JP3058651B2 - Method and apparatus for mass spectrometry - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method and an apparatus for mass spectrometry, and more particularly to a mass spectrometer characterized by setting the voltage of a plurality of electrodes used to guide generated ions to a mass spectrometer. Method and apparatus.

[Conventional technology]

When the sample is atomized and guided to an ionization section with an inert gas such as nitrogen gas or argon, and given microwave energy thereto, the sample and gas ionize and a plasma is formed. Attempts are being made to introduce ions in the plasma to a mass spectrometer for mass spectrometry.

In such microwave plasma ion mass spectrometry, it is desirable to guide the generated ions in a focused state at a predetermined position, and then to introduce the ions into a mass spectrometry unit. In order to guide ions in the plasma to a predetermined position in a focused state, it is desirable to dispose a plurality of electrodes between the plasma generating unit and the predetermined position.

The sample is atomized and guided to an ionization section together with an inert gas such as argon, and the sample and the inert gas are ionized also by exposure to a high-frequency magnetic field to generate plasma. Therefore, if the ions in the plasma are extracted and introduced into the mass spectrometer, mass analysis can be performed in the same manner. This is known as inductively coupled high frequency plasma ion mass spectrometry, and is described in, for example, Japanese Patent Application Laid-Open No. 62-20231 (corresponding US Pat. No. 4,740,696) and US Pat. No. 4,804,838. .

Also in the case of such inductively coupled high-frequency plasma ion mass spectrometry, it is desirable to guide the ions in the plasma in a focused state at a predetermined position and then to introduce the ions into the mass spectrometry unit. It is desirable to arrange a plurality of electrodes for guiding ions in a focused state between the predetermined positions.

[Problems to be solved by the invention]

Generally, different voltages are applied to a plurality of electrodes,
It is not easy to set those voltages to optimal values.

Accordingly, it is an object of the present invention to provide a method and apparatus for mass spectrometry in which a plurality of electrode voltages can be easily set.

Another object of the present invention is to provide a method and apparatus for mass spectrometry suitable for enabling accurate setting of a plurality of electrode voltages.

Another object of the present invention is to provide a mass spectrometry method and apparatus suitable for preventing a change in sensitivity based on drifts of a plurality of electrode voltages.

Still another object of the present invention is to provide a method and apparatus for mass spectrometry suitable for preventing an erroneous voltage setting based on saturation of an ion detection output.

Still another object of the present invention is to provide a method and an apparatus for mass spectrometry capable of increasing the counting efficiency.

It is still another object of the present invention to provide a mass spectrometry method and apparatus which can clarify any mass number error.

[Means for solving the problem]

In the present invention, the voltages applied to the plurality of electrodes are respectively scanned, and the voltage is automatically set to a predetermined value based on the ion detection output obtained through the scanning.

According to one aspect of the invention, the mass number scan is performed repeatedly, and the voltage scan is performed for each mass number scan prior to the mass number scan.

According to another aspect of the invention, the voltages applied to the plurality of electrodes are scanned in a predetermined order and are automatically set based on the ion detection output obtained through the scan. Subsequently, the voltage of at least one selected electrode of the plurality of electrodes is scanned, and the voltage of the selected electrode is automatically checked and reset based on the ion detection output obtained through the scan. You.

[Action]

According to the present invention, the voltage applied to the plurality of electrodes is scanned, and the voltage is automatically set to a predetermined value based on the ion detection output obtained through the scanning, thereby facilitating the setting. .

According to one aspect of the present invention, the mass number scan is performed repeatedly, and the electrode voltage scan is performed prior to the mass number scan for each mass number scan, so that the sensitivity change based on the drift of the electrode voltage that occurs during mass analysis. Is prevented.

According to another aspect of the invention, after scanning the plurality of electrode voltages, at least one of the selected electrode voltages is re-scanned, so that the electrode voltages can be brought closer to an appropriate value. Become like That is,
More accurate setting of the electrode voltage becomes possible.

〔Example〕

Referring to FIG. 1, microwave power is supplied from a microwave power generator (for example, 2.45 GHz, 1 kilowatt) 1 to a plasma generator through a waveguide 2. At this time, the sample 4 is atomized by a nebulizer 5 together with the discharge gas 3 of N (nitrogen) and introduced into the plasma generating section to generate plasma. The ions in the plasma are guided to the mass spectrometer 6 in a vacuum. That is, the ions are focused by the ion lens system 6A, further subjected to mass analysis by the quadrupole mass filter 6B, which is a mass analyzer, and the mass-analyzed ions are detected by the ion detector 7. The output of the detector 7 is amplified by a pulse amplifier 8 and introduced into a discriminator 9. The discriminator 9 has a discrimination level automatically set by the master CPU 29, and only a pulse signal exceeding this level enters the gate circuit 12.

On the other hand, from the user by the personal computer 31,
The analysis condition setting information is passed through the interface 30 to the master
The CPU 29 enters the CPU 29, and the CPU converts the information signal into a command signal in the register 28 and transfers the command signal to the slave CPU 20. The slave CPU 20 can store the data of the voltage corresponding to the mass number to be scanned in the scan memory 18 for the mass number scan by the quadrupole mass filter 6B of the ion mass spectrometer in accordance with the command signal of the register 28. It has become.
Therefore, the slave CPU 20 sends the address signal to the latch circuit 16.
The address is set in the scan memory 18 via the up-down counter 17. Scan data (mass number) corresponding to the address is stored in the scan memory 18 through the buffer 19. By repeating this operation, a series of scanning mass number data is stored in the scan memory 18.
Ends the storage.

After that, when the personal computer 31 starts the measurement, a start signal is sent from the slave CPU 20 to the scan timing circuit 15, and the circuit 15 opens the gate of the gate circuit 12 according to the scan mode. At the same time, a clock pulse signal is input to the up-down counter 17, the counter of the up-down counter 17 is incremented, the memory address of the scan memory 18 is scanned, the mass number data is output, and the data is temporarily stored in the latch circuit 22. Is input to the D / A converter 23. The D / A converter 23 drives the high-frequency power supply 24 to step scan the quadrupole mass filter 6B. That is, mass number scanning is performed. Ion pulses within the time of the step are counted by a pulse counter 13 through a gate circuit 12, sent to a latch circuit 14, and the count value for each step scan is stored in a memory unit in the slave CPU 20. This scan is repeated several times, and the master CPU 29 calculates the average value of each ion count corresponding to the scan position, that is, the mass number, calculates the spectrum, and displays the spectrum on the screen of the personal computer 31. Buffer 21
, The data of the scan memory is simultaneously sent to the slave CPU 20, and the mass number currently scanned is compared with the mass number stored in advance, and the mass number is displayed on the horizontal axis of the screen of the personal computer 31. If they do not match, a mismatch display is displayed on the screen of the personal computer 31.

Further, the master CPU 29 has a function of controlling the D / A converter 11 and setting the voltage of the ion detector high-voltage generator 10 to a position where the multiplication sensitivity is good.

FIG. 2 shows an electrode arrangement of the ion lens system of FIG.
The ions in the plasma are extracted by the extraction electrode 50 through the sampling cone electrode 50 ′, and further accelerated by the acceleration electrode 51 through the extraction electrode 50 and the acceleration electrode 51. The ions accelerated in this way pass through the gap between these electrode elements by means of a collecting electrode 54 consisting of a central electrode element 52 arranged on the center line of the lens system and a peripheral electrode element 53 arranged around it. The ions passed in this way are focused by the focusing electrode 55 through the differential aperture electrode 56 to the position of the slit electrode 57, and the focused ions enter the quadrupole mass filter 6B through the slit electrode 57 for mass analysis. .

When plasma is generated, light is emitted, and the emitted light is shielded by the center electrode element 52.

The sampling cone electrode 50 ', the extraction electrode 50 and the slit electrode 57 are grounded.
51, collecting electrode 54, focusing electrode 55, and differential aperture electrode 56 have -350 volts, +8 volts, and +31 volts, respectively.
Volts and +20 volts are applied. These voltages should be set so that the output of the ion detector 7 is maximized, but the adjustment is practically difficult. Therefore, in the present invention, the adjustment is automatically performed to facilitate the adjustment.

FIG. 3 shows only two of the accelerating electrode 51, the collecting electrode 54, the focusing electrode 55, and the differential aperture electrode 56 to which a voltage is applied. 2 shows a flow for automatically adjusting and setting the applied voltage. Here, the two selected electrodes are A and B.

In this electrode adjustment, the mass adjustment of the quadrupole mass filter is not performed, and the voltage adjustment is performed while fixing the mass at a certain mass. As an example, the voltage adjusting method will be described of using mass number of isotopic ion of nitrogen N ¹⁵ in the discharge gas.

Referring to Figure 3, the mass spectrum of the first N ¹⁵ is measured. During this measurement, the initial voltages of the electrodes A and B are set. Subsequently, n = 1 is set, the voltage of the electrode A is scanned, and it is determined whether or not the output of the ion detector at that time is maximum. This decision step is repeated until the maximum peak value is obtained. When the maximum peak value is obtained, the voltage _VPA1 of the electrode A at that time is set.

Next, a determination is made as to whether n = 1. Since the setting of n = 1 has been performed, it is determined in this step that n = 1 is correct, and the process proceeds to the voltage scanning of the electrode B. It is determined whether the output signal obtained from the ion detector through this voltage scan is the maximum. This decision is repeated until the maximum peak value is obtained. When the maximum peak value is obtained, the voltage V _PB1 of the electrode B at that time is set. Thereafter, it is determined whether or not n = 1. In this case, since n = 1 is correct, it is then determined whether or not n = 2. Obviously, n is not 2, so in this case the value of n is incremented by "1". That is, the setting of n = 2 is performed. In this state, the voltage scanning of the electrode A is performed again. Then, it is determined whether or not n = 1 is correct. In this case, since n = 2, it is determined that n = 1 is not correct, and when the ion detector output obtained through the second scan of the electrode B is maximum. Electrode A voltage V
_It is determined whether _PA2 is equal to the first set voltage _VPA1 . Maximum two Although the equal, the voltage of electrode A if remains set to V _PA1, if not equal, and the first ion detector output maximum peak value P _A1 of the second ion detector output The comparison of the peak value _PA2 is performed.
When the former is larger than the latter, the set voltage of electrode A is V _PA1
In the opposite case, the voltage of the electrode A becomes V _PA2
Is reset to.

The setting of the voltage of the electrode B is similarly performed as can be seen from the flow of FIG. Finally, it is determined whether or not n = 2. In this case, since n = 2 is correct, the voltage setting adjustment of the electrodes A and B is completed.

When there are three or more electrodes, the number of steps is increased by the number of the electrodes, and the concept of voltage setting is exactly the same as in the case of two electrodes.

A plurality of electrodes can be considered to constitute one lens. Since the voltage adjustment is performed for each electrode,
This causes a voltage change between the electrodes, which has an overall effect on the output of the ion detector. Therefore, if the adjustment of each electrode voltage is performed a plurality of times, each electrode voltage can be brought close to the optimum voltage value that gives the maximum ion detection output as a whole.

The degree of the effect of each electrode voltage on the ion detection output is not necessarily the same, and a certain electrode voltage may have a very small effect on the ion detection output as compared to other electrode voltages. In this case, the adjustment may be performed in the order of the electrode voltage having the largest influence. In some cases, only one check of the electrode voltage having a very small influence is required.

FIG. 4 to FIG. 6 are flow charts for adjusting and setting three electrode voltages with n = 3. However, this is an example in which the first and second electrode voltage checks are performed three times, but the third electrode voltage check is performed twice. The three electrodes will be described as A, B and C respectively.

First, the spectral measurement of the N ¹⁵ is performed. Through the measurement, the initial voltages of the electrodes A, B and C are set. Thereafter, the setting of n = 1 is performed, and the voltage scanning of the electrode A is performed in this state. This scan is repeated until the ion detector output gives the maximum peak value. When the output gives the maximum peak, the voltage _VPA1 of the electrode A at that time is set. Thereafter, it is determined whether or not n = 1. In this case, since n = 1 is correct, the process then proceeds to the voltage check and setting of the electrode B (FIG. 5). This check and setting are performed in the same manner as the voltage of the electrode A. Then, n = 1
It is determined whether or not n = 1 is correct. Then, the process proceeds to check and set the voltage of the electrode C (FIG. 6). The check and setting are the same as those of the voltage of the electrode A.
Finally, it is determined whether n = 3. In this case, n
Since = 3 is not correct, the process returns to checking and setting the voltage of the electrode A again. That is, the value of n is incremented by “1”. The method of checking and setting is the same as that of the first time.

Next, it is determined whether or not n = 1, but n = 2.
Is correct, then it is determined whether n = 2. In this case, since n = 2 is correct, a second comparison between the voltage V _PA2 of the electrode A and the first voltage V _PA1 is performed. If they are equal, V _PA1 is not changed, but if they are not equal, a comparison is made between the first ion detector output P _A1 and the second ion detector output P _A2 . If the former is greater than the latter, V _PA1 is not changed, but in the opposite case, the voltage on electrode A is reset to V _PA2 instead of V _PA1 .

Thereafter, the same operation is performed for the electrodes B and C. Finally, it is determined whether n = 3,
Since n = 2, the process returns to the check setting of the voltage of the electrode A again. That is, n is further incremented by “1”. As a result, a check of the voltage of the electrode A,
The settings are made again. The first and second steps
It is the same as the first time. Thereafter, it is determined whether or not n = 1. However, since n = 1 is not satisfied, then n = 2.
A determination is made whether or not. Of course, since n = 3 instead of n = 2, this time the voltage V _{PA3 of the} third electrode A
It is determined whether or not is equal to the second set voltage _VPA1 or _VPA2 . If they are equal, the voltage setting of the electrode A is not changed, but if they are not equal, the third ion detector output _PA3 and the second ion detector output _PA1 or P2.
A big or small decision is made with _A2 . Of course the former is not performed voltage changes if a small electrode A, the voltage of electrode A when the opposite is re-set to V _PA3.

Thereafter, the process goes to the third check and setting step of the voltage of the electrode B (FIG. 5). The method is the same as that for the electrode A. When this is completed, a determination is made as to whether n = 3. Of course, since n = 3, the adjustment and setting of the voltages of all the electrodes are completed.

The above-described adjustment and setting of the electrode voltage is performed before each mass number scan. That is, when the high-frequency scanning of the high-frequency power supply 24 and the scanning of the mass number are performed as shown in FIG. 7 (b), the mass spectrum is obtained as shown in FIG. 7 (c). Adjustment and setting are performed during the preceding stage T (see FIG. 7A) of the repetitive mass number scan. Thereby, the change in sensitivity due to the drift of the electrode voltage is reduced.

Isotope abundance ratio in the nitrogen gas is 99.6% N ¹⁴ 0.37% N
There are ¹⁴ . Adjustment of the electrode voltage, if high or if the sample concentration, the voltage with the use of N ¹⁵ ions for setting may sometimes ion detection output is saturated. This problem can be solved by using N ¹⁵ ions having a low abundance ratio. In addition, argon, which is an inert gas, includes Ar ³⁶ having an isotope ratio of 0.337%, Ar ^{38 having} 0.063%, and Ar ⁴⁰ having an isotopic ratio of 99.6%. Therefore, when using argon for micro-liquid plasma generation, Ar ³⁶ or Ar ³⁸ having a small abundance ratio may be used.

When gas ions for generating microwave plasma are used for adjusting and setting the electrode voltage, there is also an advantage that it is not necessary to prepare a special standard sample.

FIG. 8 is an example of a display screen displayed when adjusting the electrode voltage. The voltage values of electrodes A, B, C and D are displayed, and N
The count values of the pulse output from the ion detector for the ¹⁵ spectra are shown in a bar graph superimposed for each electrode voltage. Thereby, the adjustment and setting status of the electrode voltage can be visually confirmed.

FIG. 9 is a block diagram for setting the discrimination level of the discriminator. The output ion signal from the ion detector 7 is amplified by the pulse amplifier 8 and input to the discriminator 9. The master CPU controls the D / A converter 11 'and sets the discrimination level of the discriminator 9. At this time, as shown in FIG. 10, the noise N is counted by the pulse counter 13 as the number of pulse counters at each equally spaced point in the pulse peak value adjustment range LDL to HDL before plasma generation, and stored in the memory 32.
Next, a plasma is generated, N ¹⁵ ions are measured, the count number of the signal S at the same point is counted, and the S / N ratio of each point is calculated by the master CPU 29. Then, the maximum peak value DL of the S / N curve is obtained, and the discrimination level of the discriminator 9 is set at this position. Thereby, the pulse counting efficiency is increased, and the detection sensitivity is increased.

Note that the horizontal axis in FIG. 10 is the pulse peak value, which represents the magnitude of the pulse, which is the vertical axis in FIG.

〔The invention's effect〕

 According to the present invention, the following effects can be expected.

(1) Setting of a plurality of electrode voltages is easy.

(2) It is possible to accurately set the electrode voltage.

(3) A change in sensitivity based on the drift of the electrode voltage is prevented.

(4) An erroneous voltage setting based on the saturation of the ion detection output is prevented.

(5) High counting efficiency is achieved.

(6) Mass number error can be confirmed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a mass spectrometer showing one embodiment of the present invention, FIG. 2 is a layout diagram of the lens system of FIG. 1, and FIG. 3 is an example of adjustment and setting of an electrode voltage based on the present invention. Flow diagram,
4 to 6 are flow charts of another example of adjustment and setting of the electrode voltage according to the present invention, and FIG. 7 shows one relationship between the mass number scanning and the adjustment and setting of the electrode voltage according to the present invention. FIG. 8 is a display example of one of the screens for adjusting and adjusting the electrode voltage according to the present invention, FIG. 9 is a circuit diagram of the discriminator in FIG. 1, and FIG. 10 is a diagram for setting a discrimination level. FIG. 11 is a diagram for explaining the horizontal axis of FIG. 6 mass spectrometer, 9 discriminator, 18
Scan memory, 20: slave CPU, 29: master CP
U, 31 ... Personal computer.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-27553 (JP, A) JP-A-2-64449 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 49/26 G01N 27/62 H01J 49/12

Claims (6)

(57) [Claims] 1. A method for generating ions, focusing the ions at a predetermined position using a plurality of electrodes, guiding the ions to a mass spectrometer, and performing mass scanning while performing a mass number scan. A mass spectrometry method for analyzing, detecting ions subjected to mass spectrometry, and displaying a detection result on a display means, wherein each of the voltages applied to the plurality of electrodes is scanned prior to the mass number scanning, and the scanning is performed. Mass spectrometry, wherein the voltage is set based on the ion detection output obtained through step (a), and the applied voltage value to each electrode after the setting is displayed on the display means. 2. The mass spectrometric method according to claim 1, wherein each of the voltages to be set is a value when the ion detection output is substantially maximum. 3. The mass spectrometry method according to claim 3, wherein the ions used for voltage scanning of the plurality of electrodes are ions of the isotope having a smaller abundance ratio among the isotopes present. . 4. An ionization section for ionizing a sample to be measured, a plurality of electrodes for guiding ions generated in the ionization section to a predetermined position in a focused state, and a voltage application for applying a voltage to each of the electrodes. Unit, a mass spectrometry unit that repeats mass number scanning so as to perform mass analysis on the guided ions, a detection unit that detects mass-analyzed ions, and a display unit that displays a detection result of the detection unit, A mass spectrometer having a control unit that controls the operation of each unit, wherein the control unit scans a voltage applied from the voltage application unit to the plurality of electrodes prior to the mass number scan, and scans the voltage. A voltage applied to the plurality of electrodes is set based on an output of the detection unit obtained through the detection unit, and the set voltage value is displayed on the display unit. 5. The mass spectrometer according to claim 4, wherein each of the voltages to be set is a value when the output of the detection unit is substantially maximum. 6. The mass fractionating device according to claim 4, wherein the voltage value to each electrode after setting, which is displayed on the display section, is displayed in a bar graph shape for each electrode.