Ion detection device and method for controlling the ion detection device

Abstract

A method for controlling an ion detector is provided. The method includes converting a received ion into emitted electrons, multiplying the emitted electrons, detecting the multiplied electrons, generating a detection signal in response, and determining the ion intensity from the detection signal.

Description

Technical Field

[0001] The present invention relates to an ion detection device such as a secondary electron multiplier (SEM) detection device. The SEM detection device can be used to detect ions emerging from a mass analyzer of a mass spectrometer.

Background Art

[0002] FIG. 1 illustrates a simple SEM detection device 10 according to the prior art. The SEM detection device 10 is composed of a multiplier section 12 and a detection section 13. The ion beam IB is guided by a (optional) deflector 11 and enters the multiplier section. When an ion hits the first dynode of the multiplier section, i.e., the "conversion dynode" 12A, the conversion dynode emits secondary electrons. The conversion dynode is under a suitable potential so that charged ions are accelerated towards the conversion dynode 12A. The subsequent electron emission is called secondary emission.

[0003] Electrons emitted from ion collisions with a conversion dynode are attracted by the electric field towards the next dynode, and the charge causes the emission of secondary electrons. A potential difference exists between the conversion dynode and the next dynode, and as a result, the next dynode emits secondary electrons for each collision of charge received from the conversion dynode (the second dynode may have a positive potential difference compared to the first dynode, the third dynode may have a positive potential difference compared to the second dynode, and so on). In multiple consecutive stages, it is foreseeable that each dynode may have a less negative (more positive) potential, and secondary electrons emitted from the previous dynode are attracted to the next dynode which may have a more positive potential. Thus, a single ion entering the front of the SEM detector can result in more than one million electrons exiting the multiplication section 12, which are directed to the detection section 13 of the detector 10 where a pulse detection signal P is generated. In this way, charge multiplication by secondary electrons makes it possible to register even a very small number of ions, or even a single ion input, into the SEM detector.

[0004] In the case of inductively coupled plasma mass spectrometry (ICP-MS), for example, the detector should be able to operate over a 9-digit dynamic range, and ideally, have a target of operating over an 11-digit dynamic range. This is because the detector is suitable for detecting the major and minor components of the sample.

[0005] U.S. Patent No. 7,220,970(B2) describes examples of SEM detectors and SEM detector operating modes. SEM detectors may be used in counting (or pulse) mode and current (or analog) mode. As illustrated in Figure 1, in an ICP-MS, the detector generally has different sections, with the first section used for analog mode, while the second section is used for pulse mode (or is switched off). To record electrons in current (or analog) mode, an analog signal is taken from one of the central dynodes or multiplication stages in the multiplication section (as illustrated in Figure 1). Counting (or pulse) mode records electrons arriving at the final stage of the SEM detector (i.e., the detection section or detector 13). High ion currents can be measured using analog mode, while counting mode evaluates relatively small ion currents. The SEM may operate selectively in analog mode and counting mode to provide a wider dynamic measurement range.

[0006] Another option for increasing the dynamic range of the detection device is employed in Thermo Fisher Scientific's iCAP®Q instrument, where electrons after the first n dynodes are taken out of the multiplication section of the SEM and redirected into a Faraday cup where the current is measured.

[0007] In counting mode, where all multiplied electrons are supplied to the detector, the detector has been observed to degrade over time due to constant exposure to a relatively high electron flow. Furthermore, in higher counting ranges (above approximately 10⁶ cps), the counting mode is affected by the detector's saturation effect. After each peak (a burst of multiplied secondary electrons caused by the incidence of ions into the SEM), the detector needs to be switched off for a certain period of time because, after the pulse, the detector may not be adapted to support the charge for the next pulse, and / or the detector is susceptible to "ringing" that leads to double pulses. Also, each pulse may have a different duration (or "pulse width") depending on its intensity. Additional pulses falling during this time cannot be detected. To mitigate these effects, counting mode may require a "dead time," during which the detector is "switched off" for a specific period after the detection of each peak. The pulse width may differ for different pulses. Therefore, the dead time must be at least the same length as the longest pulse width to be considered. During the time the detector is "switched off," the electronic equipment may be switched to a mode in which no counts are registered (although power may still be supplied).

[0008] Compared to counting mode, analog mode has a faster response time and is therefore sensitive to changes in ion current. However, since the analog signal depends on the aging of the dynode, drift can occur over time. Thus, there is an inherent lack of stability and accuracy in analog mode detection. Furthermore, the average amplification of each dynode may depend on the work function, and therefore, residual gas near the detector can cause further drift.

[0009] When both modes are employed to increase the dynamic range of the detection device, the drift of the analog mode and the degradation effects linked to the counting mode must be considered. This is usually done by performing cross-calibration between the counting signal and the analog signal. Due to unknown drift / degradation effects, cross-calibration should be performed periodically. Furthermore, cross-calibration is also element-dependent and requires calibrators.

[0010] Another technique to increase the dynamic range of the detector is to down-control the current in the first dynode by adjusting the dynode voltage to attenuate high-level signals in a mode known as the attenuation mode. However, the voltage required in this mode can change over time, which can lead to reduced stability. Furthermore, the required voltage depends on the mass of the sample ions. If the amplification voltage is increased during this mode, the problem of detector aging may be exacerbated (and the instrument may exhibit a relatively poor plateau). Moreover, the advantages of the counting mode (high accuracy, linearity, low drift) are not present in the attenuation mode. Therefore, this mode is not better than the analog mode. In fact, this mode may lack the fast response of the analog mode (e.g., for detector protection) and does not offer overlap between modes where measurements are performed simultaneously with different attenuations. Instead, a single pre-measurement is required to measure the intensity and then determine the attenuation of the mode. If a pre-measurement is not performed, there is a risk of detector damage due to high signals. The plateau curve also depends on the attenuation voltage. Therefore, providing an attenuation mode is not a preferred solution to this problem.

[0011] The advantages arising from the counting mode are negated by introducing the effects seen in the analog mode. Additionally, attenuation must be pre-set before the analysis. Therefore, this method may suffer from the drawbacks of both modes. [Overview of the project]

[0012] The present invention aims to provide an ion detection device, such as a SEM detector, having an increased dynamic range. The present invention also aims to provide an ion detection device having an improved lifespan. The present invention further aims to provide an ion detection device having the ability to measure ion current in an analog range, as well as accuracy and stability of the counting range. In addition, the present invention aims to provide a control method for an ion detection device.

[0013] Therefore, a method for controlling an ion detector is provided. Converting received ions into emitted electrons, The process of multiplying the emitted electrons, Detecting multiplied electrons, In response to that, generate a detection signal, This includes determining the ion intensity from the detected signal.

[0014] The method involves controlling the number of multiplied electrons that reach the detector by intermittent gating, depending on the ionic strength.

[0015] According to the present invention, intermittent gating is provided (for example, by gating secondary electrons in the multiplication section of the SEM detector and / or by gating the ion beam before ions collide with the conversion element). Advantageously, the proposed method and apparatus can achieve an improved dynamic range while addressing the shortcomings of the counting modes of the conventional apparatus described above. The stability and reproducibility of the SEM detector can also be improved.

[0016] The proposed method provides a detector that can offer a large dynamic range by using only the counting mode of the detector and without requiring the analog (current) mode. As a result, cross-calibration between the analog mode and the counting mode may not be necessary. Therefore, a detector calibration solution for cross-calibration may not be required, potentially saving time spent on performing detector cross-calibration.

[0017] By providing intermittent gating, it may be possible to switch the ion flow and / or secondary electron flow on or off (or at least significantly attenuate the flow). By implementing a probabilistic algorithm, it may be possible to reduce the overall exposure of the detector to electrons, which reduces detector degradation.

[0018] Additionally, by implementing intermittent gating according to the proposed method, the switching time can allow for intermittent "dead periods" during which electrons are not received by the detector. In this way, useful readings can be obtained from the detector while maintaining the overall readiness of the detector. As long as the "off" time due to gating is at least the same length as the detector's dead period, the detector will not be overloaded with electrons.

[0019] By implementing the present invention, the detection device does not necessarily require an analog section. Therefore, the technical complexity and cost of the detection device can be reduced. Furthermore, calibration of the analog mode may not be required, which significantly increases the efficiency of the device's preparation and operation (and eliminates the costs associated with calibration, such as the use of calibrators). In addition, compared to prior art dual-mode SEM detection devices, the overall device (i.e., the mass spectrometer) is also advantageous because analog electronics, as well as a dedicated analog high-voltage power supply and some feedthroughs, may be omitted.

[0020] If the counting unit of a detection device detects an excessive number of counts, it will degrade. As the number of counts on the detector decreases, the detector life increases. Therefore, by applying intermittent gating to the detection device to improve the range and keep the number of detected events low, the detector life can be improved by the proposed method. The proposed measurement technique can also be used for intensity, which is conventionally used for count detection and involves fewer events required to make accurate measurements.

[0021] In the high counting range (e.g., the range exceeding 1 million counts per second) and in the higher range conventionally measured by the analog mode, the proposed method can determine the ion intensity with fewer detector events. The results may be more accurate and reliable than the methods provided by the prior art.

[0022] As mentioned above, the detection device may not require an analog section. This means that fewer components may be required for the detection device (the detection device can be made less expensive). In addition, this may also mean that fewer components are required for the overall instrument because the following components may no longer be required: · Analog electronics, · Dedicated analog high voltage, and · Some feed-throughs.

[0023] According to the proposed method, the way ions are counted is changed from pure counting to "variable chopping technology". By providing a variable "chopping" time and using this "chopping" time for data evaluation, the proposed method provides significant benefits as described above.

[0024] Determining the ion intensity from the detection signal may include determining the ion count value. The number of multiplied electrons reaching the detector of the detection device can be controlled by intermittent gating depending on the count value.

[0025] The conversion element may be referred to as a conversion electrode or a conversion dynode. The multiplication unit may include a plurality of electrodes that may also be referred to as dynodes.

[0026] The multiplication unit may be for multiplying the electrons emitted downstream of the conversion element. The detector may be downstream of the multiplication unit.

[0027] The method is When receiving ions, electrons are emitted (by the conversion element), The process involves multiplying the emitted electrons (by a multiplier unit), The process involves detecting amplified electron pulses and generating a detection signal in response (by a detector), This may further include determining the count value from the detection signal (by the detector or by a processor communicating with the detector).

[0028] The number of electron pulses reaching the detector can be controlled by intermittent gating, depending on the count value.

[0029] The emission of the multiplied electrons can be controlled by intermittent gating based on the determined count value.

[0030] The number of received ions can be controlled by intermittent gating based on the determined count value.

[0031] Intermittent gating can occur after electron emission. In one example, gating may be performed after the transformation element but before the multiplication unit. In another example, gating may occur after the first few dynodes (e.g., after the first, second, third, fourth, or fifth dynode).

[0032] Emitted electrons can be attenuated by gating.

[0033] The gate can control the propagation of secondary electrons within the multiplication section of the detector by attenuating the secondary electrons.

[0034] Controlling the number of multiplied electrons reaching the detector through intermittent gating may involve intermittently switching the charging grid to attenuate emitted electrons or redirect ions away from the conversion element.

[0035] Controlling the number of multiplied electrons reaching the detector through intermittent gating may involve intermittently switching the ion optics positioned upstream of the conversion element to redirect ions away from the conversion element.

[0036] Controlling the number of multiplied electrons reaching the detector through intermittent gating may involve intermittently switching one of several dynodes within the multiplier unit to attenuate all multiplied electrons within the multiplier unit.

[0037] A potential may be applied to one of the dynodes (preferably one of the first few downstream dynodes in the multiplication section) so that all electrons are attenuated by this dynode, which acts as a gate.

[0038] Intermittent gating may occur before the emitted electrons are doubled.

[0039] Intermittent gating can occur before electrons are released.

[0040] The gate may be a grid before or within the detection device. For example, the grid may be before the transformation element, before the multiplication section (immediately downstream of the transformation element), or between the dynodes of the multiplication section.

[0041] Ions can be deflected by gate control.

[0042] A gate can control ion collisions to a conversion dynode by attenuating the ion flow guided to the conversion dynode. Alternatively, a gate can control electron collisions to the next dynode after a conversion dynode by attenuating the electron flow between the conversion dynode and the next dynode.

[0043] Intermittent gating can be achieved by gates that provide an "energy barrier" to block, attenuate, or redirect electrons or ions.

[0044] Intermittent gating may be implemented when repeating a gating cycle, which includes an "off" time during which the number of multiplied electrons reaching the detector is attenuated (or during which the emission of multiplied electrons is stopped) and an "on" time during which the number of multiplied electrons reaching the detector is not attenuated (or during which the emission of multiplied electrons is continued).

[0045] The duration of the "on" time can be controlled during intermittent gating, depending on the count value. The duration of the "off" time can be controlled during intermittent gating, depending on the count value, and / or based on the required dead time of the detector.

[0046] The gate may operate in a cycle that switches between an "on" time when the gate allows the passage of ions or secondary electrons and an "off" time when the gate decays the ions or secondary electrons.

[0047] The multiplied electrons can be detected as pulses of multiplied electrons. These pulses of multiplied electrons can be detected by the detector (also called a "counting detector") of an ion detector.

[0048] The "off" time for each gating cycle may be longer than the detector's dead time.

[0049] If the count value of the previous gating cycle was zero, the "on" time of the gating cycle may be increased.

[0050] The "on" time can be set to a duration such that the probability of the count value exceeding zero during the "on" time of at least one gating cycle is 0.2 to 0.8, preferably 0.4 to 0.6.

[0051] If one or more count values ​​in the previous gating cycle were zero, the "on" time may be increased from the pre-set duration.

[0052] If the ionic intensity of one or more of the previous gating cycles exceeds zero, the "on" time can be kept constant for at least one gating cycle.

[0053] If the probability of the count value being greater than zero is greater than 0.5 (or alternatively, greater than 0.6, 0.7, 0.8, or 0.9), the "on" time may be reduced after a predetermined number of gating cycles.

[0054] If the probability of the count value being zero is less than 0.5 (or alternatively, less than 0.4, 0.3, 0.2, or 0.1), the "on" time may be reduced after a predetermined number of gating cycles.

[0055] If the probability of the count value being greater than zero is less than 0.5 (or alternatively, less than 0.4, 0.3, 0.2, or 0.1), the "on" time may be increased after a predetermined number of gating cycles.

[0056] If the probability of the count value being zero is greater than 0.5 (or alternatively, greater than 0.6, 0.7, 0.8, or 0.9), the "on" time may be increased after a predetermined number of gating cycles.

[0057] The "on" time of the initial gating cycle may be extended until the count value is determined from the detection signal (also called the "counting signal" or "voltage signal").

[0058] The detection device may be kept in "on" mode (the gate allows the passage of ions or secondary electrons) until one count is detected, and then switched to "off" mode (the gate decays ions or secondary electrons). The duration of the "on" mode until a count is detected may be used as an initial value for the "on" time for the initial gating cycle.

[0059] The "on" time can vary between an upper and lower threshold with respect to the number of gating cycles, and the probability of the count value exceeding zero is determined from the determined count value determined during the number of cycles.

[0060] The present invention also relates to an ion detection device, - A conversion element for converting received ions into emitted electrons, - A multiplier unit for multiplying emitted electrons, - An ion detection device is provided, comprising a detector for detecting multiplied electrons, generating a detection signal in response to the detection of multiplied electrons, and determining a count value from the detection signal.

[0061] The ion detector further includes a controller configured to control the number of multiplied electrons reaching the detector by intermittently switching a gate depending on the ion intensity determined by the detector.

[0062] The multiplication unit can be located downstream of the conversion element.

[0063] The detector may be located downstream of the amplification unit.

[0064] Intermittent switching of the gate can be used to control the number of multiplied electrons that reach the detector from the multiplication unit.

[0065] The gate can be located upstream of the transformation element.

[0066] The gate can be located downstream of the transformation element.

[0067] The gate may be a chargeable grid. The grid may be configured to be switched intermittently to attenuate and / or redirect charged particles. Specifically, the grid may be configured to be switched intermittently to attenuate emitted electrons. Alternatively, the grid may be configured to be switched intermittently to redirect ions away from the conversion element (and / or attenuate ions arriving at the conversion element).

[0068] The gate may be an ion optical element positioned upstream of the conversion element. The ion optical element may be positioned to be intermittently switched to redirect ions away from the conversion element.

[0069] The multiplication unit may be a secondary electron multiplier tube. One of several dynodes within the multiplication unit may be arranged to be intermittently switched to attenuate all the multiplied electrons within the multiplication unit.

[0070] The controller may be configured to perform the method of controlling the ion detector described above.

[0071] The present invention also provides a mass spectrometer equipped with an ion detector as described above.

[0072] The detection device may comprise a multiplication section having a plurality of dynodes and a detection section having an electron detector. Each of the plurality of dynodes may be configured to emit an electron when an ion and / or electron collides with the dynode. Each dynode may have a smaller negative potential than the aforementioned dynodes. The detector may comprise a gate located upstream of or inside the multiplication section, which is configured to intermittently attenuate ions or secondary electrons in the ion stream.

[0073] The detection device may be an SEM detection device. The gate may be upstream of the detection's conversion dynode (upstream of the detection's first dynode). One of the dynodes (preferably one of the first few downstream dynodes in the multiplication section) may act as a gate. A grid before or between the dynodes of the detection device may act as a gate. The grid may include a two-dimensional grid and / or parallel wires. Gate control may be provided by an AC voltage on adjacent wires.

[0074] The mass spectrometer may be equipped with a detection device. A mass filter (preferably a quadrupole) upstream of the detection device may function as a gate. [Brief explanation of the drawing]

[0075] [Figure 1] An example of a SEM detection device based on prior art is provided. [Figure 2] An example of a prior art SEM detection device is provided in which the gate is located at the beginning of the counting or pulse section after the analog section. [Figure 3] A schematic example of a SEM detection setup using prior art is provided. [Figure 4] One embodiment of the SEM detection setup according to the present invention is schematically illustrated. [Figure 5] Another embodiment of the SEM detection setup according to the present invention is schematically illustrated. [Figure 6] Further embodiments of the SEM detection setup according to the present invention are schematically illustrated. [Figure 7] Further embodiments of the SEM detection setup according to the present invention are schematically illustrated. [Figure 8] A gating signal that may be used in the present invention. [Figure 9] A schematic representation of some of the gating signals that may be used in the present invention is shown below. [Modes for carrying out the invention]

[0076] In mass spectrometry, secondary electron multiplier tubes can be used to assist in the detection of ions separated by a mass spectrometer. A secondary electron multiplier tube comprises multiple electrodes called dynodes. Each dynode multiplies the incident charge through a process called secondary emission, in which a single charged particle (e.g., an electron) incident on a secondary emission material can induce the emission of more electrons (e.g., 1 to 10 electrons). A potential is applied between each electrode in the SEM and the next electrode. Electrons emitted by the first electrode are accelerated to the next electrode, inducing secondary emission of more electrons from that electrode. This can be repeated multiple times (e.g., using 8 to 14 secondary electrode dynodes), resulting in a large number of multiplied electrons emitted from the last electrode. The multiplied electrons can be detected as an electric current, or pulses of multiplied electrons can be capacitively or inductively coupled and counted by a counting electronic circuit.

[0077] Typically, the analog mode (where current is measured) is used for low-amplification modes, while the counting mode uses high gain. By providing dual modes, a linear dynamic range of up to nine orders of magnitude can be achieved, allowing the principal and minor components of a sample to be measured in a single run.

[0078] In some dual-mode detectors, a gate is provided at the beginning of the counting section after the analog section. This is illustrated in Figure 2. This gate is located after the analog section and can switch off the electron beam entering the counting section. Switching off the electron beam entering the counting section can help increase the lifetime of the pulse detector 13.

[0079] Figure 2 illustrates a schematic configuration of a SEM detector device 10 used in some prior art mass spectrometers. The ion detector device 10 comprises a multiplication section 12 containing a plurality of dynodes 12A to 12L, and a detection section or detector 13. Figure 2 also illustrates a schematic diagram of the gating principle. The ion beam IB is directed to the transforming dynode 12A of the multiplication section 12. A deflector 11 may be used to direct the ion beam (although this is optional). When the ion beam IB strikes the transforming dynode 12A, the transforming dynode emits secondary electrons. The transforming dynode 12A of the detector may be set to a moderate voltage, e.g., 2kV in a typical ICP instrument, or higher in other instruments. Electrons emitted from the transforming dynode are attracted by the electric field towards the next dynode, causing a multiplied emission of secondary electrons. Because a potential difference exists between adjacent dynodes, each dynode emits electrons in response to collisions of charges received from the previous dynode (or, in the case of a conversion dynode, from the ion beam). In counting mode, electrons emitted by the last dynode 12L are directed to the detection section of the detection device 10 or detector 13, which generates a detection signal output P. Thus, counting mode records the electrons arriving at the final stage of the multiplication section. In analog mode, an analog signal "A" is measured from the central dynode 12G in the multiplication section. Relatively high ion currents can be measured using analog mode, while relatively small ion currents can be measured using counting mode. Combinations of these modes are intended to provide a wider dynamic measurement range.

[0080] Figure 3 schematically illustrates an SEM detector configuration that may include the SEM detection device 10 shown in Figure 2. The SEM detector configuration 1 in Figure 3 comprises a conversion dynode 12A, an electron multiplier tube (multiplier section) 12, a pulse detector (or counter) 13, a current detector 14, and a processor 15. The multiplier section 12 includes a gate 16 positioned between an analog section and a pulse section. The analog section may supply electrons to the current detector 14, while the pulse section may supply pulses to the pulse detector 13. The processor 15 provides a gating signal to the gate 16, which may be based on a current I detected by the current detector or a pulse P counted by the pulse detector 13. The gating signal may switch off the pulse / count section during periods when pulse counting is not desired.

[0081] The need for an analog detection mode is the source of problems in the prior art. Eliminating analog detection (at least for accuracy measurement) would result in an improved system. However, prior art devices rely on analog mode in combination with counting mode to increase the dynamic range of the detector to the required level. A gating function may be provided in a dual-mode detector to switch between modes. In the detection device of Figure 2, a gating signal "G" is applied to the dynode 12I of the multiplication section 12 (downstream of the dynode 12G where the analog signal "A" is measured). In the prior art device, the gate (consisting of the dynode 12I in Figure 2) is provided after the analog section and is used to switch off the electron beam entering the counting (or pulse) section. This is done by varying at least one voltage of the dynode between a first voltage through which electrons pass the dynode and a second voltage through which electrons do not pass. Thus, the gating signal blocks or attenuates electrons moving down the multiplication section, preventing large electron pulses from reaching the detector 13.

[0082] As described above, gating functions are used in prior art devices to switch between analog mode and counting mode. In contrast, the present invention proposes applying a voltage oscillation to the gate based on the count value determined by the detector. This voltage oscillation (which may include a voltage that repeatedly switches between two states and / or substantially sinusoidal oscillations) will have an "on" time (during which electrons pass through the gate normally) and an "off" time (during which electrons are attenuated or blocked). Preferably, the voltage oscillation is a rectangular voltage oscillation, as illustrated in Figure 8. However, the voltage change may not be instantaneous, and the sides of the wave may exhibit a slight slope. In the example shown in Figure 8, each pulse defines the "on" time (OT). In the example in Figure 8, the average voltage V A This is correct.

[0083] During the "on" time, the detector can count the number of pulses (e.g., 0, 1 or more).

[0084] The number of pulses received by the detector follows approximately a Poisson distribution. The Poisson distribution represents the probability of the number of pulses (0, 1, 2, etc.) occurring during the "on" time. Pulse events occur at a nearly constant average rate and are largely independent of the time elapsed since the last pulse.

[0085] The detector may count pulses during the "on" time but may not count pulses during the "off" time (either because gating prevents electrons from reaching the detector or because the number is sufficiently attenuated).

[0086] The detector may be further configured to adjust the length of the "on" time so that pulse events occur in the detector only for a portion of the "on" time. These pulse events are also referred to as "counts" or simply "events." The probability of an event not occurring follows Poisson statistics.

[0087]

number

[0088] In the formula, I is the intensity in counts per second, and t オン This is "on" time.

[0089] This can be rewritten as follows:

[0090]

number

[0091] Therefore, given duration t オン For each "on" time (also referred to as an "on" period), the inventors predict a specific probability that zero counts exist during that period, and a complementary probability (the probability of zero counts being present minus 1) that at least one count exists during that period. In other words, each "on" period can be classified by one of the following complementary events: Events in which zero pulses are detected, and An event in which one or more pulses are detected.

[0092] "On" time オン The probability of a zero pulse occurring during this period can be estimated from the detection results, and the intensity of the ion beam can be calculated using equation 2.

[0093] One way the probability can be estimated is by repeatedly checking the fixed-duration "on" time over several cycles and counting the proportion of time during which a zero pulse is detected. This proportion is an estimate of the probability, and it becomes more accurate with more cycles.

[0094] Having a probability of approximately 0.5 is advantageous, and therefore, the "on" time t オン Preferably, this value is set such that no count is detected during the "on" period, and approximately half of the "on" period has elapsed.

[0095] One reason why a probability of approximately 0.5 is preferable is that when a probability is determined to be approximately 0.5, it is more likely to be accurate with fewer experiments ("on" time). If the probability is close to 0, the number of periods during which one or more pulses are obtained is small, and therefore the estimate of the probability may not be very accurate.

[0096] The present invention also provides a method for setting the duration of the “on” time. As described above, the “on” time is set to adjust the probability to be close to 0.5. Typically, it is preferable to set the duration of the “on” time to a suitable value before the intensity measurement is determined (although the “on” time can also be adjusted in a similar manner while the measurement is being determined). There are several different ways in which the duration of the “on” time can be set, which are described individually below.

[0097] In the first preferred method, measurements from the last on-time are used, and the duration of on-time is continuously fitted.

[0098] Advantageously, the first method is possible whenever the detection signal increases (e.g., from a lower counting area region to a higher counting area region) or decreases (e.g., from a higher counting area region to a lower counting area region). Thus, this method can provide updated intensity readings in response to ion beams of changing intensity.

[0099] In the second method, the analog detector output is used to determine the on-time duration. This method requires a cross-calibration coefficient. Due to the discrepancy between the expected probability and the observed probability, the cross-calibration coefficient can be continuously updated without user interaction. The analog output may be used for cross-calibration when setting the "on" time in counting mode, but the intensity of the detection signal should preferably be measured in counting mode rather than using analog mode.

[0100] A third method involves attenuating the ion beam with any lens within the mass spectrometer. For example, a quadrupole lens can be used to attenuate the ion beam arriving at the detector. In this case, the following steps may be performed: If the detection signal is higher than the normal counting range, the ion beam is attenuated until the detection signal falls within the counting range. As explained above, the attenuation is gradually reduced while adjusting the on-time to the required duration. Alternatively, the stored dependency between the Q-focus voltage and attenuation can be used to directly provide an approximate "on" time.

[0101] In other words, the lens of the mass spectrometer upstream of the SEM detector (e.g., a quadrupole) may be used to attenuate the ion beam in order to reduce the count to the detector downstream of the multiplication section. This may be done in addition to switching the gate. Once the gate timing is established to yield the desired detection probability within each cycle, the attenuation is reduced and the gate timing is readjusted.

[0102] A fourth method is to increase the "on" time until a suitable value is found. This method may involve performing a "sweep." The following steps may be performed. 1. Start with a short "on" time. 2. Perform several cycles and estimate the probability of zero counts. (For example, 100 cycles). 3. If the probability of zero counts is not approximately 0.5, increase the "on" time. 4. Proceed to step 2.

[0103] Sweeping can be performed in step 3 by increasing the "on" time by a predetermined process size. Alternatively, the "on" time can be continuously adjusted.

[0104] Furthermore, if the estimated probability is close to the probability target of 0.5, the sweep process may be performed with a reduced process size. Conversely, if the estimated probability is far from the probability target of 0.5, the process size may be increased.

[0105] In one example of continuous adjustment of the "on" time, it may not be necessary to perform a fixed number of cycles to estimate the probability. Instead, the "on" duration may increase with each zero-count period and decrease with each period of one or more counts. The duration of the "on" time should eventually settle around an "on" time that gives a probability of approximately 0.5.

[0106] A moving average of the intensity can be taken, and the probabilities can be estimated using the formula provided in equation (2). The sweep can be stopped when the estimate of the probability falls within a predetermined threshold of the target probability (the desired accuracy is reached). This may be the case if shot noise statistics are assumed:

[0107]

number

[0108]

number

[0109] The desired accuracy could be, for example, 1% (0.01).

[0110] In another example, the process size can be based on the estimated probability for the previous "on" time. Therefore, if the estimated probability p(0) = 0.9, a larger process size may be applied (e.g., increasing the "on" time by ln(0.5) / ln(0.9) = 6.56 times), and if the estimated probability p(0) = 0.6, a smaller process size may be applied (e.g., increasing the "on" time by ln(0.5) / ln(0.6) = 1.36 times). Generally,

[0111]

number

[0112] Because the number of cycles required to determine the probability is small, the estimate may not be accurate. The new "on" time can then be used to increase the number of cycles to provide updated probabilities and updated tones, etc.

[0113] Sweeping can also be done in reverse (starting with a longer "on" time and decreasing). However, starting with a shorter "on" time and increasing is more likely to reach the desired "on" time faster than starting with a longer "on" time and decreasing.

[0114] The number of "on" time events can also change over time. In other words, the number of cycles may be quite low at the start if the "on" time is likely to require considerable adjustment, and may increase as the "on" time approaches the desired value (when the probability is close to 0.5) to obtain a more accurate estimate of the probability.

[0115] When the "on" time is selected, measurement may be performed by counting the number of cycles in which zero or one or more counts are detected. The result of the count may be used to adjust the "on" time setting for the next series of cycles.

[0116] In the fourth method, a sweep is performed over a range of "on" time. The "on" time varies (arbitrarily swept) between an upper and lower threshold with respect to the number of cycles, and the electron detection probability is determined from the detection results for the number of cycles.

[0117] In the fifth method, if no electrons are counted by the counting detector during at least one switching cycle of the gate, the on time is increased from a preset duration. The "on" time is kept constant for at least one cycle if electrons were counted by the counting detector during the previous switching cycle of the gate. The "on" time is increased after a predetermined number of switching cycles of the gate if the probability of detecting electrons is less than 0.5.

[0118] In the sixth method, the probability for a fixed number of cycles (e.g., 100) is determined, and the "on" time is increased if the probability is less than 0.5. The detection probability is calculated for a number of cycles with a constant "on" time, and the "on" time is increased if the detection probability is less than 0.5.

[0119] The seventh method involves mixing pulses of different lengths and evaluating the results.

[0120] For example, the "on" period can vary incrementally from an upper limit to a lower limit (one cycle for each process size). The preferred duration can be estimated from the results. This can be done as a sweep over possible "on" times. Alternatively, a statistical mixture of "on" times can be used (in other words, larger processes between less likely values ​​and smaller processes between more likely values).

[0121] A sufficient "off" time is required between each iteration. The "off" time is preferably as short as possible so as to reduce the required time. In one example, the "off" time may be set to a dead period. Alternatively, the "off" time may be shorter than (and as short as possible) the dead period, as long as at least one count is not observed (in this case, the "off" time should be the dead period). This technique can reduce the overall required time. This can be applied to any of the methods 1-7 mentioned above, and can also be applied when taking measurements.

[0122] In the eighth method, the time from when the gate opens until the first pulse arrives is measured. This is repeated multiple times (e.g., 10 times). In this case, there is a risk that several pulses may pass through the detector before the gate is closed. This is because the execution time effect in the detection device can cause a delay between the electrons passing through the gate and the detection signal that will be detected. Furthermore, there may be a time lag between the detection of the detection signal and the closing of the gate. Therefore, a second pulse may pass through the gate before it is closed.

[0123] In the eighth method, the gate is kept open until an electron is detected. The "on" time is determined by measuring the time it takes for the first secondary electron to be detected by the detector while the gate is kept open to allow the passage of ions or secondary electrons.

[0124] Another option is to end the "on" time immediately after the count is registered. In this case, the execution time of the pulse passing through the detector should also be taken into consideration.

[0125] Examples of setting the "on" time (according to the fourth, fifth, and sixth methods) are provided below.

[0126] First, we assume an intensity of 2 million counts / second (cps). We start with an "on" time of 1 nanosecond (ns) and an "off" time of 1 ns. The probability of 0 counts during the "on" period is 1.00 (or more precisely, 0.998) from equation 1. After 10 "on" periods with no events (totaling 20 ns), it is determined that the "on" time should be increased.

[0127] Next, the "on" time is increased to 10 ns. If no events occur, the "off" time is set to 1 ns; if one or more events occur, the "off" time is set to 20 ns. The probability of zero events occurring during the "on" time is 0.98. Therefore, after 10 "on" periods, the inventors expect that 10 "on" periods without counts have been registered. This takes 110 ns.

[0128] Next, the "on" time is increased to 100 ns. Then, the probability of zero events occurring during the "on" period is 0.82. The expected number of periods in which events occur is 1.8. In this example, we assume that there were two "on" periods in which at least one event occurred. The total duration of the "on" and "off" times is 100 ns × 10 ("on" time) + 20 ns × 2 ("off" time after an event) + 1 ns × 8 ("off" time after no event) = 1.048 microseconds (μs). Based on the detection results, the probability is estimated to be 0.8. Therefore, the intensity determined from the estimated probability is 2.23 million cps (from equation 2). The calculated "on" time (based on the detection results) required to make the probability 0.5 is provided by the following equation.

[0129]

number

[0130] This gives a true probability of 0.537 for zero events in this example (from Equation 1). This can be sufficiently accurate depending on the requirements set in this method. In a preferred example, probabilities of 0.4 to 0.6 are acceptable. Therefore, t オン This value provides the probability that it has an acceptable range.

[0131] Overall, determining the correct "on" time took 1.168 μs in this example.

[0132] Alternatively, instead of measuring over a fixed time, measurements can be taken until the shot noise limit reaches a predetermined value. If an accuracy of 1% is desired, this would mean 10,000 counts. This is because, when the noise is shot noise limited, the noise is proportional to the square root of the ions. Therefore, to obtain a 1% RMS, 1 / square(0.01) = 10,000 ions are required. For 2 million cps, this would require less than 0.03 seconds. The "on" time can be fine-tuned during this measurement.

[0133] Theoretically, the proposed method could provide a detector with an infinite dynamic range. As long as ions reach the second dynode as separate events, the dynamic range is limited only by the electronic equipment and its ability to set very short and precise "on" and "off" times. When the electronic equipment is at its limits, for example, when the pulse shape deviates from a rectangular shape, this can be calibrated.

[0134] An illustrative example is provided in Figure 9, which illustrates a graph of (a portion of) the gating voltage over time. The pulses received by the detector are illustrated on the same axis. During the "on" time OT, two events occurred, namely the detector pulses.

[0135] If the start / initiation of an electronic pulse occurs during the "on" time, the pulse will continue to be detected even after the gate is closed again. Closing the gate can prevent further pulses from reaching the detector, but pulses initiated during the "on" time may not be blocked.

[0136] For this reason at least, it is preferable to set the "off" time to be at least the same length as the detector dead time. If the start of the pulse coincides with the end of the "on" period, the event may be detected during the "off" period (when the discriminator levels cross). In this case, the duration of the "off" period may need to be slightly longer than the detector dead time by at least the duration of the pulse.

[0137] There may also be a delay required for the pulse to propagate through the detector. This delay is not illustrated in Figure 9. Those skilled in the art will understand the modifications necessary to explain this delay.

[0138] Ionic strength can also be calculated using the following alternative methods. 1. Set the gate to the "ON" position. 2. Measure the time it takes for the first ionic event to be registered. 3. Set the gate to the "off" position during the dead period. 4. Repeat the above steps and calculate the average time between ionic events.

[0139] Intermittent gating can be performed at several different locations within the SEM detection setup. In the first embodiment, gating can be performed using an existing gate located at an existing location within the multiplication section. However, in contrast to prior art devices (which use gates to switch between analog and counting modes), the proposed method applies an intermittent gating signal to the gate based on the determined count value. The first embodiment is illustrated in Figure 4.

[0140] The SEM detection configuration 1 in Figure 4 also comprises a conversion dynode 12A, a multiplication section 12, a pulse detector or counter 13, and a processor 15, similar to the prior art configuration in Figure 3. The multiplication section 12 also comprises a gate 16 located between the analog and pulse sections or the multiplication section, which are components referred to as analog and pulse sections in the prior art apparatus (in the detector configuration according to the present invention, only pulses are detected, not currents, and consequently, a current detector may be omitted). According to the present invention, the processor 15 provides an intermittent gating signal to the gate 16, which is based on pulses P counted by the pulse detector 13. The intermittent gating signal periodically switches off the downstream portion of the multiplication section.

[0141] As schematically illustrated in Figure 4, gating may be performed at a different dynode of the detector (such as a conversion dynode or one of the first few dynodes) rather than using a gate at a location present in the dual-mode detector (a dynode after the analog section AS). Figure 5 illustrates a second embodiment of the present invention. As seen in Figure 5, the gate is moved from a conventional location after the analog section, as in the prior art (Figure 3), to a location toward the front of the electron multiplier tube 12. This location may correspond, for example, to a second, third, or fourth dynode. The current detector (14 in Figure 3) is no longer needed and has been removed. The gating signal previously switched between analog mode and counting mode is replaced by an intermittent gating signal based on the determined count value. By using intermittent gating as described herein, the dynamic range of the counting mode is such that the analog mode is not required (at least for measuring ionic intensity, although the analog mode is used in some of the ways described above). オン It can be increased (which can be used to set)

[0142] Alternatively or additionally, a grid may be provided to switch the electron beam or ion beam on / off. Thus, the gate function may be provided by a separate grid instead of by a dynode. The grid may be provided before the electron multiplier tube (as shown in the third embodiment illustrated in Figure 6) or before the ion converter (as shown in the fourth embodiment illustrated in Figure 7).

[0143] In the embodiment shown in Figure 6, the multiplication section 12 does not have a gate. Instead, a gate 16 (composed of a grid) is located here immediately downstream of the conversion dynode 12A and before the multiplication section 12. In this embodiment, an intermittent gating signal generated by the processor 15 is used to intermittently prevent electrons generated by the conversion dynode 12A from reaching the multiplication section 12. As a result, the multiplier section 12 intermittently generates electrons, and the detector 13 intermittently detects electrons.

[0144] In the embodiment shown in Figure 7, the multiplier section 12 also has no gate. Instead, a gate 16 (composed of a grid) is located here in front of the conversion dynode 12A. In this embodiment, an intermittent gating signal generated by the processor 15 is used to intermittently prevent ions from reaching the conversion dynode. As a result, the multiplier section 12 intermittently generates electrons.

[0145] The principles described above in relation to this method can be applied wherever gating is implemented.

[0146] It is preferable to perform gating in the electron multiplier tube earlier than when gating is present in prior art devices. This is because earlier gating can reduce the power consumption of the device, since electron multiplication does not occur when the gate is off. Furthermore, gating should preferably be performed when the pulse is as short as possible. If gating is performed with ions before the SEM, the pulse becomes a delta function with zero pulse width.

[0147] In some cases, it may be advantageous to retain the gate's position after the analog section rather than moving the gate upstream. In this way, existing devices can be modified to implement the invention with fewer changes than would be possible if the gate's position were moved.

[0148] It may be advantageous to provide multiple gates to different stages of the device and drive all gates with the same gating signal. If the gates can only attenuate charged particles rather than completely block them, this can further reduce the total number of multiplied electrons arriving at the detector in off mode.

[0149] As explained above, the following alternative embodiments exist. In one embodiment (first embodiment), the dynode after the analog output provides a gating function. In an embodiment where one of the first dynodes is used to provide a gating function (preferably one of the first five dynodes, in a second embodiment), Embodiments in which the grid is provided inside the SEM (third embodiment), and Embodiments providing a different type of energy filter (which may be provided by a combination of grids, deflection plates, ion choppers, special grids, etc.).

[0150] In a second embodiment, one of the dynodes preceding the analog output (where the analog output is held) may be used to provide a gating function. In another example, the gating function may be provided by a conversion dynode, a second dynode, a third dynode, a fourth dynode, or a fifth dynode.

[0151] Where used herein, including in the claims, singular terms are interpreted as including plural forms unless otherwise indicated by context. For example, unless otherwise indicated by context, singular references in the claims, such as "a" or "an" (e.g., analog-to-digital converter), mean "one or more" (e.g., one or more analog-to-digital converters). Throughout the specification and claims of this disclosure, words such as "comprise," "including," "having," and "contain," as well as variations thereof, such as "comprising" and "comprises" or similar, mean "including but not limited to," and are not intended to exclude other components.

[0152] The embodiments described herein are described with reference to a specific type of apparatus and application (specifically, an SEM detector deployed within an ICP-MS or ICAP(trademark) apparatus), and while the embodiments have particular advantages in such cases as considered herein, the methods described herein may be applicable to other types of apparatus and / or applications. Specific structural details of the apparatus may be substantially modified to arrive at an apparatus having similar or identical operation, although this may be potentially advantageous (particularly considering the limitations and capabilities of known systems). Each feature disclosed herein may be replaced by alternative features serving the same, equivalent, or similar purpose unless otherwise described. Thus, unless otherwise described, each feature disclosed is merely an example of a general set of equivalent or similar features.

[0153] Any and all examples or illustrative language provided herein (such as “for instance,” “such as,” “for example,” etc.) are used solely to better illustrate the invention and not to limit its scope, and unless otherwise claimed, they are within the scope of the invention. Nothing in this specification should be construed as indicating an element not claimed to be essential to the practice of the invention.

[0154] Any process described herein may be performed in any order or simultaneously, unless otherwise specified or the context requires.

[0155] All embodiments and / or features disclosed herein can be combined in any combination, except for any combination in which at least some of such features and / or processes are mutually exclusive. There may be specific combinations of embodiments that yield further benefits, such as embodiments for determining a set of compensation parameters and applying the set of compensation parameters to a measurement, as described herein. Specifically, preferred features of the present invention are applicable to all embodiments of the present invention and can be used in any combination. Similarly, features described in non-essential combinations can be used separately (rather than in combination).

Claims

1. A method for controlling an ion detection device, Converting received ions into emitted electrons, The emission of the aforementioned electrons is multiplied, To detect the multiplied electrons, In response to that, generate a detection signal, This includes determining the ion strength from the detection signal, A method characterized by controlling the number of multiplied electrons reaching the detector by intermittent gating depending on the ion strength.

2. The method according to claim 1, wherein determining the ion intensity from the detection signal includes determining the ion count value, and the number of multiplied electrons reaching the detector is controlled by intermittent gating depending on the count value.

3. The aforementioned intermittent gating is Before releasing the aforementioned electron, After releasing the aforementioned electrons, and / or The method according to claim 1 or 2, which is performed after the emission of the electrons but before multiplying the emitted electrons.

4. The emitted electrons are attenuated by the gating, and / or The method according to any one of claims 1 to 3, wherein the ions are deflected by the gating.

5. Controlling the number of multiplied electrons reaching the detector by intermittent gating is, Intermittently switching the charging grid to attenuate the emitted electrons or redirect the ions from the conversion element, The ion optical system located upstream of the conversion element is intermittently switched to redirect the ions away from the conversion element, The method according to any one of claims 1 to 4, comprising one or more of the following: intermittently switching one of a plurality of dynodes in a multiplication unit to attenuate all multiplied electrons in the multiplication unit.

6. The aforementioned intermittent gating is carried out by repeating the gating cycle. The aforementioned gating cycle is During the "off" time, the number of amplified electrons reaching the detector is attenuated by the intermittent gating. The method according to any one of claims 1 to 5, comprising an "on" time during which the number of amplified electrons reaching the detector is not attenuated by the intermittent gating.

7. The duration of the "on" time and the duration of the "off" time are controlled during the intermittent gating depending on the ion intensity, and / or The method according to claim 6, wherein the “off” time of each gating cycle is greater than or equal to the dead time of the detector.

8. The aforementioned "on" time is If the ionic intensity in the previous gating cycle was zero, the "on" time of the gating cycle is increased, The "on" time is set to a duration such that the probability of the ion intensity being greater than zero during the "on" time of at least one gating cycle is 0.2 to 0.8, preferably 0.4 to 0.

6. If the ion intensity for one or more previous gating cycles was zero, the "on" time is increased from a preset duration. If the ionic intensity of one or more previous gating cycles exceeds zero, the "on" time is kept constant for at least one gating cycle. If the probability that the ionic intensity exceeds zero is greater than 0.5, the "on" time is reduced after a predetermined number of gating cycles. The "on" time of the initial gating cycle is extended until the ion intensity is determined from the detection signal, The method according to claim 6 or 7, wherein the "on" time is set based on one or more of the following: that the "on" time varies between an upper threshold and a lower threshold with respect to the number of gating cycles, and that the probability of the ion intensity being greater than zero is determined from the determined ion intensity determined over a certain number of cycles.

9. An ion detection device, A conversion element for converting received ions into emitted electrons, A multiplier unit for multiplying the emitted electrons, A detector, The amplified electrons are detected, In response, a detection signal is generated, The system includes a detector for determining ion intensity from the aforementioned detection signal, The ion detection device further comprises a controller configured to control the number of multiplied electrons reaching the detector by intermittently switching a gate depending on the ion intensity determined by the detector.

10. The gate is located upstream of the conversion element, or The ion detection apparatus according to claim 9, wherein the gate is located downstream of the conversion element.

11. The ion detection apparatus according to claim 9 or 10, wherein the gate is a charged grid, and the grid is configured to be switched intermittently so as to attenuate emitted electrons or redirect ions from the conversion element.

12. The ion detection device according to claim 9, wherein the gate is an ion optical system located upstream of the conversion element, and the ion optical system is arranged to be intermittently switched to redirect ions from the conversion element.

13. The ion detection apparatus according to claim 9, wherein the multiplication unit is a secondary electron multiplier tube, and one of a plurality of dynodes in the multiplication unit is arranged to be intermittently switched to attenuate all the multiplied electrons in the multiplication unit.

14. The ion detection apparatus according to any one of claims 9 to 13, wherein the controller is configured to perform the method according to any one of claims 1 to 8.

15. A mass spectrometer comprising an ion detection device according to any one of claims 9 to 14.

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