Patent Grant
11101127
This application is a National Stage of International Application No. PCT/JP2017/039691 filed Nov. 2, 2017.
The present invention relates to a time-of-flight mass spectrometer, and more specifically, to a time-of-flight mass spectrometer configured to repeatedly and periodically perform a measurement operation which includes ejecting ions from an ion ejector and detecting the ions after making them fly in a flight space.
In a time-of-flight mass spectrometer (which is hereinafter appropriately called the “TOFMS”), various ions originating from a sample are ejected from an ion ejector, and the time of flight required for each of those ions to fly a specific distance is measured. Since the speed of a flying ion depends on the mass-to-charge ratio m/z of the ion, the time of flight also depends on the mass-to-charge ratio of the ion. Accordingly, the mass-to-charge ratio can be determined from the time of flight.
In
The reflector 3 includes a plurality of ring-shaped reflection electrodes 31 and a back plate 32. Predetermined DC voltages are applied from a reflection voltage generator 8 to the reflection electrodes 31 and back plate 32, respectively. Those voltages create a reflecting electric field within the space surrounded by the reflection electrodes 31. The ions are reflected by this electric field and once more fly in the flight space 2 to ultimately reach a detector 4. The detector 4 generates an ion intensity signal corresponding to the amount of ions which have reached the detector 4, and sends the signal to a data processor 5. The data processor 5 creates a time-of-flight spectrum showing the relationship between the time of flight and ion intensity signal, with the point of ejection of the ions from the ion ejector 1 defined as a time-of-flight value of zero, and calculates a mass spectrum by converting the time of flight into mass-to-charge ratio based on previously determined mass calibration information.
In such an OA-TOFMS, it is necessary to apply a high-voltage pulse having a considerably short duration and a magnitude of kV order to the push-out electrode 11 and/or extraction electrode 12 in the ion ejector 1 when ejecting the ions. In order to generate such a high-voltage pulse, a power unit as disclosed in Patent Literature 1 (which is called the “pulsar power source” in the same literature) has conventionally been used.
This power unit includes the following components: a pulse generator configured to generate a pulse signal for controlling the timing of the generation of the high-voltage pulse; a pulse transformer configured to transmit the pulse signal from a control system circuit to a power system circuit while providing electrical insulation between the control system circuit which operates at a low voltage and the power system circuit which operates at a high voltage; a drive circuit connected to the secondary winding of the pulse transformer; a high-voltage circuit configured to generate a high DC voltage; and a switching element employing a MOSFET configured to generate voltage pulses by turning on/off the DC voltage from the high voltage circuit according to a control voltage given through the drive circuit. Such a circuit is commonly used for generating high-voltage pulses and not limited to the TOFMS (see Patent Literature 2, 3 or other related documents).
In the case of an LC-TOFMS which includes a liquid chromatograph (LC) located in front of an OA-TOFMS having an atmospheric pressure ion source, such as an electrospray ion source, it is necessary to detect all kinds of compounds contained in a sample liquid which is continuously introduced from the exit port of the LC column into the atmospheric pressure ion source of the OA-TOFMS. To this end, the previously described measurement operation for creating a mass spectrum is repeatedly performed with a predetermined period in the OA-TOFMS. The shorter the measurement repetition period is, the shorter the time intervals of the measurement points on the created chromatogram become, which improves the accuracy of the peak waveform of the target compound and increases the reliability of the quantitative determination. Accordingly, in order to achieve the shortest possible time intervals of the measurement points on the chromatogram, a conventional control method is configured so that a relatively short measurement period is set for the measurement of the ions included within a low mass-to-charge-ratio range which corresponds to relatively short times of flight, while a relatively long measurement period is set for the measurement of the ions included within a high mass-to-charge-ratio range which corresponds to relatively long times of flight.
Specifically, in a conventional device of this type, a switching control is performed in which, for example, the measurement period is set at 125 μs for a low mass-to-charge-ratio range of m/z 2000 or lower, 250 μs for a medium mass-to-charge-ratio range of m/z 2000-10000, or 500 μs for a high mass-to-charge-ratio range of m/z 10000-40000.
The switching of the measurement period is performed by changing the time interval of the generation of the high-voltage pulse applied to the push-out electrode 11 and extraction electrode 12 in the ion ejector 1. That is to say, when the measurement period is changed, the parameters other than the time interval of the generation of the high-voltage pulse, such as the pulse duration (pulse application time), are fixed regardless of the measurement period. In the aforementioned power unit for the high-voltage pulse generation, a small amount of temporal delay inevitably occurs between the point in time where the pulse signal inputted to the pulse transformer begins to rise and the point in time of the start of the output of the high-voltage pulse from the power unit. In principle, this temporal delay should be constant and unaffected by the measurement period as long as the voltage value of the high-voltage pulse (pulse peak value) is the same. However, as described in Patent Literature 4, changing the measurement period in a conventional OA-TOFMS causes a temporal fluctuation of the start of the output of the high-voltage pulse from the power unit. It should be noted that the “start of the output of the high-voltage pulse” means a change in voltage which triggers the ejection of the ions in the ion ejector 1, as will be described later.
In TOFMSs, the time of flight of each ion is measured from the point in time of the ejection or acceleration of the ions. Therefore, in order to improve the accuracy of the measurement of the mass-to-charge ratio, the point in time of the beginning of the time-of-flight measurement must be maximally coincide with the timing at which the high-voltage pulse for ion ejection is actually applied to the push-out electrode 11 or the like. If a temporal fluctuation of the start of the output of the high-voltage pulse occurs depending on the measurement period as described earlier, a variation in time of flight occurs by an amount corresponding to the temporal discrepancy between the point in time of the beginning of the measurement and the point in time of the ejection of the ions caused by the temporal fluctuation, so that a mass discrepancy occurs even when the mass-to-charge ratio of the ion is the same. Consequently, a change in measurement period leads to a decrease in mass accuracy.
Patent Literature 1: JP 2001-283767 A
Patent Literature 2: JP H05-304451 A
Patent Literature 3: U.S. Pat. No. 4,511,815 B
Patent Literature 4: WO 2017/122276 A
In order to solve the previously described problem, the OA-TOFMS described in Patent Literature 4 has the function of changing the voltage applied from the primary-side drive circuit of the power unit to the two ends of the primary winding of the pulse transformer according to the measurement period of the repetitive measurement, whereby a shift of the timing at which the gate voltage of the MOSFET for switching the high voltage reaches the threshold voltage can be corrected regardless of the measurement period. As a result, the timing of the start of the output of the high-voltage pulse can be consistently maintained regardless of the measurement period, and a high level of mass accuracy can be achieved. This technique has the advantage that it can deal with various measurement periods as well as consistently maintain the timing of the start of the output of the high-voltage pulse with a considerably high level of accuracy. However, a voltage source which allows the output voltage to be changed with a high level of accuracy is required as the voltage source for generating the voltage applied to the two ends of the primary winding of the pulse transformer. Furthermore, for a high-speed switching of the measurement period, the output voltage of the voltage source also needs to be switched at high speeds. Due to those requirements, it is difficult to suppress the cost increase.
Needless to say, the polarity of the high-voltage pulse should be changed according to whether the high-voltage pulse is applied to the push-out electrode 11 or extraction electrode 12. Furthermore, in the case of applying the high-voltage pulse to the push-out electrode 11 (or extraction electrode 12), the polarity of the high-voltage pulse must be changed with a change in polarity of the target ion. Specifically, for the measurement of a positive ion, the high-voltage pulse is basically a positive voltage (a voltage with the positive polarity) when applied to the push-out electrode 11, whereas the high-voltage pulse is a negative voltage (a voltage with the negative polarity) when applied to the extraction electrode 12. On the other hand, for the measurement of a negative ion, the high-voltage pulse is basically a negative voltage (a voltage with the negative polarity) applied to the push-out electrode 11, whereas the high-voltage pulse is a positive voltage (a voltage with the positive polarity) when applied to the extraction electrode 12. In the present description, such a change in voltage of the high-voltage pulse for triggering the ejection of the ions is defined as the “start of the output of the high-voltage pulse”, regardless of whether the voltage is changed from a positive voltage to a negative voltage or vice versa. Additionally, as for the change in the gate voltage of the MOSFET for switching the high voltage, a voltage change from a level lower than a threshold voltage of the MOSFET to a level higher than the threshold voltage (i.e. a voltage change which turns on the MOSFET) is defined as the “rise” of the voltage, while a voltage change from a voltage higher than the threshold voltage of the MOSFET to a level lower than the threshold voltage (i.e. a voltage change which turns off the MOSFET) is defined as the “fall” of the voltage.
The present invention has been developed to solve such a problem. Its objective is to provide a time-of-flight mass spectrometer which allows for a change in measurement period of a repetitive measurement and yet can achieve a high level of mass accuracy regardless of the measurement period by reducing the temporal discrepancy between the point in time of the beginning of the time-of-flight measurement and the point in time of the ejection of the ions while suppressing the cost increase.
The present invention developed for solving the previously described problem is a time-of-flight mass spectrometer including an ion ejector configured to eject a measurement-target ion into a flight space by imparting acceleration energy by an effect of an electric field created by a voltage applied to an electrode, and a high-voltage pulse generator configured to apply a high-voltage pulse for ion ejection to the electrode, where the high-voltage pulse generator includes:
a) a DC power source configured to generate a high DC voltage;
b) a switch circuit including a switching element configured to generate the high-voltage pulse by switching the high DC voltage generated by the DC power source, and to output the high-voltage pulse to a voltage-output end;
c) a switching element driver configured to turn on/off the switching element according to a pulse signal for ejecting ions; and
d) an adjustment circuit including at least a resistor inserted in series with a control terminal of the switching element on a signal path extending from the switching element driver to the control terminal, the adjustment circuit configured to make the voltage at the control terminal be a voltage having a predetermined transient characteristic.
As one mode of the present invention, the switching element driver may include: a transformer including a primary winding and a secondary winding; a primary-side driver section configured to receive a pulse signal for ejecting ions and supply a driving current to the primary winding of the transformer in response to the pulse signal; and a secondary-side driver section connected to the secondary winding of the transformer, where the secondary-side driver section is configured to turn on/off the switching element.
The time-of-flight mass spectrometer according to the present invention is suitable for a device configured to repeatedly perform, with a predetermined measurement period, a measurement in which ions are ejected from the ion ejector and detected after being made to fly in a flight space, and in which the measurement period is variable. The reason for this suitability will be evident from the following explanation.
As is explained in Patent Literature 4, a cause of the temporal fluctuation of the start of the output of the high-voltage pulse from the power unit which occurs in a conventional OA-TOFMS when the measurement period is changed is the overshoot of the voltage sent to the control terminal (gate terminal) of the switching element used for turning on/off the high DC voltage. For example, in the switching element driver in the previously described mode of the present invention, when the pulse signal is sent to the primary-side driver section in order to eject ions from the ion ejector, a voltage is applied to the control terminal of the switching element through the transformer and secondary-side driver section. This causes an overshoot of the voltage at the control terminal of the switching element due to the LC resonance circuit which is mainly formed by the leakage inductance L of the transformer and the input capacitance C of the control terminal of the switching element. After the overshoot, the voltage (in absolute value) gradually decreases with the passage of time. However, an overshoot which occurred in the previous measurement is not yet sufficiently settled at the moment when ions are about to be ejected for the next measurement. Therefore, changing the measurement period causes a change in voltage value at the beginning of the rise of the voltage applied to the control terminal, which leads to a fluctuation of the period of time from the point in time where the voltage begins to rise to the point in time where the voltage reaches the threshold voltage for the switching element. This is the cause of the temporal fluctuation of the start of the output of the high-voltage pulse due to a change in measurement period.
In the OA-TOFMS described in Patent Literature 4, which is based on the assumption that the overshoot inevitably occurs, the voltage value at the point in time where the voltage applied to the control terminal of the switching element begins to rise is changed depending on the measurement period so that the timing at which the voltage reaches the threshold voltage is consistently maintained even when the measurement period is changed. By comparison, the present invention is configured to directly suppress the overshoot of the voltage applied to the control terminal of the switching element, which is the root of the temporal fluctuation of the start of the output of the high-voltage pulse, by providing a simple adjustment circuit including a resistor which gives a predetermined transient characteristic to the voltage at the control terminal of the switching element so that the timing at which the voltage reaches the threshold voltage is consistently maintained regardless of the measurement period.
Specifically, for example, the overshoot of the voltage at the control terminal of the switching element is suppressed by an adjustment circuit formed by a resistor inserted in series immediately before the control terminal of the switching element on a signal path extending from the secondary-side driver section in the switching element driver to the control terminal. Needless to say, the suppression of the overshoot should not cause a decrease in steepness of the rise and fall of the voltage at the control terminal of the switching element, otherwise it would conversely cause a temporal discrepancy of the high-voltage pulse. Therefore, it is preferable that the transient characteristic of the voltage at the control terminal of the switching element be appropriately determined so as to prevent an excessive overshoot while maximally avoiding the decrease in steepness of the rise and fall of the voltage.
Thus, in one preferable mode of the present invention, the resistance value of the resistor in the adjustment circuit is determined so as to substantially satisfy critical damping conditions.
This configuration sufficiently suppresses the overshoot of the voltage at the control terminal of the switching element while allowing the voltage to quickly rise and fall. The temporal discrepancy of the start of the output of the high-voltage pulse is thereby reduced even in the case of using different measurement periods, so that a high level of mass accuracy is achieved regardless of the measurement period.
It should be noted that a gate capacitor having an appropriate capacitance may be added if there is a considerable decrease in voltage at the control terminal of the switching element due to the natural electric discharge.
The present invention may additionally be configured as follows:
the switch circuit includes one or more plus-side switching elements and one or more minus-side switching elements connected in series, where each of the plus-side switching elements is configured to output a plus-side voltage generated by the DC power source to the voltage-output end when in the ON state, and each of the minus-side switching elements is configured to output a minus-side voltage generated by the DC power source to the voltage-output end when in the ON state;
the switching element driver includes a first switching element driver configured to respond to a first pulse signal and electrically charge the control terminal to a voltage which turns on the plus-side switching element or a voltage which maintains the plus-side switching element in the ON state, as well as a second switching element driver configured to respond to a second pulse signal and electrically charge the control terminal to a voltage which turns on the minus-side switching element or a voltage which maintains the minus-side switching element in the ON state; and
the time-of-flight mass spectrometer further includes a controller configured to generate the first pulse signal and the second pulse signal in addition to the pulse signal for starting the output of the high-voltage pulse so as to recharge the control terminal of the plus-side switching element or the minus-side switching element which is in the ON state.
In the present description, the “plus-side voltage and minus-side voltage” do not mean voltages having positive and negative polarities; they mean that the former voltage is higher than the latter. Accordingly, for example, it is possible that both the plus-side voltage and minus-side voltage are positive, or both the plus-side voltage and minus-side voltage are negative. The term “plus-side” in the “plus-side switching element” means, for example, that this switching element is located between the plus-side voltage and the voltage-output end so that the plus-side voltage is outputted to the voltage-output end when this switching element is in the ON state. Similarly, the term “minus-side” in the “minus-side switching element” means, for example, that this switching element is located between the minus-side voltage and the voltage-output end so that the minus-side voltage is outputted to the voltage-output end when this switching element is in the ON state.
According to this configuration, for example, when the first or second pulse signal for recharging is sent from the controller to the primary-side driver section in the switching element driver, the control terminal of the plus-side or minus-side switching element is recharged to a positive voltage (i.e. a voltage which turns on the switching element). The switching element is thereby maintained in the ON state, and the voltage of the high-voltage pulse is maintained in the previous state. Therefore, for example, even when the measurement period is extremely long and the time interval of the ion ejection is wide, an exact high-voltage pulse corresponding to that time interval can be generated. However, since the voltage at the charged or recharged control terminal gradually decreases due to the natural electric discharge, the voltage value at the point in time at which the voltage applied to the control terminal begins to rise varies depending on how much time has elapsed since the last charging or recharging operation when the high-voltage pulse is generated. Such a variation in voltage value leads to a temporal fluctuation of the start of the output of the high-voltage pulse.
Accordingly, as the first mode of the time-of-flight mass spectrometer having the previously described configuration, it is preferable to configure the controller to generate the second pulse signal for recharging and thereby recharge the control terminal of the minus-side switching element a specific length of time earlier than the point in time of the generation of the pulse signal for starting the output of the high-voltage pulse, when starting the output of the high-voltage pulse of the plus-side voltage, as well as generate the first pulse signal for recharging and thereby recharge the control terminal of the plus-side switching element a specific length of time earlier than the point in time of the generation of the pulse signal for starting the output of the high-voltage pulse, when starting the output of the high-voltage pulse of the minus-side voltage.
According to this configuration, the length of time elapsed from the last recharging operation to the generation of the high-voltage pulse is consistently maintained regardless of the measurement period, so that the voltage value at the point in time where the voltage applied to the control terminal of the switching element begins to rise becomes almost the same. Therefore, the charging voltage for the control terminal barely undergoes the decreasing effect due to the natural electric discharge, so that the temporal fluctuation of the start of the output of the high-voltage pulse can be even more reduced.
As the second mode of the time-of-flight mass spectrometer having the previously described configuration, a plurality of measurement periods may be set to be substantially equal to integer multiples of the shortest ion ejection period, and the resistance value of the resistor in the adjustment circuit may be determined according to the shortest ion ejection period and a control-terminal-recharging period with which the controller repeatedly sends the pulse signal for recharging.
As is evident from the previous description, the extent of the overshoot of the voltage applied to the control terminal of the switching element as well as the extent of the decrease in steepness of the waveform in the rising or falling phase of the voltage can be regulated by adjusting the resistance value of the resistor in the adjustment circuit. As in the first mode, even if it is impossible to synchronize the timing of sending the pulse signal for starting the output of the high-voltage pulse and the timing of sending the pulse signal for recharging, i.e. even if it is impossible to send the pulse signal for recharging a fixed length of time earlier than the point in time where the pulse signal for starting the output of the high-voltage pulse is sent, the voltage at the control terminal should gradually change with the repetition of the recharging if the relationship between the shortest ion ejection period and the control-terminal-recharging period is previously determined. Therefore, it is possible to almost equalize the voltage value of the voltage applied to the control terminal in generating the high-voltage pulse, by adjusting the extent of the overshoot of the voltage applied to the control terminal of the switching element or the extent of the decrease in steepness of the waveform in the rising or falling phase of the voltage according to the state of change in the voltage. By such an operation, as in the first mode, the temporal fluctuation of the start of the output of the high-voltage pulse can be even more reduced.
In a specific example of the second mode, the control-terminal-recharging period is shorter than the shortest ion ejection period, and the resistance value of the resistor in the adjustment circuit is determined so that the state of overdamping occurs. In some cases, the control-terminal-recharging period may be longer than the shortest ion ejection period, and the resistance value of the resistor in the adjustment circuit may be determined so that the state of insufficient damping occurs.
The present invention is applicable in any type of time-of-flight mass spectrometer configured to accelerate ions and send them into a flight space by an electric field created by applying a high-voltage pulse to an electrode in an ion ejector. That is to say, the present invention is not only naturally applicable in an orthogonal acceleration type of time-of-flight mass spectrometer but is also applicable in an ion trap time-of-flight mass spectrometer configured to accelerate ions held within an ion trap and send them into a flight space, as well as in a time-of-flight mass spectrometer configured to accelerate ions generated from a sample by matrix assisted laser desorption/ionization (MADLI) or similar methods and send them into a flight space.
According to the present invention, the timing to apply the high-voltage pulse to the electrode for ejecting ions can be consistently maintained even when the measurement period of a repetitive measurement is changed, so that a high level of mass accuracy can be realized regardless of the measurement period. To achieve such an effect, the present invention merely requires adding, to a conventional device, a simple circuit formed by a resistor and other inexpensive circuit elements. Thus, it is possible to suppress the cost increase while ensuring a high level of mass accuracy which is independent of the measurement period.
An OA-TOFMS as one embodiment of the present invention is hereinafter described with reference to the attached drawings.
In the OA-TOFMS according to the present embodiment, the acceleration voltage generator 7 includes a primary-side driver section 71, pulse transformer 72, secondary-side driver section 73, switch circuit 74, high-voltage power source 75, and primary-side power source 76. The controller 6 control the switching operation in the switch circuit 74 to control the primary-side driver section 71.
As shown in
The pulse transformer 72 is a ring-core transformer. One ring core is provided for the gate terminal of the MOSFET 741 in each stage of the switch circuit 74. The secondary winding 72b wound on each ring core is connected to the transformer load resistor 730 and the MOSFETs 731 and 732 in the secondary-side driver section 73. A single turn of cable passed through the ring core is used as the primary winding 72a. For this cable, a high-voltage insulated wire is used, which electrically insulates the primary side from the secondary side. The number of turns of the secondary winding may be appropriately determined.
The primary-side driver section 71 includes a plurality of MOSFETs 711, 712 and 715-718, as well as a plurality of transformers 713 and 714. Pulse signals a and b are sent from the controller 6 to a plus-side pulse signal input end 771 and minus-side pulse signal input end 772, respectively. The term “plus-side” in the “plus-side pulse signal input end 771” means that an input of a high-level signal to this input end turns on the plus-side MOSFET 741 (or maintains it in the ON state), as in a circuit operation which will be described later. Similarly, the term “minus-side” in the “minus-side pulse signal input end 772” means that an input of a high-level signal to this input end turns on the minus-side MOSFET 741 (or maintains it in the ON state), as in a circuit operation will be described later.
As shown in
Thus, a predetermined voltage is induced between the two ends of each secondary winding 72b of the pulse transformer 72. The voltage applied to the gate terminal of each MOSFET 741 via the transformer load resistor 730, MOSFETs 731 and 732, and gate discharge resistor 733 in the secondary-side driver section 73 as well as an adjustment circuit 742 in the switch circuit 74 (this voltage is hereinafter called the “gate voltage”) can be approximately expressed by the following equation:
[Gate Voltage]≈{[Primary-Side Voltage of Pulse Transformer 72]/[Number of MOSFETs 741 Serially Connected in Switch Circuit 74]}×[Number of Turns of Secondary Winding of Pulse Transformer 72] (1)
For example, if the primary-side voltage (VDD) of the pulse transformer 72 is 175 V, number of MOSFETs 741 connected in series in the switch circuit 74 is 12, and number of turns of the secondary winding of the pulse transformer 72 is one, a voltage which is approximately 174/12=14 V is applied to the gate terminal of each MOSFET 741.
When the voltage is applied in the forward direction between the gate terminal and source terminal of the six plus-side MOSFETs 741 in the switch circuit 74, those MOSFETs 741 simultaneously turn on. Meanwhile, the voltage is also applied in the reverse direction between the gate terminal and source terminal of the six minus-side MOSFETs 741 in the switch circuit 74, those MOSFETs 741 turns off. As a result, the voltage-supply end +V from the high-voltage power source 75 is almost directly connected to the voltage-output end 79, and a voltage of +V=+2500V is outputted to the voltage-output end 79.
At time t1, the pulse signal a inputted to the plus-side pulse signal input end 771 is changed to the low level (voltage zero). Then, the voltage between the two ends of each primary winding 72a in the pulse transformer 72 becomes zero. However, the gate voltage of the MOSFET 741 is roughly maintained at the same level by the electric charges already accumulated in the input capacitance of the gate terminal of the MOSFET 741, i.e. by the charging voltage for the gate terminal. The output voltage from the voltage-output end 79 is maintained at +V=+2500V. At a later point in time t2, the pulse signal b inputted to the minus-side pulse signal input end 772 is changed to the high level. This time, the MOSFET 712 turns on, which turns on the MOSFETs 717 and 718. A voltage is thereby applied between the two ends of the primary winding 72a in the pulse transformer 72 in the direction opposite to the previous direction, thereby producing an electric current flowing in the opposite direction. Consequently, a voltage is induced between the two ends of each secondary winding 72b of the pulse transformer 72 in the direction opposite to the previous direction, so that the six plus-side MOSFETs 741 in the switch circuit 74 turn off, while the six minus-side MOSFETs 741 turn on. Consequently, the output voltage from the voltage-output end 79 becomes zero (i.e. the value of −V).
When the pulse signal b inputted to the minus-side pulse signal input end 772 is changed to the low level (voltage zero), the voltage between the two ends of the primary winding 72a of the pulse transformer 72 becomes zero. However, the gate voltage of the MOSFETs 741 is roughly maintained at the same level by the electric charges already accumulated in the input capacitance of the gate terminal of each of the six minus-side MOSFETs 741, i.e. by the charging voltage for the gate terminal. Consequently, the output voltage from the voltage-output end 79 is maintained at 0V.
By the previously described basic operation, the acceleration voltage generator 7 generates a high-voltage pulse with a pulse peak value of +2500V at the timing corresponding to the pulse signals a and b inputted to the plus-side pulse signal input end 771 and minus-side pulse signal input end 772. As is evident from
In advance of a description concerning how the adjustment circuit 742 located between the secondary-side driver section 73 and the gate terminals of the MOSFETs 741 functions in the previously described operation, a specific description will be hereinafter given concerning a problem which occurs if the adjustment circuit 742 is not present, i.e. a problem with the conventional circuit.
In the secondary-side circuit of the pulse transformer 72, a resonance occurs in the LC circuit including the leakage inductance L of the pulse transformer 72 and the input capacitance C of the control terminal of the MOSFET 741. Therefore, an overshoot as shown in
As noted earlier, the timing of the start of the output of the high-voltage pulse is determined by the timing at which the MOSFETs 741 in the switch circuit 74 turn on/off, i.e. the timing of the rise/fall of the gate voltage of those MOSFETs 741. For example, in the case of the waveform shown in
In principle, the rising/falling waveform of the gate voltage should be unaffected by the measurement period of the repetitive measurement. However, when the ion ejection period is changed in order to change the measurement period, the phenomenon of a slight change in the rising/falling waveform of the gate voltage is observed.
In the present example, the gate terminal of the MOSFET 741 is charged from ˜19.0V to a predetermined positive voltage when the measurement period is 125 μs, whereas the gate terminal is charged from ˜18.3V to the predetermined positive voltage when the measurement period is 500 μs. That is to say, the voltage at the point in time where the gate voltage begins to rise varies depending on the measurement period. This is due to the effect of the overshoot mentioned earlier. The measurement period is one order of magnitude shorter than the settling time of the overshoot, which is a few to several milliseconds. Accordingly, it is necessary to generate the high-voltage pulse for the next measurement while the voltage is gradually decreasing (toward the target voltage) after the overshoot as shown in
Such a variation in the voltage at the point in time where the gate voltage begins to rise causes a shift of the point in time where the gate voltage reaches the threshold voltage, as shown in
In the OA-TOFMS according to the present embodiment, the adjustment circuit 742 including the gate resistor 742a has the function of eliminating the temporal discrepancy of the output voltage waveform depending on a change in measurement period which occurs due to the previously described cause.
As shown in
As shown in
In general, the elements used in each stage of the secondary-side drive section 73 or switch circuit 74 are the same. Therefore, the gate voltages respectively applied to the gate terminals of the MOSFETs 741 in those stages are almost equal to each other. Accordingly, in normal cases, the resistance values of the gate resistors 742a in those stages may also be the same. The appropriate resistance value of those gate resistors 742a can be experimentally determined by the manufacturers who offer the present device.
As just described, the timing discrepancy of the generation of the high-voltage pulse due to a change in measurement period can be sufficiently decreased by providing the adjustment circuit 742 having a simple configuration on the wiring connected to the gate terminal of the MOSFET 741 in each stage. In the OA-TOFMS according to the present embodiment, the timing discrepancy of the generation of the high-voltage pulse due to the change in measurement period can be even more reduced by additionally performing a control which will be hereinafter described.
As shown in
The temporal interval of the high-voltage pulse shown in
In order to perform the previously described control, it is necessary to generate either the dummy-pulse signal b′ at the timing which is the specific period of time tc earlier than the point in time of the generation of the high-voltage pulse, or to initially generate the dummy-pulse signal b′ in response to a command for the execution of the measurement and subsequently generate the high-voltage pulse after the passage of the specific period of time tc. Therefore, the previously described control cannot be performed in the case where the generation of the high-voltage pulse and that of the dummy-pulse signal b′ are not synchronized with each other in the controller 6. In such a case, it is preferable to perform the following control.
In this case, the plurality of ion ejection periods (measurement periods) are set to be equal to integer multiples of the shortest ion ejection period tp. For example, if the shortest ion ejection period tp is 125 μs, the ion ejection periods are set at 125 μs, 250 μs and 500 μs. On the other hand, the gate-charging period tgc, which is the period of the generation of the dummy pulse signal b′, is set to be slightly shorter or longer than the shortest ion ejection period tp. For example, if the shortest ion ejection period tp is 125 μs, the gate-charging period tgc is set at 105 μs or 150 μs.
[Case 1] Shortest Ion Ejection Period tp>Gate-Charging Period tgc (
That is to say, in the present embodiment, the gate voltage is given a predetermined transient characteristic as shown in
It should be noted that a gate capacitor may be added as needed to control the decrease in charging voltage due to the natural electric discharge so that the gate voltage immediately before the generation of the high-voltage pulse will be even more consistently maintained.
[Case 2] Shortest Ion Ejection Period Tp<Gate-Charging Period Tgc (
For example, this is the case where the shortest ion ejection period tp is 125 μs and the gate-charging period tgc is 150 μs. Since the shortest ion ejection period tp<the gate-charging period tgc, the longer the measurement period is, the shorter the period of time becomes from the point in time of the last charging of the gate terminal to the point in time of the generation of the high-voltage pulse, as shown in
That is to say, in the present embodiment, the gate voltage is given a predetermined transient response as shown in
As with the previously described case, a gate capacitor may be added as needed to control the decrease in charging voltage due to the natural electric discharge so that the gate voltage immediately before the generation of the high-voltage pulse will be even more consistently maintained.
The descriptions thus far have been concerned with the case where the measurement-target ion is a positive ion. In the case where the measurement-target ion is a negative ion, the ion is ejected by applying a high-voltage pulse having a pulse peak value −V (e.g. −2500V) to the push-out electrode 11. It is evident that such a high-voltage pulse can be generated in the acceleration voltage generator 7 by setting +V=0 and −V=−2500V as well as appropriately changing the timing of the pulse signals a and b.
It should be noted that the previously described embodiment is a mere example of the present invention, and any change, addition or modification appropriately made within the spirit of the present invention will naturally fall within the scope of claims of the present application.
For example, as opposed to the previously described embodiment in which the present invention is applied in an OA-TOFMS, the present invention may also be applied in other types of TOFMS, such as an ion trap time-of-flight mass spectrometer in which ions held within a three-dimensional quadrupole type or linear type of ion trap are accelerated and sent into a flight space, or a time-of-flight mass spectrometer in which ions generated from a sample by a MALDI ion source or similar device are accelerated and sent into a flight space.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/039691 | 11/2/2017 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2019/087347 | 5/9/2019 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4511815 | Wood | Apr 1985 | A |
| 9870910 | Haufler | Jan 2018 | B2 |
| 20130214148 | Albeanu | Aug 2013 | A1 |
| 20190006168 | Mizutani | Jan 2019 | A1 |
| Number | Date | Country |
|---|---|---|
| 5-304451 | Nov 1993 | JP |
| 2000-134075 | May 2000 | JP |
| 2001-283767 | Oct 2001 | JP |
| 2017122276 | Jul 2017 | WO |
| Entry |
|---|
| International Search Report for PCT/JP2017/039691 dated Feb. 6, 2018 [PCT/ISA/210]. |
| Written Opinion for PCT/JP2017/039691 dated Feb. 6, 2018 [PCT/ISA/237]. |
| Number | Date | Country | |
|---|---|---|---|
| 20210202229 A1 | Jul 2021 | US |
