Detection of positive and negative ions

An ion detector comprises an ion guide with electrodes arranged about a first axis; a positive ion detection device with an ion inlet at a first side of the ion output section offset from and at an angle to the first axis; and a negative ion detection device with an ion inlet at a second side opposite the first side, offset from and at an angle to the first axis. A negative voltage bias applied to the positive ion device accelerates positive ions toward the inlet along a path including a component along a second axis orthogonal to the first axis. A positive voltage bias applied to the negative ion detection device accelerates negative ions toward the inlet along a path that includes a component along the second axis orthogonal to the first axis in a direction generally opposite to the path of the positive ions.

FIELD OF THE INVENTION

The present invention relates generally to the detection of ions which finds use, for example, in fields of analytical chemistry such as mass spectrometry. More particularly, the present invention relates to selectively detecting positive or negative ions, including sequentially or simultaneously as desired.

BACKGROUND OF THE INVENTION

An ion detector is a type of transducer that converts ion current (ion flux, ion beam, etc.) to electrical current and thus is useful in technologies entailing the processing, transport, or manipulation of ions, such as for example mass spectrometry (MS), electronics fabrication, coating or surface treatment of articles of manufacture, etc. An ion detector is commonly employed in an MS system. Generally, an MS system converts the ionizable components of a sample material into ions and resolves (sorts, separates, or “analyzes”) the ions according to their mass-to-charge ratios, thereby producing an output of mass-discriminated ions that is transmitted to the ion detector. The information represented by the ion output received by the ion detector is thus encoded as electrical signals to enable data processing by analog and/or digital techniques. The MS system processes the resulting electrical current outputted from the ion detector as needed to produce a mass spectrum, which may entail processing/conditioning by a signal processor, storage in memory, and presentation by a readout/display means. Typically, a mass spectrum is a series of peaks indicative of the relative abundances of the detected ions as a function of mass-to-charge ratio. A trained analyst can then interpret the mass spectrum to obtain information regarding the sample material processed by the MS system.

A typical ion detector includes, as a first stage, an ion-to-electron conversion device. Ions from the mass analyzer or other type of ion source are focused toward the ion-to-electron conversion device by an appropriately applied acceleration (bias) voltage. The ion-to-electron conversion stage typically includes a surface that emits electrons in response to impingement by ions. The conversion efficiency is different for each ion mass and its energy state at the time of impact. The ion conversion stage may be followed by an electron multiplier stage. In this case, a voltage potential is impressed across the length of a containment structure of the electron multiplier. The electrical current resulting from the ion-to-electron conversion is amplified in the multiplier stage through multiplication of liberated electrons. The gain of this multiplication can be influenced by the applied voltage potential. An anode positioned at the end of the multiplier collects the multiplied flux of electrons and the resulting electrical output current is transmitted to subsequent processes. Hence, the output of an ion detector equipped with an electron multiplier is an amplified electrical current proportional to the intensity of the ion current fed to the ion detector, the ion-to-electron conversion rate, and the gain of the electron multiplier. The entrance into the electron multiplier may be biased at a fixed acceleration voltage to draw ions into the electron multiplier, as is the case of the 3×0 triple quadrapole systems available from Varian, Inc., Palo Alto, Calif. As an example, the acceleration voltage at the input of the ion detector may be ±5 kV depending on the polarity of the ions to be detected, and the gain on the signal multiplier may range up to 2 kV. This results in the output of the ion detector ranging from 3-7 kV. The output current from the ion detector can be processed as needed to yield a mass spectrum that can be displayed or printed by the readout/display means as noted above. Typically, the output current is converted to a voltage signal, digitized, and then transmitted to ground-based circuitry for further processing.

Many ion detectors are capable of detecting ions of only one polarity, that is, either positive ions or negative ions. Some ion detectors, however, have been designed to detect both positive and negative ions. Typically, the entrance into the signal multiplier is aligned on-axis with the incoming ion beam, which is disadvantageous in that neutral (uncharged) particles of no analytical value enter the ion detector and contribute to problems such as varying signal noise, reduced sensitivity, fouling, etc. Moreover, to be able to detect either positive ions or negative ions, the ion detector requires electronics that enable to polarity of the acceleration voltage to be switched. This switching requires a large voltage swing on which the gain voltage and the operating voltage of the detector's electronics ride on top. Consequently, the maximum switching speed is limited (typically 200-2000 ms) and the fast-switching circuitry required is complex and costly.

In one example of an ion detector capable of detecting either positive and negative ions, U.S. Pat. No. 4,267,448, discloses an electron multiplier inherently designed to detect positive ions. The first dynode that leads into the electron multiplier is continuously biased at −2 kV. A shutter-type acceleration electrode is positioned in front of the first dynode and can be selectively biased at either a positive or negative voltage. To detect negative ions, the acceleration electrode is biased at a positive voltage and hence operates as a conversion dynode. Negative ions impact the acceleration electrode, are converted to positive ions, and then are accelerated to the first dynode under the influence of its negative voltage bias. To detect positive ions, a high-voltage power supply connected to the acceleration electrode must be switched to a negative voltage. Another example, U.S. Pat. No. Re 33,344, similarly provides a conversion dynode in front of an electron multiplier to convert incoming negative ions to positive ions. Ion detectors such as disclosed in U.S. Pat. Nos. 4,627,448 and Re 33,344 suffer from the disadvantages noted above in that they require complex and costly switching hardware and switching between polarities causes undesirable delay. Additionally, these types of ion detectors do not adequately prevent neutral particles from entering the ion detector.

Some ion detectors have been designed to detect both positive and negative ions simultaneously. In one example, U.S. Pat. No. Re 33,344 also discloses a positively-biased conversion dynode and a negatively-biased first-stage dynode in front of a single, continuous-dynode electron multiplier. A plate is in turn positioned in front of the conversion dynode and the first-stage dynode. One aperture of the plate is aligned with the conversion dynode and another aperture of the plate is aligned with the first-stage dynode. Negative ions are attracted through the first aperture of the plate to the conversion dynode where they are converted to positive ions and subsequently flow into the electron multiplier. Positive ions are attracted through the second aperture of the plate to the first-stage dynode and subsequently flow into the remaining portion of the electron multiplier. In another example, U.S. Pat. No. 4,066,894 discloses the use of two separate ion detectors with two respective electron multipliers. The electron multipliers are arranged adjacent to each other, both in the direction of the axis of incoming ions. One ion detector is configured to detect positive ions and the other ion detector is configured to detect negative ions. Ion detectors such as disclosed in U.S. Pat. Nos. Re 33,344 and 4,066,894 also suffer from the disadvantages noted above in that they do not adequately prevent neutral particles from entering the ion detector. Moreover, they do not adequately ensure that an acceptable number of ions of a given polarity strike the corresponding first dynode and are detected.

In another example, U.S. Pat. No. 4,810,882 discloses utilizing a negatively-biased conversion electrode positioned off-axis on one side of the incoming ion flight path and a positively-biased transmission/conversion electrode positioned off-axis on the opposite side of the ion flight path. A single photomultiplier with an electron-to-photon conversion electrode is located downstream of the transmission/conversion electrode. Positive ions are deflected off-axis and strike the conversion electrode, thus releasing secondary electrons. Negative ions are deflected off-axis and strike the transmission/conversion electrode, thus releasing secondary electrons. In both cases, the secondary electrons are accelerated in the same direction through the transmission/conversion electrode toward the electron-to-photon conversion electrode of the photomultiplier. This type of ion detector is disadvantageous in that, like the other ion detectors mentioned above, the ion detector requires at least one conversion dynode. Conversion dynodes require high acceleration voltages, are prone to producing a corona discharge, and contribute to background signal noise.

Accordingly, there continues to be a need for improved ion detectors capable of detecting positive and negative ions.

SUMMARY OF THE INVENTION

To address the foregoing problems, in whole or in part, and/or other problems that may have been observed by persons skilled in the art, the present disclosure provides methods, processes, systems, apparatus, instruments, and/or devices, as described by way of example in implementations set forth below.

According to one implementation, an ion detector for selectively detecting positive and negative ions includes an ion guide, a positive ion detection device, and a negative ion detection device. The ion guide includes a plurality of electrodes arranged about a first axis and configured to apply an RF field to constrain ions to motions generally about the first axis. The positive ion detection device includes a positive ion inlet disposed at a first side of the ion output section, the positive ion inlet being offset from and at an angle to the first axis. The positive ion detection device is configured to apply a negative voltage bias and accelerate positive ions along a positive ion path directed from the ion guide into the positive ion inlet. The positive ion path includes a component directed along a second axis orthogonal to the first axis. The negative ion detection device includes a negative ion inlet disposed at a second side of the ion output section opposite the first side, the negative ion inlet being offset from and at an angle to the first axis. The ion detection device is configured to apply a positive voltage bias and accelerate negative ions along a negative ion path directed from the ion guide into the negative ion inlet. The negative ion path includes a component directed along the second axis generally opposite to the component of the positive ion path.

According to another implementation, a method is provided for selectively detecting positive and negative ions. A plurality of particles is guided in an ion guide generally along a first axis by applying an RF voltage to a plurality of electrodes of the ion guide to generate an RF field in the ion guide and constrain ions of the plurality of particles to motions focused along the first axis. A first ion detector is negatively biased and any positive ions of the plurality of particles are accelerated to flow along a positive ion path from the ion guide toward the first ion detector, the positive ion path including a component directed along a second axis orthogonal to the first axis. A second ion detector is positively biased and any negative ions of the plurality of particles are accelerated to flow along a negative ion path from the ion guide into the second ion detector, the negative ion path including a component directed along the second axis generally opposite to the component of the positive ion path.

According to various implementations of the method, either or both ion detectors may be selectively operated simultaneously or sequentially to detect positive and/or negative ions simultaneously or sequentially.

DETAILED DESCRIPTION OF THE INVENTION

The subject matter disclosed herein generally relates to the detection of ions and associated ion processing. Examples of implementations of methods and related devices, apparatus, and/or systems are described in more detail below with reference toFIGS. 1-5. These examples are described in the context of mass spectrometry (MS). However, any process that involves the detection of ions may fall within the scope of this disclosure. Additional examples include, but are not limited to, vacuum deposition and other fabrication processes such as may be employed to manufacture materials, electronic devices, optical devices, and articles of manufacture.

FIG. 1is a schematic view of an example of an ion detector (or ion detection apparatus, assembly, or system)100according to an implementation of the present disclosure. The ion detector100includes a first ion detection device or unit102and a second ion detection device or unit104. One of the ion detection devices102or104is configured to detect ions of one polarity (positive or negative) and the other ion detection device104or102is configured to detect ions of the opposite polarity (negative or positive). In the illustrated example, the first ion detection device102is a positive ion detection device and the second ion detection device104is a negative ion detection device. The positive ion detection device102generally includes a housing106and a positive ion inlet108. The negative ion detection device104likewise generally includes a housing110and a negative ion inlet112. Each housing106and110includes components and circuitry as needed to convert ions received at the respective positive ion inlet108and negative ion inlet112into electrical currents indicative of ion intensity as appreciated by persons skilled in the art.FIG. 1illustrates a detector output current114produced by the positive ion detection device102and a detector output current116produced by the negative ion detection device104. The positive ion detection device102includes a voltage source (not shown) for negatively biasing the positive ion inlet108to accelerate or attract positive ions to flow into the positive ion inlet108. The negative ion detection device104likewise includes a voltage source (not shown) for positively biasing the negative ion inlet112to accelerate or attract negative ions to flow into the negative ion inlet112. The positive ion inlet108may be biased at a voltage falling within any suitable range of negative voltage values for attracting positive ions, and the negative ion inlet112may be biased at a voltage falling within any suitable range of positive voltage values for attracting negative ions. In one non-limiting example, the positive ion inlet108may be biased at a voltage of −5 kV or thereabouts and the negative ion inlet112may be biased at a voltage +5 kV or thereabouts. In a typical implementation, these biasing voltages are fixed during operation. Each ion detection device102and104may also include a signal multiplier such as an electron multiplier for multiplying electrons as needed to produce an amplified electrical detector output current114or116representative of detected ion intensity. When provided with a signal multiplier, the electronics in each ion detection device102and104include circuitry for applying a gain voltage across the signal multiplier to control the multiplication factor as appreciated by persons skilled in the art.

FIG. 1illustrates both the positive ion detection device102and the negative ion detection device104installed in the ion detector100. In other implementations, only one of the ion detection devices102or104may be installed. That is, the ion detector100may be configured to detect positive ions only, negative ions only, or both positive and negative ions.

For illustrative purposes,FIG. 1shows two references axes, a first axis120and a second axis122, which by example may be referred to as a z-axis and a y-axis respectively.FIG. 1illustrates a flow of particles124directed generally along the first axis120. The particle flow124may include a flow of positive and/or negative ions (e.g., an ion beam) as well as neutral particles (e.g., gas molecules, liquid droplets, etc.). The ion detector100further includes an output (or ion diverting) section or region130through which the first axis120runs. As described below and illustrated inFIG. 2, the ion detector100may include a detector ion guide located in the output section130. The detector ion guide includes means for trapping or focusing ions in the output section130whereby the motions of the ions are constrained to an ion field132concentrated generally along the first axis120. The positive ion detection device102and the negative ion detection device104are spaced at a distance from each other on opposite sides of the output section130relative to the second axis122. At least the positive ion inlet108of the positive ion detection device102and the negative ion inlet112of the negative ion detection device104are each arranged in an offset or transverse, angled relation to the first axis120. In the illustrated example, the positive ion inlet108and the negative ion inlet112are each arranged about the second axis122. Hence, the positive ion inlet108and the negative ion inlet112are disposed orthogonal to the first axis120and orthogonal to the particle flow at least at a location134where the particle flow enters the output region130. As used herein, the term “orthogonal” is taken to encompass “substantially orthogonal” to account for implementations in which the positive ion inlet108and the negative ion inlet112are not oriented exactly 90 degrees relative to the first axis120.

As also illustrated inFIG. 1, an upstream ion processing device140may be located at an input side of the output section130. All or part of the upstream ion processing device140may be arranged about the first axis120. In particular, an axial outlet142of the upstream ion processing device140is located at (directly on or near) the first axis120such that a particle outlet flow132is emitted from the outlet142and into the output section130generally along the first axis120. While the upstream ion processing device140and the first axis120are illustrated as being straight or linear, it will be understood that all or part of the upstream ion processing device140may be curvilinear or include straight sections that are angled or orthogonal to the illustrated horizontal first axis120. That is, when the first axis120is considered as corresponding to the particle flow through the upstream ion processing device140, it will be understood that this part of the first axis120may likewise be straight or linear, curvilinear, or include straight sections that are angled or orthogonal to the illustrated horizontal first axis120. An example of a 180-degree curved arrangement is disclosed in U.S. Pat. No. 6,576,897, assigned to the assignee of the present disclosure. In some implementations, all or part of the upstream ion processing device140or at least its outlet142may be considered as being part of the ion detector100.

In addition to the upstream ion processing device140, a downstream ion processing device150may be located at an axial output side of the output section130. All or part of the downstream ion processing device150may be arranged about the first axis120and like the upstream ion processing device140may be linear, curved, or have sections oriented in differing directions. An axial inlet152of the downstream ion processing device150may be located at (directly on or near) the first axis120such that a particle flow is emitted from the output section130and into the inlet152generally along the first axis120. In some implementations, all or part of the downstream ion processing device150or at least its inlet152may be considered as being part of the ion detector100. Examples of ion processing devices140and150are described below.

All or part of the ion detection devices102and104(particularly the positive ion inlet108and the negative ion inlet112) and all or part of the upstream ion processing device140(if provided) and the downstream ion processing device150(if provided) may be enclosed in a suitable housing or structural enclosure160. Depending on the type of MS system or other ion processing system contemplated, the enclosure may provide an evacuated, low-pressure, or ambient pressure environment. The output region130, being located between the ion detection devices102and104, is also enclosed in the enclosure160. Accordingly, the output region130may be considered as structurally defined at least in part by the volume between the positive ion inlet108and the negative ion inlet112, with the enclosure of the output region130being completed by the schematically illustrated enclosure160.

In operation, the particle outlet flow134, which may be provided from an upstream device or ion source140as noted elsewhere in this disclosure, enters the output section130generally along the first axis120. The particle outlet flow134may include positive ions, negative ions and/or neutral particles. The detector ion guide in the output section130is operated to focus the ions along the first axis120as generally depicted by the focused ion beam132. If the positive ion detection device102is installed and activated, then any positive ions in the particle flow132are accelerated toward the positive ion inlet108under the influence of the negative bias voltage applied to the positive ion inlet108. The positive ion detection device102converts received positive ions into electrical current and outputs this signal over the detector output line114. If the negative ion detection device104is installed and activated, then any negative ions in the particle outlet flow132are accelerated toward the negative ion inlet112under the influence of the positive bias voltage applied to the negative ion inlet112. The negative ion detection device104converts received negative ions into electrical current and outputs this signal over the detector output line116. Signals over the detector output lines114and/or116are then processed as desired to derive useful information regarding the positive and/or negative ions detected.

Due to the off-axis orientation of the positive ion detection device102, positive ions of the ion beam132are diverted from the first axis120and follow a positive ion path generally depicted by way of example by an arrow166inFIG. 1. Similarly, due to the off-axis orientation of the negative ion detection device104, negative ions of the ion beam132are diverted from the first axis120and follow a negative ion path generally depicted by way of example by another arrow168inFIG. 1generally having an orientation opposite to that of the positive ion path166. Here, the schematic nature ofFIG. 1should be emphasized, as no specific limitation is intended for the precise trajectories of the positive ion path166and the negative ion path168. Generally, the positive ion path166deviates from the first axis120, runs to a surface of the positive ion inlet108, and includes a component in a direction of the second axis122orthogonal to the first axis120. Similarly, the negative ion path168deviates from the first axis120, runs to a surface of the negative ion inlet112, and includes a component in the direction of the second axis122opposite to the direction of the second-axis component of the positive ion path166. The trajectory of each ion path166and168may range from being somewhat linear but angled relative to the first axis120and second axis122, or curved according to some radius of curvature (which may vary along the ion path166or168), or substantially orthogonal to the first axis120in the nature of a 90-degree turn relative to the first axis120, or may include a combination of two or more of the foregoing types of trajectories. The precise shape of each ion path166and168and the point along the first axis120at which the ion path166or168begins to diverge from the first axis120may depend on a variety of factors, such as, for example, the mass-to-charge ratio of the ions, the strength of the voltage bias, the time at which the voltage bias is applied relative to the time at which the ions enter the output section130, whether both ion detection devices102and104are operating such that both the positive and negative voltage biases may affect the motion of positive and negative ions, the shape of the surface(s) associated with the ion inlets108and112, the positions of the ion inlets108and112relative to the first axis120or to the second axis122, the presence or absence of an ion focusing or trapping field in the output section130and the operating parameters (voltage amplitude, frequency, RF-only or RF/DC) of that field, etc.

The arrangement of opposing dual ion detection devices102and104orthogonal or substantially orthogonal to the first axis120may provide a number of advantages, including the following. First, the use of two separate ion detection devices102and104for individual ion polarities eliminates the complexity and cost of components and circuitry conventionally required when employing a single detection unit to detect either positive or negative ions. Examples of such complexity and/or cost include the electronics associated with switching the polarity of the acceleration (bias) voltage, the large voltage swings involved with switching, the delay occurring with such switching, and the need for fast switching circuitry to minimize the delay. Second, only one type (positive or negative) of ion detection device102or104needs to be installed if desired, thus offering a low-cost ion detection solution that requires only one +5 kV or −5 kV power supply. Third, the arrangement eliminates the need for providing the ion detector100with conversion dynodes that convert the polarity of an impinging ion to the opposite polarity. Elimination of conversion dynodes allows for lower acceleration voltages, thereby reducing background noise and the risk of a corona discharge. Fourth, the arrangement is able to detect small negative ions very efficiently, which conventionally has been difficult to do. Fifth, uncharged (neutral) particles flowing through the output section130are unaffected by the off-axis ion detection devices102and104, even when only one of the ion detection devices102or104is installed or being utilized. Because the ion detection devices102and104are offset by a distance and an angle from the first axis120, the flow of uncharged particles is completely unimpeded. Uncharged particles continue to fly straight through the output section130generally along the first axis120as generally depicted by an arrow172, and thus do not produce any signal, thereby eliminating or at least significantly reducing noise attributed to uncharged particles.

Sixth, if the power supply to the ion detection devices102and104is turned off, the detector ion guide in the output section130can still be operated to focus the ions. The detector ion guide facilitates passing these ions to the downstream ion processing device150, which may be another MS system.

Seventh, due to the provision and orientation of the two ion detection devices102and104, the operation of both ion detection devices102and104simultaneously can be utilized to facilitate the detection of either positive or negative ions. This is because while one ion detection device102or104may function to attract ions of a given polarity the other ion detection device104or102may function to repel the same ions. Positive ions may be accelerated toward the positive ion inlet108of the positive ion detection device102under the “pulling” influence of the negative bias voltage applied to the positive ion inlet108and, additionally, under the “pushing” influence of the positive bias voltage applied to the negative ion inlet112of the negative ion detection device104. Likewise, negative ions may be accelerated toward the negative ion inlet112of the negative ion detection device104under the “pulling” influence of the positive bias voltage applied to the negative ion inlet112and, additionally, under the “pushing” influence of the negative bias voltage applied to the positive ion inlet108of the positive ion detection device102.

Eighth, the arrangement enables a variety of different operational modes for the ion detector100. For instance, the particle flow may include both positive and negative ions. The ion detector100may be operated to detect positive ions only, negative ions only, both positive and negative ions simultaneously, or positive and negative ions sequentially. In another example, depending upon the configuration and operation of the upstream ion processing device140, which may include a combination of two or more different types of ion processing devices, the particle flow may consist of time-sequenced groups or packets of positive and/or negative ions. The two ion detection devices102and104may be operated simultaneously or sequentially to detect ions of a selected polarity from each incoming packet.

As previously noted, the detection ion guide in the output section130between the two ion detection devices102and104may be configured to generate a two-dimensional RF ion trapping or focusing field that imparts a restoring force on the ions toward the first axis120. The focusing field may be utilized for a variety of purposes, including controlling ion paths prior to detection or downstream processing. In the case of ion detection, the biasing voltage of the ion detection device102or104must be strong enough to impart enough energy to ions of a given polarity to enable those ions to overcome the restoring force of the RF field.

FIG. 1also illustrates an example of an ion processing system180in which the ion detector100may be implemented if desired. The ion processing system180may, for example, be a mass spectrometry (MS) system (or apparatus, device, etc.) configured to perform a desired MS technique (e.g., single-stage MS, tandem MS or MS/MS, MSn, etc.). The ion processing system180may include a sample introduction device, which inFIG. 1is schematically depicted as a sample input line182, and an ion source or ionization device184. The sample introduction device182introduces a sample material to be ionized into the ion source184. In “hyphenated” techniques, the sample input line182may be the output of an analytical separation instrument such as employed for chromatography, electrophoresis, solid-phase extraction, or other techniques. The ion source184is then operated to ionize the sample according to any ionization technique and may be configured to produce an output particle stream124of positive and/or negative ions as well as neutral species. The particle flow124resulting from the ion source184may be transmitted directly into the output region130of the ion detector100, in which case the depicted particle stream portions124and132may be one and the same and the particle exit of the ion source184corresponds to the outlet142leading into the output section130of the ion detector100. Alternatively, the particle stream124may first be directed into the afore-mentioned upstream ion processing device140.

The illustrated upstream ion processing device140may represent a single type of ion processing device configured to perform one or a few primary ion processing functions such as mass filtering, ion guiding or focusing, etc. Alternatively, the illustrated upstream ion processing device140may represent a combination of different types of ion processing modules configured to perform a variety of ion processing operations, as indicated schematically by partition lines186inFIG. 1. Examples of ion processing devices or modules include, but are not limited to, an ionizing device (in a case where the external atmospheric-pressure ionization device184is not employed), an ion storage or trapping device including the type applying an RF (or RF/DC) trapping field, a mass-sorting or mass-analyzing device for mass-discrimination of ions, an ion fragmenting device such as a collision cell or ion trap, ion optics such as one or more grids, lenses or apertured plates, etc.

The illustrated downstream ion processing device150may likewise represent a single type of ion processing device or a combination of different types of ion processing modules. Examples of ion processing devices or modules include, but are not limited to, a particle collection device, an ion storage or trapping device including the type applying an RF (or RF/DC) trapping field, a mass-sorting or mass-analyzing device for mass-discrimination of ions, an ion fragmenting device such as a collision cell or ion trap, ion optics such as one or more grids, lenses or apertured plates, a vent to an ambient environment, etc.

In an example of tandem MS that utilizes both an upstream ion processing device140and a downstream ion processing device150, the upstream ion processing device140may perform mass analyzing operations on precursor (parent) ions. The downstream ion processing device150may then perform fragmentation of precursor ions to produce product (daughter) ions and then mass-analyze the product ions. In this regard, it will be appreciated that the ion processing system180may include another ion detector downstream of the downstream ion processing device150, which may structured similarly to the illustrated ion detector100. More generally, the ion processing system180may include any number of ion detectors100and ion processing devices140or150. It will also be understood that the ion detector100need not include any downstream ion processing device150. Both undetected charged particles as well as neutral particles may simply flow through the output section130generally along the first axis120to an environment external to the ion detector100.

The particle stream124resulting from operation of the ion source184or the particle stream resulting from operation of the upstream ion processing device140is flowed into the output section130of the ion detector100where the ions are focused as an ion beam132by the detector ion guide. As described above, one or both of the positive ion detection device102and negative ion detection device104are selectively operated to detect positive and/or negative ions as desired. To accomplish this, the ion detector100creates the off-axis positive ion path166and/or off-axis negative ion path168as described above. As a result, the positive ion detection device102produces a detector output signal that may be transmitted over lines114and188to a system controller190, which in some implementations may be referred to as MS electronics. The negative ion detection device104likewise produces a detector output signal that may be transmitted over the line116to the system controller190.

The system controller190may include, for example, signal processing and/or detector control devices or circuitry, a data acquisition device or circuitry, etc. The system controller190may include a main computer that includes a terminal, console or the like for enabling interface with an operator of the ion processing system180, and/or one or more modules or units that have dedicated functions such as instrument control and data acquisition and processing. In addition to performing signal processing and conditioning and data acquisition, the system controller190may be configured to control the operations of the ion detector100such as, for example, the timing and application of the acceleration voltages at the positive ion inlet108and negative ion inlet112, the monitoring of the ion signal received at the positive ion inlet108and negative ion inlet112, the control and adjustment of gain voltages applied to respective signal multipliers of the ion detection devices102and104, the application and control of an ion focusing field in the output section130, etc. However, at least some of the foregoing ion detector control operations may be performed directly by electronics provided with the ion detection device102or104itself. In addition, the system controller190may represent an electronic controller configured to control the operations of other components of the ion processing system180such as, for example, the sample introduction system182, the ion source184, and the ion processing devices140and150. The system controller190may transmit signals over a data line192to a readout or display device194configured to produce information196pertaining to the detected ions such as a mass spectrum.

FIG. 2illustrates an example of an ion detector200and an ion processing device240supplying ions to the ion detector200. A mutually orthogonal first axis220and second axis224are again shown for reference purposes. The ion detector200includes a positive ion detection device202and a negative ion detection device204arranged on opposing sides of an output section230generally about the second axis222normal (or substantially normal) to the first axis220of incoming particle flow as described above. The ion processing device240in this example includes three multipole (e.g., quadrupole) sections: an RF-only pre-filter241, a mass filter243, and an RF-only detector ion guide245. As appreciated by persons skilled in the art, such multipole sections include a plurality of electrodes (or rods) elongated along the first axis220of a main or resultant particle flow and spaced from each other about the first axis220(and usually all parallel to the first axis220). The RF-only pre-filter241is configured to apply a controlled RF field between its electrodes to focus ions along the first axis220in preparation for mass filtering. The mass filter243is configured to apply a controlled RF field and typically also a DC field between its electrodes to separate ions based on mass-to-charge ratio in accordance with well-known principles. The detector ion guide245is configured to apply a controlled RF field between its electrodes to focus the ions received from the mass filter243along the first axis220in preparation for ion detection or further downstream processing (not shown). For these purposes, appropriate voltage sources are provided, as schematically depicted by an RF voltage source251communicating with the RF-only pre-filter241, an RF voltage source253and a DC voltage source254communicating with the mass filter243, and an RF voltage source255communicating with the detector ion guide245. The electrodes of the detector ion guide245extend into the output section230of the ion detector200. Thus, the output section230may generally be considered as including all or a portion of the detector ion guide245, and an end257of the mass filter243leading into the detector ion guide245may generally be considered as the inlet into the output section230. The detector ion guide245generally includes an axial inlet247communicating with the mass filter243for receiving ions and an axial outlet249communicating with an MS device or other downstream device or environment for discharging from the detector ion guide245neutral particles and ions not deflected by the ion detection devices202and204.

The ion detector200includes means for switching the ion detector200between an ion detecting mode and a non-detecting mode. In the ion detecting mode, the ion detection devices202and/or204are active such that positive and/or negative ions are diverted along positive and/or negative ion paths for detection as described above. In the non-detecting mode, all species of the particle stream flow through the detector ion guide245generally along the first axis220and through its exit249, without being deflected off-axis. The switching means may include the power supplies and associated circuitry (seeFIG. 3and description below) that apply the ion-accelerating voltage biases described above, and may be controlled by a suitable controller such as the system controller190schematically represented inFIG. 1.

FIG. 3illustrates an example of an ion detection device302. Two such ion detection devices302may be utilized as a positive ion detector and a negative ion detector in an off-axis ion detector as described above. The ion detection device302includes a housing or body304, which may be constructed from a suitable electrically insulative material such as, for example, epoxy resin. An O-ring or gasket306may be provided on the housing304for creating a vacuum seal. The ion detection device302further includes an electronics board308protected within the housing304. The ion detection device302further includes a signal multiplier, which in the present example is provided as an electron multiplier (EM)310. The EM310may extend from the housing304to an ion inlet312of the ion detector302. The EM310may include a tapered or funnel-shaped inlet section314that opens at the ion inlet312and transitions to a narrower-bore tube section316, which in turn terminates at an anode318in signal communication with circuitry of the electronics board308. The inner surface of the inlet section314of the EM310may be biased at an acceleration voltage of desired magnitude and polarity via a contact pin322communicating with a high-voltage power supply324(for example, +5 kV for a negative ion detector, −5 kV for a positive ion detector) provided with the electronics board308. In this example, a grounded outer shield332surrounds the inlet section314.

As appreciated by persons skilled in the art, the EM310converts the ion signal received at the ion inlet312into an electrical signal (current) indicative of and proportional to the intensity of the received ion signal, and amplifies the current signal pursuant to a controlled gain. Here, the intensity of the ion signal may be given in ion counts per second, and the resulting output electrical signal may be given in Coulombs per second (amperes, or A). The circuitry of the electronics board308may include an EM voltage driver such as a DC amplifier that provides a gain voltage across the length of the EM310and thereby determines the overall gain of the EM310. In one example, the output (or gain) voltage of the EM voltage driver may be varied from about 600 V to about 2000 V. The circuitry of the electronics board308may include signal processing functionality for collecting data. In one example, the circuitry includes an electrometer (including, for example, a current-to-voltage amplifier) or other component configured to convert the current signal transmitted from the anode318to a voltage signal and an analog-to-digital converter to digitize the voltage signal. The circuitry may also include components for scaling and filtering the collected data in preparation for further processing. The circuitry may also include components for calibration and for controlling/adjusting/optimizing the gain on the EM310. The circuitry may include an analog and/or digital controller for controlling the various operations and functions of the circuitry and other components of ion detection device302.

FIG. 4is a cross-sectional elevation end view, taken in a plane coincident with the second axis224shown inFIG. 2, of an example of an ion detector400. A mutually orthogonal first axis420and second axis422are again shown for reference purposes. The ion detector400includes a positive ion detection device402and a negative ion detection device404arranged on opposing sides of an output section430. The positive ion detection device402includes a positive ion inlet408and the negative ion detection device404includes a negative ion inlet412. In this example, the positive ion inlet408and the negative ion inlet412are oriented about the second axis422, i.e., the axis normal to the axis420of incoming particle flow. The ion detection devices402and404may include respective EMs414and416or other types of signal multipliers and may otherwise be configured as described above and illustrated inFIG. 3. A negative bias voltage applied to the positive ion inlet408establishes a positive ion path having a directional or vector component466along (or parallel to) the second axis422. A positive bias voltage applied to the negative ion inlet412establishes a negative ion path having a directional or vector component468along (or parallel to) the second axis422in the direction opposite to the positive ion path. In this example, the output section430includes a set445of four electrodes configured to generate an RF-only ion trapping or focusing field, as described above in the context of an RF-only post-filter extending from an upstream ion processing device. The positive ion path runs between the two upper electrodes and the negative ion path runs between the two lower electrodes. In one implementation, each electrode of the electrode set445is semicircular in cross-sectional shape and may be either hollow as shown or solid. By this configuration, the electrode set445takes up less space in the outlet section430and thus the spacing between the positive ion inlet408and the negative ion inlet412can be reduced. Alternatively, the cross-sections of the electrodes may be truncated in some other suitable manner to achieve the same purpose.

FIG. 5is a cross-sectional elevation view of an ion processing system580that includes an ion detector500. The ion detector500includes a detector ion guide545with an electrode set and an offset ion detection device502for detecting positive or negative ions as described above. The ion detection device502includes an ion inlet508arranged about an axis orthogonal to or at some other angle to the longitudinal axis of the electrode set of the detector ion guide545, as also described above. Another offset ion detection device (not shown) may also be provided for detecting ions of opposite polarity as described above.FIG. 5further illustrates an ion trajectory or flow path525through the ion processing system500. The ion trajectory525was calculated by the software tool SIMION™ developed at the Idaho National Engineering and Environmental Laboratory, Idaho Falls, Id. In this example, the ion trajectory525was calculated for low-mass ions (18 amu) at certain non-optimal operating conditions of the ion detector500(e.g., the parameters of the RF voltage applied to the electrode set, the bias voltage applied to the ion detection device502, etc.). As illustrated, the majority of the ions is deflected too early and strike the inactive part of the ion detection device502, and therefore are not collected for detection. This problem may occur when the high-voltage bias field from the side of the ion detection device502penetrates between the electrodes of the detector ion guide545and deflects the ions. The problem may be ameliorated somewhat by applying a higher RF voltage to the electrodes, but less than 100% detection may still result.

FIG. 6is a cross-sectional elevation view of the same ion processing system500. In this example, the ion trajectory625was calculated for high-mass ions (1036 amu). As illustrated, the majority of the ions are not deflected enough to reach the ion detection device502and therefore are not collected for detection. This problem may occur when the RF voltage applied to the electrodes is so high that the RF voltage in effect annihilates the effect of the high-voltage field penetration of the ion detection device502.

FIG. 7is a cross-sectional elevation view of an example of an ion processing system700that addresses the problem described above associated with the detection of low-mass ions. In this implementation, an electrically conductive ion shield755is positioned generally between the electrodes of the detection ion guide745and the ion inlet708of the offset ion detection device702. Although not specifically shown, another offset ion detection device may be provided to detect ions of opposite polarity as described above, in which case another shield may be positioned generally between the electrodes and the ion inlet of this second ion detection device. The shield755may be positioned, and may have any suitable shape or configuration, so as to shield ions from impinging on the inactive portions of the ion detection device702. For this purpose in the illustrated example, the shield755is plate-shaped and has an opening757generally surrounding the ion inlet708of the ion detection device702. The opening757may be arranged concentrically with the ion inlet708and about the second axis.FIG. 7also illustrates a simulated ion trajectory725for ions having a mass of 18 amu, with the ion path passing through the opening757of the shield755and into the ion detection device702. In comparison withFIG. 6, it can be seen that when the shield755is provided, low-mass ions, in response to the bias voltage applied to the ion detection device702, are tightly focused on the center of the ion inlet708of the ion detection device702and thereby greatly improve detection efficiency.

FIG. 8, as an example, is a bottom plan view of a shield855looking toward the ion inlet of an ion detection device802from the perspective of the electrodes of the detector ion guide. The shield855may have a single-piece construction with an opening857having a fully closed-boundary geometry completely surrounding the ion inlet. Alternatively, as illustrated inFIG. 9, the shield955may be a multi-piece construction with small gaps between the parts of the shield955and between the edges or boundaries defining the opening957that surrounds the ion inlet of the ion detection device902. As a further alternative, a conductive shield such as the shield855or955may not need to completely surround the ion inlet of the ion detection device802or902. For instance, it may be sufficient that the shield cover the area that is generally axially between the ion inlet and the mass filter (or other ion processing device upstream of the ion detector) to prevent early-deflected ions from striking the inactive part of the ion detection device outside of the ion inlet.

FIG. 10is a perspective view of an example of an ion processing system1080that addresses the problem described above associated with the detection of high-mass ions. The ion processing system1080may include one or more upstream ion processing devices1040and an ion detector1000as described above. The ion detector1000may include a detector ion guide1045and one or more off-set ion detection devices1002and1004as described above. In this implementation, the electrode set of the detector ion guide1045includes a pair of electrodes1061and1063spaced apart from each other and located proximate to one ion detection device1002, and another pair of electrodes1067and1069spaced apart from each other and from the first pair of electrodes1061and1063. If a second ion detection device1004is provided as illustrated in this example, the second pair of electrodes1067and1069is located proximate to the second ion detection device1004, similar to the configuration illustrated inFIG. 4. As best shown inFIG. 11, each electrode1061,1063,1067,1069has a respective cut-out section1171,1173,1177,1179. For the first pair of electrodes1061and1063, the respective cut-out sections1171and1173face each other and are oppositely disposed relative to the axis of the ion detection device1002. For the second pair of electrodes1067and1069, the respective cut-out sections1177and1179face each other and are oppositely disposed relative to the axis of the other ion detection device1004if provided. Accordingly, each corresponding pair of cut-out sections1171,1173and1177,1179is aligned about the axis of the ion detection device(s)1002and1004and thus is aligned with the ion inlet(s)1008and1012. By this configuration, each pair of cut-out sections1171,1173and1177,1179forms a respective electrode hole1257of the electrode set as best shown inFIG. 12, which is a bottom plan of one pair of electrodes1061and1063looking toward the electrode set from the perspective of the ion inlet1008of the ion detection device1002.

Referring again toFIGS. 11 and 12and additionally to the end view of the electrode set illustrated inFIG. 13, in this implementation conductive ion shields1355and1365may be provided as structures integrated with the electrode set. In the illustrated example, the shield1355extends from the electrode1061generally toward the electrode1063and spans the space between this pair of electrodes1061and1063. Likewise, the shield1365extends from the electrode1069generally toward the electrode1067and spans the space between this pair of electrodes1067and1069. In the specifically illustrated example, the shields1355and1365cover the spaces upstream of the electrode holes1257(FIG. 12) and corresponding ion inlets. However, in other implementations additional shield structures may be provided on other sides of the electrode holes or may completely surround the electrode holes, similar to the examples illustrated inFIGS. 7-9. The shields1355and1365do not adversely affect the two-dimensional RF field applied by the electrode set, and may reduce RF field faults.

FIG. 14is a cross-sectional elevation view of the ion processing system1480similar to that illustrated inFIG. 10.FIG. 14illustrates a simulated ion trajectory1425for ions having a mass of 3000 amu, with the ion path passing through the electrode hole defined by the cut-outs (FIGS. 10-12) of the pair of electrodes proximal to the detector ion guide1445and into the corresponding ion detection device1402. In comparison withFIG. 7, it can be seen that when the electrode hole is provided, high-mass ions, in response to the bias voltage applied to the ion detection device1402, are tightly focused on the center of the ion inlet1408of the ion detection device1402and thereby greatly improve detection efficiency. The electrode hole greatly increases the penetration of the electrical field established by the bias voltage applied to the ion detection device1002.FIG. 14also illustrates that plate-type shield(s)1455may be provided in combination with the electrode hole(s), in which case the deflected ions pass through an opening1457of the shield1455as well as the electrode hole. That is, the deflected ions pass around or adjacently to the structure of the shield1455. Alternatively, shields that are integrated with the electrodes as shown inFIGS. 11-13may be provided. Ion simulations have demonstrated that the off-axis detectors taught in the present disclosure work well for ions in the mass range of 10-3000 amu. It is believed that these off-axis detectors will also work well for even higher ion masses.

It will be understood that the methods and apparatus described in the present disclosure may be implemented in an ion processing system such as an MS system as generally described above by way of example. The present subject matter, however, is not limited to the specific ion processing systems illustrated herein or to the specific arrangement of circuitry and components illustrated herein. Moreover, the present subject matter is not limited to MS-based applications, as previously noted.

In general, terms such as “communicate” and “in . . . communication with” (for example, a first component “communicates with” or “is in communication with” a second component) are used herein to indicate a structural, functional, mechanical, electrical, signal, optical, magnetic, electromagnetic, ionic or fluidic relationship between two or more components or elements. As such, the fact that one component is said to communicate with a second component is not intended to exclude the possibility that additional components may be present between, and/or operatively associated or engaged with, the first and second components.