WO2015104572A1

WO2015104572A1 - Detector current amplification with current gain transformer followed by transimpedance amplifier

Info

Publication number: WO2015104572A1
Application number: PCT/IB2014/002676
Authority: WO
Inventors: Mikhail DOLGANOV
Original assignee: Dh Technologies Development Pte. Ltd.
Priority date: 2014-01-08
Filing date: 2014-12-06
Publication date: 2015-07-16

Abstract

Systems and methods are provided for converting ion strikes to electrical voltage in a mass spectrometer system using a current transformer. Ion strikes are received and electrons are emitted in response to the ion strikes using an electron multiplier. The emitted electrons are received and the emitted electrons are converted to electrical current using a collector. The electrical current is received in a primary winding and a secondary current is produced in a secondary winding in response to the electrical current using a current transformer. The secondary current is received and an electrical voltage is generated using a transimpedance amplifier. The current transformer provides a current gain between the electrical current in the primary winding and the secondary current in the secondary winding. The current gain allows a voltage bias of the electron multiplier to be reduced thereby increasing the lifetime of the electron multiplier.

Description

DETECTOR CURRENT AMPLIFICATION WITH CURRENT GAIN TRANSFORMER FOLLOWED BY TRANSIMPEDANCE AMPLIFIER

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Serial No. 1/925,098, filed January 8, 2014, the content of which is incorporated by reference herein in its entirety.

INTRODUCTION

Typically, systems for pulse counting applications are more sensitive at low counts but are unable to achieve the type of high counts that systems for analog counting applications can typically achieve. For example, in pulse counting detectors that comprise a chain of dynodes, the increased ion flux at the detector can lead to carbon stitching of later dynodes, which can, in turn, reduce the gain of the later dynodes and hence the overall gain of the detector. A bias voltage applied to the detector can be increased to compensate for the decreased gain of the later dynodes. However, as the amount of carbon stitching increases over time, progressively higher bias levels are needed to compensate for the decreased gain. Such high bias levels can cause rapid aging of the detector, and hence reduce the detector's lifetime. Complications related to carbon stitching, rapid aging detectors, and reduced detector lifetime can also affect other types of detectors, including, by way of non-limiting example, continuous dynode detectors.

It is believed that carbon stitching also negatively impacts the count rate of detectors. Conventional systems tend to saturate at count rates above a few million counts per second in pulse counting mode, thus decreasing their accuracy and providing for a limited dynamic range. While efforts have been made to increase the dynamic range of conventional systems, including the use of multiple channels twisted together for continuous dynode detectors to allow a

multiplication effect to occur over multiple channels and decreasing the impedance of a continuous dynode detector to allow for a faster replenishing of the detector bias current, such efforts have had limited success. Accordingly, improved detection systems, devices, and methods are desired.

SUMMARY

A detector is disclosed for use in a mass spectrometer system. The detector includes an electron multiplier, a collector, a current transformer, and a trans impedance amplifier. The electron multiplier receives ion strikes and emits electrons in response to the ion strikes. The collector is disposed downstream of the electron multiplier, receives the electrons emitted by the electron multiplier, and converts the electrons emitted to electrical current. The current transformer is electrically connected to the collector, receives the electrical current from the collector in a primary winding, and produces a secondary current in a secondary winding in response to the electrical current. The transimpedance amplifier is electrically connected to the secondary winding of the current transformer, receives the secondary current, and generates an electrical voltage.

A method is disclosed for converting ion strikes to electrical voltage in a mass spectrometer system. Ion strikes are received and electrons are emitted in response to the ion strikes using an electron multiplier. The emitted electrons are received and converted to electrical current using a collector. The electrical current is received in a primary winding and a secondary current is produced in a secondary winding in response to the electrical current using a current transformer. The secondary current is received and an electrical voltage is generated using a transimpedance amplifier.

These and other features of the applicant's teachings are set forth herein. BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

Figure 1 is a general block diagram of an exemplary mass spectrometry system.

Figure 2 is a schematic diagram of a detection section of an exemplary mass spectrometer that includes an electron multiplier capacitively coupled to a transimpedance amplifier.

Figure 3 is a schematic diagram of a detector of a mass spectrometer that includes an electron multiplier coupled to a transimpedance amplifier using a current transformer, in accordance with various embodiments.

Figure 4 is an exemplary plot of input referred noise versus input pulse width for various permutations of current transformer current transfer ratios and transimpedance amplifier feedback resistances, in accordance with various embodiments.

Figure 5 is an exemplary plot of output pulse widening versus input pulse width for various permutations of current transformer current transfer ratios and transimpedance amplifier feedback resistances, in accordance with various embodiments. Figure 6 is a flowchart showing a method for converting ion strikes to electrical voltage in a mass spectrometer system, in accordance with various embodiments.

Before one or more embodiments of the present teachings are described in detail, one skilled in the art will appreciate that the present teachings are not limited in their application to the details of construction, the arrangements of components, and the arrangement of steps set forth in the following detailed description or illustrated in the drawings. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DESCRIPTION OF VARIOUS EMBODIMENTS

Figure 1 is a general block diagram of an exemplary mass spectrometry system 100. While the systems, devices, and methods described herein can be used in conjunction with many different mass spectrometry systems, mass spectrometry system 100 provides a general framework for describing various embodiments of the applicant's teachings. As shown, in some embodiments, a mass spectrometer 100 includes ion source 110, mass analyzer 120, and detector 130. Ion source 100 emits ions that pass through mass analyzer 120, which allows the passage of certain of those ions, e.g., ions having a mass-to-charge ratio (m/z ratio) in a selected range, to detector 130. As discussed below, the detector 130 can be implemented according to various embodiments of the applicant's teachings.

As described above, in pulse counting detectors that include a chain of dy nodes, the overall gain of the detector can be reduced by increased ion flux due to carbon stitching. In order to compensate for the reduced gain, the bias voltage applied to the detector can be increased. However, increased bias voltage applied to the detector can reduce the detector's lifetime. As a result, improved detection systems that can improve or maintain the overall gain of the detector while, at the same time, reducing the bias voltage are desired.

An exemplary detection system that provides a desired amplification of the output signal while, at the same time, allowing a lower bias voltage is described in International Publication No. WO2013/098597. In this exemplary detection system, the detector includes an electron multiplier and transimpedance amplifier. The signal gain provided by the transimpedance amplifier allows the gain associated with the electron multiplier to be reduced, e.g., by operating the electron multiplier at a lower bias voltage, while obtaining the desired amplification of the output signal generated in response to the incident positive or negative ions. The lowering of the bias voltage applied to the electron multiplier can enhance its lifetime, e.g., by reducing the rate of carbon stitching. Reducing the gain of the electron multiplier by lowering the bias voltage can also lead to a longer lifetime because gain reduction can result in fewer electrons being created within the multiplier and less charge being depleted from the multiplier.

Figure 2 is schematic diagram of a detection section 200 of an exemplary mass spectrometer that includes an electron multiplier capacitively coupled to a transimpedance amplifier. Detection section 200 shows the interface between mass analyzer 210 and detector 220. Ions exiting the last ion guide 212 of mass analyzer are 210 focused via lens 214 and are directed to a high energy conversion dynode (HED) 230 comprising an HED electrode to generate positive ions or electrons in response to impact of negative ions or positive ions, respectively, thereon. HED 230 can be part of detector 220 or it can be implemented as a separate component disposed between mass analyzer 210 and detector 220.

The polarity of HED 230 can be selected (i.e., either positive or negative) based on the polarity of ions to be detected. For example, negative ions exiting the last ion guide 212 can be transmitted toward HED 230 to strike the HED electrode that is maintained at a high positive potential, e.g., in a range of about +5 kV to about +20 kV, though other voltages can also be used. The impact of the negative ions on the HED electrode can cause emission of secondary particles in the form of positive ions, which are directed to a continuous dynode detector or channel electron multiplier (CEM) 222 of detector 220.

CEM 222 can be biased for generating a current signal in response to incident positive ions or electrons/negative ions, respectively. As shown in Figure 2, a high voltage (HV) is applied to the input end of CEM 222 and the output end of the CEM 222 is grounded. In some embodiments, a bias voltage in a range of about 1 kV to about 3kV can be applied across CEM 222. Although in the embodiment shown in Figure 2 a CEM is employed, in other embodiments other types of electron multipliers can be employed. Collector 224 receives a shower of electrons generated by CEM 222 to generate a current signal {e.g., in the form of series of current pulses.

Voltage signal generator 226 receives the current signal generated by the collector 224 and generates a voltage signal based on the current signal. Voltage signal generator 226 includes trans impedance amplifier 227 that is capacitively coupled via signal coupling capacitor 228 to the collector 224. As the output end of the CEM 222 is grounded in Figure 2, capacitor 228 does not necessarily need to be a high voltage capacitor. Capacitor 228 can, nevertheless, act as a filter and can help protect the circuit by limiting the amount of energy that is discharged to the transimpedance amplifier 227 should the energy levels rise above desired levels.

Transimpedance amplifier 227 converts the current signal it receives via capacitive coupling to collector 224 into a voltage signal. Transimpedance amplifier 227 is shown, for example, as an operational amplifier with one of its input ports resistively coupled to its output port and another of its input ports grounded. Transimpedance amplifier 227 can, however, be implemented using any type of transimpedance amplifier circuitry. Further, a voltage signal generated at the output of transimpedance amplifier can be applied to downstream circuits, signal processing, and/or output devices, such as additional amplification stages, computers, and/or display units. By way of non- limiting example, as shown, resistor 229 couples the voltage signal generated by the transimpedance amplifier 227 to the subsequent circuits, signal processing, and/or output devices and can help match the output impedance of transimpedance amplifier 227 with an input impedance of the next stage of signal processing. As described above, the use of transimpedance amplifier 227 allows the bias and gain of the CEM 222 to be reduced, thereby increasing its dynamic range and its lifetime.

In various embodiments, the bias and gain of CEM 222 is further reduced by replacing coupling capacitor 228 with an isolation current gain transformer. In other words, the dynamic range and lifetime of CEM 222 are increased by replacing the capacitive coupling between collector 224 and transimpedance amplifier 227 with transformer coupling. Detector Using a Current Transformer

In various embodiments, a circuit provides detector current amplification using a current transformer followed by transimpedance amplifier to improve signal-to-noise (S/N) ratio of the detector signal and hence the overall instrument sensitivity. For example, the circuit increases overall sensitivity for low mass ions when a detector is floated at high voltage in, for example, QTrap instruments. Lower threshold on detecting signals allows lower detector bias voltage (i.e., CEM's or microchannel plate's (MCP's) operating voltage) that in turn translates into longer lifespan of detectors in, for example, time-of-flight (TOF) applications. One skilled in the art will appreciate that the circuit prolongs lifespan of detectors in other mass spectrometry applications. As a result, customers do not need to change or service detectors as often as they used to.

Figure 3 is a schematic diagram of a detector 300 of a mass spectrometer that includes an electron multiplier coupled to a transimpedance amplifier using a current transformer, in accordance with various embodiments. Detector 300 includes electron multiplier 323, collector 324, current transformer 340, and transimpedance amplifier 327.

Electron multiplier 323 receives ion strikes. Electron multiplier 323 emits electrons in response to the ion strikes. Collector 324 is located downstream of electron multiplier 323. Collector 324 receives the electrons emitted by electron multiplier 323. Collector 324 converts the electrons emitted by the electron multiplier to electrical current.

Current transformer 340 is electrically connected to collector 324. Current transformer 340 receives the electrical current from the collector in primary winding 341. Current transformer 340 produces a secondary current in secondary winding 342 in response to the electrical current. Current transformer 340 electrically isolates collector 324 from trans impedance amplifier 327, blocking direct current between collector 324 and transimpedance amplifier 327. The number of turns in primary winding 341 and secondary winding 342 can be the same, for example.

Transimpedance amplifier 327 is electrically connected to secondary winding 342 of current transformer 340. Transimpedance amplifier 327 receives the secondary current and generates an electrical voltage.

In various embodiments, current transformer 340 also provides a current gain between the electrical current in primary winding 341 and the secondary current in secondary winding 342. The current gain allows a voltage bias of electron multiplier 323 to be reduced, for example. The current gain is produced by providing primary winding 341 with more turns than secondary winding 342. Primary winding 341 can have twice as many turns (2: 1 current transfer ratio) than secondary winding 342, for example. Or, primary winding 341 can have three times as many turns (3: 1 current transfer ratio) as secondary winding 342, for example. As one skilled in the art can appreciate, current transformer 340 is not limited to any specific current transfer ratio.

Comparison tests show how input referred noise is reduced with increasing transformer's current transfer ratio and pulse width dependencies (bandwidth limitations). These tests also show that input referred noise decreases with increasing resistance of feedback resistor 350 of transimpedance amplifier 327.

Also, for electron multipliers, such as electron multiplier 323, in floating configuration (its output is floated- not grounded), the current transformers, such as current transformer 340, are built to withstand very high bias voltage, e.g. 20 kV, with very little inter-winding capacitance. As a result, there is less chance of damaging transformer coupled transimpedance amplifiers by discharge events than capacitive coupled transimpedance amplifiers. Detector 300, however, can in various embodiments still include decoupling capacitor 328. For example, in general transimpedance amplifiers require the input common voltage to be not at ground potential (galvanically decoupled).

Figure 4 is an exemplary plot 400 of input referred noise versus input pulse width for various permutations of current transformer current transfer ratios and transimpedance amplifier feedback resistances, in accordance with various embodiments. Curves 41 1, 412, and 413 show the values for input referred noise as a function of input pulse width when the feedback resistance of a

transimpedance amplifier is 200 ohm and the current transfer ratios of a current transformer are 1 : 1, 2: 1, and 3: 1, respectively. A comparison of curves 411 and 412 shows that there is a large decrease in inferred noise by increasing current transfer ratio from 1 : 1 to 2: 1. However, a comparison of curves 412 and 413 shows that there is much less of a decrease in inferred noise by increasing current transfer ratio from 2: 1 to 3: 1.

Curves 421 , 422, and 423 show the values for input referred noise as a function of input pulse width when the feedback resistance of a transimpedance amplifier is 681 ohm and the current transfer ratios of a current transformer are 1 : 1, 2: 1, and 3: 1, respectively. A comparison of curves 421 , 422, and 423 with curves 411 , 412, and 413 shows that an increase in feedback resistance of a transimpedance amplifier can also decrease the inferred noise.

However, increasing the current transfer ratio and the feedback resistance is not without cost. Increases in both the current transfer ratio and the feedback resistance can widen the output pulse width produced by the transimpedance amplifier reducing the overall bandwidth of the detector.

Figure 5 is an exemplary plot 500 of output pulse widening versus input pulse width for various permutations of current transformer current transfer ratios and transimpedance amplifier feedback resistances, in accordance with various embodiments. Curves 51 1, 512, and 513 show the values for increased output pulse width as a function of input pulse width when the feedback resistance of a transimpedance amplifier is 200 ohm and the current transfer ratios of a current transformer are 1 : 1, 2: 1, and 3: 1, respectively. A comparison of curves 511, 512, and 513 shows that there is a larger increase in output pulse width going from current transfer ratio 2 : 1 to 3 : 1 than from 1 : 1 to 2 : 1.

Curves 521, 522, and 523 show the values for increased output pulse width as a function of input pulse width when the feedback resistance of a

transimpedance amplifier is 681 ohm and the current transfer ratios of a current transformer are 1 : 1, 2: 1, and 3: 1, respectively. A comparison of curves 521, 522, and 523 with curves 511, 512, and 513 shows that an increase in feedback resistance of a transimpedance amplifier can also increase the output pulse width. Method for Converting Ion Strikes to Voltage Using a Current Transformer

Figure 6 is a flowchart showing a method 600 for converting ion strikes to electrical voltage in a mass spectrometer system, in accordance with various embodiments.

In step 610 of method 600, ion strikes are received and electrons are emitted in response to the ion strikes using an electron multiplier.

In step 620, the emitted electrons are received and the emitted electrons are converted to electrical current using a collector. In step 630, the electrical current is received in a primary winding and a secondary current is produced in a secondary winding in response to the electrical current using a current transformer.

In step 640, the secondary current is received and an electrical voltage is generated using a transimpedance amplifier.

While the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art.

Further, in describing various embodiments, the specification may have presented a method and/or process as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the various embodiments.

Claims

WHAT IS CLAIMED IS:

1. A detector for use in a mass spectrometer system, comprising:

an electron multiplier that receives ion strikes and emits electrons in response to the ion strikes;

a collector disposed downstream of the electron multiplier that receives the electrons emitted by the electron multiplier and converts the electrons emitted to electrical current;

a current transformer electrically connected to the collector that receives the electrical current from the collector in a primary winding and produces a secondary current in a secondary winding in response to the electrical current; and

a transimpedance amplifier electrically connected to the secondary winding of the current transformer that receives the secondary current and generates an electrical voltage.

2. The detector of any combination of the preceding claims of a detector, wherein the current transformer electrically isolates the collector from the transimpedance amplifier.

3. The detector of any combination of the preceding claims of a detector, wherein the primary winding and the secondary winding have the same number of turns.

4. The detector of any combination of the preceding claims of a detector, wherein the current transformer provides a current gain between the electrical current and the secondary current.

5. The detector of any combination of the preceding claims of a detector, wherein the current gain allows a voltage bias of the electron multiplier to be reduced.

6. The detector of any combination of the preceding claims of a detector, wherein the primary winding has more turns than the secondary winding.

7. The detector of any combination of the preceding claims of a detector, wherein the primary winding has twice as many turns as the secondary winding.

8. The detector of any combination of the preceding claims of a detector, wherein the primary winding has three times as many turns as the secondary winding.

9. A method for converting ion strikes to electrical voltage in a mass spectrometer system, comprising:

receiving ion strikes and emitting electrons in response to the ion strikes using an electron multiplier;

receiving the emitted electrons and converting the emitted electrons to electrical current using a collector;

receiving the electrical current in a primary winding and producing a secondary current in a secondary winding in response to the electrical current using a current transformer; and

receiving the secondary current and generating an electrical voltage using a

transimpedance amplifier.

10. The method of any combination of the preceding claims of a method, wherein the current transformer electrically isolates the collector from the transimpedance amplifier.

1 1. The method of any combination of the preceding claims of a method, wherein the primary winding and the secondary winding have the same number of turns.

12. The method of any combination of the preceding claims of a method, wherein the current transformer provides a current gain between the electrical current and the secondary current.

13. The method of any combination of the preceding claims of a method, wherein the primary winding has more turns than the secondary winding.

14. The method of any combination of the preceding claims of a method, wherein the primary winding has twice as many turns as the secondary winding.

15. The method of any combination of the preceding claims of a method, wherein the primary winding has three times as many turns as the secondary winding.