High dynamic range ion detector for mass spectrometers

Abstract

The invention relates to the linear dynamic range of ion abundance measurement devices in mass spectrometers, such as time-of-flight mass spectrometers. The invention solves the problem of ion current peak saturation by producing a second ion measurement signal at an intermediate stage of amplification in a secondary electron multiplier, e.g. a signal generated between the two multichannel plates in chevron arrangement. Because saturation effects are observed only in later stages of amplification, the signal from the intermediate stage of amplification will remain linear even at high ion intensities and will remain outside saturation. In the case of a discrete dynode detector this could encompass, for example, placement of a detection grid between two dynodes near the middle of the amplification chain. The invention uses detection of the image current generated by the passing electrons.

Claims

1. An ion detector system for mass spectrometers, comprising a secondary electron multiplier having at least two consecutive multiplication stages that produce an avalanche of secondary electrons being used to generate a final signal at the end of the multiplication stages, the ion detector system further comprising a grid-like detection element which is installed between the multiplication stages and in which an image current is induced, the image current being used to generate an intermediate signal at intermediate amplification.

2. The ion detector system according to claim 1, further comprising a second grid-like detection element at the end of the multiplication stages to generate the final signal based on an image current induced in the second grid-like detection element.

3. The ion detector system according to claim 2, wherein the detection elements are conducting plates with holes having an open area ratio which allows an electron transmission efficiency of 90% or greater.

4. The ion detector system according to claim 3, wherein an aspect ratio of the holes, i.e. depth divided by diameter, is approximately unity.

5. The ion detector system according to claim 3, wherein the holes form a hexagonal array.

6. The ion detector system according to claim 3, wherein the detection elements are enclosed on two sides by shielding grids.

7. The ion detector system according to claim 1, further comprising a processor that receives the final signal and the intermediate signal and calculates a value proportional to an impinging ion current, the processor calculating said value from the final signal when the final signal is not in saturation, and calculating said value from the intermediate signal when the final signal is in saturation.

8. The ion detector system according to claim 1, further comprising a processor that receives the final signal and the intermediate signal, uses scaled data from the intermediate signal to replace saturated data from the final signal and calculates a value proportional to an impinging ion current from the final signal thusly corrected.

9. The ion detector system according to claim 1, wherein the grid-like detection element is a wire grid having a transmission higher than 90 percent.

10. The ion detector system according to claim 9, wherein the intermediate signal is based on the image current at this wire grid.

11. The ion detector system according to claim 1, further comprising amplifiers and digitizers for both the final signal and the intermediate signal.

12. A time-of-flight mass spectrometer having an ion detector system for mass spectrometers, the ion detector system comprising a secondary electron multiplier having at least two consecutive multiplication stages that produce an avalanche of secondary electrons being used to generate a final signal at the end of the multiplication stages, wherein the ion detector system further comprises a grid-like detection element which is installed between the multiplication stages and in which an image current is induced, the image current being used to generate an intermediate signal at intermediate amplification.

13. The ion detector of claim 1, wherein the detection element has holes an aspect ratio of which, i.e., depth divided by diameter, is approximately unity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 presents a state-of-the-art MCP ion detector using two microchannel plates (MCP) in chevron arrangement. Under normal operation conditions, each of the two microchannel plates will amplify by a factor of about 1000, resulting in a total amplification of 10.sup.6, i.e. a million secondary electrons will be emitted for each ion impinging the plates. If more than 10.sup.4 ions arrive within the digitizing period of about a quarter of a nanosecond, the second MCP can no longer deliver the more than 10.sup.10 secondary electrons required for a signal which is proportional to the ion current. The linear dynamic range thus is restricted to a maximum of about 1:10.sup.4. If the MCPs are adjusted in such a manner that one ion yields about 30 counts of the digitizer, the linear dynamic range is reduced to 1:300 only. With a 8 bit digitizer, the linear dynamic range is further reduced to 1:8 only; even using a most modern digitizer with 12 bit, the linear dynamic range is still reduced to about 1:100. The grid with high transmission in front of the anode serves (in a known way) to screen the anode from induced image currents by the incoming electron pulse which would lead to the deterioration of the shape of short ion pulses.

(2) FIG. 2 illustrates an improvement of the linear dynamic range known in the state of the art. In addition to the high transmission screening grid 2 in front of the anode, a grid 1 is installed with about 50% transmission between the two microchannel plates MCP 1 and MCP 2. About 50% of the electrons from the first MCP fall on the grid and produce signal 1 while the remaining 50% of electrons impinge on MCP 2 for further amplification. The electrons from MCP 2 are collected by the anode and produce signal 2. Under preferred operation conditions, signal 2 would be about 1000 times higher than signal 1. But whereas signal 2 is exposed to saturation, signal 1 remains linearly proportional to the incoming ion current. The separate amplification and digitization of signals 1 and 2 allows for the generation of a combined signal with high linear dynamic range.

(3) FIG. 3 depicts an embodiment in accordance with principles of the present invention. The electron avalanches after MCP 1 and MCP 2 induce image currents of greatly different strength in the two high transmission grid-like detection elements 1 and 2, the image currents of which are amplified and used for the generation of a combined signal with high linear dynamic range.

(4) In FIG. 4, three multichannel plates are used to generate the secondary electron avalanche, and the two high-transmission grid-like detection elements are placed between MCP 2 and MCP 3, thus generating image current signals in a different relation.

(5) FIG. 5 shows the use of hexagonal array detection elements instead of wire grids to optimize the induction of image currents.

(6) FIG. 6 depicts shielding grids before and after the hexagonal array detection elements to sharpen the image current signals.

(7) FIGS. 7A-B illustrate schematically time-of-flight mass spectrometers that may be equipped with ion detector systems according to principles of the present disclosure.

DETAILED DESCRIPTION

(8) While the invention has been shown and described with reference to a number of different embodiments thereof, it will be recognized by those of skill in the art that various changes in form and detail may be made herein without departing from the scope of the invention as defined by the appended claims.

(9) In FIG. 3, two grid-like detection elements are placed before and after the second MCP 2 of an arrangement that would normally be used in an MCP detector. The detection elements may, for example, be configured for 90% transmission so that 90% of the electrons from the first MCP pass through detection element 1 and strike MCP 2 for further amplification. The electrons produce in detection element 1 an image current called signal 1. Electrons from MCP 2 pass through detection element 2 and produce an image current called signal 2. (See, for instance, M. A. Park and J. H. Callahan, Rapid Com. Mass Spectrom. 8 (4), 317, 1994). The passing electrons are neutralized at the anode. Signals 1 and 2 may be recorded independentlyi.e., in separate channels of a digitizerand then recombined in-silico or in a processor to produce a spectrum of higher dynamic range. The measurement of the image current for both signal 1 and signal 2 via substantially identical detection elements has the advantage, that both image currents have the same profile in time.

(10) If an array of thin wires is used as the detection element, there is a danger that the signal could be somewhat distorted by electrons impinging on the wires. If the electrons are absorbed, there is an additional electron current, but if the impingement causes secondary electrons to leave the wire, the image current is reduced by this current of leaving electrons. It is, therefore, advantageous to reduce the formation of secondary electrons at the wires of the grid by methods known to those of skill in the art. For example, one may make the wires of the detection element from conductors known to have a high work functione.g. platinumor known to form thin oxide layers known to have high work functionse.g. tungsten oxide. Higher work functions will lead to lower rates of electron emission. Ideally, absorbed electrons and generated secondary electrons should be in balance.

(11) In an alternate embodiment, the current generated in the anode by the impinging electrons can be measured instead of the image current of detection element 2, and then compared and/or combined with signal 1 in a processor, for instance.

(12) Still other embodiments may comprise double MCPs instead of a single MCP, as shown in the example of FIG. 4. In this case, the MCP.sub.1,2 should be operated by a lower voltage to avoid early saturation, but this arrangement allows the option of a higher gain before further amplification by MCP 3.

(13) The generation of image currents may be optimized by using detection elements with holes having high aspect ratios, as shown by way of example in FIG. 5. The aspect ratio may be defined as the depth of the holes divided by their diameter. According to the embodiment of FIG. 5, the detection element encompasses a thin conducting plate having a high open area ratiothe open area consisting of holes having high aspect ratios. The high open area ratio allows for high electron transmission efficiency, preferably 90% or greater. The aspect ratio of the holesthe depth of the holes divided by their diameteris preferably such that at some point during the transit of electrons through the detection element, near 100% of the field lines of the electrons terminate on the detection element, thus, guaranteeing the maximum possible image current. It should be noted, however, that an excessively high aspect ratio will result in a non-Gaussian, flat top signal of the image current. Thus, there is a preferred aspect ratio whereby the maximum induced signal occurs when, and only when, the electron is exactly half way through the detection element.

(14) In one preferred embodiment, the aspect ratio is approximately onei.e. the thickness of the detection element is about the same as the diameter of the holes there-through, generating a short image current pulse of nearly maximum strength. In the embodiment of FIG. 5 such a high open area ratio, high aspect ratio detection element takes the form of a hexagonal array of holes in a conducting plate. Such detection elements may be produced from metal sheets by chemical etching, or by laser etching. A further method is 3D-printing from metal powder, e.g. Titanium powder. This method is known in the aircraft industry.

(15) The detection elements may be enclosed by high transmission grids to shield them from incoming and departing electrons and thereby avoiding long leading and trailing edges in the signals. This embodiment is presented in FIG. 6.

(16) FIG. 7A shows a MALDI time-of-flight mass spectrometer 100 that includes a pulse laser 6. Samples are located on a sample support plate 1 opposite accelerating electrodes 2 and 3, and can be ionized by a beam of laser light pulses 4. The laser unit 6 supplies the laser light pulses whose profiles are shaped favorably and as required by beam shaping device 5. The resultant ions are accelerated by the accelerating electrodes 2 and 3 to create an ion beam 8, which passes through a gas cell 9 which may be filled with collision gas, if required, a parent ion selector 10, a daughter ion post-acceleration unit 11 and a parent ion suppressor 12, and is then reflected from the reflector 13 onto the ion detector 14 which may be embodied as an ion detector system according to principles of the present disclosure.

(17) The ion detector system according to principles of the present disclosure may also be part of a mass spectrometer like that shown in FIG. 7B. Ions are generated at atmospheric pressure in an ion source 21 with a spray capillary 22, and these ions are introduced into the vacuum system through a capillary 23. A conventional RF ion funnel 24 guides the ions into a first RF quadrupole rod system 25, which can be operated both as a simple ion guide and also as a mass filter for selecting a species of parent ion to be fragmented. The unselected or selected ions are fed continuously through the ring diaphragm 26 and into the storage device 27; selected parent ions can be fragmented in this process by energetic collisions. The storage device 27 has an almost gastight casing and is charged with collision gas through the gas feeder 28 in order to focus the ions by means of collisions and to collect them in the axis. Ions are extracted from the storage device 27 through the switchable extraction lens 29. This lens, together with the einzel lens 30, shapes the ions into a fine primary beam 31 and sends them to the ion pulser 32. The ion pulser 32 periodically pulses out a section of the primary ion beam 31 orthogonally into the high-potential drift region 33, which is the mass-dispersive region of the time-of-flight mass spectrometer, thus generating the new ion beam 34 each time. The ion beam 34 is reflected in the reflector 35 with second-order energy focusing, and is measured in the ion detector system 36 that may operate according to principles of the present disclosure. The mass spectrometer is evacuated by the pumps 37. The reflector 35 represents a two-stage Mamyrin reflector in the example shown featuring a first strong deceleration field, followed by a weaker reflection field.

(18) The invention concerns an ion detector system for mass spectrometers, based on a secondary electron multiplier having at least two consecutive multiplication stages that produce an avalanche of secondary electrons being used to generate a final signal at the end of the multiplication stages. The detector system has a grid-like detection element installed between the multiplication stages which generates an intermediate signal at an intermediate amplification, wherein at least the intermediate signal is based on an image current induced in the grid-like detection element.

(19) The detector system may further comprise a second grid-like detection element at the end of the multiplication stages to generate the final signal based on image currents induced in the second grid-like detection element (just like the intermediate signal). The detection elements can be conducting plates with holes having high open area ratio. In preferred embodiments, an aspect ratio of the holes, i.e. depth divided by diameter, is approximately unity (optimized for maximum image current and short image current pulses). In some embodiments, the holes can form a hexagonal array. It is possible to enclose the detection elements on two sides by high transmission shielding grids.

(20) The detector system may further comprise a processor that uses the final signal to calculate a value proportional to an impinging ion current when the final signal is not in saturation and uses the intermediate signal to calculate a value proportional to the impinging ion current when the final signal is in saturation. In an alternative embodiment, the processor could use scaled data from the intermediate signal to replace saturated data from the final signal and could calculate a value proportional to an impinging ion current from the final signal thusly corrected.

(21) In preferred embodiments, the grid-like detection element may be a high transmission wire grid. Preferably, the wire grid has a transmission higher than 90 percent, and the intermediate signal can be based on the image current at this wire grid.

(22) The detector system may further comprise amplifiers and digitizers for both the final signal and the intermediate signal.

(23) The invention has been shown and described above with reference to a number of different embodiments thereof. It will be understood, however, by a person skilled in the art that various aspects or details of the invention may be changed, or various aspects or details of different embodiments may be arbitrarily combined, if practicable, without departing from the scope of the invention. Generally, the foregoing description is for the purpose of illustration only, and not for the purpose of limiting the invention which is defined solely by the appended claims, including any equivalent implementations, as the case may be.