first pair of transistors T1 , T4 and a second pair of transistors T2, T3 are connected in series and in each case form a half-bridge. The two pairs T1 , T4 and T2, T3 or the two half-bridges are arranged in a parallel circuit. A voltage source 1 1 serves for generating a supply voltage U_D for the full-bridge module 10 and is connected in parallel with the first pair of transistors T1 , T4 and with the second pair of transistors T2, T3. A first tap A of the full-bridge module 10 between the first and fourth transistors T1 , T4 is connected to the ring electrode 3, and a second tap B between the second and third transistors T2, T3 is connected to earth potential. The voltage U_AB dropped between the taps A, B corresponds to the RF storage signal U_RF shown in Figure 1.

The four transistors T1 to T4 of the full-bridge module 10 are driven with the aid of four control signals 12a-d, which are digital signals in the example shown. During a period duration T of the RF storage signal U_RF, the control signals 12a- d are chosen such that the first switching state a) illustrated in Figure 2a is assumed during the first half of the period duration T and the switching state b) likewise illustrated in Figure 2a is assumed during the second half of the period duration T.

As is evident on the right-hand side in Figure 2a, a bipolar voltage UAB and thus an RF storage signal U_RF having a rectangular shape is generated between the two taps A, B in this way. In order to be able to generate the bipolar voltage U_AB in a floating fashion, the supply voltage U_D must be floating, which can be achieved by the use of a voltage source 11 in the form of a battery or possibly by the use of an isolated, i.e. galvanically decoupled, DC/DC converter.

Figure 2b shows a signal generator 5, the full-bridge module 10 of which differs from the full-bridge module 10 shown in Figure 2a in that a first series resistor 13a is arranged between the first tap A and the first and fourth transistors T1 , T4 and a second series resistor 13b is arranged between the second tap B and the second and third transistors T2, T3. As can likewise be discerned in Figure 2b, the two series resistors 13a, 13b flatten the switching edges of the voltage UAB between the taps A, B or the switching edges of the RF storage signal U_RF, since the current rise (dl/dt) is slowed down, which facilitates the compensation of crosstalk currents. Moreover, the series resistors 13a, 13b make it possible to reduce the power loss arising in the transistors T1 to T4 or to transfer it to the series resistors 13a, 13b, from which said power loss can be dissipated more easily than from the transistors T1 to T4.

For the case shown in Figures 2a, b where the full-bridge module 10 is driven digitally and a possibly smoothed square-wave voltage U_AB is generated in the process, switching losses or shunt currents can occur in a respective half-bridge or in a respective pair of transistors T1 , T4 and/or T2, T3. In order to avoid them, the full-bridge module 10 from Figures 2a, b can be driven in the manner shown in Figure 2c:

In addition to the first and second switching states shown in Figures 2a, b, a third switching state is generated, in which the control signals 12a-d are chosen such that either the third transistor T3 and the fourth transistor T4 or the first transistor T1 and the second transistor T2 are switched on simultaneously. In the third switching state, which is typically assumed during a partial interval ΔΤ₀ of the period duration T of the RF storage signal U_RF of generally approximately 2% to 10%, the RF storage signal U_RF or the voltage U_AB between the two taps A, B has a zero amplitude or a vanishing amplitude, as can be discerned in Figure 2c. Besides the reduction of switching losses, with the aid of such a partial interval ΔΤ₀ or with the aid of a plurality of partial intervals in which the voltage U_AB in each case has a constant value, it is possible to improve the ionization in the ion trap 2, as is described in greater detail further below.

The frequency f_RF of the RF storage field URF can be varied by the transistors T1 to T4 being switched on and off at suitable points in time by means of the control signals 12a-d. However, the possibility also exists of driving the transistors T1 to T4 in linear operation with analogue control signals 12a-d in order in this way to generate a fundamentally arbitrary signal shape, for example a sinusoidal or triangular RF storage signal URF, wherein for this purpose it is possible to use a control, if appropriate, for example as described below in association with in Figure 4.

In order to be able to set the (maximum) amplitude A_RF of the RF storage signal URF with the use of the full-bridge module 10, the voltage source 11 can be designed to set the value of the supply voltage U_D. Alternatively, as is illustrated in Figure 3, a plurality of full-bridge modules 10 can be connected in series in order to increase the (maximum) amplitude A_RF of the RF storage signal URF. In the circuit diagram shown in Figure 3, the ring electrode 3 is illustrated in the form of a capacitance C^ng, across which the RF storage signal U_RF generated by the series-connected full-bridge modules 10 of the signal generator 5 is dropped. Each full-bridge module 10 has a dedicated voltage source 11 for generating a supply voltage Ubatt, the value of which, on account of the plurality of full-bridge modules 10 present, is typically lower than with the use of a signal generator 5 having only a single full-bridge module 10.

Figure 4 shows a further possibility for generating an RF storage signal URF with a storage signal generator 5 having a power analogue amplifier 15. The power analogue amplifier 15 serves for amplifying an analogue control signal U_Set_Point, in order to generate an analogue RF storage signal U_RF having a (maximum) amplitude A_RF that may be for example of the order of magnitude of e.g. 100 V or more. For this purpose, the power analogue amplifier 15 has a half-bridge module 16 (also called output stage) having two driveable electronic

components in the form of transistors T1 , T2, to put it more precisely in the form of n-channel power MOSFETs. The ring electrode 3 from Figure 1 is illustrated by a capacitance CRi_ng in Figure 4 analogously to Figure 3, said capacitance being connected between the two series-connected transistors T1 , T2 of the half-bridge module 16. The two transistors T1 , T2 of the half-bridge module 16 are driven in linear operation, wherein the gate current of the transistors T1 , T2 is controlled via two gate drivers 17a, 17b with a current-controlled output. The two gate drivers 17a, 17b are galvanically isolated from the control of the output voltage or the control of the RF storage signal U_RF via a respective optocoupler 18a, 18b. For the control, an operational amplifier 19 is used in the case of the power analogue amplifier 15 shown in Figure 4. The two drivers 17a, 17b are supplied with a supply voltage in each case via a dedicated isolated DC/DC converter 20a, 20b. The control by means of the operational amplifier 19 is carried out via a first and second resistor R1 , R2, such that the RF storage signal U_RF is proportional to the control signal Usetpoint, that is to say that the following holds true: U_RF = a Usetpoint, wherein the following holds true for the factor a in the example shown: a = (1 + R₂ / Ri).

As was described further above in association with Figure 1 , the ion current l|_0ni , hon2 generated at the cap electrodes 7a, 7b on account of the excitation of the ions 4a, 4b in the ion trap 2 is measured, in order to detect the ions 4a, 4b in the ion trap 2 or in order to record a mass spectrum. On account of the spatial proximity of the cap electrodes 7a, 7b to the ring electrode 3, between these there is a capacitive coupling with a coupling capacitance Ccross, as illustrated in Figure 5. As is shown for the first cap electrode 7a in Figure 5, the coupling capacitance Cooss leads to an interference or crosstalk current l|_nt that is added to the ion current l|_0ni . The interference current l|_nt may possibly exceed the ion current l_loni by up to approximately eleven orders of magnitude and overdrive the charge amplifier 8a, 8b or possibly an amplifier of a different type that is used for amplifying the ion current l|_0ni.

As can be discerned in Figure 5, in order to compensate for the interference current m, there is applied to the charge amplifier 8a a compensation signal for example in the form of a compensation current lcomp, which is a signal which is the inverse of the interference current _nt and which is suitably scaled, such that lcomp = - lint holds true. In the case of the example shown in Figure 5, the compensation signal lcomp is fed together with the ion signal l|_0n to the charge amplifier 8a, to put it more precisely to an operational amplifier 21 of the charge amplifier 8a, at its inverting input, such that only the ion current l|_0ni is present at the latter. As usual in the case of charge amplifiers, the charge amplifier 8a additionally has a compensation capacitance Ccom , for example in the form of a capacitor, which is connected in parallel with the inverting input of the

operational amplifier 21 and the output thereof, in order to generate at the output of the charge amplifier 8a a voltage for which it holds true that: U_out = - Qion / Ccomp = - lion Δί / C_Comp, wherein Qjon denotes the induced charge generated at the first cap electrode 7a upon the excitation of the ions 4a, 4b and At denotes the time duration of the ion measurement.

In the case of the example shown in Figure 5, in addition to a respective cap electrode 7a, 7b at which the ion current l|_0n is measured, further typically rotationally symmetrical cap electrode parts 22a, 22b having hyperbolic geometry are provided, which are adjacent to the radially outer edge of the cap electrodes 7a, 7b with the formation of a gap. Between the further cap electrode parts 22a, 22b, which are typically at earth potential, and the rotationally symmetrical ring electrode 3, a capacitance C_Ri_ng arises in a manner governed by the geometry. The two further cap electrode parts 22a, 22b serve to ensure a good quadrupole field in the ion trap 2. For the case where the two further cap electrode parts 22a, 22b are at earth potential, they are typically not active, that is to say that their contribution to the ion current detection is both small and undesired. Via a switch S1 , the excitation signal Us«mi of the (first) excitation device 6a can be fed to the first cap electrode 7a in order to excite ions 4a, 4b in the ion trap 2.

Since the signal generator 5 in the present case is designed to generate the RF storage signal URF with a variable frequency fRF into the MHz range and with a fundamentally arbitrary waveform, it is necessary to design the compensation signal lcomp for a correspondingly broadband compensation. Various

possibilities for realizing a compensation device 25 which makes it possible to generate such a compensation signal lcomp are explained in detail below.

In the case of the example illustrated in Figure 6, the compensation device 25 for generating a compensation signal Ucomp has a voltage source 26 designed to generate the compensation signal Ucomp on the basis of the RF storage signal U_RF or to derive the compensation signal Ucom from the RF storage signal U_RF. The compensation signal U_Comp is a voltage which is inverted (typically phase-shifted by 180°) with respect to the RF storage signal U_RF and is suitably scaled and which is fed via a compensation capacitance Ccomp to the charge amplifier 8a in order to generate there a compensation current lcomp that is inverted with respect to the interference current Lt. For this purpose, the voltage source 26 of the compensation device 25 and the signal generator 5 or the voltage source thereof are synchronized with the aid of the compensation device 25, that is to say that the latter is connected to the storage signal generator 5 in terms of signalling for this purpose.

The signal profile of the RF storage signal U_RF and the signal profile of the compensation signal Ucomp must be accurately adapted to one another for an optimum compensation. If appropriate, it may be necessary to record the crosstalk current portion lu that flows through the capacitance C_Ring between the ring electrode 3 and a respective cap electrode part 22a, 22b at earth potential (cf. Figure 5) and to use it as a reference for the generation of the

compensation signal Ucomp.

If appropriate - in contrast to the illustration shown in Figure 6 - the

compensation device 25 does not necessarily require a dedicated voltage source; rather, in the case of the signal generator 5 illustrated in Figure 4 for the generation of the RF storage signal URF by the amplification of an analogue control signal Usetpoint, the latter itself can be used for the generation of the compensation signal U_Com_P. In this case, the control signal U_Set_Point merely has to be inverted and possibly amplified.

Figure 7 shows the fundamental principle of a measurement of the crosstalk current portion lu between the ring electrode 3, which is represented by a capacitance C_Ring in Figure 7, with respect to earth, which is present at one of the two further cap electrode parts 22a, 22b from Figure 5, with the aid of an ammeter 27. As can likewise be discerned in Figure 7, the RF storage signal UR_F is fed to the ring electrode 3 via a series resistor R_v. In the case of the example shown in Figure 7 the ammeter 27 together with an adjustable current source 28 forms the compensation device 25 for generating the compensation signal in the form of a compensation current lcomp that is fed to the input of the charge amplifier 8a. The adjustable current source 28 serves to invert the measured crosstalk current portion lu and to scale it appropriately with respect to the interference current in order to compensate for the influence thereof on the detection of the ions 4a, 4b.

For the realization of the principle shown in Figure 7 for generating the compensation signal l_COmp on the basis of the measured crosstalk current portion lu, a number of realization possibilities exist, which differ in the type of measurement of the crosstalk current portion lu and in the type of generation of the compensation signal lcomp- A description is given below, with reference to Figures 8a-c, of a number of possibilities for realizing a compensation device 25 based on the principle illustrated in association with Figure 7.

Figure 8a shows a compensation device 25 having a transconductance amplifier 29 for generating the compensation signal l_Com_P, said

transconductance amplifier being used simultaneously for measuring the crosstalk current portion lu and for generating the compensation signal lcom - The transconductance amplifier 29 is operated in a common-base connection and has a base-emitter voltage of 0 V, for which reason the ring electrode 3 or the capacitance 0¾_η9 is kept at earth potential via the transconductance amplifier 29 and the electric storage field E in the ion trap 2 is not influenced. In the case of the compensation device 25 shown in Figure 8a, the crosstalk current portion lu is measured with respect to earth, for which reason, in this example, it is necessary to interrupt the earth connection in the ion trap 2 or within the storage cell.

Figure 8b shows a compensation device 25 designed to measure the current l_RF in an electrical lead 30 for coupling the RF storage signal URF into the ring electrode 3 outside the ion trap 2. For this purpose, the compensation device 25 has a (broadband) transformer 31 , the primary winding of which is arranged in the electrical lead 30 and across the secondary winding of which a voltage is dropped, which, in the case of the example shown in Figure 8b, is converted, by means of a capacitance Ccomp, into a compensation signal in the form of a compensation current l_RF, which is fed to the charge amplifier 8a. In contrast to what is shown in Figure 8a, it may possibly be necessary to additionally adapt the voltage generated at the secondary winding of the transformer 31 with the aid of an active circuit (not illustrated in more specific detail), in order to obtain an optimum compensation.

Figure 8b shows a compensation device 25 in which, as in the case of the compensation device shown in Figure 8a, the current l_RF through the ring electrode 3 is measured on an electrical lead 30 for coupling the RF storage signal URF into the ring electrode 3. In the case of the solution shown in Figure 8b, the compensation device 25 is designed for contactlessly measuring the current l_RF through the electrical lead 30 and has a coil 32 for this purpose. In the example shown, the coil 32 is a Rogowski coil suitable for current measurement at high frequencies fR_F of the RF storage field URF in the MHz range. A signal amplifier 33 serves for amplifying and for inverting the signal supplied by the coil 32, which signal is converted, by means of a capacitance Ccom , into a compensation signal in the form of a compensation current IRF, which is fed to the input of the charge amplifier 8a.

Figures 9a-c finally show a compensation device 25 having an auxiliary capacitance C_Aux in the form of a capacitor, which is fitted between the ring electrode 3 and the first cap electrode 7a (outside the interior of the ion trap 2). The auxiliary capacitance C_Aux is thus arranged in parallel with the coupling capacitance Ccross generated by the electric storage field E. In this example, the compensation device 25 is designed to measure the current l_Aux through the auxiliary capacitance C_Aux, which typically has a similar value to the coupling capacitance Ccross of the order of magnitude of several hundred fF, for example around approximately 700 fF.

The current l_Aux through the auxiliary capacitance C_Aux corresponds in terms of its waveform to the crosstalk current portion lu, but has a significantly lower absolute value, with the result that the requirements made of the current sources needed for the compensation can be significantly reduced. In Figure 9a, as in the case of the example shown in Figure 8a, the compensation device 25 has a transconductance amplifier 29 in order to measure the current l_Aux through the auxiliary capacitance C_Aux. The transconductance amplifier 29 simultaneously serves as an adjustable current source for generating the compensation signal in the form of a compensation current lcomp. A capacitor can serve for realizing the auxiliary capacitance 33, said capacitor being mounted on the outer side of the ion trap 2 and contacting firstly the ring electrode 3 and secondly the upper cap electrode 7a. The capacitor can be arranged as a discrete capacitor, e.g. in the form of a film capacitor, outside the vacuum chamber in which the ion trap 2 is usually arranged or operated. The arrangement of the auxiliary capacitance 33 outside the vacuum chamber or at a location at a distance from the ion trap 2 makes it possible to shorten the electrical connecting lines to the charge amplifier 8a in order to minimize possible coupled-in interference.

On account of the comparatively low intensity of the current ux through the auxiliary capacitance C_Aux, instead of the (discrete) transconductance amplifier 29 it is possible to use an operational amplifier circuit for measurement, as is shown by way of example in Figure 9c for a current feedback operational amplifier 34. The voltage U_a = - ux dt / C_Aux generated at the output of the current feedback operational amplifier 34 is proportional to the current ux through the auxiliary capacitance C_Aux. By means of a (small) compensation capacitance Ccomp connecting the inverting input of the current feedback operational amplifier 34 to the output, as described further above, the output voltage of the current feedback operational amplifier 34 is converted into a compensation signal in the form of a compensation current lcomp-

It generally holds true that the generation of the compensation signal U_Comp, lcomp as described further above can be carried out, and in general is actually carried out, not only for the first cap electrode 7a or the first charge amplifier 8a but also for the second cap electrode 7b or for the second charge amplifier 8b. On account of the symmetrical geometry of the ion trap 2, it typically suffices to measure the crosstalk current portion lu, the current IRF, l_Aux and/or the RF storage signal U_RF just once and to feed one and the same compensation signal Ucomp, lcomp both to the first charge amplifier 8a and to the second charge amplifier 8b for the compensation. Various possibilities exist for generating the ions 4a, 4b in the ion trap 2: The ions 4a, 4b can be generated outside the ion trap 2 and be fed to the ion trap 2 after the ionization. Alternatively, the gas 4 to be examined can be fed to the ion trap 2 in a charge-neutral state and can be ionized in situ within the ion trap 2. An electron beam 36 can be used for this purpose, said electron beam being generated by an electron beam generating device 35 (electron gun), as is illustrated in Figure 10a. In the example shown, the electron beam generating device 35 is fitted outside the ion trap 2 on the ring electrode 3 and the electron beam 36 is fed to the interior of the ion trap 2 via a hole 37 in the ring electrode 3.

In the example shown, the electron beam generating device 35 has a filament 38a (incandescent wire) that is heated by means of a heating device (not illustrated in more specific detail) in order to liberate electrons and to generate the electron beam 36. The hole 37 in the ring electrode 3, a corresponding hole in a Wehnelt cylinder 38b, which is likewise illustrated in Figure 10a, and also a hole in an anode 38c surrounding the Wehnelt cylinder are arranged along a line of sight. In the example shown, the electron beam 36 is fed into a plane of symmetry or central plane (XY-plane) of the hyperbolic ring electrode 3, which runs mirror-symmetrically with respect to the plane of symmetry in the Z- direction. Ideally, ions 4a, 4b are generated within the plane of symmetry, to put it more precisely into the centre of the ion trap 2, at which the plane of symmetry is intersected by the axis 39 of symmetry of the ring electrode 3 running in the Z-direction, the ring electrode 3 having a rotational symmetry with respect to said axis of symmetry.

As soon as the electron beam 36 enters the interior of the ion trap 2, it is modulated with the electric storage field E, as a result of which the electrons of the electron beam 36 obtain different energies. The optimum ionization energy for the impact ionization by the electron beam 36 is generally approximately 70 eV, with the result that the detection or the determination of the quantity of ions present in the ion trap 2 is made significantly more difficult by the variation of the energy of the electrons of the electron beam 36. In addition, on account of the electric storage field E, the electrons of the electron beam 36 are distributed in an undesired manner in the interior of the ion trap 2 and thus generate ions 4a, 4b at different locations in the interior of the ion trap 2. Ideally, however, the ions 4a, 4b should be generated only in the plane of symmetry (XY-plane) of the (hyperbolic) ring electrode 3, which in the example shown is designed in rotationally symmetrical and mirror-symmetrical fashion with respect to the plane of symmetry, since an interior of the ion trap 2 that is "filled" with ions 4a, 4b typically leads to a worsened sensitivity and resolution during the detection of the ions 4a, 4b.

In order to reduce this problem, the zero phase described further above in association with Figure 2c can be used for feeding the electron beam 36 into the ion trap 2, that is to say that the electron beam 36 can be fed into the ion trap 2 only during a partial interval ΔΤ, , ΔΤ_ί+ of the period duration T of the RF storage signal URF in which the RF storage signal U_RF and thus the electric storage field E has a vanishing amplitude (zero amplitude).

In this case, the electron beam generating device 35 can be designed to generate a pulsed electron beam 36, for example by the use of deflection electrodes that deflect the electron beam 36 in the partial intervals ΔΤ, , ΔΤ_ί+ι in which said electron beam is not intended to pass into the ion trap 2 in such a way that said electron beam cannot pass through the hole 37 in the ring electrode 3. Alternatively or additionally, for this purpose the voltage at the anode 38c of the electron beam generating device 35 can be switched rapidly between two voltage values, wherein, in the case of one of the two voltage values, the energy of the electrons of the electron beam 36 is insufficient to pass into the ion trap 2. The electron beam generating device 35 can additionally be synchronized with the storage signal generator 5 in order to feed the pulsed electron beam 36 to the ion trap 2 only during a respective desired partial interval ΔΤ,, ΔΤ,+ι of the period duration T of the RF storage signal U_RF in which the latter has a vanishing amplitude, as is illustrated in Figure 10b. The feeding only during the partial interval ΔΤ,, ΔΤ,+ι or during the zero phase ensures that the electrons of the electron beam 36 in the ion trap 2 firstly have a defined energy and secondly move exclusively on the centre axis or in the plane of symmetry of the ion trap 2 and only there generate ions 4a, 4b, such that the number of ions 4a, 4b generated in the ion trap 2 can be determined with higher accuracy.

Alternatively or additionally, it is possible to feed the electron beam 36 to the ion trap 2 also in partial intervals ΔΤ,, ΔΤ,+ι of the period duration T in which the RF storage signal U_RF has a constant amplitude different from zero, as is illustrated in Figure 10c for two (additional) partial intervals ΔΤ,, ΔΤ_ί+ι. In this case, the RF storage signal U_RF can serve for decelerating or for accelerating the electrons of the electron beam 36 along the path between the anode 38a of the electron beam generating device 35 and the ring electrode 3. In this case, the fact of whether the electrons of the electron beam 36 are accelerated depends on whether the potential between the anode 38c and the RF storage signal of the ring electrode 3, said RF storage signal being constant in the respective partial interval ΔΤ,, ΔΤ|₊ , is positive or negative. The electron energy of the electrons fed into the ion trap 2, or of the electron beam 36, can thus be modulated in a targeted manner by means of the RF storage signal U_RF or the waveform of the RF storage signal U_RF can be adapted suitably for the ionization.

For the quantity of ions n_i0n,i , n_i0n, M generated in a respective time interval ΔΤ,, ΔΤΜ it holds true that: n_ion = (AT_i+1), wherein η, , η,·_+ί denote the respective ionization rate or ionization efficiency. Accordingly, the following holds true for the quantity of ions N_i0n,tot generated in total during a period duration T of the RF storage signal URF with N partial intervals in which the amplitude of the RF storage field URF is constant in each case:

It goes without saying that, in order to determine the total number of ions 4a, 4b generated in the ion trap 2, the quantity of ions N_ion,tot has to be multiplied by the number of period durations in which the electron beam 36 is fed to the ion trap 2.

Particularly for the waveform shown in Figure 10b, it is evident that for each zero phase a well-defined ionization energy can be achieved by means of the potential Δφ_β between the anode 38c and the ring electrode 3 and a practically constant ion subset can be achieved by means of the ionization rate η, and the ionization duration ΔΤ,. It is thus possible to generate a defined total quantity of ions 4a, 4b consisting of ion subsets of different sizes and having different ionization energies and/or different ionization rates η,. As can be discerned in Figure 10c, even in the case of a waveform of the RF storage signal U_RF which has partial intervals AT,, AT_i+i in which the RF storage signal U_RF has a constant amplitude Δφ, different from zero, well-defined ionization energies Δφ_β +Δφ, can be achieved for different ion subsets.

Figure 1 1 shows an ion trap 2 in the form of a Paul trap such as can be used for example in the apparatus 1 shown in Figure 1. The ion trap 2 has two integral cap electrodes 7a, 7b and a ring electrode 3, which, in the example shown, is divided into two (ring) electrode parts 3a, 3b, to both of which the RF storage signal U_RF of the signal generator 5 is fed (cf. Figure 1 ). The two electrode parts 3a, 3b are arranged mirror-symmetrically with respect to a central plane of the ion trap 2, which, as is shown in Figure 1 1 , overall is designed to be

substantially rotationally symmetrical about the Z-axis as axis 39 of symmetry of the ion trap 2.

In the case of the example shown in Figure 1 1 , the excitation signal U_stimi is fed from an excitation device 6a, 6b (cf. Figure 1) to the cap electrodes 7a, 7b in order momentarily to excite ions 4a, 4b or an ion population in the ion trap 2, wherein the cap electrodes 7a, 7b simultaneously serve as measurement electrodes for carrying away the ion current lj₀n , -

In the case of the example shown in Figure 1 1 , as excitation signal U_stimi a (short) excitation pulse in the form of a square-wave pulse having a pulse duration T_P and an amplitude U_P is applied, to be precise differentially or in the form of a differential voltage, i.e. for the excitation signal U_stimi at the upper cap electrode 7a it holds true that Ustimi = +Up, while for the excitation signal U_stimi at the lower cap electrode 7b it holds true that: -U_stimi = -U_P. The potential φι generated here in the ion trap 2 can likewise be discerned in Figure 11 , wherein an RF storage signal URF having a value of 0 V was assumed for simplification.

In the case of the example shown in Figure 1 1 , the cap electrodes 7a, 7b have a dual functionality since they serve firstly for the excitation of ions 4a, 4b and secondly for the measurement of the induced ion signals l_i0ni , lion2- Such a dual use of the two cap electrodes 7a, 7b for excitation and for measurement is absolutely necessary in conventional ion traps 2 because the RF storage signal URF in conventional ion traps 2 is designed with a constant frequency fR_P in a very narrow frequency band in order to enable the interruption-free

compensation of the crosstalk current l_int in the first place or in order to configure said compensation as simply as possible.

If, as described further above, the apparatus 1 then contains a compensation device 25 which enables a broadband compensation of the crosstalk current lint, in addition to the (in this case for example constant) RF storage signal URF, an excitation signal U_stim2 can be fed to the ring electrode 3 or the two electrode parts 3a, 3b, as is illustrated in Figure 12. As in the case of the two cap electrodes 7a, 7b, the excitation signal U_stim2 is fed to the two electrode parts 3a, 3b in the form of a differential voltage, that is to say that the excitation signal Ustim2 is added to the RF storage signal URF at the first electrode part 3a and is subtracted from the RF storage signal URF at the second electrode part 3b, such that the doubled amplitude Up of the excitation signal U_stim2 embodied in the form of a square-wave pulse is present between the two electrode parts 3a, 3b. The potential φ₂ generated here in the ion trap 2 is shown in Figure 12 in addition to the potential φι from Figure 11 , wherein a vanishing RF storage signal (U_RF = 0 V) was again assumed for simplification.

As can be discerned in Figure 12, in the case of small deflections of the excited ions 4a, 4b from the rest position where z = 0 at the centre of the ion trap 2 to a first approximation (given suitable scaling) the potential <j>₂ corresponds to the potential φι which was generated upon the excitation signal U_stimi being fed to the cap electrodes 7a, 7b from Figure 1 1. Therefore, the relationships regarding the potential φι that are known in association with the excitation of ions 4a, 4b substantially hold true for the potential φ₂ as well. It goes without saying that, in contrast to what is shown in Figure 12, the excitation signal U_stim2 can be fed only to the first electrode part 3a, that is to say that U_RF + U_stim2 holds true for the voltage fed to the first electrode part 3a, while in this case only the RF storage voltage U_RF as AC voltage is fed to the second electrode part 3b (or vice versa).

By means of the broadband compensation of the interference or crosstalk current l,_nt with the aid of the compensation device 25 (not illustrated in Figure 12), it is possible to compensate for the influence of the excitation signal U_stim2 coupled into the two electrode parts 3a, 3b on the ion current lj_0ni, - In this example, the two cap electrodes 7a, 7b are used only to detect the respective ion current l_ioni, on2 and thus are always at virtual zero potential (+ 0 V). At the same time, the excitation signal U_stim2 can be fed to the two electrode parts 3a, 3b in order to excite ions 4a, 4b present in the ion trap 2 (cf. Figure 1 ).

In the case of the ion trap 2 illustrated in Figure 12, too, the ions 4a, 4b excited in the ion trap 2 can be excited (in a broadband manner) with an excitation signal U_stim2 in the form of a square-wave pulse having a (large) amplitude Up and an (in comparison with the amplitude very short) time duration T_P, i.e. which is practically a Dirac pulse.

For the broadband excitation of ions 4a, 4b by means of such a square-wave pulse in the operating mode shown in Figure 11 , the following holds true in accordance with the thesis "Ein Beitrag zur breitbandigen Massenspektrometrie mit elektrischen lonenresonanzzellen [A contribution to broadband mass spectrometry using electron ion resonance cells]" (EIRZ) by M. Aliman, 1998, which is incorporated by reference in the content of this application, in accordance with equation (5.16), page 59:

_{Tp ≤} ^ 0 4444 _

I ιοη,τηαχ wherein fi₀n,z,max denotes the oscillation frequency of the fastest ion 4a, 4b to be excited and Tp denotes the pulse duration of the square-wave pulse.

In the operating mode of the ion trap 2 as shown in Figure 12, the following accordingly results:

T_P≤ (2)

/ ίοη,τηαχ

In addition, for the differential stimulus or for the excitation signal U_stim2, it is necessary to comply with a condition regarding the "Pulse-time integral" Up x T_P of the excitation signal U_stim2 that prevents the oscillation amplitude of the ions 4a, 4b from increasing during the excitation to such an extent that said ions collide with the cap electrodes 7a, 7b and are thus removed from the ion trap 2. For the standard operating mode of the ion trap 2 as shown in Figure 11 , in the case of a sinusoidal RF storage signal or a sinusoidal storage voltage having amplitude V_RF and frequency f_RF, in accordance with page 89, equation (5.55) from the above thesis, the following results:

For the operating mode of the ion trap 2 as shown in Figure 12, the following correspondingly holds true:

The results regarding the excitation at the cap electrodes 7a, 7b that are described regarding the broadband excitation in the thesis cited further above can thus readily be applied to the excitation at the ring electrode 3 as described in Figure 12.

The feeding of the excitation signal U_stim2 to the ring electrode 3, to put it more precisely to the two electrode parts 3a, 3b, and the simultaneous detection of the ion current l_i0ni , Ii₀n2 can advantageously be used to permanently store ion populations in the ion trap 2 in a targeted manner by virtue of said ion populations being kept on paths with a substantially constant oscillation amplitude, or to remove said ion populations from the ion trap 2 in a targeted manner by virtue of the oscillation amplitude (in the Z-direction) being chosen with a magnitude such that the excited ions 4a, 4b collide with the cap electrodes 7a, 7b or possibly with the ring electrode 3 and are thus removed from the ion trap 2. As was described further above, for keeping the excited ions 4a, 4b on paths with a constant oscillation amplitude it is necessary to satisfy the two

inequalities (2) and (3) regarding the pulse duration Tp and the pulse-time integral of the excitation signal U_stim2. Particularly if inequality (3) is not satisfied, the excited ions 4a, 4b typically have an oscillation amplitude with a magnitude such that the excited ions 4a, 4b are removed from the ion trap 2. It goes without saying that inequality (3) can be generalized from a Dirac pulse to arbitrary waveforms of the excitation signal U_stim2, cf. for example equation (5.56) in the thesis cited above.

In order to permanently store the excited ions 4a, 4b in the ion trap 2, it is therefore necessary for the excitation signal U_stim2, to put it more precisely the amplitude U_P thereof and the pulse duration T_P thereof, for the excited ions 4a, 4b to satisfy conditions (2) and (3) described further above. Since the excitation signal U_stim2 applied to the ring electrode 3, on account of the compensation device 25, which is not illustrated in Figure 12, leads to an interference current lint that is compensated for, such that the latter has no influence on the ion signal lj_0ni , lion2, the excitation signal U_stim2 can in principle assume an arbitrary waveform. The averaged "pulse integrals" U_P T_P of the excitation signal U_stim2 can therefore be chosen in particular such that they satisfy condition (3) above.

In order to achieve this, it is possible to use a control device 40, which is described below by way of example with reference to Figure 13 for the ion signal l_ioni = Hon that is tapped off at the first cap electrode 7a. It goes without saying that such control can also be carried out on the basis of the ion signal lion2 = - lion at the second cap electrode 7b or that both ion signals lj_0ni , lion2, in particular also the difference between them, can be used for the control.

In the case of the example shown in Figure 13, the control device 40 has a (frequency-dependent) charge amplifier 8a, which generates an amplified ion signal Uj_0n in the form of an AC voltage. The use of a frequency-dependent charge amplifier 8a is advantageous in order, in a targeted manner, to suppress the amplification of frequency components in the ion signal l_ioni to be amplified which are attributable to the crosstalk currents. In general, such a frequency discrimination is necessary or advantageous since a signal component of the ion signal l|_0ni that is attributable to crosstalk currents can be up to twelve orders of magnitude greater than a signal component of the ion signal l|_0ni that is generated by the ions 4a, 4b. The ion signal U_i0n amplified by the charge amplifier 8a is fed to a (broadband) comparison device 41 , which is likewise part of the control device 40. Reference signals 42 are also fed to the comparison device 41 , said reference signals in each case containing an excitation frequency fi_0ni , ■ · · , fion of the excitation signal U_siim2 or consisting of the excitation frequency f_ioni , fionN-

The excitation frequencies f_lon, i,■ ·■ of the excitation signal U_stim2 serve for selectively exciting ions 4a, 4b in the ion trap 2 which belong in each case to an ion population having a specific mass-to-charge ratio. In this case, the excitation frequency f_ion of the excitation signal U_stim2 typically corresponds to an oscillation frequency or resonant frequency f_ion of the ions 4a, 4b to be excited in the ion trap 2 for which they can be kept stably in the ion trap 2 both in the radial direction and in the z-direction. In the case of the ion trap 2 described here in the form of a hyperbolic Paul trap, the ion movements e.g. in the z-direction are described by the Mathieu differential equations. Figure 15 shows a stability diagram of the Mathieu differential equations for which the stability condition for the characteristic frequencies β = f(a,q), as generally customary, is represented as a function of the dimensionless parameters a and q. The parameter a is proportional to a common-mode voltage which is possibly present and which is applied to the cap electrodes 7a, 7b, and the parameter q is proportional to the RF storage signal U_RF. For a respective ion frequency fj_0n it holds true that: f_ion = (a,q) x f_RF / 2. The stability criterion is met in the areas represented in a hatched fashion in Figure 15, that is to say that the ions 4a, 4b move on stable paths in the hatched areas. As can be discerned in Figure 15, in the stability diagram a plurality of hatched areas exist which are attributable to higher-order

resonances. With reference to Figure 15 it is evident that the ion frequencies fi_0n for any recorded value of the parameter a can be stabilized virtually arbitrarily more highly, i.e. at higher frequencies f_RF of the RF storage signal f_RF.

As was described further above, in the case of the example shown in Figure 13, a plurality of reference signals 42 are present which have different excitation frequencies fi_0ni, , - in order, in a targeted manner, to excite ions 4a, 4b with different mass-to-charge ratios in the ion trap 2. The broadband, adjustable comparison device 41 also serves as a frequency mixer and mixes the excitation frequencies fj_0ni, fion2,■·· of the reference signals 42 with the RF storage signal U_RF and with the ion signal Uj₀n amplified by the charge amplifier 8a. The comparison device 41 can be set such that the degree of excitation, in particular the product U_P T_P of amplitude Up and the pulse duration T_P of the excitation signal U_stim2, for a respective excitation frequency f_i0ni, ■· , satisfies condition (3), i.e. the excitation is tracked to the trajectories or the oscillation amplitudes A of the excited ions 4a, 4b in the ion trap 2 such that these ion packets remain in the ion trap 2. Ions 4a, 4b or ion packets which have this or possibly a different excitation frequency f_ioni, ... can be excited with the aid of the comparison device 41 such that they are removed from the ion trap 2 in a targeted manner, if this is desired.

The reference signals 42 having the respective excitation frequency fi_0ni, ... can be generated by respective tuneable oscillators that are part of a respective excitation device 6a, 6b (cf. Figure 1) or of a common excitation device 6. It is also possible to generate the reference signals 42 or the respective excitation frequency f_ioni, ... from the excitation signal U_stim2, which are fed to the two electrode parts 3a, 3b by virtue of said signal being filtered with the aid of a filter device, for example with the aid of a filter device in the form of a phase-locked loop 43, as will be described below with reference to Figure 14, or with the aid of a filter device based on a filtering by means of an FFT transformation.

The apparatus 1 illustrated in Figure 14 is a particularly simple and thus cost- effective apparatus 1 for ion detection or for mass analysis in which the compensation device 25 described further above can be dispensed with. In the case of the apparatus 1 shown in Figure 14, the ion signal l_ioni , lion2 is tapped off as usual at a respective cap electrode 7a, 7b, wherein only the ion signal lj₀ni tapped off at the first cap electrode 7a is illustrated in Figure 14 for the sake of simplification. The excitation signal U_stimi is fed (with a positive and respectively a negative sign) to a respective outer cap electrode part 22a, 22b, which is separated from the actual cap electrode 7a, 7b by a gap. Unlike in Figure 13, the ring electrode 3 of the apparatus 1 shown in Figure 14 is embodied in an integral fashion. As in the case of the apparatus 1 shown in Figure 13, a simultaneous excitation and detection of ions 4a, 4b in the ion trap 2 can be carried out in the case of the apparatus 1 shown in Figure 14 as well.

Just like the apparatus 1 shown in Figure 13, the apparatus 1 in Figure 14 has a comparison device 41 and a charge amplifier 8a for generating an amplified ion signal U_i0ni , which is fed to the comparison device 41 which forms a frequency mixer in the case of the example shown in Figure 14. A reference signal 42 is fed to the comparison device 41 , said reference signal being generated from the excitation signal U_stimi by virtue of the latter being filtered in a filter device in the form of a phase-locked loop 43. For this purpose, the phase-locked loop 43 has a comparator 44 in the form of a frequency mixer, an integrator 45 and an oscillator 46 for generating the reference signal 42. The phase-locked loop 43 makes it possible to match the phase of the reference signal 42 to the phase of the amplified ion signal U_i0ni . The apparatus 1 shown in Figure 14 makes it possible, in a particularly simple manner, to detect ions 4a, 4b having a specific ion mass in the ion trap 2: For this purpose, all that is necessary is to feed the output signal 47 of the comparison device 41 in the form of the frequency mixer to a low-pass filter 48, which filters the summation signal out of the output signal 47, such that the output signal 47 forms the difference frequency between the oscillation frequency f_ion and the reference frequency f_ref of the oscillator 46 or of the reference signal 42, that is to say that the following holds true: f_out = I fion - fref I - The output signal 47 is converted into a digital output signal DC by an analogue-to-digital converter 49. The digital output signal DC has a value different from zero only if the oscillation frequency fi_0n corresponds to the reference frequency f_ref, which corresponds to the excitation frequency, or if the oscillation frequency f_ion lies in the capture range of the phase-locked loop 43. Therefore, the mass or the mass-to-charge ratio of the excited ions 4a, 4b can be determined if the numerical values of the other variables in equation (1) are known.

In order to record a frequency spectrum with the apparatus 1 shown in Figure 14, the excitation frequency f_i0n or the reference frequency f_ref can be varied, i.e. for example run through continuously. If, in the case of the apparatus 1 shown in Figure 14, use is made of a signal generator 5 for generating the RF storage signal U_RF, the amplitude A_RF and/or the frequency f_RF of which are adjustable, and provision is made of a broadband compensation device 25, such as was described further above, then the amplitude A_RF and/or the frequency f_RF of the RF storage signal U_RF can be varied in order to excite ions 4a, 4b having different mass-to-charge ratios in the ion trap 2 in accordance with equation (1 ). It goes without saying that the variation of the RF storage signal U_RF can be combined with the variation of the excitation frequency f_ion or of the reference frequency f_ref, in order to record a broadband mass spectrum. In the case of the apparatus 1 shown in Figure 14, too, a control device 40 can be provided in order to permanently keep the excited ions 4a, 4b in the ion trap 2 by virtue of said ions being permanently excited with the aid of a suitably chosen excitation signal U_stimi , as described further above in association with Figure 13. Both in the case of the apparatus 1 shown in Figure 13 and in the case of the apparatus 1 shown in Figure 14, it is possible to compensate for dampings on account of collisions between the ions 4a, 4b, such that requirements that have to be made of the vacuum installation or the vacuum environment in which the ion trap 2 is typically operated are less stringent than is the case for conventional ion traps 2. In particular, the operating pressure in the ion trap 2 can be chosen to be higher than in a conventional ion trap 2, which leads to reduced pump powers of the vacuum pumps required for generating the vacuum and therefore enables a very compact construction of the entire apparatus 1.

Claims

1 Patent Claims

1. Apparatus (1 ) for detecting ions (4a, 4b), comprising:

an ion trap (2) having a first electrode, preferably a ring electrode (3) and also having at least one second electrode, preferably a cap electrode (7a, 7b),

a storage signal generator (5) for generating an RF storage signal (U_RF), which can be coupled into the at least one first electrode (3; 3a, 3b) in order to generate an electric storage field (E) in the ion trap (2),

an excitation device (6a, 6b) for generating an excitation signal (U_stimi , U_stim2) for exciting ions (4a, 4b) stored in the ion trap (2), and also

a detector (9) for detecting an ion signal (l|_0n-i, ), generated by the excited ions (4a, 4b)

characterized

in that the storage signal generator (5) is designed to set an amplitude (ARF) and/or a frequency (f_RF) of the RF storage signal (URF).

2. Apparatus according to Claim 1 , wherein the storage signal generator (5) for setting the frequency (f_RF) and/or the amplitude (A_RF) of the RF storage signal (UR_F) has at least one full-bridge module (10) having at least four electronic components (T1 , T2, T3, T4) which are driveable by means of a respective control signal (12a-d).

3. Apparatus according to Claim 2, wherein the full-bridge module (10) has at least one series resistor (13a, 13b) for reducing the power loss of the driveable electronic components (T1 , T2, T3, T4).

4. Apparatus according to any of the preceding claims, wherein the storage signal generator (5) is designed to generate an RF storage signal (U_RF) which has a constant amplitude during at least one partial interval (ΔΤ,,

ΔΤ_ί+ι) of its period duration (T). 2

5. Apparatus according to any of the preceding claims, wherein the storage signal generator (5) for generating the RF storage signal (U_RF) has at least one power analogue amplifier (15) for amplifying an analogue control signal (Usetpoint). which preferably comprises a half-bridge module (16) having at least two electronic components which are driveable by means of the analogue control signal (12).

6. Apparatus according to any of the preceding claims, wherein the storage signal generator (5) has at least one isolated DC/DC converter (20a, 20b).

7. Apparatus according to any of the preceding claims, further comprising: a compensation device (25) for generating a compensation signal (Ucomp, lcomp) for compensating for an interference current ( ) that is generated by the electric storage field (E) at the second electrode (7a, 7b).

8. Apparatus according to Claim 7, wherein the compensation device (25) is designed to generate the compensation signal (Ucomp) on the basis of the RF storage signal (U_RF) that is coupled into the first electrode (3) by the storage signal generator (5).

9. Apparatus according to Claim 8, wherein the compensation device (25) for generating the compensation signal (U_COmp) has a voltage source (26).

10. Apparatus according to Claim 8 or 9, wherein the compensation device (25) is designed to generate the compensation signal (Ucomp) on the basis of at least one analogue control signal (U_Setpoint) that serves for driving at least one driveable electronic component (T1 , T2) of the storage signal generator (5). 3

11. Apparatus according to Claim 7, wherein the compensation device (25) is designed for generating the compensation signal (lcomp) with the aid of a current (l_RF) flowing through the first electrode (3) or with the aid of a crosstalk current portion (lu) between the first electrode (3) and a further cap electrode part (22a, 22b).

12. Apparatus according to Claim 1 1 , wherein the compensation device (25) is designed to measure the current (l_RF) through the first electrode (3) on an electrical lead (30) for coupling the RF storage signal (U_RF) into the first electrode (3).

13. Apparatus according to Claim 12, wherein the compensation device (25) has a transformer (31 ) or a device (32) for contactlessly measuring the current (IRF) in the electrical lead (30).

14. Apparatus according to Claim 7, wherein the compensation device (25) has an auxiliary capacitance (33) fitted between the first electrode (3) and the second electrode (7a, 7b) and is designed for measuring the current (l_Aux) through the auxiliary capacitance (33).

15. Apparatus according to Claim 14, wherein the compensation device (25) for measuring the current (l_Aux) through the auxiliary capacitance (33) has an operational amplifier circuit, in particular a current feedback operational amplifier (34).

16. Apparatus according to any of Claims 1 1 to 15, wherein the compensation device (25) for measuring the crosstalk current portion (lu) or the current (IA_UX) through the auxiliary capacitance (33) has a transconductance amplifier (29).

£-£-£- 4

17. Apparatus according to any of the preceding claims, further comprising: an electron beam generating device (35) for generating an electron beam (36) that can be fed to the ion trap (2) in order to generate ions (4a, 4b).

18. Apparatus according to Claim 17, wherein the electron beam generating device (35) is designed for feeding the electron beam (36) into the ion trap (2) in a pulsed manner, where the electron beam generating device (35) is preferably synchronized with the storage signal generator (5) in order to feed the electron beam (36) to the ion trap (2) only during at least one partial interval (ΔΤ,, ΔΤ,+ι) of the period duration (T) of the RF storage signal (URF) in which the RF storage signal (URF) has a constant amplitude.

19. Apparatus according to the preamble of Claim 1 , in particular according to any of the preceding claims, which is designed simultaneously to generate an excitation signal (U_stimi , U_stj_m2) for exciting ions (4a, 4b) present in the ion trap (2) and to detect an ion signal (li_0ni, ) of the ions (4a, 4b) excited in the ion trap (2).

20. Apparatus according to any of the preceding claims, wherein the detector (9) for feeding the ion signal (l_ioni , Ik>n2) is connected to the at least one second electrode (7a, 7b), and wherein the excitation device (6a, 6b) is designed to feed the excitation signal (U_stimi , U_sti_m2) for exciting the ions (4a, 4b) stored in the ion trap (2) to the first electrode (3a, 3b) or to a further electrode (22a, 22b) of the ion trap (2).

21. Apparatus according to any of the preceding claims, wherein the first

electrode is split into at least two electrode parts (3a, 3b), to each of which the RF storage signal (URF) can be fed, wherein the excitation signal (U_stimi , Ustim2) can preferably be fed to at least one of the electrode parts (3a, 3b). 5

22. Apparatus according to any of the preceding claims, wherein the excitation device (6a, 6b) is designed to generate an excitation signal (U_stimi , Ustim2) having at least one excitation frequency (f_ion, f_ion1, ...) for selectively exciting ions (4a, 4b) in the ion trap (2), which preferably corresponds to an oscillation frequency (f_ion) of the ions (4a, 4b) to be excited in the ion trap (2).

23. Apparatus according to any of the preceding claims, further comprising: an amplifier connected to the at least one second electrode (7a, 7b) for feeding an amplified ion signal (U_ioni , U_ion2) to the detector (9), preferably an in particular frequency-dependent charge amplifier (8a, 8b).

24. Apparatus according to Claim 23, further comprising: a comparison device (41 ) for comparing the at least one excitation frequency (f_i0m fion,i _> ·■■) of the excitation signal (U_stim,i , U_stim,2) with the amplified ion signal (Ui_oni , U|_0n2)-

25. Apparatus according to Claim 24, wherein the comparison device (41 ) is designed for mixing at least one reference signal (42) containing the at least one excitation frequency (f_i0n, fioni _> ■■■ , fionNi) with the amplified ion signal

(Uioni > Uion2)-

26. Apparatus according to Claim 25, further comprising: a filter device (43) for filtering the reference signal (42) prior to feeding it to the comparison device

(41) , in particular a phase-locked loop (43) for generating a reference signal

(42) that is phase-synchronous with the amplified ion signal (U_i0ni , U_ion2)-

27. Apparatus according to either of Claims 25 and 26, wherein the detector (9) has a low-pass filter (48) for filtering an output signal (47) supplied by the comparison device (41 ).

28. Apparatus according to any of Claims 22 to 27, which has a control device (40) for selectively storing or removing the ions (4a, 4b) excited by means of

£-£-£- 6 the excitation signal (U_stimi , U_stim2) in the ion trap (2), wherein the control device (40) preferably comprises the amplifier, in particular the frequency- dependent charge amplifier (8a, 8b), and also the comparison device (41 ).

29. Method for mass-selective detection of ions (4a, 4b) in an ion trap (2), in particular an ion trap (2) in an apparatus (1 ) according to any of the preceding claims, comprising:

generating an excitation signal (U_stimi , U_stim2) having at least one excitation frequency (fi_0n, ioni , ··) for exciting ions (4a, 4b) in the ion trap (2), and also simultaneously detecting an ion signal (l_i0ni , generated upon the excitation of the ions (4a, 4b) in the ion trap (2), wherein detecting preferably comprises comparing the preferably amplified ion signal (U_i0ni , U,on2) with a reference signal (42) containing at least one excitation frequency (fj_0n, fiom , ..) of the excitation signal (U_stimi , U_stim2).

30. Method according to Claim 29, wherein a control of an amplitude (Up) and/or of a pulse duration (T_P) of the excitation signal (U_ioni , U_ion2) is effected for storing the excited ions (4a, 4b) in the ion trap (2) and/or for removing the excited ions (4a, 4b) from the ion trap (2), wherein the control comprises a preferably frequency-dependent amplification of the ion signal (l_j0ni . lion2) in order to form an amplified ion signal (U_ioni , U_i0n2), and also a comparison of the amplified ion signal (U_i0ni , Uion2) with a reference signal (42) containing at least one excitation frequency (f_ion, fioni , ··) of the excitation signal (U_stimi ,

U_stim2)-