Ion analyzer

Abstract

A microchannel plate (MCP) 41 in an ion detection section 4 multiplies electrons. An anode 42 detects those electrons and produces a current signal. An amplifier 44 converts this signal into a voltage signal. A low-pass filter 5A acting as a smoothing section 5 is located at the output end of the amplifier 44. A waveform-shaping time adjuster 6 adjusts the time constant of the low-pass filter 5A beforehand according to the response time of the MCP 41, mass-to-charge ratio of an ion species to be subjected to the measurement, and duration of the spread of the ion species which depends on device-specific parameters. A plurality of peaks which sequentially appear in the detection signal corresponding to one ion species are thereby smoothed into a single broad peak. Thus, the distinguishability between signal waves and noise components is improved.

Claims

1. An ion analyzer having an ion detection section for generating a detection signal corresponding an amount of incident ion, the ion analyzer comprising: a) a signal waveform shaping section which is a smoothing circuit for reducing a higher-frequency component of the detection signal; and b) a time constant adjuster for adjusting a time constant of the smoothing circuit according to at least a duration of an ion species of a same mass-to-charge ratio incident on the ion detection section.

2. The ion analyzer according to claim 1, wherein: the time constant adjuster is configured to adjust the time constant of the smoothing circuit according to the duration of the ion species of the same mass-to-charge ratio incident on the ion detection section and an output response time which is a characteristic value of the ion detection section.

3. The ion analyzer according to claim 2, wherein: the time constant adjuster is configured to adjust the time constant of the smoothing circuit to approximately (t.sub.1.sup.2+t.sub.2.sup.2), where t.sub.2 is the duration of the ion species of the same mass-to-charge ratio incident on the ion detection section and t.sub.1 is the output response time of the ion detection section.

4. The ion analyzer according to claim 1, wherein: the time constant adjuster is configured to adjust the time constant of the smoothing circuit to a value approximately equal to the duration of the ion species of the same mass-to-charge ratio incident on the ion detection section.

5. The ion analyzer according to claim 4, wherein: the time constant adjuster is configured to adjust the time constant of the smoothing circuit to a value approximately equal to the duration of the ion species of the same mass-to-charge ratio incident on the ion detection section when t.sub.2>2t.sub.1 is satisfied, where t.sub.2 is the duration of the ion species of the same mass-to-charge ratio incident on the ion detection section and t.sub.1 is the output response time of the ion detection section.

6. The ion analyzer according to claim 1, wherein: the ion detection section is a microchannel-plate detector.

7. The ion analyzer according to claim 6, wherein: the ion analyzer is a time-of-flight mass spectrometer.

8. The ion analyzer according to claim 7, further comprising: a multiturn time-of-flight mass separator; and a controller for controlling the time constant adjuster to change the time constant of the smoothing circuit according to one or more values selected from a mass-to-charge-ratio range, amount, number of turns, flight distance, and flight time of an ion to be introduced into and analyzed by the multiturn time-of-flight mass separator.

9. The ion analyzer according to claim 1, wherein: the ion detection section is a Faraday-cup detector.

10. The ion analyzer according to claim 9, wherein: the ion analyzer is an ion mobility spectrometer.

11. The ion analyzer according to claim 2, wherein: the ion detection section is a microchannel-plate detector.

12. The ion analyzer according to claim 3, wherein: the ion detection section is a microchannel-plate detector.

13. The ion analyzer according to claim 4, wherein: the ion detection section is a microchannel-plate detector.

14. The ion analyzer according to claim 5, wherein: the ion detection section is a microchannel-plate detector.

15. The ion analyzer according to claim 11, wherein: the ion analyzer is a time-of-flight mass spectrometer.

16. The ion analyzer according to claim 12, wherein: the ion analyzer is a time-of-flight mass spectrometer.

17. The ion analyzer according to claim 13, wherein: the ion analyzer is a time-of-flight mass spectrometer.

18. The ion analyzer according to claim 14, wherein: the ion analyzer is a time-of-flight mass spectrometer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic block configuration diagram of a multiturn TOFMS according to one embodiment of the present invention.

(2) FIG. 2 is a schematic configuration diagram of the ion detection section and the smoothing section in the present embodiment.

(3) FIG. 3 is a schematic configuration diagram of the ion detection section and the smoothing section in another embodiment.

(4) FIG. 4 is a waveform diagram for explaining an operation in the ion detection section and the smoothing section in the present embodiment.

(5) FIG. 5 is a schematic block configuration diagram of an ion mobility spectrometer according to another embodiment of the present invention.

(6) FIGS. 6A and 6B are model diagrams for explaining the state of the spread of ions flying in a common multiturn TOFMS.

(7) FIG. 7 is a graph showing one example of the waveform of the detection signal of a conventional microchannel-plate detector.

(8) FIGS. 8A-8C are graphs showing simulation results of the signal waveform of a detection signal produced by a microchannel-plate detector, with a random noise component superposed on the signal.

(9) FIGS. 9A and 9B are graphs showing simulation results of the signal waveform of a detection signal produced by a microchannel-plate detector, with a random noise component superposed on the signal.

DESCRIPTION OF EMBODIMENTS

(10) One embodiment of the ion analyzer according to the present invention is hereinafter described with reference to the attached drawings.

(11) FIG. 1 is a schematic block configuration diagram of a multiturn TOFMS according to the present embodiment. FIG. 2 is a schematic configuration diagram of an ion detection section and a smoothing section. FIG. 4 is the waveform diagram for explaining an operation in the ion detection section and the smoothing section.

(12) The multiturn TOFMS according to the present embodiment includes: an ionizer 1 for ionizing each component in a sample; an ion trap 2 for temporarily holding ions generated by the ionizer 1; a multiturn mass separator 3 for receiving various ions released from the ion trap 2 and for making those ions fly along a predetermined path to separate them according to their mass-to-charge ratios; an ion detection section 4 for sequentially detecting ions separated from each other by the multiturn mass separator 3; a smoothing section 5 for smoothing detection signals produced by the ion detection section 4; a waveform-shaping time adjuster 6 for adjusting the time constant in the smoothing section 5; a controller 7 for controlling the operations of the previously mentioned components; and an input unit 8 for allowing a user to set measurement conditions and other necessary items of information.

(13) A brief description of the measurement operation in the present TOFMS is as follows: The ionizer 1 ionizes various components in a sample introduced into it. The generated ions are temporarily held within an ion trap 2, which is either a linear type or three-dimensional quadrupole type of ion trap. Within the ion trap, for example, the ions which fall within a mass-to-charge-ratio range to be subjected to the measurement are selected. Since there is a limitation on the amount of ions that can be introduced into the multiturn mass separator 3, an operation for reducing the amount of ions by partially discharging the ions is also performed in the ion trap 2 if there is an excessive amount of ions to be subjected to the measurement. An operation for dissociating an ion by collision induced dissociation or similar techniques may also be performed in the ion trap 2.

(14) The ions temporarily held in the ion trap 2 are ejected from the ion trap 2 at a predetermined timing. The ejected ions are introduced into the multiturn mass separator 3. After flying along the flight path formed by the multiturn mass separator 3, the ions enter the ion detection section 4. While flying along the flight path, ion species having different mass-to-charge ratios are separated from each other and sequentially enter the ion detection section 4, having a time difference from each other. There are various possible configurations and structures for the multiturn mass separator 3, as described in Patent Literature 1, 2 or 4 for example. That is to say, there is no specific limitation on the shape of the path in which the ions are made to fly as well as the shape, structure, number and other aspects of the electrodes which form an electric field for making the ions fly. The ion detection section 4 produces a detection signal corresponding to the amount of incident ions. The smoothing section 5 smooths the detection signal and outputs the obtained signal, as will be described later. Though not shown, this output signal is sent to a data processor. The data processor converts the time of flight into mass-to-charge ratio and creates a mass spectrum showing the relationship between the mass-to-charge ratio and the ion intensity.

(15) The configuration and operation of the ion detection section 4 and the smoothing section 5 are hereinafter described in detail.

(16) The ion detection section 4 includes: a microchannel plate (MCP) 41 for generating electrons in response to an incident ion and multiplying the generated electrons; an anode 42, which is a flat metal plate, for collecting electrons released from the MCP 41; a floating power source 43 for generating a high direct-current voltage for driving the MCP 41; and an amplifier 44 for converting a current signal generated by the electrons which have reached the anode 42 into a voltage signal and amplifying the voltage signal. The MCP 41 in the present embodiment has a two-stage structure. The smoothing section 5 in FIG. 1 is a low-pass filter 5A located at the output end of the amplifier 44. The waveform-shaping time adjuster 6 adjusts one or both of the resistance value of the resistor (variable resistor) VR and the capacitance value of the capacitor (variable capacitor) VC constituting the low-pass filter 5A. The controller 7 sends the waveform-shaping time adjuster 6 the mass-to-charge ratio of an ion to be subjected to the measurement as well as other necessary information when performing an analysis according to the analysis program of the TOFMS.

(17) An operation of the ion detection section 4 and the low-pass filter 5A is hereinafter described.

(18) The following description assumes the case where a precise mass-to-charge ratio for an ion having a roughly known mass-to-charge ratio is determined by using a multiturn TOFMS.

(19) Upon receiving a command from the controller 7, the power source 43 applies a high appropriately-adjusted direct-current voltage to the MCP 41. As shown, when ions enter the MCP 41, the electrons generated in response to the ions are multiplied, and a large number of electrons are released. Those ions strike the anode 42, and an electric current whose amount corresponds to that of the electrons is sent to the amplifier 44. The amplifier 44 converts the current signal into a voltage signal and outputs this voltage. Such a response of the ion detection section 4 is speedy. Therefore, if a cloud of ions having the same mass to-charge ratio is spread in their direction of travel in the multiturn TOFMS as shown in FIG. 6B before entering the MCP 41, the waveform of the detection signal produced by the amplifier 44 will have multiple discrete peaks, for example, as indicated by the solid line in FIG. 4.

(20) The duration of the same ion species entering the MCP 41 depends on specific parameters to the TOFMS and those related to the control performed in the TOFMS for the measurement, such as the flight distance (number of turns of the loop path), configuration of the ion ejection source (e.g. ion trap 2), and method of application of the voltage for ejecting ions from the ion ejection source. The duration also depends on the mass-to-charge ratio of the ion to be subjected to the measurement. The controller 7 informs the waveform-shaping time adjuster 6 of a rough mass-to-charge ratio (mass-to-charge-ratio range) of the ion species as the measurement target beforehand (before the execution of the measurement), for example, based on the information set by a user through the input unit 8.

(21) The waveform-shaping time adjuster 6 calculates the time constant for the ion species to be subjected to the measurement, based on a calculation formula (or the like) which is previously determined according to the device-specific parameters mentioned earlier and other pieces of information. The waveform-shaping time adjuster 6 adjusts one or both of the resistance value of the resistor VR and the capacitance value of the capacitor VC constituting the low-pass filter 5A, so as to achieve the calculated time constant. More specifically, the waveform-shaping time adjuster 6 performs the following processing.

(22) Let t.sub.1 denote the output response time of the MCP 41. For an ion species which is to be subjected to the measurement, the waveform-shaping time adjuster 6 estimates the duration t.sub.2 of the ion species based on the rough mass-to-charge ratio (or mass-to-charge-ratio range) of the ion species. Using the following equation (1), the waveform-shaping time adjuster 6 determines the time constant tc of the low-pass filter 5A and calculates the resistance value of the resistor VR and/or the capacitance value of the capacitor VC from the time constant tc.
tc=(t.sub.1.sup.2+t.sub.2.sup.2)(1)
Subsequently, the waveform-shaping time adjuster 6 adjusts the resistance value of the resistor VR and/or the capacitance value of the capacitor VC to the calculated value or values.

(23) If a signal having a waveform with a series of peaks as shown by the solid line in FIG. 4 is passed through the low-pass filter 5A whose time constant has been adjusted in the previously described manner, its higher-frequency components are cut (in other words, the signal components are integrated), and a smoothed output signal as shown by the long-dashed short-dashed line in FIG. 4 is obtained. That is to say, the output signal forms a single large peak, i.e. a broad peak in an entirely integrated form, with the series of peaks corresponding to one ion species barely observable. The signal whose waveform is shaped in this manner by the low-pass filter 5A is fed to the subsequent circuits. Such a signal originating from the ions which have entered the MCP 41 can be easily distinguished from pulsed signals which may possibly be mixed in the signal.

(24) The previously described waveform shaping is needed when the ratio of the duration t.sub.2 of the ion species to the output response time t.sub.1 of the MCP 41 is equal to or higher than a certain threshold. To determine this threshold, a simulation using a random function has been performed as follows: Consider the situation in which a signal having a random Gaussian waveform has been detected with a waveform expressed by equation (2):

(25) $\begin{array}{cc}\mathrm{rg}\left(x,n,a\right)=n\exp \left(-4\log 2{\left(\frac{x}{a}\right)}^2\right)-\sqrt{n\exp \left(-4\log 2{\left(\frac{x}{a}\right)}^2\right)}+2R\left(x\right)\sqrt{n\exp \left(-4\log 2{\left(\frac{x}{a}\right)}^2\right)}+R\left(x\right)& \left(2\right)\end{array}$

(26) Equation (2) simulates a signal having intensity n and half-value width a with respect to variable x, on which a noise signal expressed by random function R(x) having an intensity of 1 and duration of 0.5 ns is superposed. The duration of the random function is assumed to be equal to the output response time t.sub.1 of the MCP 41. On the assumption that the SN ratio is 3 and the signal is detected at t=0, the waveform has been calculated under the condition that the duration t.sub.2 of the ion species is set to 0.5 ns, 1 ns, 1.5 ns, 2 ns and 5 ns. The results are as shown in FIGS. 8A, 8B and 8C as well as FIGS. 9A and 9B, respectively.

(27) Those graphs demonstrate that the detection signal can be satisfactorily distinguished from the noise signal when the duration t.sub.2 of the ion species is approximately equal to or shorter than 1 ns, i.e. two times the output response time t.sub.1 of the MCP 41. By comparison, when t.sub.2 is approximately two to three times t.sub.1 or greater, the waveform of the detection signal is split into multiple peaks. In such a situation, it may be difficult to distinguish some of those peaks from noise peaks. Accordingly, as a rough guide, it is reasonable to consider that the previously described smoothing process using the low-pass filter is useful when t.sub.2 is approximately equal to or greater than two times t.sub.1.

(28) It should be noted that using an excessively small time constant tc for the ion duration t.sub.2 or output response time t.sub.1 causes the problem that the waveform of the series of peaks cannot be sufficiently smoothed, whereas using a time constant tc which is slightly larger than an optimum value causes no practical problem. Therefore, when t.sub.2 is approximately equal to or larger than two times t.sub.1, it is possible to ignore t.sub.1, which means that equation (1) can be changed into an extremely simple form: tc=t.sub.2. That is to say, the time constant tc of the low-pass filter 5A can be roughly set to be the estimated value of the ion duration t.sub.2.

(29) If the duration t.sub.2 of the ion species having the same mass-to-charge ratio does not substantially change, it is unnecessary to adjust the time constant of the low-pass filter 5A. By comparison, if the duration t.sub.2 of the ion species having the same mass-to-charge ratio can significantly change, it is preferable to adjust the time constant of the low-pass filter 5A according to the parameters which affect the duration t.sub.2. Specifically, the duration t.sub.2 of the same ion species in a multiturn TOFMS tends to increase with an increase in the time of flight of that ion species. Furthermore, the duration t.sub.2 of the same ion species also tends to increase with the amount of ions, since the influence of the space-charge effect increases with the amount of ions. With these factors considered, it is preferable to configure the controller 7 to inform the waveform-shaping time adjuster 6 of the mass-to-charge-ratio range of the ions to be subjected to the measurement, flight distance (number of turns of the flight path), time of flight, amount of ions and other pieces of information so that the waveform-shaping time adjuster 6 can appropriately change the time constant of the low-pass filter 5A according to the provided information.

(30) In a measurement using a multiturn TOFMS, when a mass spectrum covering a wide range of mass-to-charge ratios needs to be obtained, it is often the case that the entire mass-to-charge-ratio range is divided into a plurality of narrower mass-to-charge-ratio ranges, and the measurement is performed for each of the narrow mass-to-charge-ratio ranges. In that case, the flight distance, or the number of turns of the flight path, may be varied for each narrow mass-to-charge-ratio range so that the time of flight will be roughly equalized regardless of the narrow mass-to-charge-ratio range. In that case, it is preferable that the time constant tc for a longer flight distance, i.e. for a narrow mass-to-charge-ratio range within which the ions to be subjected to the measurement have relatively large mass-to-charge ratios, be set at a larger value than for a narrow mass-to-charge-ratio range within which the ions to be subjected to the measurement have relatively small mass-to-charge ratios. Such a setting appropriately reduces the influence of the spatial dispersion of the same ion species and enables the measurement to be performed with a high level of sensitivity and accuracy regardless of the narrow mass-to-charge-ratio range.

(31) FIG. 3 is a schematic configuration diagram of the ion detection section and the smoothing section in another embodiment of the present invention. The components which are identical or correspond to those shown in FIG. 2 are denoted by the same reference signs. In the present embodiment, a low-pass filter 5B employing an operational amplifier is used as the smoothing section 5 in FIG. 1. Similar to the previous embodiment, the time constant of the low-pass filter 5B can be adjusted through the resistance value of the resistor VR and the capacitance value of the capacitor VC. The output voltage will also be basically similar to the previously described one.

(32) The waveform-shaping time adjuster 6 may be a mechanism for allowing an operator to manually adjust the variable resistor and variable capacitor constituting the low-pass filter 5A or 5B. In order to allow for the adjustment of the time constant according to the measurement parameters and other related factors in the previously described manner, the waveform-shaping time adjuster 6 must be configured so that the time constant of the low-pass filter 5A or 5B (smoothing section 5) can be adaptively (dynamically) adjusted.

(33) In the case where the time constant of the smoothing section 5 is frequently changed, it is preferable to configure the smoothing section 5 which includes an analogue-to-digital converter capable of a high-speed operation and a digital filter, instead of using the low-pass filter 5A or 5B which is an analogue circuit including circuit elements whose constants are variable. In that case, a computer or digital signal processor configured to set a plurality of coefficients which determine the frequency characteristics of the digital filter can be used as the waveform-shaping time adjuster 6. This waveform-shaping time adjuster 6 can shape the waveform of the detection signal by controlling the frequency characteristics of the digital filter according to pre-installed software or firmware. Although the low-pass filters used in the configurations shown in FIGS. 1 and 3 are first-order filters, it is naturally possible to use a second-order or higher-order filter.

(34) The previously described effect obtained by smoothing the detection signal from the ion detection section 4 by the smoothing section 5 can be recognized in any type of TOFMS in which the temporal spread of a cluster of ions becomes considerably large as compared to the response time of the ion detection section 4, although the smoothing is particularly useful in a multiturn TOFMS or other types of TOFMS in which ions are made to fly a long distance. If the previously described technique is applied in such a TOFMS, detection signals produced by the ion detection section upon detecting a cluster of ions from the mass separator can be distinguished from noise components and correctly evaluated. The previously described problem of the detection signals being difficult to be distinguished from noise components is not limited to multiturn TOFMSs; such a problem always occurs at high mass-to-charge ratios if a high-speed ion detector is selected in order to improve the mass-resolving power for ions with low mass-to-charge ratios. A conventional solution to this problem is to increase the number of accumulations of the detection signals to make the signals more distinguishable from noise components. If the present invention is used, the distinction between the detection signals and noise components can be achieved with a smaller number of accumulations.

(35) Accordingly, the present invention is applicable not only in a TOFMS employing a microchannel-plate detector but also in a TOFMS in which a detector employing an electron multiplier or avalanche photodiode is installed.

(36) The present invention is not limited to mass spectrometers. It may also be applied in an ion mobility spectrometer including a Faraday-cup detector capable of a high-speed response. FIG. 5 is a schematic block configuration diagram of an ion mobility spectrometer according to one embodiment of the present invention (for example, see Patent Literature 5).

(37) In this ion mobility spectrometer, various ions generated in an ionizer 1 are temporarily blocked by a shutter gate 12 and collected in front of this gate. When the shutter gate 12 is subsequently opened for a short period of time, the collected ions are simultaneously introduced into an ion drift section 13 and fly through the drift space. During this flight, the ions are separated from each other according to the ion mobility which mainly depends on the size of the ion. The separated ions sequentially reach an ion detection section 14. As in the previous embodiment, the same species of ions (i.e. a kind of ions having the same ion mobility) which should simultaneously reach the ion detection section 14 have a certain duration, and the detection signal originating from those ions has a waveform showing a series of peaks, as described earlier. Such a detection signal corresponding to the same ion species can be converted into a signal forming a single large peak by appropriately selecting the time constant in the smoothing section 5. Thus, the detection signals can be distinguished from noise components and correctly evaluated.

(38) Needless to say, the present invention can evidently be applied in an ion mobility-mass spectrometer in which ions are initially separated according to their ion mobilities and further separated according to their mass-to-charge ratios, if the previously described problem similarly occurs due to an ion detection section having an excessively short response time as compared to the duration of the same kind of ions at the point of time where the ions enter the ion detector.

(39) Any of the previously described embodiments and their variations is a mere example of the present invention, and any modification, change or addition appropriately made within the spirit of the present invention will evidently fall within the scope of claims of the present application.

REFERENCE SIGNS LIST

(40) 1 . . . Ionizer 2 . . . Ion Trap 3 . . . Multiturn Mass Separator 4, 14 . . . Ion Detection Section 41 . . . Microchannel Plate (MCP) 42 . . . Anode 43 . . . Power Source 44 . . . Amplifier 5 . . . Smoothing Section 5A, 5B . . . Low-Pass Filter 6 . . . Waveform-Shaping Time Adjuster 7 . . . Controller 8 . . . Input Unit 12 . . . Shutter Gate 13 . . . Ion Drift Section