阅读说明：本技术 改进的带电粒子检测器 (Improved charged particle detector ) 是由 W.谢尔斯 R.斯特雷索 K.宏特 于 2018-06-01 设计创作，主要内容包括：本发明总体上涉及科学分析设备的部件，并且涉及分析设备的完整条目。更具体地，本发明涉及在质谱应用中用于检测离子的装置和方法。该装置可以包括具有高灵敏度部段和低灵敏度部段的电子倍增器，或者电子倍增器与单独供电的转换倍增电极(特别是高能转换倍增电极)的组合，或者物理地结合在电子倍增器内或周围的转换倍增电极的组合。(The present invention relates generally to components of scientific analysis equipment and to complete items of analysis equipment. More particularly, the present invention relates to apparatus and methods for detecting ions in mass spectrometry applications. The device may comprise an electron multiplier having a high sensitivity section and a low sensitivity section, or a combination of an electron multiplier and a separately powered conversion dynode, in particular a high energy conversion dynode, or a combination of conversion dynodes physically incorporated in or around the electron multiplier.)

1. An apparatus for detecting charged particles, the apparatus comprising:

a conversion dynode configured to emit one or more secondary electrons or ions upon impact by a particle,

an electron multiplier configured to form an amplified electron signal output from the one or more secondary electrons or ions emitted by the conversion dynode.

2. The apparatus of claim 1, wherein the electron multiplier has a relatively low sensitivity section and a relatively high sensitivity section.

3. The apparatus of claim 1 or claim 2, wherein the conversion dynode is separately powered from the electron multiplier and/or is not electrically coupled to a dynode of the electron multiplier.

4. The apparatus of any of claims 1-3, wherein the conversion dynode is a high energy conversion dynode.

5. The apparatus of any of claims 1-4, wherein the conversion dynode is physically incorporated within or around the electron multiplier.

6. The apparatus of any one of claims 1 to 5, wherein the electronic signal output of the relatively high sensitivity section is a relatively high gain electronic signal output compared to the electronic signal output of the relatively low sensitivity section.

7. The apparatus of any one of claims 1 to 6, wherein the relatively low sensitivity section is an analog section and the relatively high sensitivity section is a digital section configured to output a series of pulse heights.

8. The apparatus of claim 7, wherein the digital section output is configured to be usable as an input in an electronic counting circuit.

9. The apparatus of any one of claims 1 to 8, wherein the relatively high sensitivity section and the relatively low sensitivity section each comprise one or more discrete dynodes, wherein the electron multiplier is configured such that the relatively low sensitivity section provides a relatively low gain electronic signal output and the relatively high sensitivity section provides a relatively high gain electronic signal output.

10. The apparatus of any one of claims 1 to 9, wherein the relatively high sensitivity section and/or the relatively low sensitivity section is configured to operate at an output current of at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, or 100 μ Α.

11. The apparatus of any of claims 1-10, wherein the conversion dynode has an applied voltage of greater than about +1, +2, +3, +4, +5, +6, +7, +8, +9, +10, +15, or +20kV or less than about-1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -15, or-20 kV.

12. The apparatus of any one of claims 1 to 11, wherein the voltage applied to the conversion dynode is decoupled from the voltage applied to the relatively low sensitivity section of the electron multiplier.

13. The apparatus of any one of claims 1 to 12, wherein the relatively low sensitivity section comprises at least about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16 discrete dynodes.

14. The apparatus of any one of claims 1 to 13, wherein the relatively high sensitivity section comprises at least about 10, 11, 12, 13, 14, 15, 16, 17, or 18 discrete dynodes.

15. The apparatus of any one of claims 1 to 14, wherein the conversion dynode is integral with, immediately adjacent to, or within the structure of the electron multiplier.

16. The apparatus of claim 15, in which the bias applied to the conversion dynode is lower than that applied with the same apparatus, even if a conversion dynode is provided distal to the same apparatus, this lower voltage causes any deleterious effects normally associated with bringing a conversion dynode close to the electron multiplier to be reduced or eliminated.

17. The apparatus of claim 15 or claim 16, wherein the bias voltage applied to the conversion dynode is less than about 5, 4, 3, 2, or 1 kV.

18. A mass spectrometry instrument comprising the apparatus of any one of claims 1 to 17.

19. The mass spectrometry instrument of claim 18, configured to detect target particles present at a concentration of less than about 0.1ppb, 0.01ppb, 1ppt, 0.1ppt, 0.01ppt, or 1 ppq.

20. A mass spectrometry instrument according to claim 18 or claim 19, configured to perform inductively coupled plasma mass spectrometry.

21. A method of performing mass spectrometry analysis on a sample, the method comprising the steps of: introducing a sample for analysis into the mass spectrometry instrument of any one of claims 18 to 20; and operating the instrument to provide one or more electronic signal outputs.

22. The method of claim 21, wherein the mass spectrometry is capable of detecting target particles present at a concentration of less than about 0.1ppb, 0.01ppb, 1ppt, 0.1ppt, 0.01ppt, or 1 ppq.

23. The method of claim 21 or claim 22, wherein the mass spectrometry is inductively coupled plasma mass spectrometry.

Technical Field

The present invention relates generally to components of scientific analysis equipment and to complete items of analysis equipment. More particularly, but not exclusively, the invention relates to apparatus and methods for detecting ions in mass spectrometry applications.

Background

In many scientific applications, amplifying electronic signals is necessary. For example, in a mass spectrometer, an analyte is ionized to form a series of charged particles (ions). The resulting ions are then separated according to their mass-to-charge ratio, typically by acceleration and exposure to an electric or magnetic field. The separated signal ions strike the ion detector surface to generate one or more secondary electrons. The results are shown as spectra of the relative abundance of detected ions as a function of mass-to-charge ratio.

In other applications, the particles to be detected may not be ions, and may be neutral atoms, neutral molecules, electrons, or photons. In any case, a detector surface is still provided, which particles hit.

Secondary electrons generated by the impact of the input particles on the impact surface of the detector are typically amplified by an electron multiplier. Electron multipliers typically operate by secondary electron emission, whereby the impact of a single or multiple particles on the multiplier impact surface causes a single or (preferably) multiple electrons associated with the atoms impacting the surface to be released.

For some applications, a particle detector with a very high sensitivity level is required in order to allow detection of individual ions in other species. For example, inductively coupled plasma mass spectrometry (ICP-MS) converts the atoms being analyzed into ions (at the ICP source). The ions so formed are then separated and detected by a mass spectrometer. ICP-MS typically requires the use of a specialized electron multiplier to handle extremely wide dynamic range outputs. In the prior art, a series of multipliers are known which can process very high level signals generated by a plurality of ions while still being able to detect very low signals generated by the impact of a single ion.

Regardless of the sensitivity level, there remains a need in the art for further improvements in dynamic range. To the best of the applicant's knowledge, since the 90 s of the 20 th century introduced these instruments, their dynamic range did not substantially improve.

Improvements in detection efficiency, response linearity, gain stability, differential drift, and lifetime are also generally desired in the art.

It is also desirable in the art to simplify the construction of mass spectrometry instruments and also to facilitate maintenance and replacement of any conversion surfaces and/or electron emission surfaces.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Disclosure of Invention

In a first, but not necessarily broadest, aspect of the invention there is provided apparatus for detecting charged particles, the apparatus comprising: a conversion dynode configured to emit one or more secondary electrons or ions upon impact by a particle; an electron multiplier configured to form an amplified electron signal output from the one or more secondary electrons or ions emitted by the conversion dynode.

In an embodiment of the first aspect, the electron multiplier has a relatively low sensitivity section (section) and a relatively high sensitivity section.

In an embodiment of the first aspect, the conversion dynode is powered separately from the electron multiplier.

In an embodiment of the first aspect, the conversion dynode is a high energy conversion dynode.

In one embodiment of the first aspect, the conversion dynode is physically incorporated within or around the electron multiplier.

In a second aspect, the present invention provides an apparatus for detecting charged particles, the apparatus comprising: a conversion dynode configured to emit one or more secondary electrons or ions upon impact by a particle; an electron multiplier configured to form an amplified electron signal output from one or more secondary electrons or ions emitted by a conversion dynode, wherein the conversion dynode is physically incorporated within or around the electron multiplier.

In one embodiment of the second aspect, the conversion dynode is separately powered from the electron multiplier and/or is not electrically coupled to the dynode of the electron multiplier.

In one embodiment of the second aspect, wherein the electron multiplier has a relatively low sensitivity section and a relatively high sensitivity section.

In one embodiment of the second aspect, the conversion dynode is separately powered from the electron multiplier and/or is not electrically coupled to the dynode of the electron multiplier.

In one embodiment of the second aspect, the conversion dynode is a high energy conversion dynode.

In a third aspect, the present invention provides an apparatus for detecting charged particles, the apparatus comprising: a conversion dynode configured to emit one or more secondary electrons or ions upon impact by a particle; an electron multiplier configured to form an amplified electron signal output from one or more secondary electrons or ions emitted by the conversion dynode, wherein the conversion dynode is separately powered from the electron multiplier and/or is not electrically coupled to a dynode of the electron multiplier.

In one embodiment of the third aspect, the conversion dynode is a high energy conversion dynode.

In one embodiment of the third aspect, the conversion dynode is physically incorporated within or around the electron multiplier.

In one embodiment of the third aspect, the electron multiplier has a relatively low sensitivity section and a relatively high sensitivity section.

In an embodiment of the first or second or third aspect, the electronic signal output of the relatively high sensitivity section is a relatively high gain electronic signal output compared to the electronic signal output of the relatively low sensitivity section.

In an embodiment of the first or second or third aspect, the relatively low sensitivity section is an analog section and the relatively high sensitivity section is a digital section configured to output a series of pulse heights.

In an embodiment of the first or second or third aspect, the digital section output is configured to be available as an input in an electronic counting circuit.

In an embodiment of the first or second or third aspect, the relatively high sensitivity section and the relatively low sensitivity section each comprise one or more discrete dynodes, wherein the electron multiplier is configured such that the relatively low sensitivity section provides a relatively low gain electronic signal output and the relatively high sensitivity section provides a relatively high gain electronic signal output.

In an embodiment of the first or second or third aspect, the relatively high sensitivity section and/or the relatively low sensitivity section is configured to operate at an output current of at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75 or 100 μ Α.

In an embodiment of the first aspect or the second aspect or the third aspect, the conversion dynode has an applied voltage of greater than about +1, +2, +3, +4, +5, +6, +7, +8, +9+10kV, +15kV or +20kV or less than about-1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -15 or-20 kV.

In an embodiment of the first or second or third aspect, the voltage applied to the conversion dynode is decoupled from the voltage applied to the relatively low sensitivity section of the electron multiplier.

In an embodiment of the first or second or third aspect, the relatively low sensitivity section comprises at least about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16 discrete dynodes.

In an embodiment of the first or second or third aspect, the relatively high sensitivity section comprises at least about 10, 11, 12, 13, 14, 15, 16, 17 or 18 discrete dynodes.

In one embodiment of the first or second or third aspect, the conversion dynode is integral with, immediately adjacent to, or within the structure of the electron multiplier.

In an embodiment of the first or second or third aspect, the bias applied to the conversion dynode is lower than the bias applied in the case of the same device, even if the conversion dynode is provided on the far side of the same device, the lower voltage being such that any detrimental effects normally associated with bringing the conversion dynode close to the electron multiplier are reduced or eliminated.

In an embodiment of the first or second or third aspect, the bias voltage applied to the conversion dynode is less than about 5, 4, 3, 2 or 1 kV.

In a fourth aspect, the present invention provides a mass spectrometry instrument comprising the apparatus of any embodiment of the first aspect.

In one embodiment of the fourth aspect, the mass spectrometry instrument is configured to detect target particles present at a concentration of less than about 0.1ppb, 0.01ppb, 1ppt, 0.1ppt, 0.01ppt, or 1 ppq.

In an embodiment of the fourth aspect, the mass spectrometry instrument is configured to perform inductively coupled plasma mass spectrometry.

In a fifth aspect, the present invention provides a method of mass spectrometry of a sample, the method comprising the steps of introducing a sample for analysis into a mass spectrometry instrument of any embodiment of the first aspect, and operating the instrument to provide one or more electronic signal outputs.

In one embodiment of the fifth aspect, the mass spectrometry is capable of detecting target particles present at a concentration of less than about 0.1ppb, 0.01ppb, 1ppt, 0.1ppt, 0.01ppt, or 1 ppq.

In an embodiment of the fifth aspect, the mass spectrum is an inductively coupled plasma mass spectrum.

Drawings

FIG. 1A diagrammatically shows a preferred apparatus of the invention, which is useful in the context of a detector for a mass spectrometry apparatus.

FIG. 1B diagrammatically shows a preferred arrangement in which the conversion dynode is incorporated into the structure of an electron multiplier.

FIG. 1C diagrammatically shows a preferred arrangement in which the conversion dynode is incorporated into the structure of an electron multiplier. The device does not have the high gain section and the low gain section present in other embodiments of the invention, but only comprises a single gain section. This embodiment may be used in conjunction with analog or pulse count detection electronics.

Fig. 2A diagrammatically shows the device of fig. 1, but showing the path of the charged particles: the impact point of the positive ions onto the high energy conversion dynode, and the dynode of the electron multiplier from which the electrons are converted.

Fig. 2B diagrammatically shows the device of fig. 2A configured to detect negative ions. Negative ions travel through the input aperture to the conversion dynode, from which positive ions are emitted. The positive ions travel to the first dynode of the electron multiplier.

FIG. 2C diagrammatically shows the apparatus of FIG. 1B configured to detect positive ions and having a conversion dynode contained within the structure of an electron multiplier.

FIG. 2D diagrammatically shows the device of FIG. 1B configured to detect negative ions and having conversion dynodes contained within the structure of an electron multiplier.

Fig. 3 is a photograph of a prototype of a preferred apparatus of the invention, showing the main physical features of the external presentation.

Fig. 4 shows model operating parameters of the prototype apparatus of fig. 3. The letters "m" to "v" are used only for identifying the curves.

Fig. 5A is a graph showing the yield of secondary electrons from ion collisions as a function of ion mass and energy. The variation of secondary electron yield with mass produces a "mass shift" effect in the spectrum. The letters "m" to "v" are used only for identifying the curves.

Fig. 5B is a graph showing poisson probability distribution functions with mean values of 0.8, 2.0, and 4.0. The maximum possible detection efficiency is limited by poisson statistics.

Fig. 6A is a gain curve generated from the low gain (analog) section of the prototype detector of fig. 3.

Fig. 6B is a gain curve generated from the high gain (pulse output) section of the prototype detector of fig. 3.

FIG. 7 shows the plateau curve generated by the prototype detector of FIG. 3, in which the high energy conversion dynode was biased to-5 kV and-10 kV. The letters "m" and "n" are used only for identifying the curves.

FIG. 8A is a simulated gain curve for an electron multiplier having split dynodes with an Open Area Ratio (OAR) of 30% compared to a split dynode with an open area ratio of 75%.

FIG. 8B is a pulse gain curve for an electron multiplier having split dynodes with an open area ratio of 30% compared to a split dynode with an open area ratio of 75%. For both electron multipliers, the high energy conversion dynode was biased to-10 kV.

Detailed Description

After considering this description, it will become apparent to one skilled in the art how to implement the invention in various alternative embodiments and alternative applications. However, while various embodiments of the present invention will be described herein, it should be understood that they have been presented by way of example only, and not limitation. Accordingly, the description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention. Moreover, statements of advantages or other aspects apply to particular exemplary embodiments and not necessarily to all embodiments covered by the claims.

Throughout the description and claims of this specification, the word "comprise" and variations of the word, such as "comprises" and "comprising", are not intended to exclude other additives, components, integers or steps.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may.

It is to be understood that not all embodiments of the invention described herein have all of the advantages disclosed herein. Some embodiments may have a single advantage, while other embodiments may have no advantage at all, and are merely useful alternatives to the prior art.

The present invention is based, at least in part, on applicants' following findings: an electron multiplier having a high sensitivity section and a low sensitivity section, or a combination of an electron multiplier and a separately powered conversion dynode (and in some embodiments a high energy conversion dynode), or a combination of conversion dynodes physically incorporated within or around the electron multiplier, provides a detector arrangement with certain improvements over prior art detector arrangements.

As used herein, the term "conversion dynode" is intended to include any invention capable of emitting secondary electrons (or ions) upon impact by a particle, such as a charged or uncharged atom, a charged or uncharged molecule, a charged or uncharged subatomic particle such as a neutron or proton or electron or photon. According to the present invention, the conversion dynode can be operated to have a relatively high potential compared to a dynode dedicated to amplification.

In some embodiments, the dynode is a "high energy conversion dynode". The potential may be measured relative to ground, or suitably relative to another component of the apparatus. Although there may be some correlation in defining the high energy conversion dynode, typically, the high energy conversion dynode and the first dynode of the low gain section of the electron multiplier are typically grounded, and the high energy conversion dynode is biased to a voltage further away from zero than the first dynode of the low gain section of the electron multiplier.

As understood by those skilled in the art, the efficiency of ion-to-electron (and ion-to-ion) conversion generally increases with the velocity at which ions strike the surface of the conversion dynode. Therefore, conversion dynodes are typically designed to increase the velocity of the incident ions in order to optimize the conversion efficiency as much as possible.

The incorporation of separately powered discrete conversion dynodes in the detector arrangement allows the dynodes to be individually biased and to a higher voltage than the dynodes of the electron multiplier, particularly the first dynode of the low gain section. An advantage of this arrangement is that it avoids the need to raise the bias voltage of the electron multiplier section beyond that required for electron amplification to occur. In prior art detectors for ICP-MS, the initial voltage of the electron multiplier is raised to about-1600V to ensure sufficient ion-to-electron conversion efficiency. However, in the present invention, the bias of the conversion dynode is raised (in order to ensure the desired conversion efficiency), while the electron multiplier (particularly the low gain section) can be biased to a lower voltage. Thus, the low gain electron multiplier section (operating at a lower voltage) has greater voltage "headroom", which in turn provides a longer service life.

Extending the useful life of the detector may provide the additional advantage of slowing the rate of gain change and/or slowing the differential drift rate over time.

High energy conversion dynodes typically have a dedicated power supply, substantially independent of the power supply of the electron multiplier section of the device. The use of separate power supplies may allow for better independent control of the voltage and/or current applied to the high energy conversion dynode. The use of a separate power supply may also allow for better control of the voltage and/or current applied to the electron multiplier section.

The power source used in conjunction with the device may be of a fixed voltage or adjustable voltage type. The connection location of any power supply to any dynode in the dynode chain of the high energy conversion dynode or electron multiplier section may be selected according to the linearity or gain requirements of the device, or indeed any other requirement. In some embodiments, the power supply may be configured to apply a voltage to only a single dynode or a group of dynodes.

As understood by those skilled in the art, a chain of voltage dividers may be used to distribute the voltage from the power supply to a set of dynodes. The divider chain may comprise a series of resistors arranged between dynodes. The voltage divider chain may be purely passive, consisting of only resistive elements, or it may contain active components in voltage regulation, such as diodes or transistors. When a terminal dynode is involved, a resistor is typically placed between the terminal dynode and ground or a reference voltage. Alternatively, a zener diode may be used at this location.

In the context of a mass spectrometer, ions that have passed through a mass separator are accelerated onto a conversion multiplier electrode to which a high voltage is applied. Electrons (or ions) emitted from the conversion dynode by the incident ions then enter the first dynode of the electron multiplier where secondary electrons are emitted from the secondary emission surface.

In this case, the skilled person is fully familiar with the material, physical and functional configuration of the emission surface, an exemplary type being provided by dynodes.

As is conventional in electron multipliers, a first electron emission surface (the surface of the first dynode in a series of dynodes) is provided that is configured to receive an input particle and emit one or more electrons in response to impingement of the input particle. In the case of multiple electrons being emitted, which is typical, this results in amplification of the input signal. A series of second and subsequent electron emitting surfaces are provided, also as is conventional. The function of these emission surfaces is to amplify the electrons emitted from the first emission surface. It will be appreciated that amplification will typically occur at each subsequent emission surface of the series of emission surfaces. Typically, the secondary electrons emitted by the final emission surface are directed onto the anode surface, and the current formed in the anode is fed to a signal amplifier and subsequently to an output device.

In the present invention, the electron multiplier may have a low-sensitivity section configured to detect ions present at a relatively high concentration, and a high-sensitivity section configured to detect ions present at a relatively low concentration.

Differential sensitivity may be provided by any means deemed appropriate by those skilled in the art, including the use of a signal amplifier, for example, whereby the signal output of the high sensitivity section is amplified, while the signal output of the low sensitivity section is not amplified. Alternatively, when the dynode is constituted by discrete dynodes, the secondary electron emissivity level of the dynodes may be higher in the high-sensitivity section than in the low-sensitivity section. For example, the dynodes in the high-sensitivity section may be made of a material having a higher emissivity or a larger impact area than the dynodes in the low-sensitivity section.

More typically, however, the differential sensitivity of the electron multiplier section is produced by multiplying the gains of the high and low sensitivity sections. A higher gain of the high sensitivity section is achieved because it is multiplied by the gain of the previous low sensitivity section.

In the electron multiplier of the device, the low sensitivity section may be an analog section (which may be considered a low gain section) and the high sensitivity section may be a digital section (which may be considered a high gain section) capable of pulse counting. The effective gain of the digital section output is approximately the product of the gains of the analog and digital sections. Thus, it will be appreciated that in the isolated case, the two sections may have the same gain, but the digital section has a higher output gain because its gain is multiplied by the gain of the preceding analog section. In other words, the digital section may be referred to as a high gain section because the signal provided by it has a higher gain than the signal provided by the analog section because it is the product of the gains of the two (analog and digital) sections. The gain signal provided by the digital section is the total gain-the product of the gains of the two (analog and digital) sections.

In any case, the high sensitivity (digital) section is suitable for detecting ions present in small amounts. The output of the digital section comprises pulses generated by single ions that have been provided with sufficient gain so that all of these ions can be detected by the pulse count detection electronics. Electronic timers or counters are typically used to process the output pulses. As one example, each time a signal is generated within a predetermined time window, a counter associated with the predetermined time window may be incremented.

In operation, the pulse counting section has a limited range due to saturation caused by very high ion flux, in which case the analog section of the multiplier provides a useful output signal. In the present device, these two sections can be operated simultaneously by two sets of dynodes arranged in series, with an intermediate "split" dynode, a grounded dynode and a guard (gate) dynode. The output signal of the analog section is extracted through a "split" dynode onto an analog collector. The portion of the signal that passes through the aperture of the split dynode is the analog output. The part of the signal that does not pass through the aperture is passed to the digital (pulse) section of the electron multiplier to further amplify the gain.

The analog and pulsed sections of the electron multiplier may be described with reference to a "split ratio". For example, the first electron multiplier may have a split dynode with an open area ratio (i.e., the total area of all holes in the dynode) of 30%. In this case, the split multiplier electrode provides a nominal 30% signal extraction for the analog section and 70% transmission to the pulse section. The second electron multiplier may have split dynodes with an open area ratio of 75%. In this case, the split dynode would provide a nominal 75% signal extraction for the analog section and 25% transmission to the pulse section. As shown in fig. 8, the differential splitting ratio of the first and second electron multipliers results in a differential gain.

Ions impinging on the first dynode stage produce an electron signal, a portion of which is collected in an analog collector, producing an analog signal output. The voltage on the guard dynode is set at a level such that the remaining electrons pass through the second stage to produce a digital (pulse count) output signal. When the pulse signal rises to a predetermined level (a level that occurs in response to a relatively high ion flux), the rising pulse signal causes a suitable voltage to be applied to the guard dynode to prevent electrons from entering the second stage and damaging the detector.

In some embodiments, the use of an electron multiplier having a low sensitivity section and a high sensitivity section contributes to a significant enhancement of the dynamic range of the detector. Applicants have discovered, among other things, that reducing the internal resistance of the detector arrangement (including the high gain section and the low gain section of the electron multiplier) contributes to the overall enhanced dynamic range of ion detection. Other advantages are provided as discussed further below with respect to certain preferred embodiments.

In some embodiments of the apparatus, the conversion dynode is physically disposed within or around the electron multiplier. The conversion dynode may be considered within an electron multiplier in which it is disposed (fully or partially) within a bounding volume defined by the hard surfaces of the electron multiplier. Alternatively, the conversion dynode may be proximate to any outwardly facing surface of the electron multiplier, the term "proximate" including any distance less than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mm.

The term "in.. or around" in this context may be defined by reference to the distance between the first electron emission surface of the electron multiplier and the conversion dynode. The distance may be less than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mm.

In some embodiments, the conversion dynode is immediately adjacent to the first dynode of the electron multiplier.

As will be explained further below, advantages are obtained when the conversion dynode is powered separately or electrically decoupled from the electron multiplier, allowing the conversion dynode to be disposed within or around the electron multiplier.

Reference is now made to fig. 1A and 1B, which illustrate an exemplary apparatus of the present invention in highly diagrammatic form. In this embodiment, positive ions are accelerated and focused by the quadrupole rods to pass through the input aperture of the device. The ions then travel to a high energy conversion dynode that emits secondary electrons (or ions). The curved emission surfaces of the switching dynodes focus secondary electrons (or ions) into the electron multiplier and first to the leftmost dynode (considered to be the first dynode of the low gain section of the electron multiplier), which then emits further secondary electrons deflected to the adjacent dynode, which then emits further secondary electrons deflected to the (right) adjacent dynode, and so on.

Continuing with the embodiment of fig. 1A and 1B, for positive ion detection, the high energy conversion dynode will have a high bias voltage hv (a) that is higher (or of significantly greater magnitude, or further from zero) than the bias voltage of the first dynode having the high bias voltage hv (B). The bias voltage for detection of negative ions is shown in fig. 2B and 2D.

The path of the charged particles through the device is clearly shown in fig. 2A, 2B, 2C and 2D.

In the embodiment of fig. 2A and 2B, the first five dynodes (counting from the first dynode of the left low gain section) constitute the low gain section of the electron multiplier. The output signals generated by the electrons emitted and amplified by the first three dynodes are analog.

The sixth and subsequent dynodes form the high gain portion of the electron multiplier. The output signal produced by the electrons emitted and amplified by the sixth and subsequent dynodes is a pulsed (digital) signal that can be used (by other electronics not shown) to provide a counting output. As described below, a preferred form of the device has a higher number of dynodes, e.g., 12 dynodes in the low gain section and 17 dynodes in the high gain section.

In the preferred embodiment of fig. 1A and 1B, it will be noted that the conversion dynode is biased to a high negative voltage (e.g., -10kV), which is necessary if the incident ions are positively charged. As will be understood by those skilled in the art, when the incident ions are negatively charged, the conversion dynode is biased to a high positive voltage (e.g., +10 kV). When the particles emitted by the high energy conversion dynode are negative (i.e., electrons), the first dynode in the low gain section of the electron multiplier is negative (e.g., -2 kV). When the incident ions are negative, the high energy conversion dynode that releases positive ions upon striking the first dynode is also negative (e.g., -2 kV).

According to the present invention, in which the high energy conversion dynode is biased to a negative voltage, the first dynode of the low gain section of the electron multiplier is biased to a larger negative voltage (i.e., closer to zero). When the high-energy conversion dynode is biased to a positive voltage, the first dynode of the low-gain section of the electron multiplier is normally biased to a negative voltage.

As will be understood from fig. 1B, 2C, and 2D, the function of the high-energy conversion dynode shown in fig. 1A, 2A, and 2B may be replaced by a conversion dynode physically located within or around the electron multiplier. This provides the advantages of ease of construction, space saving and also makes it possible to use a combined converter/multiplier, thereby minimising any difficulty in component replacement. For embodiments with conversion dynodes physically located within or around the electron multiplier, the conversion dynodes may have a relatively modest applied bias (e.g., less than about 5, 4, 3, 2, or 1 kV). Typically, a conversion dynode bias of less than about 3kV allows the conversion dynode to be physically close to the electron multiplier without any significant detrimental effect on the multiplier function.

As can be understood from fig. 1C, the electron multiplier section of some embodiments of the apparatus is a single gain multiplier. In this case, all dynodes of the electron multiplier form a single gain section, unlike the high gain section and the low gain section of other embodiments disclosed herein. In the embodiment of fig. 1C, advantages are obtained by switching dynodes (i) physically disposed within the electron multiplier and (ii) separately powered or electrically decoupled from the electron multiplier.

In one aspect, the invention also provides a replacement component or replaceable component for use with a particle detection apparatus (e.g. a mass spectrometer) comprising a conversion dynode and a dynode chain, wherein the replacement component or replaceable component is configured to allow the conversion dynode to be powered separately from the dynode chain.

The resistance of the electron multiplier, i.e. the combination of the high-sensitivity section and the low-sensitivity section, is set to a level below that indicated for the prior art electron detector, thereby enhancing the dynamic range of the electron multiplier.

In addition to or instead of reducing the internal resistance, or as a natural result of reducing the internal resistance, the apparatus is configured such that the high-sensitivity section and the low-sensitivity section of the electron multiplier are operable at a linear output current of at least about 50, 75, or 100 μ Α. For the high sensitivity section and the low sensitivity section of the electron multiplier this means an increase of about 10 times the current used in the prior art device.

It should be noted that when a differential current flowing through each dynode is required, the use of independent power supplies configured to apply bias voltages of different magnitudes to the selected dynodes is one means for achieving the differential current.

Without wishing to be bound by theory in any way, because the dynamic range of the detector is the product of the dynamic ranges of the low and high gain sections, the overall increase in the dynamic range of the detector may be at least one or two orders of magnitude higher than typical commercial detectors in some embodiments.

Referring now to fig. 3, fig. 3 shows a prototype apparatus of the present invention broadly constructed in accordance with the scheme of fig. 1A. A high energy conversion dynode is shown at 100 and an electron multiplier is shown at 110. Electron multiplier 110 has a low gain section near 120 and a high gain section near 130.

Turning now to fig. 4, certain operating parameters of the apparatus of the present invention have been estimated by modeling.

Without wishing to be bound by theory in any way, it is suggested to increase the dynamic range by reducing the internal resistance of the detector. Both the pulse counting section (i.e. the high-sensitivity section) and the analog section (i.e. the low-sensitivity section) are designed to operate at a linear output current of at least 50 μ Α. This is about an order of magnitude higher for each section than typical prior art detectors that are commercially available.

Since the dynamic range of the detector is the product of the dynamic ranges of the analog and pulse counting sections, the overall increase in dynamic range of the new detector is in principle two orders of magnitude higher than that of a typical commercial detector.

To improve the lifetime of the device, gain stability and reduce the differential rate, the number of dynodes in each section is increased: 12 dynodes are used in the analog (i.e., low sensitivity) section and 17 dynodes are used in the pulse counting (i.e., high sensitivity) section.

It is expected that the benefit of using separately powered conversion dynodes is to provide additional useful life for the detector. As noted above, in prior art detectors for applications such as inductively coupled mass spectrometry, the starting voltage (-HV) of the analog section is typically raised to about-1600V to ensure adequate ion-to-electron conversion efficiency. With the introduction of high energy conversion dynodes, the conversion dynode voltage can be decoupled from the voltage required to provide analog gain. Thus, the analog section of the multiplier can operate at a much lower initial-HV voltage, providing additional voltage overhead for longer life.

With respect to the detection efficiency of incident ions, when energetic ions are incident on a surface, secondary electrons are emitted according to a poisson distribution, the mean of which depends on the mass and energy of the ions and the material of the emitting surface. The secondary ion yield corresponds to the mean of the poisson distribution of the number of emitted ions. The trends and values in fig. 5A are typical trends and values that can be expected for stainless steel and magnesium/silver conversion dynodes.

The poisson distribution of emitted electrons from the ion-electron conversion process is a determining factor in the shape of the Pulse Height Distribution (PHD) from the electron multiplier, which also places a fundamental limit on detection efficiency.

For a secondary electron emission distribution with a low mean value, there is a high probability that zero electrons will be emitted when ions are incident on the conversion surface. For a distribution with an average of 0.8, the probability that zero secondary electrons will be emitted is about 0.45 (see fig. 5B), i.e. the maximum possible detection efficiency for a species with an average secondary electron yield of 0.8 is about 55%. For species with yield of 2, the probability of zero secondary electrons drops to about 0.14, increasing the maximum possible detection efficiency to about 86%.

When the yield is increased to 4, a detection efficiency of 98% can be produced, which is highly desirable, especially when low concentrations of the target ion are present in the sample. With the incorporation of a high energy conversion dynode, a yield of 4 is achievable. This provides an advantage in terms of achievable detection efficiency over detectors without a high-energy conversion dynode disposed before the first dynode of the low gain region of the electron multiplier.

For a given impact energy, the variation of secondary electron yield with mass leads to mass deviations in the spectrum. The data shown in fig. 5A shows that for an impact energy of 2keV, the secondary electron yield varies from a maximum of about 2 to a minimum of about 0.8. That is, the highest yield (at 5 amu) was about 2.5 times higher than the lowest yield (at 140 amu). For an impact energy of 10keV, the secondary electron yield varies from a maximum of about 5.1 to a minimum of about 4.2. In this case, the maximum yield is only about 1.2 times higher than the minimum yield.

To obtain the same or similar signal level as a multiplier with a secondary electron yield of about 4.2 for 140amu ions, a multiplier with a yield of about 0.8 for 140amu ions needs to operate with a gain of about 5.2 times higher than the detector (═ 4.2/0.8). The need to operate at a higher gain may reduce the lifetime of the detector, which is a disadvantage of the prior art detectors compared to the present arrangement.

The linearity level of the prototype can be measured. To obtain an analog output current of 50 μ A with a gain of 3e3, an input ion current of greater than about 16nA is required. Ion currents of this magnitude can be obtained, for example, from mass spectrometry instruments.

For the prototype detector shown in fig. 3, gain curves for the low gain (analog) section (see fig. 6A) and the high gain (pulse count) section (see fig. 6B) have been generated. To obtain an analog gain of about 3e3, a starting voltage of about 1100V is required. This is about 500V below the start voltage typically applied to the analog section of the ICP-MS detector.

Advantageously, the prototype detector shown in FIG. 3 has been shown to produce a high quality plateau curve for voltages of-5 kV or-10 kV applied to the high energy conversion dynode (see FIG. 5). This is due to the improved behavior of the pulse height distribution due to the increased yield of secondary electrons from the conversion dynode. A high quality plateau curve enables a reliable setting of the pulse count operating voltage.

The incorporation of a separate high energy conversion dynode provides flexibility in the mechanical design of the detector, in addition to any functional improvement in operation. In some embodiments, the electron multiplier can be rotated in any direction above the conversion dynode, allowing axial or radial orientation with respect to the quadrupole rods. Referring to fig. 3, fig. 3 shows a prototype detector with a high-energy conversion dynode 100, the high-energy conversion dynode 100 being operatively connected to a discrete dynode electron multiplier 110 having a high-sensitivity section and a low-sensitivity section.

The apparatus is particularly useful as a detector component in mass spectrometry instruments, including instruments used in applications requiring the detection of extremely low abundance (abundance) ions. Such applications include inductively coupled mass spectrometry. Accordingly, in one aspect, the present invention provides a combination of an inductively coupled mass spectrometry instrument and an apparatus as described herein.

The instrument may include means for ionizing the sample by inductively coupled plasma.

Inductively coupled plasma is plasma that is typically generated by heating a gas with a solenoid coil, the gas containing a sufficient concentration of ions and electrons to make it electrically conductive. The plasma is usually maintained in the torch of the instrument, which consists of three concentric tubes (usually quartz), the ends of which are arranged inside the induction coil. The apparatus generally comprises means for introducing argon between the two outermost tubes of the torch, with an electric spark being applied intermittently to introduce free electrons into the gas stream. The electrons are accelerated and may collide with argon atoms to cause electron release, which in turn is accelerated. This process continues until the rate of release of new electrons in the collision is balanced by the rate of recombination of electrons with argon ions (atoms that lose electrons). The temperature of the plasma is about 10,000K.

The apparatus may be physically and/or structurally configured to operate with existing commercially available ICP-MS instruments. By way of example only, the present apparatus may be configured to operate as an electron multiplier in any ICP-MS instrument supplied by the following suppliers: agilent^TMFor example, model numbers 7800, 7900, 8900 triple quadrupole rods, 8800 triple quadrupole rods, 7700e, 7700x, and 7700 s; or PerkinElmer^TMFor example, models NexION2000, N8150045, N8150044, N8150046 and N8150047; or ThermoFisher Scientific, such as models iCAP RQ, iCAP TQ and Element Series; or Shimadzu, for example model ICPMS-2030.

The electron multiplier components of the present apparatus have been exemplified by means of a linear, discrete dynode multiplier. Given the benefit of this description, the skilled artisan is able to routinely test whether other types of multiplier types are suitable for the present invention. For example, a continuous (channel) dynode may be used instead of a discrete dynode electron multiplier. In this case, the device may include a high energy conversion dynode in combination with a continuous dynode.

Furthermore, although the present apparatus is particularly advantageous with respect to ICP-MS instruments and components thereof, it is not intended that the scope of the present application be so limited. It is contemplated that at least some features of the present invention may be applied to non-ICP-MS instruments and components thereof. For example, the use of high-gain and low-gain section dynodes in an electron multiplier may still provide advantages, as well as the integration of separately powered conversion dynodes into the structure of the electron multiplier. One skilled in the art can use conventional methods to test the usefulness of the present invention for a range of existing mass spectrometry instruments and their components, even for applications unrelated to mass spectrometry.

It should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment.

Moreover, although some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are intended to be within the scope of the invention and form different embodiments, as will be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments may be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. Functions may be added to or deleted from the figures and operations may be interchanged among functional blocks. Steps may be added to or deleted from the described methods within the scope of the invention.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.